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The evolution of the Thor-Odin gneiss dome and related geochronological studies Duncan, Ian James 1982

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C-l THE EVOLUTION OF THE THOR-ODIN GNEISS DOME AND RELATED GEOCHRONOLOGICAL STUDIES by Ian James Duncan B.A. [Hons. I ] , M a c q u a r i e U n i v e r s i t y , 1973 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n THE FACULTY OF GRADUATE STUDIES (De p a r t m e n t o f G e o l o g i c a l S c i e n c e s ) We a c c e p t t h i s t h e s i s as c o n f o r m i n g to t h e r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA A p r i l 1982 © Ian James Duncan, 1982 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of &e*olo^iCcd{ Sct€MC&3 The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date DE-6 (3/81) i i ABSTRACT The T h o r - O d i n g n e i s s dome i s one o f s e v e r a l s t r u c t u r a l c u l m i n a t i o n s a l o n g the e a s t e r n m a r g i n o f t h e Shuswap t e r r a i n . The g n e i s s domes have p r e v i o u s l y been a s c r i b e d b o t h t o d i a p i r i c u p r i s e o f g r a n i t i c g n e i s s e s and t o i n t e r f e r e n c e o f l a t e - s t a g e b u c k l e f o l d s . The ages o f r o c k s i n the g n e i s s domes have a l s o been t h e s u b j e c t o f c o n t r o v e r s y . Age e s t i m a t e s range f r o m A r c h e a n t o M e s o z o i c . T h i s s t u d y p r o v i d e s t h e b a s i s f o r a d e t a i l e d model f o r a new i n t e r p r e t a t i o n o f t h e s t r u c t u r a l e v o l u t i o n o f t h e T h o r - O d i n g n e i s s dome. P r i o r t o doming, t h i s a r e a was c h a r a c t e r i z e d by t h e f o r m a t i o n o f l a r g e - s c a l e nappe s t r u c t u r e s and i m b r i c a t i o n o f A r c h e a n basement r o c k s w i t h t h e c o v e r r o c k s e q u e n c e . The f i r s t p e r i o d o f d e f o r m a t i o n (Phase One) c o n s i s t e d o f l a r g e - s c a l e i n f o l d i n g o f t h e c o v e r r o c k s e q u e n c e i n t o the basement r o c k s . The P i n g s t o n f o l d i n t h e c o r e o f t h e dome i s a p r o d u c t o f t h i s e v e n t . The s e c o n d p e r i o d o f d e f o r m a t i o n (Phase Two) was marked by t h e f o r c i n g o f wedges o f basement i n t o t h e c o r e s o f n o r t h e r l y - m o v i n g nappes. The t h i r d p e r i o d o f d e f o r m a t i o n (Phase T h r e e ) was c o - a x i a l w i t h Phase Two and c o n s i s t e d o f i m b r i c a t i o n and r e f o l d i n g o f t h e upper l e v e l s o f t h e s t a c k o f Phase Two nappes. S t r a i n a n a l y s i s b a s e d on f o l d s h a p e , d i s t o r t e d l i n e a t i o n p a t t e r n s and e l l i p t i c a l s t r a i n markers d e m o n s t r a t e t h a t f i n i t e s t r a i n s a r e h i g h e s t i n t h e i m b r i c a t e d zone between the Basement C o r e d Nappe Domain and t h e C o v e r Rock Domain. F l a t t e n i n g s t r a i n s a r e a n o m a l o u s l y low i n t h e A u t o c h t h o n o u s C o r e G n e i s s Domain. A n a l y s i s o f Phase Two f o l d axes d i s t o r t e d i n t o f l a t t e n e d s m a l l c i r c l e s by Phase T h r e e f o l d s shows t h a t Phase T h r e e s t r a i n magnitude (A 9/X 1) v a r i e s from 0.81 t o 0.49. i i i Rb-Sr whole r o c k geochronometry has r e v e a l e d e v i d e n c e f o r t h r e e P r e c a m b r i a n events w i t h i n t h e basement g n e i s s e s . Unmigmatized m e t a s e d i -mentary basement g n e i s s e s form a s i x - p o i n t e r r o r c h r o n w i t h an age o f 2 . 7 3 + 0 . 2 0 Ga. G r a n i t i c g n e i s s e s p r e v i o u s l y d a t e d a t 1.96 Ga by the U-Pb z i r c o n t e c h n i q u e , g i v e r e s e t ages f o r g n e i s s / v e i n whole r o c k s u i t e s w i t h ages i n the range 750 t o 860 Ma. Common l e a d s y s t e m a t i c s o f s t r a t a b o u n d s u l f i d e d e p o s i t s i n the c o v e r r o c k sequence s u g g e s t t h a t t h e c o v e r r o c k s a r e Cambrian i n age, i n a g r e e -ment w i t h e a r l i e r l i t h o l o g i c a l c o r r e l a t i o n s . The l e a d i s o t o p e r a t i o s s u g g e s t t h e d e p o s i t s were u l t i m a t e l y d e r i v e d from an a p p r o x i m a t e l y 2.0 Ga basement t e r r a i n . The o u t c r o p p i n g g n e i s s i c basement r o c k s o f the Shuswap a r e d o m i n a n t l y g r a n i t i c g n e i s s e s which g i v e 2.0 t o 2.2 Ga Rb-Sr whole rock dates.; D u c t i l e d e f o r m a t i o n i n t h e t e r r a i n a p p e a r s t o have ended by 150 Ma, as t h i s c o r r e s p o n d s t o t h e ages o f s e v e r a l p o s t - t e c t o n i c i n t r u s i o n s which c r o s s c u t Phase Thre e f o l d s and impose a c o n t a c t metamorphic o v e r p r i n t on t h e r e g i o n a l metamorphic p a t t e r n . The N e l s o n B a t h o l i t h i s a p o s t - t e c t o n i c b a t h o l i t h which i n t r u d e s both t h e Shuswap t e r r a i n and the Kootenay A r c t o t h e e a s t . Two sample s u i t e s were c o l l e c t e d from t h e d i f f e r e n t b a tho-l i t h i c phases and a n a l y z e d by Rb-Sr whole r o c k methods w i t h the f o l l o w i n g r e s u l t s : S u i t e I - P o r p h y r i t i c monzonite w i t h a l k a l i f e l d s p a r m e g a c r y s t s from t h e West Arm o f Kootenay Lake g i v e a s e v e n - p o i n t i s o c h r o n w i t h an age o f 158 + 16 Ma and an i n i t i a l r a t i o o f 0.7069 + 0 . 0 0 0 1 . S u i t e I has a Rb-Sr f e l d -s p a r , d a t e o f 153 + 26 Ma, a K-Ar h o r n b l e n d e d a t e o f i v 143 +5 Ma, and a K-Ar b i o t i t e d a t e o f 60 + 2 Ma. I t seems l i k e l y t h a t the K-Ar d a t e s have been d i s t u r b e d by thermal e v e n t s s u b s e q u e n t t o emplacement o f the b a t h o l i t h . S u i t e I I - Medium g r a i n e d g r a n o d i o r i t e from the s o u t h e r n t a i l o f the N e l s o n B a t h o l i t h g i v e s a n i n e - p o i n t whole r o c k i s o c h r o n w i t h an age o f 162 + 6 w i t h an i n i t i a l r a t i o o f 0.7065 + 0.0001. Taken t o g e t h e r , t h e s e d a t a p r o v i d e s i g n i f i c a n t new i n f o r m a t i o n on t h e n a t u r e and t i m i n g o f t h e e v e n t s t h a t formed the metamorphic c o r e zone o f the s o u t h e r n C a n a d i a n C o r d i l l e r a . V TABLE OF CONTENTS Page ABSTRACT i i L I S T OF TABLES i x LI S T OF FIGURES x ACKNOWLEDGMENTS . . . . . x i v GENERAL INTRODUCTION 1 CHAPTER 1: STRUCTURAL EVOLUTION OF THE THOR-ODIN GNEISS DOME . . 4 INTRODUCTION 5 GEOLOGIC AND TECTONIC SETTING 8 GEOLOGY OF THE THOR-ODIN GNEISS DOME 10 L i t h o l o g i e s 13 Basement G n e i s s e s 13 Cov e r Rocks 18 G e o c h r o n o l o g y . . . . . 20 Metamorphism 22 S t r u c t u r a l G e o l o g y 24 Phase One S t r u c t u r e s 25 Phase Two S t r u c t u r e s 32 Phase T h r e e S t r u c t u r e s 32 Phase Four S t r u c t u r e s 32 Phase F i v e S t r u c t u r e s 35 Phase S i x S t r u c t u r e s 35 STRUCTURAL ANALYSIS OF FAULT-BOUNDED DOMAINS 35 D e l i n e a t i o n o f Domain B o u n d a r i e s 35 Au t o c h t h o n o u s Core G n e i s s Domain . . . . . 35 Basement C o r e d Nappe Domain 52 T r a n s i t i o n a l Domain o f I m b r i c a t e T h r u s t i n g 64 The C o v e r Rock Domain 70 The B i g Ledge Subdomain 70 Mount F o s t h a l l Synform 71 The Upper F a u l t B l o c k 76 STRAIN ANALYSIS 78 v i Page STRUCTURAL SYNTHESIS 85 CONCLUSIONS 93 CHAPTER 2: Rb-Sr GEOCHRONOLOGY OF BASEMENT GNEISSES FROM THE THOR-ODIN GNEISS DOME 95 INTRODUCTION 96 GENERAL GEOLOGY AND TECTONIC SETTING OF THE SHUSWAP COMPLEX . . 96 PREVIOUS GEOCHRONOLOGY 99 GEOLOGY OF THE THOR-ODIN GNEISS DOME 100 Rb-Sr GEOCHRONOLOGY 104 Sample S e l e c t i o n and A n a l y t i c a l T e c h n i q u e 104 R e s u l t s and I n t e r p r e t a t i o n 105 DISCUSSION I l l CONCLUSIONS . . . . ' 113 CHAPTER 3: COMMON LEAD SYSTEMATICS OF LEAD-ZINC DEPOSITS IN THE SHUSWAP COMPLEX AND KOOTENAY ARC, SOUTHEASTERN BRITISH COLUMBIA 115 INTRODUCTION 116 REGIONAL GEOLOGY 119 GEOLOGY OF STRATABOUND SULFIDE DEPOSITS 121 LEAD ISOTOPE DATA 124 A n a l y t i c a l Methods and P r e c i s i o n 124 R e s u l t s 125 I n t e r p r e t a t i o n 133 DISCUSSION 139 Shuswap Type D e p o s i t s 139 Salmo Type D e p o s i t s 141 G o l d s t r e a m Type D e p o s i t s 142 I m p l i c a t i o n s f o r Sediment S o u r c e s and P a l e o g e o g r a p h y . . . 143 SUMMARY 144 CHAPTER 4: GEOCHRONOLOGY AND S r ISOTOPE GEOCHEMISTRY OF THE NELSON BATHOLITH: A POST-TECTONIC INTRUSIVE COMPLEX IN SOUTHEAST BRITISH COLUMBIA 146 INTRODUCTION 147 T e c t o n i c and G e o l o g i c S e t t i n g 150 v i i Page G e o l o g y o f t h e B a t h o l i t h 151 P r e v i o u s G e o c h r o n o l o g y 153 Rb-Sr GEOCHRONOLOGY 154 Sample S e l e c t i o n 154 A n a l y t i c a l Method and R e s u l t s 155 K-Ar GEOCHRONOLOGY 156 A n a l y t i c a l Method and R e s u l t s . . . . 164 FISSION TRACK GEOCHRONOLOGY 164 A n a l y t i c a l Method and R e s u l t s 164 DISCUSSION 167 S r I s o t o p e G e o c h e m i s t r y 167 T i m i n g o f Emplacement o f t h e B a t h o l i t h 168 Thermal H i s t o r y 168 CONCLUSIONS 170 REFERENCES 171 * * * * * APPENDICES 196 APPENDIX I : STRUCTURAL ANALYSIS OF THE THOR-ODIN GNEISS DOME . 196 I n t r o d u c t i o n 197 T e c h n i q u e s o f S t r u c t u r a l A n a l y s i s 198 A u t o c h t h o n o u s Core G n e i s s Domain 198 Basement C o r e d Nappe Domain 216 T r a n s i t i o n a l Domain o f I m b r i c a t e T h r u s t i n g 229 C o v e r Rock Domain 247 B i g Ledge Subdomain 248 Mount F o s t h a l l Subdomain . 267 The Upper F a u l t B l o c k Subdomain 272 D i s c u s s i o n 278 APPENDIX I I : STRAIN ANALYSIS 279 I n t r o d u c t i o n 280 H u d l e s t o n ' s F o l d M o r p h o l o g y Method 280 Ramsay's t„ Method 284 v i i i Page D i s t o r t e d L i n e a t i o n P a t t e r n s 284 S t r a i n A n a l y s i s o f E l l i p t i c a l O b j e c t s 292 R e s u l t s 294 H u d l e s t o n ' s Method . 2 9 4 Ramsay's t a Method 294 D i s t o r t e d L i n e a t i o n P a t t e r n s 301 E l l i p s o i d a l S t r a i n M a r k ers 313 D i s c u s s i o n 314 C o n c l u s i o n s 321 APPENDIX I I I : METAMORPHIC ASSEMBLAGES AND TEXTURAL RELATIONSHIPS IN THE THOR-ODIN GNEISS DOME 322 I n t r o d u c t i o n 323 Q u a r t z o - f e l d s p a t h i c A s semblages 323 P e l i t i c Assemblages 326 G e d r i t e - A l u m i n o s i l i c a t e Assemblages 327 C o n c l u s i o n s 329 APPENDIX IV: METAMORPHIC PHASE EQUILIBRIA AND THE EVOLUTION OF THE THOR-ODIN GNEISS DOME 330 I n t r o d u c t i o n 331 P e l i t i c M i n e r a l Assemblages . 331 G e d r i t e - C o r d i e r i t e Assemblages 333 C o n c l u s i o n s 336 APPENDIX V: AN EXPERIMENTAL STUDY OF THE EFFECT OF WATER ON THE PHASE RELATIONS OF MAGNESIUM CORDIERITE . . . 338 I n t r o d u c t i o n 339 E x p e r i m e n t a l T e c h n i q u e 339 R e s u l t s 342 REFERENCES 345 i x L I S T OF TABLES T a b l e Page 2- 1 Rb-Sr I s o t o p i c R e s u l t s . . . . 106 3- 1 L e a d I s o t o p e Data 126 3- 1I Two-Stage Model P a r a m e t e r s f o r Shuswap Type D e p o s i t s . . 140 4- 1 Rb-Sr I s o t o p e D a t a 157 4-11 K-Ar Data 165 4-111 F i s s i o n T r a c k Data 166 IV- I B i o t i t e - G a r n e t Geothermometry R e s u l t s . 334 V- I D i f f u s i v i t y o f Water i n C o r d i e r i t e and Z e o l i t e s . . . . 340 V - I I B r a c k e t e d H y d r a t i o n Data f o r Mg C o r d i e r i t e 343 X L I S T OF FIGURES F i g u r e Page 1-1 T e c t o n i c map o f t h e C a n a d i a n C o r d i l l e r a 6 1-2 G e o l o g i c a l map o f t h e T h o r - O d i n g n e i s s dome 11 1-3 L o c a t i o n map f o r a r e a s o f d e t a i l e d s t u d y 14 1-4 D e t a i l e d map o f a l a r g e - s c a l e Phase One c l o s u r e f r o m t h e n o r t h s i d e o f F o s t h a l l R i d g e 26 1-5 M e s o s c o p i c Phase One f o l d s 30 1-6 M e s o s c o p i c Phases Two and T h r e e f o l d s 33 1-7 G e o l o g y o f the west s i d e o f t h e T h o r - O d i n g n e i s s dome w i t h t h e t o p o g r a p h i c d i s t o r t i o n e l i m i n a t e d by p r o j e c t i o n onto a h o r i z o n t a l p l a n e 36 1-8 Map o f f a u l t - b o u n d e d domains 38 1-9 G e o l o g i c a l map o f t h e A u t o c h t h o n o u s Core G n e i s s Domain . 41 1-10 C r o s s - s e c t i o n s o f t h e A u t o c h t h o n o u s Core G n e i s s Domain . 43 1-11 The g e o l o g y o f t h e P i n g s t o n f o l d , T h o r Ridge . . . . 45 1-12 C r o s s - s e c t i o n a c r o s s t h e P i n g s t o n f o l d 48 1-13 E q u a l - a n g l e s t e r e o n e t p l o t s showing o r i e n t a t i o n d a t a f r o m t h e a r e a shown i n F i g u r e 1-11 53 1-14 G e o l o g i c map o f t h e Basement C o r e d Nappe Domain . . . 55 1-15 C r o s s - s e c t i o n o f the Basement C o r e d Nappe Domain . . . 60 1-16 O r i e n t a t i o n d a t a f o r m e s o s c o p i c f o l d s from t h e Gunnarsen Nappe 62 1-17 S t r u c t u r a l map o f a p o r t i o n o f the T r a n s i t i o n a l Domain o f I m b r i c a t e T h r u s t i n g i n the C a r i b o u A l p a r e a . . . . 65 1-18 C r o s s - s e c t i o n o f the T r a n s i t i o n a l Domain o f I m b r i c a t e T h r u s t i n g 67 1-19 G e o l o g i c a l map o f t h e F o s t h a l l R i d g e a r e a showing o r i e n t a t i o n o f m e s o s c o p i c Phases Two and T h r e e s t r u c t u r e s 72 1-20 C r o s s - s e c t i o n s o f t h e F o s t h a l l Ridge a r e a 74 x i F i g u r e Page 1-21 Harmonic a n a l y s i s o f Phases One and Two f o l d s u s i n g t h e v i s u a l t e c h n i q u e s o f H u d l e s t o n (1973a) 79 1-22 t a p l o t o f Ramsay (1962) f o r Phases One and Two f o l d p r o f i l e s 83 1- 23 S t r u c t u r a l model f o r the T h o r - O d i n g n e i s s dome . . . . 89 2- 1 T e c t o n i c map o f s o u t h e a s t e r n B r i t i s h Columbia . . . . 97 2-2 G e o l o g i c map o f t h e T h o r - O d i n g n e i s s dome 101 2-3 Rb-Sr i s o c h r o n d i a g r a m f o r unmigmatized m e t a s e d i m e n t a r y g n e i s s e s 107 2- 4 A Rb-Sr i s o c h r o n d i a g r a m f o r t h e O d i n G l a c i e r and K e l l e y R i d g e m i g m a t i t e s 109 3- 1 L o c a t i o n s o f l e a d - z i n c d e p o s i t s i n s o u t h e a s t e r n B r i t i s h C o l u m b i a 117 3-2 2 0 7 P b / 2 0 4 P b - 2 0 6 P b / 2 0 4 P b d i a g r a m f o r Shuswap t y p e d e p o s i t s 127 3-3 2 0 7 P b / 2 0 4 P b - 2 0 6 P b / 2 0 4 P b d i a g r a m f o r G o l d s t r e a m t y p e d e p o s i t s 129 3-4 2 0 7 P b / 2 0 4 P b - 2 0 6 P b / 2 0 4 P b d i a g r a m f o r Salmo t y p e d e p o s i t s . 131 3- 5 N u m e r i c a l s i m u l a t i o n o f S a l mo-Metal l i n e d a t a 137 4- 1 Summary g e o l o g i c map o f s o u t h e a s t e r n B r i t i s h C o l umbia showing s e t t i n g o f N e l s o n B a t h o l i t h 148 4-2 Rb-Sr i s o c h r o n d i a g r a m f o r the NB-I ( p o r p h y r i t i c m o n z o n i t e ) s u i t e 158 4-3 Rb-Sr i s o c h r o n d i a g r a m f o r t h e NB-II ( b i o t i t e g r a n o d i o r i t e ) s u i t e 160 4-4 A p p a r e n t c o o l i n g h i s t o r y o f NB-I l o c a l i t y 162 1-1 The g e n e r a l g e o l o g y o f t h e T h o r - O d i n g n e i s s dome . . . 199 1-2 The g e n e r a l g e o l o g y o f t h e A u t o c h t h o n o u s Core G n e i s s Domain 201 1-3 The g e o l o g y o f t h e P i n g s t o n f o l d , T h o r Ridge . . . . 204 1-4 C r o s s - s e c t i o n a c r o s s t h e P i n g s t o n f o l d 207 x i i F i g u r e Page 1-5 Phase One f o l d s on upper l i m b o f P i n g s t o n f o l d . . . . 211 1 - 6 E q u a l - a n g l e s t e r e o n e t p l o t s showing o r i e n t a t i o n d a t a f r o m the a r e a shown i n F i g u r e 1 - 3 213 1 - 7 S t r u c t u r a l map o f a p o r t i o n o f t h e Gunnarsen and O d i n Nappes 217 1 - 8 P r o j e c t e d map view o f c h e v r o n - s t y l e Phase F i v e f o l d s on t h e w e s t s i d e o f t h e dome 222 1-9 S t e r e o n e t p l o t s o f o r i e n t a t i o n d a t a f o r Basement C o r e d Nappe Domain 224 1-10 M e s o s c o p i c f o l d s from t h e Basement Cored Nappe Domain . . 226 1-11 C r o s s - s e c t i o n o f Gunnarsen and O d i n Nappes 230 1-12 G e o l o g i c a l map o f C a r i b o u A l p , showing t y p i c a l g e o l o g y o f t h e T r a n s i t i o n a l Domain o f I m b r i c a t e T h r u s t i n g . . . 232 1 - 1 3 M e s o s c o p i c f o l d s f r o m Nappes I , I I , I I I and IV . . . . 235 1-14 S t e r e o n e t p l o t s f o r t h e f o u r nappes f r o m t h e T r a n s i t i o n a l Domain o f I m b r i c a t e T h r u s t i n g 239 1-15 C r o s s - s e c t i o n o f t h e T r a n s i t i o n a l Domain o f I m b r i c a t e T h r u s t i n g / S k e t c h o f b u l l d o z e r c u t i n banded c a l c - s i l i c a t e s c h i s t s 4 km e a s t o f Empress Lake 243 1-16 S t r u c t u r a l map o f the B i g Ledge subdomain 249 1-17 M e s o s c o p i c f o l d s from t h e C o v e r Rock Domain 252 1-18 C r o s s - s e c t i o n s o f the B i g Ledge subdomain 256 1 - 1 9 S k e t c h o f an u p r i g h t Phase Two a n t i f o r m o v e r p r i n t e d by Phase T h r e e f o l d s w i t h s h a l l o w l y - d i p p i n g a x i a l p l a n e s , on t h e e a s t f l a n k o f Mt. Symonds 258 1-20 D e t a i l e d s t r u c t u r a l map o f Phase T h r e e t e c t o n i c s l i d e s s o u t h o f Empress Lake 260 1 - 2 1 S t e r e o n e t p l o t s o f o r i e n t a t i o n d a t a f o r t h e B i g Ledge subdomain 263 1-22 O r i e n t a t i o n d a t a c o l l e c t e d f r o m the Phase One h i n g e zone shown i n t h e n o r t h - c e n t r a l p o r t i o n o f F i g u r e 1 - 2 3 , n o r t h o f the f a u l t 265 x i i i F i g u r e Page 1-23 S t r u c t u r a l map o f t h e Mt. F o s t h a l l subdomain . . . . 268 1-24 C r o s s - s e c t i o n s a c r o s s t h e Mt. F o s t h a l l s y n f o r m . . . . 270 1-25 S t e r e o n e t p l o t s o f o r i e n t a t i o n d a t a f o r t h e Mt. F o s t h a l l subdomain 273 I - 26 C h e v r o n - s t y l e Phase Four f o l d s s o u t h e a s t o f t h e T h o r - O d i n dome 276 I I - 1 G r i d o f i d e a l f o l d shapes b a s e d on s p e c t r a l s t u d i e s o f n a t u r a l f o l d shapes 282 I1-2 P l o t o f f o l d t h i c k n e s s p a r a m e t e r t v e r s u s l i m b d i p a . . 285 I I - 3 E q u a l - a n g l e s t e r e o n e t p r o j e c t i o n o f t h e l i n e a t i o n l o c i r e s u l t i n g from t he s u p e r p o s i t i o n o f a f l a t t e n i n g s t r a i n on a s m a l l c i r c l e 288 11-4 Shape d a t a f o r Phases One and Two f o l d s p l o t t e d on H u d l e s t o n ' s f o l d shape g r i d 295 I1-5 Shape d a t a f o r Phase T h r e e f o l d s f r o m t h e f o u r t h r u s t s h e e t s mapped w i t h i n the T r a n s i t i o n a l Domain o f I m b r i c a t e T h r u s t i n g 297 11-6 t a p l o t s f o r Phases One, Two and T h r e e f o l d s from v a r i o u s domains 299 I I - 7 Phase One l i n e a t i o n s deformed by Phase Two f o l d s w i t h s u p e r p o s e d f l a t t e n i n g s t r a i n s 302 11-8 Phase Two f o l d axes from t h e C a r i b o u Nappe deformed by Phase T h r e e f o l d i n g 308 11-9 S t r a i n c a l c u l a t e d f r o m e l l i p s o i d a l q u a r t z - f e l d s p a r , q u a r t z and e p i d o t e - c a l c i t e - q u a r t z , p l o t t e d on a Hsu d i a g r a m 315 11-10 F l a t t e n e d g a r n e t s i n q u a r t z o - f e l d s p a t h i c g n e i s s f o l d e d by Phase T h r e e f o l d s 318 111 — 1 Metamorphic assemblage map f o r the T h o r - O d i n g n e i s s dome . 324 x i v LIST OF PLATES P l a t e 1 G e o l o g y o f t h e Mt. Gunnarsen a r e a 2 Geology o f the C a r i b o u A l p a r e a 3 G e o l o g y o f t h e F o s t h a l 1 R i d g e a r e a k G e o l o g y o f t h e Mt. Symonds-Empress Lake a r e a P l a t e s a r e l o c a t e d -rnrpock'et^a-t^baclr"6f™^So°in' in ' s ^ c e i f l J C a \ l e e r i on*? • XV ACKNOWLEDGMENTS T h i s t h e s i s would n o t have been c o m p l e t e d w i t h o u t t h e h e l p , e n couragement and e n t h u s i a s m o f many p e o p l e . In p a r t i c u l a r , my a d v i s o r Hugh Greenwood p r o v i d e d a c o n s t a n t example o f e x c e l l e n c e i n r e s e a r c h and words o f encouragement when I r e a l l y needed i t . My f i e l d a s s i s t a n t s Tony Hodge, D i t a R u n k l e , Joan G r e t t e , Randy P a r r i s h , Mark and Susan H a r r i s o n , and Graham N i x o n , a l l b r a v e d snow, h a i l s t o r m s and b e a r s t o make t h e f i e l d work p o s s i b l e . I owe my s u r v i v a l , and much more, t o G a i l and S t u a r t A s h l e y , who t a u g h t me how t o use an i c e a x e , e s c a p e f r o m c r e v a s s e s , and o t h e r s k i l l s u s e f u l f o r a l p i n e f i e l d w o r k . Hugh Greenwood, P e t e r Read, T r i g Hoy and Lee Pi gage v i s i t e d me i n t h e f i e l d and p r o v i d e d u s e f u l i d e a s . The i s o t o p e a n a l y s e s c o u l d n o t have been c o m p l e t e d w i t h o u t t h e mass s p e c t r o m e t e r s k e p t r u n n i n g by D i c k A r m s t r o n g and B a r r y Ryan. K r i s t a S c o t t h e l p e d w i t h Rb-Sr c h e m i s t r y t e c h n i q u e s . B a r r y Ryan p a t i e n t l y showed me how t o do l e a d c h e m s i t r y . Randy P a r r i s h h e l p e d w i t h t h e Rb-Sr s t u d y i n C h a p t e r 4. P a t r i c k S h o r e , B a r r y Ryan, Tim Ahern and Kathy A l d e n w r o t e computer programs t o p r o c e s s t h e Rb-Sr and Pb i s o t o p e d a t a . B a r r y Ryan d i s c u s s e d t h e s i g n i f i c a n c e o f the Pb d a t a w i t h me. D i s c u s s i o n s on Shuswap g e o l o g y and g e o c h r o n o l o g y w i t h Kent N i e l s e n , Mark H a r r i s o n , Randy P a r r i s h , B a r r y Ryan and T r i g Hoy have been most u s e f u l . However, t h e i d e a s p r e s e n t e d i n t h i s t h e s i s a r e t h o s e o f t h e a u t h o r , who t a k e s f u l l r e s p o n s i b i l i t y f o r the o p i n i o n s h e r e i n . Many p e o p l e have r e a d and commented on v a r i o u s p a r t s o f the m a n u s c r i p t , i n c l u d i n g B i l l B a r n e s , Dave P e r r y , John Ross, Kent N i e l s e n , D i c k A r m s t r o n g , xv i P a t r i c k S h o r e , B a r r y Ryan, A l S i n c l a i r and Hugh Greenwood. B e l l K o b l i n t z and R i c k K e l l e y h e l p e d w i t h t h e d r a f t i n g . The s t a f f o f t h e S.M.U. Dedman C o l l e g e Word P r o c e s s o r p e r f o r m e d much o f t h e f i n a l m a n u s c r i p t p r e p a r a t i o n . The t h e s i s was t y p e d by N a z l e e Coburn and C i n d y Gwinn. I owe a g r e a t d e b t t o C i n d y Gwinn, who e d i t e d t h e f i n a l m a n u s c r i p t and g r e a t l y f a c i l i t a t e d i t s c o m p l e t i o n . To a l l t h e s e p e o p l e and t o my o t h e r f r i e n d s whom I n e g l e c t e d w h i l e t h i s was b e i n g done, my s i n c e r e t h a n k s . 1 GENERAL INTRODUCTION T h i s s t u d y was i n i t i a t e d t o r e s o l v e s e v e r a l l o n g - s t a n d i n g c o n t r o v e r s i e s ' r e g a r d i n g t he o r i g i n and e v o l u t i o n o f g n e i s s domes. T h e o r i e s o f t h e g e n e s i s o f g n e i s s domes have v a r i e d f r o m g r a v i t a t i o n a l l y - d r i v e n u p w e l l i n g o f l o w e r c r u s t a l m a t e r i a l ( F l e t c h e r , 1972) t o an o r i g i n f r o m l a t e - s t a g e c r o s s f o l d i n g o f u p r i g h t f o l d s ( R o s s , 1968). An o r i g i n as a c o n f i n e d metamorphic h o t s p o t has been s u g g e s t e d (D. N o r t o n , p e r s o n a l c o m m u n i c a t i o n , 1978), and t h e e x i s t e n c e o f m a n t l i n g p e g m a t i t e zones has been e m p h a s i z e d by s e v e r a l a u t h o r s ( R e e s o r and Moore, 1971; Wheeler, 1965). On t h e b a s i s o f s u c h w i d e l y d i f f e r i n g g e n e t i c m o d e l s , t h e t e c t o n i c s i g n i f i c a n c e o f g n e i s s domes remains a n - i n t r i g u i n g q u e s t i o n . The Shuswap g n e i s s domes p r o v i d e an e x c e l l e n t o p p o r t u n i t y t o a t t a c k t h e p r o b l e m o f t h e o r i g i n o f g n e i s s domes. E x t e n s i v e t h r e e - d i m e n s i o n a l o u t c r o p s a r e a v a i l a b l e i n t h e g l a c i a l l y - s c o u r e d Monashee M o u n t a i n s o f s o u t h e a s t e r n B r i t i s h C o l u m b i a . D i a p i r i c u p w e l l i n g o f t h e Shuswap t e r r a i n as a whole has been s u g g e s t e d as t h e d r i v i n g f o r c e f o r t h e f o r m a t i o n o f t h e Rocky M o u n t a i n s t h r u s t b e l t t o t h e e a s t ( P r i c e and M o u n t j o y , 1970). Thus an u n d e r s t a n d i n g o f the n a t u r e and t i m i n g o f t h e f o r m a t i o n o f t h e s e g n e i s s domes i s a p r o b l e m o f c o n s i d e r a b l e t e c t o n i c i n t e r e s t . The age o f t h e r o c k s o f t h e Shuswap complex has been i n . i t s e l f a s o u r c e o f c o n s i d e r a b l e d i s a g r e e m e n t . When t h i s s t u d y was begun, l i t t l e was known o f t h e ages o f t h e o l d e r , p r e - M e s o z o i c r o c k s o f t h e complex. O n l y s c a n t g e o c h r o n o l o g i c a l i n f o r m a t i o n was a v a i l a b l e on s t r u c t u r a l p l u t o n i c a n d / o r metamorphic e v e n t s o l d e r t h a n t he C r e t a c e o u s . In c o m p a r i s o n to t h e Kootenay A r c - S e l k i r k t e r r a i n t o t h e e a s t , l i t t l e was known o f the c h r o n o l o g y o f e v e n t s w i t h i n and a d j a c e n t t o the g n e i s s domes. 2 T h i s t h e s i s p r e s e n t s t h e r e s u l t s o f a s t r u c t u r a l and g e o c h r o n o l o g i c a l s t u d y o f t h e T h o r - O d i n g n e i s s dome. A d e t a i l e d s t r u c t u r a l a n a l y s i s o f t h i s dome i s p r e s e n t e d i n C h a p t e r One. A l t h o u g h t h e l a c k o f a p p r o p r i a t e s t r a i n markers p r e v e n t e d an e x h a u s t i v e a n a l y s i s o f s t r a i n , a v a i l a b l e s t r a i n i n d i c a t o r s have been u t i l i z e d t o g e t h e r w i t h o r i e n t a t i o n d a t a f r o m m e s o s c o p i c s t r u c t u r e s t o p r e s e n t a d e t a i l e d model f o r t h e s t r u c t u r a l e v o l u t i o n o f t h e dome. The s u c c e s s o f g e o c h r o n o l o g i c a l s t u d i e s i n t h e Shuswap complex has been hampered by t h e e f f e c t s o f r e c e n t ( T e r t i a r y ) r e s e t t i n g o f i s o t o p i c s y s t e m s . The ages o f the u n i t s c o m p r i s i n g t h e complex have been s u g g e s t e d t o d a t e back t o t h e A r c h e a n (Dawson, 1898), and a m a j o r metamorphic p l u t o n i c e v e n t was t h o u g h t t o have o c c u r r e d i n the M i d d l e J u r a s s i c ( W h e e l e r , 1970). However, d e f i n i t i v e e v i d e n c e on the t i m i n g o f t h e s e u n i t s and e v e n t s was l a c k i n g . P r e v i o u s p u b l i s h e d Rb-Sr g e o c h r o n o l o g i c a l s t u d i e s i n t h e Shuswap t e r r a i n have f a i l e d t o p r o v i d e e v i d e n c e f o r o l d ( g r e a t e r t h a n 750 Ma) c r u s t . The i n t e r p r e t a t i o n o f an o l d e r z i r c o n age (Wanless and R e e s o r , 1975) has been r e g a r d e d as p r o b l e m a t i c b e c a u s e o f t h e p o s s i b i l i t y o f a s e d i m e n t a r y d e t r i t a l o r i g i n . Samples c o l l e c t e d i n t h i s s t u d y , b a s e d on g e o l o g i c a l and p e t r o l o g i c a i c r i t e r i a , have y i e l d e d Rb-Sr whole r o c k d a t a s u g g e s t i n g t h e p r e s e n c e o f A r c h e a n basement i n the c o r e o f t h e dome. C h a p t e r Two o f t h i s t h e s i s p r e s e n t s t h e Rb-Sr g e o c h r o n o l o g i c a l d a t a and i n t e r p r e t s t h e i r s i g n i f i c a n c e i n terms o f t h e s t r u c t u r a l e v o l u t i o n p r o p o s e d i n C h a p t e r One. C h a p t e r T h r e e p r e s e n t s an a t t e m p t t o d a t e t h e r o c k sequence o v e r l y i n g the c o r e g n e i s s o f t h e domes. The common l e a d s y s t e m a t i c s o f l e a d - z i n c d e p o s i t s a s s o c i a t e d w i t h c a l c a r e o u s metamorphic r o c k s i n t h e c o v e r r o c k sequence a r e d i s c u s s e d and t h e i r g e o c h r o n o l o g i c a l s i g n i f i c a n c e a s s e s s e d . 3 C h a p t e r F o u r p r e s e n t s an age f o r a p o s t - t e c t o n i c b a t h o l i t h i n t r u d i n g t h e Shuswap complex. To draw t o g e t h e r the main c o n c l u s i o n s f r o m the s t r u c t u r a l and g e o c h r o n o l o g i c a l s t u d i e s o u t l i n e d above, a s h o r t o u t l i n e i s g i v e n i n t h e G e n e r a l C o n c l u s i o n . The A p p e n d i c e s p r o v i d e a d d i t i o n a l d a t a and a n a l y s e s t h a t s u p p o r t t h e c o n c l u s i o n s o f the f i r s t f o u r c h a p t e r s , b u t w h i c h d i d n o t f i t w e l l w i t h i n the p u b l i c a t i o n f o r m a t c h o s e n f o r t h i s t h e s i s . Some o f t h e A p p e n d i c e s t h e m s e l v e s c o n s t i t u t e s m a l l p a p e r s b e i n g s u b m i t t e d f o r p u b l i c a t i o n . To h e l p r e a d i n g o f the t h e s i s , r e f e r e n c e s t o t h e s e A p p e n d i c e s have been i n s e r t e d i n t o the main body o f t h e t h e s i s where a p p r o p r i a t e . 4 CHAPTER 1 STRUCTURAL EVOLUTION OF THE THOR-ODIN GNEISS DOME 5 INTRODUCTION The T h o r - O d i n g n e i s s dome i s one o f f o u r domes i d e n t i f i e d by Re e s o r (1970) i n t h e e a s t e r n p a r t o f t h e Shuswap metamorphic complex, t h e c o r e zone o f t h e s o u t h e r n C a n a d i a n C o r d i l l e r a ( F i g . 1 ) . Re e s o r (1970) r e f e r r e d t o t h e s e s t r u c t u r e s as m a n t l e d g n e i s s domes and s u g g e s t e d they were d i a p i r i c i n o r i g i n . R e e s o r and Moore (1971) p r e s e n t e d metamorphic e v i d e n c e from t h e T h o r - O d i n g n e i s s dome which t h e y i n t e r p r e t e d as s u p p o r t i n g a d i a p i r i c o r i g i n . T h i s c o n c l u s i o n has c o n s i d e r a b l e t e c t o n i c s i g n i f i c a n c e , as P r i c e and Mountjoy (1970) have s u g g e s t e d t h a t d u c t i l e u p w e l l i n g o r d i a p i r i s m i n t h e Shuswap complex i n i t i a t e d f o r e l a n d t h r u s t i n g i n t h e Rocky M o u n t a i n s t o t h e e a s t . An a l t e r n a t i v e model f o r the o r i g i n o f t h e Shuswap domes has been p r o -p o s e d by Ross ( 1 9 6 8 ) , who s u g g e s t e d t h a t they were i n t e r f e r e n c e s t r u c t u r e s r e s u l t i n g from l a t e - s t a g e r e f o l d i n g o f e a r l y nappes c o n t a i n i n g s l i c e s o f basement g n e i s s w i t h i n t h e i r c o r e s . I f t h i s were t h e c a s e t he domes would have a d i f f e r e n t t e c t o n i c s i g n i f i c a n c e t h a n i m p l i e d by t h e P r i c e - M o u n t j o y model. C o n t r o v e r s y r e g a r d i n g t h e o r i g i n o f m a n t l e d g n e i s s domes (Hobbs e t a l . , 1976, p. 428) has r e s u l t e d i n a s e r i e s o f t h e o r e t i c a l and e x p e r i m e n t a l s t u d i e s i n an a t t e m p t t o e s t a b l i s h t h e n a t u r e o f the s t r a i n p a t t e r n s a s s o -c i a t e d w i t h d i a p i r i s m . F l e t c h e r (1972) used a n u m e r i c a l a p p r o a c h t o p r e -d i c t t h e w a v e l e n g t h and f i n i t e s t r a i n p a t t e r n s f o r domes. Ramberg (1967) and D i x o n (1975) have u t i l i z e d e x p e r i m e n t a l c e n t r i f u g e t e c h n i q u e s t o s t u d y t h e same prob l e m . U n f o r t u n a t e l y , t h e s e s t u d i e s have n o t r e s u l t e d i n a c o h e r e n t p i c t u r e o f t h e n a t u r e o f d i a p i r i s m i n g n e i s s e s ; r a t h e r , they have 6 FIGURE 1. T e c t o n i c map o f the C a n a d i a n C o r d i l l e r a . The major t e c t o n i c s u b d i v i s i o n s f o r t h e C o r d i l l e r a a r e shown. The c i r c l e shows t h e l o c a t i o n o f the s t u d y a r e a shown i n F i g u r e 2. 7 8 g e n e r a t e d a c o n f u s i n g a r r a y o f v a r y i n g boundary c o n d i t i o n s , d u c t i l i t y c o n t r a s t s and s t r a i n p a t t e r n s . To p r o v i d e d e t a i l e d s t r u c t u r a l and s t r a i n d a t a and t o a d d r e s s t h e p r o b l e m o f g n e i s s dome f o r m a t i o n , a s t u d y o f t h e T h o r - O d i n g n e i s s dome was i n i t i a t e d (Duncan, 1978a). E x t e n s i v e ( o v e r 2,000 me t e r s o f r e l i e f ) , r e c e n t l y g l a c i a t e d o u t c r o p and an e x c e l l e n t d a t a base a v a i l a b l e from R e e s o r and Moore's (1971) s t u d y made t h i s a r e a an o b v i o u s c h o i c e . T h i s c h a p t e r summarizes t h e g e o l o g y o f t h e dome, i n c l u d i n g new d a t a on t h e l i t h o l o g i e s , g e o c h r o n o l o g y and metamorphism; p r e s e n t s a new a n a l y s i s o f t h e m e s o s c o p i c s t r u c t u r e ; a n a l y z e s s t r a i n v a r i a t i o n ; and p r e s e n t s a new model f o r the o r i g i n and s t r u c t u r a l d evelopment o f t h e T h o r - O d i n g n e i s s dome. GEOLOGIC AND TECTONIC SETTING The g e o l o g i c and t e c t o n i c s e t t i n g o f t h e Shuswap g n e i s s domes has been summarized by Reesor ( 1 9 7 0 ) , W h e e l e r ( 1 9 7 0 ) , R e e s o r and Moore ( 1 9 7 1 ) , and Read and Brown ( 1 9 8 1 ) . The Shuswap complex, a p a r t o f t h e Omineca B e l t , i s an e l o n g a t e metamorphic t e r r a i n 500 km l o n g and 100 km wide i n s o u t h e a s t e r n B r i t i s h C o l u m b i a and n o r t h e r n Washington S t a t e ( F i g . 1 ) . I t i s bounded on t h e west by t h e I n t e r m o n t a n e B e l t , a l a t e P a l e o z o i c t o M e s o z o i c i s l a n d a r c - o c e a n f l o o r t e r r a i n , and on the e a s t by l o w - g r a d e , P r o t e r o z o i c t o J u r a s s i c m e t a s e d i m e n t s o f the Kootenay A r c - n o r t h e r n S e l k i r k - C a r i b o o . M o u n t a i n s t e r r a i n . The Shuswap complex i s an a r e a o f v a r i a b l e , y e t g e n e r a l l y h i g h , meta-m o r p h i c g r a d e . E s t i m a t e s o f t e m p e r a t u r e s and p r e s s u r e s o f 450 t o 750°C and 400 t o 700 MPa a r e t y p i c a l ( R e e s o r and Moore, 1971; H i l l , 1975; Ghent e t a l . , 1977; Pi gage, 1979; F l e t c h e r and Greenwood, 1979). The r o c k s range 9 from h i g h - g r a d e m e t a s e d i m e n t a r y g n e i s s e s , a m p h i b o l i t e s , c a l c - s i l i c a t e s and m i g m a t i t e s t o s c h i s t s , s l a t e s , m e t a v o l c a n i c s and l i m e s t o n e s . The u n i f y i n g f e a t u r e o f t h e complex i s t h e p r e s e n c e o f g e n e r a l l y e a s t - w e s t t r e n d i n g , recumbent, t i g h t - t o - i s o c l i n a l f o l d s ( J o n e s , 1959). These s t r u c t u r e s t y p i -c a l l y dominate t h e o u t c r o p p a t t e r n and a r e a s s o c i a t e d w i t h a s h a l l o w l y d i p p i n g f o l i a t i o n and c o m p o s i t i o n a l l a y e r i n g . The s t r u c t u r a l g e o l o g y o f the Shuswap complex has been summarized by Ross ( 1 9 7 3 ) , Ryan ( 1 9 7 3 ) , N i e l s e n ( 1 9 7 8 ) , Ross and C h r i s t i e ( 1 9 7 9 ) , Read ( 1 9 8 0 ) , Read and Brown ( 1 9 8 1 ) , and o t h e r s . In most a r e a s o f t h e complex t h e e a r l i e s t phase o f f o l d i n g (Phase One) i s i s o c l i n a l and o f t e n r o o t l e s s . Phase Two f o l d s c o n s i s t o f g e n e r a l l y e a s t - w e s t t r e n d i n g , t i g h t - t o -i s o c l i n a l , recumbent f o l d s . Phase T h r e e f o l d s a r e t y p i c a l l y more u p r i g h t and open i n s t y l e t h a n Phase Two f o l d s , b u t a r e f r e q u e n t l y o b s e r v e d t o be c o - a x i a l w i t h Phase Two. Phase T h r e e f o l d s a r e o f t e n i n c l i n e d t o o v e r -t u r n e d , w i t h s h a r p h i n g e s and s t r a i g h t l i m b s . These t h r e e phases o f d e f o r -m a t i o n a p p e a r t o be p r e s e n t o v e r a l a r g e p e r c e n t a g e o f t h e complex. The d e g r e e o f d e v e l o p m e n t , s t y l e , and t o some e x t e n t , o r i e n t a t i o n v a r i e s from one a r e a t o the n e x t . The sequence o f phases o f f o l d i n g , however, a p p e a r s t o be c h a r a c t e r i s t i c o f t h e complex. In some a r e a s o f t h e complex, n o r t h w e s t - t r e n d i n g , u p r i g h t , open Phase F o u r f o l d s a r e d e v e l o p e d . In the n o r t h e a s t p a r t o f t h e complex ( N i e l s e n , 1978) t h e s e f o l d s have an a x i a l p l a n e c r e n u l a t i o n c l e a v a g e . In l o w e r - g r a d e a r e a s t h e i r s t y l e a p p e a r s t o be o f a more b r i t t l e , f l e x u r a l - s l i p n a t u r e ( N i e l s e n , 1978; Duncan and N i e l s e n , 1977). Two phases o f n o r t h e r l y and n o r t h e a s t e r l y t r e n d i n g l a t e - s t a g e , b r i t t l e , f l e x u r a l - s l i p f o l d i n g can g e n e r a l l y be r e c o g n i z e d . These f o l d s ( P h a s e s F i v e and S i x ) a r e c h a r a c -10 t e r i z e d by a n o n - p e n e t r a t i v e f r a c t u r e c l e a v a g e p a r a l l e l t o the a x i a l p l a n e s ( R o s s , 1974). I t s h o u l d be n o t e d t h a t n o t a l l t h e s e phases have been mapped i n e v e r y a r e a ; c o n s e q u e n t l y , s e v e r a l d i f f e r e n t f o l d n o m e n c l a t u r e s e x i s t i n t h e l i t e r a t u r e . The a d j a c e n t Kootenay A r c - n o r t h e r n S e l k i r k s - C a r i b o o M o u n t a i n s t e r r a i n has a d i s t i n c t l y d i f f e r e n t s t r u c t u r a l sequence (Read, 1973; Read and Wheeler, 1975; Wheeler, 1965; Brown and T i p p i t , 1978; Hoy, 1977a) con-s i s t i n g o f t h r e e p h a s e s o f g e n e r a l l y c o - a x i a l f o l d i n g . F i r s t phase s t r u c -t u r e s a r e t y p i c a l l y l a r g e - s c a l e recumbent nappes r e f o l d e d by more open, u p r i g h t , s e c o n d and t h i r d phase s t r u c t u r e s . No phases o f e a s t - w e s t o r i e n -t a t i o n have been i d e n t i f i e d , i n c o n t r a s t t o Phases Two and T h r e e s t r u c t u r e s i n t h e Shuswap complex ( F i g . 1 ) . A l o n g t h e e a s t e r n m a r g i n o f the Shuswap complex, t h e Frenchman Cap and T h o r - O d i n g n e i s s domes o c c u r near t h e boundary between t h e Kootenay A r c and n o r t h e r n S e l k i r k s . The domes c o n s i s t o f a c e n t r a l c o r e o f m e t a s e d i m e n t a r y and m i g m a t i t i c basement g n e i s s e s o v e r l a i n by a c o v e r r o c k sequence o f p o l y p h a s e deformed q u a r t z i t e s , s c h i s t s , a m p h i b o l i t e s and c a i c - s i l i c a t e s . R e g i o n a l mapping by Wheeler (1965) and R e e s o r and Moore (1971) s u g g e s t s t h a t t he c o r e zones o f t h e s e g n e i s s domes a r e c h a r a c t e r i z e d by n o r t h e r l y t o n o r t h w e s t - t r e n d i n g s t r u c t u r e s . In c o n t r a s t , t h e c o v e r r o c k sequence i s d o m i n a t e d by e a s t - w e s t t r e n d i n g s t r u c t u r e s ( F y l e s , 1970; M c M i l l a n , 1973; Hoy, 1979; R e e s o r and Moore, 1971). GEOLOGY OF THE THOR-ODIN GNEISS DOME The g e n e r a l g e o l o g y o f t h e T h o r - O d i n g n e i s s dome i s shown i n F i g u r e 2. The a r e a shown i n F i g u r e 2 has been e x t e n s i v e l y remapped d u r i n g t h e c o u r s e 1 1 FIGURE 2. G e o l o g i c a l map o f t h e T h o r - O d i n g n e i s s dome. The g e o l o g i c a l d a t a a r e t a k e n f r o m t h e p r e s e n t s t u d y e x c e p t f o r a r e a s n o t i n c l u d e d i n d e t a i l e d maps i n Ap p e n d i x I . The r e m a i n d e r i s from Reesor and Moore ( 1 9 7 1 ) . The f a u l t s a r e drawn on t h e b a s i s o f mapping and i n t e r p r e t a t i o n s c a r r i e d o u t i n the p r e s e n t s t u d y . In some c a s e s , t h e s e l a r g e l y c o r r e s p o n d t o f a u l t s mapped by R e e s o r and Moore ( 1 9 7 1 ) . 12 5 0 ° 38' KM QUATERNARY ALLUVIUM MARBLE BANDED CALC-SILICATE SCHIST LINEATED QUARTZ MONZONITE AMPHIBOLITE QUARTZ-FELDS.-BIOT.-GA. PARAGNEISS PELITIC AND CALC-SILICATE SCHIST 1 - 1 0 ^ . QUARTZITE BIOT. GRANODIORITE QUARTZ MONZONITE HORNB. GRANODIORITE METASEDIMENTARY GNEISS THRUST FAULT ICE AND SNOW 13 o f t h i s s t u d y . A r e a s o f new d e t a i l e d mapping c o m p l e t e d on a s c a l e o f 4" = 1 m i l e a r e shown i n F i g u r e 3. A p p r o x i m a t e l y 75% o f t h e a r e a i n F i g u r e 2 has been remapped d u r i n g t h i s s t u d y . Changes t o the map p a t t e r n a r e e x t e n s i v e f o r 25% o f t h e a r e a ; d e t a i l e d g e o l o g i c a l and s t r u c t u r a l o b s e r -v a t i o n s were made i n an a d d i t i o n a l 30%. The r e m a i n i n g 20% was t r a v e r s e d ; t h e l i t h o l o g i e s were remapped and r e p r e s e n t a t i v e s t r u c t u r a l d a t a were c o l l e c t e d . S t e e p t e r r a i n , g l a c i e r s , and bad weather p r e v e n t e d a c c e s s t o some a r e a s as shown i n F i g u r e 3. F i g u r e 3 a l s o p r o v i d e s a l o c a t i o n g u i d e t o s u b s e q u e n t f i g u r e s showing d e t a i l e d g e o l o g i c a l mapping. The e x t e n s i v e p r o p o r t i o n o f c l e a n o u t c r o p i n t h e a r e a meant t h a t t r a c i n g o f u n i t s i n con-s i d e r a b l e d e t a i l was p o s s i b l e . Thus, t h e summary maps r e p r e s e n t a conden-s a t i o n o f t h e o r i g i n a l f i e l d d a t a . S t r u c t u r a l d a t a shown, f o r example, commonly r e p r e s e n t t h e a v e r a g e o f f i v e t o t e n measurements t a k e n a t an i n d i v i d u a l o u t c r o p ( s e e Appendix I ) . More d e t a i l e d v e r s i o n s o f t h e maps a r e p r e s e n t e d i n t h e p o c k e t a t t h e back o f t h e t h e s i s . T h i s g e o l o g y i s c o n t i n u o u s t o t h e n o r t h w i t h t h a t o f t h e C r a n b e r r y M o u n t a i n a r e a ( M u t t i , 1978). L a r g e f o l d e d s l i v e r s o f c o v e r r o c k s o c c u r w i t h i n t h e basement g n e i s s e s i n t h e T h o r - O d i n and C r a n b e r r y a r e a s which a p p e a r t o be a b s e n t i n t h e Frenchman Cap dome t o t h e n o r t h . The v a r i o u s r o c k t y p e s w i t h i n the T h o r - O d i n dome have been d e s c r i b e d i n d e t a i l by R e e s o r and Moore (1971) and M u t t i ( 1 9 7 8 ) , who d i v i d e d the l i t h o l o g i e s i n t o c o r e and m a n t l i n g r o c k s ( e q u i v a l e n t t o the basement and c o v e r r o c k s , r e s p e c t i v e l y , o f t h i s s t u d y ) . L i t h o l o g i e s Basement G n e i s s e s - The basement r o c k s a r e d o m i n a n t l y m i g m a t i t i c , b i o t i t e -q u a r t z - f e l d s p a r g n e i s s e s . The term "basement" has been used i n t h i s s t u d y 14 FIGURE 3. L o c a t i o n map f o r a r e a s o f d e t a i l e d s t u d y . 15 16 t o d e s c r i b e t h e g n e i s s e s and g r a n i t i c r o c k s t h a t a r e o v e r l a i n by t h e c l e a r l y m e t a s e d i m e n t a r y q u a r t z i t e s , s c h i s t s , m a r b l e s , e t c . o f t h e c o v e r r o c k s e q u e n c e . The o c c u r r e n c e o f r o o t l e s s f o l d h i n g e s w i t h i n i n c l u s i o n s o f s c h i s t i n g r a n i t i c o r t h o g n e i s s e s may r e f l e c t an e a r l i e r phase o f d e f o r -m a t i o n i n t h e basement. However, t h e e x i s t e n c e o f a phase o f d e f o r m a t i o n o l d e r t h a n t h e c o v e r r o c k sequence c o u l d n o t be documented. Two b r o a d g r o u p i n g s can r e a d i l y be d i s c e r n e d i n t h e f i e l d : h e t e r o g e n e o u s , m e t a s e d i -m entary g n e i s s e s and r e l a t i v e l y homogeneous g r a n i t i c g n e i s s e s . The h e t e r o -geneous g n e i s s e s ( u n i t C l o f R e e s o r and Moore, 1971) c o n s i s t o f b i o t i t e -q u a r t z - f e l d s p a r g n e i s s , q u a r t z - f e l d s p a r g n e i s s , and m i n o r p e l i t i c g n e i s s e s i n t e r l a y e r e d on a s c a l e o f c e n t i m e t e r s t o m e t e r s . L e n s e s and b o u d i n s o f a m p h i b o l i t e and g a r n e t a m p h i b o l i t e a r e l o c a l l y abundant b u t make up o n l y a few p e r c e n t o v e r a l l . The g n e i s s o s i t y i s l o c a l l y o b s c u r e d by m i g m a t i z a t i o n and i n j e c t i o n o f g r a n i t i c v e i n l e t s . The g r a n i t i c g n e i s s e s range i n c o m p o s i t i o n from h o r n b l e n d e - b i o t i t e g r a n o d i o r i t e ( u n i t C2 o f Reesor and Moore, 1971), t o b i o t i t e g r a n o d i o r i t e (C3a) t o l e u c o c r a t i c q u a r t z m o n z o n i t e ( b o t h C3b and C 3 c ) . The g r a n i t i c g n e i s s e s o c c u r as d i s c r e t e s h e e t s o r l e n s e s . A l l have been e x t e n s i v e l y m i g m a t i z e d , b e i n g c h a r a c t e r i z e d by 5 t o 20 cm l e u c o c r a t i c l a y e r s i n more m a f i c - r i c h g n e i s s e s . These u n i t s a r e v a r i a b l e on a s c a l e o f c e n t i m e t e r s , b u t a r e u n i f o r m i n o v e r a l l a p p e a r a n c e f o r k i l o m e t e r s a l o n g s t r i k e . M i n o r l e n s e s o f m a r b l e and c a l c - s i l i c a t e r o c k s d i s c o v e r e d d u r i n g t h i s s t u d y and by M u t t i (1978) w i t h i n the h e t e r o g e n e o u s g n e i s s e s s u p p o r t R e e s o r and Moore's (1971) c o n t e n t i o n t h a t t h i s u n i t i s m e t a s e d i m e n t a r y . A l t h o u g h i s o c l i n a l f o l d i n g i s w e l l d e v e l o p e d and no c o n v i n c i n g e v i d e n c e o f sedimen-t a r y s t r u c t u r e s i s p r e s e r v e d , some o f t h e l a y e r i n g i n t h e s e g n e i s s e s may be p r i m a r y . 17 O t h e r m i n o r r o c k t y p e s w i t h i n t h e h e t e r o g e n e o u s g n e i s s e s i n c l u d e a d i s t i n c t i v e a s s o c i a t i o n o f g e d r i t e - c o r d i e r i t e - k y a n i t e - s i l l i m a n i t e s c h i s t , k y a n i t e - s i l 1 i m a n i t e - b i o t i t e s c h i s t and p h l o g o p i t e - g e d r i t e s c h i s t . These o c c u r as e l o n g a t e l e n s e s and b o u d i n s (5 t o 50 m i n l e n g t h ) , t y p i c a l l y a l i g n e d a l o n g the f o l i a t i o n . T h i s r a t h e r unusual magnesium- and aluminum-r i c h b u l k c o m p o s i t i o n may be t h e p r o d u c t o f h y d r o t h e r m a l a l t e r a t i o n o f m a f i c v o l c a n i c r o c k s , as s u g g e s t e d by V a l l a n c e ( 1 9 6 7 ) . Whatever i t s o r i -g i n , t h e d i s t r i b u t i o n o f t h i s r o c k t y p e a p p e a r s t o d e f i n e a pseudo-s t r a t i g r a p h y w i t h i n t h e h e t e r o g e n e o u s g n e i s s e s . The b o u d i n s o f g e d r i t e -r i c h r o c k s o c c u r a t two s t r u c t u r a l l e v e l s w i t h r e s p e c t t o the l o w e s t q u a r t -z i t e o f t h e c o v e r r o c k sequence. -The f i r s t l e v e l i s 5 t o 30 m e t e r s below t h e q u a r t z i t e , t h e s e c o n d 150 t o 250 m e t e r s l o w e r . These d a t a have s i g n i -f i c a n t i m p l i c a t i o n s , as t h e y c o u l d be i n t e r p r e t e d t o r e p r e s e n t t h e l i m b s o f l a r g e - s c a l e i s o c l i n a l f o l d s w i t h i n t h e basement sequence. The g r a n i t i c g n e i s s e s o c c u r as d i s c r e t e s h e e t s up t o 100 m e t e r s t h i c k and s e v e r a l km l o n g . The f o u r c o m p o s i t i o n a l v a r i a n t s d i s t i n g u i s h e d by R e e s o r and Moore were c o n f i r m e d by t h i s s t u d y , a l t h o u g h M u t t i (1978) had d i f f i c u l t y t r a c i n g t h e s e u n i t s t o t h e n o r t h . These r o c k s have a g n e i s s i c f o l i a t i o n and m i g m a t i t i c v e i n i n g . I n c l u s i o n s o f o t h e r r o c k t y p e s a r e r a r e , a l t h o u g h s l i v e r s o f b i o t i t e - r i c h g n e i s s and b i o t i t e a m p h i b o l i t e s c o n t a i n i n g r o o t l e s s r e c l i n e d i s o c l i n a l f o l d s were f o u n d i n the h o r n b l e n d e g r a n o d i o r i t e u n i t . A l t h o u g h i t c o u l d n o t be d e m o n s t r a t e d t h a t t h e s e g r a n i t i c s h e e t s have c r o s s c u t a p r e - e x i s t i n g l i t h o l o g i c a l s e q u e n ce, the map p a t t e r n and f i e l d r e l a t i o n s s u p p o r t the i n f e r e n c e t h a t t h e y a r e i n t r u s i v e s . T h i s has been s u p p o r t e d by Rb-Sr whole roc k d a t i n g (Duncan, 1978b, see C h a p t e r 2 o f t h i s t h e s i s ) . 18 M i g m a t i t i c v e i n i n g i s p e r v a s i v e i n t h e g r a n i t i c g n e i s s e s , b u t i s v a r i a b l y d e v e l o p e d i n t h e h e t e r o g e n e o u s m e t a s e d i m e n t a r y g n e i s s e s . T h i s v a r i a b i l i t y p r o b a b l y r e f l e c t s t h e d e v i a t i o n o f t h e b u l k c o m p o s i t i o n o f t h e r o c k s from t h e minimum m e l t c o m p o s i t i o n . The o r i g i n o f the e x t e n s i v e l e u c o c r a t i c v e i n i n g i n the h o r n b l e n d e g r a n o d i o r i t e u n i t has been i n v e s t i g a t e d by B l a t t n e r (1971) u s i n g t r a c e e l e m e n t g e o c h e m i s t r y and oxygen i s o t o p e work. He c o n c l u d e d t h a t t h e v e i n s were p r o d u c e d by a c o m b i n a t i o n o f p a r t i a l m e l t i n g and s h o r t range h y d r o t h e r m a l t r a n s f e r . P a t c h y d e v e l o p m e n t o f m i g m a t i z a t i o n i n t h e h e t e r o g e n e o u s g n e i s s e s a p p e a r s t o be c o n t r o l l e d by b u l k c o m p o s i t i o n , though l o c a l v a r i a t i o n s i n w a t e r f u g a c i t i e s may be i m p o r t a n t . S e v e r a l g e n e r a t i o n s o f l e u c o c r a t i c v e i n s , s i l l s and d i k e s c r o s s c u t t h e basement g n e i s s e s . E a r l i e r p r e - o r s y n t e c t o n i c v e i n s t e n d t o be g r a n i t o i d o r p e g m a t o i d i n c h a r a c t e r , whereas p o s t - t e c t o n i c s i l l s and d i k e s t y p i c a l l y a r e a p l i t i c . F i e l d r e l a t i o n s o f t h e s e l a t e - s t a g e i n t r u s i v e phases a r e complex. In some examples, s i l l s w i t h i n d i s t i n c t b o u n d a r i e s can be t r a c e d c o n t i n u o u s l y i n t o d i k e s t h a t c l e a r l y have i n t r u d e d a l o n g u p r i g h t , b r i t t l e f r a c t u r e s . Cover Rocks - The c o v e r r o c k s have p r e s e r v e d more e v i d e n c e o f t h e i r o r i g i -n a l s t r a t i g r a p h y t h a n t h e basement g n e i s s e s , p r e s u m a b l y b e c a u s e o f t h e i r l e s s complex s t r u c t u r a l and metamorphic h i s t o r y . The l o w e s t u n i t o f the c o v e r rock sequence i s a q u a r t z i t e w hich v a r i e s i n t h i c k n e s s from a few m e t e r s t o f i f t y m e t e r s . A q u a r t z i t e u n i t , c o n g l o m e r a t i c i n p l a c e s , i s f o u n d above the basement g n e i s s e s i n t h e C r a n b e r r y Mountain-Mount B e g b i e a r e a ( M u t t i , 1978; C r a i g , 1966; Read, 1980) and around t h e Frenchman Cap 19 dome ( F y l e s , 1970; M c M i l l a n , 1973; Hoy and M c M i l l a n , 1979; Brown and P s u t k a , 1979). A l t h o u g h no a n g u l a r d i s c o r d a n c e has been i d e n t i f i e d , t h i s r e l a t i o n s h i p seems b e s t i n t e r p r e t e d as an e r o s i o n a l u n c o n f o r m i t y . Above t h e b a s a l q u a r t z i t e i s a h e t e r o g e n e o u s a s s o c i a t i o n o f p e l i t i c and c a l c a r e o u s s c h i s t s and g n e i s s e s ( U n i t s M4, M5 and M6 o f R e e s o r and Moore, 1971). In most a r e a s t h i s u n i t i s d o m i n a n t l y a q u a r t z - f e l d s p a r -b i o t i t e m e t a s e d i m e n t a r y g n e i s s ; however, c a l c - s i l i c a t e s c h i s t s a r e dominant i n t h e n o r t h e a s t p a r t o f t h e map a r e a . T h e s e g n e i s s e s and s c h i s t s a r e t y p i c a l l y w e l l - l a y e r e d on a s c a l e o f t e n s o f c e n t i m e t e r s , p o s s i b l y an o r i -g i n a l s e d i m e n t a r y f e a t u r e . S t r u c t u r a l l y above t h e h e t e r o g e n e o u s p e l i t i c and c a l c a r e o u s s c h i s t s and g n e i s s e s i s a s i m i l a r a s s o c i a t i o n o f r o c k t y p e s i n t e r l a y e r e d w i t h q u a r t z i t e s , a m p h i b o l i t e s and m a r b l e . These t h r e e groups o f l i t h o l o g i e s f o r m mappable u n i t s t h a t can be t r a c e d f o r s e v e r a l km a l o n g s t r i k e ( F i g . 