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The origin of Georgia depression and the Coast Plutonic Complex/Insular Belt province boundary on Hardwicke… Nelson, JoAnne Lee 1976

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THE ORIGIN OF GEORGIA DEPRESSION ; AND THE COAST- PLUTONIC COMPLEX/ INSULAR BELT PROVINCE BOUNDARY ON HARDWICKE AND WEST THURLOW ISLANDS, B.C. by JoAnne Lee Nelson B.Sc., U n i v e r s i t y of Washington 1973 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of GEOLOGICAL SCIENCES We accept t h i s t h e s i s as conforming to the r e q u i r e d standard: THE UNIVERSITY OF BRITISH COLUMBIA J u l y 1976 © JoAnne Lee Nelson, 1976 In p resent ing t h i s t he s i s in p a r t i a l f u l f i l m e n t o f the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree that permiss ion for ex ten s i ve copying o f t h i s t he s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r ep re sen ta t i ve s . It i s understood that copying or p u b l i c a t i o n of t h i s t he s i s f o r f i n a n c i a l gain s h a l l not be al lowed without my w r i t t e n permis s ion. The U n i v e r s i t y of B r i t 2075 Wesbrook P l a c e Vancouver, Canada V6T 1W5 Depa rtment ABSTRACT G e o r g i a D e p r e s s i o n i s a major 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 a l t r o u g h t h a t l i e s between the C o a s t M o u n t a i n s and V a n c o u v e r I s l a n d , a l o n g t h e w e s t e r n b o u n d a r y of t h e C o a s t P l u t o n i c Complex. F i e l d work on a p a r t of t h i s b o u n d a r y on H a r d w i c k e and West T h u r l o w I s l a n d s i n J o h n s t o n e S t r a i t e s t a b l i s h e d i t as an i n t r u -s i v e c o n t a c t between an Upper J u r a s s i c q u a r t z d i o r i t e and s t r a t -i f i e d r o c k s of t h e V a n c o u v e r Group. The V a n c o u v e r Group i n t h e v i c i n i t y of t h e c o n t a c t i s b r o k e n by two major w e s t - n o r t h w e s t t r e n d i n g p r e p l u t o n i c f a u l t s . C o n t a c t metamorphic c o n d i t i o n s d e t e r m i n e d from b a s i c and c a r b o n a t e a s s e m b l a g e s a r e P ,-3 kb. t o t a l and T . = (650 - 720°C . P o s t - p l u t o n i c f r a c t u r e s and d i k e s maximum t r e n d n o r t h e a s t . The main r e s u l t of t h e f i e l d s t u d y was to c o n f i r m f o r a p a r t i c u l a r a r e a t h a t t h e w e s t e r n m a r g i n of t h e C o a s t P l u t o n i c Complex i s e s s e n t i a l l y a magmatic f r o n t . G e o r g i a D e p r e s s i o n began to s u b s i d e i n t h e Upper C r e t a c e -ous as t h e C o a s t P l u t o n i c Complex r o s e . I t i s p r o p o s e d t h a t t h e i n i t i a l s u b s i d e n c e of G e o r g i a D e p r e s s i o n was complementary to t h e u p l i f t of t h e C o a s t M o u n t a i n s . G e o r g i a D e p r e s s i o n i s p a r t of t h e C o a s t a l T r o u g h of w e s t e r n N o r t h A m e r i c a , an e l o n g a t e d e p r e s s i o n t h a t l i e s between a s e r i e s of c o l i n e a r magmatic a r c s ( t h e C o a s t P l u t o n i c Complex, t