2 ) . A d e f i n i t e s t r a t i g r a p h i c sequence f o r t h e c o v e r r o c k s i n t h i s a r e a c o u l d n o t be e s t a b l i s h e d due t o t h e a p p a r e n t a b s e n c e o f s e d i m e n t a r y s t r u c t u r e s s u i t a b l e f o r f a c i n g d e t e r m i n a t i o n . The p o s s i b i l i t y o f l i t h o l o g i c a l r e p e t i -t i o n r e l a t e d t o u n r e c o g n i z e d f o l d s and the i m b r i c a t i o n o f the c o v e r r o c k s by t h r u s t i n g f u r t h e r c o m p l i c a t e any a t t e m p t t o e s t a b l i s h a s t r a t i g r a p h y . Read (1980) has r e c e n t l y p r o p o s e d a c o v e r r o c k s t r a t i g r a p h y f o r the B o u l d e r Mountain-Mount B e g b i e a r e a a l o n g the s o u t h e r n f l a n k o f t h e Frenchman Cap dome. The T h o r - O d i n c o v e r r o c k s a r e s u f f i c i e n t l y d i f f e r e n t from t h i s a r e a t h a t t h e two sequences p r o b a b l y can o n l y be r e l a t e d by s t r u c t u r a l mapping o f t h e i n t e r v e n i n g 50 km. R e e s o r and Moore (1971) c o r r e l a t e d the q u a r t z i t e and m a r b l e u n i t s w i t h i n t h e c o v e r r o c k sequence w i t h t h e Lower Cambrian Hamil q u a r t z i t e and 20 B a d s h o t l i m e s t o n e o f t h e Kootenay A r c . More r e c e n t l y , Hoy (1977a) has s u g g e s t e d more d e t a i l e d c o r r e l a t i o n s between t h e c o v e r r o c k s and the e s t a b l i s h e d l o w e r P a l e o z o i c s t r a t i g r a p h y . Read (1979 and 1980), Brown (1980) and Brown and P s u t k a (1979) have made d i s t i n c t l y d i f f e r e n t c o r r e l a -t i o n s . They c o n c l u d e t h a t t he c o v e r r o c k sequence i s c o r r e l a t i v e w i t h P u r c e l l - B e l t s t r a t i g r a p h y and i n p a r t w i t h t h e H o r s e t h i e f Creek Group. I t i s d o u b t f u l t h a t t h i s d i f f e r e n c e o f o p i n i o n can be r e s o l v e d w i t h o u t i n d e -p e n d e n t g e o c h r o n o l o g i c a l e v i d e n c e . G e o c h r o n o l o g y I n i t i a l a t t e m p t s t o d a t e t h e r o c k s o f t h e T h o r - O d i n dome by t h e G e o l o g i c a l S u r vey o f Canada u s i n g t he K-Ar t e c h n i q u e gave r e s u l t s i n t h e ra n g e 81 t o 61 Ma f o r m u s c o v i t e s and b i o t i t e s ( R e e s o r and Moore, 1971). R e e s o r and Moore i n t e r p r e t e d t h e s e d a t e s as r e p r e s e n t i n g t he c o o l i n g and u p l i f t o f t h e dome. A s u b s e q u e n t a t t e m p t t o d a t e p o s s i b l e basement g n e i s s e s from t he C I a c h n a c u d a i n n S a l i e n t e a s t o f Revel s t o k e was made by B l e n k i n s o p ( 1 9 7 2 ) , u s i n g Rb-Sr whole r o c k t e c h n i q u e s . T h i s a t t e m p t was u n s u c c e s s f u l , g i v i n g s c a t t e r e d r e s u l t s s u g g e s t i v e o f p a r t i a l r e s e t t i n g o r open system b e h a v i o r . S i m i l a r p roblems o f r e s e t t i n g o f Sr i s o t o p e systems were e n c o u n t e r e d by Ryan (1973) i n h i s a t t e m p t s t o d a t e g n e i s s i c g r a n i t e s a l o n g t h e w e s t e r n m a r g i n o f the Shuswap. These r o c k s may be c o n s i d e r a b l y o l d e r than t he M e s o z o i c d a t e s o f Ryan (1973) ( A r m s t r o n g , p e r s o n a l com-m u n i c a t i o n , 1982). In an a t t e m p t t o overcome t h e pro b l e m s i n h e r e n t i n K-Ar and Rb-Sr d a t i n g t e c h n i q u e s , Wanless and Reesor (1975) d a t e d z i r c o n s from g r a n i t e basement g n e i s s ( u n i t C2) from t he c o r e o f the T h o r - O d i n dome. U s i n g the 21 U-Pb t e c h n i q u e , t h e y o b t a i n e d f o u r p o i n t s l y i n g a l o n g a c h o r d w i t h an upper i n t e r c e p t o f 1,960 Ma and a l o w e r i n t e r c e p t o f 175 Ma. On t h i s b a s i s t h e y c o n c l u d e d t h a t the basement g n e i s s e s were, i n p a r t , Hudsonian basement w h i c h had r i s e n d i a p i r i c a l l y . Brown (1978) had q u e s t i o n e d t h e i n t e r p r e t a -t i o n o f t h e s e d a t a , s u g g e s t i n g t h a t t h e z i r c o n s c o u l d be d e t r i t a l i n o r i -g i n . R e c e n t Rb-Sr whole r o c k d a t i n g (Duncan, 1978b; t h i s t h e s i s , C h a p t e r 2) has c o n f i r m e d P r e c a m b r i a n ages f o r t h e basement g n e i s s e s . A s i x - p o i n t Rb-Sr i s o c h r o n f o r samples f r o m a s i n g l e o u t c r o p a r e a o f m e t a s e d i m e n t a r y g n e i s s g i v e s an age o f 2.7 ± 0.2 Ga (Duncan, 1978b, C h a p t e r 2 ) . S i m i l a r r e s u l t s have r e c e n t l y been r e p o r t e d f o r a u n i t w i t h i n the M a l t o n G n e i s s by C h a m b e r l a i n ^ t a l _ . ( 1 9 7 8 ) . Rb-Sr d a t i n g o f m i g m a t i t i c g r a n i t e basement g n e i s s e s (Duncan, 1978b, C h a p t e r 2) d e m o n s t r a t e d t h a t h i g h - g r a d e meta-morphism and m i g m a t i t e f o r m a t i o n i n t h e basement g n e i s s e s o c c u r r e d a t 750 Ma. D e s p i t e t h e r e s e t t i n g o f t h e Rb-Sr c l o c k by l o c a l p a r t i a l m e l t i n g i n t h e s e g r a n i t i c g n e i s s e s , t h e r o c k s s t i l l r e t a i n an i m p r i n t o f t h e i r o r i -87 86 87 ftfi g i n a l age. By r e c o n s t r u c t i n g the a v e r a g e S r / S r and Rb/ S r f o r l a r g e (10 m ) sample a r e a s , t h e d a t a were f o u n d t o l i e a p p r o x i m a t e l y on a 2.0 Ga r e f e r e n c e i s o c h r o n . Thus t h e Sr i s o t o p i c d a t a can be r e g a r d e d as con-s i s t e n t w i t h t h e U-Pb z i r c o n d a t e ( C h a p t e r 2 ) . S i m i l a r ages o f 2.0 t o 2.5 Ga a r e s u g g e s t e d by Rb-Sr whole roc k d a t a from t h e Frenchman Cap basement g n e i s s e s (R.L. A r m s t r o n g , p e r s o n a l c o m m u n i c a t i o n , 1982). The c o v e r r o c k sequence has been d a t e d a t 5 50 t o 500 Ma ( t h i s t h e s i s , C h a p t e r 3) by a 2 0 7 P b / 2 0 4 P b - 2 0 6 P b / 2 0 4 P b s e c o n d a r y i s o c h r o n o b t a i n e d from s y n g e n e t i c l e a d s u l f i d e s from w i t h i n t h e c o v e r r o c k sequence. T h i s sup-p o r t s e a r l i e r l i t h o l o g i c a l c o r r e l a t i o n s o f t h e c o v e r r o c k sequence w i t h 22 Cambrian s t r a t a i n t h e Kootenay A r c ( R e e s o r and Moore, 1971; Hoy, 1977a). However, A r m s t r o n g ( p e r s o n a l c o m m u n i c a t i o n , 1982) s u g g e s t s t h a t t h e c o v e r r o c k sequence t o t h e n o r t h n e a r Frenchman Cap may be 1.5 t o 2.5 Ga. Metamorphism The g e n e r a l p a t t e r n o f metamorphism i n t h e T h o r - O d i n dome has been i n v e s t i g a t e d by Re e s o r and Moore ( 1 9 7 1 ) , who c o n c l u d e d t h a t the basement g n e i s s e s and c o v e r r o c k sequence were o f r e l a t i v e l y u n i f o r m upper amphi-b o l i t e g r a d e . These a u t h o r s mapped one i s o g r a d ( w i t h i n t h e map a r e a i n F i g . 2) between s i l l i m a n i t e + a l k a l i f e l d s p a r and m u s c o v i t e + q u a r t z . M u s c o v i t e - q u a r t z a s s e m b l a g e s o c c u r i n t h e n o r t h e a s t c o r n e r o f t h e a r e a a r o u n d t h e P i n g s t o n f o l d . As t h e r e i s f i e l d and p e t r o g r a p h i c e v i d e n c e f o r a minimum o f t h r e e metamorphic e v e n t s ( A p p e n d i x I I I ) , t h e o c c u r r e n c e o f p o l y m e t a m o r p h i c e f f e c t s p r e s e n t s a major i n t e r p r e t a t i o n a l p r o b l e m . Because some o f t h e phases o c c u r r i n g i n t h e s e r o c k s a r e " r e f r a c t o r y " ( e . g . , g a r n e t , s i l l i m a n i t e , k y a n i t e and corundum), metamorphic r e a c t i o n s f r e q u e n t l y have f a i l e d t o p r o c e e d t o c o m p l e t i o n . As a r e s u l t , t e x t u r a l e v i d e n c e o f d i s e q u i l i b r i u m i s common. F i e l d e v i d e n c e f o r polymetamorphism i n c l u d e s t h e o c c u r r e n c e o f s i l l i m a n i t e (and r a r e r k y a n i t e ) l i n e a t i o n s a l o n g Phase One f o l d a x e s , t h e o c c u r r e n c e o f k y a n i t e i n p o s t - P h a s e Two peg m a t o i d v e i n s , and t h e o c c u r r e n c e o f a n d a l u s i t e i n p o s t - P h a s e T h r e e v e i n s . T e x t u r a l e v i d e n c e f o r po l y m e t a -morphism i n c l u d e s g a r n e t s w i t h c o r e s c o n t a i n i n g k y a n i t e i n c l u s i o n s , rimmed by i n c l u s i o n - f r e e g a r n e t i n a q u a r t z - f e l d s p a r - b i o t i t e - s i l l i m a n i t e m a t r i x . The e a r l i e s t metamorphic e v e n t unambiguously e s t a b l i s h e d i s the h i g h -g r a d e metamorphism t h a t r e s u l t e d i n t h e m i g m a t i z a t i o n o f the basement 23 g n e i s s e s a t 750 Ma (Duncan, 1978b; C h a p t e r 2 ) . I t i s p o s s i b l e t h a t t h e g e d r i t e - k y a n i t e - c o r d i e r i t e a s s e m b l a g e s f o u n d as b o u d i n s w i t h i n t h e meta-s e d i m e n t a r y basement g n e i s s e s r e p r e s e n t p r o d u c t s o f t h i s P r e c a m b r i a n meta-morphism. G r a n t (1968) has s u g g e s t e d t h a t such a s s e m b l a g e s may be p r o d u c e d as a r e s i d u u m l e f t b e h i n d a f t e r e x t r a c t i o n o f a l e u c o c r a t i c m e l t from a g n e i s s i c p a r e n t r o c k . R e g a r d l e s s o f t h e v a l i d i t y o f t h i s h y p o t h e s i s , t h e s e r o c k s show abundant e v i d e n c e o f polymetamorphism. L a r g e k y a n i t e c r y s t a l s , commonly o c c u r r i n g as r a d i a t i n g r o s e t t e s up t o 30 cm a c r o s s , a r e t y p i c a l l y r e p l a c e d by b u n d l e s o f f i b r o l i t i c s i 11 imam*te. V e i n i n g and c o r o n a t e x t u r e s u g g e s t t h a t t h e r e a c t i o n s : (1) k y a n i t e + g e d r i t e + q u a r t z = c o r d i e r i t e , and (2) g e d r i t e + k y a n i t e = c o r d i e r i t e + corundum have o c c u r r e d i n t h e s e r o c k s , p r o b a b l y i n r e s p o n s e t o a d e c r e a s e i n p r e s s u r e ( s e e Appendix I V ) . F o r example, s i l l i m a n i t e t y p i c a l l y o c c u r s rimmed by corundum and g r a i n s o f c o r d i e r i t e i n a g e d r i t e m a t r i x . These a s s e m b l a g e s have been i n t e r p r e t e d by R e e s o r and Moore (1971) and S c h r e y e r (1973) as r e p r e s e n t i n g p r e s s u r e s o f 750 t o 950 MPa and t e m p e r a t u r e s o f 700 t o 7 5 0 ° C . E x p e r i m e n t a l d e t e r m i n a t i o n o f r e a c t i o n (1) by Green and Vernon (1974) a p p e a r s t o c o n f i r m p r e s s u r e s o f 800 t o 1000 MPa f o r t h e s e a s s e m b l a g e s . However, more r e c e n t t h e o r e t i c a l and e x p e r i m e n t a l work by Duncan and Greenwood (1977) and Newton and Wood (1979) has shown t h a t l o w e r water f u g a c i t i e s move the c o r d i e r i t e s t a b i l i t y f i e l d to l o w e r p r e s s u r e s ( s e e Appe n d i x I V ) . E s t i m a t e s o f w a t e r f u g a c i t i e s f o r the dome ( b a s e d on phase e q u i l i b r i u m s t u d i e s by F r o e s e , 1973, and the i n t e r s e c t i o n o f t h e m u s c o v i t e + q u a r t z breakdown w i t h t he g r a n o d i o r i t e s o l i d u s as a f u n c t i o n o f water 24 p r e s s u r e ) s u g g e s t t h a t p r e s s u r e s o f 500 t o 600 MPa and t e m p e r a t u r e s o f 600 t o 700°C a r e r e a s o n a b l e e s t i m a t e s f o r t h e s e c o r d i e r i t e e q u i l i b r i a . T h i s e s t i m a t e o f p r e s s u r e and t e m p e r a t u r e i s c o n s i s t e n t w i t h t h a t o f Ghent e t a l . (1977) f o r the n o r t h e r n f l a n k o f t h e C r a n b e r r y dome 50 km n o r t h o f t h e s t u d y a r e a . Thus, i t i s n o t n e c e s s a r y t o h y p o t h e s i z e a n o m a l o u s l y h i g h p r e s s u r e s , c o r r e s p o n d i n g t o l o w e r c r u s t a l c o n d i t i o n s , t o e x p l a i n t h e o b s e r v e d metamorphic a s s e m b l a g e s . Metamorphism a l s o accompanied Phase One d e f o r m a t i o n as s i l l i m a n i t e , k y a n i t e and h o r n b l e n d e l i n e a t i o n s were d e v e l o p e d . However, t h e e f f e c t s o f t h i s e v e n t have been s t r o n g l y o v e r p r i n t e d by t h e metamorphism a s s o c i a t e d w i t h Phases Two and T h r e e . U n f o r t u n a t e l y , t h e abse n c e o f w e l l - d e v e l o p e d c l e a v a g e s i n t h e s e r o c k s p r e c l u d e s d e f i n i t i v e r e l a t i o n o f phases o f d e f o r -m a t i o n t o metamorphic a s s e m b l a g e s . I t seems l i k e l y , on the b a s i s o f t h e i n f e r r e d l o w - p r e s s u r e c o n d i t i o n s , t h a t t h e p e l i t i c a s s e m b l a g e s s t u d i e d by F r o e s e (1970, 1973) and M u t t i (1978) r e p r e s e n t phase e q u i l i b r i a e s t a b l i s h e d d u r i n g t h i s t h i r d metamorphic e v e n t . C a l c u l a t i o n o f b i o t i t e - g a r n e t Fe/Mg d i s t r i b u t i o n c o e f f i c i e n t s from F r o e s e ' s d a t a ( A p p e n d i x IV) s u g g e s t s tempera-t u r e s o f 850 ± 50°C b a s e d on the e x p e r i m e n t a l l y c a l i b r a t e d geothermometer o f F e r r y and Spear ( 1 9 7 8 ) . The abse n c e o f s t a u r o l i t e , t h e c o e x i s t e n c e o f s i l l i m a n i t e and a l k a l i f e l d s p a r , and t h e p r e s e n c e o f t e x t u r a l e v i d e n c e f o r t h e r e a c t i o n s i l l i m a n i t e + b i o t i t e = g a r n e t + c o r d i e r i t e + a l k a l i f e l d s p a r a r e c o n s i s t e n t w i t h t e m p e r a t u r e s above 700°C and p r e s s u r e s o f 400 t o 500 MPa. S t r u c t u r a l Geology The complex m e s o s c o p i c s t r u c t u r e s p r e s e n t i n most o f t h e T h o r - O d i n dome and t h e e f f e c t s o f r o t a t i o n o f s t r u c t u r e s by dome f o r m a t i o n n e c e s s i t a t e 25 great care in i d e n t i f y i n g phases of deformation. The sequence of folding outlined below was established by i d e n t i f y i n g consistent overprinting r e l a -tions rather than by observation of s t y l e or orientation. Since excellent outcrop was ava i l a b l e , overprinting relations could be established both within individual structural domains and l o c a l l y on p a r t i c u l a r ridges. Within a local area, orie n t a t i o n , and to some extent s t y l e , were consistent for a p a r t i c u l a r phase of deformation. The following description of the various phases of deformation i s generalized for the whole area, but provides a framework for the l a t e r detailed analysis. Phase One Structures - Phase One structures are t y p i c a l l y rootless, appressed, i s o c l i n a l f o l d s , with f o l d axes oriented in a broadly north-south d i r e c t i o n . Interference between these and l a t e r phases of defor-mation produces complex dome and basin structures on an outcrop scale. Local areas of intense development of mesoscopic Phase One folds are interpreted as the hinge zones of large-scale f o l d s . A large, well exposed Phase One f o l d crops out on the north flank of Fosthall Ridge and has been mapped in detail (Fig. 4). This structure confirms the scale and impor-tance of Phase One folding. Phase One folds deform an e a r l i e r cleavage and have a crenulation cleavage approximately p a r a l l e l to t h e i r axial surfaces. Figure 5 shows an example of a boudinaged layer of c a l c - s i l i c a t e rock, wrapped around a Phase One f o l d hinge. The boudinage and crenulated cleavage apparently formed p r i o r to Phase One. Elsewhere, rare appressed rootless hinges are folded by Phase One. The nature of t h i s e a r l i e r deformation i s not c l e a r , but i t could be related to soft-sediment folding. 26 FIGURE 4. D e t a i l e d map o f a l a r g e - s c a l e Phase One c l o s u r e from t h e n o r t h s i d e o f F o s t h a l l R i d g e . (a) D e t a i l e d map o f o u t c r o p . Note t h a t a l l c o n t a c t s a r e d e f i n i t e due t o t h e e x t e n s i v e e x p o s u r e o f t h i s f o l d h i n g e . The arrows show t h e a z i m u t h and p l u n g e o f s m a l l -s c a l e p a r a s i t i c f o l d s . O n l y r e p r e s e n t a t i v e o r i e n t a t i o n d a t a a r e shown. The a x i a l t r a c e has n o t been drawn on th e map so as n o t t o o b s c u r e t h e f o l d geometry. 7900 6900 ,< 6800 -6800 \30 6700 Marble Gritty Brown Marble B io t i te -S i l l . -Garnet Gneiss Leucocratic Grani te Gneiss Quartz - Felds. Augen Gneiss Biotite - Quartz - Felds. -Gar. Schist 50 100 Meters r o 28 FIGURE 4 ( c o n t i n u e d ) . (b) E q u a l - a n g l e s t e r e o n e t p l o t o f t h e o r i e n t a t i o n d a t a f r o m the a r e a shown i n ( a ) . Shown a r e : p o l e s t o m e s o s c o p i c Phase One a x i a l p l a n e s (A); p o l e s t o c o m p o s i t i o n a l l a y e r i n g ( • ) ; and m e s o s c o p i c Phase One f o l d axes and l i n e a t i o n s ( ° ) . The f o l d axes shown as * a l l were measured from a s i n g l e o u t c r o p o f a few s q u a r e meters i n t h e h i n g e zone o f t h e f o l d . The l o c a l Phase Two f o l d axes (•) have a d i s t i n c t l y d i f f e r e n t o r i e n t a t i o n and r e f o l d Phase One l i n e a t i o n s . The s o l i d g r e a t c i r c l e i s t h e g i r d l e formed by the p o l e s t o c o m p o s i t i o n a l l a y e r i n g . The p o l e t o t h i s g r e a t c i r c l e i s the modal Phase One f o l d a x i s ( A ) . The dashed g r e a t c i r c l e i s t h e modal a x i a l p l a n e o f the Phase One f o l d . 29 30 FIGURE 5. M e s o s c o p i c Phase One f o l d s . (a) Phase One f o l d i n q u a r t z o - f e l d s p a t h i c basement g n e i s s e s d e f o r m i n g a b o u d i n a g e d a m p h i b o l i t e l a y e r . Note t h e d e v e l o p m e n t o f a c r e n u l a t i o n c l e a v a g e i n t h e h i n g e zone p a r a l l e l t o t h e a x i a l p l a n e . The d r a w i n g i s t a k e n f r o m a p h o t o g r a p h f r o m C a r i b o u A l p . (b) Phase One f o l d s from Gunnarson Nappe on t h e e a s t f a c e o f Mount Gun n a r s o n . A l l f o l d s k e t c h e s a r e o r i e n t e d w i t h n o r t h t o the r i g h t and s o u t h t o the l e f t , e x c e p t as marked. A l l f o l d s a r e viewed down the p l u n g e where p o s s i b l e . 31 32 Phase Two S t r u c t u r e s - Phase Two f o l d s ( F i g . 6) a r e t i g h t - t o - i s o c l i n a l , w i t h f o l d axes g e n e r a l l y o r i e n t e d e a s t - w e s t ; t h e y dominate t h e o u t c r o p p a t -t e r n and m e s o s c o p i c s t r u c t u r e s o f much o f t h e dome a r e a . Phase Two f o l d s d e form Phase One f o l d s and p e g m a t i t e v e i n l e t s w h i c h a r e s y n t e c t o n i c t o Phase One. E l l i p s o i d a l , f l a t t e n e d g a r n e t s , s i m i l a r t o t h o s e d e s c r i b e d by Ross ( 1 9 7 3 ) , may be f o u n d o r i e n t e d p a r a l l e l t o t h e a x i a l s u r f a c e o f Phase Two f o l d s . L a r g e - s c a l e , Phase Two recumbent nappe s t r u c t u r e s c h a r a c t e r i s t i c a l l y have zones o f i n t e n s e f l a t t e n i n g on t h e l o w e r l i m b s o f a n t i f o r m s . The t h r u s t f a u l t s shown i n F i g u r e 2 a r e i n t e r p r e t e d as Phase Two s t r u c t u r e s . Phase T h r e e S t r u c t u r e s - Phase T h r e e f o l d s ( F i g . 6) range i n s t y l e from open t o i s o c l i n a l and a r e a p p r o x i m a t e l y c o - a x i a l w i t h Phase Two f o l d s . Numerous o u t c r o p s c o n t a i n Phase T h r e e f o l d s t h a t r e f o l d Phase Two f o l d s , w h i c h i n t u r n f o l d Phase One l i n e a t i o n s a nd/or f o l d s . Where Phase T h r e e f o l d s o v e r p r i n t Phase Two f o l d s , t h e f o r m e r a r e more open i n s t y l e and g e n e r a l l y have more s h a l l o w l y d i p p i n g a x i a l s u r f a c e s . Phase T h r e e f o l d s d e f o r m t h e f l a t t e n e d g a r n e t f a b r i c and s i l l i m a n i t e l i n e a t i o n s a s s o c i a t e d w i t h Phase Two. Phase T h r e e f o l d s a r e a s s o c i a t e d w i t h t e c t o n i c s l i d e s , t h r u s t s and l a g s t r u c t u r e s ( W h i t t e n , 1966) i n some a r e a s . Phase F o u r S t r u c t u r e s - Phase F o u r f o l d s a r e t y p i c a l l y c h e v r o n - s h a p e d and have v e r t i c a l , n o r t h w e s t - t r e n d i n g a x i a l s u r f a c e s . M e s o s c o p i c Phase Four f o l d s a r e r a r e , b u t show a c o n s i s t e n t r e v e r s a l o f v e r g e n c e s s o u t h w e s t t o n o r t h w e s t a c r o s s t h e dome from T h o r R i d g e to Gates Ledge. A s p o r a d i c a l l y d e v e l o p e d , w i d e l y s p a c e d f r a c t u r e c l e a v a g e o c c u r s p a r a l l e l t o the a x i a l s u r f a c e . L i t t l e r e c r y s t a l l i z a t i o n o f m i c a s was o b s e r v e d i n t h e Phase Four h i n g e s . 33 FIGURE 6. M e s o s c o p i c Phases Two and T h r e e f o l d s . (a) S o u t h e r l y - v e r g e n t Phase Two f o l d s r e f o l d e d by s h a l l o w l y - d i p p i n g , n o r t h e r l y - v e r g e n t Phase T h r e e f o l d s . From the n o r t h s i d e o f the Gates Ledge i n f o l d . (b) T y p i c a l s t r u c t u r a l s t y l e o f Phase Two f o l d s and r e l a t e d s h e a r zones i n q u a r t z o - f e l d s p a t h i c basement g n e i s s e s f r o m t h e upper l i m b o f t h e Gunnarson Nappe 1 km e a s t o f G l a d s h e i m L a k e . ( c ) Phase Two f o l d s wiith h i n g e - t y p e v e r g e n c e a r e deformed by n o r t h - v e r g i n g Phase T h r e e f o l d s wiith s h a l l o w e r - d i p p i n g a x i a l p l a n e s . The o u t c r o p i s from t h e h i n g e zone o f a major Phase T h r e e s y n f o r m on t h e e a s t e r n t i p o f F o s t h a l l R i d g e . (d) Phase T h r e e f o l d s o v e r p r i n t i n g Phase Two f o l d s f r o m the B i g Ledge Subdomain o f t h e C o v e r Rock Domain. Note the t h r u s t zone t y p i c a l l y a s s o c i a t e d w i t h Phase T h r e e i n t h i s domain. 34 35 Phase F i v e S t r u c t u r e s - N o r t h e a s t - t r e n d i n g , u p r i g h t , Phase F i v e f o l d s o c c u r on two s c a l e s : as b r o a d b u c k l e s w i t h a w a v e l e n g t h o f 10 t o 30 m e t e r s and l a r g e - s c a l e c h e v r o n f o l d s w i t h a w a v e l e n g t h o f 2 t o 3 km ( F i g . 7 ) . Phase F i v e a l s o has a v a r i a b l y d e v e l o p e d , w i d e l y s p a c e d , v e r t i c a l f r a c t u r e c l e a v a g e . Mutual i n t e r f e r e n c e o f Phase F i v e s t r u c t u r e s w i t h t h e main Phase F o u r a n t i f o r m c r e a t e d t h e domal c u l m i n a t i o n t h a t c h a r a c t e r i z e s t h e a r e a . Phase S i x S t r u c t u r e s - N o r t h - s o u t h t r e n d i n g Phase S i x f o l d s o c c u r as b r o a d , u p r i g h t , b u c k l e f o l d s , o f t e n w i t h v e r t i c a l f a u l t s a l o n g t h e i r a x i a l s u r f a c e . STRUCTURAL ANALYSIS OF FAULT-BOUNDED DOMAINS D e l i n e a t i o n o f Domain B o u n d a r i e s Mapping by R e e s o r and Moore (1971) and Duncan (1978a) r e v e a l e d t h e e x i s t e n c e o f a number o f ma j o r f a u l t s w i t h i n t h e T h o r - O d i n g n e i s s dome ( F i g . 8 ) . C o n s i s t e n t d i f f e r e n c e s i n t h e o r i e n t a t i o n o f s t r u c t u r a l e l e m e n t s a c r o s s t h e s e f a u l t s make them n a t u r a l c h o i c e s f o r t h e b o u n d a r i e s o f s t r u c -t u r a l domains. D e l i n e a t i o n o f subdomains was n o t f o u n d t o be n e c e s s a r y t o i n t e r p r e t t h e o r i e n t a t i o n d a t a . The s t r u c t u r a l geometry o f each domain ( F i g . 8 ) , t o g e t h e r w i t h j u s t i f i -c a t i o n o f t h e i r b o u n d a r i e s , w i l l be d i s c u s s e d i n t u r n b e g i n n i n g w i t h t h e c e n t r a l c o r e zone and moving o u t w a r d s . A u t o c h t h o n o u s C o r e G n e i s s Domain The c e n t r a l c o r e o f t h e dome r e p r e s e n t s t h e l o w e s t s t r u c t u r a l l e v e l e x p o s e d . P a r a d o x i c a l l y , t h i s i s s t r u c t u r a l l y t h e s i m p l e s t domain. F o l d s i n t h e l a y e r e d g n e i s s e s o f t h i s domain a r e t y p i c a l l y t i g h t r a t h e r than 36 FIGURE 7. G e o l o g y o f t h e west s i d e o f t h e T h o r - O d i n g n e i s s dome w i t h t h e t o p o g r a p h i c d i s t o r t i o n e l i m i n a t e d by p r o j e c t i o n o n t o a h o r i z o n t a l p l a n e . Comparison w i t h F i g u r e 2 shows th e s i g n i f i c a n t e f f e c t o f t o p o g r a p h y on t h e map p a t t e r n o f t h e r o c k u n i t s w i t h i n t h e dome. METASEDIMENTARY BASEMENT GNEISSES HORNBLENDE -BIOTITE GRANODIORITE GNEISS BIOTITE GRANODIORITE BASAL QUARTZITE MIXED PELITIC AND CALC-SILICATE SCHISTS IMBRICATED BASEMENT GNEISSES AND COVER ROCKS DIP AND STRIKE OF LAYERING TRACE of COMPOSITIONAL LAYERING SYNFORMAL AXIAL TRACE ANTIFORMAL AXIAL TRACE 0 1 2 3 I KM C O •sj 38 FIGURE 8. Map o f f a u l t - b o u n d e d domains. 40 i s o c l i n a l , as seen s t r u c t u r a l l y h i g h e r . The o r i e n t a t i o n o f the dominant phase o f recumbent f o l d s and t h e l i n e a t i o n p a t t e r n i n t h e domain i s s h a r p l y j u x t a p o s e d a g a i n s t t h e o v e r l y i n g Basement C o r e d Nappe Domain. In t h e c o r e g n e i s s domain, t h e dominant m e s o s c o p i c recumbent f o l d s and l i n e a t i o n s t r e n d n o r t h - s o u t h t o n o r t h w e s t e r l y , as can be seen i n F i g u r e 9. In t h e o v e r l y i n g domain t h e dominant f o l d s t r e n d e a s t - w e s t . The boundary between t h e s e domains a p p e a r s t o be a f a u l t w h i c h t r u n c a t e s l i t h o l o g i e s ( F i g s . 8 and 10) and j u x t a p o s e s two a r e a s o f d i f f e r i n g s t r u c t u r a l e v o l u t i o n . T h i s c o r e g n e i s s i s assumed t o be a u t o c h t h o n o u s , and t r a n s l a t i o n a l s t r a i n i n t h e o v e r l y i n g r o c k s i s r e f e r r e d t o t h i s domain as i f i t were a s t a t i o n a r y r e f e r e n c e . The c o r e domain i s d o m i n a t e d by t h e P i n g s t o n f o l d o r i g i n a l l y mapped by R e e s o r and Moore ( 1 9 7 1 ) . T h i s s t r u c t u r e i s a l a r g e - s c a l e , recumbent f o l d ( F i g . 11) o f c o v e r r o c k s , c o r e d by c a l c - s i l i c a t e s and p e l i t i c s c h i s t s and o u t l i n e d by q u a r t z i t e s . On t h e s o u t h e r n c l i f f f a c e o f Thor R i d g e , l a r g e -s c a l e f o l d s i n w e l l - l a y e r e d c o r e g n e i s s e s can be seen t o v e r g e e a s t e r l y , a p p a r e n t l y c o n s i s t e n t w i t h t h e i r p o s i t i o n on t h e l o w e r l i m b o f the P i n g s t o n f o l d . V e r g e n c e s o f m i n o r f o l d s s u g g e s t t h a t t h e P i n g s t o n f o l d i s a n o r t h -t o - n o r t h w e s t e r l y p l u n g i n g s y n f o r m as e x p o s e d on T h o r Ridge ( F i g . 1 1 ) . The o u t c r o p p a t t e r n has been c o m p l i c a t e d due t o r e f o l d i n g by l a r g e - s c a l e n o r t h w e s t - t r e n d i n g P h a s e F o u r f o l d s . R e e s o r and Moore (1971) have t r a c e d the P i n g s t o n f o l d from T h o r Ridge t o t h e s o u t h e a s t n e a r Mount Burnham. Here m i n o r f o l d s on the upper l i m b o f t h e P i n g s t o n s t r u c t u r e v e r g e e a s t e r l y and have s h a l l o w p l u n g e s t o the s o u t h ( F i g . 9 ) . Thus t h e s e f o l d s have t h e wrong v e r g e n c e to be p a r a s i t i c t o the 41 FIGURE 9. G e o l o g i c a l map o f t h e A u t o c h t h o n o u s Core G n e i s s Domain ( m o d i f i e d from R e e s o r and Moore, 1971). The a r e a s remapped d u r i n g t h e p r e s e n t s t u d y a r e shown i n F i g u r e 3. The s h o r t dashes show t h e a x i a l t r a c e o f t h e Phase One P i n g s t o n f o l d . The l o n g dashed l i n e s show t h e a x i a l t r a c e s o f Phase Two s y n f o r m s w i t h i n the Basement C o r e d Nappe Domain. The dominant phase o f d u c t i l e f o l d i n g shown as arrows a r e d i f f e r e n t i a t e d i n t o phases ( a c c o r d i n g t o t h e c r i t e r i a d i s c u s s e d i n the t e x t ) as f o l l o w s : • — i — * * Phase One, n o r t h e r l y v e r g e n t ; • — n — P h a s e Two, h i n g e v e r g e n c e ; —m— * - Phase T h r e e . F o l d s shown as — i — a r e S e t II group f o l d s and — i — S e t I group f o l d s ( s e e t e x t and c a p t i o n t o F i g . 1 3 ) . 5 0 1 K M THRUST FAULTS P L U N G E OF D O M I N A N T DUCTILE FOLDS STRIKE A N D DIP OF LAYERING C o v e r R o c k s PELITIC A N D CALCAREOUS SCHISTS A N D GNEISSES BASAL QUARTZITE ICE A N D S N O W B a s e m e n t R o c k s H O R N B L E N D E G R A N O D I O R I T E BIOTITE G R A N O D I O R I T E M E T A S E D I M E N T A R Y A R C H E A N GNEISSES 1X5 43 FIGURE 10. C r o s s - s e c t i o n s o f t h e A u t o c h t h o n o u s Core G n e i s s Domain. (a) G e o l o g i c a l c r o s s - s e c t i o n f o r l i n e X — X ' on F i g u r e 9. The symbols a r e the same as f o r F i g u r e 9. Some a d d i t i o n a l d a t a f o r Phase T h r e e nappes i n the C a r i b o u A l p a r e a have been added. (b) S t r u c t u r a l s e c t i o n o f (a) a b o v e , showing i n t e r p r e t a -t i o n o f basement c o r e d nappes d i s c u s s e d i n t h e t e x t . 45 FIGURE 11. The g e o l o g y o f the P i n g s t o n f o l d , T h o r R i d g e . Shown a r e t h e c o v e r r o c k s c o r i n g t he f o l d ( shaded p a t t e r n ) , the l o w e r q u a r t z i t e s ( d o t t e d p a t t e r n ) , and the basement g n e i s s e s ( u n p a t t e r n e d ) . A l s o shown a r e t h e p l u n g e , a z i m u t h and v e r g e n c e o f S e t I f o l d s ( • — i — • ) , S e t II f o l d s ( • — I — • )» and Phase F o u r f o l d s ( — r m — • )• .46 47 P i n g s t o n f o l d . T h i s s u g g e s t s t h a t t h e y r e p r e s e n t a l a t e r phase o f d e f o r -m a t i o n s u p e r p o s e d on t h e P i n g s t o n f o l d . T h i s l a t e r phase o f d e f o r m a t i o n has n o t been a d e q u a t e l y documented by t h i s s t u d y . Due t o a c o m b i n a t i o n o f w e a t h e r c o n d i t i o n s , d i f f i c u l t y o f a c c e s s and the p r e c i p i t o u s t e r r a i n , o n l y a s m a l l p o r t i o n o f t h e A u t o c h t h o n o u s Core G n e i s s Domain on t h e n o r t h s i d e o f T h o r R i d g e was mapped i n d e t a i l d u r i n g t h i s s t u d y . U n f o r t u n a t e l y , t h e s o u t h e r n p o r t i o n o f the P i n g s t o n f o l d to t h e e a s t o f Mount Burnham was n o t mapped. Due t o t h e p a u c i t y o f d e t a i l e d i n f o r m a t i o n on t h i s domain, no d e f i n i t i v e c o n c l u s i o n s can be made on the l a r g e - s c a l e s t r u c t u r a l geometry. I n s t e a d , two a l t e r n a t i v e models c a n be s u g g e s t e d : Model I - The P i n g s t o n f o l d i s assumed to be a Phase Two f o l d s i m i l a r t o t h e s y n f o r m s i n t h e o v e r l y i n g domain. The f o l d i s o v e r p r i n t e d by b r o a d l y c o - a x i a l n o r t h e a s t - v e r g e n t Phase T h r e e f o l d s . T h i s i s c o n s i s t e n t w i t h t h e o v e r p r i n t i n g r e l a t i o n s shown by R e e s o r and Moore (1971, f i g u r e 16b) and documented by t h e i r P l a t e XVI A ( R e e s o r and Moore, 1971, page 84). T h i s model i m p l i e s t h a t t h e r e i s a s i g n i f i c a n t r o t a t i o n between the a u t o c h t h o n o u s basement and t h e o v e r l y i n g P hase Two nappes o f t h e Basement C o r e d Nappe Domain. I f t h i s model i s c o r r e c t , t h e n the s t r u c t u r e o f t h e P i n g s t o n f o l d t o the n o r t h o f T h o r R i d g e can be i n t e r p r e t e d as an i n t e r -f e r e n c e s t r u c t u r e c o m p r i s i n g a Phase One s y n f o r m , a Phase T h r e e recumbent s y n f o r m , and an u p r i g h t Phase F o u r a n t i f o r m ( F i g . 1 2 ) . Model II - The P i n g s t o n f o l d i s i n t e r p r e t e d as a Phase One s t r u c t u r e . T h i s i s c o n s i s t e n t w i t h t h e a b s e n c e o f o v e r p r i n t e d Phase One l i n e a t i o n s ( c h a r a c t e r i s t i c o f t h e o v e r l y i n g domain) and t h e c o r r e s p o n d e n c e between Phase One f o l d o r i e n t a t i o n s t r u c t u r a l l y h i g h e r and the o r i e n t a t i o n o f the P i n g s t o n f o l d ( i . e . , g e n e r a l l y n o r t h e r l y t r e n d s ) . E v i d e n c e from m e s o s c o p i c 48 FIGURE 12. C r o s s - s e c t i o n a c r o s s t h e P i n g s t o n f o l d . The c r o s s - s e c t i o n l i n e i s shown as A — A ' i n F i g u r e 11. The complex shape o f t h e P i n g s t o n f o l d r e p r e s e n t s , i n p a r t , t h e s u p e r p o s i t i o n o f Phase F o u r f o l d s on t h e Phase One nappe s t r u c t u r e . The r o c k s i n t h e c o r e o f t h e f o l d a r e d u c t i l e s c h i s t s and c a l c - s i l i c a t e s and thus have been f l a t t e n e d d u r i n g Phase Four b u c k l i n g . Two a l t e r n a t i v e models p r o p o s e d i n the t e x t f o r t h e P i n g s t o n f o l d a r e i l l u s t r a t e d : (a) The P i n g s t o n f o l d i s i n t e r p r e t e d as an i n t e r f e r e n c e s t r u c t u r e between Phase One, Phase T h r e e and Phase F o u r f o l d s . The f l a t upper l i m b o f the P i n g s t o n f o l d on t h e n o r t h s i d e o f T h o r Ridge would thus be t h e l o w e r l i m b o f a Phase T h r e e s y n f o r m . 49 N0I1VA313 50 FIGURE 12 ( c o n t i n u e d ) . (b) The P i n g s t o n f o l d F o u r i n t e r f e r e n c e i n t e r p r e t a t i o n o f i s i n t e r p r e t e d as a Phase One/Phase s t r u c t u r e . T h i s i s s i m i l a r t o the R e e s o r and Moore ( 1 9 7 1 ) . 51 52 r e f o l d e d f o l d s s u g g e s t s t h i s s t r u c t u r e may have been o v e r p r i n t e d by two s u b s e q u e n t e v e n t s . One i s t h e u p r i g h t Phase F o u r f o l d s which a r e w e l l -documented b o t h on a m e s o s c o p i c and map s c a l e . The s e c o n d e v e n t i s r e p r e -s e n t e d by t i g h t f o l d s w i t h a n g u l a r h i n g e s and g e n e r a l l y s h a l l o w l y - d i p p i n g a x i a l p l a n e s . They have been f o u n d i n t h i s s t u d y a l o n g t h e upper l i m b o f t h e P i n g s t o n f o l d n o r t h e a s t o f Mt. T h o r and i n t h e c e n t r a l c o r e zone 2-5 km n o r t h - n o r t h w e s t o f Mt. O d i n . The n o r t h e a s t - v e r g i n g f o l d s on t h e s o u t h c l i f f f a c e o f T h o r R i d g e may b e l o n g t o t h i s phase. The l a r g e - s c a l e s t r u c -t u r a l geometry o f t h e s e s t r u c t u r e s was i n a d e q u a t e l y documented i n t h i s s t u d y . The geometry o f t h e P i n g s t o n f o l d on t h e n o r t h s i d e o f T h o r R i d g e , i n t h i s i n t e r p r e t a t i o n , i s c o n s i d e r e d d o m i n a n t l y a Phase One/Phase F o u r i n t e r f e r e n c e s t r u c t u r e . However, t h e b u l b o u s n a t u r e o f t h e h i n g e zone i s d i f f i c u l t t o e x p l a i n i f t h e m e s o s c o p i c f o l d s f o u n d on t h e upper l i m b a r e p a r a s i t i c Phase One s t r u c t u r e s . The o r i e n t a t i o n o f m e s o s c o p i c s t r u c t u r e s and l i n e a t i o n s a s s o c i a t e d w i t h t h e P i n g s t o n f o l d a r e deformed by t h e major Phase F o u r a n t i f o r m t h a t t r e n d s n o r t h w e s t e r l y a c r o s s t h e domain ( F i g . 9 ) , i n e f f e c t f o r m i n g the dome. The a x i a l p l a n e s o f m e s o s c o p i c f o l d s form a g r e a t c i r c l e g i r d l e a r o u n d the Phase F o u r f o l d a x i s ( F i g . 1 3 ) . Basement C o r e d Nappe Domain To t h e s o u t h and west o f t h e A u t o c h t h o n o u s Core G n e i s s Domain, t h e s t r u c t u r e o f t h e basement g n e i s s e s i s more complex, b e i n g dominated by t i g h t - t o - i s o c l i n a l , w e s t - t o - n o r t h w e s t t r e n d i n g Phase Two f o l d s ( F i g s . 9 and 1 4 ) . Two i n f o l d s o f q u a r t z i t e , c a l c - s i l i c a t e and p e l i t i c s c h i s t o f c o v e r r o c k sequence were mapped by R e e s o r and Moore ( 1 9 7 1 ) . D e t a i l e d s t r u c t u r a l 53 FIGURE 13. E q u a l - a n g l e s t e r e o n e t p l o t s showing o r i e n t a t i o n d a t a f r o m t h e a r e a shown i n F i g u r e 1-11. (a) Data f o r m e s o s c o p i c Phase F o u r f o l d s . Open diamonds a r e f o l d a x e s , c r o s s e s a r e l i t h o l o g i c a l l a y e r i n g . The dashed l i n e i s a r e p r e s e n t a t i v e a x i a l p l a n e . (b) Data f o r t i g h t - t o - i s o c l i n a l m e s o s c o p i c f o l d s . S e t I : f o l d axes ( * ) ; p o l e s to a x i a l p l a n e s ( * ) . S e t I I : f o l d axes (A); p o l e s t o a x i a l p l a n e s (A). The dashed g r e a t c i r c l e i s t h e modal a x i a l p l a n e o f S e t II f o l d a x e s . The s o l i d g r e a t c i r c l e i s the TT g i r d l e o f p o l e s to c o m p o s i t i o n a l l a y e r i n g shown i n (a) above. The l o n g dashes r e p r e s e n t a s m a l l c i r c l e l o c u s f o r r o t a t i o n o f Li_ by Phase F o u r . 55 FIGURE 14. G e o l o g i c map o f the Basement Co r e d Nappe Domain, (a) Map l e g e n d . GEOLOGY OF THE MT. GUNNARSEN AREA COVER ROCKS Ga-Ky-S i11 S c h i s t s ± Ca1careous S c h i s t s +. A m p h i b o l i t e Q u a r t z i te BASEMENT GNEISSES B i o t - Q t z - F e l d s G n e i s s e s , M i n o r M i g m a t i t e and A m p h i b o l i t e , Rare C a 1 c - S i 1 i c a t e and S c h i s t L i g h t - C o l o r e d Q t z - F e l d s G n e i s s V C o r d i e r i t e - G e d r i t e R i c h Rocks + + + + + + B i o t i t e G r a n o d i o r i t e P l u n g e o f Phase Three f o l d w i t h s o u t h e r l y v e r g e n c e S t r i k e and d i p o f c o m p o s i t i o n a l 50 l a y e r i n g T h r u s t f a u l t s ''''itm'^ Snow 57 FIGURE 14 ( c o n t i n u e d ) . (b) G e o l o g y o f the Mt. Gunnarsen a r e a . 58 59 mapping s u g g e s t e d the i n f o l d s t o be s y n c l i n e s on the upper l i m b s o f two n o r t h w a r d - v e r g e n t nappe s t r u c t u r e s ( t h e Gunnarson and Odin Nappes), w i t h basement g n e i s s e s f o r m i n g t h e a n t i c l i n a l c o r e s ( F i g . 1 4 ) . The s t r u c t u r a l l y h i g h e s t Gunnarson Nappe has been s t u d i e d i n some de-t a i l ( F i g s . 14 and 1 5 ) . I s o c l i n a l , n o r t h w a r d - v e r g e n t Phase Two f o l d s a r e e x t e n s i v e l y d e v e l o p e d on an o u t c r o p s c a l e . The n o r t h w a r d o r upper l i m b v e r g e n c e o f t h e s e f o l d s i s c o n s i s t e n t from t h e t o p o f t h e s t r u c t u r e ( t h e t h r u s t f a u l t a t t h e t o p o f t h e Gl adsheim Lake i n f o l d ) t o w i t h i n 100 m e t e r s o f t h e Mt. Skade i n f o l d , a t w h i c h p o i n t Phase Two f o l d s become e x t r e m e l y f l a t t e n e d and a t t e n u a t e d . The a b s e n c e o f southward v e r g e n c e s c o n s i s t e n t w i t h t h e Mt. Skade i n f o l d s u g g e s t s t h a t t h e l o w e r c o n t a c t o f t h e nappe i s a f a u l t . The a n t i f o r m a l s t r u c t u r e o f t h e Gunnarson Nappe, p r o p o s e d on the b a s i s o f m i n o r f o l d v e r g e n c e s , i s s u p p o r t e d by an a p p a r e n t l i t h o l o g i c a l r e p e t i t i o n . Two b o u d i n a g e d u n i t s o f g e d r i t e - c o r d i e r i t e rock o c c u r on b o t h t h e upper and l o w e r l i m b s o f t h e s t r u c t u r e ; t h e ones on t h e l o w e r l i m b a r e f l a t t e n e d and a t t e n u a t e d . A s i m i l a r r e l a t i o n s h i p a p p e a r s t o e x i s t i n t h e l o w e r Odin Nappe. Phase One f o l d s a r e r a r e l y p r e s e r v e d i n the Gunnarson Nappe; however, a s o u t h e r l y - t r e n d i n g s i l l i m a n i t e l i n e a t i o n i s s t r o n g l y d e v e l o p e d . A l t h o u g h Phase Two f o l d axes show c o n s i s t e n t t r e n d s l o c a l l y , w i t h i n the domain as a whole t h e y have been r o t a t e d a l o n g s m a l l c i r c l e s ( F i g . 1 6 ) , a p p r o p r i a t e t o t h e i r p o s i t i o n on the s o u t h w e s t e r n l i m b o f t h e main Phase F o u r a n t i f o r m . M i n o r Phase F i v e f l e x u r e s o c c u r on a s c a l e o f up t o 10 m e t e r s , b u t do n o t f o r m c o n t i n u o u s l y mappable s t r u c t u r e s . Phase T h r e e f o l d s w i t h s h a l l o w l y -d i p p i n g a x i a l p l a n e s (15 t o 20° t o t h e south) r e f o l d c o - a x i a l Phase Two f o l d s 60 FIGURE 15. C r o s s - s e c t i o n o f t h e Basement C o r e d Nappe Domain. The s t r u c t u r a l geometry i s i n f e r r e d f r o m t h e o u t c r o p p a t t e r n and t h e s t y l e and v e r g e n c e o f m e s o s c o p i c f o l d s . 62 FIGURE 16. O r i e n t a t i o n d a t a f o r m e s o s c o p i c f o l d s f r o m t h e Gunnarsen Nappe. Phase Two f o l d axes ( o ) ; p o l e s t o a x i a l p l a n e s ( • ) . Phase F o u r f o l d a x i s ( • ) ; p o l e t o a x i a l p l a n e ( • ) . Phase Two a x i a l p l a n e s form a g r e a t c i r c l e d i s t r i b u t i o n due t o r e f o l d i n g by Phase F o u r . The dashed g r e a t c i r c l e i s t h e a x i a l p l a n e o f a m e s o s c o p i c Phase Four f o l d . The Phase Two f o l d axes s c a t t e r a b o u t a s m a l l c i r c l e l o c u s . 63 64 and Phase One f o l d s and l i n e a t i o n s ( s e e Appendix I ) . However, t h e s e f o l d s a r e p o o r l y d e v e l o p e d and have o n l y m i n o r l o c a l e f f e c t s . The geometry o f t h e Mt. Skade and G l a d s h e i m Lake i n f o l d s ( F i g s . 14, 15) i s n o t c e r t a i n , as t h e y a r e i s o c l i n a l and t h e h i n g e zones a r e e i t h e r c o v e r e d by g l a c i a l d r i f t o r e l i m i n a t e d by f a u l t i n g . T h e s e f o l d s a r e i n t e r p r e t e d as b e i n g Phase Two s y n f o r m s b e c a u s e Phase Two f o l d s dominate on the o u t c r o p s c a l e and show a p p r o p r i a t e v e r g e n c e r e v e r s a l s ( F i g s . 14, 1 5 ) . I f t h e s e a r e Phase Two f o l d s and t h e P i n g s t o n f o l d and r e l a t e d s t r u c t u r e s a r e Phase One f o l d s , i t a p p e a r s t o be anomalous t h a t no l a r g e - s c a l e r e f o l d e d Phase One h i n g e s a r e p r e s e r v e d . A p o s s i b l e e x p l a n a t i o n i s t h a t t h e Phase One nappe s t r u c t u r e s were d e v e l o p e d on a v e r y l a r g e s c a l e , t h e Phase Two f o l d s o v e r p r i n t i n g t h e upper l i m b o f a s i n g l e l a r g e - s c a l e a n t i f o r m . T r a n s i t i o n a l Domain o f I m b r i c a t e T h r u s t i n g T h i s domain can be d e f i n e d on b o t h a l i t h o l o g i c a l and a s t r u c t u r a l b a s i s . R e e s o r and Moore (1971) mapped two u n i t s o f c o r e g n e i s s e s s e p a r a t e d by g n e i s s e s and s c h i s t s o f t h e i r m a n t l i n g g n e i s s e s ( c o v e r r o c k s ) . On the b a s i s o f s y s t e m a t i c and a b r u p t changes i n Phases Two and T h r e e f o l d a x i s o r i e n t a t i o n s and t r u n c a t i o n o f l i t h o l o g i e s , f o u r t h r u s t s l i c e s have been i d e n t i f i e d ( F i g . 1 7 ) . The uppermost t h r u s t s l i c e , t h e C a r i b o u Nappe, c o r r e s p o n d s w i t h t h e "zone o f complex f o l d i n g " mapped by R eesor and Moore. The C a r i b o u Nappe i s a l a r g e , recumbent a n t i f o r m ( F i g . 1 8 ) , f o l d i n g w e l l -l a y e r e d q u a r t z o - f e l d s p a t h i c and g a r n e t - s i 1 1 i m a n i t e g n e i s s e s o f t h e c o v e r r o c k sequence. M i n o r f o l d s w i t h i n t h e nappe have e a s t - w e s t o r i e n t e d f o l d axes and a r e a l m o s t e x c l u s i v e l y n o r t h w a r d - v e r g e n t . As the b a s a l t h r u s t i s a p p r o a c h e d , t h e f o l d s show h i n g e t y p e v e r g e n c e , t h e n s o u t h e r l y ( l o w e r l i m b ) 65 FIGURE 17. S t r u c t u r a l map o f a p o r t i o n o f t h e T r a n s i t i o n a l Domain o f I m b r i c a t e T h r u s t i n g i n the C a r i b o u A l p a r e a . The g e o l o g y c o n s i s t s o f f o u r main t h r u s t s h e e t s which i m b r i -c a t e t h e basement g n e i s s e s and the c o v e r r o c k s e q u e n c e . The s o u t h e r n , s t r u c t u r a l l y h i g h e s t t h r u s t s h e e t i s t h e C a r i b o u Nappe, w h i c h i s a Phase T h r e e recumbent a n t i f o r m . The a x i a l t r a c e o f t h i s f o l d i s shown by t h e d a s h e d l i n e . The arrows show the t r e n d and p l u n g e f o r the a v e r a g e f o l d axes f o r s m a l l a r e a s . Phases One, Two and T h r e e f o l d s a r e d i s t i n g u i s h e d by one, two and t h r e e b a r s on t h e p l u n g e a r r o w , r e s p e c t i v e l y . The l o c a l v e r g e n c e o f a l l f o l d s examined i s n o r t h e r l y , e x c e p t f o r the s m a l l wedge w i t h i n the C a r i b o u Nappe n o r t h o f t h e a x i a l t r a c e marked on t h e map. Here Phase T h r e e f o l d s show a s o u t h e r l y v e r g e n c e . BASEMENT GNEISS QUARTZITE COVER ROCKS KM STRIKE AND PLUNGE OF PHASES 1,2,3 FOLDS THRUST FAULTS AXIAL T R A C E OF V PHASE 3 NAPPE CTi 01 67 FIGURE 18. C r o s s - s e c t i o n o f t h e T r a n s i t i o n a l Domain o f I m b r i c a t e T h r u s t i n g . The s e c t i o n l i n e C — C i s shown i n F i g u r e 17. The m i n o r f o l d s d e p i c t e d r e f l e c t t h e s t y l e and o r i e n t a t i o n o f o b s e r v e d m e s o s c o p i c f o l d s . S.W. N.E. 69 v e r g e n c e , and become p r o g r e s s i v e l y a t t e n u a t e d f o r t h e l a s t 10 t o 20 m e t e r s . To t h e e a s t o f C a r i b o u A l p , t h e a r e a o f t h e nappe w i t h l o w e r l i m b v e r g e n c e expands as t h e a x i a l t r a c e o f t h e C a r i b o u Nappe d i v e r g e s from the t h r u s t zone ( F i g . 1 7 ) . The t h r u s t i s w e l l e x p o s e d , w i t h r u s t y w e a t h e r e d g a r n e t -s i l l i m a n i t e g n e i s s e s s h a r p l y t r u n c a t e d a g a i n s t more homogeneous s t r e a k y , g r e y augen g n e i s s . The f i r s t 15 m e t e r s below t h e t h r u s t c o n t a i n a few f l a t t e n e d , i s o c l i n a l f o l d s and b o u d i n a g e d p e g m a t i t e v e i n l e t s . P o s t -d e f o r m a t i o n metamorphism has r e c r y s t a l 1 i z e d what was p r o b a b l y o r i g i n a l l y a m y l o n i t i c f a b r i c . S t r u c t u r a l l y below and t o t h e n o r t h o f t h e C a r i b o u Nappe, t h r e e a d d i -t i o n a l t h r u s t s l i c e s have been mapped ( F i g s . 17, 1 8 ) . The f o l d axes o f t h e d o minant m e s o s c o p i c recumbent f o l d s show changes i n o r i e n t a t i o n a c r o s s e ach f a u l t . Toward t h e base o f e a c h f a u l t s l i c e t h e r o c k s become p r o g r e s s i v e l y f l a t t e n e d , as i n d i c a t e d by b o u d i n a g e o f p e g m a t i t e v e i n s and a m p h i b o l i t e l a y e r s . F o l d s i n t h e s e zones a r e t y p i c a l l y r o o t l e s s and s t r o n g l y a t t e -n u a t e d e x c e p t where r a r e i s o c l i n a l f o l d s deform t h i s f l a t t e n e d l a y e r i n g . S m a l l - s c a l e t e c t o n i c s l i d e s ( o r t h r u s t s ) and l a g s t r u c t u r e s o c c u r w i t h i n t h e s e z o n e s . D e t a i l e d s t r u c t u r a l mapping s u g g e s t s t h a t the C a r i b o u Nappe and r e l a t e d s t r u c t u r e s a r e a c t u a l l y Phase T h r e e s t r u c t u r e s . T h e r e a r e numerous examples o f i s o c l i n a l Phase Two f o l d s c o - a x i a l l y r e f o l d e d by more open, recumbent Phase T h r e e f o l d s w h i c h form the l a r g e r , more o b v i o u s s t r u c t u r e s . Phase One k y a n i t e and s i l l i m a n i t e l i n e a t i o n s a r e r e f o l d e d by Phases Two and T h r e e f o l d s . S m a l l - s c a l e i n t e r f e r e n c e o f Phases One and Two f o l d s c r e a t e s a p p r e s s e d dome and b a s i n s t r u c t u r e s . 70 To summarize, t h i s domain r e p r e s e n t s a zone o f t h r u s t i n g a s s o c i a t e d w i t h Phase T h r e e d e f o r m a t i o n which has i m b r i c a t e d basement g n e i s s and c o v e r r o c k s c h i s t s , q u a r t z o - f e l d s p a t h i c g n e i s s e s and q u a r t z i t e s . The domain marks t h e t r a n s i t i o n between dominant Phase Two f o l d i n g and t h r u s t i n g i n t h e u n d e r l y i n g basement c o r e d nappes and l a r g e - s c a l e Phase T h r e e f o l d i n g f o u n d i n t h e o v e r l y i n g c o v e r r o c k s ( F i g . 1 8 ) . R e e s o r and Moore's (1971) c h a r a c t e r i z a t i o n o f t h e C a r i b o u A l p a r e a as a zone o f complex f o l d i n g i s t h u s v e r y a p p r o p r i a t e . The C o v e r Rock Domain The s t r u c t u r e o f t h e C o v e r Rock Domain has been d i s c u s s e d by Hoy (1977b) and Duncan ( 1 9 7 8 a ) . The C o v e r Rock Domain i s d e f i n e d by t h e a b s e n c e o f i m b r i c a t e t h r u s t f a u l t i n g c h a r a c t e r i s t i c o f the t r a n s i t i o n a l domain d e s c r i b e d above. A l l s i x phases o f d e f o r m a t i o n have been i d e n t i f i e d w i t h i n t h e C o v e r Rock Domain. The a r e a o f t h e C o v e r Rock Domain mapped i n t h i s s t u d y can be d i v i d e d i n t o t h r e e sub-domains s e p a r a t e d by two major f a u l t s ( F i g . 8 ) . The B i g Ledge Subdomain - W i t h i n t h i s subdomain Phases One, Two and T h r e e have a s i g n i f i c a n t e f f e c t on t h e s t r u c t u r a l geometry. The l a r g e - s c a l e f o l d s o u t l i n e d by r e p e t i t i o n o f p a r t s o f t h e l i t h o l o g i c a l sequence a r e a l s o d e f i n e d by v e r g e n c e r e v e r s a l s i n r a r e m e s o s c o p i c Phase Two i s o c l i n a l f o l d s . Thus Hoy (1977b) has s u g g e s t e d t h a t t h e s e l a r g e - s c a l e Phase Two f o l d s have an o v e r a l l n o r t h w a r d v e r g e n c e . A l t h o u g h t h i s c o n c l u s i o n i s s u p p o r t e d i n p a r t by t h e v e r g e n c e s o f s m a l l - s c a l e f o l d s , i t c a n n o t be c o n f i r m e d by mapping o f t h e f o l d h i n g e s due t o the h o r i z o n t a l o r i e n t a t i o n o f the Phase Two f o l d axes and the poor o u t c r o p i n t h e v a l l e y s . 71 The p r e d o m i n a n t m e s o s c o p i c f o l d s a r e t i g h t - t o - o p e n Phase T h r e e f o l d s , c o - a x i a l w i t h Phase Two, b u t w i t h s h a l l o w , s o u t h - d i p p i n g a x i a l s u r f a c e s . Phase T h r e e f o l d s have a c o n s i s t e n t southward l o w e r - l i m b v e r g e n c e t h r o u g h o u t t h e domain. Hinge t y p e v e r g e n c e s a r e f o u n d on t h e top o f Mt. Symonds. O v e r p r i n t i n g o f t h e i s o c l i n a l Phase Two f o l d s by t h i s m a j o r Phase T h r e e s t r u c t u r e a p p e a r s t o have l e d t o s e v e r e room problems w i t h i n t he Phase T h r e e a n t i f o r m ( R o s s , 1973). L o c a l l y , t h e sequence i s d i s r u p t e d i n t e r n a l l y by n o r t h e r l y - d i r e c t e d t h r u s t f a u l t s o r s l i d e s w h i c h a p p e a r t o be l a t e - s t a g e Phase T h r e e s t r u c t u r e s . T h r u s t f a u l t i n g i s p a r t i c u l a r l y common i n i n t e r l a y e r e d m a r b l e - q u a r t z o - f e l d s p a t h i c g n e i s s s e q u e n c e s which have a h i g h d u c t i l i t y c o n t r a s t . Mount F o s t h a l l Synform - The Mount F o s t h a l l s y n f o r m i s a m a j o r Phase T h r e e s t r u c t u r e j u x t a p o s e d a g a i n s t t h e s o u t h e r n edge o f the B i g Ledge subdomain by a f a u l t ( F i g s . 8 and 1 9 ) . The a x i a l p l a n e o f the s y n f o r m d i p s 50 t o 70° t o t h e s o u t h , i n c o n t r a s t t o an a v e r a g e d i p o f 10 t o 30° f o r Phase T h r e e a x i a l p l a n e s i n t h e B i g Ledge subdomain. T h i s change i n o r i e n t a t i o n o f a x i a l p l a n e s t a k e s p l a c e a c r o s s a s h a r p boundary which c o i n c i d e s w i t h a change o f 5 t o 15° i n t h e d i p o f t h e c o m p o s i t i o n a l l a y e r i n g and a s t r i n g o f e l o n g a t e l e n s e s o f u l t r a m a f i c r o c k s ( F i g . 2 0 ) . The n a t u r e and magnitude o f movement a l o n g t h i s f a u l t a r e n o t known. However, s i g n i f i c a n t d i s p l a c e -ment ( a t l e a s t t h o u s a n d s o f m e t e r s ) can be i n f e r r e d from the t r u n c a t i o n o f t h e e x t e n s i v e o u t c r o p s o f banded c a l c - s i 1 i c a t e s c h i s t i m m e d i a t e l y e a s t o f t h e a x i a l t r a c e o f the Mount F o s t h a l l s y n f o r m . Note t h a t t he main synform i s c o r e d by banded c a l c - s i l i c a t e r o c k s s i m i l a r t o t h o s e i n the s o u t h o f the 72 FIGURE 19. G e o l o g i c a l map o f t h e F o s t h a l l R i d g e a r e a showing o r i e n t a t i o n o f m e s o s c o p i c Phases Two and T h r e e s t r u c t u r e s . Note t h e l a r g e Phase One c l o s u r e mapped n o r t h o f t h e f a u l t i n the upper c e n t r a l p o r t i o n o f the map. The f a u l t may have normal movement on i t . See t e x t f o r d i s c u s s i o n . BANDED CALC-SILICATE SCHIST QTZ - FELDS-BIOT-GA ROCK PLUNGE OF PHASE 3 FOLD IMPURE BROWN MARBLE PHASE SIX FAULT AXIAL TRACE PHASE 3 SYNFORM 74 FIGURE 20. C r o s s - s e c t i o n s o f t h e F o s t h a l l R i d g e a r e a . The c r o s s -s e c t i o n l i n e s and symbols a r e shown i n P l a t e I Y . The m i n o r s t r u c t u r e s shown r e f l e c t t h e s t y l e o f o b s e r v e d m i n o r f o l d s , though t h e d e e p e r s t r u c t u r e , such as b e n e a t h t he F o s t h a l l s y n f o r m , i s s c h e m a t i c . 75 o KM 10 ,000 •MOUNT FOSTHALL SUBDOMAIN- .BIG LEDGE . SUBDOMAIN MOUNT FOSTHALL SYNFORM DIPS of PHASE THREE AXIAL PLANES j 16 26 29 17 25 G' MOUNT . F O S T H A L L -SUBDOMAIN •10,000 <u I < > LU _J LU •BIG L E D G E SUBDOMAIN-PHASE THREE THRUST ? ) KM H' 76 B i g Ledge subdomain and t h a t b o t h a r e u n d e r l a i n by a t h i n , banded amphibo-l i t e ( F i g . 1 9 ) . However, t h e l a r g e - s c a l e s t r u c t u r e s f o r m i n g t h e o u t c r o p p a t t e r n i n t h e B i g Ledge a r e a a r e Phase Two s t r u c t u r e s , whereas t h e Mount F o s t h a l l s y n f o r m i s a Phase T h r e e s t r u c t u r e . No o b v i o u s h y p o t h e s i s o f s m a l l - s c a l e movement a l o n g t h e f a u l t s e p a r a t i n g t h e s e two domains can r e c o n c i l e t h e s e d i f f e r e n c e s . M e s o s c o p i c f o l d s i n t h i s subdomain a r e d o m i n a n t l y Phase T h r e e s t r u c -t u r e s , p a r a s i t i c t o t h e F o s t h a l l s y n f o r m . These p a r a s i t i c f o l d s c h a r a c -t e r i s t i c a l l y have s t e e p , o v e r t u r n e d s o u t h e r n l i m b s , s h a l l o w - d i p p i n g n o r t h e r n l i m b s , and t i g h t a n g u l a r h i n g e s . In d e t a i l , t h e o u t c r o p p a t t e r n i s c o m p l i c a t e d b e c a u s e t h e e n v e l o p i n g s u r f a c e o f o u t c r o p - s c a l e f o l d s i s s u b p a r a l l e l w i t h t h e s l o p e o f much o f t h e s o u t h e r n s i d e o f t h e F o s t h a l l r i d g e ( F i g . 2 0 ) . Phase Two f o l d s w i t h i n t h i s subdomain a r e t i g h t - t o - i s o c l i n a l i n s t y l e and a r e b r o a d l y c o - a x i a l w i t h Phase T h r e e . Phase Two f o l d axes c h a r a c -t e r i s t i c a l l y d i v e r g e i n a z i m u t h from Phase T h r e e o r i e n t a t i o n s by 5 t o 3 0 ° . B o t h p l u n g e 5 t o 2 0 ° t o t h e w e s t . S i l l i m a n i t e l i n e a t i o n s p a r a l l e l t o Phase Two a x i a l t r e n d s a r e r e f o l d e d i n t o s m a l l c i r c l e p a t t e r n s by Phase T h r e e f o l d s r e f l e c t i n g t h e dominant f l e x u r a l - s l i p c h a r a c t e r o f Phase T h r e e i n t h e subdomain. The Upper F a u l t B l o c k - S t r u c t u r a l l y above t h e Mount F o s t h a l l s y n f o r m i s a n o t h e r f a u l t b l o c k b r o a d l y s i m i l a r i n n a t u r e . The e x i s t e n c e o f a f a u l t e d s o u t h e r n boundary o f t h e Mount F o s t h a l l synform can be i n f e r r e d f r o m t h r e e o b s e r v a t i o n s : ( i ) t h e r i d g e t o t h e s o u t h o f Mount F o s t h a l l i s a p p a r e n t l y formed by a Phase T h r e e s y n f o r m and an i n t e r v e n i n g a n t i f o r m i s 77 a b s e n t ; ( i i ) as shown i n F i g u r e s 2 and 19, t h e l i t h o l o g i e s i n t h e Mount F o s t h a l l b l o c k a r e t r u n c a t e d t o t h e e a s t by p o o r l y o u t c r o p p i n g q u a r t z o -f e l d s p a t h i c g n e i s s e s and p e g m a t i t e s w h i c h r e c o n n a i s s a n c e mapping s u g g e s t s a r e p a r t o f t h i s upper f a u l t b l o c k ; ( i i i ) m i n o r o u t c r o p s o f u l t r a m a f i c r o c k s o c c u r i n t h e v a l l e y a l o n g t h e i n f e r r e d f a u l t t r a c e . T h i s subdomain was n o t i n c l u d e d w i t h i n t h e a r e a o f d e t a i l e d mapping which forms t h e b a s i s f o r t h i s s t u d y . However, r e c o n n a i s s a n c e d a t a , combined w i t h p r e v i o u s mapping by Re e s o r and Moore, s u g g e s t t h a t t h e s t r u c t u r a l s t y l e e x h i b i t e d by t h e Mount F o s t h a l l subdomain may be r e p e a t e d by a t l e a s t two f a u l t b l o c k s w i t h i n 10 km t o t h e s o u t h o f Mount F o s t h a l l . The l i n e a t e d q u a r t z m o n z o n i t e mapped by Re e s o r and Moore on t h e e a s t e r n s i d e o f t h i s upper f a u l t b l o c k subdomain i s dom i n a t e d by an e a s t - w e s t t r e n d i n g l i n e a t i o n , p a r a l l e l t o Phase T h r e e ( a nd Phase Two) a x i a l t r e n d s . The q u a r t z m o n z o n i t e a p p e a r s t o be b e s t i n t e r p r e t e d as a n o t h e r f a u l t - b o u n d e d b l o c k emplaced a f t e r Phase T h r e e d e f o r m a t i o n . T h i s r o c k i s n o t f o u n d e l s e w h e r e i n t h e s t u d y a r e a , a l t h o u g h a p o s s i b l e c o r r e l a t i v e r o c k t y p e has been mapped 30 km t o the n o r t h on t h e e a s t s i d e o f C r a n b e r r y dome by M u t t i ( 1 9 7 8 ) . These r o c k s have been c o r r e l a t e d w i t h t h e Mt. F o w l e r Pl uton 100 km t o t h e n o r t h w e s t by O k u l i t c h e t aj_. ( 1 9 7 5 ) . They d a t e d t h e Mt. F o w l e r p l u t o n , by t h e U-Pb z i r -c o n t e c h n i q u e , a t 430 Ma. T h i s c o r r e l a t i o n a t p r e s e n t r e m a i n s s p e c u l a t i v e and u n d e r s t a n d i n g o f t h e s i g n i f i c a n c e o f t h i s r o c k u n i t a w a i t s f u r t h e r work. In summary, the s t r u c t u r a l geometry o f the T h o r - O d i n dome can be s e p a r a t e d i n t o two components: f i r s t , t h e domal s t r u c t u r e formed by the i n t e r f e r e n c e o f Phases F o u r , F i v e and S i x s t r u c t u r e s ; and s e c o n d , t h e i m b r i c a t e d s l i c e s t h a t e x i s t e d p r i o r t o dome f o r m a t i o n . The f o l l o w i n g 78 s e c t i o n s a n a l y z e t h e f i n i t e s t r a i n p a t t e r n o f the dome and d i s t i n g u i s h between t h e s e two s t r u c t u r a l components. STRAIN ANALYSIS A s t r i k i n g f e a t u r e a l l u d e d t o i n t h e p r e v i o u s s e c t i o n i s t h e p r o g r e s s i v e change i n s t y l e o f f o l d i n g ( i . e . , shape o f f o l d s i n p r o f i l e ) m oving s t r u c t u r a l l y upwards. T h i s t r e n d has been q u a n t i f i e d u s i n g two t e c h n i q u e s : t h e q u a l i t a t i v e harmonic a n a l y s i s o f f o l d shape ( H u d l e s t o n , 1973a) and t h e t a p l o t o r i g i n a t e d by Ramsay (1967) t o measure f l a t t e n i n g s t r a i n s . H u d l e s t o n (1973a) p r o p o s e d a method o f g e o m e t r i c c l a s s i f i c a t i o n o f f o l d s b a s e d on h armonic a n a l y s i s o f a s i n g l e f o l d e d s u r f a c e . He d e m o n s t r a t e d t h a t t h e f i r s t two odd terms o f a s i n e s e r i e s can c l o s e l y match t h e shape o f n a t u r a l f o l d s . He p r o p o s e d a v i s u a l method o f a n a l y z i n g f o l d shape i n terms o f t h e s p e c t r u m o f i d e a l p r o f i l e s g e n e r a t e d by two v a r i a b l e s , shape and a m p l i t u d e , c o r r e s p o n d i n g t o t h e two c o e f f i c i e n t s i n t h e s i n e s e r i e s . Each f o l d i s c h a r a c t e r i z e d by one o f s i x shape c a t e g o r i e s (A t o F) and f i v e a m p l i t u d e c a t e g o r i e s (1 t o 5 ) . T h i s method, s u c c e s s f u l l y a p p l i e d t o f o l d s o f t h e Moine r o c k s o f Monar, S c o t l a n d by H u d l e s t o n ( 1 9 7 3 b ) , was u s e d i n t h i s s t u d y t o a n a l y z e the shape o f 545 Phase One f o l d s , 218 Phase Two f o l d s , and 209 Phase T h r e e f o l d s . The r e s u l t s , w h i c h were a n a l y z e d s e p a r a t e l y f o r each r o c k t y p e and f o r each o f the s t r u c t u r a l domains d e l i n e a t e d i n t h e p r e v i o u s s e c t i o n , a r e shown i n F i g u r e 21. The modal shape o f Phase One f o l d s ( i n q u a r t z o - f e l d s p a t h i c g n e i s s e s ) c h a n g e s fro m D3-E3 w i t h i n t h e A u t o c h t h o n o u s C o r e G n e i s s Domain, compared w i t h E4-D4 i n t h e Basement C o r e d Nappe Domain, t o C5-D5 i n the T r a n s i t i o n a l 79 FIGURE 21. Harmonic a n a l y s i s o f Phases One and Two f o l d s u s i n g t he v i s u a l t e c h n i q u e s o f H u d l e s t o n ( 1 9 7 3 a ) . A l l f o l d s shown a r e i n q u a r t z o - f e l d s p a t h i c g n e i s s e s . The numbers r e f e r t o t h e f r e q u e n c y o f o c c u r r e n c e o f each shape t y p e . Autochthonous Basement Domain Cored Nappes Transitional Domain Cover Rock Domain A B C D E F A B C D E F A B C D E F A B C D E F 1 2 3 8 II 8 2 II 5 8 7 5 Summary of Phase I I 2 3 4 5 I 2 3 4 5 A B C D E F A B C D E F A B C D E F 1 5 4 2 8 6 4 3 4 2 1 7 II 7 4 27 17 13 8 16 9 2 A B C D E F A B C D E F Autoch. Nappe Trans. 1 33% Theoretical 2 53% Trend 3 69% (Chappie, 1968) 4 897. 5 81 Zone o f I m b r i c a t e T h r u s t i n g , and C4 i n t h e C o v e r Rock Domain. A s i m i l a r t r e n d can be seen i n t h e d a t a f o r Phase Two f o l d s . C h a p p i e (1968) p r e d i c t s t h a t b u c k l e d l a y e r s w i l l change t h e i r shape w i t h p r o g r e s s i v e s h o r t e n i n g t o become more rounded i n t h e c r e s t s . C h a p p i e ' s n u m e r i c a l e x p e r i m e n t s have been a n a l y z e d by H u d l e s t o n (1973a) t o d e t e r m i n e t h e v a r i a t i o n i n t h e two harmonic c o e f f i c i e n t s as a f u n c t i o n o f p r o g r e s s i v e d e f o r m a t i o n . As can be seen i n F i g u r e 21, t h e t r e n d i n f o l d shape m o d i f i c a t i o n p r e d i c t e d by C h a p p i e ' s c a l c u l a t i o n s i s o f t h e same form as t h a t o b s e r v e d i n t h i s d a t a s e t . P a r r i s h et a]_. (1976) have shown t h e i m p o r t a n c e o f t e m p e r a t u r e i n the t r a n s i t i o n f r o m b u c k l i n g t o f l a t t e n i n g i n f o l d d e v elopment. A l t h o u g h t h e above a n a l y s i s o f f o l d shape v a r i a t i o n s s u p p o r t s t h e c o n c l u s i o n s o f the m e s o s c o p i c s t r u c t u r a l a n a l y s i s , t h e a p p r o a c h can o n l y be r e g a r d e d as s e m i - q u a n t i t a t i v e . A n o t h e r a p p r o a c h t o a n a l y z i n g t h e f l a t t e n i n g i n v o l v e d i n m o d i f y i n g t h e f o l d shape i s the t a p l o t o f Ramsay (1962, 1967). T h i s method e n a b l e s c a l c u l a t i o n o f t h e f l a t t e n i n g s t r a i n i n t h e p r o f i l e p l a n e f o r f o l d s , b a s e d on t h e a s s u m p t i o n s t h a t t h e i n i t i a l f o l d was c o n c e n t r i c i n s t y l e and t h a t one o f t h e axes o f p r i n c i p a l s t r a i n i s t h e f o l d a x i s . The f i r s t a s s u m p t i o n i s c o n s i s t e n t w i t h c u r r e n t l y a c c e p t e d t h e o r i e s o f f o l d f o r m a t i o n ( s e e f o r example P a r r i s h ^ t a l _ . , 1976). The s e c o n d a s s u m p t i o n i s u n l i k e l y t o be t r u e i n g e n e r a l ( F l i n n , 1962) b u t i s p r o b a b l y a r e a s o n a b l e a s s u m p t i o n f o r Phases Two and T h r e e f o l d s b e c a u s e : ( i ) l e n s - l i k e b u n d l e s o f s i l l i m a n i t e and e l l i p s o i d a l a g g r e g a t e s o f a l k a l i f e l d s p a r and q u a r t z t y p i c a l l y p a r a l l e l P hases Two and T h r e e f o l d axes; ( i i ) t h e two d e f o r m a t i o n s a r e e s s e n t i a l l y c o - a x i a l ; and ( i i i ) Phase T h r e e a p p e a r s t o have been t h e l a s t d e f o r m a t i o n t o i n v o l v e s i g n i f i c a n t d i s t o r -t i o n a l s t r a i n o f the q u a r t z o - f e l d s p a t h i c g n e i s s e s . 82 R e p r e s e n t a t i v e f o l d s from t h e f o u r s t r u c t u r a l domains were a n a l y z e d u s i n g Ramsay's (1967) method ( A p p e n d i x I I ) and t h e r e s u l t s a r e shown i n F i g u r e 22. These d a t a s u g g e s t t h a t s u p e r i m p o s e d f l a t t e n i n g s t r a i n s f o r P h a s e s Two and T h r e e i n c r e a s e s i g n i f i c a n t l y from t h e Basement C o r e d Nappe Domain t o t h e T r a n s i t i o n a l Zone o f I m b r i c a t e T h r u s t i n g and then drop t o l o w e r v a l u e s i n t h e o v e r l y i n g C o v e r Rock Domain. A s i m i l a r change o c c u r s i n t h e ^ 3 / ^ v a l u e s f o r Phase One f o l d s , w h i c h d e c r e a s e s i g n i f i c a n t l y f r o m t h e Basement C o r e G n e i s s Domain t o t h e Basement C o r e d Nappe Domain. S t r a i n e s t i m a t e s from t h e s e f o l d p r o f i l e s a r e c o n s i s t e n t w i t h d i s t o r t i o n a l s t r a i n s c a l c u l a t e d from l i n e a t i o n p a t t e r n s u s i n g a method s u g g e s t e d by Ramsay (1967) ( s e e A p p e n d i x I I ) . The t h r e e - d i m e n s i o n a l f i n i t e s t r a i n c o u l d o n l y be e s t i m a t e d i n a quan-t i t a t i v e way i n a s m a l l a r e a o f t h e T r a n s i t i o n a l Domain o f I m b r i c a t e T h r u s t i n g ( s e e Appendix I I ) . P r i n c i p a l a x i s l e n g t h s o f e l l i p t i c a l q u a r t z and c a l c - s i l i c a t e " c l a s t s " i n a q u a r t z o - f e l d s p a t h i c g n e i s s were measured. A l t h o u g h t h e o r i g i n o f t h i s p s e u d o - c o n g l o m e r a t e i s n o t u n d e r s t o o d , t h e d a t a s u g g e s t t h a t t h e f i n i t e s t r a i n s t a t e v a r i e s from c o n s t r i c t i o n a l t o t r u e f l a t t e n i n g . The geometry o f b o u d i n a g e o f p e g m a t i t i c v e i n s p a r a l l e l t o Phase T h r e e a x i a l p l a n e s s u g g e s t s the Phase T h r e e s t r a i n f i e l d was one o f p l a n e s t r a i n [x^=x^). In t h i s t r a n s i t i o n a l domain t h e s e b o u d i n s c h a r a c -t e r i s t i c a l l y have t h e geometry o r i g i n a l l y termed by Wegmann " c h o c o l a t e t a b l e t s t r u c t u r e " (Ramsay, 1967), c o n s i s t e n t w i t h a s t a t e o f p l a n e s t r a i n . I t s h o u l d be n o t e d t h a t b o u d i n a g e d a m p h i b o l i t e l a y e r s , i n c o n t r a s t , t y p i -c a l l y have a r o d - l i k e geometry w i t h t h e d i r e c t i o n o f the r o d d i n g p a r a l l e l i n g P hase One f o l d a x e s . T h r e e - d i m e n s i o n a l s t r a i n d a t a f o r t h e o t h e r domains a r e l i m i t e d i n q u a l i t y and q u a n t i t y b u t a r e c o n s i s t e n t w i t h 83 FIGURE 22. t a p l o t o f Ramsay (1962) f o r Phases One and Two f o l d p r o f i l e s . The p r o f i l e s c an be used t o e s t i m a t e the magnitude o f f l a t t e n i n g s t r a i n s s u p e r p o s e d on an o r i g i n a l c o n c e n t r i c g eometry. A l l f o l d s measured were i n q u a r t z o f e l d s p a t h i c g n e i s s e s . 84 85 p l a n e s t r a i n c o n d i t i o n s f o r t h e s t r a i n s a s s o c i a t e d w i t h Phases Two and T h r e e d e f o r m a t i o n . In summary, t h e p a t t e r n o f d i s t o r t i o n a l s t r a i n s u g g e s t e d above i s con-s i s t e n t w i t h t h e c o n c l u s i o n s o f t h e m e s o s c o p i c s t r u c t u r a l a n a l y s i s p r e -s e n t e d i n t h e p r e v i o u s s e c t i o n . The low s t r a i n s measured i n t h e A u t o c h t h o -nous Core G n e i s s Domain a p p a r e n t l y r e f l e c t t h e f a c t t h a t Phases Two and T h r e e d e f o r m a t i o n s o n l y a f f e c t e d t h e o v e r l y i n g domains above a b a s a l t h r u s t f a u l t ( F i g . 1 8 ) . D u r i n g Phase T h r e e d e f o r m a t i o n , t h e low e r m o s t detachment b e i n g a c t i v a t e d s t e p p e d upwards. Thus Phase T h r e e d e f o r m a t i o n a p p e a r s e s s e n t i a l l y a b s e n t below t h e T r a n s i t i o n a l Domain o f I m b r i c a t e T h r u s t i n g . C o n s e q u e n t l y t h i s domain has t h e l a r g e s t amount o f p o s t - P h a s e Two f l a t -t e n i n g . Thus, i t can be i n f e r r e d t h a t t h e d i s t o r t i o n a l s t r a i n a s s o c i a t e d w i t h a p a r t i c u l a r phase o f d e f o r m a t i o n i n c r e a s e s downwards tow a r d t h e b a s a l d e c o l l e m e n t a l o n g which t h e main t r a n s l a t i o n a l component o f t h e d e f o r m a t i o n was t a k i n g p l a c e . STRUCTURAL SYNTHESIS T h i s s t u d y has s u g g e s t e d t h a t t h e T h o r - O d i n g n e i s s dome may r e p r e s e n t t h e s t r u c t u r a l l y d e e p e s t e x p o s e d l e v e l o f t h e Shuswap complex. Thus, t h i s a r e a p r o v i d e s a u n i q u e i n s i g h t i n t o t h e d e f o r m a t i o n a l p r o c e s s e s t h a t have shaped t h e complex. In p a r t i c u l a r , i t e l u c i d a t e s t h e i n t e r a c t i o n s which have o c c u r r e d between t h e basement and c o v e r r o c k s . T h i s s e c t i o n a t t e m p t s t o s y n t h e s i z e t h e s t r u c t u r a l e v o l u t i o n o f the dome i n the c o n t e x t b o t h o f a r e g i o n a l p e r s p e c t i v e and as a t e s t c a s e f o r t h e o r i e s o f g n e i s s dome f o r m a t i o n . 86 The f i r s t phase o f d e f o r m a t i o n i d e n t i f i e d i n t h i s s t u d y (Phase One) a p p e a r s t o be p r e s e n t a t a l l s t r u c t u r a l l e v e l s . Phase One f o l d s a r e b e s t p r e s e r v e d i n t h e A u t o c h t h o n o u s C o r e G n e i s s Domain and a r e e x t e n s i v e l y o v e r p r i n t e d i n t h e o v e r l y i n g domains. I t i s t e m p t i n g t o c o r r e l a t e t h e s e f o l d s w i t h Phase One f o l d s mapped by Read and Wheeler (1975) t o the e a s t a c r o s s t h e C o l u m b i a R i v e r i n t h e Kootenay A r c . R e g a r d l e s s o f t h i s s p e c u l a t i o n , t h e w i d e s p r e a d o c c u r r e n c e o f Phase One f o l d s t h r o u g h o u t t h i s s t u d y a r e a and t h e d i s c o v e r y o f t h e l a r g e - s c a l e Phase One f o l d on t h e F o s t h a l l r i d g e (shown i n F i g s . 4 and 19) d e m o n s t r a t e t h a t Phase One f o l d s a r e w i d e s p r e a d and c a n n o t be i g n o r e d i n i n t e r p r e t i n g the l i t h o l o g i c a l d i s t r i b u t i o n a r o u n d the dome. On t h e b a s i s o f t h e l a r g e s c a l e o f t h e P i n g s t o n Nappe i t seems r e a s o n a b l e t h a t Phase One s t r u c t u r e s had a l a r g e w a v e l e n g t h ( p o s s i b l y s e v e r a l km). T h i s , t o g e t h e r w i t h the i n t e n s i t y o f P h a s e s Two and T h r e e d e f o r m a t i o n , may e x p l a i n t h e absence o f more l a r g e P h ase One c l o s u r e s w i t h i n t h e s t u d y a r e a . T h i s a p p a r e n t l a r g e s c a l e o f Phase One f o l d s i s c o n s i s t e n t w i t h r e g i o n a l i n t e r p r e t a t i o n s by Brown ( 1 9 8 0 ) . The o c c u r r e n c e o f s i l l i m a n i t e , k y a n i t e and h o r n b l e n d e l i n e a t i o n s p a r a l l e l t o Phase One f o l d axes s u g g e s t s t h a t a m p h i b o l i t e f a c i e s metamorphism accomp a n i e d t h i s d e f o r m a t i o n . H i g h t e m p e r a t u r e s can a l s o be i n f e r r e d by t h e d u c t i l e s t y l e o f f o l d i n g i n t h e q u a r t z i t e s i n t h e P i n g s t o n Nappe and t h e l a c k o f a s t r o n g c o n t r a s t i n m e c h a n i c a l p r o p e r t i e s between the g n e i s s i c basement and t h e c o v e r r o c k s , s u g g e s t e d by t h e i n f o l d i n g o f t h e two. T h i s c o n c l u s i o n , t h a t h i g h - g r a d e metamorphism accompanied the f i r s t phase o f d e f o r m a t i o n , i s c o n t r a r y t o t h a t r e a c h e d by Ross (1979) f o r t h e west s i d e o f t h e Shuswap complex and by Brown and T i p p e t t (1978) f o r the S e l k i r k 87 M o u n t a i n s . In b o t h c a s e s t h e a u t h o r s c o n c l u d e d t h a t g r e e n s c h i s t f a c i e s c o n d i t i o n s accompanied e a r l y nappe f o r m a t i o n . T h i s d i f f e r e n c e may be a r e s u l t o f t h e d e e p e r s t r u c t u r a l l e v e l r e p r e s e n t e d by t h e T h o r - O d i n a r e a . The age o f Phase One d e f o r m a t i o n i s n o t w e l l c o n s t r a i n e d . L i m i t e d s t r u c t u r a l d a t a s u g g e s t a phase o f d e f o r m a t i o n i n t h e n o r t h e r n Shuswap p r i o r t o t h e emplacement o f t h e Mt. F o w l e r P l u t o n a t 430 Ma ( O k u l i t c h et a l . , 1 9 75). Read and Wheeler (1975) d e m o n s t r a t e d t h a t the f i r s t phase o f d e f o r m a t i o n i n t h e K o o t e n a y A r c was p r e - m i d d l e M i s s i s s i p p i a n i n age. A maximum age l i m i t o f Cambrian i s s u g g e s t e d by t h e i n v o l v e m e n t o f t h e c o v e r r o c k sequence i n Phase One d e f o r m a t i o n . Brown (1980) has s u g g e s t e d t h a t t h i s d e f o r m a t i o n i s a p r o d u c t o f the. D e v o n i a n t o e a r l y M i s s i s s i p p i a n C a r i b o o a n o r A n t l e r Orogeny. However, t h e C a r i b o o a n o r o g e n y i s n o t a f i r m l y e s t a b l i s h e d e v e n t ( N i l s e n and S t e w a r t , 1980) and i t s c o r r e l a t i o n w i t h t h e A n t l e r Orogeny i s t e n u o u s . P h a s e s Two and T h r e e f o l d s a p p e a r t o r e p r e s e n t two s i m i l a r d e f o r m a t i o n s , p e r h a p s r e l a t e d t o r e a c t i v a t i o n o f t h e same s t r e s s f i e l d . The absence o f t h e s e two p h a s e s o f d e f o r m a t i o n i n t h e a d j a c e n t Kootenay A r c t e r r a i n prompted Duncan and N i e l s e n (1977) t o s u g g e s t the e x i s t e n c e o f a t r a n s -c u r r e n t f a u l t a l o n g the C o l u m b i a R i v e r . Though d e t a i l s o f t h i s model a r e beyond t h e scope o f t h i s p a p e r , some e x p l a n a t i o n i s r e q u i r e d t o e x p l a i n t h e p e r v a s i v e e a s t - w e s t , recumbent nappe s t r u c t u r e s i n the Shuswap and t h e i r a b s e n c e i n t h e Kootenay A r c . Phase Two d e f o r m a t i o n may have been i n i t i a t e d by t h e p r o t r u s i o n o f wedges o f basement g n e i s s e s i n t o t h e c o v e r rock sequence. The basement c o r e d nappes d e s c r i b e d i n t h i s p a per a r e s i m i l a r i n s c a l e and geometry t o c l a s s i c s t r u c t u r e s o f the European A l p s (Ramsay, 1963). The dominant 88 n o r t h w a r d v e r g e n c e o f t h e s e Phase Two s t r u c t u r e s s u g g e s t s t h a t movement o f t h e s e g n e i s s wedges was f r o m s o u t h t o n o r t h ( F i g . 2 3 ) . Comparison o f t h e mapping i n t h i s s t u d y w i t h t h a t o f M u t t i (1978) and Read (1980) s u g g e s t s t h a t a minimum o f 15 t o 2 5 km o f t r a n s l a t i o n t o t h e n o r t h o c c u r r e d d u r i n g Phase Two. T h i s f i g u r e i s a p p r o x i m a t e s i n c e s u p e r i m p o s e d f l a t t e n i n g has e x a g g e r a t e d t h e e f f e c t o f t r a n s l a t i o n o f t h e nappes. The l o c a t i o n o f the r o o t zone o f t h e s e nappes i s unknown. Phase T h r e e f o l d s become more open i n s t y l e w i t h d e c r e a s i n g s t r u c t u r a l d e p t h ( F i g . 2 3 ) . More p r e c i s e l y , t i g h t - t o - i s o c l i n a l Phase T h r e e f o l d s a p p e a r t o be l i m i t e d t o the T r a n s i t i o n a l Domain o f I m b r i c a t e T h r u s t i n g and, t o a l e s s e r e x t e n t , t h e B i g Ledge subdomain o f the C o v e r Rock Domain. The Mount F o s t h a l l Synform a p p e a r s t o be t y p i c a l o f t h e s t y l e o f Phase T h r e e f o l d s r e g i o n a l l y d e v e l o p e d i n t h e Shuswap complex. S e p a r a t i o n o f Phases Two and T h r e e f o l d s w i t h i n t h e s t u d y a r e a i s d i f -f i c u l t b e c a u s e o f t h e c o - a x i a l n a t u r e o f the two d e f o r m a t i o n s and the f a c t t h a t t h e p h a s e s a r e seldom d e v e l o p e d t o g e t h e r on t h e same s c a l e . As a r e s u l t t h e r e i s no l a r g e - s c a l e r e f o l d i n g o f Phase Two by Phase T h r e e , o u t l i n e d by t h e d i s t r i b u t i o n o f l i t h o l o g i e s , w i t h i n the T h o r - O d i n dome. Th e s e phases can o n l y be d i s t i n g u i s h e d by c a r e f u l s t r u c t u r a l a n a l y s i s . To t h e n o r t h o f t h e T h o r - O d i n dome, F y l e s (1970) has mapped two p hases o f b r o a d l y e a s t - w e s t o r i e n t e d f o l d i n g i n the c o v e r rock sequence a l o n g t h e s o u t h e r n m a r g i n o f t h e Frenchman Cap g n e i s s dome. The s i m i l a r i t y i n s t y l e and o r i e n t a t i o n between t h e s e f o l d s and Phases Two and T h r e e o f t h e p r e s e n t s t u d y make a c o r r e l a t i o n a t t r a c t i v e . P h a s e s Two and T h r e e d e f o r m a t i o n s a r e o l d e r t h a n p o s t - t e c t o n i c p l u t o n s i n t r u d i n g the Shuswap complex. These have been d a t e d as 150-170 Ma (Duncan 89 FIGURE 23. S t r u c t u r a l model f o r t h e T h o r - O d i n g n e i s s dome. T h i s s c h e m a t i c n o r t h - s o u t h c r o s s - s e c t i o n summarizes t h e changes i n s t r u c t u r a l geometry a c r o s s t h e dome as d i s c u s s e d i n the t e x t . The f i g u r e i s a p p r o x i m a t e l y t o s c a l e . The basement g n e i s s e s a r e shown by c r o s s e s and t h e q u a r t z i t e s by d o t s . " S 3 " shows t h e l o c a l o r i e n t a t i o n o f Phase T h r e e a x i a l p l a n e s . SOUTH PHASE THREE I S I T K U M I MT . FOSTHALLl BIG L E D G E I R O O T I I M B R I C A T E L A K E I S Y N F O R M | A N T I F O R M | Z O N E | T H R U S T I N G S Y N F O R M NORTH PHASE TWO BASEMENT CORED NAPPES Ml. Odin Mt. Thor o 91 _et a j _ . , 1979; Duncan and P a r r i s h , 1979; C h a p t e r 4 o f t h i s t h e s i s ) . A t l e a s t Phase T h r e e d e f o r m a t i o n a p p e a r s t o p o s t d a t e t h e M i d d l e J u r a s s i c as e a s t - w e s t o r i e n t e d f o l d s deform s e d i m e n t s o f t h i s age i n t h e S l o c a n S y n c l i n e , an a r c u a t e b e l t o f l o w - g r a d e m e t a s e d i m e n t s e x t e n d i n g a c r o s s t h e Shuswap complex ( R o s s and K e l l e r h a l s , 1968; Read, 1980; N i e l s e n , 1982). In t h e absence o f d e f i n i t i v e e v i d e n c e , i t seems l i k e l y t h a t Phase Two d e f o r -m a t i o n (which c o r r e s p o n d s w i t h t h e metamorphic maximum) p r e d a t e s the f o r -m a t i o n o f t h e S l o c a n S y n c l i n e . E l s e w h e r e , i n the west o f t h e complex, Ross (1973, 1979, 1981) has documented b r o a d l y c o - a x i a l Phases Two and T h r e e f o l d s which a r e s i m i l a r i n s t y l e and o r i e n t a t i o n t o t h o s e i n t h e T h o r - O d i n dome. However, c o r r e l a t i o n o f f o l d s e q u e n c e s o v e r such d i s t a n c e s must be r e g a r d e d as s p e c u l a t i v e u n t i l t h e i n t e r v e n i n g a r e a has been s t r u c t u r a l l y mapped. The f i n a l t h r e e phases o f d e f o r m a t i o n have t h r e e f e a t u r e s i n common: an open, u p r i g h t s t y l e ; v e r t i c a l a x i a l p l a n e s ; and an a p p a r e n t f l e x u r a l -s l i p mechanism o f f o r m a t i o n . The main p a r t o f t h e dome geometry was c r e a t e d by t h e l a r g e - s c a l e , n o r t h w e s t - t r e n d i n g Phase F o u r a n t i f o r m . P a r a s i t i c f o l d s a s s o c i a t e d w i t h t h i s s t r u c t u r e have e n v e l o p i n g s u r f a c e s w h i c h d i p a t 20 t o 4 0 ° . These d a t a g i v e a more a c c u r a t e i n d i c a t i o n o f t h e s t y l e and i n t e r l i m b a n g l e o f Phase F o u r f o l d s than the o r i e n t a t i o n o f the c o m p o s i t i o n a l l a y e r i n g ( w h i c h i s i n p a r t a p r o d u c t o f t h e e a r l i e r phases o f d e f o r m a t i o n ) . The weak development o f c r e n u l a t i o n c l e a v a g e a s s o c i a t e d w i t h P hase F o u r d e f o r m a t i o n c o n t r a s t s w i t h f r a c t u r e c l e a v a g e s a s s o c i a t e d w i t h P h a s e s F i v e and S i x . The l a r g e - s c a l e Phase F i v e f o l d s shown i n F i g u r e 7 were c l e a r l y s u p e r i m p o s e d on the s o u t h w e s t l i m b o f t h e Phase F o u r a n t i f o r m . The 92 r e l a t i v e t i m i n g o f Phases F i v e and S i x i s l e s s r e a d i l y e s t a b l i s h e d . I n t e r -f e r e n c e o f f o l d s r e l a t e d t o t h e s e two e v e n t s forms domes and b a s i n s o c c u r r i n g on a s c a l e up t o 10 m e t e r s , b u t g i v e s no c l e a r e v i d e n c e o f r e l a -t i v e age. The n o r t h e r l y - t r e n d i n g Phase S i x f o l d s a r e b e l i e v e d t o be y o u n g e r th a n Phase F i v e f o l d s on t h e b a s i s o f s h e a r d i s p l a c e m e n t s a l o n g c r o s s c u t t i n g f r a c t u r e c l e a v a g e s and t h e n o r t h - t r e n d i n g f a u l t s ( a s s o c i a t e d w i t h t h e Phase S i x a x i a l p l a n e s ) w hich a p p e a r t o c u t Phase F i v e s t r u c t u r e s . A n d e s i t i c and b a s a l t i c d i k e s i n t r u d e d a l o n g t h e Phase S i x f r a c t u r e c l e a v a g e a r e o f p r o b a b l e T e r t i a r y age on t h e b a s i s o f K-Ar d a t i n g and r e g i o n a l i n f o r m a t i o n ( R o s s , 1974, 1979). T h u s , t h e e v o l u t i o n o f the T h o r - O d i n g n e i s s dome was c h a r a c t e r i z e d by two s e r i e s o f d e f o r m a t i o n a l e v e n t s . The f i r s t c o n s i s t e d o f t h r e e p hases o f t i g h t , a p p a r e n t l y recumbent f o l d i n g w h i c h r e s u l t e d i n a r e l a t i v e l y f l a t -l y i n g s t a c k o f nappes, t h e s t r u c t u r a l l y l o w e r ones b e i n g c o r e d by s l i c e s o f basement. These d e f o r m a t i o n a l f o l d i n g and t h r u s t i n g e v e n t s were e a r l y P a l e o z o i c t o M i d d l e J u r a s s i c i n age. The secon d s e r i e s o f d e f o r m a t i o n s o c c u r r e d under upper c r u s t a l c o n d i t i o n s and c o n s i s t e d o f t h r e e phases o f f l e x u r a l - s l i p f o l d i n g t h a t formed t he b a s i c domal s t r u c t u r e . These were p r o b a b l y C r e t a c e o u s t o T e r t i a r y i n age. Few o t h e r a r e a s c o n t a i n i n g c l a s s i c " m a n t l e d g n e i s s domes" have been s t u d i e d u s i n g t h e d e t a i l e d s t r u c t u r a l and s t r a i n a n a l y s i s t e c h n i q u e s employed i n t h i s s t u d y (Hobbs e t a l _ . , 1976, p. 428-430). N i s b e t (1976) has c o m p l e t e d a s t r u c t u r a l a n a l y s i s o f t h e n o r t h e r n C h e s t e r dome i n e a s t -c e n t r a l Vermont. N i s b e t (1976) i d e n t i f i e d two e a r l y phases o f recumbent i s o c l i n a l f o l d i n g . A l a t e r phase, more open i n s t y l e , o v e r p r i n t s t h e e a r l i e r two and forms t h e r e g i o n a l doming. N i s b e t d e l i n e a t e d f i v e mappable 9 3 u n i t s w i t h b o u n d a r i e s p a r a l l e l t o t h e s c h i s t o s i t y . The s t r u c t u r a l l y h i g h e s t and l o w e s t u n i t s b o t h a p p e a r t o be basement r o c k s . The m i d d l e t h r e e u n i t s c o u l d n o t be i d e n t i f i e d s t r a t i g r a p h i c a l l y b u t a r e c h a r a c t e r i z e d by w i d e s p r e a d t r a n s p o s i t i o n b o u d i n a g e , i s o c l i n a l f o l d i n g and abundance o f l o w - a n g l e f a u l t s . N i s b e t (1976) i n t e r p r e t s t h e b o u n d a r i e s o f t h e s e t h r e e u n i t s as a d i s l o c a t i o n zone a c r o s s w h i c h r e l a t i v e l y l a r g e d i s p l a c e m e n t s have o c c u r r e d . I t i s i n t e r e s t i n g t o note t h e s i m i l a r i t y between t h e s e c o n c l u s i o n s and t h o s e r e a c h e d i n t h e p r e s e n t s t u d y . S i g n i f i c a n t l y , b o t h t h e T h o r - O d i n dome and t h e C h e s t e r dome had p r e v i o u s l y been r e g a r d e d i n the l i t e r a t u r e as t y p e examples o f dome f o r m a t i o n due t o g r a v i t y - d r i v e n d i a p i r i s m ( F l e t c h e r , 1972; Thompson ^ t a l _ . , 1968; Hobbs ^ t a l _ . , 1976). CONCLUSIONS T h i s s t u d y was i n i t i a t e d i n an a t t e m p t t o r e s o l v e t h e c o n t r o v e r s y a b o u t t h e o r i g i n o f t h e T h o r - O d i n g n e i s s dome. The r e s u l t s o f a d e t a i l e d s t r u c t u r a l a n a l y s i s and a p r e l i m i n a r y s t r a i n a n a l y s i s l e a d t o the c o n c l u -s i o n t h a t t h e dome i s t h e p r o d u c t o f h i g h - l e v e l , T e r t i a r y b u c k l e f o l d i n g . I t seems l i k e l y t h a t b u c k l e f o l d s were p r e f e r e n t i a l l y d e v e l o p e d i n the T h o r - O d i n a r e a as a r e s u l t o f t h e s t r o n g d u c t i l i t y c o n t r a s t c r e a t e d by t h e i m b r i c a t i o n o f basement and c o v e r r o c k s d u r i n g Phase Two d e f o r m a t i o n . T h i s c o n c l u s i o n s u p p o r t s t h e model o r i g i n a l l y s u g g e s t e d by Ross ( 1 9 6 8 ) . The a p p l i c a b i l i t y o f g r a v i t a t i o n a l i n s t a b i l i t y models, such as t h o s e p r o p o s e d by F l e t c h e r (1972) and D i x o n (1974, 1975), t o the T h o r - O d i n dome i s q u e s t i o n a b l e . The domal s t r u c t u r e was formed by t h r e e d i s t i n c t phases o f f l e x u r a l - s l i p s t y l e f o l d i n g . The c h e v r o n s t y l e o f t h e s e f o l d s and the o v e r a l l dome geometry a r e i n c o n s i s t e n t w i t h a d i a p i r i c model f o r dome 94 f o r m a t i o n . I t i s c l e a r t h a t any a t t e m p t t o compare t h e t o t a l f i n i t e s t r a i n s i n t h e T h o r - O d i n r o c k s w i t h t h o s e p r e d i c t e d by d i a p i r m o d e l i n g s t u d i e s ( D i x o n , 1975) a r e i n v a l i d . The d i s t o r t i o n a l s t r a i n s measured i n t h i s s t u d y were d o m i n a n t l y a p r o d u c t o f d e f o r m a t i o n p r i o r t o dome f o r m a t i o n . T h i s c o n c l u s i o n a p p e a r s t o a p p l y t o t h e o t h e r g n e i s s domes i n t h e Shuswap complex ( R o s s , 1968; Ro s s , 1981). S e v e r a l l i n e s o f e v i d e n c e s u g g e s t t h a t t h e domal s t r u c t u r e formed under upper c r u s t a l c o n d i t i o n s r a t h e r t h a n b e i n g a p r o d u c t o f d i a p i r i s m and u p w e l l i n g o f t h e l o w e r c r u s t . The b r i t t l e n a t u r e o f the dome-forming d e f o r m a t i o n s , and t h e C r e t a c e o u s t o T e r t i a r y age i n f e r r e d f o r dome f o r -m a t i o n ( R o s s , 1974), s u g g e s t t h e dome formed a t low t e m p e r a t u r e s and s h a l l o w d e p t h s . The d a t a p r e s e n t e d i n t h i s p a per s u p p o r t t h e c o n c l u s i o n t h a t t h e T h o r - O d i n g n e i s s dome i s n o t t h e p r o d u c t o f some un i q u e o r unusual p r o c e s s ; r a t h e r , i t i s an i n t e g r a l p a r t o f t h e e v o l u t i o n o f t h e metamorphic complex as a whole. CHAPTER 2 Rb-Sr GEOCHRONOLOGY OF BASEMENT GNEISSES FROM THE THOR-ODIN GNEISS DOME 96 INTRODUCTION The age o f t h e g n e i s s i c r o c k s w i t h i n t he Shuswap complex has been a s u b j e c t o f s p e c u l a t i o n e v e r s i n c e t h e p i o n e e r i n g g e o l o g i c a l s u r v e y s o f Dawson (1898) and D a l y ( 1 9 1 5 ) . Dawson (1898) c o n c l u d e d t he Shuswap was A r c h e a n i n age. T h i s c o n c l u s i o n was d i s p u t e d by C a i r n e s ( 1 9 3 9 ) , who s u g g e s t e d t h a t t h e Shuswap i n c l u d e d a v a r i e t y o f r o c k u n i t s as young as M e s o z o i c , a l l o f which were a f f e c t e d by M e s o z o i c metamorphism. More r e c e n t s t u d i e s by J o n e s ( 1 9 5 9 ) , Wheeler (1970) and R e e s o r and Moore (1971) have c o n c l u d e d t h a t t he Shuswap complex p r o b a b l y r a n g e s i n age from l a t e P r o t e r o z o i c t o M i d d l e J u r a s s i c and has undergone major meta-morphism and d e f o r m a t i o n d u r i n g t h e J u r a - C r e t a c e o u s Columbian Orogeny. Ross (1970) a s s e r t e d t h a t t h e c o r e s o f t h e g n e i s s domes i n t h e Shuswap complex were composed o f P r e c a m b r i a n basement r o c k s o f Hudsonian ( e a r l y P r o t o z e r o i c ) age. R e e s o r (1970) c o r r e l a t e d t h e s e same r o c k s w i t h t h e upper P r o t e r o z o i c H o r s e t h i e f Creek Group. K-Ar d a t i n g , t h e t e c h n i q u e most e x t e n s i v e l y u t i l i z e d i n the p a s t , i s p a r t i c u l a r l y s u s c e p t i b l e t o r e s e t t i n g and g e n e r a l l y r e c o r d s t h e d a t e o f t h e most r e c e n t thermal e v e n t . T h i s paper summarizes t e c t o n i c s e t t i n g o f t h e complex; d i s c u s s e s t he p r e v i o u s Rb-Sr and U-Pb z i r c o n s t u d i e s ; r e v i e w s t h e g e o l o g y o f the T h o r - O d i n g n e i s s dome; and p r e s e n t s t h e r e s u l t s o f a Rb-Sr whole rock s t u d y . GENERAL GEOLOGY AND TECTONIC SETTING OF THE SHUSWAP COMPLEX The g e o l o g i c and t e c t o n i c s e t t i n g o f t h e Shuswap complex ( F i g . 1) i s summarized by Wheeler ( 1 9 7 0 ) , Ross (1973) and Brown ( 1 9 7 8 ) . The Shuswap 97 FIGURE 1. T e c t o n i c map o f s o u t h e a s t e r n B r i t i s h C o l u m b i a . Shown a r e : t h e m a j o r t e c t o n i c d i v i s i o n s ; t h e Mai t o n G n e i s s (M), t h e Frenchman Cap (FC) g n e i s s dome, t h e T h o r - O d i n (TO) g n e i s s dome, and t h e V a l h a l l a (V) g n e i s s dome; and t h e N e l s o n ( N B ), O l i v e r ( 0 1 ) , and Kuskanax (Kus) b a t h o l i t h s . 98 99 complex c o n s i s t s o f p o l y d e f o r m e d g n e i s s e s , s c h i s t s and me t a s e d i m e n t s e x t e n s i v e l y i n t r u d e d by M e s o z o i c p l u t o n s . The metamorphic grade i s v a r i a b l e b u t g e n e r a l l y i n t h e a m p h i b o l i t e f a c i e s . F i v e p h a s e s o f d e f o r -m a t i o n have been i d e n t i f i e d w i t h i n t h e complex by Ross ( 1 9 7 3 ) . The Shuswap complex can be d e f i n e d e i t h e r as t h e a r e a o f s i l l i m a n i t e meta-morphism o r as t h e a r e a o f metamorphic r o c k s deformed by e a s t - w e s t t r e n d i n g Phases Two and/or T h r e e f o l d s . B oth o f t h e s e phases o f d e f o r -m a t i o n p r e d a t e t h e O l i v e r G r a n i t e which i n t r u d e s t h e w e s t e r n boundary o f t h e complex and has an age o f 150 t o 160 Ma (Duncan e t a l _ . , 1979; A r m s t r o n g , w r i t t e n c o m m u n i c a t i o n , 1982). A l o n g t h e e a s t e r n boundary o f t h e Shuswap complex t h e s e p o l y d e f o r m e d h i g h - g r a d e metamorphic r o c k s a r e i n f a u l t c o n t a c t w i t h t h e l o w e r - g r a d e metamorphic r o c k s o f t h e Kootenay A r c (Brown, 1978; Read, 1979). T h i s e a s t e r n m a r g i n o f t h e Shuswap i s c h a r a c t e r i z e d by t h r e e domal complexes a t a p p r o x i m a t e l y 80 km i n t e r v a l s : t h e Frenchman Cap dome t o the n o r t h -w e s t o f Revel s t o k e ; t h e T h o r - O d i n dome west o f Arrow Lake; and t h e V a l h a l l a dome f u r t h e r s o u t h . These domes have h i g h - g r a d e m i g m a t i t i c g n e i s s e s i n t h e i r c o r e s . The t e c t o n i c s i g n i f i c a n c e o f t h e s e s t r u c t u r e s i s c o n t r o v e r s i a l . Ross (1968) has s u g g e s t e d t h a t t h e y a r e the p r o d u c t o f i n t e r f e r e n c e o f l a t e - s t a g e , u p r i g h t f l e x u r a l - s l i p f o l d s , whereas Reesor (1970) s u g g e s t e d t h a t t h e domes formed by d i a p i r i c u p r i s e o f l o w e r c r u s t a l r o c k s . The age o f the g n e i s s e s w i t h i n t h e Shuswap complex i s 1 i k e w i s e d i s p u t e d . PREVIOUS GEOCHRONOLOGY The f i r s t a t t e m p t t o do a modern g e o c h r o n o l o g i c a l s t u d y o f g n e i s s i c r o c k s o f t h e Shuswap complex was t h e Rb-Sr whole r o c k s t u d y o f the 100 C I a c h n a c u d a i n n s a l i e n t e a s t o f R e v e l s t o k e , by B l e n k i n s o p (1972). T h i s work and t h e f o l l o w - u p s t u d y by B i r n i e (1975) r e s u l t e d i n d a t a w i t h a 87 Hfi R7 ftfi l a r g e d e g r e e o f s c a t t e r on a S r / S r - Rb/ S r p l o t and no e v i d e n c e f o r ages o l d e r t h a n 750 Ma. I t i s l i k e l y t h a t t h e Sr i s o t o p e systems i n t h e s e r o c k s have been s i g n i f i c a n t l y d i s t u r b e d by y o u n g e r metamorphic e v e n t s . Even y o u n g e r ages have been o b t a i n e d f o r s y n t e c t o n i c g r a n i t i c r o c k s from t he w e s t e r n m a r g i n o f t h e complex i n the s o u t h e r n Okanagan V a l l e y n e a r Osoyoos by Ryan (1973). He d e t e r m i n e d Rb-Sr whole rock ages o f 145 and 176 Ma f o r g r a n i t i c s h e e t s i n v o l v e d i n p o l y p h a s e d e f o r m a t i o n and a r g u e d t h a t t h e s e d a t e s d i d n o t r e p r e s e n t t h e age o f i n t r u s i o n , b u t r a t h e r r e p r e s e n t e d t h e e f f e c t s o f a J u r a s s i c thermal e v e n t . E a r l i e r , Menzer (1970) d e t e r m i n e d a Rb-Sr whole r o c k age o f 129 Ma f o r an o r t h o g n e i s s from t h e Okanagan Range, Washington s t a t e . As i n the Osoyoos a r e a , Menzer (1970; p e r s o n a l c o m m u n i c a t i o n , 1981) r e g a r d e d t h i s d a t e as an o m a l o u s l y young. U-Pb z i r c o n d a t i n g p r o v i d e s a t e c h n i q u e f o r o v e r c o m i n g some o f the prob l e m s i n h e r e n t i n Rb-Sr d a t i n g . Wanless and Re e s o r (1975) d a t e d z i r -con from o r t h o g n e i s s i n t h e T h o r - O d i n dome. F o u r z i r c o n s from a ho r n b l e n d e g r a n o d i o r i t e g n e i s s p l o t t e d on a c h o r d w i t h an upper c o n c o r d i a i n t e r c e p t o f 1960 Ma and a lo w e r i n t e r c e p t o f 175 Ma. They c o n c l u d e d t h a t t h e s e basement g n e i s s e s r e p r e s e n t e d Hudsonian basement r o c k s . Brown (1978) has q u e s t i o n e d t he v a l i d i t y o f t h i s i n t e r p r e t a t i o n , s u g g e s t i n g t h a t t h e z i r c o n s c o u l d be d e t r i t a l i n o r i g i n . GEOLOGY OF THE THOR-ODIN GNEISS DOME The T h o r - O d i n g n e i s s dome i s l o c a t e d on the e a s t e r n m a r g i n o f the Shuswap complex, 60 km s o u t h o f R e v e l s t o k e . The g e o l o g y ( F i g . 2) o f t h e 101 FIGURE 2. G e o l o g i c map o f the T h o r - O d i n g n e i s s dome. M o d i f i e d a f t e r R e e s o r and Moore ( 1 9 7 1 ) . The l o c a t i o n o f t h e m e t a s e d i m e n t a r y basement g n e i s s s a m p l e s , t h e O d i n G l a c i e r m i g m a t i t e s and t h e K e l l e y R i d g e m i g m a t i t e s a r e shown on the map as 1, 2 and 3 r e s p e c t i v e l y . KM Q U A T E R N A R Y A L L U V I U M M A R B L E B A N D E D CALC-SIL ICATE SCHIST L I N E A T E D Q U A R T Z M O N Z O N I T E A M P H I BOLITE Q U A R T Z - F E L D S . - B I O T . - G A . P A R A G N E I S S PELITIC A N D C A L C - S I L I C A T E SCHIST Q U A R T Z I T E r T l B I O T . G R A N O D I O R I T E Q U A R T Z M O N Z O N I T E H O R N B . G R A N O D I O R I T E M E TA S E D I M E N T A R Y G N E I S S T H R U S T F A U L T ICE A N D S N O W 103 dome has been d e s c r i b e d by R e e s o r and Moore (1971) and more r e c e n t l y by Hoy ( 1 9 7 7 b ) , and Duncan ( 1 9 7 8 a ) . On t h e b a s i s o f l a r g e - s c a l e s t r u c t u r e s and t h e d i s t r i b u t i o n o f l i t h o l o g i e s , t h e T h o r - O d i n dome can be d i v i d e d i n t o t h r e e major domains: 1. The A u t o c h t h o n o u s Core G n e i s s Domain, do m i n a t e d by m e t a s e d i -m entary q u a r t z o - f e l d s p a t h i c g n e i s s e s and i n t r u d e d by s h e e t s o f g r a n i t i c g n e i s s . These r o c k s a p p e a r t o be a metamorphic base-ment t o t h e o v e r l y i n g m e t a s e d i m e n t a r y s e q u e n c e . 2. The Basement C o r e d Nappe Domain, c h a r a c t e r i z e d by s l i c e s o f basement g n e i s s c o r i n g l a r g e - s c a l e nappes o f the c o v e r r o c k s e q u e n c e . 3. The C o v e r Rock Domain, c o n s i s t i n g o f an a l l o c h t h o n o u s sequence o f c o m p l e x l y f o l d e d and i m b r i c a t e d m e t a s e d i m e n t s . T h i s sequence o f q u a r t z i t e s , m a r b l e s , c a l c - s i l i c a t e s , s c h i s t s and a m p h i b o l i t e s has been c o r r e l a t e d w i t h t h e l o w e r P a l e o z o i c sequence i n the Kootenay A r c by Re e s o r and Moore (1971) and Hoy ( 1 9 7 7 b ) . The dome a r e a i s o f r e l a t i v e l y u n i f o r m upper a m p h i b o l i t e g r a d e . Most o f t h e a r e a i s c h a r a c t e r i z e d by t h e assemblage a l k a l i f e l d s p a r and s i l l i m a n i t e , r a t h e r t h a n m u s c o v i t e and q u a r t z . E v i d e n c e f o r po l y m e t a -morphism comes from metamorphic t e x t u r e s and the o c c u r r e n c e o f a l l t h r e e a l u m i n o s i l i c a t e polymorphs v a r i o u s l y a s s o c i a t e d w i t h f a b r i c e l e m e n t s such as f o l d l i n e a t i o n s , c r o s s c u t t i n g v e i n s and p e g m a t i t e s . P a r t i a l m e l t i n g and m i g m a t i z a t i o n have o c c u r r e d i n a l l t h r e e domains b u t a r e p e r v a s i v e i n g r a n i t i c b u l k c o m p o s i t i o n s w i t h i n t h e c o r e g n e i s s e s . T h i s s t u d y i s an a t t e m p t t o d a t e t h e t i m i n g o f e v e n t s i n the basement g n e i s s e s u s i n g t h e Rb-Sr whole r o c k t e c h n i q u e . 104 Rb-Sr GEOCHRONOLOGY Sample S e l e c t i o n and A n a l y t i c a l T e c h n i q u e The p r e v i o u s l a c k o f s u c c e s s i n d a t i n g Shuswap r o c k s u s i n g t h e Rb-Sr whole r o c k t e c h n i q u e s u g g e s t e d t h a t g r e a t c a r e i n s a m p l i n g would be n e c e s s a r y . The f o l l o w i n g s t r a t e g i e s were employed i n s a m p l i n g : ( i ) l a r g e samples were t a k e n (5 kg o r more); ( i i ) sample s u i t e s were c o l l e c t e d from s i n g l e o u t c r o p a r e a s ( w i t h i n a few t e n s o f s q u a r e m e t e r s ) ; and ( i i i ) r o c k s w i t h v e i n s o r o t h e r e v i d e n c e o f i n j e c t i o n o f g r a n i t i c m a t e r i a l were n o t sampled. A s u i t e o f m e t a s e d i m e n t a r y c o r e g n e i s s e s were c o l l e c t e d from the e a s t s l o p e o f Mount Gunnarson ( F i g . 2 ) . These samples f e l l i n t o two g r o u p s : t h o s e w i t h e v i d e n c e o f p a r t i a l m e l t i n g , and t h o s e l a c k i n g any e v i d e n c e o f m i g m a t i z a t i o n . Sample s u i t e s from m i g m a t i t i c g r a n i t o i d g n e i s s e s were c o l l e c t e d from t h e O d i n G l a c i e r a r e a and K e l l e y R i d g e . The l o c a t i o n s a r e shown i n F i g u r e 2. The samples were c o l l e c t e d from s i n g l e o u t c r o p s and c o n s i s t e d o f 15 t o 20 kg samples o f g n e i s s and l e u c o c r a t i c v e i n m a t e r i a l . The samples c h o s e n f o r a n a l y s i s were o f g n e i s s c o n t a i n i n g 10 t o 30 cm wide l e u c o c r a t i c v e i n s . These were c a r e f u l l y s e p a r a t e d from t h e i r g n e i s s i c h o s t by sawin g . Any m a f i c s e l v a g e m a t e r i a l was i n c l u d e d w i t h t h e v e i n sample. Rb and Sr c o n c e n t r a t i o n s were d e t e r m i n e d by r e p l i c a t e a n a l y s i s o f p r e s s e d powder p e l l e t s u s i n g X - r a y f l u o r e s c e n c e . U.S. G e o l o g i c a l Survey r o c k s t a n d a r d s were used f o r c a l i b r a t i o n ; mass a b s o r p t i o n c o e f f i c i e n t s were o b t a i n e d from Mo K a Compton s c a t t e r i n g measurements. Rb/Sr r a t i o s 105 have a p r e c i s i o n o f 2% (la) and c o n c e n t r a t i o n s have a p r e c i s i o n o f 5% ( l a ) . Sr i s o t o p i c c o m p o s i t i o n was measured i n the l a b o r a t o r y o f R. L. A r m s t r o n g (U.B.C.) on u n s p i k e d samples p r e p a r e d u s i n g s t a n d a r d i o n exchange t e c h n i q u e s . The mass s p e c t r o m e t e r (60° s e c t o r , 30 cm r a d i u s , s o l i d s o u r c e ) i s o f U.S. N a t i o n a l Bureau o f S t a n d a r d s d e s i g n , m o d i f i e d by H. ' F a u l . Data a c q u i s i t i o n i s d i g i t i z e d and automated u s i n g a NOVA com-86 88 p u t e r . E x p e r i m e n t a l d a t a have been n o r m a l i z e d t o a S r / S r r a t i o o f 0.1194 and a d j u s t e d so t h a t t h e NBS s t a n d a r d S r C 0 3 (SRM987) g i v e s a 87 86 S r / S r r a t i o o f 0.71022 ± 2, and the Eimer and Amend Sr s t a n d a r d g i v e s a r a t i o o f 0.70800 ± 2. Rb-Sr d a t e s a r e b a s e d on a Rb decay c o n s t a n t o f I . 42 x 1 0 " ^ y ~ * . The r e g r e s s i o n s a r e c a l c u l a t e d a c c o r d i n g t o the t e c h -n i q u e o f York ( 1 9 6 9 ) . R e s u l t s and I n t e r p r e t a t i o n The r e s u l t s o f t h e Rb-Sr a n a l y s e s a r e g i v e n i n T a b l e I and shown i n F i g u r e s 3 and 4. The s i x non- m i g m a t i z e d m e t a s e d i m e n t a r y g n e i s s e s f a l l on a 2.73 ± 0.21 Ga e r r o r c h r o n w i t h a 8 7 S r / 8 6 S r i n i t i a l r a t i o o f 0.701 ± 0.002. I t i s c l e a r t h a t t h e b a s i c i s o c h r o n a s s u m p t i o n o f a c l o s e d s y s t e m has n o t 87 86 been met f o r t h e s e samples. The h i g h S r / S r r a t i o s o f s e v e r a l o f t h e s e s a m p l e s , however, p r o v i d e s t r o n g e v i d e n c e f o r a P r e c a m b r i a n , p r o b a b l y A r c h e a n , age f o r t h e s e g n e i s s e s . The r e s u l t s f o r t h e m i g m a t i t i c g r a n i t o i d r o c k s s u g g e s t a c o m p l i c a t e d h i s t o r y f o r t h e Sr i s o t o p e e v o l u t i o n o f t h e s e r o c k s . Samples from the two s i t e s f a l l on s u b p a r a l l e l i s o c h r o n s w i t h d i s t i n c t l y d i f f e r e n t i n i t i a l r a t i o s . The two i s o c h r o n s g i v e ages o f 744 ± 53 Ma f o r the Odin G l a c i e r m i g m a t i t e s and 848 + 220 Ma f o r the K e l l e y Ridge m i g m a t i t e s ( F i g . 4 ) . A TABLE I Rb-Sr ISOTOPIC RESULTS C o r e G n e i s s M e t a s e d i m e n t s Sample ppm S r ppm Rb 8 7 R b / 8 6 S r 8 7 S r / 8 6 S r TOMS-2 127 73 1.65 ( 0 . 0 3 ) * 0.7580 (2) TOMS-3 209 129 1.96 (0.03) 0.7769 (2) TOMS-4 260 68 0.75 (0.02) 0.7283 (5) TOMS-5 219 218 2.81 (0.04) 0.8085 (2) TOMS-6 806 91 0.33 (0.03) 0.7198 (2) TOMS-7 396 109 0.80 (0.03) 0.7305 (4) O d i n G l a c i e r M i g m a t i t e s OG-1 209 292 4.07 (0.06) 0.8000 (2) OG-2 178 188 3.08 (0.04) 0.7891 (2) OG-3 231 293 ' 3.71 (0.04) 0.7957 (2) OG-4 195 196 2.94 (0.04) 0.7883 (3) OG-5 220 245 3.27 (0.05) 0.7902 (2) K e l l e y R idge M i g m a t i t e s C2-1A 239 28 0.34 (0.02) 0.7367 (3) C2-1B 278 68 0.71 (0.02) 0.7390 (3) C2-1C 222 86 1.12 (0.03) 0.7459 (2) C2-2A 278 68 0.25 (0.02) 0.7390 (3) C2-2B 250 82 0.85 (0.03) 0.7464 (2) * E r r o r s a r e based on t h e r e p r o d u c i b i l i t y o f samples r u n a t d i f f e r e n t t i m e s . The numbers c a l c u l a t e d a r e w e i g h t e d means u s i n g t h e i n v e r s e o f t h e s q u a r e o f t h e s t a n d a r d d e v i a t i o n . 107 FIGURE 3. Rb-Sr i s o c h r o n d i a g r a m f o r unmigmatized m e t a s e d i m e n t a r y g n e i s s e s . The e r r o r b a r s a r e s m a l l e r t h a n the symbols used ( s e e T a b l e I ) . The age c a l c u l a t e d from the r e g r e s s i o n l i n e shown i s 2.730 + 0 . 