h e C a s c a d e s , and t h e S i e r r a Nevada) and t h e w e s t e r n m a r g i n of t h e N o r t h A m e r i c a n p l a t e . The v a r i o u s s u b - b a s i n s of t h e C o a s t a l T r o u g h formed a t d i f f e r e n t t i m e s , from the Upper J u r a s s i c to t h e L a t e T e r t i a r y , t h a t c o r r e s -pond to t h e t i m e s of u p l i f t of t h e a d j a c e n t i g n e o u s c o m p l e x e s . i i i . T h i s c o i n c i d e n c e s u g g e s t s t h a t t h e s u b s i d e n c e of t h e w h o l e C o a s t a l T r o u g h , i n c l u d i n g G e o r g i a D e p r e s s i o n , was c o u p l e d t o t h e u p l i f t o f t h e m a g m atic a r c s o f N o r t h A m e r i c a . The C o a s t a l T r o u g h may be a n a l o g o u s t o t h e m a r g i n a l s y n c l i n e s d e v e l o p e d i n Ramberg's (1967) d i a p i r e x p e r i m e n t s . . iv, TABLE OF CONTENTS SECTION I : I n t r o d u c t i o n and R e g i o n a l S y n t h e s i s 1 I n t r o d u c t i o n 1 CHAPTER I : The H i s t o r y o f G e o r g i a D e p r e s s i o n and V i c i n i t y 4 i . T h e I n s u l a r B e l t 4 i i . T h e C o a s t P l u t o n i c Complex 9 i i i . G e o r g i a D e p r e s s i o n 20 iv.Summary and E v a l u a t i o n of R e g i o n a l S y n t h e s i s 24 SECTION I I : F i e l d Study 30 I n t r o d u c t i o n - G e n e r a l G e o l o g y o f t h e Map-Area 30 CHAPTER I I : S t r a t i g r a p h y o f t h e V a n c o u v e r Group 33 i . K a r m u t s e n F o r m a t i o n 33 i i . T h e V a n c o u v e r Group s e d i m e n t s : Q u a t s i n o , P a r s o n Bay and H a r b l e d o w n F o r m a t i o n s 38 i i i . B o n a n z a V o l c a n i c s 40 i v . S t r a t i g r a p h y and M a j o r S t r u c t u r e 41 CHAPTER I I I : P l u t o n i c Rocks 44 i . O l d e r B o d i e s 44 i i . Q u a r t z D i o r i t e and A s s o c i a t e d H o r n b l e n d e Gabbros 48 i i i . R a d i o m e t r i c D a t i n g : Q u a r t z D i o r i t e 66 CHAPTER IV: P r e - and S y n - P l u t o n i c S t r u c t u r e and D i k e s 73 i . P r e - P l u t o n i c D i k e s 73 i i . P h a s e One ( P r e - P l u t o n i c ) : The M a j o r F a u l t s ' 75 i i i . P h a s e T w o : S y n - P l u t o n i c D e f o r m a t i o n 77 iv.Summary 86 CHAPTER V: Metamorphism 89 i . B u r i a l Metamorphism 89 i i . C o n t a c t Metamorphism 90 a) A s s e m b l a g e s i n B a s i c Rocks 91 b) C a r b o n a t e M e t a m o r p h i c A s s e m b l a g e s 107 c) C o n c l u s i o n s 113 V . CHAPTER XI: P o s t - P l u t o n i c S t r u c t u r e and Dikes 119 Summary of F i e l d Results 126 SECTION I I I : The C o a s t a l Trough 130 CHAPTER VII: Geologic S e t t i n g of the C o a s t a l Trough 130 i.Hecate Depression 130 i i . T h e Puget and W i l l i a m e t t e Depressions 134 i i i . T h e Great V a l l e y 145 CHAPTER VIII:Deep S t r u c t u r e of the C o a s t a l Trough 149 i . B r i t i s h Columbia 149 ii.Washington and Oregon 153 i i i . C a l i f o r n i a 156 iv.Summary 159 CHAPTER IX: Synthesis and Conclusions 160 LIST OF TABLES vi, T a b l e 1. C o m p a r i s o n between l a y e r i n g i n h o r n b l e n d e g a b b r o s , c u m u l a t e and s c h l i e r e n l a y e r i n g . T a b l e 2. R a d i o m e t r i c age d a t a from q u a r t z d i o r i t e . T a b l e 3. C a r b o n a t e p a r a g e n e s e s . T a b l e 4. P e t r o g r a p h i c D a t a - P o s t - P l u t o n i c D i k e s . 