2 1 0 Ga. The e r r o r i n t h e age i s c a l c u l a t e d from t h e York (1969) r e g r e s s e d s l o p e and the r e l a t i o n : 2 _ (1.42 x I P " 5 ) 2 2 t (1 + s l o p e ) 2 s l o p e * 108 109 FIGURE 4. A Rb-Sr i s o c h r o n d i a g r a m f o r the O d i n G l a c i e r and K e l l e y R i d g e m i g m a t i t e s . The O d i n G l a c i e r s u i t e g i v e s an age o f 750 + 60 Ma and an i n i t i a l r a t i o o f 0.756 + 0.003. The K e l l e y R i d g e m i g m a t i t e s l i e on the l o w e r s u b p a r a l l e l i s o c h r o n w i t h an age o f 850 + 200 Ma and an i n i t i a l r a t i o o f 0.732 + 0.002. I l l s e p a r a t e g n e i s s - v e i n p a i r from t h e K e l l e y R i d g e l o c a t i o n ( T a b l e I) g i v e s an age o f 863 Ma. T h e s e r e s u l t s can be i n t e r p r e t e d i n t h e f o l l o w i n g way. A t a p p r o x i -m a t e l y 2.0 Ga, a s u i t e o f g r a n i t i c s i l l s was i n j e c t e d i n t o t h e deformed m e t a s e d i m e n t a r y g n e i s s e s . The s t r o n t i u m i s o t o p e r a t i o s o f t h e g r a n i t i c r o c k s t h e n e v o l v e d p r e s u m a b l y as a c l o s e d s y s t e m up t o 750 Ga, when a m a j o r t h e r m a l e v e n t l e d t o e x t e n s i v e p a r t i a l m e l t i n g o f t h e basement g n e i s s t e r r a i n . The g r a n i t i c g n e i s s e s , b e i n g o f q u a r t z m o n z o n i t e and g r a n o d i o r i t e c o m p o s i t i o n s , were p a r t i a l l y m e l t e d t o p r o d u c e l e u c o c r a t i c v e i n s , t y p i c a l l y w i t h b i o t i t e s e l v a g e s . B I a t t n e r (1971) has s t u d i e d t h e s e r o c k s i n d e t a i l . B I a t t n e r d e m o n s t r a t e d t h a t t r a c e e l e m e n t abun-d a n c e s show c o n s i d e r a b l e v a r i a t i o n between a d j a c e n t p e g m a t o i d and g r a n i t i c v e i n s . T h i s i s c o n s i s t e n t w i t h l o c a l p a r t i a l m e l t i n g and i n s i t u m i g m a t i z a t i o n . The e f f e c t o f p a r t i a l m e l t i n g on the Rb-Sr system 8 7 86 was t o c r e a t e l o c a l h o m o g e n i z a t i o n o f S r / S r r a t i o s (due t o enhanced d i f f u s i o n r a t e s o f Sr i n m e l t s ) and t o f r a c t i o n a t e Rb i n t o t h e m e l t . The o r i g i n a l Sr i s o t o p e g e o c h e m i s t r y p r i o r t o m e l t i n g can be e s t i m a t e d by 87 86 making a w e i g h t e d a v e r a g e o f t h e Rb/ S r r a t i o s o f the l e u c o c r a t i c v e i n s and g n e i s s i c m a t e r i a l . Such a v e r a g e s combined w i t h the i n i t i a l 87 86 87 86 r a t i o s o f t h e m i g m a t i t e i s o c h r o n s g i v e a l o c u s on a S r / S r - Rb/ S r p l o t c o n s i s t e n t w i t h an age o f a p p r o x i m a t e l y 2 Ga f o r the time o f the g r a n i t i c i n t r u s i o n . DISCUSSION The new Rb-Sr d a t a , f i r s t p r e s e n t e d by Duncan (1978 b ) , a r e e v i d e n c e f o r r o c k s o l d e r than 2.0 Ga i n t h e Shuswap complex. P r e l i m i n a r y Rb-Sr 112 d a t a f o r t h e M a l t o n G n e i s s ( C h a m b e r l a i n et a j _ . , 1978) s u g g e s t t h a t r o c k s as o l d as 3.0 Ga may f o r m an e x t e n s i v e basement t e r r a i n w i t h i n t h e Omineca c r y s t a l l i n e b e l t . Whether t h i s basement r e p r e s e n t s a p a r t o f t h e N o r t h American c r a t o n , o r i s an e x o t i c t e r r a i n a c c r e t e d d u r i n g t h e P r e c a m b r i a n , r e m a i n s s p e c u l a t i v e . I t has always been assumed, l a r g e l y on t h e b a s i s o f t r e n d s i n a e r o m a g n e t i c a n o m a l i e s ( P r i c e , 1979; D o u g l a s e t a l . , 1970), t h a t t h e basement t e r r a i n b e n e a t h t h e Rocky M o u n t a i n s i s H u d s o n i a n i n age. I t i s p o s s i b l e t h a t t h e s e r o c k s a r e a c t u a l l y a 2.7 t o 3.0 Ga t e r r a i n as i n d i c a t e d i n t h i s work, w i t h a s t r o n g Hudsonian o v e r p r i n t . Z i r c o n d a t i n g o f basement g n e i s s e s from d r i l l h o l e s i n A l b e r t a m i g h t be a way t o t e s t whether o r n o t t h e two basement t e r r a i n s may be c o n t i n u o u s . The Rb-Sr d a t a p r e s e n t e d i n t h i s p a p e r a l s o s u p p o r t t h e U-Pb z i r c o n d a t e o f 1.96 Ga on t h e g r a n o d i o r i t e u n i t o b t a i n e d by Wanless and R e e s o r ( 1 9 7 5 ) . The c o h e r e n c y o f t h e s e two d a t a s e t s s u p p o r t s R e e s o r ' s i n t e r p r e -t a t i o n t h a t t h i s u n i t i s i g n e o u s i n o r i g i n and t h a t t h e z i r c o n s d a t e from t h e time o f c r y s t a l l i z a t i o n ( R e e s o r and Moore, 1971; Wanless and R e e s o r , 1975). C a u t i o n must be a d v i s e d , however, i n a c c e p t i n g t h e s e r e s u l t s as d e f i n i t i v e . The Rb-Sr " d a t e " p r o p o s e d i n t h i s s t u d y i s o b v i o u s l y an a p p r o x i m a t i o n and g a i n s c r e d i b i l i t y o n l y when s u p p o r t e d by t h e U-Pb z i r -c o n d a t e . A l t h o u g h t h e r e i s no b a s i s t o s u s p e c t t h e a n a l y t i c a l r e s u l t s , W anless and R e e s o r (1975) do m e n t i o n t h a t a p o r t i o n o f the z i r c o n s show o v e r g r o w t h s a r o u n d e u h e d r a l c o r e s ( s e e F i g . 3 o f Wanless and R e e s o r , 1975), s u g g e s t i n g the p o s s i b i l i t y t h a t t h e i r c h o r d r e p r e s e n t s a m i x i n g l i n e between o l d and new z i r c o n components. I f t h i s i s so, t h e n the 113 upper i n t e r c e p t o f t h e c h o r d would n o t n e c e s s a r i l y have any geochrono-l o g i c s i g n i f i c a n c e . D e s p i t e t h e s e u n c e r t a i n t i e s , t h e e x i s t e n c e o f a m a j o r i g n e o u s e v e n t w i t h an age o f 2.0 t o 2.5 Ga i s becoming e s t a b l i s h e d by r e c e n t Rb-Sr whole r o c k d a t i n g by R.L. A r m s t r o n g ( A r m s t r o n g , 1979; p e r s o n a l communi-c a t i o n , 1982). These d a t e s a r e c o n s i s t e n t w i t h a l a r g e a r e a o f g r a n i t i c g n e i s s e s e x p o s e d t o the n o r t h o f t h e T h o r - O d i n dome and i n the Frenchman Cap dome, b e i n g o f t h i s age (2.0 t o 2.2 G a ) . The g e o l o g i c a l mapping o f Brown (1980) i n t h e Frenchman Cap dome s u g g e s t s t h a t , i n c o n t r a s t t o the T h o r - O d i n dome, g r a n i t i c g n e i s s e s form t h e b u l k o f t h e basement g n e i s s e s . I t i s l i k e l y t h a t much o f t h i s basement t e r r a i n has been a f f e c t e d by p r o c e s s e s t h a t have s i g n i f i c a n t l y d i s t u r b e d t h e Sr i s o t o p e s y s t e m a t i c s , p a r t i c u l a r l y i n r o c k s t h a t have p a r t i a l l y m e l t e d . The Shuswap g n e i s s e s a r e v e r y s u s c e p t i b l e t o such d i s t u r b a n c e s , due t o t h e i r S r - r i c h geochem-87 Rfi i s t r y w h i c h r e s u l t s i n a narrow s p r e a d i n Rb/ S r r a t i o s . The n e t 87 86 r e s u l t i s t h a t h i g h S r / S r r a t i o s a r e n o t t y p i c a l l y g e n e r a t e d even i n 2.0 Ga o r o l d e r r o c k s . Thus r e l a t i v e l y s m a l l d e g r e e s o f open-system b e h a v i o r can d e s t r o y any e v i d e n c e o f t h e a n t i q u i t y o f a t y p i c a l Shuswap g n e i s s . The 750-860 Ma d a t e s o b t a i n e d i n t h i s s t u d y f r o m t h e m i g m a t i t i c g n e i s s e s a p p e a r t o r e p r e s e n t a m a j o r h i g h - g r a d e metamorphic e v e n t i n the C o r d i l l e r a . CONCLUSIONS The Rb-Sr whole r o c k s t u d i e s r e p o r t e d i n t h i s paper have p r o v i d e d e v i d e n c e f o r t h r e e P r e c a m b r i a n e v e n t s i n the basement g n e i s s e s o f t h e 114 T h o r - O d i n g n e i s s dome. The m e t a s e d i m e n t a r y g n e i s s e s o f t h e basement p r e s e r v e e v i d e n c e o f an A r c h e a n age. A l t h o u g h t h e Rb-Sr system has been d i s t u r b e d such t h a t t h e s c a t t e r o f t h e d a t a i s l a r g e r t h a n c a n be o n QC. e x p l a i n e d by a n a l y t i c a l p r e c i s i o n , t h e h i g h S r / S r v a l u e s s t r o n g l y s u g g e s t t h e p r e s e n c e o f r o c k s c o n s i d e r a b l y o l d e r t h a n t h e g r a n i t i c g n e i s s e s w h i c h i n t r u d e them. The Rb-Sr r e s u l t s f o r t h e g r a n i t i c g n e i s s e s r e f l e c t e x t e n s i v e r e d i s t r i b u t i o n o f Sr d u r i n g a h i g h - g r a d e metamorphic e v e n t a t 860 t o 750 Ma. Thus o n l y t h e a v e r a g e o f many samples from an i n d i v i d u a l o u t c r o p a p p r o x i m a t e a 2.0 Ga r e f e r e n c e i s o c h r o n . T h i s 2.0 Ga a p p r o x i m a t e age i s c o n s i s t e n t w i t h an upper i n t e r c e p t age o f 1.96 Ga f o r a z i r c o n s u i t e e x t r a c t e d from t h e g r a n o d i o r i t e g n e i s s . P r e c a m b r i a n basement o c c u r s w i t h i n t h e Omineca c r y s t a l l i n e b e l t . T h i s may r e p r e s e n t an e x t e n s i o n o f t h e C a n a d i a n S h i e l d under t h e Rocky M o u n t a i n s and i s a p p a r e n t l y t h e w e s t e r n m o s t e x p o s u r e o f A r c h e a n r o c k s i n Canada. CHAPTER 3 COMMON LEAD SYSTEMATICS OF LEAD-ZINC DEPOSITS IN THE SHUSWAP COMPLEX AND KOOTENAY ARC, SOUTHEASTERN BRITISH COLUMBIA 116 INTRODUCTION The ages o f t h e metamorphic r o c k s i n t h e Shuswap complex, t h e c o r e zone o f t h e S o u t h e r n C a n a d i a n C o r d i l l e r a ( F i g . 1), have been t h e s u b j e c t o f an o n g o i n g c o n t r o v e r s y . Dawson (1898) c o n c l u d e d t h a t t h e " d i s t u r b e d m e t a m o r p h i c s " o f t h e Shuswap complex were A r c h e a n i n age. D a l y (1915) s u g g e s t e d t h a t P r e c a m b r i a n B e l t Group s e d i m e n t s l a y u n c o n f o r m a b l y o v e r t h e Shuswap t e r r a i n . C a i r n e s (1939) q u e s t i o n e d Dawson's i n t e r p r e t a t i o n , s u g g e s t i n g t h a t t h e Shuswap i n c l u d e d a v a r i e t y o f r o c k u n i t s , as young as M e s o z o i c i n age, w h i c h were metamorphosed by M e s o z o i c p l u t o n s . In b r o a d o u t l i n e , t h e s e two d i v e r g e n t p o i n t s o f view have c o n t i n u e d t o the p r e s e n t . A t a 1969 c o n f e r e n c e on t h e t e c t o n i c s o f the S o u t h e r n C a n a d i a n C o r d i l l e r a ( W h e e l e r , 1970), J.V. Ross a s s e r t e d t h a t t h e g n e i s s domes i n t h e Shuswap were c o r e d by P r e c a m b r i a n basement g n e i s s e s o f p r o b a b l e Hudsonian ( e a r l y P r o t e r o z o i c ) age. A t t h e same c o n f e r e n c e J . E . Reesor s u g g e s t e d t h e s e g n e i s s e s were c o r r e l a t i v e w i t h t h e upper P r o t e r o z o i c H o r s e t h i e f Creek Group. In an a t t e m p t t o r e s o l v e t h i s c o n t r o v e r s y , a m u l t i f a c e t e d s t u d y o f t h e g e o l o g y and g e o c h r o n o l o g y o f t h e T h o r - O d i n g n e i s s dome and r e l a t e d a r e a s o f the Shuswap t e r r a i n was i n i t i a t e d (Duncan and N i e l s e n , 1977; Duncan, 1978a, 1978b). P r e l i m i n a r y Rb-Sr whole r o c k g e o c h r o n o l o g y (Duncan, 1978b) on samples o f basement g n e i s s e s c o n f i r m e d t h e e x i s t e n c e o f A r c h e a n r o c k s i n t h e Shuswap. Ross' s (1970) s u g g e s t i o n o f a Hudsonian age f o r t h e g n e i s s e s was b a s e d i n p a r t on S i n c l a i r ' s (1966) i n t e r p r e t a t i o n o f anomalous l e a d s from s u l f i d e d e p o s i t s a l o n g the e a s t e r n m a r g i n o f t h e Shuswap. T h e r e f o r e a s t u d y was 117 FIGURE 1. L o c a t i o n s o f l e a d - z i n c d e p o s i t s i n s o u t h e a s t e r n B r i t i s h C o l u m b i a . The open c i r c l e s show the l o c a t i o n s o f s t r a t a -bound d e p o s i t s . The Shuswap t y p e d e p o s i t s shown a r e B i g Ledge, R i v e r J o r d a n , C o t t o n B e l t , Ruddock C r e e k , J and L, and Wigwam. The l o c a t i o n s o f t h e G o l d s t r e a m d e p o s i t s and t h e r e g i o n o f o c c u r r e n c e o f s m a l l p r o s p e c t s o f t h e G o l d s t r e a m t y p e , a n a l y z e d i n t h i s s t u d y , a r e shown i n the n o r t h p a r t o f t h e map. The Salmo t y p e d e p o s i t s shown a r e Duncan, Re e v e s - M a c d o n a l d , H.B. and J e r s e y . 118 4 9 ° . _ ] . . _ MONT. 119 begun o f t h e l e a d i s o t o p e s y s t e m a t i c s o f s t r a t a b o u n d s u l f i d e d e p o s i t s i n m e t a s e d i m e n t s o v e r l y i n g and a d j a c e n t t o t h e g n e i s s domes. T h i s p a p e r p r e s e n t s d a t a on t h e l e a d i s o t o p e c h a r a c t e r i s t i c s o f t h r e e groups o f s t r a t a b o u n d s u l f i d e d e p o s i t s from t h e Shuswap complex and t h e Kootenay A r c and S e l k i r k M o u n t a i n s t o t h e e a s t . These r e s u l t s a r e used t o e s t a b l i s h t h e age o f basement and c o v e r r o c k s u t i l i z i n g m u l t i s t a g e l e a d m o d e l s and known g e o l o g i c a l c o n s t r a i n t s . REGIONAL GEOLOGY The g e o l o g i c and t e c t o n i c s e t t i n g o f t h e Shuswap Complex ( F i g . 1) has been summarized by R e e s o r and Moore ( 1 9 7 1 ) , Ross ( 1 9 7 3 ) , Duncan ( 1 9 7 8 a ) , Read (1979) and Brown and Read ( 1 9 7 9 ) . The Shuswap t e r r a i n c o n s i s t s o f a v a r i e t y o f g n e i s s e s , m e t a s e d i m e n t s and a m p h i b o l i t e s e x t e n s i v e l y i n t r u d e d by M e s o z o i c p l u t o n s . Metamorphic g r a d e i s v a r i a b l e b u t e s t i m a t e d p r e s s u r e s and t e m p e r a t u r e s above 400 MPa and 600°C a r e c h a r a c t e r i s t i c . A minimum-of f i v e p h a s e s o f d e f o r m a t i o n has been r e c o g n i z e d w i t h i n t he complex ( R o s s , 1974; Duncan and N i e l s e n , 1977). The f i r s t t h r e e phases a r e t i g h t - t o - i s o c l i n a l recumbent f o l d s . The s e c o n d and t h i r d phases have d o m i n a n t l y e a s t - w e s t t r e n d i n g f o l d axes and s h a l l o w l y d i p p i n g a x i a l s u r f a c e s ; t h e s e dominate l o c a l and r e g i o n a l o u t c r o p p a t t e r n s . The l a s t two phases o f d e f o r m a t i o n , which a r e b r o a d , open f l e x u r a l -s l i p f o l d s , p o s t d a t e most metamorphic r e c r y s t a l l i z a t i o n . I n t e r f e r e n c e o f t h e s e two f o l d s e t s l e d t o the c r e a t i o n o f b r o a d domes and b a s i n s ( R o s s , 1968; Ross, 1970). T h r e e o f t h e s e domes, T h o r - O d i n , C r a n b e r r y and Frenchman Cap ( F i g . 1 ) , a r e c o r e d by g r a n i t i c and g n e i s s i c r o c k s and have been termed " m a n t l e d g n e i s s domes" by R e e s o r and Moore ( 1 9 7 1 ) . R e c e n t 120 Rb-Sr whole r o c k d a t i n g s t u d i e s (Duncan, 1978b) s u g g e s t t h a t t he Thor-O d i n g r a n i t i c and g n e i s s i c r o c k s a r e P r e c a m b r i a n basement. M e t a s e d i -mentary g n e i s s e s g i v e an age o f 2 . 7 _+ 0.2 Ga whereas g r a n i t i c g n e i s s e s g i v e ages o f 2.0 t o 2.25 Ga ( t h i s t h e s i s , C h a p t e r 2 ) . The g n e i s s e s have been v a r i a b l y a f f e c t e d by h i g h - g r a d e metamorphism and m i g m a t i z a t i o n a t 750 Ma. The Shuswap complex can be d e f i n e d as t h e a r e a o f metamorphic r o c k s deformed by e a s t - w e s t t r e n d i n g Phase Two a n d / o r Phase T h r e e f o l d s (Duncan and N i e l s e n , 1977). Both o f t h e s e phases o f d e f o r m a t i o n p r e d a t e t h e O l i v e r G r a n i t e which i n t r u d e s t h e Shuswap complex's w e s t e r n boundary and has an Rb-Sr age o f 150 t o 160 Ma (Duncan ^ t a l _ . , 1979; A r m s t r o n g , w r i t t e n c o m m u n i c a t i o n , 1982). A t l e a s t one o f t h e s e e a s t - w e s t o r i e n t e d phases o f d e f o r m a t i o n a f f e c t s r o c k s as young as T r i a s s i c (Hyndman, 1968). To t h e west o f t h e Shuswap complex, t h e I n t e r m o n t a n e B e l t ( F i g . 1) c o m p r i s e s a l a t e P a l e o z o i c t o M e s o z o i c i s l a n d a r c - m a r g i n a l b a s i n - o c e a n i c t e r r a i n (Monger e t a j _ . , 1972). T h i s t e r r a i n o f l o w - g r a d e metamorphic r o c k s i s e v e r y w h e r e i n f a u l t c o n t a c t w i t h t h e Shuswap complex. The e a s t e r n m a r g i n o f t h e Shuswap ( F i g . 1) has f a u l t c o n t a c t s w i t h the p o l y p h a s e deformed, v a r i a b l y metamorphosed, Kootenay A r c - S e l k i r k t e r r a i n ( R o s s , 1970; Read and Wheeler, 1975; Read, 1977; Read, 1979; Brown and Read, 1979; Hoy, 1979). T h i s t e r r a i n c o m p r i s e s a w e l l - u n d e r s t o o d s t r a -t i g r a p h i c sequence r a n g i n g from P r o t e r o z o i c P u r c e l l - B e l t r o c k s t h r o u g h t h i c k upper P r o t e r o z o i c and P a l e o z o i c c l a s t i c s e q u e n c e s , t o T r i a s s i c - -J u r a s s i c v o l c a n i c s and f i n e - g r a i n e d e l a s t i c s (Read and Wheeler, 1975). The Kootenay Arc has been deformed by t h r e e phases o f c o - a x i a l d e f o r -m a t i o n t h a t have b r o a d l y n o r t h w e s t e r l y - t r e n d i n g f o l d axes i n the 121 n o r t h e r n p a r t o f t h e a r c . The t h i r d phase o f d e f o r m a t i o n o c c u r r e d p r i o r t o the i n t r u s i o n o f the N e l s o n B a t h o l i t h a t 150 Ma (Duncan and P a r r i s h , 1979; t h i s t h e s i s , C h a p t e r 4 ) . C o r r e l a t i o n o f s t r a t i g r a p h y between t h e Kootenay A r c and the Shuswap complex i s h i n d e r e d by h i g h metamorphic g r a d e , complex d e f o r -m a t i o n and s c a r c i t y o f f o s s i l s i n t h e Shuswap. R e e s o r and Moore (1971) and F y l e s (1970) s u g g e s t e d t h a t t h e q u a r t z i t e s , c a l c - s i l i c a t e s , m a r b l e s and s c h i s t s o v e r l y i n g t h e c o r e g n e i s s e s o f t h e T h o r - O d i n and Frenchman Cap g n e i s s domes a r e p o s s i b l e e q u i v a l e n t s o f a 1 i t h o l o g i c a l l y s i m i l a r s e quence o f r o c k s o f Cambrian age i n t h e Kootenay A r c . T h i s c o r r e l a t i o n has been s u p p o r t e d by more r e c e n t d e t a i l e d mapping by Hoy ( 1 9 7 9 ) . Such l i t h o l o g i c c o r r e l a t i o n s must, by n a t u r e , be t e n t a t i v e . On t h e b a s i s o f r e c o n n a i s s a n c e mapping and r e g i o n a l t e c t o n i c m o d e l i n g , Read ( 1 9 7 9 ) , Brown and P s u t k a ( 1 9 7 9 ) , Brown and Read (1979) and O k u l i t c h (1979) s u g g e s t e d t h a t t he r o c k s o v e r l y i n g t h e c o r e g n e i s s e s a r e p r o b a b l y o f P u r c e l l - B e l t age. They i n f e r t h a t t h e upper P r o t e r o z o i c and P a l e o z o i c s t r a t a have been i n p a r t t e c t o n i c a l l y removed by l o w - a n g l e normal f a u l t i n g t o form t h e Kootenay A r c - S e l k i r k t e r r a i n . GEOLOGY OF STRATABOUND SULFIDE DEPOSITS The g e o l o g y o f s t r a t a b o u n d s u l f i d e d e p o s i t s i n t h e Shuswap and K o o t e n a y A r c - S e l k i r k t e r r a i n s has been r e v i e w e d by F y l e s ( 1 9 6 6 ) , Muraro (1966) and Hoy (1977c, d; 1979). These s t u d i e s s u g g e s t t h a t f o u r t y p e s o f d e p o s i t s c an be d i s t i n g u i s h e d : 1. S a l mo Type - p y r i t e , s p h a l e r i t e and g a l e n a i n low-grade meta-morphosed and d o l o m i t i z e d Lower Cambrian l i m e s t o n e s . 122 2. M e t a l l i n e Type - p y r i t e , s p h a l e r i t e and g a l e n a i n z o nes p a r a l l e l t o b e d d i n g i n unmetamorphosed Lower Cambrian d o l o s t o n e . Numerous t r a n s g r e s s i v e f e a t u r e s o c c u r , i n c l u d i n g d i s s e m i n a t e d s u l f i d e s i n s e d i m e n t a r y b r e c c i a s and p i p e - l i k e b o d i e s a t h i g h a n g l e s t o b e d d i n g . 3. Shuswap Type - s p h a l e r i t e , p y r r h o t i t e , g a l e n a and p y r i t e i n t h i n m a s s i v e - s u l f i d e l a y e r s a n d / o r d i s s e m i n a t i o n s i n c a l c a r e o u s m e t a s e d i m e n t s , m a r b l e and b i o t i t e - s i l l i m a n i t e s c h i s t s . D e s p i t e a complex d e f o r m a t i o n h i s t o r y many o f the s u i f i d e - r i c h u n i t s can be t r a c e d f o r s e v e r a l k i l o m e t e r s . 4. G o l d s t r e a m Type - p y r r h o t i t e and c h a l c o p y r i t e w i t h v a r y i n g amounts o f s p h a l e r i t e i n t h i n , m a s s i v e - s u l f i d e l a y e r s c l o s e l y a s s o c i a t e d w i t h g r e e n s t o n e s and c h l o r i t i c m e t a s e d i m e n t s . The o r i g i n o f t h e Salmo and M e t a l l i n e t y p e d e p o s i t s was r e v i e w e d by M uraro ( 1 9 6 6 ) , McConnel and A n d e r s o n ( 1 9 6 8 ) , and MacDonald ( 1 9 7 3 ) . McConnel and A n d e r s o n (1968) s u g g e s t e d t h a t the l e a d i n the M e t a l l i n e d e p o s i t s may have been i n t r o d u c e d d u r i n g s e d i m e n t a t i o n and d i a g e n e s i s . A d d i e (1970) a l s o f a v o r e d s y n g e n e t i c d e p o s i t i o n o f p y r i t e , b u t w i t h much l a t e r i n t r o d u c t i o n o f t h e l e a d . MacDonald (1973) c o n c l u d e d t h a t a M i s s i s s i p p i V a l l e y t y p e model was a p p r o p r i a t e f o r t h e Salmo d e p o s i t s . Whishaw (1954) s u g g e s t e d t h a t t h e l e a d i n t h e Salmo d e p o s i t s was d e r i v e d f r o m o v e r l y i n g b l a c k a r g i l l i t e s . D e s p i t e t h e u n c e r t a i n t y o f o r i g i n , d e t a i l e d s t u d i e s o f the Salmo d e p o s i t s by Muraro (1966) and MacDonald (1973) d e m o n s t r a t e d t h a t the s u l f i d e s were emplaced p r i o r t o the f i r s t p hase o f d e f o r m a t i o n and metamorphism o f the Kootenay A r c . T h i s f i r s t p hase has been d a t e d by Rb-Sr t e c h n i q u e s a t a p p r o x i m a t e l y 430 Ma (Read 123 and Wheeler, 1975). A r m s t r o n g ( w r i t t e n c o m m u n i c a t i o n , 1982), however, s u g g e s t s t h a t t h i s d a t e i s b e s t i n t e r p r e t e d as a p r o v e n a n c e age. The h i g h metamorphic g r a d e and complex d e f o r m a t i o n t y p i c a l o f t h e Shuswap t y p e d e p o s i t s have o b s c u r e d many o f t h e i r p r i m a r y c h a r a c t e r i s t i c s . However, t h e s t r a t i f o r m n a t u r e o f the s u l f i d e l e n s e s and the o c c u r r e n c e o f s u l f i d e s i n t h e same s t r a t i g r a p h i c u n i t f o r up t o 10 km a l o n g s t r i k e (Duncan, 1978a; Hoy, 1977c) s u g g e s t t h e d e p o s i t s a r e s y n g e n e t i c . F y l e s (1966) s u g g e s t e d a s y n g e n e t i c o r i g i n f o r t h e Shuswap t y p e d e p o s i t s b u t r e f e r r e d t o p r e l i m i n a r y l e a d i s o t o p e e v i d e n c e which d i d n o t s u p p o r t such a model. The Wigwam and " J and L" d e p o s i t s , i n c l u d e d i n the Shuswap group i n t h i s s t u d y , o c c u r i n the n o r t h e r n Kootenay A r c and S e l k i r k M o u n t a i n s . The Wigwam has been p r e v i o u s l y g r o u p e d w i t h t h e Shuswap d e p o s i t s ( F y l e s , 1966; Muraro, 1966) and the "J and L" has a s i m i l a r g e o l o g y . As t h e Shuswap complex i s t h e p r o d u c t o f m i d d l e P a l e o z o i c t o M e s o z o i c e v e n t s (Duncan and N i e l s e n , 1977), i t i s n o t s u r p r i s i n g t h a t t h e e x t e n t o f t h e complex i s n o t d i r e c t l y r e l a t e d t o t h e o r i g i n o r d i s t r i b u t i o n o f t h e s e d e p o s i t s o f p r o b a b l e Cambrian age. The G o l d s t r e a m t y p e d e p o s i t s have been d e s c r i b e d b r i e f l y by Hoy (1 9 7 7 d ) . The l a r g e s t d e p o s i t o f t h i s t y p e , t h e " G o l d s t r e a m , " o c c u r s as an e x t e n s i v e m a s s i v e - s u l f i d e l a y e r i n c h l o r i t e - s e r i c i t e p h y l l i t e s , q u a r t z p h y l l i t e s and l i m e s t o n e a l o n g s t r i k e from a l e n s - s h a p e d u n i t o f g r e e n s t o n e . Hoy (1977d) s u g g e s t s t h a t t h e G o l d s t r e a m t y p e d e p o s i t s a r e s i m i l a r t o the B e s s h i d e p o s i t s i n J a p a n ( K a n e h i r a and T a t s u m i , 1970) which a r e g e n e t i c a l l y r e l a t e d to nearby o r e n c l o s i n g g r e e n s t o n e s . 124 LEAD ISOTOPE DATA E a r l i e r l e a d i s o t o p e d a t a from d e p o s i t s i n s o u t h e a s t e r n B r i t i s h C o l u m b i a were d i s c u s s e d by S i n c l a i r (1964, 1966), R e y n o l d s and S i n c l a i r (1971) and L e C o u t e u r (1973, 1979). R e y n o l d s and S i n c l a i r (1971) p r e -s e n t e d r e l i a b l e l e a d i s o t o p e d a t a f o r seven Salmo t y p e d e p o s i t s . L e C o u t e u r (1973) r e a n a l y z e d t h e s e samples t o g e t h e r w i t h f i v e Shuswap t y p e and f o u r M e t a l l i n e t y p e d e p o s i t s . As no l e a d i s o t o p e d a t a e x i s t e d f o r G o l d s t r e a m d e p o s i t s and o n l y l i m i t e d d a t a were a v a i l a b l e f o r t h e Shuswap t y p e , seven new Shuswap samples and f i v e G o l d s t r e a m samples were a n a l y z e d f o r t h i s s t u d y . A n a l y t i c a l Methods and P r e c i s i o n Powdered s u l f i d e samples were d i s s o l v e d i n 6M HCl + 6M HNO^. In t h e c a s e o f t h e Shuswap t y p e d e p o s i t s , g a l e n a c r y s t a l s were h a n d - p i c k e d from t h e o r e . The G o l d s t r e a m t y p e d e p o s i t s d i d n o t c o n t a i n g a l e n a i n suf-f i c i e n t q u a n t i t i e s t o s e p a r a t e by hand; however, min o r g a l e n a was p r e s -e n t i n a l l samples w i t h t h e e x c e p t i o n o f MOP, w h i c h was a p y r r h o t i t e . The f i l t r a t e was d r i e d , r e d i s s o l v e d i n HBr and added t o a n i o n exchange col u m n s . The s o l u t i o n was e v a p o r a t e d , t h e p r e c i p i t a t e t a k e n up i n d i l u t e HNO-j and added t o p l a t i n u m e l e c t r o d e e l e c t r o p l a t i n g b e a k e r s . L e a d was p l a t e d onto t h e anode u s i n g a p o t e n t i a l o f 1.9 v o l t s and removed u s i n g a d i l u t e H 2 0 2 + HN0 3 s o l u t i o n . A p p r o x i m a t e l y 1 yg o f t h e p l a t e d l e a d was l o a d e d onto a Re f i l a m e n t u s i n g t h e s t a n d a r d s i l i c a g e l -p h o s p h o r i c a c i d t e c h n i q u e . I s o t o p i c a n a l y s e s were made u s i n g a 1 2 - i n c h , 9 0 ° , s i n g l e - f o c u s i n g mass s p e c t r o m e t e r i n t e r f a c e d w i t h an I n t e r d a t a Model F o u r computer. 125 W i t h i n r u n , l a p r e c i s i o n s f o r i s o t o p i c r a t i o s were m a i n t a i n e d a t 0.1% o f t h e r a t i o v a l u e o r b e t t e r . The a v e r a g e r a t i o s o f t w e l v e d e t e r m i n a t i o n s o f "Broken H i l l No. 1" s t a n d a r d made d u r i n g t h e c o u r s e o f t h i s s t u d y a r e 2 0 6 / 2 0 4 p D = 15.987 (0.017%), 2 0 7 / 2 0 4 p b = 15.369 (0.024%), 2 0 8 / 2 0 4 P D = 35.575 (0.051%). By co m p a r i s o n w i t h t he a b s o l u t e r a t i o s o f Coo p e r £ t aj_. (1969), an appro-p r i a t e f r a c t i o n a t i o n c o r r e c t i o n was c a l c u l a t e d and a p p l i e d t o a l l sample a n a l y s e s . The a c c u r a c y o f i s o t o p e r a t i o s o f samples i s 0.15% o r b e t t e r , b a s e d on u s i n g t he e q u a t i o n °sample = " ^ w i t h i n r u n ^ + ( C TBroken H i l l S t a n d a r d ) 2 . The a n a l y s i s o f samples by gas s o u r c e mass s p e c t r o m e t r y was d e s c r i b e d by L e C o u t e u r (1973). R e s u l t s The r e s u l t s o f t h e s o l i d s o u r c e a n a l y s e s o b t a i n e d i n t h i s s t u d y a r e p r e s e n t e d i n T a b l e I, and t h e s e d a t a t o g e t h e r w i t h t h e gas s o u r c e d a t a . o f L e C o u t e u r (1973) a r e p l o t t e d i n F i g u r e s 2, 3 and 4. The r e s u l t s f o r 2 0 7 P b / 2 0 4 P b - 2 0 6 P b / 2 0 4 P b from Shuswap t y p e d e p o s i t s shown i n F i g u r e 2 d e m o n s t r a t e t h e e x c e l l e n t agreement between the s o l i d and gas s o u r c e d a t a . A l s o shown i n F i g u r e 2 i s t h e a v e r a g e o f t e n i s o t o p i c d e t e r m i n a t i o n s o f B i g Ledge l e a d u s i n g s o l i d s o u r c e t e c h n i q u e s by Cumming and G u d j u r g i s (1973). T h e i r d a t a a p p e a r t o have f r a c t i o n a t i o n e r r o r s an o r d e r o f magnitude l a r g e r than t h e i r q u o t e d p r e c i s i o n s . However, t he av e r a g e o f t h e i r v a l u e s i s i n good agreement w i t h t h e r e s u l t s i n T a b l e I. The G o l d s t r e a m t y p e d e p o s i t s ( F i g . 3) p l o t c o n s i s t e n t l y below t h e l i n e d e f i n e d by the Shuswap t y p e d a t a p o i n t s . A l t h o u g h t h i s d i f f e r e n c e i s n o t 126 TABLE I LEAD ISOTOPE DATA L o c a t i o n Sample No. 206/204 207/204 208/204 SHUSWAP TYPE DEPOSITS B i g Ledge BL-80 19.579(+ 0 . 1 ) * 15.851(+ 0.07) 40.101(+ 0.12) 1 J and L' JL-1 19.007(+ 0.2) 15.765(+ 0.08) 38.902(+ 0.23) JL- 2 19.039(+ 0.09) 15.778(+ 0.11) 39.007(+ 0.18) J L - 3 19.004(+ 0.12) 15.767(+ 0.14) 39.077(+ 0.1) J o r d a n R i v e r J - l 17.888(+ 0.06) 15.585(+ 0.06) 37.874(+ 0.05) •SL' SL-1 19.008(+ 0.04) 15.754(+ 0.16) 38.808(+ 0.3) * 1 a w i t h i n - r u n p r e c i s i o n s GOLDSTREAM TYPE DEPOSITS G o l d s t r e a m GL-0 18.867(+ 0.09) 15.699(+ 0.09) 38.645(+ 0.01) G o l d s t r e a m MOP 18.708(+ 0.12) 15.683(+ 0.16) 38.302(+ 0.14) 'ML' ML-1 18.904(+ 0.3) 15.703(+ 0.21) 38.947(+ 0.09) 'RY' RY-0 18.744(+ 0.08) 15.693(+ 0.08) 38.646(+ 0.07) •HW' HW-0 18.727(+ 0.13) 15.679(+ 0.11) 38.458(+ 0.16) 127 FIGURE 2. 2 0 7 P b / 2 0 4 P b - 2 0 6 P b / 2 0 4 P b d i a g r a m f o r Shuswap t y p e d e p o s i t s . The d a t a p l o t t e d a r e f r o m T a b l e One (•^•), w i t h t h e a d d i t i o n o f d a t a f o r Wigwam, C o t t o n B e l t , R i v e r J o r d a n , and Ruddock Creek fro m L e C o u t e u r ( 1 9 7 3 ) , p l o t t e d as (-j-). The s q u a r e p o i n t i s t h e a v e r a g e B i g Ledge v a l u e d e t e r m i n e d by Cumming and G u d j u r g i s ( 1 9 7 3 ) . The e r r o r b a r s r e p r e s e n t l a r e p r o d u c i b i l i t y f o r t h e d a t a i n T a b l e One and f o r t h e d a t a f r o m L e C o u t e u r ( 1 9 7 3 ) . A l s o shown i s t h e t w o - s t a g e l e a d e v o l u t i o n c u r v e o f S t a c e y and Kramers ( 1 9 7 5 ) . 8 Z I 129 FIGURE 3. t u / P b / t u * P b - t u u P b / t u ' t P b d i a g r a m f o r G o l d s t r e a m t y p e d e p o s i t s . Data p l o t t e d a r e f r o m T a b l e One. The r e g r e s s i o n l i n e i s f o r Shuswap t y p e d e p o s i t s , as shown i n F i g u r e 2. A l s o shown i s t h e t w o - s t a g e l e a d e v o l u t i o n c u r v e o f S t a c e y and Kramers ( 1 9 7 5 ) . o CD 18.0 18.5 19.0 19.5 20.0 20.5 P b 2 0 6 / P b 2 0 4 131 FIGURE 4. ^ ' P b / ^ P b - ^ P b / ^ P b d i a g r a m f o r Salmo t y p e d e p o s i t s . Salmo d a t a f r o m Le C o u t e u r (1973) a r e shown as ( \ ) , w i t h l a e r r o r b a r s . The d a t a o f R e y n o l d s and S i n c l a i r (1971) a r e shown as open c i r c l e s , and t h e d a t a p o i n t s f o r t h e M e t a l l i n e t y p e d e p o s i t s ( L e C o u t e u r , 1973) a r e shown as s o l i d c i r c l e s . The r e g r e s s i o n l i n e i s f o r t h e Salmo d a t a o f L e C o u t e u r ( 1 9 7 3 ) . A l s o shown i s the t w o - s t a g e l e a d e v o l u t i o n c u r v e o f S t a c e y and Kramers ( 1 9 7 5 ) . 1 3 3 s i g n i f i c a n t a t 2 a , t h e c o n s i s t e n c y o f t h e r e s u l t s s u g g e s t s t h a t t h e d i f -f e r e n c e i s r e a l . The d a t a f o r t h e Salmo and M e t a l l i n e d e p o s i t s from L e C o u t e u r ( 1 9 7 3 ) and R e y n o l d s and S i n c l a i r ( 1 9 7 1 ) a r e p l o t t e d i n F i g u r e 4 . Because o f t h e i r h i g h e r p r e c i s i o n , o n l y t h e d a t a p o i n t s from L e C o u t e u r ' s s t u d y were use d i n the s u b s e q u e n t a n a l y s e s . The r e g r e s s i o n l i n e shown i s f o r t h e Salmo d a t a , w h i c h were r e g r e s s e d s e p a r a t e l y from t h e M e t a l l i n e d a t a b e c a u s e o f t h e i r g e o g r a p h i c s e p a r a t i o n and t h e p r e v i o u s l y n o t e d d i f f e r e n -c e s i n g e o l o g i c s e t t i n g . I n t e r p r e t a t i o n The i n t e r p r e t a t i o n o f anomalous o r m u l t i s t a g e l e a d s has r e c e n t l y been d i s c u s s e d by R u s s e l l ( 1 9 7 2 ) , G a l e and M u s s e t t ( 1 9 7 3 ) , Doe and S t a c e y ( 1 9 7 4 ) , S t a c e y and Kramers ( 1 9 7 5 ) , F a u r e ( 1 9 7 7 ) , and Doe and Zartman ( 1 9 7 9 ) . F o r t h e p u r p o s e s o f t h e f o l l o w i n g a n a l y s i s , the i n t e r p r e t a t i o n o f t h e growth c u r v e as g i v e n by S t a c e y and Kramers has been a c c e p t e d . The l e a d i s o t o p e r a t i o s o f a l l groups o f d e p o s i t s d i s c u s s e d h e r e i n c a n be c o n s i d e r e d anomalous ( R u s s e l l and F a r q u h a r , 1 9 6 0 ) . A l t h o u g h i s o -t o p i c c o m p o s i t i o n w i t h i n an i n d i v i d u a l d e p o s i t i s c o n s t a n t ( e . g . , r e s u l t s f r o m "J and L" and B i g Ledge i n T a b l e I ) , t h e d a t a p l o t a l o n g l i n e a r t r e n d s w i t h most d e p o s i t s h a v i n g s i n g l e - s t a g e model ages f a r y o u n g e r than t h e i r age o f f o r m a t i o n . The s i m p l e s t t h e o r y f o r i n t e r p r e t i n g such p a t -t e r n s i s t h e t w o - s t a g e model which has been w i d e l y a p p l i e d to i n t e r p r e t i n g common l e a d s y s t e m a t i c s (Doe and S t a c e y , 1 9 7 4 ) . The two-s t a g e model assumes t h a t a t t h e end o f t h e f i r s t s t a g e ( t ^ ) the system i n q u e s t i o n i s c h a r a c t e r i z e d by u n i f o r m v a l u e s f o r 2 0 7 P b / 2 0 4 P b , 2 0 6 P b / 2 0 4 P b 134 and t u o p b / T b . T h i s would be t h e c a s e i f t h e l e a d had a s i n g l e - s t a g e d e v e l o p m e n t u n t i l t h i s p o i n t i n time ( t ) , b u t t h i s i s n e i t h e r n e c e s s a r y n o r l i k e l y . A t t h e time o f t h e t e r m i n a t i o n o f t h e f i r s t s t a g e ( t ^ ) , an e v e n t o c c u r s w h i c h c r e a t e s subsystems w i t h v a r y i n g U/Pb o r p v a l u e s . D e p e n ding on t h e y v a l u e o f an i n d i v i d u a l s u b s y s t e m , t h e l e a d i s o t o p e r a t i o s e v o l v e u n t i l t h e age o f m i n e r a l i z a t i o n ( t 2 ) , when t h e l e a d i s removed i n t o a z e r o e n v i r o n m e n t , i . e . , g a l e n a . The e v o l u t i o n o f t h e i s o t o p e s from t o t f o r t h e i t h subsystem i s g i v e n by t h e e q u a t i o n s : [ 2 0 6 / 2 0 4 p b ] t 2 = r206/204 p b ] t i + ^ 1*1 . ^1*2, ( 1 ) [ 2 0 7 / 2 0 4 P b ] t 2 - [ 2 0 7 / 2 0 4 P b ] t l + P i/137.88 (e* 2* 1 - e*2*2) (2) [ 2 0 8 / 2 0 4 P b ] t 2 = [ 2 ° 8 / 2 0 4 P b ] t l + K i ( e A 3 t l - e* 3 ' 2) (3) where y = 2 3 8 U / 2 0 4 P b , K = 2 3 2 T h / 2 0 4 P b and t h e decay c o n s t a n t s a r e \1 ( 2 3 8 U ) = 0.155125 x 1 0 " 9 y r " 1 , \ 2 ( 2 3 5 U ) = 0.98485 x 1 0 " 9 y r " 1 and ,23? -Q _1 X 3 rocJh) = 0.049475 x 10 y r ( J a f f e y e t a l _ . , 1971; LeRoux and G l e n d e n i n , 1963). C o m b i n i n g E q u a t i o n s (1) and (2) g i v e s t h e e q u a t i o n o f the anomalous l e a d l i n e o r s e c o n d a r y i s o c h r o n : - 2 0 7 / 2 0 4 p b ] _ [ 2 0 7 / 2 0 4 p b ] A 2 t 2 J t 2 L itx i ( e e ) (4) r 2 0 6 / 2 0 4 n w l r 2 0 6 / 2 0 4 n k l 137.88 , Xj.tj. A i t 2 . t P b ] t 2 " [ P M t l ( e - e 1 z ) 135 E q u a t i o n (4) can be s o l v e d i t e r a t i v e l y f o r t o r i f one o f t h e t i m e s ( t ) i s known o r assumed. The u v a l u e a t t i o f t h e a p p a r e n t s i n g l e -s t a g e growth c u r v e can be c a l c u l a t e d from t h e e q u a t i o n : r206/204 D. i r206/204 D. -i ^ , x l t 0 x l t l , A , x l t l x l t 2 , [ P b ] t z = [ P b ] ^ + y Q ( e - e ) + M l ( e - e ) (5) where t i s t h e age o f t h e e a r t h ( o r 3.7 Ga f o r t h e S t a c e y and Kramers m o d e l ) , by s e t t i n g \iQ = L ^ . Both t and t can be c a l c u l a t e d i n d e p e n -d e n t l y i f a u f o r a p a r t i c u l a r s i n g l e - s t a g e growth c u r v e i s known o r c a n be assumed. L e a s t s q u a r e s l i n e a r r e g r e s s i o n ( Y o r k , 1969) o f t h e Shuswap d a t a i n F i g u r e 2 g i v e s a s l o p e o f 0.16231 +_ 0.00406 and an i n t e r c e p t o f 12.676 +_ 0.075. The goodness o f f i t s u p p o r t s , b u t does n o t p r o v e , t h e a p p l i c a b i l i t y o f t h e t w o - s t a g e model. I f t h e i n i t i a l s ystem up t o t i s a p p r o x i m a t e d by t h e S t a c e y and Kramers (1975) growth c u r v e (p = 9.74), t h e n a s o u r c e age ( t ^ ) o f 2.25 Ga, and a m i n e r a l i z a t i o n age ( t 2 ) o f 501 Ma c a n be c a l c u l a t e d . These d a t e s seem r e a s o n a b l e i n terms o f t h e g e o l o -g i c a l c o n s t r a i n t s . The r e s u l t s w i l l be d i s c u s s e d f u r t h e r i n the n e x t s e c t i o n . L i n e a r r e g r e s s i o n o f t h e Salmo d a t a g i v e s a s l o p e o f 0.0959 _+ 0.0102 and an i n t e r c e p t o f 13.9110 _+ 0.1955. The Salmo l i n e does n o t i n t e r s e c t t h e S t a c e y and Kramers growth c u r v e , a f a c t w h i c h may r e f l e c t a complex l e a d e v o l u t i o n . 136 The d i s t r i b u t i o n o f i s o t o p e r a t i o s i n F i g u r e 4 re s e m b l e s t h e p a t t e r n c a l c u l a t e d f o r t h e " f r e q u e n t l y mixed" m u l t i s t a g e l e a d s o f R u s s e l l e t a l . (1966). The " f r e q u e n t l y mixed" model assumes t h a t t h e l e a d i s t r a n s f e r r e d a t v a r i o u s t i m e s between e n v i r o n m e n t s o f d i f f e r i n g y . The l e a d i s o t o p e e v o l u t i o n o f s u c h a sys t e m can be e x p r e s s e d by m o d i f y i n g E q u a t i o n s (1) and (2) as f o l l o w s : [ 2 0 6 / 2 0 4 P b ] t = [ 2 0 6 / 2 0 4 P b ] t + I ^ . ( e ^ 1 ' - 1 " e V i ) (6) n 1 i=l [ 2 0 7 / 2 0 4 P b L = [ 2 0 7 / 2 0 4 P b ] t + I ^ ^ { e ^ 1 - e ^ 1 " ) (7) ^n  Z l 1=1 137.88 In u s i n g t h e s e e q u a t i o n s t o s i m u l a t e l e a d i s o t o p e e v o l u t i o n , t h e v a l u e s a n d / o r t h e i n t e r v a l between m i x i n g e v e n t s can be s e l e c t e d a t r a n -dom f r o m an assumed normal o r l o g normal d i s t r i b u t i o n . A v a r i e t y o f such s i m u l a t i o n s shows t h a t t h e r e s u l t s s c a t t e r a r o u n d a s t r a i g h t l i n e , t h e s l o p e o f w hich depends on t h e t i m e o f commencement o f t h e f i r s t s t a g e , and t h e end o f the l a s t s t a g e , t n ( R u s s e l l _et a l _ . , 1966). U t i l i z i n g t h i s model t h e Salmo d a t a have been s i m u l a t e d u s i n g the f o l l o w i n g p a r a m e t e r s : t . = 1.22 6a, t n = 0.5 Ga, l e n g t h o f n t h t i m e i n c r e m e n t = 0.10 + 0.02 (la) Ga, m =17+4 ( l a ) , and 2 0 6 P b / 2 0 4 P b and 207 204 Pb/ Pb v a l u e s a t tl e q u i v a l e n t t o t w o - s t a g e l e a d s w i t h a y ^ o f 9.98 (S t a c e y and Kramers, 1975). The r e s u l t s a r e shown i n F i g u r e 5. Such s i m u l a t i o n s a r e u s e f u l i n t h a t t h e y c o n f i r m t h a t a m u l t i p l e m i x i n g model i s f e a s i b l e and, more i m p o r t a n t l y , they i d e n t i f y t h e range o f e n v i r o n m e n t s n e c e s s a r y t o p r o d u c e t h e o b s e r v e d r a n g e s o f i s o t o p i c r a t i o s . 137 FIGURE 5. N u m e r i c a l s i m u l a t i o n o f S a l m o - M e t a l l i n e d a t a . " " P b / ^ P b -206p|=)y204p|;i d i a g r a m showing t h e r e s u l t s o f an a t t e m p t t o n u m e r i c a l l y s i m u l a t e t h e d a t a f o r t h e Salmo and M e t a l l i n e t y p e d e p o s i t s u s i n g a m u l t i p l e m i x i n g model. The g e n e r a l a p p r o a c h o u t l i n e d by R u s s e l l e t a l _ . (1966) was f o l l o w e d , t-j ( t h e b e g i n n i n g o f t h e f i r s t s t a g e ) was s e t a t 1.22 Ga, t ( t h e time o f o r e f o r m a t i o n ) was s e t a t 0.5 Ga, t h e time i n c r e m e n t n as 0.10 + 0 . 0 2 (1 a), and t h e as 17 +_ 4 (1 a). The a c t u a l n and u . f o r each i n c r e m e n t were c h o s e n randomly u s i n g a Monte C a r l o a p p r o a c h . 18.0 18.5 19.0 19.5 20.0 p b 2 0 6 / p b 2 0 4 20.5 CO 00 139 The r a t h e r h i g h s e c o n d s t a g e ps i n d i c a t e d by t h e Salmo d a t a c o u l d r e p r e -s e n t e n r i c h m e n t o f uranium i n l o c a l l a t e P r o t e r o z o i c s e d i m e n t s . An a l t e r -n a t i v e i n t e r p r e t a t i o n would be t h a t l e a d which was more r a d i o g e n i c t h a n a v e r a g e was p r e f e r e n t i a l l y s c a v e n g e d from t h e s e d i m e n t s d u r i n g f o r m a t i o n o f t h e d e p o s i t s i n Cambrian t i m e s . T h i s a p p e a r s t o be a c h a r a c t e r i s t i c o f many M i s s i s s i p p i V a l l e y t y p e d e p o s i t s . Much more d e t a i l e d i s o t o p i c and g e o c h e m i c a l s t u d i e s would be r e q u i r e d t o t e s t t h i s h y p o t h e s i s f o r t h e Salmo d e p o s i t s . The G o l d s t r e a m d a t a form an e l o n g a t e c l u s t e r ( F i g . 3) which s u g g e s t s no o b v i o u s i n t e r p r e t a t i o n i n terms o f t h e models o u t l i n e d above. These 207 204 d e p o s i t s do have c o n s i s t e n t l y l o w e r Pb/ Pb t h a n t h e Shuswap t y p e . DISCUSSION As w i t h most o t h e r l e a d i s o t o p e s t u d i e s , t h e d a t a p r e s e n t e d h e r e i n c a n n o t be i n t e r p r e t e d u n i q u e l y . However, by c o n s i d e r i n g r e l e v a n t g e o l o -g i c a l and g e o c h e m i c a l c o n s t r a i n t s t o g e t h e r w i t h d a t i n g by o t h e r geo-c h r o n o l o g i c a l t e c h n i q u e s , t h e i n t e r p r e t a t i o n s s u g g e s t e d c a n be r e f i n e d . Shuswap Type D e p o s i t s As t h e Shuswap t y p e d e p o s i t s a r e o f p r o b a b l e s y n g e n e t i c o r i g i n , t h e age o f m i n e r a l i z a t i o n s h o u l d be t h a t o f t h e h o s t r o c k s . As shown above, a s s u m i n g a r e a s o n a b l e n f o r the f i r s t s t a g e ( t t o t ^ , f o l l o w i n g t h e model o f S t a c e y and Kramers ( 1 9 7 5 ) , r e s u l t s i n a t age o f 501 Ma and a t o f 2.25 Ga. To t e s t t h e s e n s i t i v i t y o f t h i s c a l c u l a t i o n , a r a n g e o f t v a l u e s were us e d t o c a l c u l a t e d t . The r e s u l t s a r e g i v e n i n T a b l e I I . Note t h a t I 3 l o w e r P a l e o z o i c m i n e r a l i z a t i o n ages p r e d i c t t s between 2.20 t o 2.25 Ga. T h i s range i s i n good agreement w i t h Rb-Sr whole rock and U-Pb z i r c o n 140 TABLE II TWO-STAGE MODEL PARAMETERS FOR SHUSWAP TYPE DEPOSITS t 2 t i y * 400 Ma 2.303 Ga 9.845 501 Ma 2.252 Ga 9.74** 600 Ma 2.199 Ga 9.647 * Model u f o r f i r s t s t a g e u s i n g a p p r o a c h o f S t a c e y and Kramers (1975), ** ti and t 2 c a l c u l a t e d a s s u m i n g S t a c e y and Kramers' p r e f e r r e d v a l u e f o r f i r s t s t a g e u o f 9.74. 141 age d a t a from g r a n i t i c r o c k s i n u n d e r l y i n g basement t e r r a i n (Duncan, 1978b; A r m s t r o n g , u n p u b l i s h e d d a t a ; Wanless and R e e s o r , 1975). As g r a n i t i c r o c k s make up t h e b u l k o f t h e basement t e r r a i n p a r t i c u l a r l y i n t h e C r a n b e r r y and Frenchman Cap domes ( M u t t i , 1978; Wheeler, 1965), i t i s p o s s i b l e t h a t t h e l e a d i n t h e s e d e p o s i t s had i t s s o u r c e i n t h e basement t e r r a i n . I m p l i c i t i n a c c e p t i n g a t w o - s t a g e model f o r t h e s e d e p o s i t s i s t h e a s s u m p t i o n t h a t t h e l e a d i n each was e x t r a c t e d from a subsystem t h a t had n o t been g e o c h e m i c a l l y d i s t u r b e d between t and t^. The immediate s o u r c e o f m e t a l s i s p r o b a b l y t h e f i n e - g r a i n e d m e t a s e d i m e n t s i n t h e Cambrian s t r a t a , t h e m e t a l s b e i n g e x t r a c t e d and t r a n s p o r t e d by b r i n e s o r f o r m a t i o n w a t e r s ( J a c k s o n and B e a l e s , 1967; Solomon, 1976). T h u s , t h e c u r r e n t l y a c c e p t e d g e n e t i c model f o r s t r a t a b o u n d s e d i m e n t a r y s u l f i d e d e p o s i t s (Solomon, 1976) i s n o t s t r i c t l y c o m p a t i b l e w i t h the two-s t a g e l e a d model. N u m e r i c a l c a l c u l a t i o n s have been made which t a k e i n t o a c c o u n t l o c a l m i x i n g d u r i n g Cambrian e r o s i o n and s e d i m e n t a t i o n . I f t h e p v a l u e s i n t h e s e d i m e n t a r e s i m i l a r t o t h o s e i n t h e basement s o u r c e r o c k s ( e . g . , +_ 2 ) , t h e n t h e d e v i a t i o n from a t w o - s t a g e e v o l u t i o n i s n e g l i g i b l e . Salmo Type D e p o s i t s S i n c l a i r (1966) p r o p o s e d a m o d i f i e d v e r s i o n o f t h e t w o - s t a g e model t o e x p l a i n l e a d i s o t o p e s y s t e m a t i c s from d e p o s i t s i n s o u t h e a s t e r n B r i t i s h C o l u m b i a . He c o n c l u d e d t h a t S u l l i v a n o r e l e a d s were s t a t i s t i c a l l y i n d i s t i n g u i s h a b l e from l e a d s p l o t t i n g on h i s Kootenay A r c anomalous l e a d l i n e ( i n c l u d i n g Salmo and Shuswap d e p o s i t s ) . He s u g g e s t e d t h a t Kootenay A r c l e a d s were formed by m i x i n g o f S u l l i v a n t y p e l e a d and a r a d i o g e n i c component e v o l v e d i n a 1.7 Ga l o w e r P u r c e l l basement. 1.7 Ga i s a two-142 s t a g e t b a s e d on a m i n e r a l i z a t i o n age o f 170 Ma. S i n c l a i r s u g g e s t e d t h a t m i x i n g o f t h e two l e a d s took p l a c e by l o c a l m e l t i n g and e v e n t u a l p r o d u c t i o n o f h y d r o t h e r m a l f l u i d s . R e y n o l d s and S i n c l a i r (1971) r e f i n e d t h i s model, s u g g e s t i n g t h a t a r a d i o g e n i c component d e r i v e d from 1.53 Ga u p p e r c r u s t a l r o c k s was mixed w i t h l a r g e r amounts o f S u l l i v a n t y p e l e a d 150 Ma ago. L e C o u t e u r (1973) f i r s t p o i n t e d o u t t h a t t h e S u l l i v a n t y p e l e a d s do n o t p l o t on t h e same r e g r e s s i o n l i n e as t h e Salmo and Shuswap t y p e s . H i s c o n c l u s i o n has been f u r t h e r s u p p o r t e d by t h e a n a l y s i s p r e s e n t e d i n t h i s s t u d y . A p p l i c a t i o n o f t h e t w o - s t a g e model t o t h e Salmo d a t a s u g g e s t s a s o u r c e age f o r t h e l e a d o f 1.17 t o 1.21 Ga i f t h e d e p o s i t s a r e s y n g e n e t i c o r d i a g e n e t i c i n o r i g i n , and t h e r e f o r e Cambrian i n age. T h i s range o f s o u r c e ages i s s i m i l a r t o Rb-Sr whole r o c k ages f o r upper P u r c e l l - B e l t s e d i m e n t s , a l t h o u g h t h e s e d a t e s a r e y o u n g e r t h a n the p r o b a b l e age o f t h e s e s e d i m e n t s (Ryan, 1973; O b r a d o v i c h and Peterman, 1968). I f t h e Salmo d e p o s i t s have a l o w e r P a l e o z o i c M i s s i s s i p p i V a l l e y t y p e o r i g i n as s u g g e s t e d by MacDonald ( 1 9 7 3 ) , t h e n t h e age o f m i n e r a l i z a t i o n c o u l d be 100 Ma o r so y o u n g e r and t h e basement s o u r c e age m a r g i n a l l y o l d e r . A p p l i c a t i o n o f t h i s m u l t i p l e m i x i n g model t o t h e s e d e p o s i t s i s more a p p r o p r i a t e t o s i m u l a t i o n o f the d a t a and a l s o t o the s t r a t i g r a p h i c con-t e x t o f t h e d e p o s i t s . However, i f v a r i a t i o n s i n u were random then the c a l c u l a t e d basement s o u r c e ages a r e i d e n t i c a l ( R u s s e l l ^ t a l _ . , 1966). G o l d s t r e a m Type D e p o s i t s The o r i g i n o f B e s s h i t y p e d e p o s i t s has been d i s c u s s e d by K a n e h i r a and T a t s u m i (1970) and i n a g e n e r a l sense by Solomon ( 1 9 7 6 ) . The a s s o c i a t i o n 143 o f a l t e r e d b a s i c v o l c a n i c s and f i n e - g r a i n e d c l a s t i c s e d i m e n t s w i t h C u - Z n - F e - r i c h m a s s i v e s u l f i d e s i s c o n s i s t e n t w i t h t h e g e n e t i c model p r o -p o s e d by Solomon (1976) f o r v o l c a n o g e n i c d e p o s i t s . H y d r o t h e r m a l c i r -c u l a t i o n systems s e t up by t h e emplacement o f h o t v o l c a n i c r o c k s i n t o w a t e r - s a t u r a t e d s e d i m e n t s l e a c h m e t a l s from t h e c o u n t r y r o c k s . The o b s e r v e d l e a d i s o t o p e c h a r a c t e r i s t i c s o f t h e G o l d s t r e a m d e p o s i t s can be e x p l a i n e d i n t h e c o n t e x t o f t h i s g e n e t i c model. B a s a l t i c r o c k s c h a r a c -t e r i s t i c a l l y have l o w e r Pb/ Pb t h a n a v e r a g e c r u s t a l r o c k s ( t h e Pb anomaly o f R u s s e l l , 1972) r e f l e c t i n g t h e low a v e r a g e y v a l u e s o f t h e m a n t l e (Doe and Zartman, 1979). Assuming G o l d s t r e a m t y p e l e a d s t o be a m i x t u r e o f a b a s a l t i c l e a d component (tlaken from Doe and Zartman 1 s model m a n t l e growth c u r v e , 1979) and l o c a l s e d i m e n t a r y l e a d ( t a k e n t o be t h e same as l e a d from t h e "J and L" d e p o s i t s ) , t h e n t h e r a t i o o f s e d i m e n t a r y t o b a s a l t i c l e a d can be c a l c u l a t e d as 1.6 t o 1. T h i s r a t i o seems r e a s o -n a b l e c o n s i d e r i n g t h e r e l a t i v e abundance o f t h e two r o c k t y p e s i n t h e immediate a r e a a r o u n d t h e d e p o s i t . I m p l i c a t i o n s f o r Sediment S o u r c e s and P a l e o g e o g r a p h y A l t h o u g h t h e l e a d i s o t o p e d a t a p r e s e n t e d i n t h i s p a per a r e n o t sub-j e c t t o u n i q u e i n t e r p r e t a t i o n s , t h e most l i k e l y models i n v o l v e e x t r a c t i o n o f s e d i m e n t a r y l e a d . Thus, t h e d i f f e r e n c e s between t h e i s o t o p i c p a t t e r n s o f Shuswap and Salmo t y p e d e p o s i t s a r e i n h e r i t e d from t h e i r s e d i m e n t a r y h o s t s . The l e a d i s o t o p e models d i s c u s s e d i m p l y the e x i s t e n c e o f two d i s t i n c t s e d i m e n t s o u r c e s . To t h e west, i n and near t h e Shuswap t e r r a i n , t h e Cambrian s e d i m e n t s have been d e r i v e d from t h e u n d e r l y i n g 2.0 t o 2.25 Ga P r e c a m b r i a n basement such as i s e x p o s e d i n t h e T h o r - O d i n and Frenchman 144 Cap g n e i s s domes. To t h e e a s t i n t h e K o o t e n a y A r c t h e s e d i m e n t s have been d e r i v e d u l t i m a t e l y from P u r c e l l basement. The s t r a t a c o n t a i n i n g t h e Shuswap t y p e d e p o s i t s u n c o n f o r m a b l y o v e r l i e t h e P r e c a m b r i a n basement r o c k s o f t h e Shuswap g n e i s s domes. The b a s a l u n i t i s t y p i c a l l y a q u a r t z i t e , a l t h o u g h c o n g l o m e r a t e s have been r e p o r t e d by F y l e s (1970) and M u t t i ( 1 9 7 8 ) . The l e a d i s o t o p e d a t a p r e -s e n t e d i n t h i s p a per s u g g e s t a Cambrian age f o r t h e s e s t r a t a , as had been p r e v i o u s l y s u g g e s t e d by R e e s o r and Moore (1971) and Hoy (1977c) on t h e b a s i s o f l i t h o l o g i c a l c o r r e l a t i o n . SUMMARY The l e a d i s o t o p e c h a r a c t e r i s t i c s o f t h e Shuswap, Salmo and G o l d s t r e a m t y p e s o f d e p o s i t s a r e d i s t i n c t i v e . The Shuswap d a t a p r o v i d e an e x c e l l e n t g r a p h i c a l f i t t o a t w o - s t a g e model w i t h a t o f 2.25 Ga and a t o f 500 Ma. 1 2 The v a l i d i t y o f t h i s t w o - s t a g e i n t e r p r e t a t i o n i s s u p p o r t e d by i n d e p e n d e n t g e o l o g i c a l c o r r e l a t i o n s , and t h e o c c u r r e n c e o f t h e d e p o s i t s i n r o c k s p r e -sumably d e r i v e d from a g n e i s s i c basement t e r r a i n d ominated by 2.0 t o 2.25 Ga g r a n i t e s . The degree o f s c a t t e r shown by t h e Sal mo-Metal l i n e d a t a s u p p o r t s a m u l t i p l e m i x i n g model f o r t h e s e d a t a . The age o f m i n e r a l i z a t i o n can be g e o l o g i c a l l y c o n s t r a i n e d as 500 +_ 100 Ma, g i v i n g a t w o - s t a g e t ^ age o f 1.21 _+ 0.07 Ga. S t r a t i g r a p h i c d a t a a r e c o n s i s t e n t w i t h the d e r i v a t i o n o f t h e Lower Cambrian s t r a t a i n t h e Salmo a r e a by e r o s i o n and r e w o r k i n g o f t h e upper P u r c e l l d u r i n g l a t e P r o t e r o z o i c and E a r l y Cambrian t i m e s . The G o l d s t r e a m d a t a s u p p o r t a v o l c a n o g e n i c o r i g i n f o r t h e s e d e p o s i t s , w i t h m i x i n g o f v o l c a n i c and s e d i m e n t a r y l e a d by h y d r o t h e r m a l c o n v e c t i o n 145 c e l l s . T h i s e x p l a i n s t h e l e s s u r a n o g e n i c c h a r a c t e r o f t h e i r l e a d s com-p a r e d w i t h t h e Shuswap t y p e d e p o s i t s d e s p i t e t h e g e o g r a p h i c p r o x i m i t y o f t h e two i n h o m o t a x i a l Cambrian s e d i m e n t s . The l e a d i s o t o p e d a t a s t r o n g l y s u p p o r t t h e e x i s t e n c e o f an e x t e n s i v e P r e c a m b r i a n basement t e r r a i n w h ich was a s i g n i f i c a n t w e s t e r l y s e d i m e n t s o u r c e d u r i n g t h e l a t e P r o t e r o z o i c and Cambrian. CHAPTER 4 GEOCHRONOLOGY AND S r ISOTOPE GEOCHEMISTRY OF THE NELSON BATHOLITH A POST-TECTONIC INTRUSIVE COMPLEX IN SOUTHEAST BRITISH COLUMBIA 147 INTRODUCTION The N e l s o n B a t h o l i t h i s a l a r g e , c o m p o s i t e b a t h o l i t h o f g r a n o d i o r i t e t o m o n z o n i t e c o m p o s i t i o n i n s o u t h e a s t B r i t i s h C o l u m b i a ( F i g . 1 ) . The age o f t h e b a t h o l i t h i s s i g n i f i c a n t as i t i n t r u d e s b o t h t h e Shuswap metamorphic complex and t h e Kootenay A r c , and i s p o s t - t e c t o n i c t o the l a s t p hases o f d u c t i l e d e f o r m a t i o n i n t h e s e t e r r a i n s . P r e v i o u s a t t e m p t s t o d a t e t h e b a t h o l i t h u s i n g the K-Ar method have r e s u l t e d i n d a t e s r a n g i n g from 49-171 Ma. The o l d e s t o f t h e s e d a t e s was somewhat c o n t r a d i c t o r y t o g e o l o g i c a l mapping by L i t t l e ( 1 9 6 0 ) , who s u g g e s t e d t h a t t h e N e l s o n B a t h o l i t h i n t r u d e s deformed r o c k s as young as B a j o c i a n . T h i s r e l a t i o n s h i p , however, p r o v i d e d t h e p o s s i b i l i t y o f p l a c i n g a t i g h t c o n s t r a i n t on t h e t i m i n g o f t h e l a s t phase o f d u c t i l e d e f o r m a t i o n . T h i s study was i n i t i a t e d t o r e s o l v e t h e a m b i g u i t i e s e x i s t i n g i n t h e p u b l i s h e d g e o c h r o n o l o g i c a l r e s u l t s and t o d e t e r m i n e the time span o v e r w h i c h t h e b a t h o l i t h was i n t r u d e d . To a c h i e v e t h i s o b j e c t i v e two s u i t e s o f samples were c o l l e c t e d from d i s t i n c t i v e p h a s e s o f t h e b a t h o l i t h . T h e s e were a n a l y z e d u s i n g the Rb-Sr, f i s s i o n t r a c k and K-Ar d a t i n g t e c h -n i q u e s . T h i s c h a p t e r summarizes t h e g e o l o g y o f t h e b a t h o l i t h , r e v i e w s t h e p r e v i o u s g e o c h r o n o l o g i c a l r e s u l t s and p r e s e n t s new d a t a on the age o f t h e b a t h o l i t h , i t s u p l i f t and c o o l i n g h i s t o r y . These d a t a a r e used to p l a c e c o n s t r a i n t s on models f o r t h e t e c t o n i c e v o l u t i o n o f the Shuswap complex. 148 FIGURE 1. Summary g e o l o g i c map o f s o u t h e a s t e r n B r i t i s h Columbia showing s e t t i n g o f t h e N e l s o n B a t h o l i t h . M a j o r t e c t o n i c d i v i s i o n s shown a r e : t h e Rocky M o u n t a i n t h r u s t b e l t , t h e Puree!1 a n t i c l i n o r i u m , t h e Kootenay A r c and t h e Shuswap metamorphic complex. A l s o shown a r e t h e l o c a t i o n s o f : t h e Frenchman Cap ( F C ) , T h o r - O d i n ( T O ) , Mai t o n (M) and V a l h a l l a (VA) g n e i s s domes; t h e N e l s o n ( N e i ) and t h e Kuskanex (Kus) i n t r u s i v e c o mplexes; and r o c k u n i t s o f P u r c e l l - B e l t ( P - B ) , Windemere (W), l o w e r P a l e o z o i c (HP) and upper P a l e o z o i c - T r i a s s i c - J u r a s s i c age (uP-"fc- J ) . \ 149 150 T e c t o n i c and G e o l o g i c S e t t i n g The t e c t o n i c s e t t i n g o f t h e N e l s o n B a t h o l i t h i s shown i n F i g u r e 1. As o r i g i n a l l y d e f i n e d by L i t t l e (1960) t h e N e l s o n B a t h o l i t h i n c l u d e d a l a r g e , h e t e r o g e n e o u s a r e a o f g r a n i t i c and g n e i s s i c r o c k s between S l o c a n L a k e and Okanagan La k e . G a b r i e l s e and R e e s o r (1964) r e s t r i c t t h e name t o r e f e r t o t h e l a r g e l y p o s t - t e c t o n i c p l u t o n i c r o c k s , d o m i n a n t l y g r a n o d i o r i t e and m o n z o n i t e , between S l o c a n Lake and K o o t e n a y Lake. The N e l s o n B a t h o l i t h i s bounded on the e a s t by t h e Kootenay A r c ( C r o s b y , 1968; F y l e s , 1967; Read, 1977), a t e r r a i n o f g e n e r a l l y low metamorphic g r a d e . On t h e west t h e V a l h a l l a Complex ( F i g . 1) i s an a r e a o f p o l y p h a s e deformed g n e i s s e s , o f u n c e r t a i n age, which c o m p r i s e s p a r t o f t h e Shuswap complex ( R e e s o r , 1965). The N e l s o n B a t h o l i t h i s one o f s e v e r a l l a r g e c o m p o s i t e b a t h o l i t h s o f M e s o z o i c age i n s o u t h e a s t e r n B r i t i s h C o l u m b i a which appear t o be p o s t -t e c t o n i c ( G a b r i e l s e and R e e s o r , 1974). The t e c t o n i c s e t t i n g o f the N e l s o n B a t h o l i t h has been d i s c u s s e d by G a b r i e l s e and R e e s o r ( 1 9 7 4 ) . To t h e n o r t h , i n the S e l k i r k M o u n t a i n s , t h e boundary between t h e Shuswap complex and t h e l o w e r - g r a d e r o c k s o f t h e n o r t h e r n e x t e n s i o n o f t h e Kootenay A r c (Brown, 1978) i s d e v o i d o f l a r g e p l u t o n s (Wheeler ^ t a j _ . , 1972). G a b r i e l s e and R e e s o r (1974) s u g g e s t t h a t t h e p r e s e n c e o f t h e N e l s o n and Kuskanax B a t h o l i t h s i n the c e n t r a l s e c t i o n o f t h e Kootenay A r c has e x t e n s i v e l y m o d i f i e d t h e s t r u c t u r a l p a t t e r n s i n t h i s p a r t o f t h e f o l d b e l t . I t can be seen i n F i g u r e 1 t h a t t h e p o s i t i o n and shape o f t h e N e l s o n B a t h o l i t h i s such t h a t i t d e f i n e s , i n p a r t , the a r c u a t e shape o f the c e n t r a l and s o u t h e r n Kootenay A r c . R e g i o n a l mapping ( C r o s b y , 1968; F y l e s , 1967) shows t h a t f o l d axes i n the a r c t r e n d r o u g h l y 151 p a r a l l e l t o t h e e a s t m a r g i n o f the b a t h o l i t h and swing a r o u n d t o an e a s t -w e s t o r i e n t a t i o n i n t h e S l o c a n S y n c l i n e t o t h e n o r t h o f the b a t h o l i t h (Ross and K e l l e r h a l s , 1968; Read and Wheeler, 1975). Geology o f t h e B a t h o l i t h The c o n t a c t r e l a t i o n s o f t h e b a t h o l i t h a r e v a r i a b l e and o f t e n complex. On t h e n o r t h e r n m a r g i n t h e c o n t a c t s a r e c l e a r l y i n t r u s i v e ( S i n c l a i r and L i b b y , 1969) and s u p e r i m p o s e a l o w - p r e s s u r e c o n t a c t meta-morphism on t h e r e g i o n a l l y metamorphosed c o u n t r y r o c k s ( C h i l d s , 1968). On t h e b a s i s o f t h e o c c u r r e n c e o f a n d a l u s i t e - m u s c o v i t e - q u a r t z i n t h e c o n t a c t a u r e o l e o f t h e Mt. C a r l y l e S t o c k , ' a s a t e l l i t e s t o c k t o t h e n o r t h o f t h e main N e l s o n B a t h o l i t h , C h i l d s e s t i m a t e d a c o n t a c t meta-m o r p h i c p r e s s u r e o f 300-500 MPa ( o r 15 t o 25 km d e p t h o f b u r i a l ) . On i t s west s i d e , t h e b a t h o l i t h i s i n f a u l t c o n t a c t w i t h m y l o n i -t i z e d g r a n i t i c r o c k s and g n e i s s e s o f t h e e a s t e r n m a r g i n o f t h e V a l h a l l a G n e i s s Dome ( R e e s o r , 1965; Ross and K e l l e r h a l s , 1968). However, t h e o r i g i n a l n a t u r e o f t h i s c o n t a c t i s n o t known. A l o n g t h e e a s t e r n m a r g i n o f t h e b a t h o l i t h t h e r e i s a b o r d e r zone o f c r u s h e d p o r p h y r i t i c g r a n i t e ( C a i r n e s , 1934; C r o s b y , 1968). The c o n t a c t metamorphic a u r e o l e , as shown by F y l e s ( 1 9 6 7 ) , i s o n l y a b o u t t e n m e t e r s w ide a d j a c e n t t o t h e c r u s h e d zone. T h i s i s i n c o n t r a s t t o t h e c o n t a c t zone f u r t h e r n o r t h where t h e zone o f c o n t a c t metamorphism e x t e n d s up t o 1 km. In t h e s o u t h , t h e e l o n g a t e t a i l o f t h e b a t h o l i t h ( F i g . 1) i s b r o a d l y c o n f o r m a b l e w i t h r e g i o n a l t r e n d s . G a b r i e l s e and Reesor (1964) have d e s c r i b e d a s t r o n g r e c r y s t a l 1 i z e d f a b r i c i n t h i s s o u t h e r n a r e a o f 152 t h e b a t h o l i t h , which shows an i n t e n s e d e v e l o p m e n t o f l i n e a t i o n p a r a l l e l t o f o l d axes i n t h e c o u n t r y r o c k . T h i s s u g g e s t s t h a t t h e s o u t h e r n t a i l a r e a may be an e a r l i e r syn- o r p r e - t e c t o n i c phase and s h o u l d be con-s i d e r e d a s e p a r a t e e n t i t y . With t h e e x c e p t i o n o f t h i s s o u t h e r n a r e a , s t r u c t u r a l mapping by C r o s b y (1968) and F y l e s (1967) s u g g e s t s t h a t t h e N e l s o n B a t h o l i t h i s y o u n g e r t h a n a l l t h r e e phases o f d u c t i l e d e f o r m a t i o n i n t he K o o t e n a y A r c ( c f . Read, 1977). Due i n p a r t t o t h e r u g g e d n a t u r e o f t h e t e r r a i n , no d e t a i l e d i n f o r -m a t i o n i s a v a i l a b l e on t h e s p a t i a l v a r i a t i o n o f l i t h o l o g y w i t h i n the b a t h o l i t h . A v a i l a b l e p e t r o g r a p h i c d a t a ( G a b r i e l s e and R e e s o r , 1964; C r o s b y , 1968; S i n c l a i r and L i b b y , 1969) and o b s e r v a t i o n s made d u r i n g t h i s s t u d y s u g g e s t t h a t t h e dominant roc k t y p e i s a m a s s i v e , p o r p h y r i -t i c , h o r n b l e n d e - b i o t i t e g r a n o d i o r i t e . A few p e r c e n t o f l a r g e pheno-c r y s t s o f m i c r o c l i n e (up t o 10 cm i n l e n g t h ) g i v e t h i s rock a d i s t i n c -t i v e a p p e a r a n c e . N o n - p o r p h y r i t i c g r a n o d i o r i t e , m o n z o n i t e s and d i o r i t e s o c c u r , a p p a r e n t l y as s e p a r a t e p l u t o n s . These phases a r e more common i n t h e s o u t h and s o u t h w e s t p a r t s o f t h e b a t h o l i t h . G a b r i e l s e and R e e s o r (1974) s u g g e s t t h a t l e u c o - q u a r t z m o n z o n i t e w i t h o n l y a few p e r c e n t b i o t i t e a nd/or m u s c o v i t e c u t s t h e main p o r p h y r i t i c g r a n o d i o r i t e . T h i s may r e p r e s e n t a l a t e r phase o f i n t r u s i v e a c t i v i t y ( G a b r i e l s e and R e e s o r , 1974). In summary, t h e N e l s o n B a t h o l i t h i s a c o m p o s i t e , p o s t - t e c t o n i c i n t r u s i o n o f d o m i n a n t l y g r a n o d i o r i t e c o m p o s i t i o n . I t i s n o t c l e a r t h a t a l l o f t h e c o m p o s i t i o n a l v a r i a n t s o b s e r v e d r e p r e s e n t d i s t i n c t i v e i n t r u -s i v e p h a s e s . 153 P r e v i o u s G e o c h r o n o l o g y P r e v i o u s a t t e m p t s t o d a t e t h e N e l s o n B a t h o l i t h have m o s t l y r e l i e d on t h e K-Ar method on h o r n b l e n d e and b i o t i t e s e p a r a t e s . The r e s u l t s o f G e o l o g i c a l S u r vey o f Canada d e t e r m i n a t i o n s p r i o r t o 1964 have been d i s -c u s s e d by G a b r i e l s e and R e e s o r ( 1 9 6 4 ) . El even K-Ar b i o t i t e d a t e s a v a i l a b l e t o t h e s e a u t h o r s r a n g e d from 171 Ma ( f r o m a h o r n b l e n d e - b i o t i t e g r a n o d i o r i t e from t h e c e n t r a l a r e a o f t h e b a t h o l i t h ) t o 49 Ma ( f r o m a g r a n o d i o r i t e from t h e s o u t h e r n h a l f o f t h e b a t h o l i t h ) . T hese d a t a posed two problems t o G a b r i e l s e and R e e s o r . F i r s t , t h e o l d e r ages were incom-p a t i b l e w i t h t h e i n t r u s i o n o f t h e b a t h o l i t h i n t o Lower J u r a s s i c s e d i -ments and K u l p ' s (1961) t i m e s c a l e . Second, t h e range o f ages a p p e a r e d t o r e p r e s e n t t o them a p r o t r a c t e d emplacement e v e n t . T h i s was i n c o n t r a s t w i t h most o t h e r C o r d i l l e r a n i n t r u s i v e s . G a b r i e l s e and R e e s o r (1964) s u g g e s t e d a t h r e e - s t a g e model f o r t h e emplacement o f t h e b a t h o l i t h . E a r l y c o n s o l i d a t i o n a t d e p t h , b e f o r e 171 Ma, was s u g g e s t e d t o be f o l l o w e d by r e i n t r u s i o n o f a r e l a t i v e l y p a s s i v e mass a t a p p r o x i m a t e l y 130 Ma. T h i s , i n t u r n , was f o l l o w e d by f u r t h e r emplacement o f m o n z o n i t e i n and a r o u n d t h e f r i n g e o f the b a t h o l i t h . A l t h o u g h r e c a l c u l a t i o n o f t h e s e ages u s i n g t h e c u r r e n t l y a c c e p t e d decay c o n s t a n t s would i n c r e a s e them by a p p r o x i m a t e l y 2.4%, use o f t h e new r e v i s e d t ime s c a l e ( A r m s t r o n g , 1978) r e l a x e s t h e s t r a t i g r a p h i c c o n s t r a i n t . Thus t h e need f o r a t w o - s t a g e emplacement model i s o b v i a t e d . S u b s e q u e n t t o G a b r i e l s e and R e e s o r ' s s t u d y , Nguyen ^ t a]_. (1968) p r e s e n t e d t h e r e s u l t s o f seven K-Ar d e t e r m i n a t i o n s from t h e n o r t h e r n N e l s o n B a t h o l i t h and one K-Ar h o r n b l e n d e d a t e from t h e Mt. C a r l y l e 154 S t o c k . On t h e b a s i s o f t h e s e d a t a Nguyen _et a l _ . c o n c l u d e d t h a t b o t h t h e b a t h o l i t h and i t s s a t e l l i t e s t o c k were emplaced a t l e a s t 160 Ma ago. T h r e e h o r n b l e n d e K-Ar samples gave an a v e r a g e a p p a r e n t age o f 158 ± 7 Ma, whereas f o u r f r e s h b i o t i t e samples gave an a v e r a g e age o f 153 ± 10 Ma. Thus t h e s e a u t h o r s c o n c l u d e d t h a t t h e p l u t o n i c e v e n t s they had mapped o c c u r r e d o v e r a s h o r t t i m e span, s h o r t e r t h a n t h e r e s o l u t i o n o f t h e i r a n a l y t i c a l t e c h n i q u e s . A l t h o u g h Nguyen ^ t aj_.'s d a t a were i n t e r n a l l y c o n s i s t e n t , one o f t h e i r d a t e s a p p e a r s anomalous i n t h e c o n t e x t o f more r e c e n t s t u d i e s . They r e p o r t e d a d a t e o f 169 ± 6 Ma f o r a b i o t i t e f r o m a l a m p r o p h y r e d i k e c r o s s c u t t i n g t h e n o r t h e r n N e l s o n B a t h o l i t h . In l i t h o l o g y and f i e l d o c c u r r e n c e t h i s d i k e a p p e a r s t o be c o r r e l a t i v e w i t h a w i d e s p r e a d phase o f l a m p h r o p h y r e d i k e i n j e c t i o n w i t h i n t h e Ko o t e n a y A r c and a l o n g t he e a s t e r n m a r g i n o f t h e Shuswap complex. These l a m p r o p h y r e s have been d a t e d as T e r t i a r y i n a number o f l o c a l i t i e s . T h i s s u g g e s t s t h e p o s s i b l i t y t h a t t h e l a m p r o p h y r i c b i o t i t e d a t e d by Nguyen et a l _ . (1968) i s a n o m a l o u s l y o l d due t o e x c e s s a r g o n . K-Ar d a t e s o f 41 t o 58 Ma on b i o t i t e s from metamorphic r o c k s i n t r u d e d by t h e N e l s o n B a t h o l i t h have been r e p o r t e d by F a r r a r (1966) and Wanless et a l _ . ( 1 9 7 9 ) . The e x i s t e n c e o f i n t r u s i v e K-Ar d a t e s o l d e r than t h o s e o f t h e p o l y d e f o r m e d metamorphic r o c k s b e i n g i n t r u d e d i s anomalous and a g a i n r a i s e s t h e p o s s i b i l i t y o f e x c e s s a r g o n . Rb-Sr GEOCHRONOLOGY Sample S e l e c t i o n To complement e x i s t i n g K-Ar m i n e r a l d a t i n g r e s u l t s , a program o f whole r o c k Rb-Sr d a t i n g was i n i t i a t e d . In an a t t e m p t both t o d a t e t h e 155 main age o f i n t r u s i o n and t o e s t a b l i s h a minimum t i m e span f o r emplacement o f t h e b a t h o l i t h i c complex, t h e main p o r p h y r i t i c g r a n d i o r i t e and one o f t h e n o n - p o r p h y r i t i c phases were sampled. The p o r p h y r i t i c g r a n o d i o r i t e s u i t e (NB-I samples) was c o l l e c t e d from f r e s h , b l a s t e d r o c k i n a r o a d c u t a l o n g t h e n o r t h s h o r e o f t h e west arm o f Kootenay Lake n e a r Duhamel C r e e k , 10 km n o r t h o f N e l s o n . The non-p o r p h y r i t i c s u i t e (NB-II samples) was c o l l e c t e d from a r e c e n t r o a d c u t on t h e C a s t l e g a r r o a d 1 km west o f t h e c i t y o f N e l s o n . A t each o f t h e s e o u t c r o p s , m e d i u m - s i z e d (5-10 kg) samples o f t h e dominant i n t r u s i v e phase p r e s e n t were c o l l e c t e d . Samples o f m a f i c and l e u c o c r a t i c v a r i a n t s , m a f i c c l o t s , a p l i t i c and p e g m a t i t i c phases were a l s o s e l e c t e d . By s a m p l i n g a s m a l l a r e a i n d e t a i l i t was hoped t o a v o i d t h e s p a t i a l v a r i a t i o n o f i n i -t i a l s t r o n t i u m r a t i o s t h a t can be e x p e c t e d i n l a r g e b a t h o l i t h s . A n a l y t i c a l Method and R e s u l t s Each sample was c r u s h e d , s p l i t and a n a l y z e d f o r Rb and Sr i n d u p l i c a t e p r e s s e d powder p e l l e t s u s i n g X-ray f l u o r e s c e n c e . U.S. G e o l o g i c a l Survey s t a n d a r d s were used f o r c a l i b r a t i o n and mass a b s o r p t i o n c o e f f i c i e n t s were o b t a i n e d from Mo K a Compton s c a t t e r i n g measurements. Rb-Sr r a t i o s t h u s o b t a i n e d have a p r e c i s i o n o f 21 ( l a ) , whereas c o n c e n t r a t i o n s have a p r e c i s i o n o f 5% ( l a ) . Sr i s o t o p e c o m p o s i t i o n s were measured on u n s p i k e d samples p r e p a r e d u s i n g s t a n d a r d i o n exchange t e c h n i q u e s . Mass s p e c t r o -metry r u n s were done on a 6 0 ° s e c t o r , 30 cm r a d i u s , s o l i d - s o u r c e machine o f N.B.S. d e s i g n ( m o d i f i e d by H. F a u l ) . Data were a c q u i r e d d i g i t a l l y , i n an automated mode u s i n g a NOVA computer. E x p e r i m e n t a l d a t a have been Q C O O n o r m a l i z e d t o a S r / S r r a t i o o f 0.1194 and a d j u s t e d so t h a t N.B.S. 156 s t a n d a r d SRM987 g i v e s a u / S r / o u S r r a t i o o f 0.71022 + 2. Rb-Sr d a t e s a r e based on a Rb decay c o n s t a n t o f 1.42 x 1 0 " ^ y r ~ \ The r e s u l t s f o r sample s u i t e s NB-I and NB-II a r e l i s t e d i n T a b l e I and a r e p l o t t e d i n F i g u r e s 2 and 3. The b i o t i t e g r a n o d i o r i t e s u i t e (NB-II) from t h e s o u t h e r n t a i l o f the b a t h o l i t h g i v e s a 9 - p o i n t whole r o c k i s o c h r o n w i t h an age o f 162 + 6 Ma and an i n i t i a l r a t i o o f 0.7065 + 1 (as r e g r e s s e d by the method o f Yo r k , 1969). As shown i n F i g u r e 4, the m a j o r i t y o f t h e samples from t h i s o u t -87 86 87 86 c r o p c l u s t e r around a Rb/ S r r a t i o o f 0.75. Low Rb/ Sr r a t i o s such as t h e s e appear t o be c h a r a c t e r i s t i c o f the Ne l s o n B a t h o l i t h . The p o r p h y r i t i c monzonite s u i t e from the West Arm o f Kootenay Lake (NB-I) do n o t f a l l on a s i n g l e i s o c h r o n . F i v e samples (NBI-1, 2, 3L, 5, 5P) f a l l on a i s o c h r o n w i t h an age o f 158 + 16 Ma w i t h an i n i t i a l r a t i o o f 0.70695 + 0 . 0 0 1 2 . Two samples, NBI-4L and 4P, f a l l below t h i s i s o c h r o n and y i e l d an a p p a r e n t age o f 167 + 40 Ma w i t h an i n i t i a l r a t i o o f 0.7052. A whole r o c k , p l a g i o c l a s e , a l k a l i f e l d s p a r i n t e r n a l i s o c h r o n f o r NBI-5 g i v e s an age o f 153 + 26 Ma w i t h an i n i t i a l r a t i o o f 0.7069 + 1 . E i t h e r the age o f t h e NBI-4 sample p a i r i s a n o m a l o u s l y low (about 100 Ma i f r e -g r e s s e d w i t h t h e o t h e r NBI s u i t e d a t a ) o r the i n i t i a l r a t i o i s low. K-Ar GEOCHRONOLOGY To i n v e s t i g a t e t h e n a t u r e o f the d i s c r e p a n c y between the Rb-Sr whole r o c k d a t e s d e t e r m i n e d i n t h i s s t u d y and the K-Ar d a t e s r e p o r t e d i n t h e l i t e r a t u r e , K-Ar d a t e s were o b t a i n e d f o r a b i o t i t e and a ho r n -b l e n d e s e p a r a t e d from' NB-I-5. 157 TABLE I Rb-Sr ISOTOPE DATA Suite I Sample No. ppm Sr ppm Rb 8 7 R b / 8 6 S r 8 7 S r / 8 6 S r * MB 1-1 725 . 95 0.381(0.05) 0.70766(15) NB1-3L ' 683 118 0.500(0.01) 0.70820(15) NB1-4L 213 148 2.010(0.03) 0.71010(11) NB1-4P 207 171 2.390(0.03) 0.71089(03) NB1-5 678 113 0.480(0.007) 0.70790(20) NB1-5P 682 206 0.875(0.018) 0.70870(20) NB1 Plag 1003 4 0.012(0.01) 0.70696( 5) NB1 KFelds 1072 190 0.521(0.01) 0.70820( 5) Suite II NB2-1 591 145 0.709(0.01) 0.70844(09) NB2-2 630 142 0.654(0.01) 0.70786(20) NB2-3P 220 208 2.736(0.04) 0.71260(11) NB2-3M 610 144 0.685(0.01) 0.70796(14) NB2-4P 170 165 2.803(0.04) 0.71310(29) NB2-5 570 134 0.682(0.01) 0.70804(10) NB2-6M 529 146 0.801(0.01) 0.70800(20) NB2-7M 457 215 1.352(0.02) 0.70920(20) NB2-8 555 135 0.703(0.01) 0.70830(10) * Weighted a v e r a g e c a l c u l a t e d f r o m e q u a t i o n s : x = + wx^ 1 + w w 2 2 2 S l + 2 (1 + w)' 158 FIGURE 2 . Rb-Sr i s o c h r o n diagram f o r the NB-I ( p o r p h y r i t i c m o n z o n i t e ) s u i t e . 159 9 8 J S / , 8 J S 160 FIGURE 3. Rb-Sr i s o c h r o n diagram f o r NB-II ( b i o t i t e g r a n o d i o r i t e ) s u i t e . The b e s t f i t i s o c h r o n u s i n g a l l n i n e p o i n t s g i v e s an age o f 162 + 4 H a and an i n i t i a l r a t i o o f 0.7066 + 1. 161 1 6 2 FIGURE 4. A p p a r e n t c o o l i n g h i s t o r y o f NB-I l o c a l i t y . P l o t o f m i n e r a l ages v e r s u s b l o c k i n g t e m p e r a t u r e f o r K-Ar and f i s s i o n t r a c k d a t a . 6 0 0 -5 0 0 H K - A r -Hornblende f — © — i o o LU rr Z> l -< cr LU O L 4 0 0 -3 0 0 -LU h- 2 0 0 -1 0 0 -Fission Track - Sphene - Z i rcon Jt« Apat i te H ( M K - A r = Biotite 0 0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 0 A G E (Ma) 164 A n a l y t i c a l Method and R e s u l t s M i n e r a l s e p a r a t e s were a n a l y z e d f o r a r g o n by J . H a r a k a l u s i n g an MS-10 and f o l l o w i n g t he p r o c e d u r e o u t l i n e d i n W h i t e s t a l _ . ( 1 9 6 8 ) . S p l i t s o f t h e same samples were a n a l y z e d by K. S c o t t f o r K by a t o m i c a b s o r p t i o n s p e c t r o p h o t o m e t r y . The r e s u l t s a r e p r e s e n t e d i n T a b l e I I . The h o r n b l e n d e sample g i v e s a K-Ar d a t e o f 140 ± 5 Ma, and t h e b i o t i t e sample g i v e s a d a t e o f 58.8 ± 2 Ma. The o l d e r age f o r t h e h o r n b l e n d e sample i s c o n s i s t e n t w i t h t h e Rb-Sr p l a g i o c l a s e d a t e f o r t h e same sample ( N B - I - 5 ) . FISSION TRACK GEOCHRONOLOGY To a s s e s s t h e v a l i d i t y o f t h e thermal r e s e t t i n g model s u g g e s t e d by t h e K-Ar d a t a , f i s s i o n t r a c k d a t i n g was c a r r i e d o u t on samples from b o t h s u i t e s s t u d i e d i n the Rb-Sr s t u d y . F i s s i o n t r a c k d a t e s a r e s e n s i t i v e t o t h e r m a l h i s t o r y ( H a r r i s o n e t a l _ . , 1979) b u t a r e i n d e p e n d e n t o f v a r i a -t i o n s i n Ar f u g a c i t y and K + i o n a c t i v i t y t h a t may p e r t u r b t he r e s u l t s o f K-Ar d a t i n g . A p a t i t e , sphene and z i r c o n s were s e p a r a t e d from sample NB-I-5, and a p a t i t e s from NB-II-1. A n a l y t i c a l Method and R e s u l t s The method o f f i s s i o n t r a c k d a t i n g has been d e s c r i b e d by Naeser ( 1 9 7 9 ) . The s p e c i f i c t e c h n i q u e s used i n t h i s s t u d y have been documented and summarized i n H a r r i s o n et a l _ . ( 1 9 7 9 ) . The samples were i r r a d i a t e d i n t h e TRIGA R e a c t o r , U.S.G.S., Denver. The a p a t i t e samples were d a t e d by t h e p o p u l a t i o n method. The sphene and z i r c o n were d a t e d u s i n g t he e x t e r n a l d e t e c t o r method. The r e s u l t s a r e summarized i n T a b l e I I I . 165 TABLE II K - A r DATA Sample M i n e r a l % K Ar* 1* 0 / A r * 0 A r * * 0 A r * * V A g e t ; (10-Sr.c S T P / Q ) ^ K * °  NB1-5 b i o t i t e 6 . 8 9 + 0 . 1 1 % 76.2% 1.633 3.475 x I O " 3 6 0 . 0 + 2 . 0 NB1-5 h o r n b l e n d e 1 . 2 5 + 0 . 1 % 83.3% 7.213 x T O - 1 8.461 x I O - 3 1 4 3 + 5 t C a l c u l a t e d u s i n g XQ = 0.581 x 10" 1 0 a - 1 ; A g = 4.962 x 1 0 ~ 1 0 a " 1 ; K * ° / K = 1.167 x 1 0 " \ Note t h a t Ar**° r e f e r s t o r a d i o g e n i c a r g o n 40. i 166 TABLE I I I FISSION TRACK DATA*  P o p u l a t i o n Method Sample M i n e r a l S p ontaneous T r a c k s n Z x" Induced n E T r a c k s x" <j> x 1 0 1 4 f n e u t r o n s i cm* P s / P i Age NB2-1 a p a t i t e 56 511 9.29 94 237 2.52 1.77 + 0.03 3.68 38.4 +3.6t NB1-5 a p a t i t e 41 619 15.1 65 361 2.72 1.76 + 0.03 2.72 28.7 +2.6 E x t e r n a l D e t e c t o r Method Sample M i n e r a l G r a i n s Counted Mean o f S p o n t . / I n d u c e d x 2 4> x 10 1 1* n e u t r o n s ' cm 2 Age NB1-5 sphene NB1-5 z i r c o n 10 10 5.22 3.64 1.74 + 0.03 1.71 + 0.03 54.4 +6.1 37.3 +4.0 * F i s s i o n t r a c k s c o u n t e d by R.R. P a r r i s h . t S t a n d a r d e r r o r o f mean. 167 The d a t e s f o r NB-I-5 a r e : a p a t i t e , 26.8 Ma; z i r c o n , 34.8 Ma; and sphene, 50.8 Ma. Sample NB-II-1 g i v e s an a p a t i t e d a t e o f 36.3 Ma. The r e l a t i v e d a t e s f o r t h e m i n e r a l s i n NB-I-5 a r e c o n s i s t e n t w i t h the s u s c e p -t i b i l i t y o f t h e s e m i n e r a l s t o thermal a n n e a l i n g o f the f i s s i o n t r a c k s . The age d i s c r e p a n c y i n the a p a t i t e d a t e s s u g g e s t s t h a t t h e s e two s u i t e s have undergone d i f f e r e n t thermal h i s t o r i e s . DISCUSSION Sr I s o t o p e G e o c h e m i s t r y The d u a l p a r a l l e l i s o c h r o n i n t e r p r e t a t i o n f o r t h e p o r p h y r i t i c m onzonite NBI s u i t e , s u g g e s t e d above, i s not a unique i n t e r p r e t a t i o n o f the d a t a . I t i s r a t h e r s u r p r i s i n g t h a t the NB1-4 samples s h o u l d have a d i f f e r e n t i n i t i a l r a t i o than the r e s t o f t h e s u i t e . An a l t e r n a -t i v e i n t e r p r e t a t i o n i s t h a t t h e Sr i s o t o p e g e o c h e m i s t r y o f t h e s e r o c k s has been d i s t u r b e d by metasomatic e f f e c t s . F i e l d e v i d e n c e o f a l k a l i f e l d s p a r m e g a c r y s t s o v e r p r i n t i n g a p l i t i c v e i n s and m a f i c i n c l u s i o n s i s common. These o b s e r v a t i o n s s u g g e s t t h a t the m e g a c r y s t s grew m e t a s o m a t i c a l l y s u b s e q u e n t t o t h e p r i m a r y c r y s t a l l i z a t i o n o f the p l u t o n . C o n s i d e r a t i o n o f t h e s e two a l t e r n a t i v e s s u g g e s t s the metasomatic c o n t a m i n a t i o n model i s l e a s t p l a u s i b l e . I t r e q u i r e s the Sr i s o t o p e system t o be f o r t u i t o u s l y d i s t u r b e d so t h a t a l l the samples, w i t h t h e e x c e p t i o n o f NBI-4L and NBI-4P f a l l on an i s o c h r o n t h a t a g r e e s , i n age w i t h t h e NBII sample s u i t e . The age c a l c u l a t e d f o r the NBI 168 s u i t e as a whole c o n t r a d i c t s both t h e Rb-Sr m i n e r a l i s o c h r o n (NBI-5) and K-Ar h o r n b l e n d e d a t e s from the same r o a d c u t . G r a n i t i c r o c k s o f t h e Omineca b e l t a r e n o t a b l y h e t e r o g e n e o u s w i t h r e s p e c t t o Sr i s o t o p e i n i t i a l r a t i o s ( u n p u b l i s h e d c o m p i l a t i o n o f R.L. Armstrong) so t h a t i s a s u f f i c i e n t e x p l a n a t i o n o f the s t r a n g e b e h a v i o u r o f the NBI sample s u i t e . T i m i n g o f Emplacement o f the B a t h o l i t h C o n s i d e r a t i o n o f the a v a i l a b l e d a t a s u g g e s t s t h a t t he Nels o n B a t h o l i t h was emplaced a p p r o x i m a t e l y 160 Ma ago. T h i s age i s based on t h e Rb-Sr whole r o c k i s o c h r o n s f o r the two s u i t e s and i s c o n s i s t e n t w i t h t he r e c e n t G e o l o g i c a l Survey o f Canada h o r n b l e n d e K-Ar d a t e s (Wanless e t a l _ . , 1979) which a r e i n the 135 to 152 Ma ran g e . These Survey d a t e s t o g e t h e r w i t h p u b l i s h e d and u n p u b l i s h e d U n i v e r s i t y o f B r i t i s h Columbia d a t e s on t h e N e l s o n B a t h o l i t h a r e i n d i s t i n g u i s h a b l e s t a t i s t i c a l l y and g i v e a modal d a t e o f 1 5 3 + 5 Ma ( A r m s t r o n g , w r i t t e n communication, 1982). No d i f f e r e n c e i n age o f i n t r u s i o n has been d e t e c t e d between the p o r p h y r i t i c r o c k s o f t h e main b a t h o l i t h phase and the g r a n o d i o r i t e t h a t forms t he s o u t h e r n t a i l o f the B a t h o l i t h . The coh e r e n c y o f the h o r n b l e n d e K-Ar d a t e s f o r t h e whole b a t h o l i t h s u g g e s t t h a t the i n t r u s i v e complex c o o l e d through t he h o r n b l e n d e b l o c k i n g temperature as a c o h e r e n t mass. 169 Thermal H i s t o r y The i n t e r p r e t a t i o n o f f i s s i o n t r a c k and K-Ar d a t e s i n terms o f thermal h i s t o r i e s has been d i s c u s s e d by H a r r i s o n e t al_. ( 1979). Using the b l o c k i n g t e m p e r a t u r e o f the m i n e r a l (Dodson, 1974), the a p p a r e n t t e m p e r a t u r e - t i m e h i s t o r y o f the r o c k can be c o n s t r u c t e d as shown i n F i g u r e 4. The b l o c k i n g t e m p e r a t u r e s have been taken from H a r r i s o n e t a l . ( 1979), e x c e p t t h o s e f o r K-Ar b i o t i t e and K-Ar h o r n b l e n d e . These two have been c a l c u l a t e d u s i n g the method o f Dodson (1974) and t h e d a t a on Ar d i f f u s i o n i n h o r n b l e n d e ( H a r r i s o n and M c D o u g a l l , 1980) and b i o t i t e ( H a r r i s o n and Duncan, u n p u b l i s h e d d a t a ) . The d a t a shown i n F i g u r e 4 f a l l on an a p p r o x i m a t e l y l i n e a r a p p a r e n t c o o l i n g c u r v e . Such a c o o l i n g h i s t o r y i s c o n s i s t e n t w i t h u p l i f t and con-c u r r e n t c o n d u c t i v e c o o l i n g . D e s p i t e t h e a p p a r e n t s u c c e s s o f t h i s approach to i n t e r p r e t i n g b l o c k i n g t e m p e r a t u r e d a t a ( H a r r i s o n . e t a l _ . , 1979; P a r r i s h , 1980), s e v e r a l l i n e s o f e v i d e n c e s u g g e s t t h a t t h e N e l s o n B a t h o l i t h has e x p e r i e n c e d a more c o m p l i c a t e d thermal h i s t o r y . The 9.7 Ma d i f f e r e n c e i n a p a t i t e ages between NB-I and NB-II cannot be e x p l a i n e d by a s i m p l e u p l i f t - c o o l i n g model. As no major f a u l t appears to e x i s t between t h e s e two sample s i t e s , i t i s u n l i k e l y t h a t they had d i f f e r e n t u p l i f t h i s t o r i e s . A s e c o n d l i n e o f e v i d e n c e s u g g e s t i n g c o m p l i c a t i o n s i n the thermal h i s t o r y o f t h e b a t h o l i t h i s the a n o m a l o u s l y low K-Ar d a t e s t h a t a r e a v a i l a b l e from t h e metamorphic r o c k s on both t h e e a s t and west margins o f t h e b a t h o l i t h . To the west o f the b a t h o l i t h , the p o l y p h a s e deformed g n e i s s e s o f t h e V a l h a l l a g n e i s s dome g i v e K-Ar b i o t i t e and 170 h o r n b l e n d e d a t e s i n the range 40 t o 55 Ma ( G a b r i e l s e and R e e s o r , 1964; Wanl e s s e t a]_., 1 979). These young d a t e s a p p e a r t o be a m a n i f e s t a t i o n o f a T e r t i a r y t h e r m a l e v e n t p r e v i o u s l y i d e n t i f i e d i n t h e Okanagan V a l l e y t o t h e west by Ross (1974) and M e d f o r d ( 1 9 7 5 ) . The e x t e n s i v e r e s e t t i n g o f m i n e r a l ages i n t h e metamorphic r o c k s i n t r u d e d by t h e b a t h o l i t h i s b e s t e x p l a i n e d by h e a t i n g o f t h e upper c r u s t by g e o t h e r m a l c o n v e c t i o n s y s t e m s . I r r e s p e c t i v e o f t h e mechanism o f r e h e a t i n g , most, i f n o t a l l , c o n c e i v a b l e t h e r m a l models would n e c e s s i t a t e p a r t i a l r e s e t t i n g o f m i n e r a l d a t e s w i t h i n t h e b a t h o l i t h . CONCLUSIONS The Rb-Sr d a t a p r e s e n t e d i n t h i s s t u d y t o g e t h e r w i t h the new K-Ar m i n e r a l d a t e s c o n f i r m a L a t e J u r a s s i c age f o r t h e emplacement o f t h e b a t h o l i t h . The K-Ar and f i s s i o n t r a c k m i n e r a l d a t e s w i t h i n the batho-l i t h a p p e a r t o have been v a r i a b l y r e s e t by r e h e a t i n g d u r i n g a T e r t i a r y t h e r m a l e v e n t . T hese r e s u l t s d e m o n s t r a t e t h e p r o b l e m s i n h e r e n t i n r e l y i n g on a s i n g l e d a t i n g t e c h n i q u e such as K-Ar. 171 REFERENCES 172 GENERAL INTRODUCTION REFERENCES Dawson, G.M., 1898. R e p o r t on e x p l o r a t i o n i n t h e s o u t h e r n B r i t i s h C o l umbia Shuswap S h e e t . G e o l o g i c a l S u r v e y o f Canada, Map 143A. F l e t c h e r , R . C , 1972. A p p l i c a t i o n o f a m a t h e m a t i c a l model t o t h e emplace-ment o f m a n t l e d g n e i s s domes. A m e r i c a n J o u r n a l o f S c i e n c e , v. 272, p. 197-216. P r i c e , R.A. and M o u n t j o y , E.W., 1970. G e o l o g i c s t r u c t u r e o f t h e C a n a d i a n Rocky M o u n t a i n s between Bow and A t h a b a s c a R i v e r s , a p r o g r e s s r e p o r t . G e o l o g i c a l A s s o c i a t i o n o f Canada S p e c i a l P a p e r 6, p. 73-86. R e e s o r , J . E . and Moore, J.M., J r . , 1971. 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E . ; and M o u n t j o y , E.W., 1972, S t r u c t u r a l s t y l e o f t h e s o u t h e r n C a n a d i a n C o r d i l l e r a : 2 4 t h I n t e r n a t ' l . G e o l . Cong., E x c u r s i o n A-01-X01, M o n t r e a l , Canada. W h i t e , W.H.; H a r a k a l , J . E . ; and C a r t e r , N.C., 1968, K-Ar ages o f some o r e d e p o s i t s i n B r i t i s h C o l u m b i a : C.I.M.M. B u l l . , v. 61, no. 679, p. 1326-1334. Y o r k , D., 1969, L e a s t s q u a r e s f i t t i n g o f a s t r a i g h t l i n e w i t h c o r r e l a t e d e r r o r s : E a r t h P l a n e t . S c i . L e t t e r s , v. 2, p. 320-324. APPENDIX I STRUCTURAL ANALYSIS OF THE THOR-ODIN GNEISS DOME 197 I n t r o d u c t i o n The o r i g i n o f g n e i s s domes has been t h e s u b j e c t o f an o n g o i n g con-t r o v e r s y s i n c e E s k o l a (1949) d e s c r i b e d t h e " m a n t l e d g n e i s s domes" o f the F i n n i s h C a l e d o n i d e s and drew a t t e n t i o n t o t h e i r common o c c u r r e n c e i n o r o g e n i c z o n e s . A l t h o u g h g n e i s s domes have been d e s c r i b e d from t h e A p p a l a c h i a n s ( i n t h e M a r y l a n d - P e n n s y l v a n i a r e g i o n by Higgens et a l _ . , 1973, and i n New E n g l a n d by Thompson eit £l_., 1968), i n G r e e n l a n d by H a l l e r (1962) and i n B r i t i s h C o l u m b i a by Reesor ( 1 9 7 0 ) , mechanisms f o r g n e i s s dome f o r -m a t i o n r a n g i n g from g r a v i t a t i o n a l i n s t a b i l i t y t o m e t e o r i t e impact have been s u g g e s t e d . These a r e summarized by Hobbs ^ t a l _ . ( 1 9 7 6 ) . In B r i t i s h C o l u m b i a , t h e g n e i s s domes have been v a r i o u s l y a s c r i b e d t o d i a p i r i c u p w e l l i n g o f t h e l o w e r c r u s t ( R e e s o r , 1970) and an o r i g i n by i n t e r f e r e n c e o f l a t e - s t a g e , u p r i g h t b u c k l e f o l d s ( R o s s , 1968). C o n s i d e r a b l e p r o g r e s s has been made i n t h e o r e t i c a l and e x p e r i m e n t a l s t u d i e s o f g n e i s s dome f o r m a t i o n . Ramberg (1967) has modeled g n e i s s domes u s i n g c e n t r i f u g e e x p e r i m e n t s and t h e s e s t u d i e s have been f o l l o w e d up i n c o n s i d e r a b l e d e t a i l by Dixon ( 1 9 7 5 ) . D i x o n ' s m o d e l l i n g work has documented t h e s p a t i a l v a r i a t i o n s i n i n t e r n a l s t r a i n t o be e x p e c t e d i n g n e i s s domes o f d i a p i r i c o r i g i n . F l e t c h e r (1972) has pr o d u c e d a numeri-c a l s i m u l a t i o n o f t h e g r a v i t a t i o n a l i n s t a b i l i t y model and has d u p l i c a t e d t h e b r o a d s t r u c t u r a l f e a t u r e s o f some o f t h e New E n g l a n d g n e i s s domes. U n f o r t u n a t e l y , d e t a i l e d f i e l d s t u d i e s o f the m e s o s c o p i c s t r u c t u r e s i n t h e s e g n e i s s domes a r e n o t a v a i l a b l e i n the l i t e r a t u r e . To p r o v i d e such d a t a , a d e t a i l e d s t r u c t u r a l i n v e s t i g a t i o n o f the T h o r - O d i n g n e i s s dome i n B r i t i s h C o lumbia was i n i t i a t e d (Duncan, 1978a). T h i s dome was chosen b e c a u s e o f the e x c e l l e n t base o f g e o l o g i c a l knowledge p r o v i d e d by t h e 198 e a r l i e r l a r g e r - s c a l e s t u d y o f Reesor and Moore (1971) and t h e t h r e e -d i m e n s i o n a l e x p o s u r e p r o v i d e d by t h e d e e p l y d i s s e c t e d , g l a c i a l l y s c o u r e d t e r r a i n ( F i g . 1-1). T h i s a p p e n d i x p r e s e n t s t h e r e s u l t s o f a d e t a i l e d s t r u c t u r a l a n a l y s i s o f t h e T h o r - O d i n dome and d i s c u s s e s t h e s i g n i f i c a n c e o f t h e s e r e s u l t s i n terms o f t h e v a r i o u s a l t e r n a t i v e mechanisms f o r g n e i s s dome f o r m a t i o n t h a t have been p r o p o s e d . T e c h n i q u e s o f S t r u c t u r a l A n a l y s i s The t e c h n i q u e s u t i l i z e d i n t h i s s t u d y were, i n g e n e r a l , t h o s e o u t l i n e d i n Ramsay (1967) and Hobbs _e_t a l _ . ( 1 9 7 6 ) . Emphasis was p l a c e d on o u t l i n i n g l a r g e - s c a l e s t r u c t u r e b o t h by t r a c i n g s t r a t i g r a p h i c o r l i t h o l o g i c a l sequence and by p i e c i n g t o g e t h e r f o l d d a t a from i n d i v i d u a l o u t c r o p s . Data f o r m e s o s c o p i c o r o u t c r o p - s c a l e f o l d s were r e c o r d e d i n terms o f o r i e n t a t i o n o f s t r u c t u r a l e l e m e n t s ( f o l d a x e s , a x i a l p l a n e s and l i n e a t i o n s ) , t h e v e r g e n c e o r sense o f r o t a t i o n o f t h e f o l d , t h e s t y l e o f f o l d i n g and e v i d e n c e o f o v e r p r i n t i n g r e l a t i o n s . As has been shown by W i l l i a m s ( 1 9 7 0 ) , t h e s t y l e o f f o l d i n g can v a r y f r o m p l a c e t o p l a c e w i t h i n one g e n e r a t i o n o f f o l d i n g . L o c a l v a r i a t i o n s o f d u c t i l i t y , d u c t i l i t y c o n t r a s t and magnitude o f f i n i t e s t r a i n c a u s e c h a n ges i n f o l d s t y l e . The g e n e r a t i o n t o which a f o l d b e l o n g s i s b e s t e s t a b l i s h e d by o v e r p r i n t i n g l i n e a t i o n s , and more i m p o r t a n t l y , by o v e r -p r i n t e d c l e a v a g e r e l a t i o n s . The T h o r - O d i n a r e a i s w e l l - s u i t e d t o t h i s t y p e o f s t u d y as a r e s u l t o f the e x c e l l e n t o u t c r o p and e x t e n s i v e d e v e l o p -ment o f m e s o s c o p i c f o l d s . A u t o c h t h o n o u s Core G n e i s s Domain The o r i e n t a t i o n o f the dominant m e s o s c o p i c i s o c l i n a l f o l d s i n the Au t o c h t h o n o u s Core G n e i s s Domain i s shown i n F i g u r e 1-2. The rugged 199 FIGURE 1-1. The g e n e r a l g e o l o g y o f the T h o r - O d i n g n e i s s dome. The d i s t r i b u t i o n o f l i t h o l o g i e s i s m o d i f i e d f r o m R e e s o r and Moore ( 1 9 7 1 ) . T h i s map shows t h e a x i a l t r a c e s o f l a r g e - s c a l e Phase One f o l d s ( ) , Phase Two f o l d s ( ) , and Phase T h r e e f o l d s ( — • — ) . Synforms a r e shown a s - J - and recumbent s y n f o r m s as ^ """^  . A n t i f o r m s a r e s i m i l a r l y shown. The l o c a t i o n s f o r v a r i o u s s u b s e -q u e n t maps a r e o u t l i n e d and i d e n t i f i e d by t h e i r f i g u r e number. 200 1 1 8 ° 10' 5 0 ° 38 mmm KM Q U A T E R N A R Y A L L U V I U M M A R B L E B A N D E D CALC-SILICATE SCHIST L I N E A T E D Q U A R T Z M O N Z O N I T E A M P H I B O L I T E Q U A R T Z - F E L D S . - B I O T . - G A . PARAGNEISS PELITIC A N D C A L C - S I L I C A T E SCHIST Q U A R T Z I T E T T B I O T . G R A N O D I O R I T E Q U A R T Z M O N Z O N I T E H O R N B . G R A N O D I O R I T E M E T A S E D I M E N T A R Y G N E I S S T H R U S T F A U L T ICE A N D S N O W 201 FIGURE 1-2. The g e n e r a l g e o l o g y o f t h e A u t o c h t h o n o u s Core G n e i s s Domain. The d i s t r i b u t i o n o f l i t h o l o g i e s and o r i e n t a t i o n d a t a f o r c o m p o s i t i o n a l l a y e r i n g , shown by s t e r e o n e t s , i s f r o m R e e s o r and Moore ( 1 9 7 1 ) . The s t r u c t u r a l d a t a and i n t e r p r e t a t i o n s a r e ba s e d on t h e p r e s e n t s t u d y . o r\3 203 n a t u r e o f t h e t e r r a i n p r e v e n t e d d e t a i l e d s t r u c t u r a l mapping o f t h i s domain w i t h t h e e x c e p t i o n o f t h e P i n g s t o n f o l d on t h e n o r t h e a s t s i d e o f T h o r R i d g e . The f o l l o w i n g a n a l y s i s i s b a s e d on d e t a i l e d mapping o f the P i n g s t o n f o l d ( F i g . 1-3), s c a t t e r e d o b s e r v a t i o n s a l o n g t r a v e r s e s a c r o s s t h e a r e a and the o r i e n t a t i o n d a t a p r e s e n t e d i n R e e s o r and Moore (1971, F i g . 1 2 ) . T h i s domain i s d o m i n a t e d by t h e P i n g s t o n f o l d , a l a r g e - s c a l e recumbent nappe s t r u c t u r e , t h e h i n g e o f which t r e n d s n o r t h w e s t e r l y a c r o s s t h e domain. T h i s s t r u c t u r e has a c o r e o f c o v e r r o c k m e t a s e d i m e n t s , c a l c -s i l i c a t e s and p e l i t i c s c h i s t s , and i s o u t l i n e d by t h e b a s a l q u a r t z i t e u n i t o f t h e c o v e r r o c k s e q u e n c e . T h i s l a r g e Phase One s t r u c t u r e p l u n g e s n o r t h w e s t on the n o r t h e a s t l i m b o f the m a j o r Phase F o u r a n t i f o r m t h a t forms t h e domal c u l m i n a t i o n ( F i g . 1-1). The f o l d p l u n g e s s o u t h e r l y t o s o u t h e a s t e r l y on the s o u t h e r n s i d e o f the dome. The o r i e n t a t i o n o f f o l d axes o f m e s o s c o p i c f o l d s p a r a s i t i c t o the P i n g s t o n f o l d s c a t t e r a l o n g s m a l l c i r c l e l o c i c o n s i s t e n t w i t h r e o r i e n t a -t i o n by t h e l a r g e Phase F o u r a n t i f o r m . In the T h o r Ridge a r e a , the p o l e s t o a x i a l p l a n e s o f Phase One s t r u c t u r e s form a g i r d l e whose p o l e a p p r o x i -mates t h e f o l d a x i s o f Phase F o u r . The p o l e s t o c o m p o s i t i o n a l l a y e r i n g form a g i r d l e w i t h a s i n g l e maxima, c o n s i s t e n t w i t h the P i n g s t o n f o l d b e i n g i s o c l i n a l . The d e g r e e o f s c a t t e r i n g around the g i r d l e i s c a u s e d by t h e f o l d i n g a s s o c i a t e d w i t h Phase F o u r . In t h e s o u t h e a s t e r n c o r n e r o f the domain ( t o the e a s t o f Mt. Burnham), p a r a s i t i c f o l d s i n the h i n g e o f the P i n g s t o n f o l d p l u n g e s h a l l o w l y t o t h e s o u t h . . As t h i s a r e a i s a l o n g the a x i a l t r a c e o f the Phase F o u r a n t i f o r m ( F i g . 1-1), the f o l d axes show l i t t l e r e o r i e n t a t i o n . Thus they a p p r o x i m a t e the o r i e n t a t i o n o f the f o l d s p r i o r t o Phase F o u r 204 FIGURE 1-3. The g e o l o g y o f t h e P i n g s t o n f o l d , T h o r R i d g e . Shown a r e t h e c o v e r r o c k s c o r i n g t he f o l d ( s h a d e d p a t t e r n ) , th e l o w e r q u a r t z i t e s ( d o t t e d p a t t e r n ) , and t h e basement g n e i s s e s ( u n p a t t e r n e d ) . A l s o shown a r e the p l u n g e , a z i m u t h and v e r g e n c e o f S e t I f o l d s ( • — i — • ) , S e t II f o l d s ( « r — I — • ) , and Phase F o u r f o l d s ( mt >•)• 206 b u c k l i n g . The p o l e s t o c o m p o s i t i o n a l l a y e r i n g i n t h i s a r e a show a w e l l -d e v e l o p e d bimodal g i r d l e , t h e p o l e t o which i s t h e a v e r a g e Phase One f o l d a x i s . T h i s bimodal p a t t e r n i s dominated by d a t a from t i g h t , a n g u l a r f o l d s such as shown i n F i g u r e 1-5. The g e o l o g y o f t h e P i n g s t o n f o l d on t h e n o r t h s i d e o f Th o r Ridge has been mapped i n d e t a i l ( F i g . 1-4). U n f o r t u n a t e l y , much o f t h e w e s t e r n m o s t h i n g e zone a r e a i s c o v e r e d by permanent snow and m o r a i n e s , o r overhu n g by u n s t a b l e i c e c l i f f s . In t h i s a r e a t h e P i n g s t o n f o l d i s a s y n c l i n e o f c a l c - s i l i c a t e s , p e l i t i c s c h i s t s and a q u a r t z i t e / p s a m m i t i c g n e i s s / q u a r t -z i t e s e q u e n c e . T h i s sequence o u t l i n e s t he f o l d h i n g e as shown i n F i g u r e 1-4. The h i n g e zone on t h e west s i d e o f t h e map i s rounded w i t h poor d e v e l o p m e n t o f p a r a s i t i c f o l d i n g . M e s o s c o p i c f o l d s ( F i g . 1-5) a r e w e l l - d e v e l o p e d on the n o r t h e r n l i m b o f t h e P i n g s t o n f o l d ( F i g . 1-4); however, i t i s n o t c l e a r t h a t a l l , o r any, o f t h e s e s t r u c t u r e s a r e p a r a s i t i c t o the P i n g s t o n f o l d . As mapped i n t h i s s t u d y , t h e s e s t r u c t u r e s f a l l i n t o two g r o u p s : s t e e p l y p l u n g i n g , w e s t e r l y - t r e n d i n g i s o c l i n e s on t h e s t e e p p a r t o f the P i n g s t o n f o l d (shown i n F i g . 1-4), and s h a l l o w l y - p l u n g i n g , n o r t h e r l y - t r e n d i n g , more open f o l d s on the s h a l l o w l y - d i p p i n g s e c t i o n o f t h e upper l i m b . The s t e e p and s h a l l o w l i m b d i p s o f t h e P i n g s t o n f o l d a p p e a r t o be the p r o d u c t o f o v e r p r i n t i n g by l a r g e s c a l e Phase F o u r s t r u c t u r e s . The o r i e n t a t i o n o f t h e s e two groups i s shown i n F i g u r e 1-6. The f i r s t group p l o t s as a c l u s t e r . The secon d group shows some d i s p e r s i o n a r o u n d a g r e a t c i r c l e w hich r e p r e s e n t s t h e i r modal a x i a l p l a n e . These two f o l d s e t s may r e p r e s e n t a s i n g l e phase o f d e f o r m a t i o n r o t a t e d and f l a t t e n e d by Phase F o u r f o l d i n g . Numerous examples o c c u r o f o u t c r o p -207 FIGURE 1-4. C r o s s - s e c t i o n a c r o s s t h e P i n g s t o n f o l d . The c r o s s - s e c t i o n l i n e i s shown as A — A ' i n F i g u r e 1-3. The complex shape o f t h e P i n g s t o n f o l d r e p r e s e n t s , i n p a r t , t h e s u p e r p o s i t i o n o f Phase F o u r f o l d s on the Phase One nappe s t r u c t u r e . The r o c k s i n the c o r e o f t h e f o l d a r e d u c t i l e s c h i s t s and c a l c - s i l i c a t e s and thus have been f l a t t e n e d d u r i n g Phase F o u r b u c k l i n g . Two a l t e r n a t i v e models p r o p o s e d i n t h e t e x t f o r t h e P i n g s t o n f o l d a r e i l l u s t r a t e d : (a) The P i n g s t o n f o l d i s i n t e r p r e t e d as an i n t e r f e r e n c e s t r u c t u r e between Phase One, Phase T h r e e and Phase F o u r f o l d s . The f l a t u pper l i m b o f t h e P i n g s t o n f o l d on the n o r t h s i d e o f T h o r Ridge would thus be the l o w e r l i m b o f a Phase T h r e e s y n f o r m . 208 N0I1VA313 209 FIGURE 1-4 ( c o n t i n u e d ) . (b) The P i n g s t o n f o l d F o u r i n t e r f e r e n c e i n t e r p r e t a t i o n o f i s i n t e r p r e t e d as a Phase One/Phase s t r u c t u r e . T h i s i s s i m i l a r t o t h e R e e s o r and Moore ( 1 9 7 1 ) . 210 NOI1VA313 211 FIGURE 1-5. Phase One f o l d s on upper l i m b o f P i n g s t o n f o l d : (a) P a r a s i t i c Phase One f o l d s i n q u a r t z o - f e l d s p a t h i c g n e i s s e s on t h e upper l i m b o f the P i n g s t o n f o l d , f r o m t h e n o r t h s i d e o f T h o r R i d g e (b) Phase One f o l d n e a r ( a ) , r e f o l d e d by u p r i g h t Phase F o u r f o l d s . A l l f o l d s k e t c h e s a r e o r i e n t e d w i t h n o r t h t o the r i g h t and s o u t h t o t h e l e f t , e x c e p t as marked. A l l f o l d s a r e v i e w e d down the p l u n g e where p o s s i b l e . 212 213 FIGURE 1-6. E q u a l - a n g l e s t e r e o n e t p l o t s showing o r i e n t a t i o n d a t a f r o m t h e a r e a shown i n F i g u r e 1-3. (a) Data f o r m e s o s c o p i c Phase F o u r f o l d s . Open diamonds a r e f o l d a x e s , c r o s s e s a r e l i t h o l o g i c a l l a y e r i n g . The dashed l i n e i s a r e p r e s e n t a t i v e a x i a l p l a n e . (b) Data f o r t i g h t - t o - i s o c l i n a l m e s o s c o p i c f o l d s . S e t I : f o l d axes ( * ) ; p o l e s t o a x i a l p l a n e s ( x ) . S e t I I : f o l d axes ( A ) ; p o l e s t o a x i a l p l a n e s (A). The dashed g r e a t c i r c l e i s t h e modal a x i a l p l a n e o f S e t II f o l d a x e s . The s o l i d g r e a t c i r c l e i s the TT g i r d l e o f p o l e s t o c o m p o s i t i o n a l l a y e r i n g shown i n (a) a b o v e . The l o n g dashes r e p r e s e n t a s m a l l c i r c l e l o c u s f o r r o t a t i o n o f L, by Phase F o u r . 215 s t y l e i n t e r f e r e n c e p a t t e r n s w i t h Phase F o u r f o l d s o v e r p r i n t i n g what a p p e a r t o be Phase One f o l d s ( F i g . 1-5). T h i s i m p l i e s t h a t the s t e e p p o r t i o n o f t h e l i m b has been f l a t t e n e d d u r i n g Phase F i v e f o l d i n g such t h a t t h e f o l d s have been r o t a t e d toward the v e r t i c a l ( t h e d i r e c t i o n o f t h e X s t r a i n a x i s d u r i n g Phase F o u r d e f o r m a t i o n ) . A l t h o u g h t h e shape o f t h e s e f o l d s i s c o n s i s t e n t w i t h s i g n i f i c a n t f l a t t e n i n g , t h i s h y p o t h e s i s has n o t been a d e q u a t e l y documented. An a l t e r n a t i v e i n t e r p r e t a t i o n i s t h a t t h e s e two g roups r e p r e s e n t d i f f e r e n t phases o f d e f o r m a t i o n . The s t e e p l y - p l u n g i n g f o l d s c o u l d be i n t e r p r e t e d as Phase One f o l d s p a r a s i t i c t o the main P i n g s t o n s t r u c t u r e and the s h a l l o w l y - d i p p i n g f o l d s as a l a t e r phase o f s u p e r p o s e d d e f o r m a t i o n ( p o s s i b l y Phase T h r e e f o l d s ) . T h i s l a t t e r i n t e r p r e t a t i o n , t h a t a l a t e r p e r i o d o f recumbent f o l d i n g has o v e r p r i n t e d the P i n g s t o n f o l d , has been p r e v i o u s l y p r o p o s e d by R eesor and Moore (1971, f i g u r e 16B). U n f o r t u n a t e l y no m e s o s c o p i c r e f o l d e d f o l d s i n v o l v i n g t h e s e two p h a s e s were documented i n t h i s s t u d y . The two i n t e r p r e t a t i o n s o f the m e s o s c o p i c s t r u c t u r e r e s u l t i n two d i f f e r e n t i n t e r p r e t a t i o n s o f the l a r g e - s c a l e s t r u c t u r a l geometry". The f i r s t i n t e r p r e t a t i o n assumes t h a t the m e s o s c o p i c s t r u c t u r e s a r e Phase One ( o r p o s s i b l y Phase Two) s t r u c t u r e s , and t h a t the P i n g s t o n f o l d as i t o u t c r o p s n o r t h o f Mt. T h o r r e p r e s e n t s t h e e f f e c t s o f a Phase T h r e e (?) recumbent s y n f o r m r e f o l d i n g the P i n g s t o n f o l d , i n t u r n o v e r p r i n t e d by a l a r g e - s c a l e Phase F o u r s t r u c t u r e ( F i g . 1-4). The s e c o n d i n t e r p r e t a t i o n i s t h a t t h e geometry o f the P i n g s t o n f o l d i n t h i s a r e a i s the r e s u l t o f a Phase One/Phase F o u r i n t e r f e r e n c e . As o u t l i n e d above, t h i s c o u l d e x p l a i n the m e s o s c o p i c o r i e n t a t i o n d a t a i f f l a t t e n i n g o c c u r r e d on the s t e e p l i m b . The " n e c k i n g " geometry o f the l a r g e - s c a l e s t r u c t u r e on the 216 c r e s t o f T h o r Ridge i s d i f f i c u l t t o u n d e r s t a n d i n t h i s model. I t c o u l d be e x p l a i n e d i f t h e Phase One f o l d was a n t i f o r m a l and p l u n g e d s h a l l o w l y n o r t h . T h i s was t h e o r i g i n a l i n t e r p r e t a t i o n o f R e e s o r and Moore ( 1 9 7 1 ) . However, no m e s o s c o p i c f o l d s w i t h t h i s o r i e n t a t i o n have been i d e n t i f i e d . A l t e r n a t i v e l y , t h e shape c o u l d r e p r e s e n t a mega-boudinage s t r u c t u r e ; however, t h i s h y p o t h e s i s i s d i f f i c u l t t o s u b s t a n t i a t e . Note t h a t i n the f i r s t i n t e r p r e t a t i o n , t h e P i n g s t o n f o l d i s a n o r t h e r l y - t r e n d i n g s y n f o r m t h a t i s o v e r p r i n t e d by a n o r t h w e s t e r l y - t r e n d i n g Phase T h r e e s t r u c t u r e . In t h e s e c o n d i n t e r p r e t a t i o n , t h e l a r g e - s c a l e s t r u c t u r e would be a s h a l l o w l y - p l u n g i n g n o r t h e r l y - t r e n d i n g s y n f o r m deformed i n t o a s t e e p e r p l u n g e i n the h i n g e zone by the o v e r p r i n t i n g Phase F o u r s t r u c t u r e . Basement C o r e d Nappe Domain To t h e s o u t h and west o f the A u t o c h t h o n o u s Core G n e i s s Domain, s t r u c t u r a l t r e n d s a r e t r u n c a t e d by a s e r i e s o f t h r u s t - b o u n d e d nappes. These nappes t y p i c a l l y c o n s i s t o f a n t i f o r m a l c o r e s o f basement g n e i s s e s d o m i n a t e d by n o r t h e r l y - v e r g e n t Phase Two m e s o s c o p i c s t r u c t u r e s . Only n e a r t h e detachment s u r f a c e o f the nappe do h i n g e - t y p e v e r g e n c e s and, more r a r e l y , a t t e n u a t e d s o u t h e r l y ( l o w e r l i m b ) v e r g e n c e s o c c u r . The b a s a l zones o f the nappes a r e zones o f w e l l - d e v e l o p e d f o l i a t i o n , a t t e -n u a t e d r o o t l e s s f o l d s and f i n e - g r a i n e d , r e c r y s t a l 1 i z e d g n e i s s e s . In c o n t r a s t t o t h i s , s y n f o r m s o f the c o v e r r o c k s a r e p r e s e r v e d i n t a c t on the u p p e r l i m b s o f t h e nappes ( F i g . 