55 69 108 123 v i i . LIST OF FIGURES F i g u r e 1. The C o a s t a l T r o u g h 2 2. Upper C r e t a c e o u s p a l e o g e o g r a p h y 10 3. P a l e o m a g n e t i c p o l e s and f a u n a l a f f i n i t i e s , s o u t h w e s t e r n B r i t i s h C o l u m b i a 13 4. P a l e o g e o g r a p h i c e v o l u t i o n of s o u t h w e s t e r n B r i t i s h C o l u m b i a 25 5. Index map = F i e l d a r e a on H a r d w i c k e and West T h u r l o w I s l a n d s 31 6. S t r a t i g r a p h y of t h e V a n c o u v e r Group 34 7. S t r a t i g r a p h y of i n t e r v o l c a n i c s e d i m e n t a r y c y c l e , Helmcken I s l a n d 36 8. P o l e s to b e d d i n g , Domain I 42 9. P o l e s to b e d d i n g , Domain I I 44 10. a) Miner's Bay gabbro 4 6 b) W e l l b o r e C h a n n e l g n e i s s i c q u a r t z d i o r i t e 11. Q u a r t z d i o r i t e 49 12. L a r g e - s c a l e i n t r u s i v e b r e c c i a , M i n e r ' s Bay 49 13. P o l e s to f o l i a t i o n i n q u a r t z d i o r i t e 51 1«. T e x t u r a l v a r i a t i o n s i n h o r n b l e n d e g a b b r o s 54. 15. a) "Hornblende - p l a g i o c l a s e l a y e r i n g 56. b) L a y e r e d h o r n b l e n d e - r i c h gabbro t r u n c a t e d by l i g h t e r u n l a y e r e d phase 16. a) N e a r - v e r t i c a l c r o s s - b e d d i n g 58. b) A g m a t i t i c zone between n o r m a l l a y e r s 17. a) Deformed l a y e r i n g t r u n c a t e d by p l a n a r l a y e r s 59 b) L a y e r e d gabbro i n t r u d e d by gabbro d i k e v i i i F i g u r e 18. a) " C l a s t i c d i k e " 60 b) Channel 19. a) Path of melt composition i n a supercooled 62 boundary l a y e r b) E v o l u t i o n of the boundary l a y e r 20. Radiometric d a t i n g of quartz d i o r i t e 67 21. B i o t i t e i s o c h r o n s from quartz d i o r i t e 68 22. S t r i k e o r i e n t a t i o n s of p r e - p l u t o n i c dikes 74 23. R o t a t i o n a l r e l a t i o n s h i p s between homoclinal 76 domains 24. S t r i k e o r i e n t a t i o n s of epidote v e i n s 79 25. O r i e n t a t i o n s of minor f o l d s 80 26. Minor f o l d s i n i n t e r l a v a sediments, upper 81 Karmutsen Formation, West Thurlow I s l a n d 27. a) O r i e n t a t i o n s of c h a o t i c f o l d s , east of 83 Vans i t t a r t Point b) F o l d - a x i s d i s t r i b u t i o n s i n v e r t i c a l shear 28. Poles to f o l i a t i o n i n metavolcanics and 85 marbles 29. S t r u c t u r a l c r o s s - s e c t i o n , Hkusam to 87 Hardwicke I s l a n d 30. Metamorphic assemblages i n the Vancouver 92 Group 31. a) Karmutsen g r e e n s c h i s t with amygdule, 93 Helmcken I s l a n d b) G r e e n s c h i s t - hornblende h o r n f e l s t r a n s i t i o n i n amygdule i n Upper Karmutsen, West Thurlow I s l a n d 32. Schematic diagram