1-7). Two basement c o r e d nappes have been mapped and a r e shown i n F i g u r e 1-7. The s t r u c t u r a l l y h i g h e s t f a u l t e d s y n f o r m ( t h e G l a d s h e i m Lake i n f o l d o f R e e s o r and Moore, 1971) l i e s on the upper l i m b o f t h e Gunnarson Nappe d e l i n e a t e d i n t h i s s t u d y . Beneath t h e Gunnarson Nappe i n the n o r t h w e s t 217 FIGURE 1-7. S t r u c t u r a l map o f a p o r t i o n o f t h e Gunnarsen and O d i n Nappes. (a) Map l e g e n d . GEOLOGY OF THE MT. GUNNARSEN AREA COVER ROCKS G a - K y - S i l l S c h i s t s + C a l c a r e o u s Sch i s t s +. Amph i bol i t e Q u a r t z i t e BASEMENT GNEISSES B i o t - Q t z - F e l d s G n e i s s e s , M inor M i g m a t i t e and A m p h i b o l i t e , Rare C a 1 c - S i 1 i c a t e and S c h i s t L i g h t - C o l o r e d Q t z - F e l d s G n e i s s V C o r d i e r i t e - G e d r i t e R i c h Rocks + + + + + + B i o t i t e G r a n o d i o r i t e P l u n g e o f Phase Three f o l d w i t h s o u t h e r l y v e r g e n c e S t r i k e and d i p o f c o m p o s i t i o n a l 50 l a y e r i n g T h r u s t f a u l t s '"''a 1 u 1* Snow 219 FIGURE 1-7 ( c o n t i n u e d ) . (b) S t r u c t u r e and g e o l o g y o f t h e Mt. Gunnarsen a r e a . 220 221 c o r n e r o f F i g u r e 1-7 i s a s e c o n d s y n f o r m ( t h e Mt. Skade i n f o l d o f R eesor and Moore, 1971) f o r m i n g t h e upper p a r t o f t h e O d i n Nappe. A t h i r d nappe p r o b a b l y e x i s t s b e n e a t h t h e Odin Nappe, b u t t h i s has n o t been e s t a b l i s h e d . W i t h i n t h i s domain t h e p o l e s t o c o m p o s i t i o n a l l a y e r i n g l o c a l l y form unimodal c l u s t e r s . T h i s r e f l e c t s the i s o c l i n a l n a t u r e o f t h e dominant Phase Two s t r u c t u r e s , t h e i r s h a l l o w p l u n g e , and t h e r e s u l t a n t r a r i t y o f e x p o s e d h i n g e a r e a s . T h i s r e l a t i v e l y s i m p l e p a t t e r n o f h o m o c l i n a l l y -d i p p i n g l a y e r i n g , t o g e t h e r w i t h t h e s t r o n g d u c t i l i t y c o n t r a s t c r e a t e d by t h e i n t e r l e a v i n g o f basement g n e i s s e s and c o v e r r o c k s , l e a d s t o s t r o n g d e v e l o p m e n t o f Phases F i v e and S i x s t r u c t u r e s . A l o n g the west s i d e o f t h e dome t h i s domain i s c h a r a c t e r i z e d by l a r g e - s c a l e Phase F i v e f o l d s . F i g u r e 1-8 shows a map view o f t h e s e s t r u c t u r e s i n w hich t h e o u t c r o p p a t -t e r n has been c o r r e c t e d f o r t o p o g r a p h i c d i s t o r t i o n t o more a c c u r a t e l y show t h e i r s t y l e . These f o l d s ( F i g . 1-8) have a c h e v r o n s t y l e w i t h s t r a i g h t l i m b s and a n g u l a r h i n g e z o n e s . Both t h e map view and the s t e r e o n e t p l o t ( F i g . 1-9) show t h a t Phase F i v e s t r u c t u r e s c o n s i s t o f two s e t s : t h e l a r g e - s c a l e c h e v r o n f o l d s whose a x i a l p l a n e s t r e n d 8 0 ° , and s m a l l - s c a l e b u c k l e s ( s e e the s o u t h w e s t c o r n e r o f t h e map i n F i g . 1-8) t r e n d i n g 5 5 ° . M e s o s c o p i c Phase F o u r f o l d s a r e v e r y r a r e w i t h i n t h i s domain. Indeed, t h e s i m p l e d i s t r i b u t i o n o f Phase T h r e e a x i a l p l a n e s i n F i g u r e 1-9 s u g g e s t s the absence o f s m a l l - s c a l e Phase F o u r f o l d s . Phase Two f o l d s a r e the dominant m e s o s c o p i c s t r u c t u r e i n t h i s domain. Phase Two f o l d s a r e t i g h t - t o - i s o c l i n a l and t y p i c a l l y a t t e n u a t e d t o r o o t -l e s s i n s t y l e . M e s o s c o p i c Phase Two f o l d s f r e q u e n t l y have a s s o c i a t e d z o n es o f i n t e n s e f l a t t e n i n g and r e c r y s t a l l i z a t i o n ( F i g . 1-10). In 222 FIGURE 1-8. P r o j e c t e d map view o f c h e v r o n - s t y l e Phase F i v e f o l d s on the w e s t s i d e o f t h e dome. The a r e a i s l o c a t e d i n F i g u r e 1-1. The g e o l o g y has been p r o j e c t e d t o the 7 , 0 0 0 - f o o t e l e v a t i o n p l a n e t o e l i m i n a t e t h e e f f e c t s o f t o p o g r a p h i c d i s t o r t i o n . I METASEDIMENTARY BASEMENT GNEISSES HORNBLENDE-BIOTITE G R A N O D I O R I T E GNEISS BIOTITE GRANODIORITE BASAL QUARTZITE MIXED PELITIC A N D C A L C - S I L I C A T E SCHISTS IMBRICATED B A S E M E N T GNEISSES A N D COVER ROCKS DIP A N D STRIKE OF L A Y E R I N G TRACE of COMPOSIT IONAL LAYERING S Y N F O R M A L A X I A L T R A C E A N T I F O R M A L A X I A L T R A C E ro oo 224 FIGURE 1-9. S t e r e o n e t p l o t s o f o r i e n t a t i o n d a t a f o r Basement C o r e d Nappe Domain: Phase One, f o l d axes * , p o l e s t o a x i a l p l a n e s x ; Phase Two, f o l d axes o , p o l e s t o a x i a l p l a n e s o ; Phase T h r e e , f o l d axes A , p o l e s t o a x i a l p l a n e s A ; Phase F o u r , f o l d axes • , p o l e s t o a x i a l p l a n e s I ; Phase F i v e , f o l d axes ^ , p o l e s t o a x i a l p l a n e s ^ . (•a) Phase Two a x i a l p l a n e s f o r m a rr g i r d l e r e l a t e d t o r e f o l d i n g by Phases T h r e e and F o u r . The dashed g r e a t c i r c l e i s t h e a x i a l p l a n e o f a m e s o s c o p i c Phase Four f o l d from the domain. The Phase Two f o l d axes s c a t t e r a b o u t a s m a l l c i r c l e d i s t r i b u t i o n . T h i s d i s p e r s i o n i s r e l a t e d m a i n l y t o Phases T h r e e and F i v e f o l d i n g . (b) Phase T h r e e s t r u c t u r e s . The modal Phase T h r e e a x i a l p l a n e ( l o n g d a s h e s ) d i p s 40° s o u t h and s t r i k e s 3 1 2 ° . The TT g i r d l e d e f i n e d by p o l e s t o Phase Two a x i a l p l a n e s shown i n (a) i s l a b e l e d TT3. The Phase F i v e TT g i r d l e d e f i n e d by a x i a l p l a n e s t o Phase F i v e i s l a b e l e d TT5. The s h o r t dashes r e p r e s e n t the a x i a l p l a n e o f a m e s o s c o p i c Phase F i v e b u c k l e f o l d . ( c ) Phase One f o l d axes and l i n e a t i o n s . (d) Phase One s i l l i m a n i t e l i n e a t i o n s d i s t o r t e d by a m e s o s c o p i c Phase Two f o l d . 225 226 FIGURE 1-10. M e s o s c o p i c f o l d s f r o m t h e Basement C o r e d Nappe Domain. (a) Phase Two f o l d s and r e l a t e d s h e a r z o n e s , from t h e upper l i m b o f t h e Gunnarson Nappe 1 km e a s t o f G l a d s h e i m L a k e . (b) S h a l l o w l y - d i p p i n g Phase T h r e e f o l d s ( f o l d axes p l u n g e 1 6 7 3 1 7 ° ) r e f o l d t i g h t Phase Two f o l d s i n q u a r t z i t e ( f o l d axes p l u n g e 6 ° / 3 2 2 ° ) w i t h i n q u a r t z i t e s wiith t h i n l a y e r s o f b i o t i t e - a l k a l i f e l d s p a r - s i l l i m a n i t e - q u a r t z . ( c ) I s o c l i n a l Phase Two f o l d s r e f o l d e d by s h a l l o w l y -d i p p i n g Phase T h r e e f o l d s and c u t by a Phase T h r e e t h r u s t f a u l t . (d) A recumbent Phase T h r e e a n t i f o r m r e f o l d s a Phase Two a n t i f o r m . Note the Phase One s i l l i m a n i t e l i n e a t i o n s deformed by t h e Phase Two f o l d . 227 228 d e t a i l , Phase Two f o l d s a r e l o c a l l y n o n - c y l i n d r i c a l , f o r m i n g g e n t l e domes and b a s i n s . D e t a i l e d measurements o f c u r v i l i n e a r f o l d a x e s , a l o n g i n d i -v i d u a l m e s o s c o p i c f o l d s , l i e a l o n g g r e a t c i r c l e l o c i w h ich c o r r e s p o n d t o t h e a x i a l p l a n e o f the f o l d . Phase Two f o l d axes i n s m a l l a r e a s s c a t t e r a b o u t f l a t t e n e d s m a l l c i r c l e l o c i ( F i g . 1-9). The l i n e a t i o n p a t t e r n s a r e g e o m e t r i c a l l y c o n s i s t e n t w i t h r e f o l d i n g and f l a t t e n i n g by Phase T h r e e b u c k l e f o l d s . On a l a r g e r s c a l e , Phase Two f o l d axes show a more s c a t -t e r e d d i s t r i b u t i o n ( F i g . 1-9). These l i n e a t i o n t r e n d s c o n t r a s t w i t h t h e Phase One l i n e a t i o n s i n t h e P i n g s t o n f o l d a r e a o f t h e A u t o c h t h o n o u s Core G n e i s s Domain, w h i c h show o n l y t h e n o r t h w e s t h a l f o f the s m a l l c i r c l e , c o n s i s t e n t w i t h i t s p o s i t i o n on t h e n o r t h e a s t l i m b o f a l a r g e Phase F o u r f o l d . The o c c u r r e n c e o f b o t h n o r t h w e s t and s o u t h w e s t p l u n g i n g l i n e a t i o n s w i t h i n s m a l l a r e a s i n t h e Basement C o r e d Nappe Domain r e f l e c t s s m a l l -s c a l e r e f o l d i n g . The a x i a l p l a n e s o f Phase Two f o l d s a r e deformed by Phase T h r e e f o l d s , t h e p o l e s t o Phase Two a x i a l p l a n e s b e i n g s c a t t e r e d a r o u n d a g r e a t c i r c l e g i r d l e w i t h t h e modal Phase T h r e e f o l d a x i s (4°/134°) as t h e -rr p o l e . Phase T h r e e f o l d s , w h i c h range i n s t y l e from open t o i s o c l i n a l ( F i g . 1-10), w i t h s h a l l o w l y d i p p i n g a x i a l p l a n e s , o v e r p r i n t Phase Two f o l d s . Phase T h r e e f o l d s a r e c l o s e t o c o - a x i a l w i t h Phase Two. As shown i n F i g u r e 1-9, t h e s e axes show c o n s i d e r a b l e s c a t t e r r e f l e c t i n g b o t h the s u p e r p o s i t i o n o f s m a l l e r - s c a l e Phase T h r e e f o l d s on Phase Two s t r u c t u r e s and the e f f e c t o f l a r g e - s c a l e Phase F i v e b u c k l e f o l d s . The p o l e s t o the a x i a l p l a n e s o f Phase T h r e e f o l d s form a g r e a t c i r c l e g i r d l e w i t h a TT p o l e which t r e n d s 1 6 ° / 2 5 2 ° . T h i s o r i e n t a t i o n i s c o n s i s t e n t w i t h mesosco-p i c b u c k l e s and the s m a l l e r - s c a l e Phase F i v e f o l d s shown i n the s o u t h w e s t c o r n e r o f F i g u r e 1-8. 229 Thus f a r , i t has been assumed t h a t t he l a r g e - s c a l e basement c o r e d nappes o f t h i s domain a r e Phase Two s t r u c t u r e s . T h i s c o n c l u s i o n i s b a s e d on two o b s e r v a t i o n s : f i r s t l y , t h e s e nappe s t r u c t u r e s r e f o l d m i n o r f o l d s and l i n e a t i o n s i d e n t i f i a b l e w i t h Phase One; and s e c o n d l y , t h e a x i a l p l a -nes o f t h e l a r g e - s c a l e nappes ( F i g . 1-11) a r e s t e e p l y d i p p i n g compared w i t h m e s o s c o p i c Phase T h r e e f o l d s . M e s o s c o p i c s t r u c t u r e s , l i n e a t i o n p a t -t e r n s and o b s e r v e d o v e r p r i n t i n g r e l a t i o n s s u g g e s t t h a t t he main s t r u c -t u r e s a r e Phase Two. F i g u r e 1-9 shows a s t e r e o n e t p l o t o f Phase One l i n e a t i o n s , f o l d axes and a x i a l p l a n e s , showing the e f f e c t o f o v e r p r i n t i n g o f Phase Two and s u b s e q u e n t phases o f d e f o r m a t i o n . Phase One a x i a l p l a n e s show a unimodal g i r d l e r e l a t e d t o t h e l a r g e - s c a l e Phase Two f o l d s . Note t h a t Phase One f o l d axes and l i n e a t i o n s a r e s c a t t e r e d a r o u n d t h i s same g r e a t c i r c l e . T h i s i s p r o b a b l y an a r t i f a c t r e s u l t i n g from an a p p r o x i m a t e l y 90° a n g l e between the o r i g i n a l Phase One a x i s o r i e n t a t i o n and t h e axes o f t h e s u p e r i m p o s e d Phase Two f o l d s . The o r i e n t a t i o n d a t a a r e c o n s i s t e n t w i t h , and i n d e e d n e c e s s i t a t e , t h e i n t e r p r e t a t i o n t h a t t h i s domain r e p r e s e n t s i m b r i c a t i o n o f basement and c o v e r r o c k s as p a r t o f l a r g e - s c a l e , n o r t h e r l y - v e r g e n t nappes t h r u s t o v e r t he A u t o c h t h o n o u s Core G n e i s s Domain. T r a n s i t i o n a l Domain o f I m b r i c a t e T h r u s t i n g The o r i e n t a t i o n o f the dominant m e s o s c o p i c t i g h t - t o - i s o c l i n a l f o l d s i n t h i s domain i s shown i n F i g u r e 1-12. S y s t e m a t i c changes i n the o r i e n -t a t i o n o f t h e s e f o l d s a i d e d i n d e l i n e a t i n g t he t h r u s t s h e e t s mapped i n t h i s domain. R e e s o r and Moore (1971) mapped two u n i t s o f c o r e g n e i s s e s s e p a r a t e d by g n e i s s e s and s c h i s t s o f t h e i r m a n t l i n g g n e i s s e s u n i t ( c o v e r 230 FIGURE I-11. C r o s s - s e c t i o n o f Gunnarsen and O d i n Nappes. S e c t i o n l i n e shown i n F i g u r e 1-7. The s t r u c t u r a l s t y l e i s b a s e d on the o u t c r o p p a t t e r n and o b s e r v a t i o n s o f m e s o s c o p i c Phase Two f o l d s . ro co 232 FIGURE 1-12. G e o l o g i c a l map o f C a r i b o u A l p , showing t y p i c a l g e o l o g y o f t h e T r a n s i t i o n a l Domain o f I m b r i c a t e T h r u s t i n g . The s o u t h e r n and uppermost t h r u s t s h e e t , Nappe I , i s C a r i b o u Nappe. The o t h e r t h r u s t s h e e t s a r e l a b e l e d II to IV i n o r d e r o f d e s c e n d i n g s t r u c t u r a l l e v e l . The s t r u c t u r a l r e l a t i o n s a r e d i s c u s s e d i n the t e x t . 27 ' ^ BASEMENT GNEISS QUARTZITE COVER ROCKS KM sfjgf STRIKE AND PLUNGE LYTSfc^ OF PHASES 1,2,3 FOLDS <4^~\. THRUST FAULTS AXIAL TRACE OF V PHASE 3 NAPPE no co CO 234 r o c k s ) . D e t a i l e d s t r u c t u r a l mapping a c c o m p l i s h e d d u r i n g the p r e s e n t s t u d y i d e n t i f i e d t he e x i s t e n c e o f f o u r t h r u s t s l i c e s ( F i g . 1-12). On a b r o a d s c a l e , t h i s domain i s a wedge-shaped s l i c e o f m o n o c l i n a l l y - d i p p i n g g n e i s s e s and s c h i s t s which t a p e r s t o the e a s t and i s f a u l t e d o u t near t h e s o u t h e a s t c o r n e r o f t h e dome ( F i g . 1-1). The s t r u c t u r a l l y h i g h e s t t h r u s t s h e e t ( F i g . 1-12), h e r e i n c a l l e d t he C a r i b o u Nappe, i s a l a r g e , recumbent Phase T h r e e a n t i f o r m i n w e l l - l a y e r e d , q u a r t z o - f e l d s p a t h i c and g a r n e t - s i l l i m a n i t e g n e i s s e s o f the c o v e r r o c k s e q u e n c e . The C a r i b o u Nappe i s s t r u c t u r a l l y c o m p l i c a t e d on an o u t c r o p s c a l e . A l t h o u g h Phase Two f o l d s a r e t h e most e x t e n s i v e l y d e v e l o p e d m e s o s c o p i c f o l d s , t h e y a r e t y p i c a l l y o v e r p r i n t e d by Phase T h r e e s t r u c -t u r e s . T y p i c a l Phases Two/Three i n t e r f e r e n c e s t r u c t u r e s a r e shown i n F i g u r e 1-13. The a x i a l t r a c e o f t h e C a r i b o u Nappe shown i n F i g u r e 1-13 was mapped on t h e b a s i s o f the v e r g e n c e s o f m e s o s c o p i c Phase T h r e e f o l d s . To t h e s o u t h o f t h e a x i a l t r a c e , t h e upper l i m b o f the nappe i s dominated by n o r t h e r l y - v e r g e n t s t r u c t u r e s . The n o r t h e r n l o w e r l i m b o f the nappe, o u t c r o p p i n g a l o n g the e a s t e r n s i d e o f C a r i b o u A l p ( F i g . 1-12), i s c h a r a c -t e r i z e d by s o u t h e r l y - v e r g e n t , a t t e n u a t e d Phase Two f o l d s . The a x i a l t r a c e o f t h i s a n t i f o r m i s t r u n c a t e d by a zone o f i n t e n s e f l a t t e n i n g , r o o t l e s s f o l d s and s h e a r i n g . T h i s zone i s i n t e r p r e t e d as a r e c r y s t a l -1 i z e d m y l o n i t e f o r m i n g the b a s a l t h r u s t o f t h e nappe. The o r i e n t a t i o n s o f m e s o s c o p i c s t r u c t u r e s from the C a r i b o u Nappe a r e p l o t t e d on the s t e r e o n e t ( F i g . 1-14). The Phase Two f o l d axes p l o t a l o n g a m o d i f i e d s m a l l c i r c l e d i s t r i b u t i o n c o n s i s t e n t w i t h t h e i r b e i n g r e f o l d e d by Phase T h r e e f o l d s w i t h a s u p e r i m p o s e d f l a t t e n i n g s t r a i n ( s e e Ramsey, 1967, p. 4 6 6 ) . P o l e s t o Phase Two a x i a l p l a n e s f a l l a l o n g a g r e a t c i r c l e 235 FIGURE 1-13. (a) t o (d) M e s o s c o p i c f o l d s f r o m Nappe I ( C a r i b o u Nappe): (a) Phase T h r e e f o l d s w i t h h i n g e - t y p e v e r g e n c e r e f o l d a Phase Two a n t i f o r m w i t h i n q u a r t z o - f e l d s p a t h i c g n e i s s e s . (b) Open c h e v r o n - s t y l e Phase T h r e e f o l d s . ( c ) Phase Two f o l d s i n a q u a r t z i t e / q u a r t z o - f e l d s p a t h i c m u l t i - l a y e r . (d) T i g h t Phase Two f o l d s and r e l a t e d t h r u s t f a u l t s r e f o l d e d by a l a r g e r - s c a l e Phase T h r e e a n t i f o r m . (e) and ( f ) M e s o s c o p i c f o l d s from Nappe I I : (e) T i g h t - t o - i s o c l i n a l Phase T h r e e f o l d s and r e l a t e d t e c t o n i c s l i d e s i n f o l i a t e d q u a r t z o - f e l d s p a t h i c g r e y g n e i s s e s . ( f ) N o r t h e r l y - v e r g e n t u p r i g h t Phase T h r e e f o l d s deform Phase Two i s o c l i n e s i n m y l o n i t i c q u a r t z o - f e l d s p a t h i c g n e i s s e s . 236 237 FIGURE 1-13 ( c o n t i n u e d ) . (g) t o ( j ) M e s o s c o p i c f o l d s f r o m Nappe I I I : (g) Phase Two t i g h t - t o - i s o c l i n a l f o l d s and an a s s o c i a t e d Phase Two t h r u s t . (h) A t t e n u a t e d , t i g h t Phase T h r e e f o l d s and m y l o n i t i c t e c t o n i c s l i d e s . These d e f o r m Phase Two s i l l i m a n i t e l i n e a t i o n s . ( i ) Phase Two f o l d s , deformed by Phase T h r e e s t r u c t u r e s — a l a r g e r - s c a l e recumbent a n t i f o r m and a s s o c i a t e d t e c t o n i c s l i d e s . The l e n s - l i k e geometry o f l e s s deformed Phase Two s t r u c t u r e s , bounded by Phase T h r e e s l i d e z o n e s , i s c h a r a c t e r i s t i c o f t h i s nappe. ( j ) Phase Two t e c t o n i c s l i d e t h a t a p p e a r s t o have normal o r " l a g " d i s p l a c e m e n t . Q u a r t z v e i n m a t e r i a l (shown i n d a r k h a t c h u r e ) has been p r e c i p i t a t e d s u b s e q u e n t t o f a u l t i n g . (k) and (1) M e s o s c o p i c f o l d s from Nappe IV: (k) Phase T h r e e (?) f o l d s and a s s o c i a t e d f a u l t s , i n i n t e r -l a y e r e d q u a r t z i t e s and q u a r t z o - f e l d s p a t h i c g n e i s s e s . (1) A r o o t l e s s Phase Two ( ? ) f o l d i n q u a r t z - r i c h m e t a s e d i -ments fro m the l o w e r m a r g i n o f Nappe IV, a few meters above t h e boundary o f t h e G l a d s h e i m i n f o l d . 238 239 FIGURE 1-14. Stereonet plots for the four nappes from the Transitional Domain of Imbricate Thrusting: Phase One, f o l d axes * , axial planes x ; Phase Two, f o l d axes ° , axial planes • ; Phase Three, f o l d axes A , axi a l planes A . (a) Nappe I, Caribou Nappe: Phase Two f o l d axes plot on a flattened small c i r c l e around the modal Phase Three fo l d axis (plunging 12/245°). The modal Phase Three axi a l plane, shown by the dashed l i n e , dips 20° south with a s t r i k e of 300°. (b) Nappe I I : Phase Two fo l d axes scatter around a complex locus asymmetrically d i s t r i b u t e d around the modal Phase Three f o l d axis (50/249°). The modal Phase Three axial plane dips 60° south with a s t r i k e of 296°. A, B and C represent the average orientations of flattened pebbles at three locations within the nappe. (c) Nappe I I I : Phase Two f o l d axes f a l l along a complex locus asymmetrically d i s t r i b u t e d about the modal Phase Three f o l d axis (23/275°). Some Phase Two fo l d axes plot i n the southeast segment of the net and may represent folds l o c a l l y rotated by f a u l t i n g . The modal Phase Three a x i a l plane dips 40 south with a s t r i k e of 305°. (d) Nappe IV: Phases Three and/or Two f o l d c l u s t e r around 5/307° and scatter around a great c i r c l e , interpreted as the Phase Three axial plane, which dips 40° south and strikes 312°. 2 4 1 g i r d l e whose p o l e c o r r e s p o n d s w i t h t h e main c o n c e n t r a t i o n o f Phase T h r e e f o l d axes ( F i g . 1 - 1 4 ) . T h i s c l e a r l y c o n s i s t e n t p a t t e r n i s t h e p e r v a s i v e o v e r p r i n t i n g o f Phase Two f o l d s by Phase T h r e e . The "modal Phase T h r e e a x i a l p l a n e " shown i n F i g u r e 1-14 i s a b e s t - f i t l i n e c o n s t r u c t e d u s i n g t h e modal m e s o s c o p i c Phase T h r e e a x i a l p l a n e , t h e TT p o l e t o the Phase Two a x i a l p l a n e s and t h e s m a l l d egree o f g r e a t c i r c l e d i s p e r s i o n shown by the Phase T h r e e f o l d a x e s . T h i s modal a x i a l p l a n e i s s u r p r i s i n g l y s i m i l a r t o t h e o r i e n t a t i o n o f Phase T h r e e a x i a l p l a n e s i n t h e u n d e r l y i n g Basement C o r e d Nappe Domain and t h e o v e r l y i n g C o v e r Rock Domain. S t r u c t u r a l l y b e n e a t h t h e C a r i b o u Nappe i s a s e r i e s o f t h r u s t s h e e t s i n w h i c h t h e m e s o s c o p i c f o l d s a r e so a t t e n u a t e d and d i s c o n n e c t e d t h a t i t i s n o t a l w a ys p o s s i b l e t o d e t e r m i n e the o v e r a l l v e r g e n c e p a t t e r n s . Where v e r g e n c e s a r e c l e a r , t h e y a r e n o r t h e r l y o r h i n g e t y p e . T h i s , t o g e t h e r w i t h a c h a r a c t e r i s t i c n o r t h e r l y d i s p l a c e m e n t on s m a l l - s c a l e t h r u s t s , s u g g e s t s t h e t h r u s t s l i c e s a r e i s o c l i n a l , f l a t t e n e d a n t i f o r m s o r nappes. The l o w e r l i m b s and s y n f o r m s o f t h e s e s t r u c t u r e s a p p a r e n t l y have been o b l i t e r a t e d by l a r g e f l a t t e n i n g s t r a i n s . The dominant m e s o s c o p i c f o l d s a r e i s o c l i n a l Phase Two a nd/or Phase T h r e e f o l d s which a r e t y p i c a l l y d i s r u p t e d by s m a l l s c a l e t h r u s t s and l a g s t r u c t u r e s ( F i g . 1-13). F o l d s a r e c h a r a c t e r i s t i c a l l y r o o t l e s s and bounded by s h e a r zones o f augen g n e i s s . D e t a i l e d e x a m i n a t i o n r e v e a l s t h a t t h e s e augen a r e o f t e n f l a t -t e n e d f r a g m e n t s o f d i s r u p t e d p e g m a t i t e . I t i s assumed t h a t most o f t h e m e s o s c o p i c f o l d s i n t h e s e l o w e r t h r u s t s h e e t s a r e Phase Thr e e f o l d s , d e s p i t e the s t e e p d i p o f t h e i r a x i a l p l a n e s . T h i s i n t e r p r e t a t i o n i s s u p p o r t e d by t h e o b s e r v e d geometry o f m e s o s c o p i c r e f o l d e d f o l d s ( F i g . 1-13). I t a p p e a r s t h a t t h e s e Phase Thr e e 242 folds have been rotated by superimposed s t r a i n concentrated in these three thrust sheets. Similar situations have been documented in the overlying Cover Rock Domain on a mesoscopic scale. Figure 1-15 shows Phase Three structures from the Cover Rock Domain 3 km east of the Big Ledge, and a cross-section across the imbricated thrust sheets mapped in Caribou Alp (Fig. 1-12). The s i m i l a r i t y between these two structures suggests that the zones of f l a t t e n i n g and di s l o c a t i o n (thrusting) are late-stage Phase Three d u c t i l e phenomena. This interpretation i s rein-forced by the observation that on an outcrop scale, thrust and s l i d e zones are congruent with the axial planes of Phase Three structures in the lower three thrust sheets. The orientation of mesoscopic t i g h t - t o - i s o c l i n a l folds varies systematically among these four thrust sheets. The nappe immediately underlying the Caribou Nappe, designated Nappe II in t h i s study, i s characterized by attenuated i s o c l i n a l Phase Two f o l d s , refolded by north-vergent, upright Phase Three folds. As shown in Figure 1-14, Phase Three f o l d axes form a great c i r c l e d i s t r i b u t i o n (296/60S) corresponding to the modal axial plane for these folds. Phase Two f o l d axes are dispersed along a flattened small c i r c l e locus on the south side of the Phase Three axial plane. The asymmetry of the Phase Two l i n e a t i o n l o c i suggests that the principal axes of the superimposed f l a t t e n i n g strains are at an angle to the symmetry axes of the Phase Three folds (see Ramsay, 1967, p. 468). The modal Phase Three f o l d axis (275/23W) i s the -rr pole to the great c i r c l e that passes through the pole to the modal Phase Three axial plane and i s f i t through the dispersed poles to Phase Two axial planes. The next lowest thrust sheet in the sequence (designated Nappe III i n t h i s study) has a s i m i l a r structural geometry to Nappe I I . Structures 243 FIGURE 1-15. (a) C r o s s - s e c t i o n o f the T r a n s i t i o n a l Domain o f I m b r i c a t e T h r u s t i n g . The s e c t i o n l i n e C — C i s shown i n F i g u r e 1-12. 245 FIGURE 1-15 ( c o n t i n u e d ) (b) S k e t c h o f b u l l d o z e r c u t i n banded c a l c - s i l i c a t e s c h i s t s 4 km e a s t o f Empress La k e . The s t r u c t u r e s a r e a l l i n t e r p r e t e d as p r o d u c t s o f Phase T h r e e d e f o r m a t i o n . The a x i a l p l a n e s o f t h e s e f o l d s have been r o t a t e d and s t e e p e n e d w i t h i n zones o f d u c t i l e f l a t t e n i n g and detachment. The s t e r e o n e t shows o r i e n t a t i o n d a t a f o r t h e f i v e m e s o s c o p i c f o l d s i d e n t i f i e d i n t h e c r o s s - s e c t i o n . The f o l d axes a r e shown as open t r i a n g l e s ; t h e p o l e s t o t h e a x i a l p l a n e s , as s o l i d t r i a n g l e s . 246 247 i n Nappe I I I c o n t r a s t w i t h t h e o v e r l y i n g t h r u s t s h e e t i n t h a t t h e m e s o s c o p i c s t r u c t u r e s show more moderate f l a t t e n i n g and a t t e n u a t i o n . T h i s i s r e f l e c t e d i n t h e l e s s - f l a t t e n e d p a t t e r n o f Phase Two l i n e a t i o n s shown i n F i g u r e 1-14. The modal Phase T h r e e s t r u c t u r e o f Nappe I I I i s d i s t i n c t l y d i f f e r e n t t o t h e o v e r l y i n g nappe. The modal a x i a l p l a n e i s r o t a t e d n o r t h 10° and t h e d i p i s 20° s h a l l o w e r . The l o w e s t t h r u s t s h e e t (Nappe IV) i s c h a r a c t e r i z e d by r o o t l e s s , a t t e n u a t e d i s o c l i n a l f o l d s . As no o v e r p r i n t i n g r e l a t i o n s were o b s e r v e d , i t has been assumed a l l t h e f o l d s i n t h i s domain were Phase Thr e e s t r u c -t u r e s . The f o l d axes measured i n t h i s nappe f a l l a l o n g a g r e a t c i r c l e and show a l a r g e r d e g r e e o f d i s p e r s i o n than t h a t i n t h e o v e r l y i n g t h r u s t s h e e t . Phase T h r e e f o l d axes show a s t r o n g c o n c e n t r a t i o n a round the TT p o l e o r i e n t a t i o n o f 5W/307. The d i f f e r e n c e s i n Phase T h r e e o r i e n t a t i o n f o r t h e s e f o u r t h r u s t s h e e t s d e m o n s t r a t e t h e e x i s t e n c e o f r e l a t i v e r o t a -t i o n between t h e s e t h r u s t s h e e t s d u r i n g t h e i r f o r m a t i o n . T h u s , t h e T r a n s i t i o n a l Domain o f I m b r i c a t e T h r u s t i n g i s a zone o f i n t e n s e d e v e l o p m e n t o f Phase T h r e e s t r u c t u r e s . I t a p p e a r s t o r e p r e s e n t a zone o f d e c o u p l i n g between the u n d e r l y i n g Basement C o r e d Nappe Domain ( i n w h i c h Phase T h r e e f o l d s a r e o n l y weakly d e v e l o p e d i n the upper p a r t s ) and t h e o v e r l y i n g C over Rock Domain. The m agnitude o f t r a n s l a t i o n a l s t r a i n w i t h i n t h i s domain i s unknown, b u t c o u l d be s u b s t a n t i a l . C o ver Rock Domain The s t r u c t u r e o f the C o v e r Rock Domain i s c h a r a c t e r i z e d by the p r e -s e n c e o f l a r g e - s c a l e mappable Phase Two and Phase T h r e e f o l d s , b o t h o f w h i c h phases have e a s t - w e s t o r i e n t e d , s h a l 1 o w l y - p l u n g i n g f o l d a x e s . T h i s 248 domain i s d i f f e r e n t i a t e d from the u n d e r l y i n g T r a n s i t i o n a l Domain o f I m b r i c a t e T h r u s t i n g by: t h e absence o f i m b r i c a t e t h r u s t f a u l t s ; t h e a b s e n c e o f basement g n e i s s e s ; and Phase T h r e e s t r u c t u r e s which a r e more open i n s t y l e . In c o n t r a s t t o the u n d e r l y i n g domains, i n t h e C o v e r Rock Domain t h e s y n f o r m s have been p r e s e r v e d . L a r g e - s c a l e a n t i f o r m s a r e r a r e , a p p a r e n t l y b e i n g f a u l t e d o u t o r o b l i t e r a t e d by f l a t t e n i n g . O t h e r c h a r a c -t e r i s t i c s o f t h e C o v e r Rock Domain a r e : t h e p r e s e n c e o f u l t r a m a f i c l e n s e s a l o n g m a j o r f a u l t s ; a p p a r e n t l i s t r i c normal f a u l t m o t i o n on the main f a u l t s ; and the o c c u r r e n c e o f zones e x t e n s i v e l y i n t r u d e d by pegma-t i t e s . A l l s i x phases o f d e f o r m a t i o n have been i d e n t i f i e d i n o u t c r o p w i t h i n t h e C o v e r Rock Domain; however, Phase T h r e e s t r u c t u r e s dominate the o u t c r o p p a t t e r n o v e r most o f t h e a r e a . L a r g e l y on t h e b a s i s o f t h e o r i e n t a t i o n o f Phase T h r e e s t r u c t u r e s , t h r e e subdomains have been d e l i n e -a t e d w i t h i n t h e C o v e r Rock Domain. These t h r e e subdomains a r e s e p a r a t e d by m a j o r f a u l t s t h a t e x t e n d a c r o s s the map a r e a shown i n F i g u r e 1-1. A s t r u c t u r a l a n a l y s i s o f each subdomain w i l l be p r e s e n t e d , b e g i n n i n g w i t h t h e B i g Ledge Subdomain, w h i c h a b u t s t h e T r a n s i t i o n a l Domain o f I m b r i c a t e T h r u s t i n g . B i g Ledge Subdomain - The B i g Ledge Subdomain i s c h a r a c t e r i z e d by l a r g e -s c a l e Phase Two f o l d s o u t l i n e d by r e p e t i t i o n o f the l i t h o l o g i c a l sequence and v e r g e n c e r e v e r s a l s i n r a r e m e s o s c o p i c Phase Two i s o c l i n a l f o l d s . T h ese f o l d s a r e s t r o n g l y o v e r p r i n t e d by more open Phase T h r e e f o l d s w i t h s h a l l o w l y - d i p p i n g a x i a l p l a n e s . The b e s t e x p o s e d Phase Two s t r u c t u r e i s t h e s y n f o r m o u t l i n e d by t h e m a r b l e u n i t i n the s o u t h e a s t c o r n e r o f F i g u r e 1-16. T h i s s t r u c t u r e c l o s e s b o t h t o the west and e a s t , a l t h o u g h the 249 FIGURE 1-16. S t r u c t u r a l map o f t h e B i g Ledge subdomain. 250 251 e a s t e r n c l o s u r e c an o n l y be mapped by f l o a t and i s o l a t e d o u t c r o p s . Numerous m e s o s c o p i c Phase Two f o l d s show v e r g e n c e s c o n s i s t e n t w i t h a syn-f o r m a l c l o s u r e . In a d d i t i o n , t h e w e s t e r l y h i n g e zone o f t h e f o l d i s e x t e n s i v e l y e x p o s e d ( s e e F i g . 1-17). On an o u t c r o p s c a l e , t h i s f o l d i s o v e r p r i n t e d by Phase T h r e e f o l d s w i t h s h a l l o w l y - d i p p i n g a x i a l p l a n e s . To t h e n o r t h o f t h i s s t r u c t u r e , a s e r i e s o f t i g h t Phase Two f o l d s have been d e l i n e a t e d by r e p e t i t i o n s o f l i t h o l o g y (Hoy, 1977b) and by v e r g e n c e r e v e r s a l s i n r a r e Phase Two i s o c l i n e s ( F i g . 1-16). As t h e f o l d s a r e i s o c l i n a l , w i t h s h a l 1 o w l y - p l u n g i n g axes which r o u g h l y p a r a l l e l t he l a n d s u r f a c e , t h e i r h i n g e s a r e p o o r l y e x p o s e d . The h i n g e zones a r e t y p i -c a l l y o b s c u r e d by p o o r o u t c r o p i n t h e v a l l e y s . T hese s t r u c t u r e s a r e s t r o n g l y o v e r p r i n t e d by Phase T h r e e f o l d s . Phase T h r e e f o l d s w i t h i n t h i s subdomain show a c o n s i s t e n t s o u t h e r l y v e r g e n c e , i n d i c a t i n g t h a t t h e a r e a i s t h e l o w e r l i m b o f a l a r g e Phase T h r e e a n t i f o r m . M e s o s c o p i c Phase T h r e e f o l d s w i t h h i n g e - t y p e v e r g e n c e o c c u r a l o n g t h e c r e s t o f t h e f l a t - t o p p e d r i d g e t h a t g i v e s t he a r e a i t s name ( t h e B i g L e d g e ) . The top o f t h e r i d g e a p p r o x i m a t e l y p a r a l l e l s t he Phase T h r e e a x i a l p l a n e ( F i g . 1-18). Ross (1973) has documented the o c c u r r e n c e o f m y l o n i t e zones w i t h i n t h e c o r e s o f l a r g e - s c a l e Phase T h r e e f o l d s . These a r e r e l a t e d to room problems i n the c o r e s o f such s t r u c t u r e s . The h i n g e zone a r e a o f the l a r g e Phase T h r e e f o l d shown i n F i g u r e s 1-18 and 1-19 i s c h a r a c t e r i z e d by m e s o s c o p i c Phase T h r e e t h r u s t s t h a t c a u s e l o c a l i n t e r n a l d i s r u p t i o n . F i g u r e 1-20 shows a map o f a s m a l l a r e a i n which t h e s e Phase T h r e e s l i d e z o n es a r e e x t e n s i v e l y d e v e l o p e d . These zones a r e c o n c e n t r a t e d i n a r e a s w i t h l a y e r e d l i t h o l o g i e s w i t h s t r o n g d u c t i l i t y c o n t r a s t s , t y p i c a l l y i n t e r b e d d e d q u a r t z o - f e l d s p a t h i c g n e i s s e s and m a r b l e s . 252 FIGURE 1-17. M e s o s c o p i c f o l d s f r o m t h e C o v e r Rock Domain. (a) L a r g e - s c a l e Phase T h r e e f o l d s i n t h e c o r e o f t h e Mt. F o s t h a l l S y n f o r m on t h e l o w e r s o u t h e a s t s i d e o f Mt. F o s t h a l l . Note t h e f a u l t e d n a t u r e o f t h e o v e r t u r n e d l i m b . The h o r i z o n t a l s i l l s a r e l a t e -s t a g e ( T e r t i a r y ) a p l i t e s . The s y n f o r m i s c o r e d by banded c a l c - s i 1 i c a t e s u n d e r l a i n by g a r n e t - b i o t i t e -s i l l i m a n i t e s c h i s t , f a u l t e d a g a i n s t m a r b l e (no h a t c h u r e ) , and q u a r t z o - f e l d s p a t h i c g n e i s s e s t o t h e s o u t h , above t h e f a u l t . (b) Phase T h r e e f o l d ( f o l d a x i s 6/277°) r e f o l d i n g t i g h t Phase Two s t r u c t u r e s f r o m e a s t s i d e o f Mt. Symonds. Note t h e Phase T h r e e s l i d e zone. (c) R e f o l d i n g o f Phase Two f o l d s ( f o l d axes 7/280°) by Phase T h r e e f o l d s ( f o l d axes 3/284°) f r o m t h e c o r e o f th e Mt. F o s t h a l l S y n f o r m , F o s t h a l l R i d g e , 3 km e a s t o f Mt. F o s t h a l l . The r o c k t y p e s a r e i n t e r l a y e r e d c a l c - s i 1 i c a t e r i c h q u a r t z i t e s ( d o t s ) and q u a r t z -f e l d s p a r - b i o t i t e g n e i s s e s . (d) T y p i c a l Phase T h r e e f o l d s ( f o l d axes 9/277°) i n the c o r e zone o f the Mt. F o s t h a l l S y nform, F o s t h a l l R i d g e , 3 km e a s t o f Mt. F o s t h a l l . Rocks c o n s i s t o f b i o t i t e -g a r n e t - s i l l i m a n i t e s c h i s t ( u n h a t c h u r e d ) w i t h t h i n l a y e r s o f q u a r t z o f e l d s p a t h i c g n e i s s ( d o t s ) and b i o t i t e - q u a r t z - f e l d s p a r g n e i s s ( d a s h e s ) . 253 254 FIGURE 1-17 ( c o n t i n u e d ) . (e) Drawing o f t h e o u t c r o p o f the h i n g e zone o f t h e ma j o r Phase Two c l o s u r e o f m a r b l e i n t h e s o u t h e a s t c o r n e r o f F i g u r e 1-16. The m a r b l e i s shown by s h o r t d a s h e s , t h e c o r e o f b i o t i t e - g a r n e t - s i l l i m a n i t e s c h i s t by S's. Note t h e s u p e r p o s i t i o n o f s m a l l - s c a l e s h a l l o w l y - d i p p i n g Phase T h r e e s t r u c t u r e s on t h e l i m b s o f t h e main s t r u c t u r e . The a c t u a l o u t c r o p s a r e d e l i n e a t e d by s o l i d l i n e s , t h e i n f e r r e d c o n t a c t s by h e a v i l y dashed l i n e s . ( f ) T h i s shows i n d e t a i l t h e f a r l e f t o r n o r t h e r n m o s t o u t c r o p shown i n (e) above. The c r o s s e s a r e g r a n i t i c g n e i s s l e n s e s . Note t h e Phase T h r e e f o l d s ( f o l d axes 1 5 / 9 1 ° , a x i a l p l a n e 2 0 ° S/252°) deformed t h e t i g h t Phase Two a n t i f o r m ( f o l d a x i s 4 / 9 7 ° ) . 255 256 FIGURE 1-18. (a) and (b) C r o s s - s e c t i o n s o f t h e B i g Ledge subdomain. 258 FIGURE 1-19. S k e t c h o f an u p r i g h t Phase Two a n t i f o r m o v e r p r i n t e d by Phase T h r e e f o l d s w i t h s h a l l o w l y - d i p p i n g a x i a l p l a n e s , on t h e e a s t f l a n k o f Mt. Symonds. The Phase Two a n t i f o r m i s c o r e d by g r a p h i t i c s u l f i d e - r i c h s c h i s t s , a s s o c i a t e d w i t h t h e B i g Ledge-Mt. Symonds l e a d - z i n c m i n e r a l i z a t i o n . 259 10 m 260 FIGURE 1-20. D e t a i l e d s t r u c t u r a l map o f Phase T h r e e t e c t o n i c s l i d e s s o u t h o f Empress L a k e . The f i g u r e i s an e n l a r g e m e n t o f the a r e a s o u t h o f Empress Lake i n F i g u r e 1-16. The n o r t h - s o u t h o r i e n t e d f a u l t s a r e s t e e p l y - d i p p i n g Phase S i x s t r u c t u r e s . 261 262 M e s o s c o p i c Phase Two f o l d s i n t h e B i g Ledge mine a r e a (Hoy, 1977b) a r e e s s e n t i a l l y c o - a x i a l w i t h t h e s u p e r i m p o s e d Phase T h r e e f o l d s . T h i s i s shown i n t h e s t e r e o n e t p l o t i n F i g u r e 1-21. The a x i a l p l a n e s t o Phase Two a r e d i s p e r s e d a r o u n d a g r e a t c i r c l e g i r d l e by Phase T h r e e ; however, t h e f o l d axes f o r b o t h phases f a l l i n t h e same c l u s t e r . In t h e w e s t e r n p o r t i o n o f t h e B i g Ledge Subdomain, Phase T h r e e f o l d axes show a more v a r i a b l e o r i e n t a t i o n ( F i g . 1-21), s c a t t e r i n g a l o n g a g r e a t c i r c l e which r e p r e s e n t s t h e modal Phase T h r e e a x i a l p l a n e . The e l o n g a t e c l u s t e r o f p o l e s t o Phase T h r e e a x i a l p l a n e s i s a r e s u l t o f f a n n i n g o r d i v e r g e n c e , n o t r e f o l d i n g . T h i s f a n n i n g o f the a x i a l p l a n e s , t o g e t h e r w i t h a more v a r i a b l e o r i e n t a t i o n o f c o m p o s i t i o n a l l a y e r s ( r e l a t e d t o the p r e s e n c e o f Phase One f o l d s ) , e x p l a i n s t h e g r e a t e r d e g r e e o f s c a t t e r i n Phase T h r e e f o l d axes from e a s t t o west a c r o s s t h e subdomain. A l a r g e zone o f m e s o s c o p i c Phase One f o l d i n g has been mapped i n t h i s subdomain. A l a r g e f o l d h i n g e o u t l i n e d . b y m a r b l e i s shown i n the n o r t h -c e n t r a l p a r t o f F i g u r e 1-16, n o r t h o f the f a u l t . T h i s m a p - s c a l e Phase One c l o s u r e can be t r a c e d as a zone o f i n t e n s e d e v e l o p m e n t o f m e s o s c o p i c Phase One s t r u c t u r e s f o r o v e r a k i l o m e t e r t o the west. O r i e n t a t i o n d a t a f o r t h i s f o l d a r e shown i n F i g u r e 1-22. The Phase One f o l d axes and l i n e a t i o n s s c a t t e r a b o u t a g r e a t c i r c l e l o c u s t h a t i s t h e modal Phase One a x i a l p l a n e . T h i s p a t t e r n i s n o t , however, t h e p r o d u c t o f r e g i o n a l r e f o l d i n g , as t h e f u l l r ange o f v a r i a t i o n can be f o u n d i n one o u t c r o p ( F i g . 1-22). C a r e f u l e x a m i n a t i o n o f t h e d i s t r i b u t i o n o f f o l d a x i s o r i e n -t a t i o n s i n the f i e l d s u g g e s t s t h a t t he o b s e r v e d v a r i a t i o n i s c a u s e d by non-homogeneous s t r a i n , s h e a r i n g and r o t a t i o n o f m e s o s c o p i c h i n g e s w i t h t h e c o r e o f t h e l a r g e - s c a l e Phase One a n t i f o r m . 263 FIGURE 1-21. S t e r e o n e t p l o t s o f o r i e n t a t i o n d a t a f o r t h e B i g Ledge subdomain. The symbols a r e t h e same as t h o s e used i n F i g u r e 1-14. (a) Phases Two and T h r e e f o l d s f r o m a r o u n d Empress L a k e , showing c o - a x i a l n a t u r e o f t h e s e p h a s e s . G r e a t c i r c l e drawn t h r o u g h p o l e s t o Phase Two a x i a l p l a n e s ( s o l i d l i n e ) i s a TT g i r d l e , t h e p o l e t o which i s t h e modal Phase T h r e e f o l d a x i s (.10 7 9 0 ° ) . The modal Phase T h r e e a x i a l p l a n e (30°S/256°) i s shown by t h e dashed g r e a t c i r c l e . (b) Phase One f o l d s f r o m a s i n g l e o u t c r o p . The v a r i o u s f o l d axes show no s y s t e m a t i c r e l a t i o n between o r i e n t a t i o n and p o s i t i o n , s u g g e s t i n g t h e y may r e p r e s e n t r o t a t i o n o f f a u l t s l i v e r s w i t h i n t h e Phase One a x i a l p l a n e , w h i c h d i p s 40° s o u t h and s t r i k e s 3 0 8 ° . ( c ) Phases Two and T h r e e f o l d s f r o m the w e s t e r n p o r t i o n o f t h e B i g Ledge subdomain. The i n t e r p r e t a t i o n o f t h e s e d a t a i s d i s c u s s e d i n the t e x t . The g r e a t c i r c l e ( d a s h e d l i n e ) i s t h e modal Phase T h r e e a x i a l p l a n e . (d) Phase One f o l d axes and l i n e a t i o n s . 264 265 FIGURE 1-22. O r i e n t a t i o n d a t a c o l l e c t e d f r o m the Phase One h i n g e zone shown i n t h e n o r t h - c e n t r a l p o r t i o n o f F i g u r e 1-23, n o r t h o f t h e f a u l t . Phase One f o l d axes and l i n e a t i o n s ( ° ) f o r m a unimodal c l u s t e r . The p o l e s t o c o m p o s i t i o n a l l a y e r i n g ( • ) p l o t on a g r e a t c i r c l e ( s o l i d l i n e ) whose p o l e i s t h e modal f o l d a x i s f o r Phase One ( A ) . The f o l d a x i s measurements shown as an a s t e r i s k a r e from a s i n g l e t e n s q u a r e meter o u t c r o p a r e a from the h i n g e o f t h e f o l d . The modal Phase One a x i a l p l a n e ( d a s h e d l i n e ) i s drawn t h r o u g h t h e modal f o l d a x i s and the normal t o t h e a v e r a g e p o l e t o Phase One a x i a l p l a n e s ( A ) . The l o c a l Phase Two f o l d a x i s ( • ) has a d i s t i n c t l y d i f f e r e n t o r i e n t a t i o n t o Phase One and can be seen t o r e f o l d Phase One l i n e a t i o n s . 266 267 The p r e s e r v a t i o n o f t h e s e Phase One s t r u c t u r e s i s p r o b a b l y a r e s u l t o f t h e l e s s i n t e n s e d e v e l o p m e n t o f Phases Two and T h r e e i n t h e C o v e r Rock Domain. Mount F o s t h a l l Subdomain - The Mount F o s t h a l l Subdomain i s s e p a r a t e d from t h e u n d e r l y i n g B i g Ledge Subdomain by a f a u l t w h ich t r u n c a t e s l i t h o l o g i e s i n b o t h subdomains ( F i g s . 1-1 and 1-23). The f a u l t a l s o j u x t a p o s e s zones w i t h s h a l l o w l y - d i p p i n g Phase T h r e e a x i a l p l a n e s ( t h e B i g Ledge a r e a ) and more s t e e p l y - d i p p i n g a x i a l p l a n e s ( t h e Mount F o s t h a l l a r e a ) . T h i s change i s s h a r p and i s mappable. As shown i n F i g u r e 1-23, t h e t r a c e o f t h i s f a u l t c o r r e s p o n d s w i t h t h e o u t c r o p o f s e v e r a l t h i n s l i v e r s o f meta-p e r i d o t i t e . Above t h i s f a u l t t h e Mount F o s t h a l l Subdomain i s d o m i n a t e d by t h e Mount F o s t h a l l Synform, a l a r g e asymmetric Phase T h r e e s t r u c t u r e . The F o s t h a l l S y n c l i n e was o r i g i n a l l y mapped by R e e s o r and Moore (1971) as a s i m p l e Phase One s t r u c t u r e . However, m e s o s c o p i c examples o c c u r o f Phase T h r e e f o l d s , c o n g r u e n t w i t h t h e main s t r u c t u r e , o v e r p r i n t i n g b o t h Phase One and Phase Two s t r u c t u r e s ( F i g . 1-17). D e t a i l e d mapping o f the syn-form ( F i g . 1-23) r e v e a l s i t t o be a more complex s t r u c t u r e than t h a t mapped by R e e s o r and Moore ( 1 9 7 1 ) . T h i s i s shown g r a p h i c a l l y i n the c r o s s - s e c t i o n a c r o s s t h e s t r u c t u r e s shown i n F i g u r e 1-24. From west t o e a s t a c r o s s F i g u r e 1-23, t h e synform becomes more complex, b i f u r c a t i n g i n t o a s y n f o r m - a n t i f o r m - s y n f o r m s t r u c t u r e towards t h e e a s t e r n m a r g i n . C o i n c i d e n t w i t h t h i s change the s t r u c t u r e becomes more open, c h a n g i n g from a t i g h t o v e r t u r n e d s t r u c t u r e i n the west t o a more open u p r i g h t s t r u c t u r e i n t h e c e n t r a l p a r t o f F i g u r e 1-23. 268 FIGURE 1-23. S t r u c t u r a l map o f t h e Mt. F o s t h a l l subdomain. 269 270 FIGURE 1-24. C r o s s - s e c t i o n s a c r o s s t he Mt. F o s t h a l l s y n f o r m . The t h r e e c r o s s - s e c t i o n l i n e s , F — F 1 , G — G 1 and H—H', a r e shown on F i g u r e 1-23. 271 h 10,000 < > LU _J LU rf KM MOUNT FOSTHALL SYNFORM KM 10,000 •MOUNT FOSTHALL SUBDOMAIN-MOUNT FOSTHALL SYNFORM DIPS of PHASE THREE AXIAL PLANES 59 68 62 78 45 51 76 65 .BIG LEDGE SUBDOMAIN 40 26 43 42/ 17 16 26 29 17 25 MOUNT . FOSTHALL-SUBDOMAIN •10,000 CO 2 O H < > LU _J UJ / •BIG L E D G E SUBDOMAIN-PHASE THREE THRUST ? 272 The f a u l t s e p a r a t i n g t h e Mount F o s t h a l l s y n f o r m from the u n d e r l y i n g domain must have c o n s i d e r a b l e d i s p l a c e m e n t a l o n g i t . As shown i n F i g u r e 1-23, t h e f a u l t t r u n c a t e s an e x t e n s i v e u n i t o f banded c a l c - s i l i c a t e rock i n t h e s o u t h e r n p a r t o f t h e B i g Ledge Subdomain. A l t h o u g h t h e same l i t h o l o g y o c c u r s i n t h e c o r e o f t h e Mount F o s t h a l l s y n f o r m , t h i s s t r u c -t u r e i s a Phase T h r e e f o l d , whereas t h e s t r u c t u r e s t h a t dominate the B i g Ledge Subdomain a l o n g s t r i k e a r e Phase Two. O r i e n t a t i o n and v e r g e n c e d a t a o f m e s o s c o p i c f o l d s a r e c o n s i s t e n t w i t h t h e i n t e r p r e t a t i o n shown i n F i g u r e 1-24. M e s o s c o p i c Phase T h r e e f o l d s show a p p r o p r i a t e v e r g e n c e r e v e r s a l s a c r o s s t h e main f o l d c l o s u r e s , and t h e s e can be used i n mapping t h e a x i a l t r a c e s . As i n the u n d e r l y i n g subdomain, Phase T h r e e and Phase Two f o l d axes f a l l i n the same c l u s t e r ( F i g . I-25a and b ) . The p o l e s t o Phase Two a x i a l p l a n e s f a l l a l o n g a g r e a t c i r c l e g i r d l e whose p o l e i s t h e modal Phase T h r e e f o l d a x i s ( F i g . I - 2 5 a ) . The a x i a l p l a n e s o f Phase T h r e e f a n as i s shown i n the s t e r e o n e t ( F i g . I - 2 5 b ) . The c l u s t e r o f Phase T h r e e a x i a l p l a n e s i n t h i s subdomain may be r e l a t e d to t h o s e i n the u n d e r l y i n g B i g Ledge subdomain by r o t a t i o n a l movement a l o n g t h e f a u l t t h a t s e p a r a t e s them. A r o t a t i o n o f 30° would b r i n g t h e two c l u s t e r s i n t o c o i n c i d e n c e . In c o m p a r i s o n w i t h t h e u n d e r l y i n g subdomain t h e y a r e c l e a r l y s t e e p e r ( c f . F i g s . 1-21 and 1-25). The Upper F a u l t B l o c k Subdomain - S t r u c t u r a l l y above the Mount F o s t h a l l s y n f o r m i s a s e c o n d open s y n f o r m , termed the S i t k u m Lake f o l d by R e e s o r and Moore ( 1 9 7 1 ) . The a r e a t o the s o u t h o f Mount F o s t h a l l was n o t i n c l u d e d i n the a r e a o f d e t a i l e d mapping f o r t h i s s t u d y . The d a t a o f R e e s o r and Moore (1971) and b r i e f r e c o n n a i s s a n c e t r a v e r s e s d u r i n g t h i s s t u d y s u g g e s t t h a t t he b r o a d s t r u c t u r a l geometry o f t h e Sitkum Lake a r e a 273 FIGURE 1-25. S t e r e o n e t p l o t s o f o r i e n t a t i o n d a t a f o r the Mt. F o s t h a l l subdomain. The symbols a r e the same as f o r F i g u r e 1-14. (a) Phase Two s t r u c t u r e s ; the p o l e s t o Phase Two a x i a l p l a n e s form a rr g i r d l e ( s o l i d l i n e ) whose p o l e i s the modal Phase T h r e e f o l d a x i s ( A ) . The dashed l i n e i s t h e modal Phase Three a x i a l p l a n e . Phase One d a t a a r e a l s o p l o t t e d . (b) Phase Three s t r u c t u r e s . Symbols as f o r (a) above. 275 i s s i m i l a r t o t h a t o f t h e Mount F o s t h a l l synform; t h a t i s , b o t h a r e l a r g e - s c a l e Phase T h r e e s y n f o r m s . As p r e v i o u s l y p o i n t e d o u t by R e e s o r and Moore (1971, p. 9 8 ) , t h e r e i s no e v i d e n c e o f a n t i f o r m a l c l o s u r e s be-tween t h e s e two s y n f o r m s . S t r u c t u r a l l y above t h e S i t k u m Lake s y n f o r m i s a c r o s s c u t t i n g s h e e t d o m i n a t e d by p e g m a t i t e and s t r o n g l y l i n e a t e d g r a n i t i c r o c k . The l i n e a t i o n i n the p e g m a t i t e and g r a n i t i c l e n s e s t r e n d s e a s t -w e s t , b r o a d l y p a r a l l e l t o the t r e n d o f t h e Phase T h r e e f o l d a x i s . As shown i n F i g u r e 1-1, a t h i r d t h r u s t s h e e t o f l i n e a t e d q u a r t z mon-z o n i t e o v e r l i e s t h e p e g m a t i t i c r o c k s . To t h e e a s t t h i s t h r u s t b l o c k c u t s a c r o s s t h e p e g m a t i t i c r o c k s t o s t r u c t u r a l l y l o w e r l e v e l s . The r e s u l t o f t h i s geometry i s t h a t b o t h t h e S i t k u m Lake s h e e t and t h e t h r u s t s h e e t o f p e g m a t i t e a r e c u t o u t such t h a t , e a s t o f t h e B i g Ledge, the l i n e a t e d q u a r t z m o n z o n i t e i s i n f a u l t c o n t a c t w i t h the B i g Ledge subdomain. Data g a t h e r e d by R e e s o r and Moore (1971, f i g u r e 12) show t h a t t h e l i n e a t i o n i n t h e q u a r t z m o n z o n i t e i s c o n s i s t e n t w i t h the Phase T h r e e t r e n d s i n the u n d e r l y i n g f a u l t b l o c k s . The upper s e c t i o n o f t h i s t h r u s t s h e e t i s mapped by R e e s o r and Moore as a m y l o n i t e zone o v e r l a i n i n f a u l t c o n t a c t by l o w - g r a d e m e t a s e d i m e n t s t h a t c o r r e l a t e w i t h the M i s s i s s i p p i a n M i l f o r d Group. S t r u c t u r e c o n t o u r i n g t h e l o w e r c o n t a c t o f the q u a r t z m o n z o n i t e t h r u s t s h e e t ( F i g . 1-26) s u g g e s t s t h a t t h i s f a u l t has been f o l d e d by a s e r i e s o f open c h e v r o n - s t y l e f o l d s . As the a x i a l p l a n e s o f t h e s e f o l d s t r e n d 295° to 3 1 0 ° , i t i s assumed t h e s e a r e Phase F o u r s t r u c t u r e s and r e p r e s e n t l a r g e - s c a l e p a r a s i t i c f o l d s on t h e s o u t h w e s t l i m b o f the main Phase F o u r a n t i f o r m which forms t h e domal c u l m i n a t i o n . The s t r u c t u r a l r e l a t i o n s d e s c r i b e d above s u g g e s t t h a t the s t r u c t u r e s o f the upper t h r u s t b l o c k s a r e d o m i n a n t l y Phase T h r e e f e a t u r e s . The 276 FIGURE 1-26. C h e v r o n - s t y l e Phase F o u r f o l d s s o u t h e a s t o f t h e T h o r -O d i n dome. S t r u c t u r e c o n t o u r s on base o f l i n e a t e d q u a r t z m o n z o n i t e , showing t h e o u t c r o p o f t h e m o n z o n i t e and 500' t o p o g r a p h i c and s t r u c t u r e c o n t o u r s . The f o l i a t i o n d a t a a r e f r o m R e e s o r and Moore ( 1 9 7 1 ) . The dashed g r e a t c i r c l e i s t h e TT g i r d l e t o the modal (Phase T h r e e ) l i n e a t i o n o r i e n t a t i o n i n t h e domain shown by t h e d o t t e d o u t l i n e . The s o l i d g r e a t c i r c l e i s a p o s s i b l e Phase F o u r TT g i r d l e . 