of the g r e e n s c h i s t - 97 amphibolite t r a n s i t i o n 33. Epidote breakdown, T - f n space 101 °2 ix F i g u r e 34. C o n t a c t b e t w e e n L a t e J u r a s s i c q u a r t z 103 d i o r i t e and K a r m u t s e n h o r n f e l s , H a r d w i c k e I s l a n d 35. a) B o n a n z a t u f f ( ? ) i n h o r n b l e n d e h o r n f e l s zone 105 b) B o n a n z a h o r n f e l s 36. C a r b o n a t e a s s e m b l a g e s p l o t t e d i n t h e s y s t e m 109 CaO - A-120 - S i 0 2 - C 0 2 - H O 37. S c h e m a t i c T-'X _ d i a g r a m = t h e s y s t e m CaO - 112 C U 2 A 1 2 0 3 - F e 2 0 3 - S i 0 2 - H 20 - C 0 2 38. T e m p e r a t u r e v s . d i s t a n c e f r o m t h e c o n t a c t 115 39. P o l e s t o p o s t - p l u t o n i c d i k e s and j o i n t s 120 40. S t r i k e o r i e n t a t i o n s o f p o s t - p l u t o n i c d i k e s 121 41. C r o s s - c u t t i n g p o s t - p l u t o n i c d i k e s 122 42. C o m p a r a t i v e I n s u l a r B e l t s t r a t i g r a p h i e s 131 43. D i s t r i b u t i o n o f t h e Skonum F o r m a t i o n 133 44. The C o a s t a l T r o u g h and g e o t e c t o n i c p r o v i n c e s 135 o f w e s t e r n W a s h i n g t o n and Oregon 45. L i t h o l o g i c c h a r t = w e s t e r n W a s h i n g t o n 137 46. L i t h o l o g i c c h a r t = w e s t e r n O r e g o n 138 47. Upper M i o c e n e p a l e o g e o g r a p h y o f t h e C o l u m b i a n 142 Embayment 48. U p p e r J u r a s s i c - Lower C r e t a c e o u s e v e n t s : 146 w e s t e r n C a l i f o r n i a 49. a) Sh o t p o i n t s , r e c o r d i n g s t a t i o n s and u p p e r , 150 m a n t l e v e l o c i t i e s f o r r e v e r s e d s e i s m i c p r o f i l e s i n w e s t e r n B r i t i s h C o l u m b i a b) C r u s t a l p r o f i l e a c r o s s s o u t h w e s t e r n B r i t i s h Columb i a 50. a) V a r i a t i o n s i n c r u s t a l t h i c k n e s s , B r i t i s h 152 Columb i a b) R e l a t i v e c r u s t a l d e n s i t i e s , B r i t i s h C o l u m b i a X F i g u r e 51. Index map = c r u s t a l s e i s m i c s t u d i e s i n t h e 154 P a c i f i c N o r t h w e s t 52. C r u s t a l t h i c k n e s s e s b e n e a t h w e s t e r n W a s h i n g t o n 155 53. I s o s t a t i c anomaly map, W a s h i n g t o n 157 54. a) S e i s m i c p r o f i l e a c r o s s w e s t e r n C a l i f o r n i a 158 b) Model of t h e c r u s t under C a l i f o r n i a f r o m Bougner a n o m a l i e s 55. R i s i n g d i a p i r w i t h m a r g i n a l s y n c l i n e s 163 LIST OF PLATES Time-space p l o t f o r the Upper P a l e o z o i c - ( i n back Mesozoic of Vancouver I s l a n d and the ' pocket) Coast Mountains, 49° - 52°N. Geologic Map of Hardwicke and West ( i n back Thurlow I s l a n d s , B.C. pocket) xii ACKNOWLEDGEMENTS H. R. Wynne-Edwards s u g g e s t e d the p r o j e c t to me and p r o v i d e d m o r a l and f i n a n c i a l s u p p o r t t h r o u g h o u t . S t a n Reamsbottom m e n t i o n e d H a r d w i c k e I s l a n d as an i n t e r e s t i n g e x p o s u r e of the p r o v i n c e b o u n d a r y . Jenny S t a r k , Fang B e n d