277 50° 25' 278 f a u l t i n g a p p e a r s t o be a l a t e Phase T h r e e e v e n t . The n a t u r e o f d i s p l a c e -ment a l o n g t h e s e f a u l t s i s n o t known. However, due t o the p r e f e r e n t i a l p r e s e r v a t i o n o f sy n f o r m s r a t h e r than a n t i f o r m s , i t i s i n t e r e s t i n g t o spe-c u l a t e t h a t t h e Mount F o s t h a l l and S i t k u m Lake s y n f o r m s may be r e l a t e d t o l i s t r i c normal f a u l t i n g . Di s c u s s i o n The s t r u c t u r a l a n a l y s i s p r e s e n t e d i n t h i s s t u d y s u g g e s t s t h a t the o r i e n t a t i o n d a t a o f m e s o s c o p i c f o l d s i n d i c a t e t h e dome was formed by t h e i n t e r f e r e n c e o f u p r i g h t Phases F o u r , F i v e and S i x f l e x u r a l - s l i p f o l d s . T h e s e l a t e - s t a g e b u c k l e f o l d s a p p e a r t o have formed a t low t e m p e r a t u r e s , a p p a r e n t l y by a f l e x u r a l - s l i p d o m i n a t e d mechanism. They deform e a r l i e r l i n e a t i o n s i n t o s m a l l c i r c l e l o c i , have c h e v r o n g e o m e t r i e s and g e n e r a l l y have an a s s o c i a t e d f r a c t u r e c l e a v a g e . The r e g u l a r i t y o f the geometry o f t h e s e s t r u c t u r e s and t h e i r a s s o c i a t i o n w i t h b r i t t l e c l e a v a g e s and f a u l t s s u g g e s t s t h a t t h e g n e i s s dome as i t e x i s t s t o d a y i s t h e r e s u l t o f s e v e r a l s u p e r i m p o s e d c o m p r e s s i v e d e f o r m a t i o n s under s h a l l o w c r u s t a l c o n d i t i o n s . T h ese o b s e r v a t i o n s a r e c l e a r l y i n c o m p a t i b l e w i t h h y p o t h e s e s t h a t t h e dome was formed by d i a p i r i s m o r r e p r e s e n t s t h e p r o d u c t o f deep c r u s t a l p r o c e s s e s . APPENDIX II STRAIN ANALYSIS 280 I n t r o d u c t i o n T h i s a p p e n d i x o u t l i n e s i n d e t a i l t h e t e c h n i q u e s used i n t h i s s t u d y t o d e t e r m i n e t h e n a t u r e , m a g n i t u d e and s p a t i a l v a r i a t i o n o f s t r a i n w i t h i n the s t u d y a r e a . Due t o t h e s c a r c i t y o f e l l i p t i c a l s t r a i n markers w i t h i n t h e s e r o c k s , s e v e r a l i n d i r e c t methods o f e s t i m a t i n g s t r a i n were employed. Each method used i n t h i s s t u d y i s d e s c r i b e d and t h e r e s u l t s o b t a i n e d a r e p r e s e n t e d . D i s c u s s i o n o f t h e s i g n i f i c a n c e o f t h e r e s u l t s i s l a r g e l y p r e s e n t e d i n C h a p t e r One o f t h e t h e s i s , under t h e h e a d i n g " S t r a i n A n a l y s i s . " H u d l e s t o n ' s F o l d M o r p h o l o g y Method H u d l e s t o n (1973a) p r o p o s e d a method o f g e o m e t r i c c l a s s i f i c a t i o n o f f o l d s based on a F o u r i e r a n a l y s i s o f f o l d s h a p e . By a p p l y i n g t h i s a n a l y s i s both t o t h e o r e t i c a l l y p r e d i c t e d f o l d shapes ( C h a p p i e , 1968c; D i e t e r i c h and C a r t e r , 1969; P a r r i s h e t a j _ . , 1976) and t o n a t u r a l f o l d s ( H u d l e s t o n , 1973a) one c a n r e l a t e the t h e o r e t i c a l s t u d i e s t o n a t u r a l f o l d s . The shape o f a f o l d c a n be e x p r e s s e d by a F o u r i e r e x p a n s i o n g i v e n by H u d l e s t o n ( 1 9 7 3 a ) : f ( x ) = % a 0 + £ a n c o s b p s i n ^ (1) n n where a and b a r e c o n s t a n t c o e f f i c i e n t s . In t h i s e q u a t i o n , W i s the n n ^ w a v e l e n g t h o f t h e f o l d w h i c h , t o e l i m i n a t e t h e e f f e c t o f s c a l e , can be t a k e n as 2TT ( w i t h measurement o f f o l d a m p l i t u d e s c a l e d a c c o r d i n g l y ) . T h i s f u n c t i o n , f ( x ) , need n o t be p e r i o d i c but i f d e f i n e d i n t h e i n t e r v a l 0 <_ x <_ 2TT, and l e t t i n g f ( x + n2ir) = f ( x ) , where n i s a p o s i t i v e o r n e g a t i v e i n t e g e r , i t becomes p e r i o d i c w i t h a p e r i o d o f 2TT. The c o e f f i c i e n t s 281 i n E q u a t i o n (1) a r e g i v e n by: f 2TT a f ( x ) c o s mx dx, m > 0; (2a) b m TT f ( x ) s i n mx dx, m > 1. (2b) When f ( x ) i s a f o l d s u r f a c e , i t s a n a l y t i c a l form i s unknown and t h e s e i n t e g r a t i o n s c a n n o t be p e r f o r m e d . T h e r e f o r e , H u d l e s t o n (1973a) was f o r c e d t o sample f ( x ) a t d i s c r e t e i n t e r v a l s and use n u m e r i c a l methods t o d e t e r m i n e t h e c o e f f i c i e n t s . U s i n g an a p p r o p r i a t e s e l e c t i o n o f c o o r d i n a t e s , t he f u n c t i o n t o be a n a l y z e d c a n be made an odd f u n c t i o n ( H u d l e s t o n , 1973a, p. 19) and the c o s i n e terms become z e r o . In a d d i t i o n , t h e even s i n e terms go t o z e r o , as t h e y a r e as y m m e t r i c a b o u t t h e a x i a l s u r f a c e . Thus b^m = 0, where m = 1, 2, 3 and o n l y odd terms r e m a i n i n t h e s e r i e s . By a n a l y z i n g i d e a l and n a t u r a l f o l d s h a p e s , H u d l e s t o n (1973a) c o n c l u d e d t h a t t h e most d i a g n o s t i c f e a t u r e s o f f o l d shape a r e b r o u g h t o u t by t h e f i r s t two c o e f f i c i e n t s , b^ and b 3 . I t i s p a r t i c u l a r l y s i g n i f i c a n t t h a t t h e s p e c t r a o f n a t u r a l f o l d s d e s c r i b e d by H u d l e s t o n (1973b) d i s p l a y a v e r y c l o s e t o l i n e a r r e l a t i o n s h i p between t h e v a l u e s o f b . T h i s o b s e r v a t i o n p r o v i d e s m r s t r o n g s u p p o r t f o r H u d l e s t o n ' s (1973a) s u g g e s t i o n t h a t n a t u r a l f o l d shapes c an be m e a n i n g f u l l y a p p r o x i m a t e d by c o m p a r i s o n w i t h t h i r t y i d e a l f o l d f o r m s . These forms were c o n s t r u c t e d by H u d l e s t o n (1973a) by c o m b i n i n g s i x d i f f e r e n t f o l d shapes d e f i n e d by v a l u e s o f b^/b-j, each a t f i v e a m p l i t u d e s . The r e s u l t i s shown i n F i g u r e I I - l . A s i m i l a r g r i d o f i d e a l s h a p e s , based on b-j and bo, was s u g g e s t e d by S t a b l e r ( 1 9 6 8 ) . 2 8 2 FIGURE 11—1. G r i d o f i d e a l f o l d shapes based on s p e c t r a l s t u d i e s o f n a t u r a l f o l d shapes ( a f t e r H u d l e s t o n , 1973). T h i s g r i d was used t o i d e n t i f y f o l d shape t y p e s i n t h e f i e l d . 283 284 Ramsay's t Method a Ramsay (1962a, 1967) and Mukhopadhyay (1965) have examined i n d e t a i l t h e o r t h o g o n a l t h i c k n e s s i n t h e p r o f i l e p l a n e o f an i n i t i a l l y p a r a l l e l f o l d as a f f e c t e d by a s u p e r p o s e d f i n i t e homogeneous s t r a i n ( f l a t t e n i n g ) . These a u t h o r s d e v e l o p e d a t e c h n i q u e i n w h i c h a t h i c k n e s s p a r a m e t e r t i s p l o t t e d a g a i n s t l i m b d i p a. T h i s p l o t i s shown i n F i g u r e I 1 - 2 , w h i c h r e p r e s e n t s t h e geometry o f a f o l d e d l a y e r f o r v a r i o u s v a l u e s o f t h e homogeneous f l a t t e n i n g s t r a i n ( / X ^ 7 X p . The t t e c h n i q u e was a p p l i e d by Ross and C h r i s t i e (1979) t o a n a l y z e t h e m agnitude o f f l a t t e n i n g s t r a i n s s u p e r p o s e d on Phases One, Two and T h r e e f o l d s i n t h e Vaseaux Lake a r e a on the west s i d e o f t h e Shuswap complex. Ross and C h r i s t i e ' s d a t a r e v e a l e d t h a t Phase One f o l d s i n t h e g a r n e t -b i o t i t e s c h i s t s a r e a l l c l o s e t o C l a s s II f o l d s ( w i t h v a l u e s o f /X^/T^ o f l e s s t h a n 0.3 i m p l i e d by t h e i r d a t a as d i s p l a y e d i n F i g u r e 5 o f t h e i r p a p e r ) . T h e i r d a t a f o r f i f t e e n Phase Two f o l d s i n g n e i s s a p p e a r t o have / X ^ T T j -v a l u e s c o n c e n t r a t e d i n t h e r a n g e 0.6 to 0.3. They c o n c l u d e t h a t t h e s e two e a r l y phases p r o b a b l y had an e a r l y b u c k l e h i s t o r y w i t h s u b s e q u e n t h i n g e zone t h i c k e n i n g r e l a t e d t o f l a t t e n i n g . D i s t o r t e d L i n e a t i o n P a t t e r n s Ramsay (1967, p. 466) d e s c r i b e s a method t o d e t e r m i n e t h e magnitude o f s u p e r p o s e d f l a t t e n i n g s t r a i n s (/X^ TXj") on l i n e a t i o n p a t t e r n s . The l o c i o f l i n e a r s t r u c t u r e s deformed by f l e x u r a l - s l i p f o l d i n g a r e i n t h e form o f p a r t i a l s m a l l c i r c l e s a r o u n d t h e f o l d a x i s . The e f f e c t o f f l a t t e n i n g s t r a i n s i s t o r o t a t e t h e l i n e a r s t r u c t u r e s i n t h e h i n g e zone o f t h e f o l d towards t h e d i r e c t i o n o f t h e f o l d a x i s . The l i n e a t i o n s s i t u a t e d on t h e 285 FIGURE I I - 2 . P l o t o f f o l d t h i c k n e s s p a r a m e t e r t a v e r s u s l i m b d i p a. The l i n e s r e p r e s e n t t h e geometry o f f o l d e d l a y e r s f o r v a r i o u s v a l u e s o f t h e f l a t t e n i n g s t r a i n , A ^ A ^ j ( a f t e r Ramsay, 1967). 286 287 l i m b s o f t h e f o l d come t o l i e a t a h i g h e r a n g l e . When one o f t h e axes o f t h e s u p e r p o s e d s t r a i n e l l i p s e c o i n c i d e s w i t h t h e f o l d a x i s , t h e r e s u l t i s a s y m m e t r i c l i n e a t i o n l o c u s i n t h e f o r m o f a p a r t i a l e l l i p t i c a l h e l i x ( F i g u r e I 1 - 3 ) . I f t h e maximum and minimum a n g l e s ( a ) between t h e deformed l i n e a t i o n s and t h e f o l d a x i s c a n be d e t e r m i n e d from t h e f i e l d measurements, th e n t h e r a t i o o f the p r i n c i p a l s t r a i n s c an be d e t e r m i n e d f r o m t h e r e l a t i o n : t a n a 1 . c o t a ' = A 0 / X , . (3) min max 2 1 w ; T h i s method p r o v i d e s a way o f m e a s u r i n g the magnitude o f the p o s t - b u c k l i n g s t r a i n i n d e p e n d e n t l y o f t h e t a p l o t t e c h n i q u e . Ramsay (1967) has a l s o examined t h e c a s e i n w h i c h none o f the axes o f t h e s u p e r p o s e d s t r a i n e l l i p s e c o i n c i d e w i t h the f o l d a x i s . I f t h e o r i e n t a -t i o n o f t h e s u p e r p o s e d s t r a i n e l l i p s e i s known and t h e c o m p l e t e l i n e a t i o n l o c u s i s known, t h e n t h e f o l l o w i n g r e l a t i o n s h o l d : t a n x 1 = ( X 2 / X 1 ) J s t a n x (4) t a n x 2 = ( A o / A - i ) 2 t a n (x - a ) (5) t a n x 3 = ( A n A - | ) 2 t a n (x + a ) (6) (see F i g u r e 11—3 f o r e x p l a n a t i o n o f p a r a m e t e r s ) . These t h r e e e q u a t i o n s can t h e n be s o l v e d f o r t h e unknowns a ( i n i t i a l a n g l e between the l i n e a t i o n and the f o l d a x i s , /k^JT^ and x ( t h e o r i g i n a l a n g l e between the f o l d a x i s and the X - a x i s o f p r i n c i p a l s t r a i n ) . 2 8 8 FIGURE 11-3. (a) E q u a l - a n g l e s t e r e o n e t p r o j e c t i o n o f the l i n e a t i o n l o c i r e s u l t i n g f r o m t h e s u p e r p o s i t i o n o f a f l a t t e n -i n g s t r a i n on a s m a l l c i r c l e . X, Y and Z a r e t h e maximum, i n t e r m e d i a t e and minimum p r i n c i p a l s t r a i n a x e s . F o r d i s c u s s i o n , s ee t h e t e x t ( a f t e r Ramsay, 1967). f i s t h e f o l d a x i s , a i s t h e a n g l e between th e o r i g i n a l l i n e a t i o n d i r e c t i o n and the f o l d a x i s , (b) Same as (a) b u t w i t h t he f o l d a x i s d i s p l a c e d from the p r i n c i p a l s t r a i n a x i s ( a f t e r Ramsay, 1967). 289 290 F o r t h e c a s e i n which t h e f o l d a x i s c o r r e s p o n d s t o one o f t h e axes o f p r i n c i p a l s t r a i n , two e q u a t i o n s a r e p r e s e n t e d by Ramsay ( 1 9 6 7 ) : c o t a' = /A 0AI c o t a , and (7) max 2 1 ' t a n a ' m i . n = A 3 A 2 t a n a . (8) These i n v o l v e f o u r unknowns; w i t h o u t a d d i t i o n a l i n f o r m a t i o n , two a d d i t i o n a l e q u a t i o n s a r e needed f o r t h e i r s o l u t i o n . Two a s s u m p t i o n s can be made: f i r s t l y , t h a t t h e r o c k s e x p e r i e n c e d p l a n e s t r a i n ( o r t h a t ^ = 1) ; and s e c o n d l y , t h a t t h e d i l a t i o n a l s t r a i n i s z e r o ( o r t h a t ^ A ^ = 1 ) . T h e s e a s s u m p t i o n s a r e n o t uncommon i n s t r a i n a n a l y s i s s t u d i e s i n h i g h - g r a d e metamorphic t e r r a i n s . Ross (1973) and Ross and C h r i s t i e (1979) assumed z e r o d i l a t i o n a l s t r a i n i n t h e i r a n a l y s i s o f f l a t t e n e d g a r n e t s f r o m t he Shuswap t e r r a i n . The a s s u m p t i o n o f p l a n e s t r a i n i s c o n s i s t e n t w i t h t he " c h o c o l a t e t a b l e t " s t r u c t u r e (Ramsay, 1967) o f b o u d i n a g e d p e g m a t i t i c v e i n s p a r a l l e l t o Phase T h r e e a x i a l p l a n e s w i t h i n Nappes II and I I I . G i v e n t he above a s s u m p t i o n s , t h e p r i n c i p a l q u a d r a t i c e l o n g a t i o n s ( A p X^ and A g ) can be c a l c u l a t e d from t h e r e l a t i o n s A 3 = t a n a ' m i n c o t a'max ' ( 9 ) x^ = 1 and A-J = l / A ^ . The v a l u e o f a can th e n be c a l c u l a t e d by making a p p r o p r i a t e s u b s t i t u t i o n s i n t o E q u a t i o n (7) o r ( 8 ) . 291 F o r t h e c a s e i n w h i c h an a x i s o f p r i n c i p a l s t r a i n does n o t c o r r e s p o n d w i t h t h e f o l d a x i s o f t h e s e c o n d phase o f d e f o r m a t i o n , E q u a t i o n s (4), (5) and (6) c a n n o t be s o l v e d d i r e c t l y . However, t h e y c an be r e w r i t t e n i n t h e for m : t a n x, = 1 xR t a n x t a n x. t a n x„ = x R t a n a + t a n x 1 - t a n x t a n a ^ t a n a - t a n x 1 + t a n x t a n a (10) (11) (12) where xR = ( X o / X - | ) 2 . S u b s t i t u t i n g E q u a t i o n (10) i n t o E q u a t i o n s (11) and (12) and r e a r r a n g i n g g i v e s t a n a 2 -where <$> X R ( t a n x^ - t a n x-j) (xR2 + t a n x-| t a n x^) t a n x-j t a n x^ + t a n x-j t a n x^ U - D (13) (14) t a n Xr t a n x 1 t a n x^ + t a n x-| E q u a t i o n s (10), (13) and (14) can now be used t o f i n d t h e unknowns: (x^/X^), a and x ( t h e o r i g i n a l a n g l e between t h e p r i n c i p a l a x i s o f the s t r a i n e l l i p s e and t h e f o l d a x i s . I t s h o u l d be n o t e d t h a t E q u a t i o n (14) c a n n o t be e v a l u a t e d i f «j> i s l e s s t h a n one, as x R 2 would become n e g a t i v e . T h i s w i l l n o t o c c u r as l o n g as t a n x^ - 2 t a n x-j > t a n x^. N u m e r i c a l i n s t a b i l i t y a p p e a r s t o be r e l a t e d t o v a l u e s o f <j> a p p r o a c h i n g 1.0. T h i s s i t u a t i o n r e f l e c t s a p h y s i c a l l y i m p o s s i b l e c o m b i n a t i o n o f a n g l e s . 292 In b o t h t h e sym m e t r i c and a s y m m e t r i c c a s e s o u t l i n e d a b o v e , t h e c a l c u l a t e d q u a d r a t i c e l o n g a t i o n s and o r i g i n a l s m a l l c i r c l e r a d i i can be used t o c a l c u l a t e t h e t h e o r e t i c a l l o c u s o f l i n e a t i o n s . The f i t o f t h e s e t h e o r e t i c a l d i s t r i b u t i o n s t o t h e f i e l d d a t a p r o v i d e s a p o w e r f u l i n d e p e n d e n t check on t h e v a l i d i t y o f t h e m o d e l . In t h e above a n a l y s i s t h e l i n e a t i o n s a r e t r e a t e d as o r i g i n a l l y r e c t i l i n e a r e l e m e n t s . A d i f f e r e n t a n a l y s i s o f deformed l i n e a t i o n s has been p r o p o s e d by L i s l e ( 1 9 7 4 ) , who s u g g e s t s t h a t t h e y be viewed as a p p a r e n t l i n e a t i o n s p r o d u c e d by t h e l a y e r i n g i n t e r s e c t i n g an e l l i p s o i d . L i s l e (1974) c a l c u l a t e s t h e d i s p e r s i o n o f l i n e a t i o n p a t t e r n s f o r s e v e r a l t y p e s o f s u p e r p o s e d f o l d s t r a i n s ( f l a t t e n e d f l e x u r a l s l i p and h e t e r o -geneous s i m p l e s h e a r ) . A l t h o u g h L i s l e (1974) does n o t document any n a t u r a l l i n e a t i o n p a t t e r n s t h a t a r e f i n i t e s t r a i n s t r u c t u r e s , he does s u g g e s t some c r i t e r i a by w h i c h t h e y may be i d e n t i f i e d : a) t h e l i n e a t i o n i n t e n s i t y w i l l be a f u n c t i o n o f p o s i t i o n w i t h r e s p e c t t o t h e f o l d h i n g e and l i m b s , whereas i f the l i n e a t i o n i s a p a s s i v e l y r o t a t e d l i n e a r e l e m e n t , i t s d e v e l o p m e n t w i l l be i n d e p e n d e n t o f p o s i t i o n . b) t h e l a c k o f any s i m p l e g e o m e t r i c p a t t e r n s o f deformed l i n e a t i o n s . I n d e e d , most o f h i s c a l c u l a t i o n s f o r r e a l i s t i c s t r a i n models show q u i t e complex l i n e a t i o n p a t t e r n s . c ) t h e e x i s t e n c e o f a r e l a t i o n s h i p between the n a t u r e o f t h e l i n e a t i o n l o c u s and t h e a n g l e between t h e f o l d a x i s and the axes o f t h e s u p e r p o s e d s t r a i n e l l i p s e . In a d d i t i o n , i t s h o u l d be p o s s i b l e i n a t l e a s t some s i t u a t i o n s t o p h y s i c a l l y d e t e r m i n e t h e n a t u r e o f t h e l i n e a t i o n . S t r a i n A n a l y s i s o f E l l i p t i c a l O b j e c t s The use o f e l l i p t i c a l o b j e c t s as measures o f f i n i t e s t r a i n s has been d i s c u s s e d by Ramsay (1967) and Ross ( 1 9 7 3 ) . Any e l l i p t i c a l o b j e c t c a n be used as an i n d i c a t o r o f s t r a i n , though i f t h e o b j e c t s have an i n i t i a l 293 p r e f e r r e d o r i e n t a t i o n , t h e n s p e c i a l t e c h n i q u e s must be employed ( s e e Ramsay, 1967). S t r a i n can be e x p r e s s e d i n terms o f n a t u r a l s t r a i n s e where e = Hn (1 + e) and e i s t h e e l o n g a t i o n . As o u t l i n e d by Ross (1973) t h e o c t a h e d r a l s h e a r s t r a i n y 0 d e f i n e s t h e m a g n i t u d e o f t h e d i s t o r t i o n a l component o f s t r a i n and i s g i v e n by t h e e q u a t i o n : Y = 4 lie - z )2 + ( 0 J 1 2 £ - e ) 2 + (e - e ) 2] 2 2 3 3 1 (15) where e > e > e . 1 — 2 — 3 In terms o f t h e a x i a l r a t i o s , t h e o c t a h e d r a l s h e a r s t r a i n i s g i v e n by: 4 3 l o g '+ l o g + l o g (16) A s e c o n d p a r a m e t e r ( R o s s , 1973), Lodes v: v = V = 2e> £1 " e 3 , and a l s o l o g fYl Z - l o g fx) Y l o g X Z (17) (18) w h i ch d e s c r i b e s t h e shape o f t h e s t r a i n e l l i p s e , v a r i e s between +1 ( u n i -a x i a l c i g a r shape) and 0 ( p l a n e s t r a i n e l l i p s o i d ) and -1 ( u n i a x i a l p a n c a k e s ) . The two p a r a m e t e r s Y q and v can be mapped from t h r e e - d i m e n s i o n a l s t r a i n s p a c e onto t h e p l a n e ex + e2 + e3 " 0. As o u t l i n e d by Ross ( 1 9 7 3 ) , p o i n t s can be p l o t t e d on a 60° segment o f the s t r a i n p l a n e u s i n g the axes _ /3~ e s (where = j Y ) and v. Ross has used such p l o t s t o r e p r e s e n t the s t r a i n s t a t e o f f l a t t e n e d g a r n e t s f r o m m y l o n i t i c r o c k s from t h e Okanagan V a l l e y , 294 R e s u l t s H u d l e s t o n ' s Method - The t e c h n i q u e o f " v i s u a l h a r m o n i c a n a l y s i s " ( H u d l e -s t o n , 1973a) based on t h e g r i d o f f o l d shapes i n F i g u r e I I - l was used t o a n a l y z e t h e shape o f 545 Phase One f o l d s , 218 Phase Two f o l d s and 209 Phase T h r e e f o l d s . The r e s u l t s o f t h i s a n a l y s i s a r e p r e s e n t e d i n F i g u r e I1-4 w h i c h i d e n t i f i e s t h e s t r u c t u r a l domain ( a s d e f i n e d i n C h a p t e r 1) i n which t h e f o l d o c c u r r e d . O n l y t h e d a t a t a k e n f r o m q u a r t z o -f e l d s p a t h i c g n e i s s e s a r e p r e s e n t e d , i n an a t t e m p t t o e l i m i n a t e v a r i a b i l i t y r e l a t e d t o d u c t i l i t y c o n t r a s t s . F i g u r e I I - 5 shows f o l d shapes f r o m Phases Two and T h r e e f o l d s f r o m t h e T r a n s i t i o n a l Domain o f I m b r i c a t e T h r u s t i n g , w i t h f o l d s f r o m Nappes I t h r o u g h IV p l o t t e d s e p a r a t e l y . The d a t a s u g g e s t t h a t t h e modal Phase T h r e e f o l d shape shows p r o g r e s s i v e d e f o r m a t i o n s i m i l a r t o t h a t shown i n t h e t h e o r e t i c a l e v o l u t i o n p a t h i n F i g u r e I1-4. The f o l d shapes a p p e a r t o r e f l e c t s u c c e s s i v e l y g r e a t e r s h o r t e n i n g i n the s e q u e n c e : Nappe I ( l e a s t d e f o r m e d ) , Nappe I I I , Nappe I I , Nappe IV. As w i l l be d e m o n s t r a t e d l a t e r , t h i s s e q u e n c e o f i n c r e a s i n g o f i n t e n s i t y o f d e f o r m a t i o n c a n be c o n f i r m e d by o t h e r , more q u a n t i t a t i v e t e c h n i q u e s . Thus H u d l e s t o n ' s method p r o v i d e s a r a p i d f i e l d t e c h n i q u e f o r e s t a b l i s h i n g p a t t e r n s i n s t r a i n v a r i a t i o n t h a t can t h e n be s u b j e c t e d t o more c a r e f u l a n a l y s i s . As t h e o r e t i c a l and e x p e r i m e n t a l s t u d i e s o f v a r i a t i o n i n f o l d shape become a v a i l a b l e , i t may p r o v e u s e f u l t o d i g i t i z e f o l d shapes f r o m f i e l d p h o t o -graphs and c a r r y o u t a c o m p l e t e harmonic a n a l y s i s . Ramsay's t a Method - F i g u r e I1-6 shows t h e t h i c k n e s s p a r a m e t e r ( t a ) p l o t t e d v e r s u s l i m b d i p (a) f o r 23 Phase One f o l d s and 18 Phase Two f o l d s , d i s t i n g u i s h e d by domain. The modal f l a t t e n i n g s t r a i n A 9 / A - , f o r Phase One 295 FIGURE I1-4. Shape d a t a f o r Phases One and Two f o l d s p l o t t e d on H u d l e s t o n ' s f o l d shape g r i d shown i n F i g u r e 11—1. F o r i n t e r p r e t a t i o n , s e e d i s c u s s i o n i n t h e t e x t . Autochthonous Domain Basement Cored Nappes Transitional Domain Cover Rock Domain A B C D E F A B C D E F A B C D E F A B C D E F > > I 2 3 4 5 1 2 3 8 II 8 2 II 5 8 7 5 A B C D E F A B C D E F A B C D E F 1 5 4 2 8 6 4 3 4 2 1 7 II 7 4 27 17 13 8 16 9 2 Summary of Phase I A B C D E F I 2 3 4 5 Auf och. Nappe Trans . A B C D 1 Theoretical 2 Trend 3 (Chappie, 1968) 4 5 297 FIGURE 11-5- Shape d a t a f o r Phase s h e e t s mapped w i t h i n I m b r i c a t e T h r u s t i n g . T h r e e f o l d s f r o m t h e f o u r t h r u s t t h e T r a n s i t i o n a l Domain o f NAPPE EE NAPPE H I A B C D E F A B C D E F ro v> o_ 2 3 4 5 A B C D E F CvJ d> if) a JC 0_ O 2 3 4 5 NAPPE E A B C D E F NAPPE I A B C D E F A B C D E F A B C D E F ro CO 299 FIGURE I 1 - 6 . t plots for Phases One and Two folds from various CL domains within the Thor-Odin gneiss dome. 300 o m > o co > TJ m co o o O O 301 f o l d s r a n g e s f r o m 0.4-0.65 ( A u t o c h t h o n o u s Core G n e i s s Domain) t o 0.1-0.45 (Basement C o r e d Nappe Domain) t o 0.05-0.3 ( T r a n s i t i o n a l Domain o f I m b r i c a t e T h r u s t i n g ) . T h i s sequence o f f l a t t e n i n g s t r a i n i s c o n s i s t e n t w i t h the c o n c l u s i o n r e a c h e d above on the b a s i s o f H u d l e s t o n ' s shape a n a l y s i s . The shape o f d u c t i l e f o l d s f r o m t h i s a r e a i s c o n s i s t e n t w i t h an o r i g i n by b u c k l i n g w i t h a s u p e r i m p o s e d f l a t t e n i n g s t r a i n . D i s t o r t e d L i n e a t i o n P a t t e r n s - To t e s t t h e a p p l i c a t i o n o f t h i s t e c h n i q u e as a p r a c t i c a l a p p r o a c h t o s t r a i n measurement, l i n e a t i o n p a t t e r n s were measured a r o u n d i n d i v i d u a l m e s o s c o p i c f o l d h i n g e s . In s e v e r a l c a s e s , t h e l i n e a t i o n l o c i were complex, a p p a r e n t l y due t o m u l t i p l e r e f o l d i n g ; f o r example, Phase One s i l l i m a n i t e l i n e a t i o n s r e f o l d e d by a Phase Two f o l d and r e f o l d e d a g a i n by a Phase T h r e e f o l d . In o t h e r c a s e s , e i t h e r more th a n one l i n e a t i o n i s p r e s e n t o r t h e l i n e a t i o n s a r e v a r i a b l e even w i t h i n e q u i v a l e n t p o s i t i o n s on t h e f o l d . However, c a r e f u l e x a m i n a t i o n o f o u t c r o p a r e a s w i t h e x t e n s i v e f o l d d e v e l o p m e n t g e n e r a l l y r e v e a l e d an example t h a t p r o v i d e d u s e f u l d a t a . F o r d e t a i l e d a n a l y s i s , the b e s t d a t a s e t s a r e t h o s e which i n c l u d e p o i n t s c o n t i n u o u s l y d i s t r i b u t e d f r o m t h e h i n g e t o t h e i n f l e c t i o n p o i n t on t h e l i m b o f t h e s u p e r i m p o s e d f o l d . I f f o l d s a r e t i g h t w i t h s h a r p , narrow h i n g e z o n e s , t h e f u l l v a r i a t i o n t a k e s p l a c e o v e r a s m a l l d i s t a n c e , making measurement d i f f i c u l t . F o r f o l d s w i t h rounded h i n g e s and a more open s t y l e , l i n e a t i o n measurement i s e a s i e r ; however, i t i s l e s s l i k e l y t h a t t h e o u t c r o p s u r f a c e w i l l be s u f f i c i e n t l y e x t e n s i v e t o r e v e a l t h e l i n e a t i o n o r i e n t a t i o n towards t h e i n f l e c t i o n p o i n t a r e a o f the l i m b . F i g u r e I I - 7 shows t h r e e examples o f the l o c u s o f Phase One s i l l i m a n i t e l i n e a t i o n s deformed by i n d i v i d u a l m e s o s c o p i c f o l d s . The f i r s t example i s 302 FIGURE I I - 7 . Phase One l i n e a t i o n s deformed by Phase Two f o l d s w i t h s u p e r p o s e d f l a t t e n i n g s t r a i n s . In each c a s e t h e dashed l i n e i s t h e c a l c u l a t e d o r i g i n a l l o c u s f o r t h e l i n e a t i o n s , and t h e s o l i d l i n e i s the f l a t t e n e d l o c u s c a l c u l a t e d by t h e e q u a t i o n s g i v e n i n the t e x t . A l l p l o t s a r e on e q u a l -a n g l e s t e r e o n e t s . (a) S i l l i m a n i t e l i n e a t i o n s r e f o l d e d by a t i g h t Phase Two f o l d on the l o w e r n o r t h s l o p e o f F o s t h a l l R i d g e . (b) L i n e a t i o n s f r o m an a t t e n u a t e d Phase Two f o l d i n a p e l i t i c s c h i s t l a y e r w i t h i n impure m a r b l e . The l o c a l i t y i s a zone o f h i g h e r s t r a i n 300 meters s o u t h o f t h e l o c a l i t y r e p r e s e n t e d i n (a) above. ( c ) S i l l i m a n i t e l i n e a t i o n s w i t h a h i g h a n g l e t o t h e Phase Two f o l d a x i s . The f o l d i s on the upper l i m b o f t he Gunnarson Nappe, 100 meters n o r t h o f t h e b a s a l t h r u s t o f t h e Gates Ledge " i n f o l d . " 303 304 f r o m a s m a l l (40 cm w a v e l e n g t h ) Phase Two f o l d f r o m t h e B i g Ledge subdomain on t h e n o r t h s l o p e o f F o s t h a l l R i d g e . The o r i e n t a t i o n d a t a a r e shown i n F i g u r e I I - 7 . The l i n e a t i o n l o c u s shows s m a l l b u t s y s t e m a t i c d e v i a t i o n s f r o m a s m a l l c i r c l e p a t t e r n . The p a t t e r n was a n a l y z e d u s i n g t h e t e c h n i q u e o u t l i n e d above. The c a l c u l a t i o n s were b a s e d on measurements o f a ' and max a 1 . o f 33° and 29° r e s p e c t i v e l y . The v a l u e s l e a d t o t h e r e s u l t s a = 3 1 ° , min A 3 A 1 = 0.8536 and /X^/X^ - 0.9239. The t h e o r e t i c a l l i n e a t i o n l o c u s g i v e s an e x c e l l e n t f i t t o t h e o b s e r v e d d a t a and p r o v i d e s an i n d e p e n d e n t check on the v a l i d i t y o f t h e a p p r o a c h . E s t i m a t i o n o f /x^/X-J by the t a t e c h n i q u e f o r t h i s f o l d r e s u l t e d i n a s i g n i f i c a n t l y l a r g e r s t r a i n o f 0.7 +_ 0.2. E x a m i n a t i o n o f t h e f o l d s u g g e s t s t h a t t h e l a y e r shape was l e n s - l i k e and a t t e n u a t e d p r i o r to t h e f o r m a t i o n o f t h e Phase Two f o l d . T h i s d e m o n s t r a t e s an a d v a n t a g e o f t h e l i n e a t i o n l o c u s t e c h n i q u e . The method r e l i e s o n l y on t h e v a r i a t i o n o f l i n e a t i o n o r i e n t a t i o n on a s i n g l e s u r f a c e assumed t o be p l a n a r p r i o r t o b u c k l i n g . In c o n t r a s t , t h e t a t e c h n i q u e assumes t h e l a y e r has u n i f o r m t h i c k n e s s p r i o r t o d e f o r m a t i o n . I t s h o u l d be n o t e d t h a t t h e s t r a i n c a l c u l a t e d f o r t h i s example r e p r e s e n t s n o t s i m p l y the f l a t t e n i n g s t r a i n r e l a t e d t o Phase Two, b u t r a t h e r t h e sum o f t h e Phase Two and Phase T h r e e s t r a i n t e n s o r s . The A g A - j f o r t h i s t o t a l s t r a i n may be l e s s t h a n t h e i n d i v i d u a l d i s t o r t i o n a l s t r a i n s a s s o c i a t e d w i t h t h e i n d i v i d u a l d e f o r m a -t i o n s ( s e e f o r example P f f i f n e r and Ramsay, 1982). The s e c o n d example o f a symmetric f l a t t e n e d s m a l l c i r c l e l i n e a t i o n p a t t e r n ( F i g . I1-7) i s f r o m a Phase Two f o l d i n a p e l i t i c s c h i s t l a y e r w i t h i n impure m a r b l e 300 meters s o u t h o f the f o l d d e s c r i b e d i n the f i r s t example. The Phase Two f o l d r e f o l d s Phase One s i l l i m a n i t e l i n e a t i o n s , as shown i n F i g u r e 11-7. The l i n e a t i o n l o c u s shows s i g n i f i c a n t d e v i a t i o n f r o m a s m a l l c i r c l e p a t t e r n . U s i n g e s t i m a t e s f o r o.'m.n o f 30° and f o r 305 a'max °^ ^ e 1'nl't"'a'' a c a n D e e s t i m a t e d as 5 1 ° . The c a l c u l a t e d s t r a i n r a t i o s have v a l u e s o f Ag/A^ = 0.2101 and Ag/X 2 = 0.4584. A l t h o u g h the p r o f i l e o f t h i s p a r t i c u l a r f o l d c o u l d n o t be a c c u r a t e l y measured, Phase Two f o l d s i n a d j a c e n t o u t c r o p s g i v e A /^Xj" r a t i o s o f 0.35 t o 0.5, c o n s i s t e n t w i t h t h e v a l u e f o r t h e l i n e a t i o n method. The t h i r d example shown i n F i g u r e 11-7 i s o f Phase One l i n e a t i o n s d e f o r m e d by Phase Two f o l d s f r o m t h e uppermost p a r t o f t h e Gunnarson Nappe. The l i n e a t i o n l o c u s i s a t a l a r g e a n g l e t o t h e Phase Two f o l d a x i s , w hich i s t y p i c a l o f Phase One l i n e a t i o n s i n t h i s domain. Note t h a t t h e l i n e a t i o n l o c u s c e r t a i n l y does n o t f a l l on a g r e a t c i r c l e and i n d e e d i s a f l a t t e n e d s m a l l c i r c l e . U s i n g an a ' . o f 76° and an a ' „ o f 8 6 ° , t h e o r i g i n a l a J min max J i s c a l c u l a t e d t o be 8 2 ° and t h e s t r a i n r a t i o s a r e A / X ^ = 0.2805 and A j / A 2 = 0.5296. The r e s u l t i n g f i t t o t h e whole l i n e a t i o n p a t t e r n c o u l d be improved by r e f i n i n g t h e e s t i m a t e s o f a " m a x and a ' m - j n ; however, p l o t t i n g t h e s t r a i n e d l o c u s by hand i s a t e d i o u s p r o c e s s . Thus a l l o f t h e above examples a r e based on t h e i n i t i a l e s t i m a t e s f o r the i n p u t p a r a m e t e r s . S e v e r a l l i n e s o f e v i d e n c e s u p p o r t t h e i n t e r p r e t a t i o n i m p l i c i t l y assumed above, t h a t t h e s e l i n e a t i o n p a t t e r n s r e p r e s e n t r o t a t e d and f l a t t e n e d l i n e a r f a b r i c e l e m e n t s r a t h e r t h a n b e i n g f i n i t e s t r a i n i n d i c a t o r s i n the s e n s e o f L i s l e ( 1 9 7 4 ) . F i r s t l y , t h e l i n e a t i o n s a r e formed by e l o n g a t e s i l l i m a n i t e n e e d l e s , and t h e i n t e n s i t y o f t h e i r d e v e l o p m e n t i s n o t r e l a t e d t o p o s i t i o n on t h e f o l d . S e c o n d l y , t h e c a l c u l a t i o n s g i v e n above show t h a t o r i g i n a l l y e v e n l y d i s t r i b u t e d l i n e a t i o n s w i l l be p r e f e r e n t i a l l y r o t a t e d i n t o s t e e p e r l i m b p o s i t i o n s . T h i s i s s u p p o r t e d by t h e d a t a i n F i g u r e II-7. T h i r d l y , t h e e x c e l l e n t f i t f o r t h e model p r o p o s e d above s u g g e s t s t h a t L i s l e ' s more complex models a r e n e i t h e r a p p r o p r i a t e nor n e c e s s a r y . 306 The u n c e r t a i n t i e s i n e s t i m a t e s o f s t r a i n magnitude r e s u l t i n g f r o m t h i s t e c h n i q u e c a n be e v a l u a t e d by u s i n g s t a n d a r d e r r o r p r o p a g a t i o n t e c h n i q u e s . G i v e n : z = A 2 = ( t a n a' • c o t a' v ) 2 3 v min max' (19) t h e n t h e e r r o r i n z i s g i v e n by: 2 - 3 Z 3a 1 max a , a 2 + max 9z 3a1 mi n mi n (20) where: 3 Z 3a' max 2 ( t a n a ' m i n c o t "'max* " 2 t a n a mi n s i n 2 a 1 mi n (21) and: 3z mi n + i ( t a n a ' m i n c o t a , m a x ) " i ' 2 C O t a 1 max smz a mi n (22) T e s t s o f r e p r o d u c i b i l i t y o f B r u n t o n measurements under f i e l d c o n d i t i o n s s u g g e s t t h a t measurements a r e r e p r o d u c i b l e w i t h i n + 4 ° ( 2 a ) . The e r r o r s i n a ' m i n a n c' a'max ^ a v e ^ e e n e s t i m a t e d b a s e d on r e p r o d u c i b i l i t y o f measure-ment, s c a t t e r o f d a t a p o i n t s and the amount o f e x t r a p o l a t i o n n e c e s s a r y . The u n c e r t a i n t i e s c a l c u l a t e d f o r the t h r e e examples d i s c u s s e d above a r e : (a) « ' m i n = 29° ± 2 ° ; a ' m a x = 33° ± 2°; A 2 A 1 = 0.85 + 0.05; a = 31°. 307 <b> a'min " 3 0 ° ± 4 ^ a max = 70° + 6 ° ; A 9 / A , = 0.46 + 0.07; a = 5 1 ° . ( c ) a min = 76° + 4 ° ; a max = 86° + 4 ° ; A , A , = 0.5 + 0.3; a = 8 2 ° . The e r r o r a n a l y s i s d e m o n s t r a t e s t h a t t h e l i n e a t i o n method i s a s e n s i t i v e method t o d e t e r m i n e d i s t o r t i o n a l s t r a i n . The e r r o r i n t h e r e s u l t i n g s t r a i n r a t i o i s l o w e s t f o r o r i g i n a l s m a l l c i r c l e s w i t h an a o f 4 5 ° . Measurement e r r o r s become more s i g n i f i c a n t as a a p p r o a c h e s 0° o r 9 0 ° . I t s h o u l d be n o t e d t h a t t h e s e e r r o r s a r e based on u n c e r t a i n -t i e s i n a ' . and a ' o n l y . I f a l l t h e d a t a p o i n t s were i n c l u d e d i n mm max J r a l e a s t - s q u a r e s e s t i m a t i o n o f the s t r a i n p a r a m e t e r s , even l o w e r u n c e r t a i n -t i e s s h o u l d r e s u l t . The s u c c e s s o f t h e l i n e a t i o n l o c u s t e c h n i q u e a p p l i e d t o i n d i v i d u a l m e s o s c o p i c f o l d s e n c o u r a g e d the a p p l i c a t i o n o f t h i s a p p r o a c h to more complex s i t u a t i o n s . The d i s t r i b u t i o n o f Phase Two f o l d axes deformed by Phase T h r e e f o l d i n g from t h e T r a n s i t i o n a l Domain o f I m b r i c a t e T h r u s t i n g was c h o s e n f o r d e t a i l e d a n a l y s i s . F o l d axes were cho s e n r a t h e r t h a n l i n e a t i o n s b e c a u s e t h e y c o u l d be more r e a d i l y i d e n t i f i e d as b e l o n g i n g to Phase Two d e f o r m a t i o n . Both Phase One and Phase Two l i n e a t i o n s d o m i n a n t l y c o n s i s t o f s i l l i m a n i t e n e e d l e s and thus a r e d i f f i c u l t t o d i s t i n g u i s h . The l o c i o f Phase Two f o l d axes f o r Nappes I , II and I I I a r e shown i n F i g u r e I1-8. The l o c u s f o r Nappe I ( C a r i b o u Nappe) shows a s i m p l e f l a t t e n e d s m a l l c i r c l e p a t t e r n c o n s i s t e n t w i t h the Phase T h r e e a x i a l 308 FIGURE I 1 - 8 . Phase Two f o l d axes f r o m t h e C a r i b o u Nappe deformed by Phase T h r e e f o l d i n g . (a) F i e l d d a t a : Phase Two f o l d axes ( ° ) ; p o l e s t o Phase Two a x i a l p l a n e s (•); modal Phase T h r e e f o l d a x i s ( A ) ; p o l e s t o Phase T h r e e a x i a l p l a n e s (A); modal Phase T h r e e a x i a l p l a n e , dashed l i n e . (b) O r i e n t a t i o n d a t a f o r Phase Two f o l d axes w i t h the p l u n g e o f the Phase T h r e e f o l d removed and t h e Phase T h r e e a x i a l p l a n e r o t a t e d t o h o r i z o n t a l . C a l c u l a t e d l i n e a t i o n l o c u s f o r an o r i g i n a l s m a l l c i r c l e w i t h an a a n g l e o f 19.7° and A^/A^ = 0.6 i s shown as a s o l i d l i n e . The o r i g i n a l s m a l l c i r c l e l o c u s i s shown as a dashed l i n e . ( c ) O r i e n t a t i o n o f Phase Two f o l d axes from Nappe II ( w i t h d i p o f Phase T h r e e a x i a l p l a n e r o t a t e d t o v e r t i c a l ) , f i s t h e o r i g i n a l Phase T h r e e f o l d a x i s , f i s t h e o b s e r v e d modal Phase T h r e e f o l d a x i s . The dashed l i n e i s t h e o r i g i n a l s m a l l c i r c l e l o c u s o f Phase Two f o l d s . The s o l i d l i n e i s t h e f l a t t e n e d s m a l l c i r c l e d i s t r i b u t i o n c a l c u -l a t e d as d e s c r i b e d i n t h e t e x t . A, B and C r e p r e s e n t t h e a v e r a g e o r i e n t a t i o n o f the l o n g axes o f f l a t t e n e d n o d u l e s f r o m t h r e e l o c a l i t i e s w i t h i n t h e nappe. (d) O r i e n t a t i o n o f Phase Two f o l d axes f r o m Nappe I I I . The symbols a r e t h e same as i n ( c ) above. 309 310 p l a n e b e i n g t h e Y-Z p l a n e o f t h e s t r a i n and Y h a v i n g t h e o r i e n t a t i o n o f t h e Phase Thre e f o l d a x i s . U s i n g e s t i m a t e s f o r a' . o f 17° and a 1. mi n max o f 2 5 ° , a can be e s t i m a t e d as 2 0 . 2 ° , x as 1 2 . 5 ° , AJA-J = 0.66, and A 3 / A 2 = 0.81. The Phase Two f o l d axes from Nappe II f a l l on a complex and asymmetric l o c u s . T h i s i s c o n s i s t e n t w i t h t h e Phase T h r e e f o l d a x i s b e i n g d i v e r g e n t f r o m t h e axes o f p r i n c i p a l s t r a i n . The r e l a t i v e o r i e n t a t i o n o f t h e l i n e a -t i o n l o c u s and t h e Phase T h r e e f o l d a x i s s u g g e s t s t h a t t h e Phase T h r e e a x i a l p l a n e i s c o i n c i d e n t w i t h t he X-Y p l a n e o f t h e s t r a i n e l l i p s e , w i t h Y v e r t i c a l . Based on t h e s e a s s u m p t i o n s , t h e e q u a t i o n s f o r an asy m m e t r i c l i n e a t i o n l o c u s c a n be s o l v e d as o u t l i n e d above. M e a s u r i n g t he a n g l e s a n a l o g o u s t o t h o s e i n F i g u r e 11-8 r e l a t i v e t o Y g i v e s X^ = 2 4 ° , X 2 = 3 9 ° , and X 3 = 6 5 ° . Th e s e g i v e t h e r e s u l t s a = 3 4 ° , Ayx^ = 0.24, and A 3A 2 = 0.49. The p a t t e r n o f Phase Two f o l d axes from Nappe I I I shows a s i m i l a r geometry t o the l o c u s f o r Nappe II e x c e p t t h e d i s t r i b u t i o n i s l e s s f l a t t e n e d and the Phase T h r e e f o l d a x i s p l u n g e s more s h a l l o w l y . As above, t h e a n g l e s n e c e s s a r y t o c a l c u l a t e t he s t r a i n p a r a m e t e r s can be e s t i m a t e d as X-j = 3 8 ° , X 2 = 1 4 ° , and X 3 = 7 4 ° . T h i s r e s u l t s i n an a o f 2 9 ° , an x o f 4 0 ° , A-jA-, = 0.42, and A,A2 = 0.65. The u n c e r t a i n t i e s i n t h e s e c a l c u l a t i o n s can be p r o p a g a t e d as b e f o r e : f = X D 2 = ( T 2T )(T -T -2T ) - 1 + (2T x x )(T -T -2T ) _ 1 K 1 3 3 2 . 1 1 2 3 3 2 1 - (x 2T )(T -T -2T T 1 (23) 1 2 3 2 1 311 where: a 2 _ r 3 f ] 2 2 , r at- ^ 2 2 , f 3 f 1 13TJ a ^ + a z + T 2 ^ 3 J (24) where: 2 _ 3 T l I 3 x j j 2 = ( s e c x . ) * a 2, where x. = t a n x, X j 1 X j 1 1 (25) and: 2 _ f 3 x , l 3x 2) a. = ( s e c x 2 )2 a x 2, where x 2 = t a n x 2 (26) and: 3T, 3 x ; a 2 = ( s e c x 3 ) 2 a 2, where T, = t a n x. x 3 3 (27) Now: f 3z 1 ^ R 2 (28) where: /• •\ (29) 312 The r e s u l t s o f t h e s e c a l c u l a t i o n s f o r t h e two examples g i v e n above a r e : Nappe II A 2/X 1 = 0.49 + 0.14, f o r X 2 , X 3 + 2 ° ; A 2/X ] = 0.49 + 0.27, f o r X ] , X 2 , X 3 + 4 ° . Nappe I I I A 2 / X 1 = 0.65 + 0.12, f o r X 1 S X 2 , X 3 + 2 ° ; A 2 / X 1 = 0.65 + 0.25, f o r X-,, X 2 , X 3 + 4 ° . The e r r o r s i n t h e s e e s t i m a t e s o f t h e s t r a i n a r e s i g n i f i c a n t l y h i g h e r t h a n f o r t h e s y m m e t r i c c a s e . F o r example, Nappe I , which has a symmetric l i n e a t i o n l o c u s , has A ^ A - j = 0.81 + 0.07 ( f o r a 1 . = 17° + 3° and a'max = — 2 ° ^ ' however, t h e e x c e l l e n t f i t between t h e c a l c u l a t e d l i n e a t i o n p a t t e r n and t h e o b s e r v e d f o l d a x i s o r i e n t a t i o n s ( F i g u r e I I - 8 ) s u g g e s t s t h a t t h e whole d a t a s e t p r o v i d e s a much t i g h t e r c o n s t r a i n t on t h e s t r a i n r a t i o s . In f a c t , i t was f o u n d t h a t v a r y i n g X-j, X 2 , and X 3 w i t h i n r e a s o n a b l e l i m i t s r e s u l t e d i n a l a r g e v a r i a t i o n o f e l l i p s e shapes and a / A 2 / X ^ / X v a l u e s . The s e n s i t i v i t y o f t h i s f u n c t i o n t o the i n p u t p a r a m e t e r s s h o u l d thus be v i e w e d as a s t r e n g t h r a t h e r t h a n a weakness. The e r r o r s q u o t e d above f o r t h e a s y m m e t r i c c a s e a r e p r o b a b l y g r o s s o v e r e s t i m a t e s . A f u l l e r r o r a n a l y s i s , t a k i n g i n t o a c c o u n t t h e d a t a redundancy i n v o l v e d i n f i t t i n g t h e whole d a t a s e t , s h o u l d r e v e a l u n c e r t a i n t i e s s i m i l a r t o t h o s e f o r t h e symmetric c a s e . Some g e n e r a l comments a r e i n o r d e r w i t h r e g a r d t o the v a r i a t i o n i n t h e n a t u r e o f Phase T h r e e s t r a i n s a c r o s s t h e T r a n s i t i o n a l Domain o f I m b r i c a t e T h r u s t i n g . The o r i e n t a t i o n o f the Phase T h r e e s t r a i n e l l i p s e i n t h i s domain i s i n t i m a t e l y c o n n e c t e d w i t h the s t r u c t u r a l d e v e l o p m e n t 313 o f t h e i m b r i c a t e d z o ne. Though c a s u a l i n s p e c t i o n r e v e a l s no s y s t e m a t i c t r e n d i n t h e s t r a i n f i e l d f r o m nappe t o nappe, t h e f o l l o w i n g p o i n t s a p p e a r s i g n i f i c a n t . F i r s t l y , t h e Y a x i s o f t h e Phase T h r e e s t r a i n e l l i p s e i s always p a r a l l e l o r s u b p a r a l l e l t o the Phase T h r e e f o l d a x i s . S e c o n d l y , t h e s t r e s s o r i e n t a t i o n s i n f e r r e d by t h e s t r a i n f i e l d f o r Nappes II and I I I a r e i n c o n s i s t e n t w i t h t h r u s t f a u l t m o t i o n on t h e t h r u s t s h e e t s . T h i s s u g g e s t s t h a t e i t h e r the f l a t t e n i n g s t r a i n s a r e u n r e l a t e d t o t h e emplace-ment o f t h e nappe s h e e t s ( t h i s seems u n l i k e l y ) , o r t h a t i m b r i c a t i o n o f t h e t h r u s t s h e e t s has r e s u l t e d i n t r a n s l a t i o n and r o t a t i o n o f t h e s e l a r g e " s l i v e r s . " A n a l o g o u s s t r u c t u r e s a r e seen i n m e s o s c o p i c s l i d e zones w i t h i n t h e s e nappes i n w hich l e n s - l i k e , o f t e n s i g m o i d a l - s h a p e d s l i v e r s a r e r o t a t e d by t h r u s t m o t i o n a l o n g c u r v e d t h r u s t p l a n e s . E l l i p s o i d a l S t r a i n M a r k e r s - The o c c u r r e n c e o f s u i t a b l e o b j e c t s f o r s t r a i n a n a l y s i s w i t h i n t h e s e r o c k s i s l i m i t e d . F l a t t e n e d g a r n e t s o f the t y p e d e s c r i b e d by Ross (1973) o c c u r w i t h i n a l l t h e s t r u c t u r a l domains; however, t h e y a r e t y p i c a l l y embedded w i t h i n q u a r t z o - f e l d s p a t h i c g n e i s s , and t h e r e f o r e d i f f i c u l t t o measure. In c o n t r a s t , f l a t t e n e d g a r n e t s i n t h e Okanagan V a l l e y c a n o f t e n be f o u n d i n b i o t i t e - r i c h s c h i s t s and can be r e a d i l y p r i z e d f r e e . E l l i p t i c a l n o d u l e s o f q u a r t z and m i n o r s i l l i m a n i t e o c c u r w i t h i n t h e q u a r t z i t e s a t t h e base o f t h e c o v e r r o c k sequence w i t h i n t h e P i n g s t o n f o l d . A l t h o u g h R e e s o r and Moore have s u g g e s t e d t h e s e r e p r e s e n t metamorphic s e g r e g a t i o n s , some o f t h e s e e l l i p s o i d a l s t r u c t u r e s a p p e a r t o be deformed p e b b l e s . U n f o r t u n a t e l y w e a t h e r c o n d i t i o n s d u r i n g f i e l d work i n t h i s a r e a made s t r a i n a n a l y s i s s t u d i e s u n t e n a b l e . Thus s t r a i n measurement d a t a based on e l l i p t i c a l markers a r e s k e t c h y . An e x c e p t i o n t o t h i s i s Nappe II o f t h e T r a n s i t i o n a l Domain o f I m b r i c a t e T h r u s t i n g . In t h i s domain, q u a r t z o -314 f e l d s p a t h i c g r e y g n e i s s e s c o n t a i n e l l i p s o i d a l n o d u l e s o f q u a r t z , q u a r t z -f e l d s p a r and e p i d o t e - c a l c i t e - q u a r t z . The o r i g i n o f t h e s e s t r u c t u r e s i s n o t known. Ten t o f i f t e e n measurements were made a t each l o c a l i t y , and t h e l o n g a x i s o r i e n t a t i o n and l e n g t h o f t h e m a j o r axes were measured. The r e s u l t s were used t o c a l c u l a t e t h e e s and v p a r a m e t e r s o f t h e a p p a r e n t s t r a i n e l l i p s e . The r e s u l t s a r e p l o t t e d i n F i g u r e 11-9. The h i g h e s t s t r a i n s a r e measured i n t h e e p i d o t e - c a l c i t e - q u a r t z n o d u l e s , p r o b a b l y due t o t h e h i g h d u c t i l i t y o f t h i s r o c k t y p e r e l a t i v e t o t h e q u a r t z o - f e l d s p a t h i c m a t r i x . A l t h o u g h t h e q u a r t z and q u a r t z - f e l d s p a r n o d u l e s show l o w e r s t r a i n s t h a n t h e c a l c - s i l i c a t e n o d u l e s , t h e a v e r a g e f i n i t e s t r a i n s measured i n t h i s s t u d y a r e l a r g e r t h a n t h o s e measured by Ross ( 1 9 7 3 ) , Ross and C h r i s t i e (1979) and Ross ( 1 9 8 1 ) , u s i n g f l a t t e n e d g a r n e t s . Of the t h r e e d a t a s e t s shown i n F i g u r e 11-9, A and C a r e d o m i n a n t l y i n the f i e l d o f f l a t t e n i n g s t r a i n s (v p o s i t i v e ) , whereas B i s d o m i n a n t l y i n the c o n s t r i c t i o n a l s t r a i n f i e l d (v n e g a t i v e ) . The a v e r a g e o r i e n t a t i o n o f t h e l o n g axes o f t h e e l l i p s e s f o r each l o c a l i t y i s shown ' i n t h e e q u a l - a n g l e s t e r e o n e t p l o t f o r Nappe II i n F i g u r e 11-8. L o c a l i t y B, w i t h the h i g h e s t s t r a i n , has an e l l i p s e o r i e n t a t i o n s u c h t h a t the X d i r e c t i o n o f the Phase Two s t r a i n e l l i p s e has been r o t a t e d towards the X d i r e c t i o n o f t h e Phase T h r e e s t r a i n f i e l d . As t h e s e n o d u l e s a r e a measure o f t h e f i n i t e s t r a i n , t h e magnitude depends n o t o n l y on t h e m a g n i t u d e s o f t h e Phases Two and T h r e e s t r a i n (and t h e s t r a i n p r i o r t o Phase Two), b u t a l s o on the r e l a t i v e o r i e n t a t i o n o f t h e Phases Two and T h r e e s t r a i n i n c r e m e n t s . D i s c u s s i o n Though d o m i n a n t l y based on i n d i r e c t measurement o f the s t r a i n e l l i p s e , t he a p p r o a c h e s t o s t r a i n a n a l y s i s u t i l i z e d i n t h i s s t u d y have 315 FIGURE I I - 9 . S t r a i n c a l c u l a t e d f r o m e l l i p s o i d a l q u a r t z - f e l d s p a r ( • ) , q u a r t z (°) and e p i d o t e - c a l c i t e - q u a r t z ( + ) , p l o t t e d on a Hsu d i a g r a m . The d a t a a r e f o r t h r e e l o c a l i t i e s : A, B and C fro m the upp e r p a r t o f Nappe II i n the T r a n s i t i o n a l Domain o f I m b r i c a t e t h r u s t i n g . The a v e r a g e o r i e n t a t i o n o f t h e e l l i p s e s a t each l o c a l i t y i s shown i n F i g u r e 11-8. 317 some d i s t i n c t a d v a n t a g e s o v e r p r e v i o u s s t r a i n s t u d i e s i n t h e Shuswap u s i n g f l a t t e n e d g a r n e t s ( R o s s , 1973; Ross and C h r i s t i e , 1979). F i r s t l y , t h e age o f t h e g a r n e t s r e l a t i v e t o t h e phases o f d e f o r m a t i o n i s d i f f i c u l t t o e s t a b l i s h ; however, most i f n o t a l l r e c o r d t h e e f f e c t s o f b o t h Phases Two and T h r e e d e f o r m a t i o n . F o r example, two p e r i o d s o f p l a n e s t r a i n s e p a r a t e d by a r o t a t i o n a l s t r a i n c o u l d r e a d i l y r e s u l t i n a f i n i t e s t r a i n i n t h e c o n s t r i c t i o n a l f i e l d . S e c o n d l y , s t r a i n i s l i k e l y t o v a r y w i t h i n b u c k l e d l a y e r s ( l o n g i t u d i n a l t a n g e n t i a l s t r a i n ) , w h i c h c r e a t e s a s a m p l i n g p r o b l e m . T h i r d l y , t h e m a g n i t u d e o f s t r a i n measured i n g a r n e t s r e p r e s e n t s a minimum v a l u e , as g a r n e t s a r e g e n e r a l l y l e s s d u c t i l e t h a n t h e i r m a t r i x . W i t h i n t h e T h o r - O d i n a r e a , g a r n e t s i n m i c a - r i c h r o c k s show low s t r a i n s whereas t h o s e i n q u a r t z o - f e l d s p a t h i c g n e i s s e s show a h i g h d e g r e e o f f l a t t e n i n g f o l l o w e d by b u c k l i n g o f t h e f l a t t e n e d g a r n e t w i t h a s s o c i a t e d f r a c t u r i n g and b o u d i n a g e d u r i n g Phase T h r e e d e f o r m a t i o n ( F i g . 1 1 - 1 0 ) . In c o n t r a s t t o t h e s e p r o b l e m s , f o l d shape and l i n e a t i o n p a t t e r n s p r o v i d e a measure o f d i s t o r t i o n a l s t r a i n r e l a t e d t o the l a r g e - s c a l e geometry o f t h e r o c k mass. The geometry o f b o u d i n a g e f a b r i c r e l a t a b l e t o a p a r t i c u l a r d e f o r m a t i o n can p r o v i d e i n f o r m a t i o n on t h e n a t u r e o f the c a u s a t i v e s t r a i n . Thus t h e c h a r a c t e r i s t i c s o f s t r a i n a s s o c i a t e d w i t h s i n g l e d e f o r m a t i o n can be i d e n t i f i e d . The e v i d e n c e p r e s e n t e d i n t h i s s t u d y i s c o n s i s t e n t w i t h i n d i v i d u a l d e f o r m a t i o n s b e i n g d o m i n a n t l y p l a n e s t r a i n , even though f i n i t e s t r a i n r e v e a l e d by g a r n e t s and o t h e r e l l i p t i c a l m arkers i s c o n s t r i c t i o n a l i n n a t u r e . A l t h o u g h t h e s t r a i n s t u d y p r e s e n t e d i n t h i s a p p e n d i x i s n o t e x h a u s t i v e , i t does p r o v i d e s u f f i c i e n t d a t a t o compare the n a t u r e o f s t r a i n i n t h e T r a n s i t i o n a l Domain o f I m b r i c a t e T h r u s t i n g w i t h p r e v i o u s s t r a i n s t u d i e s i n i m b r i c a t e d t h r u s t z o n e s . Coward and Kim (1981) have r e c e n t l y 318 FIGURE 11-10. F l a t t e n e d g a r n e t s i n q u a r t z o - f e l d s p a t h i c g n e i s s f o l d e d by Phase T h r e e f o l d s . The g a r n e t s a r e shown i n b l a c k , and q u a r t z v e i n m a t e r i a l s as d o t t e d a r e a s . The l e n s cap g i v e s t h e s c a l e . The o u t c r o p i s f r o m t h e c r e s t o f Mt. Symonds i n the B i g Ledge Subdomain. 319 320 r e v i e w e d t h e s t a t e s o f s t r a i n i n t h r u s t s h e e t s w i t h p a r t i c u l a r r e f e r e n c e t o t h e Moine T h r u s t Zone. They u s e d t h e d i s t o r t i o n o f worm burrows i n C a m b r o - O r d o v i c i a n r o c k s a s s o c i a t e d w i t h t h e zone t o measure t h e l a y e r p a r a l l e l s h o r t e n i n g and s h e a r s t r a i n a s s o c i a t e d w i t h t h r u s t i n g . They c o n c l u d e t h a t l a y e r p a r a l l e l s h o r t e n i n g o f up t o 50% has o c c u r r e d and t h a t i n a d d i t i o n , h e t e r o g e n e o u s s h e a r s t r a i n s (y) o f up t o 0.5 a r e common w i t h i n t h r u s t s h e e t s up t o s e v e r a l k i l o m e t e r s i n t h i c k n e s s . Coward and Kim (1981) p r e s e n t an a n a l y s i s o f the e f f e c t o f c o m b i n a t i o n s o f f l a t t e n i n g and s h e a r s t r a i n s on t h e o r i e n t a t i o n and f o r m o f t h e s t r a i n e l l i p s e . T h e i r c o n c l u s i o n s a r e c o n s i s t e n t w i t h e a r l i e r s t r a i n s t u d i e s a l o n g t h e Moine T h r u s t a t G l e n c o u l , where W i l k i n s o n e t a l _ . (1975) d e s c r i b e d d e f o r m a -t i o n o f worm burrows making an a n g l e o f l e s s t h a n 2° w i t h b e d d i n g , due t o enormous s h e a r s t r a i n . However, t h e s e a n a l y s e s n o t o n l y assume p l a n e s t r a i n and no volume change ( z e r o d i s t o r t i o n a l s t r a i n ) , b u t a l s o t h a t A^ c o i n c i d e s w i t h t h e o r i e n t a t i o n o f t h e worm burrow. W i l k i n s o n e t a l _ . (1975) a l s o assume a s i m p l e s h e a r mechanism. In c o n t r a s t w i t h t h e s e s t u d i e s , J o h n s o n (1967) s u g g e s t e d i r r o t a t i o n a l s t r a i n s t o be r e s p o n s i b l e f o r the same r o c k s s t u d i e d by W i l k i n s o n e_t al_. ( 1 9 7 5 ) . The l i n e a t i o n p a t t e r n d a t a p r e s e n t e d i n t h i s s t u d y a p p e a r t o d i s c o u n t t h e e x i s t e n c e o f s i g n i f i c a n t s h e a r s t r a i n s , c e r t a i n l y s u b s e q u e n t to Phase T h r e e b u c k l i n g . Ramsay (1967, pp. 582-586) has a n a l y z e d the e f f e c t o f s u p e r i m p o s i n g s h e a r s t r a i n on s m a l l c i r c l e l i n e a t i o n p a t t e r n s . The l o c i p r e d i c t e d by Ramsay f o r t h e d e f o r m a t i o n o f s m a l l c i r c l e s by s i m p l e s h e a r b e a r no s i m i l a r i t i e s t o t h e l i n e a t i o n p a t t e r n s i n t h i s s t u d y . As p o i n t e d o u t above , t h e r e l a t i v e s t r u c t u r a l geometry i n t h e upper t h r e e t h r u s t s h e e t s i n C a r i b o u A l p a r e b e s t e x p l a i n e d by r o t a t i o n o f r i g i d t h r u s t s l i c e s r a t h e r t h a n by r o t a t i o n by s h e a r s t r a i n s . 321 C o n c l u s i o n s The main c o n c l u s i o n o f t h i s s t u d y i s t h a t deformed l i n e a t i o n p a t t e r n s p r o v i d e a p o w e r f u l t o o l f o r s t r a i n a n a l y s i s i n a r e a s o f p o l y p h a s e deforma-t i o n . Methods such as Ramsay's t method c o n t a i n a s s u m p t i o n s , t h e v a l i d i t y o f w hich c a n n o t be t e s t e d w i t h o u t i n d e p e n d e n t i n f o r m a t i o n . However, t h e l a r g e d e g r e e o f d a t a redundancy i n t h e l i n e a t i o n method, and i t s i n h e r e n t s e n s i t i v i t y t o f a i l u r e o f i t s b a s i c a s s u m p t i o n s , g i v e i t a d i s t i n c t a d v a n t a g e . F o r example, the method assumes p u r e s h e a r ( f l a t t e n i n g ) ; however, any s i g n i f i c a n t component o f s i m p l e s h e a r would l e a d t o d i s t i n c t l y d i f f e r e n t l i n e a t i o n p a t t e r n s . U s i n g the l i n e a t i o n p a t t e r n t e c h n i q u e , i t i s p o s s i b l e t o s e p a r a t e t h e e f f e c t s o f Phase Two and Phase T h r e e s t r a i n s . A p p l i e d t o f o l d a x i s p a t t e r n s o v e r a r e a s o f hundreds o f s q u a r e m e t e r s , the t e c h n i q u e p r o v i d e s a method f o r d e s c r i b i n g the b u l k o r a v e r a g e s t r a i n o f r o c k masses and t h e r e f o r e p r o v i d e s a b e t t e r measure o f r e g i o n a l s h o r t e n i n g t h a n the use o f s t r a i n markers o f u n u s u a l d u c t i l i t y and s p o r a d i c o c c u r r e n c e . APPENDIX I I I METAMORPHIC ASSEMBLAGES AND TEXTURAL RELATIONSHIPS IN THE THOR-ODIN GNEISS DOME 323 I n t r o d u c t i o n T h i s a p p e n d i x documents t h e phase a s s e m b l a g e s and t e x t u r e s o b s e r v e d i n t h i s s t u d y which a r e r e l e v a n t t o d e c i p h e r i n g t h e metamorphic h i s t o r y o f t h e T h o r - O d i n g n e i s s dome. These o b s e r v a t i o n s a r e b r o a d l y c o m p a t i b l e w i t h t h e e a r l i e r s t u d i e s by R e e s o r and Moore ( 1 9 7 1 ) , H i l l ( 1 9 7 5 ) , F r o e s e (1970) and M u t t i ( 1 9 7 8 ) . However, t h i s s t u d y p r o v i d e s a d d i t i o n a l e v i d e n c e t h a t a l l o w s a more d e t a i l e d u n d e r s t a n d i n g o f t h e v a r i a t i o n o f metamorphic grade d u r i n g e v o l u t i o n o f t h e dome and t h e r e l a t i o n s h i p between meta-morphi sm and s t r u c t u r a l e v e n t s . Q u a r t z o - f e l d s p a t h i c Assemblages The d i s t r i b u t i o n o f a l u m i n o s i l i c a t e p o l y m o r p h s , m u s c o v i t e + q u a r t z , and c o r d i e r i t e - b e a r i n g p e l i t i c m i n e r a l a s s e m b l a g e s w i t h i n t h e s t u d y a r e a i s shown i n F i g u r e I I I - l . A l t h o u g h R e e s o r and Moore (1971) mapped a m u s c o v i t e + q u a r t z = a l k a l i f e l d s p a r + s i l l i m a n i t e + H^O i s o g r a d i n t h e n o r t h e a s t c o r n e r o f t h e map a r e a , t h i s a s s e m b l a g e was f o u n d t o have a s p o t t y , v a r i a b l e d i s t r i b u t i o n i n t h i s s t u d y . In a d d i t i o n t h e assemblage m u s c o v i t e + q u a r t z + a l k a l i f e l d s p a r + s i l l i m a n i t e + p l a g i o c l a s e i s w i d e s p r e a d w i t h i n t h e T h o r - O d i n - C r a n b e r r y M o u n t a i n - B e g b i e a r e a ( H i l l , 1975; M u t t i , 1978). The p r e s e n c e o f a p a r a g o n i t e component i n the mica makes t h e m u s c o v i t e + q u a r t z breakdown r e a c t i o n d i v a r i a n t (Thompson and Thompson, 1976). The a l u m i n o s i l i c a t e polymorphs a l s o show a v a r i a b l e d i s t r i b u t i o n , and no r e a s o n a b l e i s o g r a d s can be drawn. E x a m i n a t i o n o f t h e t e x t u r a l r e l a t i o n s d i s p l a y e d by t h e assemblages s u g g e s t s t h a t polymetamorphism i s i n p a r t r e s p o n s i b l e f o r the v a r i a b i l i t y shown i n F i g u r e I I I - l . In q u a r t z - r i c h r o c k s , c l e a v a g e f r a c t u r e s c o n t a i n r o s e t t e s o f c o a r s e - g r a i n e d m u s c o v i t e up t o 20 cm i n l e n g t h . These v e i n l e t s 324 FIGURE I I 1 - 1 . M e t a m o r p h i c assemblage map f o r t h e T h o r - O d i n g n e i s s dome. T h i s map i s m o d i f i e d a f t e r R e e s o r and Moore's (1971) F i g u r e 8 . 325 GS St Si-Mu Si-Kf L E G E N D Rocks in greenscbist facias Rocks in amphibolite facies, staurolite-almandine subfacies Rocks in amphibolite facias, sillimanite-almandine-muscovite subfacies Rocks in amphibolite facies, sillimanite-almandine-orthoclase subfacies Cataclasite zone Si-Kf V, ff ,,*)*KC\\ C Retrograde metamorphism to greenschist facies K K \ X " l £ « C Isograds (defined, assumed, symbols point upgrade) Si+Mu + Kf . . . . \ v Si "-A^-, st Occurrence of Kyanite K Occurrence of andalusite A Occurrence of cordierite mantling aluminosilicates C AK st Si-Mu Si- Kf 326 c r o s s c u t t h e assemblage q u a r t z - a l k a l i f e l d s p a r - s i 1 l i m a n i t e . M u s c o v i t e a l s o o c c u r s i n f i n e - g r a i n e d r e c r y s t a l l i z e d s h e a r zones a s s o c i a t e d w i t h Phase T h r e e t e c t o n i c s l i d e s . P o l y m e t a m o r p h i c t e x t u r a l r e l a t i o n s a r e a l s o d i s p l a y e d by t h e a l u m i n o s i l i c a t e p o l y m o r p h s . The f o l l o w i n g t e x t u r a l o b s e r v a t i o n s were made d u r i n g t h i s s t u d y : 1. K y a n i t e p o r p h y r o b l a s t s a r e r e p l a c e d by b u n d l e s o f n e e d l e - l i k e c r y s t a l s o f s i l l i m a n i t e . 2. S i l l i m a n i t e c r y s t a l s o c c u r o r i e n t e d a l o n g Phase One f o l d axes and a r e l o c a l l y r e f o l d e d by Phase Two f o l d s w i t h k y a n i t e m i n e r a l l i n e a t i o n s i n t h e h i n g e . These Phase Two k y a n i t e l i n e a t i o n s a r e t h e m s e l v e s r e p l a c e d by s i l l i m a n i t e . 3. Pegmatoid v e i n s w h i c h c r o s s c u t Phase Two f o l d s have t he asse m b l a g e q u a r t z - a l k a l i f e l d s p a r - t o u r m a l i n e - k y a n i t e . 4. G a r n e t - and b i o t i t e - b e a r i n g q u a r t z o - f e l d s p a t h i c g n e i s s e s from t h e C o v e r Rock Domain show r a r e k y a n i t e i n c l u s i o n s i n t h e c o r e s o f g a r n e t s , and t h e assemblage q u a r t z - b i o t i t e - p l a g i o c l a s e -s i l l i m a n i t e i n t h e groundmass. 5. Rare c r o s s c u t t i n g v e i n l e t s i n q u a r t z i t e s have t h e assemblage q u a r t z - m u s c o v i t e - t o u r m a l i n e - a n d a l u s i t e . The a n d a l u s i t e i s c o a r s e - g r a i n e d and e u h e d r a l . S i m i l a r o b s e r v a t i o n s have been made by H i l l (1975) t o the n o r t h i n the Be g b i e a r e a . 6. S i l l i m a n i t e - p l a g i o c l a s e i n t e r g r o w t h s o c c u r i n g a r n e t p r e s s u r e shadows which a r e o r i e n t e d a l o n g Phase T h r e e f o l d axes i n t h e c e n t r a l p a r t o f t h e B i g Ledge subdomain. The above o b s e r v a t i o n s s u g g e s t t h a t metamorphic P-T c o n d i t i o n s were v a r i a b l e d u r i n g t h e e v o l u t i o n o f t h e dome. R e - e q u i l i b r a t i o n o f t h e r e f r a c t o r y a l u m i n o s i l i c a t e s i s c l e a r l y f a c i l i t a t e d by d e f o r m a t i o n and r e c r y s t a l l i z a t i o n i n s h e a r zones and f o l d h i n g e s , and by d i s s o l u t i o n i n aqueous f l u i d s w i t h s u b s e q u e n t p r e c i p i t a t i o n i n v e i n s . P e l i t i c Assemblages P e l i t i c s c h i s t s a r e a w i d e s p r e a d b u t r a r e r o c k t y p e w i t h i n t h e T h o r -O d i n g n e i s s dome a r e a . A l t h o u g h g a r n e t - b i o t i t e and g a r n e t - b i o t i t e - s i 1 1 i -m a n i t e a r e common, s t a u r o l i t e + q u a r t z i s a b s e n t and s t a u r o l i t e was o n l y 327 f o u n d i n one r o c k on t h e n o r t h e a s t p a r t o f F o s t h a l l r i d g e i n t h e c o v e r r o c k s e q u e n c e . C o a r s e - g r a i n e d p e l i t i c g n e i s s e s i n the Basement C o r e d Nappe Domain c o n t a i n c o r d i e r i t e - r i m m i n g s i l l i m a n i t e i n a b i o t i t e - r i c h m a t r i x , s u g g e s t i n g t he r e a c t i o n : s i l l i m a n i t e + b i o t i t e Z c o r d i e r i t e + a l k a l i f e l d s p a r + g a r n e t + h^O. P e l i t i c s c h i s t s and g n e i s s e s a r e c h a r a c t e r i z e d by s p o r a d i c r e t r o g r a d e metamorphism. V e i n l e t s c r o s s c u t t i n g p e l i t i c g n e i s s e s i n the C o v e r Rock Domain c o n t a i n c h l o r i t e , m u s c o v i t e and g a r n e t . R e t r o g r a d e c h l o r i t e - g a r n e t s c h i s t s o c c u r i n s h e a r zones c r o s s c u t t i n g p e l i t i c r o c k s a t the base o f the Gunnarson Nappe. G e d r i t e - A l u m i n o s i 1 i c a t e Assemblages D i s t i n c t i v e g e d r i t e - a l u m i n o s i 1 i c a t e a s s e m b l a g e s o c c u r i n b o u d i n s and l e n s e s w i t h i n t h e m e t a s e d i m e n t a r y g n e i s s e s o f t h e Basement C o r e d Nappe Domain. These a s s e m b l a g e s a r e t y p i c a l l y q u a r t z and f e l d s p a r - f r e e . Two r a r e assemblages were o b s e r v e d w i t h q u a r t z v e i n s c u t t i n g a c r o s s a g g r e g a t e s o f g e d r i t e and s i l l i m a n i t e . The q u a r t z i s rimmed by c o r d i e r i t e , s u g g e s t i n g t h e r e a c t i o n : g e d r i t e + s i l l i m a n i t e + q u a r t z t c o r d i e r i t e has t a k e n p l a c e . In o t h e r a s s e m b l a g e s , s i l l i m a n i t e i n g e d r i t e - s i l l i m a n i t e a g g r e g a t e s i s rimmed by c o r d i e r i t e and corundum. T h i s r e l a t i o n has a l s o been o b s e r v e d by R e e s o r and Moore (1971) and M u t t i ( 1 9 7 8 ) . G e d r i t e - g a r n e t -s i l l i m a n i t e a s s e m b l a g e s c o n t a i n g a r n e t p o r p h y r o b l a s t s up t o 1 cm a c r o s s , rimmed by c o r d i e r i t e . The i n t e r p r e t a t i o n o f t h i s t e x t u r e i n terms o f r e a c t i o n r e l a t i o n s i s n o t c l e a r . 328 In q u a r t z - f r e e a s s e m b l a g e s , s i l l i m a n i t e p o r p h y r o b l a s t s a r e t y p i c a l l y rimmed by c o a r s e c o r d i e r i t e and f i n e - g r a i n e d corundum. The t e x t u r a l r e l a t i o n s a r e c o m p a t i b l e w i t h t h e r e a c t i o n : g e d r i t e + s i l l i m a n i t e t c o r d i e r i t e + corundum + h^O. The b u l k c o m p o s i t i o n s r e p r e s e n t e d by t h e s e g e d r i t e - s i l l i m a n i t e assem-b l a g e s a r e u n u s u a l i n t h a t t h e y a r e r i c h i n MgO and A l ^ O ^ and r e l a t i v e l y p o or i n CaO, K^O and S i O ^ . A n a l y s e s o f b u l k c o m p o s i t i o n s o f s i m i l a r a s s e m b l a g e s i n o t h e r a r e a s do n o t c o r r e s p o n d t o t h e c h e m i s t r y o f any known non-metamorphic r o c k . Indeed S e k i (1957) has c o n c l u d e d , " C h e m i c a l l y s i m i l a r r o c k s a r e n o t known among i g n e o u s , s e d i m e n t a r y o r metamorphic r o c k s . " A l t h o u g h t h i s s t a t e m e n t i s somewhat ex t r e m e , i t i s r e a s o n a b l e t o c o n c l u d e t h a t t h e s e a r e n o t a normal s e d i m e n t a r y b u l k c o m p o s i t i o n . T h r e e t h e o r i e s have been advanced t o e x p l a i n t h e o r i g i n o f t h e s e r o c k s : 1. MgO metasomatism ( E s k o l a , 1914); 2. metamorphism o f a l t e r e d b a s a l t i c r o c k s such as s p i l i t e s ( V a l l a n c e , 1967); 3. t h e r e s i d u e f r o m p a r t i a l m e l t i n g o f a graywacke c o m p o s i t i o n w i t h e x t r a c t i o n o f a g r a n i t i c p a r t i a l m e l t ( G r a n t , 1968). F i e l d r e l a t i o n s and p e t r o g r a p h i c d a t a s u g g e s t t h a t t h e f i r s t and t h i r d t h e o r i e s a r e u n l i k e l y e x p l a n a t i o n s i n t h i s c a s e . F i r s t l y , the b o u d i n s t y p i c a l l y o c c u r w i t h i n q u a r t z o - f e l d s p a t h i c and p e l i t i c g n e i s s e s . The c o n t a c t between t h e g n e i s s e s and the g e d r i t e r o c k i s g e n e r a l l y s h a r p . C o n t a c t s w i t h p e l i t i c g n e i s s e s a p p e a r t o have b i o t i t e - g e d r i t e - s i l l i m a n i t e a ssemblages a l o n g t h e c o n t a c t , perhaps r e f l e c t i n g d i f f u s i o n o f K^O i n t o t he b o u d i n f r o m t h e c o u n t r y r o c k . A l t h o u g h t he m e t a s e d i m e n t a r y g n e i s s e s show e x t e n s i v e e v i d e n c e o f p a r t i a l m e l t i n g , g e d r i t e assemblages a r e n o t fo u n d as a r e s i d u u m . C o n v e r s e l y , no g r a n i t i c p a r t i a l m e l t s a r e f o u n d 329 i n s i d e o r e m a n a t i n g f r o m t h e margins o f t h e g e d r i t e b o u d i n s . In a d d i t i o n t o t h e s e o b s e r v a t i o n s , i t a p p e a r s s i g n i f i c a n t t h a t t h e g e d r i t e - r i c h b o u d i n s o c c u r a l i g n e d i n s p e c i f i c s t r u c t u r a l l e v e l s as s t r i n g s w i t h i n t h e l i t h o -l o g i c a l s e q u e n c e . T h i s s u g g e s t s t h a t t h e s e r o c k s a r e p a r t o f t h e " s t r a t i g r a p h y " and a r e n o t t h e p r o d u c t s o f metasomatism o r p a r t i a l m e l t i n g . C o n c l u s i o n s The metamorphic asse m b l a g e s d i s c u s s e d i n t h i s a p p e n d i x , t o g e t h e r w i t h t h e g e o l o g i c a l and g e o c h r o n o l o g i c a l d a t a d i s c u s s e d i n C h a p t e r s One and Two, s u g g e s t t h a t t h e T h o r - O d i n g n e i s s dome has had a complex, p o l y -metamorphic h i s t o r y . Rb-Sr g e o c h r o n o l o g y on h i g h - g r a d e m i g m a t i t e s i n t h e basement g n e i s s e s s u g g e s t s a m a j o r metamorphic e v e n t o c c u r r e d a t 750 Ma, p r i o r t o t h e d e p o s i t i o n o f t h e c o v e r r o c k s . The o c c u r r e n c e o f h i g h - g r a d e m i g m a t i t i c g n e i s s e s i n t h e c o v e r r o c k s d e m o n s t r a t e s a t l e a s t two major metamorphic e v e n t s . R e g i o n a l e v i d e n c e t h a t Phase One d e f o r m a t i o n i s p r e - J u r a s s i c , and t h e o b s e r v a t i o n t h a t Phase Two d e f o r m a t i o n was accompanied by upper a m p h i b o l i t e metamorphism, s u g g e s t s a t l e a s t t h r e e major meta-m o r p h i c e v e n t s have a f f e c t e d t h e a r e a . APPENDIX IV METAMORPHIC PHASE EQUILIBRIA AND THE EVOLUTION OF THE THOR-ODIN GNEISS DOME 331 I n t r o d u c t i o n R e e s o r and Moore (1971) i n t e r p r e t e d t h e metamorphic p a r a g e n e s i s o f t h e T h o r - O d i n g n e i s s dome i n terms o f the e x p e r i m e n t a l phase e q u i l i b r i a d a t a a v a i l a b l e a t t h a t t i m e . They c o n c l u d e d t h a t t h e c o r e g n e i s s e s e q u i l i b r a t e d a t p r e s s u r e s o f 750-950 MPa and t e m p e r a t u r e s o f 6 4 0 - 7 5 0 ° C . T h i s c o n c l u s i o n , t h a t t h e c o r e g n e i s s e s e q u i l i b r a t e d a t h i g h p r e s s u r e s i n t h e l o w e r c r u s t , was s u p p o r t e d by s u b s e q u e n t s t u d i e s o f c o r d i e r i t e e q u i l i b r i a by S c h r e y e r ( 1 9 7 3 ) . R e e s o r and Moore (1971) f u r t h e r s u g g e s t e d t h a t t h e r e was a 150 MPa p r e s s u r e d i f f e r e n t i a l between t h e c o r e o f t h e g n e i s s dome and t h e c o v e r r o c k s . They i n t e r p r e t t h i s p r e s s u r e d i f f e r e n c e as b e i n g a r e s u l t o f d i a p i r i c u p r i s e o f t h e c o r e g n e i s s e s . The s t r u c t u r a l i n t e r p r e t a t i o n made i n t h i s s t u d y o f the o r i g i n o f the dome i s i n c o m p a t i b l e w i t h a l o w e r c r u s t a l d i a p i r i c o r i g i n f o r the dome. T h i s a p p e n d i x r e - e x a m i n e s t h e metamorphic a s s e m b l a g e s d i s c u s s e d i n t h e p r e v i o u s a p p e n d i x i n t h e c o n t e x t o f t h e most r e c e n t e x p e r i m e n t a l phase e q u i 1 i b r i u m d a t a . P e l i t i c M i n e r a l Assemblages R e c e n t phase e q u i l i b r i u m d a t a f o r p e l i t i c m i n e r a l a s s e m b l a g e s has been r e v i e w e d by Thompson ( 1 9 7 6 ) , H a r t e ( 1 9 7 5 ) , and Holdaway and Lee ( 1 9 7 7 ) . A major m o d i f i c a t i o n t o t h e p e t r o g e n e t i c g r i d used by R e e s o r and Moore ( 1 9 7 1 ) , i n c o r p o r a t e d i n t h e s e more r e c e n t c o m p i l a t i o n s , i s t h e a c c e p t a n c e o f an a l u m i n o - s i 1 i c a t e phase d i a g r a m c o n s i s t e n t w i t h the k y a n i t e - s i l l i m a n i t e and a n d a l u s i t e - k y a n i t e d a t a o f Holdaway ( 1 9 7 1 ) . A c c e p t a n c e o f t h e s e new d a t a would move t h e p r e s s u r e e s t i m a t e s i n Re e s o r and Moore (1971, f i g u r e 11) to l o w e r v a l u e s . 332 Thompson (1976) has p r e s e n t e d a new p e t r o g e n e t i c g r i d f o r p e l i t i c m i n e r a l a s s e m b l a g e s . The common o c c u r r e n c e o f b i o t i t e - s i l l i m a n i t e and t h e e x i s t e n c e o f t e x t u r a l e v i d e n c e f o r t h e r e a c t i o n b i o t i t e + s i l l i m a n i t e = g a r n e t + c o r d i e r i t e + a l k a l i f e l d s p a r s u g g e s t s t e m p e r a t u r e s above 680°C and w a t e r p r e s s u r e s above 390 MPa, b a s e d on Thompson's g r i d . T h i s i s a p p r o x i m a t e l y 300 MPa below t h e s t a b i l i t y f i e l d o f k y a n i t e , s u g g e s t i n g t h a t t h e c o r d i e r i t e - p r o d u c i n g r e a c t i o n s o c c u r d u r i n g u p l i f t , a f t e r t h e main peak o f metamorphism. F r o e s e (1973) has used t h e a s s e m b l a g e b i o t i t e - g a r n e t - s i 11 imam*te-a l k a l i f e l d s p a r - q u a r t z t o e s t i m a t e t h e t e m p e r a t u r e and w a t e r p r e s s u r e f o r 22 samples f r o m t h e T h o r - O d i n g n e i s s dome. F r o e s e c o n c l u d e s t h a t w a t e r p r e s s u r e s f o r t h e s e r o c k s v a r y between 30 and 220 MPa, and t h a t tempera-t u r e s v a r y between 540 and 740°C. U n f o r t u n a t e l y , t h e s e e s t i m a t e s a p p e a r to be ba s e d on some q u e s t i o n a b l e a s s u m p t i o n s . F r o e s e assumed a t o t a l l i t h o s t a t i c p r e s s u r e o f 300 MPa f o r t h e s e a s s e m b l a g e s , a p r e s s u r e u n r e a l i s -t i c a l l y low f o r t h e main J u r a s s i c metamorphism. A s e c o n d p r o b l e m w i t h t h e s e c a l c u l a t i o n s i s t h a t t he f r e e e n e r g i e s and e n t r o p i e s used f o r t h e end member r e a c t i o n s : a n n i t e + s i l l i m a n i t e + q u a r t z t a l k a l i f e l d s p a r + a l m a n d i n e + w a t e r p h l o g o p i t e + s i l l i m a n i t e + q u a r t z t a l k a l i f e l d s p a r + py r o p e + w a t e r a r e c r u d e e s t i m a t e s a t b e s t . S u b s e q u e n t t o F r o e s e ' s (1973) s t u d y , F e r r y and S p e a r (1978) have p r o d u c e d an e x p e r i m e n t a l c a l i b r a t i o n f o r t h e exchange r e a c t i o n : a n n i t e + p y r o p e % p h l o g o p i t e + a l m a n d i n e . 333 On t h e b a s i s o f t h e i r e x p e r i m e n t s , t h e y e s t i m a t e t h e e n t h a l p y and e n t r o p y o f t h e exchange r e a c t i o n as 52,108 j o u l e s and 19.506 j / d e g mol r e s p e c t i v e l y . M e t a m o r p h i c t e m p e r a t u r e s have been e s t i m a t e d u s i n g F e r r y and S p e a r ' s r e s u l t s and t h e d a t a p r e s e n t e d i n F r o e s e (1970) and u t i l i z e d i n F r o e s e ' s (1973) c a l c u l a t i o n s . The r e s u l t s , p r e s e n t e d i n T a b l e I V - I , g i v e s i g n i f i c a n t l y h i g h e r t e m p e r a t u r e s (805 +_ 50°C) t h a n t h o s e e s t i m a t e d by F r o e s e . These t e m p e r a t u r e s a p p e a r t o be h i g h e r t h a n r e a s o n a b l e . T h i s may be due t o a pr o b l e m w i t h t h e geothermometer o r t o t h e a s s u m p t i o n s a n d / o r d a t a used to make t h e c a l c u l a t i o n s i n T a b l e I V - I . O t h e r s t u d i e s s u c h as F l e t c h e r and Greenwood (1979) have f o u n d t h a t t h e F e r r y - S p e a r geothermometer g i v e s r e a s o n a b l e t e m p e r a t u r e e s t i m a t e s . V a r y i n g t he e n t h a l p i e s and e n t r o p i e s o f t h e exchange r e a c t i o n w i t h i n t h e e r r o r l i m i t s g i v e n by F e r r y and S p e a r c a n change t h e t e m p e r a t u r e s i g n i f i c a n t l y ( f o r example, t h e e s t i m a t e f o r sample #1-2 v a r i e s f r o m 770 t o 8 8 5 ° C ) . In a d d i t i o n t o t h i s p r o b l e m , t h e t e m p e r a t u r e e s t i m a t e s i n T a b l e IV-I assume a t o t a l p r e s s u r e o f 550 MPa. F o r sample #1-2, the t e m p e r a t u r e e s t i m a t e would be 26°C l o w e r i f c a l c u l a t e d a t room p r e s s u r e . A major p r o b l e m i s t h a t t h e g a r n e t c o m p o s i t i o n s o f F r o e s e do n o t n e c e s s a r i l y r e p r e s e n t t h e edge c o m p o s i t i o n o f t h e g a r n e t . G e d r i t e - C o r d i e r i t e Assemblages The s t a b i l i t y r e l a t i o n s o f g e d r i t e - c o r d i e r i t e a s s e m b l a g e s have been d i s c u s s e d by R o b i n s o n and J a f f e ( 1 9 6 9 ) , Green and Vernon ( 1 9 7 4 ) , Newton (1972) and Newton and Wood ( 1 9 7 9 ) . The phase d i a g r a m f o r w a t e r - s a t u r a t e d c o n d i t i o n s p r e s e n t e d by Green and Vernon (1974) s u g g e s t s t h a t t he r e a c t i o n : c o r d i e r i t e t g e d r i t e + k y a n i t e + q u a r t z t a k e s p l a c e a t p r e s s u r e s above 1000 MPa and i s r e l a t i v e l y t e m p e r a t u r e -334 TABLE IV-I BIOTITE-GARNET GEOTHERMOMETRY RESULTS COVER ROCKS P t o t a l = 550 MPa Sample Number X F e - b i o t i t e X M g - b i o t i t e Y F e - g a r n e t V Mg-garnet T (°C) 1-2 0.398 0.600 0.668 0.301 803 9-2 0.478 0.520 0.688 0.294 974* 59-1 0.556 0.441 0.768 0.188 821 66-2 0.569 0.429 0.756 0.206 917 283-1 0.550 0.449 0.763 0.213 881 352-1 0.582 0.417 0.746 0.233 1055* 445-1 0.427 0.572 0.702 0.289 818 450-1 0.358 0.641 0.647 0.346 803 MIO-la 0.569 0.430 0.776 0.208 904 M28-2 0.451 0.548 0.702 0.286 869 M71-2 0.551 0.446 0.780 0.179 775 Ml 41-2 0.600 0.398 0.772 0.185 916 M142-2a 0.555 0.444 0.731 0.236 994* M143-3b 0.505 0.49.3 0.747 0.226 823 Ml 43-3d 0.534 0.465 0.760 0.218 858 Ml 79-2 0.253 0.738 0.441 0.377 793 Ml 79-4 0.270 0.720 0.406 0.370 881 Ml 80-3 0.490 0.509 0.712 0.263 906 M180-6 0.428 0.570 0.691 0.277 807 M-180-73 0.515 0.478 0.605 0.224 MEAN 919 853 * Mean t h e s e was c a l c u l a t e d e x c l u d i n g t h r e e h i g h v a l u e s . a + 50 CORE GNEISSES P + n + a l = 600 MPa t o t a l  205-1 0.548 0.448 0.756 0.192 827 279-3 0.557 0.432 0.682 0.151 781 MEAN 804 a + 3 2 335 i n s e n s i t i v e . T e x t u r a l e v i d e n c e f o r t h i s r e a c t i o n o c c u r r i n g i n t h e c o r e o f t h e T h o r - O d i n g n e i s s dome has been d i s c u s s e d i n A p p e n d i x I I I . However, such h i g h p r e s s u r e s a r e i n c o n s i s t e n t w i t h t h e s t r u c t u r a l e v i d e n c e , t h e o t h e r metamorphic asse m b l a g e s d i s c u s s e d a b o v e , and t h e o t h e r p r e s s u r e e s t i m a t e s f o r t h e Shuswap t e r r a i n . Newton (1972) o r i g i n a l l y s u g g e s t e d t h e p o s s i b i l i t y t h a t t h e s t a b i l i t y f i e l d o f c o r d i e r i t e m i g h t be r e d u c e d w i t h l o w e r w a t e r a c t i v i t i e s . T h i s s u g g e s t i o n was f o l l o w e d up by t h e a u t h o r (Duncan and Greenwood, 1 9 7 7 ) . Thermodynamic c a l c u l a t i o n s have been made by Newton ( 1 9 7 2 ) , based on t h e c a l o r i m e t r i c d e t e r m i n a t i o n o f t h e e n t h a l p y o f f o r m a t i o n and e n t r o p y o f an h y d r o u s c o r d i e r i t e . T hese s u g g e s t t h e s t a b i l i t y f i e l d o f c o r d i e r i t e t o be 300 t o 500 MPa l o w e r i n p r e s s u r e than t h a t o f hydrous c o r d i e r i t e . Duncan and Greenwood (1977) p r o p o s e d t h a t p a r t i a l l y h y d r a t e d c o r d i e r i t e c o u l d be c o n s i d e r e d an i d e a l s o l i d s o l u t i o n between h y d r o u s and anhydrous c o r d i e r i t e end members p a r t i c i p a t i n g i n an i n t e r n a l e q u i l i b r i u m i n the for m : u c o r d i e r i t e ^ O = u c o r d i e r i t e + u H^O . G i v e n an e n t h a l p y (AH ) and e n t r o p y ( A S r ) o f r e a c t i o n f o r t h i s e q u i l i b r i u m t h e n t h e e f f e c t o f w a t e r on c o r d i e r i t e e q u i l i b r i a c a n be c a l c u l a t e d a f t e r assuming a s o l u t i o n model f o r c o r d i e r i t e - H ^ O . Assuming t h a t t h e r e i s a maximum o f one w a t e r m o l e c u l e p e r c o r d i e r i t e u n i t c e l l ( P . Meagher, p e r s o n a l c o m m u n i c a t i o n , 1977), an i d e a l s o l u t i o n f o r m u l a t i o n c a n be made i n the form: 1 " x c o r d - H ? 0 £n K = — cord-H20 where X C Q r c j _ ^ Q 1 S t n e m o l e f r a c t i o n o f hydrous c o r d i e r i t e i n t h e s o l i d 336 s o l u t i o n . Assuming t h i s r e l a t i o n and t a k i n g a A H ° f 2 g g f ° r M g - c o r d i e r i t e -H 20 o f -289.930 k j o u l e / m o l and a A S ° f 2 9 8 o f 174.1 j / d e g mol d e r i v e d f r o m the e x p e r i m e n t a l work p r e s e n t e d i n A p p e n d i x V, t h e e f f e c t o f v a r y i n g y H 20 on any c o r d i e r i t e r e a c t i o n can be . c a l c u l a t e d . Such c a l c u l a t i o n s show t h a t the d i s c r e p a n c y between t h e c a l c u l a t e d anhydrous e q u i l i b r i a and th e e x p e r i m e n t a l l y d e t e r m i n e d hydrous e q u i l i b r i a f o r c o r d i e r i t e % e n s t a t i t e + k y a n i t e + q u a r t z , can be a c c o u n t e d f o r t o w i t h i n + 50 MPa. S i m i l a r c a l c u l a t i o n s have been made by Newton and Wood ( 1 9 7 7 , p. 3 9 8 ) . C o n s i d e r i n g t h e d r a m a t i c e f f e c t o f l o w e r i n g y H 20 on t h e s t a b i l i t y o f c o r d i e r i t e , t h e P-T s i g n i f i c a n c e o f t h e c o r d i e r i t e a s s e m b l a g e s d i s c u s s e d i n A p p e n d i x I I I must be r e - e x a m i n e d . S e v e r a l l i n e s o f e v i d e n c e s u g g e s t w a t e r p r e s s u r e s l o w e r t h a n t o t a l p r e s s u r e d u r i n g t h e metamorphic e v e n t s documented i n A p p e n d i x I I I . F i r s t l y , t h e e s t i m a t e d P-T c o n d i t i o n s and abs e n c e o f e x t e n s i v e p a r t i a l m e l t i n g o f t h e q u a r t z o - f e l d s p a t h i c m e t a s e d i -mentary g n e i s s e s s u g g e s t s t h a t low w a t e r p r e s s u r e s i n h i b i t e d p a r t i a l m e l t i n g . S e c o n d l y , F r o e s e (1973) has s u g g e s t e d P H Q l e s s t h a n P t o t a i • Use o f F r o e s e ' s e s t i m a t e s o f P^ Q would r e s u l t i n p r e s s u r e s f o r t h e c o r d i e r i t e e q u i l i b r i a o f 6 t o 7 kb, d e p e n d i n g on the t e m p e r a t u r e . Thus the e f f e c t o f w a t e r on c o r d i e r i t e e q u i l i b r i a a p p e a r s t o e x p l a i n i n p a r t the a p p a r e n t d i s c r e p a n c y between the P-T e s t i m a t e s i n f e r r e d f r o m p e l i t i c e q u i l i b r i a and t h o s e i n f e r r e d on th e b a s i s o f c o r d i e r i t e phase e q u i l i b r i a under w a t e r - s a t u r a t e d c o n d i t i o n s . C o n c l u s i o n s The phase e q u i l i b r i a d a t a s u g g e s t t h a t the p r e s s u r e s and t e m p e r a t u r e s i n f e r r e d from t h e p e l i t i c a s s e m b l a g e s a r e n o t s i g n i f i c a n t l y d i f f e r e n t f r o m t h o s e i n f e r r e d on th e b a s i s o f c o r d i e r i t e e q u i l i b r i a . P r e s s u r e s o f 5 t o 7 kb a r e r e a s o n a b l e e s t i m a t e s f o r metamorphic p r e s s u r e s . The v a l u e i s c o m p a t i b l e w i t h t h e t e c t o n i c e v o l u t i o n o f t h e dome o u t l i n e d i n Chapte One. APPENDIX V AN EXPERIMENTAL STUDY OF THE EFFECT OF WATER ON THE PHASE RELATIONS OF MAGNESIUM CORDIERITE 339 I n t r o d u c t i o n The s t a b i l i t y f i e l d o f anhydrous c o r d i e r i t e i s 300 t o 500 MPa l o w e r i n p r e s s u r e t h a n t h a t o f hy d r o u s c o r d i e r i t e . T h i s d i s c r e p a n c y may be r e c o n c i l e d by c o n s i d e r i n g t h e e f f e c t o f m o l e c u l a r w a t e r i n i t s channelways on t h e G i b b s f r e e e n e r g y o f c o r d i e r i t e . C a l c u l a t i o n o f t h e thermodynamic p r o p e r t i e s o f t h e c o r d i e r i t e - w a t e r s y s t e m r e q u i r e d a t a on the w a t e r c o n t e n t o f c o r d i e r i t e a t known t e m p e r a t u r e s and c h e m i c a l p o t e n t i a l s o f w a t e r (u HoO). E x p e r i m e n t a l T e c h n i q u e The r a p i d i t y o f t h e c o r d i e r i t e h y d r a t i o n p r o c e s s has r e s u l t e d i n t h e r e p o r t i n g o f a n o m a l o u s l y h i g h w a t e r c o n t e n t s i n s e v e r a l p r e v i o u s s t u d i e s . The p r o b l e m o f w a t e r d i f f u s i o n i n c o r d i e r i t e has been a s s e s s e d by L e p e z i n and M e l e n e v s k y (1977), who p r e s e n t D 0s and a c t i v a t i o n e n e r g i e s f o r d i f f u s i o n o f w a t e r f r o m n a t u r a l c o r d i e r i t e s . T h e i r d a t a ( T a b l e V-I) s u g g e s t a c t i v a t i o n e n e r g i e s i n the rang e 28 t o 55 k c a l / m o l . P l o t t i n g t h e a c t i v a t i o n e n e r g y v e r s u s t h e minimum neck d i a m e t e r f o r v a r i o u s z e o l i t e s ( B a r r e r , 1971, T a b l e V - I ) r e s u l t s i n a l i n e a r c o r r e l a t i o n between t h e s e o two p a r a m e t e r s . T a k i n g t he neck d i a m e t e r f o r c o r d i e r i t e t o be 1.4 A, an a c t i v a t i o n e n e r g y o f 20 +_ 2 k c a l / m o l c a n be e s t i m a t e d . L e p e z i n and Me l e n e v s k y (1977) g i v e e s t i m a t e s o f D 0 o f 4.9 x 10-1 c m 2 / s e c t o 2.2 x 10-3 c m 2 / s e c . B a r r e r ' s (1971) measurements f o r D 0s f o r z e o l i t e w a t e r r a n g e f r o m 1.2 x 10"1 t o 5.6 x 10"2. C h o o s i n g an i n t e r m e d i a t e v a l u e (1.0 x 10~2), t h e e f f e c t i v e c l o s u r e t e m p e r a t u r e f o r w a t e r i n c o r d i e r i t e can be c a l c u l a t e d u s i n g t h e t h e o r e t i c a l a n a l y s i s o f d i f f u s i v e exchange p r e s e n t e d by Dodson (1974). TABLE V-I DIFFUSIVITY OF WATER IN CORDIERITE AND ZEOLITES Sample C o r d i e r i t e #10 C o r d i e r i t e #12 C o r d i e r i t e #11 C o r d i e r i t e #17 A n a l c i t e N a t r o l i t e H e u l a n d i t e C h a b a z i t e G m e l i n i t e L i q u i d * w a t e r Ic e Neck D i a m e t e r 1.4 1.4 1.4 1.4 2.2 2.6 2.4 3.7 3.4 k c a l / m o l 38 32 28 25 17.0 15.0 11.0 8.7 8.1 4.6 13.5 Do c m 2 / s e c 4.9 x I O " 1 2.2 x I O " 3 ? ? 1.5 x I O " 1 7.6 x I O " 1 1.2 x I O " 1 2.0 x I O - 2 5.6 x I O - 2 ? 341 Dodson (1974) p r e s e n t s t h e f o l l o w i n g e q u a t i o n f o r t h e c l o s u r e t e m p e r a t u r e ( T c ) : E '-AD0~ [ R T C 2 J = £n a 2 f where E = a c t i v a t i o n e n e r g y , R = gas c o n s t a n t , A = geometry f a c t o r (A = 8.7 f o r d i f f u s i o n i n l i n e a r c h a n n e l w a y s ) , and T = t h e c o o l i n g r a t e . The f o l l o w i n g v a l u e s were used t o s i m u l a t e t h e e f f e c t o f normal c o o l i n g r a t e s f o r h y d r o t h e r m a l e x p e r i m e n t s c o o l e d i n compressed a i r : T = 0.31°C/sec and a 2 = 0.0025. F o r t h e s e v a l u e s , T c i s i n t h e rang e 470 t o 750°C f o r a c t i v a t i o n e n e r g i e s o f 16 t o 24 k c a l / m o l . T h e s e c a l c u l a t i o n s d e m o n s t r a t e t h a t t h e r e h y d r a t i o n d u r i n g normal q u e n c h i n g i s a ma j o r p r o b l e m i n e x p e r i m e n t a l s t u d i e s o f c o r d i e r i t e - w a t e r e q u i l i b r i a . To overcome t h i s p r o b l e m , Duncan and Greenwood (1977) employed two d i f f e r e n t q u e n c h i n g t e c h n i q u e s i n an a t t e m p t t o o b t a i n " r e v e r s a l s " o f t h e w a t e r - c o r d i e r i t e e q u i l i b r i a . The f i r s t quench t e c h n i q u e i s r a p i d i s o b a r i c q uench. The c a p s u l e was r un w i t h o u t a f i l l e r r o d i n a s p e c i a l l y c a l i b r a t e d c o l d s e a l w i t h a 1 0 - i n c h l o n g e x t e n s i o n . A t t h e t e r m i n a t i o n o f t h e r u n , t h e bomb was opened to a l a r g e h i g h - p r e s s u r e r e s e r v o i r , so t h a t the p r e s s u r e c o u l d be m a i n t a i n e d d u r i n g c o o l i n g o f the bomb. The bomb was t h e n removed from t h e f u r n a c e and t i p p e d v e r t i c a l l y , p l u n g i n g t h e c h a r g e t o t h e c o o l end o f the bomb. T h i s a c h i e v e d a dr o p i n t e m p e r a t u r e o f a p p r o x i m a t e l y 400°C i n l e s s t h a n a s e c o n d . The s e c o n d t y p e o f q u e n c h i n g t e c h n i q u e was an i s o t h e r m a l p r e s s u r e r e l e a s e o r "blow" q u e n c h . Samples were r u n i n normal bombs b u t w i t h c r i m p e d ( u n s e a l e d ) c a p s u l e s . The p r e s s u r e was r e l e a s e d by r a p i d l y u n s c r e w i n g the c a p i l l a r y 342 t u b e ' s c o n n e c t i o n t o t h e p r e s s u r e l i n e . D u r i n g t h i s p r e s s u r e r e l e a s e t h e t e m p e r a t u r e r e m a i n e d a t t h e c h o s e n v a l u e . Upon c o m p l e t i o n o f the p r e s s u r e r e l e a s e t h e bomb was i m m e d i a t e l y removed from t h e f u r n a c e and c o o l e d t o 300°C i n l e s s t h a n f i v e m i n u t e s u s i n g c o m p r e s s e d a i r and a f i n e s p r a y o f w a t e r . Comparison o f t h e P-T t r a j e c t o r i e s o f t h e s e two c o o l i n g p a t h s w i t h t h e P-V-T d a t a f o r w a t e r s u g g e s t s t h a t the i s o b a r i c quench w i l l r e s u l t i n h i g h e r - t h a n - e q u i l i b r i u m w a t e r c o n t e n t s . The i s o t h e r m a l quench r e s u l t s i n l o w e r - t h a n - e q u i l i b r i u m w a t e r c o n t e n t s . Thus t h e s e v a l u e s s h o u l d e f f e c t i v e l y b r a c k e t t h e l o w e r and upper bounds o f t h e e q u i l i b r i u m w a t e r c o n t e n t . I t i s a p a r a d o x o f e x p e r i m e n t a l p e t r o l o g y t h a t r e v e r s i b i l i t y o f r e a c t i o n i s most d i f f i c u l t t o d e m o n s t r a t e when r e a c t i o n r a t e s a r e v e r y h i g h . R e s u l t s The r e s u l t o f c o r d i e r i t e h y d r a t i o n e x p e r i m e n t s c o n d u c t e d i n the manner o u t l i n e d above a r e g i v e n i n T a b l e V — I I . The r e s u l t s r e f l e c t , i n p a r t , problems i n h e r e n t i n t h e p r o c e d u r e . The i s o t h e r m a l e x p e r i m e n t s t e n d e d t o be more s l o w l y quenched ( e s p e c i a l l y as t h e e x p e r i m e n t o r dodges the c l o u d o f e s c a p i n g s t e a m ) . Thus t h e e q u i l i b r i u m v a l u e s a r e more l i k e l y t o be d i s p l a c e d towards t h e i s o b a r i c r e s u l t s . A s e c o n d p r o b l e m d i s c o v e r e d towards t h e end o f t h e e x p e r i m e n t s was t h a t i n the i s o t h e r m a l e x p e r i m e n t s , t h e c h a r g e s were e x p o s e d t o a l a r g e volume o f f l u i d , as the c a p s u l e s were n o t s e a l e d . M i c r o s c o p i c e x a m i n a t i o n and X - r a y a n a l y s i s o f t h e r u n p r o d u c t s r e v e a l e d t a l c m i n e r a l s r e p l a c i n g the c o r d i e r i t e . T h i s i s i n t e r p r e t e d as r e f l e c t i n g the n o n - e q u i l i b r i u m d i s s o l u t i o n o f the c o r d i e r i t e and p r e c i p i t a t i o n o f t a l c . The i m p l i c a t i o n i s t h a t a s i g n i f i c a n t p o r t i o n 343 TABLE V - I I BRACKETED HYDRATION DATA FOR Mg CORDIERITE* 750°C 700°C 650°C 600°C 500°C I s o b a r i c 200 MPa 0.35 1* 0.44 0.57 0.63 0.68 I s o t h e r m a l 200 MPa 0.18 0.33 0.47 0.52 I s o b a r i c 100 MPa 0.46 0.52 0.53 0.56 0.70 I s o t h e r m a l 100 MPa 0.38 0.36 0.51 0.48 0.62 * S y n t h e t i c c o r d i e r i t e s t a r t i n g m a t e r i a l s u p p l i e d by Dr. H.J. H o l l a n d , C o r n i n g R e s e a r c h L a b o r a t o r y , P a i n t e d P o s t , New Y o r k . t Water c o n t e n t s were d e t e r m i n e d g r a v i m e t r i c a l l y u s i n g c a r e f u l l y c a l i b r a t e d w e i g h i n g on samples k e p t i n a d e s s i c a t o r a f t e r d r y i n g . 344 o f t h e r e s u l t s o f i s o t h e r m a l runs may have a p o r t i o n o f bound w a t e r i n l a y e r s i l i c a t e s . T h i s w a t e r would have been d r i v e n o f f d u r i n g the w a t e r a n a l y s i s and i n c l u d e d as m o l e c u l a r w a t e r i n t h e c o r d i e r i t e . The w a t e r d e t e r m i n a t i o n s were made u s i n g s t a n d a r d g r a v i m e t r i c t e c h n i q u e s . The samples were d r i e d a t 130°C f o r t w e l v e hours t o d r i v e o f f a d s o r b e d w a t e r , t h e n p u t i n a d e s s i c a t o r t o c o o l . The samples were then a c c u r a t e l y weighed and t h e b a l a n c e c h e c k e d by w e i g h i n g a p l a t i n u m r e f e r e n c e w e i g h t . The sample was t h e n h e a t e d t o 800°C f o r one hour i n a p l a t i n u m c r u c i b l e and a l l o w e d t o c o o l i n a d e s s i c a t o r . The sample was r e q e i g h e d and t h e w e i g h t l o s s r e c o r d e d . The d a t a i n T a b l e V - I I were used t o c o n s t r a i n t h e v a l u e s f o r t h e a n t h a l p y and e n t r o p y o f f o r m a t i o n o f h y d r a t e d magnesium c o r d i e r i t e . The r e s u l t s a r e c o n s i s t e n t w i t h a A G ° f 2 g g o f -241.835 k j / m o l , a A H ° f 2 g 8 o f -289.951 k j / m o l , and a A S ° ^ 2 g g o f 176 j / d e g mol. T h i s i m p l i e s a t h i r d law e n t r o p y f o r w a t e r i n t h e c o r d i e r i t e s t r u c t u r e o f 59 j / d e g m o l , i n t e r -m e d i a t e between v a l u e s f o r l i q u i d w a t e r and i c e o f 69.9 j / d e g mol and 41.84 c a l / d e g mol r e s p e c t i v e l y . The e n t h a l p y o f d e h y d r a t i o n o f h y d r o u s c o r d i e r i t e i s c a l c u l a t e d t o be 4.10 k j / m o l . T h i s v a l u e can be compared w i t h v a l u e s f o r t h e e n t h a l p y o f d e h y d r a t i o n o f z e o l i t e s o f 12.6 t o 25.1 k j / m o l . T h i s v a l u e f o r t h e d e h y d r a t i o n e n t h a l p y o f c o r d i e r i t e i s c o n s i s t e n t w i t h the s t r e n g t h o f h y d r o g e n b o n d i n g t h a t can be i n f e r r e d f r o m t h e o b s e r v e d I.R. s p e c t r a ( L a n g e r and S c h r e y e r , 1976). 345 REFERENCES APPENDIX I REFERENCES 346 D i x o n , J.M., 1975, F i n i t e s t r a i n and p r o g r e s s i v e d e f o r m a t i o n i n models o f d i a p i r i c s t r u c t u r e s . T e c t o n o p h y s i c s , v. 28, p. 88-124. Duncan, I . J . , 1978a, S t r u c t u r a l g e o l o g y o f the B i g Ledge a r e a . ln_ G e o l o g i c a l F i e l d w o r k 1977, B r i t i s h C o l umbia M i n i s t r y o f Mines and P e t r o l e u m R e s o u r c e s , p. 80-82. E s k o l a , P.E., 1949, The p r o b l e m o f m a n t l e d g n e i s s domes. G e o l o g i c a l  S o c i e t y o f London Q u a r t e r l y J o u r n a l , v. 104, p. 461-476. F l e t c h e r , R . C , 1972, A p p l i c a t i o n o f a m a t h e m a t i c a l model t o the emplace-ment o f m a n t l e d g n e i s s domes. A m e r i c a n J o u r n a l o f S c i e n c e , v. 272, p. 197-216. H a l l e r , J . , 1962, S t r u c t u r a l c o n t r o l o f r e g i o n a l metamorphism i n t h e e a s t G r e e n l a n d C a l e d o n i d e s . G e o l o g i c a l S o c i e t y o f London P r o c e e d i n g s , no. 1594, p. 21-25. H i g g e n s , M.W., G.W. F i s h e r , and I . Z i e t z , 1973, A e r o m a g n e t i c d i s c o v e r y o f a B a l t i m o r e G n e i s s Dome i n t h e Piedmont o f n o r t h w e s t e r n Delaware and s o u t h e a s t e r n P e n n s y l v a n i a . G e o l o g y , v. 1, p. 41-43. Hobbs, B.E., W.D. Means, and P.F. W i l l i a m s , 1976, An O u t l i n e o f S t r u c t u r a l G e o l o g y . John W i l e y and Sons, I n c . , New Y o r k , 571 pp. Hoy, T., 1977b, B i g Ledge 82L/8E. J j i G e o l o g y i n B r i t i s h Columbia 1975, B r i t i s h C o l umbia M i n i s t r y o f Mines and P e t r o l e u m R e s o u r c e s , p. 12-18. Ramberg, H., 1967, G r a v i t y , D e f o r m a t i o n and t h e E a r t h ' s C r u s t As S t u d i e d By C e n t r i f u g e d M o d e l s . Academic P r e s s , New Y o r k , 241 pp. Ramsay, J.G., 1967, F o l d i n g and F r a c t u r i n g o f Rocks. M c G r a w - H i l l , New Y o r k , 568 pp. 347 R e e s o r , J . E . , 1970, Some a s p e c t s o f s t r u c t u r a l e v o l u t i o n and r e g i o n a l s e t t i n g i n p a r t o f t h e Shuswap Metamorphic Complex. G e o l o g i c a l A s s o c i a t i o n o f Canada S p e c i a l Paper 6, p. 73-86. R e e s o r , J . E . , and Moore, J.M., J r . , 1971, P e t r o l o g y and s t r u c t u r e o f T h o r - O d i n g n e i s s dome, Shyswap Metamorphic Complex, B r i t i s h C o l u m b i a , G e o l o g i c a l S u r v e y o f Canada, B u l l e t i n 195, 149 pp. Ross, J.V., 1968, S t r u c t u r a l r e l a t i o n s a t the e a s t e r n m a r g i n o f the Shuswap Complex n e a r R e v e l s t o k e , s o u t h e r n B r i t i s h C o l u m b i a . C a n a d i a n J o u r n a l o f E a r t h S c i e n c e , v. 5, p. 813-849. Ross, J.V., 1973, M y l o n i t i c r o c k s and f l a t t e n e d g a r n e t s i n the s o u t h e r n Okanagan o f B r i t i s h C o l u m b i a . C a n a d i a n J o u r n a l o f E a r t h S c i e n c e , v. 10, p. 1-17. Thompson, J.B., P. R o b i n s o n , T.N. C l i f f o r d , and N.J. T r a s k , 1968, Nappes and g n e i s s domes i n west c e n t r a l New E n g l a n d . J_n S t u d i e s o f A p p a l a c h i a n G e o l o g y : N o r t h e r n and M a r i t i m e , E-An Zen, W.S. W h i t e , J.B. H a d l e y , and J.B. Thompson ( e d s . ) , W i l e y - I h t e r s c i e n c e , New Y o r k , p. 203-218. W i l l i a m s , P.F., 1970, A c r i t i c i s m o f t h e use o f s t y l e i n t h e s t u d y o f deformed r o c k s . G e o l o g i c a l S o c i e t y o f A m e r i c a B u l l e t i n , v. 81, p. 3283-3296. 348 APPENDIX II REFERENCES C h a p p i e , W.M., 1968c, A m a t h e m a t i c a l t h e o r y o f f i n i t e a m p l i t u d e f o l d i n g . G e o l o g i c a l S o c i e t y o f A m e r i c a B u l l e t i n , v. 79, p. 47-68. Coward, M.P., and Kim, J.H., 1981, S t r a i n w i t h i n t h r u s t s h e e t s . lu_ T h r u s t and Nappe T e c t o n i c s , M c C l a y , K.R., and N.J. P r i c e ( e d s . ) , G e o l o g i c a l S o c i e t y o f L o n d o n - B l a c k w e l l S c i e n t i f i c P u b l i c a t i o n s , London, E n g l a n d . D i e t e r i c h , J.H., and N.L. C a r t e r , 1969, S t r e s s - h i s t o r y o f f o l d i n g . A m e r i c a n J o u r n a l o f S c i e n c e , v. 267, p. 129-154. H u d l e s t o n , P . J . , 1973a, F o l d morphology and some g e o m e t r i c a l i m p l i c a t i o n s o f t h e o r i e s o f f o l d d e v e l o p m e n t . T e c t o n o p h y s i c s , v. 16, p. 1-46. H u d l e s t o n , P . J . , 1973b, The a n a l y s i s and i n t e r p r e t a t i o n o f m i n o r f o l d s d e v e l o p e d i n t h e Moine r o c k s o f Monar, S c o t l a n d . T e c t o n o p h y s i c s , v. 17, p. 89-132. J o h n s o n , M.R.W., 1967, M y l o n i t e zones and m y l o n i t e b a n d i n g . N a t u r e , v. 213, p. 246-247. L i s l e , R . J . , 1974, Deformed l i n e a t i o n s as f i n i t e - s t r a i n s t r u c t u r e s . T e c t o n o p h y s i c s , v. 21, p. 165-179. Mukhopadhyay, D., 1965, E f f e c t s o f c o m p r e s s i o n on c o n c e n t r i c f o l d s and mechanism o f s i m i l a r f o l d i n g . J o u r n a l o f the G e o l o g i c a l S o c i e t y o f I n d i a , v. 6, p. 27-41. P a r r i s h , D.K., A. K r i v x , and N.L. C a r t e r , 1976, F i n i t e e l e m e n t f o l d s o f s i m i l a r g e o m e t r y . T e c t o n o p h y s i c s , v. 32, p. 183-207. P f i f f n e r , O.A., and J.G. Ramsay, 1982, C o n s t r a i n t s on g e o l o g i c a l s t r a i n r a t e s : arguments fro m f i n i t e s t r a i n s t a t e s o f n a t u r a l l y deformed r o c k s . J o u r n a l o f G e o p h y s i c a l R e s e a r c h , v. 87, no. B I , p. 311-321. 349 Ramsay, J.G., 1962a, The geometry and m e c h a n i c s o f f o r m a t i o n o f " s i m i l a r " t y p e f o l d s . J o u r n a l o f G e o l o g y , v. 70, p. 309-327. Ramsay, J.G., 1967, F o l d i n g and F r a c t u r i n g o f Rocks. M c G r a w - H i l l , I n c . , New Y o r k , 568 pp. Ross, J.V., 1973, M y l o n i t i c r o c k s and f l a t t e n e d g a r n e t s i n the s o u t h e r n Okanagan o f B r i t i s h C o l u m b i a . C a n a d i a n J o u r n a l o f E a r t h S c i e n c e , v. 10, p. 1-17. Ross, J.V., 1981, A geodynamic model f o r some s t r u c t u r e s w i t h i n and a d j a c e n t t o t h e Okanagan V a l l e y , s o u t h e r n B r i t i s h C o l u m b i a . C a n a d i a n  J o u r n a l o f E a r t h S c i e n c e , v. 18, p. 1581-1598. Ross, J.V., and J . S . C h r i s t i e , 1979, E a r l y recumbent f o l d i n g i n some w e s t e r n m o s t e x p o s u r e s o f t h e Shuswap Complex, s o u t h e r n Okanagan, B r i t i s h C o l u m b i a . C a n a d i a n J o u r n a l o f E a r t h S c i e n c e , v. 16, p. 877-894. S t a b l e r , C.L., 1968, S i m p l i f i e d F o u r i e r a n a l y s i s o f f o l d s h a p e s . T e c t o n o - p h y s i c s , v. 6, no. 4, p. 343-350. W i l k i n s o n , P., N.J. S o p e r , and A.M. B e l l , S k o l i t h o s p i p e s as s t r a i n markers i n m y l o n i t e s . T e c t o n o p h y s i c s , v. 28, no. 3, p. 143-157. APPENDIX I I I REFERENCES 350 E s k o l a , P.E., 1914, On t h e p e t r o l o g y o f t h e O r i j a r v i r e g i o n i n s o u t h w e s t e r n F i n l a n d . F i n l a n d e comm. ge*ol. B u l l . 40, p. 1-277. F r o e s e , E . , 1970, Che m i c a l p e t r o l o g y o f some p e l i t i c g n e i s s e s and m i g m a t i t e s f r o m t h e T h o r - O d i n a r e a , B r i t i s h C o l u m b i a . C a n a d i a n J o u r n a l E a r t h S c i e n c e , v. 7, p. 164-175. G r a n t , J.A., 1968, P a r t i a l m e l t i n g o f common r o c k s as a p o s s i b l e s o u r c e o f c o r d i e r i t e - a n t h o p h y l l i t e b e a r i n g a s s e m b l a g e s . A m e r i c a n J o u r n a l o f S c i e n c e , v. 266, p. 908-931. H i l l , R.P., 1975, S t r u c t u r a l and p e n o l o g i c a l s t u d i e s i n t h e Shuswap Meta-m o r p h i c Complex n e a r R e v e l s t o k e , B r i t i s h C o l u m b i a . U n p u b l i s h e d M.Sc. t h e s i s , U n i v e r s i t y o f C a l g a r y , Canada. M u t t i , L . J . , 1978, S t r u c t u r e and metamorphism o f t h e C r a n b e r r y r e g i o n , T h o r - O d i n g n e i s s dome, Shuswap Metamorphic Complex, B r i t i s h C o l u m b i a . U n p u b l i s h e d Ph.D. t h e s i s , H a r v a r d U n i v e r s i t y . R e e s o r , J . E . , and Moore, J.M., J r . , 1971, P e t r o l o g y and s t r u c t u r e o f T h o r - O d i n g n e i s s dome, Shuswap Met a m o r p h i c Complex, B r i t i s h C o l u m b i a . G e o l o g i c a l S u r v e y o f Canada, B u l l e t i n 195, 149 pp. S e k i , Y., 1957, P e t r o l o g i c a l s t u d y o f h o r n f e l s e s i n t h e c e n t r a l p a r t o f t h e median zone o f t h e K i t a k a m i M o u n t a i n l a n d , Iwate P r e f e c t u r e . S c i e n t i f i c R e p o r t s o f S a i t a m a U n i v e r s i t y , S e r i e s B, v. 2, p. 307-373. Thompson, J . 8 . , J r . , and Thompson, A.B., 1976, A model s y s t e m f o r m i n e r a l f a c i e s i n p e l i t i c s c h i s t s . C o n t r i b u t i o n s t o M i n e r a l o g y and P e t r o l o g y , v. 58, p. 243-277. V a l l a n c e , T.G., 1967, M a f i c r o c k a l t e r a t i o n and i s o c h e m i c a l d e v e l o p m e n t o f some c o r d i e r i t e - a n t h o p h y l l i t e r o c k s . J o u r n a l o f P e t r o l o g y , v. 8, p a r t 1, P. 84-96. 351 APPENDIX IV REFERENCES Duncan, I . J . , and Greenwood, H.J., 1977, The e f f e c t o f m o l e c u l a r w a t e r on t h e thermodynamic p r o p e r t i e s and s t a b i l i t y r e l a t i o n s o f Mg c o r d i e r i t e . G e o l . A s s o c . Canada Ann. Mtg., P r o g , w i t h A b s t r . 2:17. F e r r y , J.M., and S p e a r , F.S., 1978. E x p e r i m e n t a l c a l i b r a t i o n o f the p a r t i t i o n i n g o f Fe and Mg between b i o t i t e and g a r n e t . C o n t r i b . M i n e r a l . P e t r o l . , v. 66, p. 113-117. F l e t c h e r , R . C , and Greenwood, H.J., 1979, Metamorphism and s t r u c t u r e o f P e n f o l d Creek a r e a n e a r Quesnel L a k e , B r i t i s h C o l u m b i a . J o u r . P e t r o l . , v. 20, p t . 4, p. 743-794. F r o e s e , E., 1970. C h e m i c a l p e t r o l o g y o f some p e l i t i c g n e i s s e s and m i g m a t i t e s f r o m t h e T h o r - O d i n a r e a , B r i t i s h C o l u m b i a . Can. J . E a r t h  S c i . , v. 7, no. 1, p. 164-175. F r o e s e , E., 1973. The assemblage q u a r t z - K f e l d s p a r - b i o t i t e - g a r n e t - s i l l i -m a n i t e as an i n d i c a t o r o f P ^ O - 1 c o n d i t i o n s . Can. J . E a r t h S c i . , v. 10, p. 1575-1579. G r e e n , T.H., and V e r n o n , R.H., 1974. C o r d i e r i t e breakdown under h i g h -p r e s s u r e , h y drous c o n d i t i o n s . C o n t r i b . M i n e r a l . P e t r o l . , v. 46, p. 215-226. H a r t e , B., 1975. D e t e r m i n a t i o n o f a p e l i t e p e t r o g e n e t i c g r i d f o r the E a s t e r n S c o t t i s h Dal r a d i a n . C a r n e g i e I n s t . Wash. Yearbook 74, p. 438-446. Holdaway, M.J., 1971. S t a b i l i t y o f a n d a l u s i t e and t h e a l u m i n u n s i l i c a t e phase d i a g r a m . Amer. J . S c i . , v. 271, p. 97-131. Holdaway, M.J., and L e e , S.M., 1977. Fe-Mg c o r d i e r i t e s t a b i l i t y i n h i g h -g r a d e p e l i t i c r o c k s based on e x p e r i m e n t a l , t h e o r e t i c a l and n a t u r a l o b s e r v a t i o n s . C o n t r i b . M i n e r a l . P e t r o l . , v. 63, p. 175-198. 352 Newton, R . C , 1972. An e x p e r i m e n t a l d e t e r m i n a t i o n o f t h e h i g h - p r e s s u r e s t a b i l i t y l i m i t s o f magnesian c o r d i e r i t e under wet and d r y c o n d i t i o n s . J . G e o l . , v. 80, p. 398-420. Newton, R . C , and Wood, B . J . , 1979. Thermodynamics o f w a t e r i n c o r d i e r i t e and some p e t r o l o g i c c o n s e q u e n c e s o f c o r d i e r i t e as a h y d r o u s p h a s e . C o n t r i b . M i n e r a l . P e t r o l . , v. 68, p. 391-405. R e e s o r , J . E . , and Moore, J.M., J r . , 1971. P e t r o l o g y and s t r u c t u r e o f T h o r - O d i n g n e i s s dome, Shuswap Metamorphic Complex, B r i t i s h C o l u m b i a . G e o l . S u r v . Can. B u l l . 195, 149 pp. R o b i n s o n , P., and J a f f e , H.W., 1969. A l u m i n o u s e n c l a v e s i n g e d r i t e -c o r d i e r i t e g n e i s s from s o u t h w e s t e r n New Hampshire. Amer. J . S c i . , v. 267, p. 389-421. S c h r e y e r , W., 1973. W h i t e s c h i s t : a h i g h - p r e s s u r e r o c k and i t s g e o l o g i c s i g n i f i c a n c e . J . G e o l . , v . 81, p. 735-739. Thompson, A.B., 1976. M i n e r a l r e a c t i o n s i n p e l i t i c r o c k s , I I : c a l c u l a t i o n o f some P-T-X (Fe-Mg) phase r e l a t i o n s . Amer. J . S c i . , v. 276, no. 4, p. 425-454. 353 APPENDIX V REFERENCES B a r r e r , R.M., 1971, I n t r a c r y s t d i f f u s i o n , j_n M o l e c u l a r S i e v e Z e o l i t e s , v. I I , C h a p t e r 1, p. 1-28, p u b l i s h e d by A m e r i c a n C h e m i c a l S o c i e t y . Dodson, M.H., 1974, T h e o r y o f c o o l i n g a g e s , vn_ J a g e r , E . , and H u n z i k e r , J . C , e d s . , L e c t u r e s i n I s o t o p e G e o l o g y , S p r i n g e r - V e r l a g , New Y o r k , 1979, p. 194-202. Duncan, I . J . , and Greenwood, H.J., 1977, The e f f e c t o f m o l e c u l a r w a t e r on t h e thermodynamic p r o p e r t i e s and s t a b i l i t y r e l a t i o n s o f Mg c o r d i e r i t e . G e o l . S o c . Canada Ann. Mtg. P r o g , w i t h A b s t r . 2:17. L a n g e r , K., and S c h r e y e r , w., 1976, A p p a r e n t e f f e c t s o f m o l e c u l a r w a t e r on t h e l a t t i c e geometry o f c o r d i e r i t e : a d i s c u s s i o n . Amer. M i n e r a l . , v. 61, p. 1036-1040. L e p e z i n , G.G., and M e l e n e v s k y , V.N., 1977, On t h e p r o b l e m o f w a t e r d i f f u s i o n i n c o r d i e r i t e s . 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