i c k s o n and my p a r e n t s , Anne and George N e l s o n , gave a b l e and u n c o m p l a i n i n g a s s i s t a n c e i n t h e f i e l d . J enny and I owe our c o m f o r t (and p e r h a p s s u r v i v a l ) to B r u c e , E d i e and H a r o l d B e n d i c k s o n and Mac and Hug C a r m i c h a e l of H a r d w i c k e I s l a n d , f a i r y g o d p a r e n t s i n d e e d . Lee P i g a g e , H.R. Wynne-Edwards and R i c h a r d A r m s t r o n g v i s i t e d H a r d w i c k e I s l a n d and p r o v i d e d u s e f u l i n s i g h t s about f i e l d r e l a t i o n s . C o n v e r s a t i o n s w i t h J a n M u l l e r , v G l e n n Woodsworth, W.R. Danner, F r a n k Danes, John G r i f f i t h s , B i l l C o u l b o u r n , I a n Duncan, and Lee P i g a g e c o n t r i b u t e d to t h e e v o l u t i o n of my i d e a s . Joe H a r a k a l , R i c h a r d A r m s t r o n g , P e t e r L e C o u t e u r and Cathy Lamb c a r r i e d o ut and/or c o a c h e d me i n a n a l y t i c a l p r o c e d u r e s f o r r a d i o -m e t r i c d a t i n g . J a n M u l l e r and G l e n Woodsworth i m p r o v e d t h e m a n u s c r i p t w i t h t h e i r s u g g e s t i o n s . J e a n n e t t e W i l l i a m s d i d t h e f i n a l t y p i n g w i t h e f f i c i e n c y and aplomb. I would l i k e to thank the g e o l o g y t e c h n i c i a n s , R.B. Coo p e r , and J o h n Payne f o r k i n d a s s i s t a n c e i n the p u r s u i t of my g o a l s . * N a t i o n a l R e s e a r c h C o u n c i l o p e r a t i n g g r a n t A8302. 1. SECTION 1: INTRODUCTION AND REGIONAL SYNTHESIS  I n t r o d u c t i o n G e o r g i a D e p r e s s i o n i s a t o p o g r a p h i c and s t r u c t u r a l low t h a t l i e s between t h e m o u n t a i n o u s r e g i o n s of V a n c o u v e r I s l a n d and t h e C o a s t M o u n t a i n s of B r i t i s h C o l u m b i a . I t i s a p a r t of the C o a s t a l T r o u g h t h a t e x t e n d s from H e c a t e S t r a i t i n t h e n o r t h , t h r o u g h t h e Puget and W i l l i a m e t t e Lowlands to t h e C e n t r a l V a l l e y of C a l i f o r n i a ( F i g u r e 1 ) . The C o a s t a l T r o u g h has been d e s c r i b e d i n g e o m o r p h o l o g i c l i t e r a t u r e (Fenneman 1931; H o l l a n d 1964) but no a t t e m p t has been made so f a r to a c c o u n t f o r i t s o r i g i n . In c o n t r a s t to p r e v i o u s s t u d i e s w h i c h have em p h a s i z e d l o c a l c a u s e s , t h i s i n v e s t i g a t i o n , a l t h o u g h p r i n c i p a l l y c o n c e r n e d w i t h t h e o r i g i n of G e o r g i a D e p r e s s i o n , has f o l l o w e d the h y p o t h e s i s t h a t s i n c e t h e C o a s t a l T r o u g h i s a c o n t i n u o u s f e a t u r e t h e n a common mode of o r i g i n s h o u l d be s o u g h t . C e r t a i n l o c a l f e a t u r e s s e t G e o r g i a D e p r e s s i o n a p a r t from t h e t r o u g h f u r t h e r s o u t h . I t i s deep: ( T i f f i n (1969) e s t i m a t e s t h a t h a l f of t h e f l o o r of G e o r g i a S t r a i t l i e s below c o n t i n e n t a l s h e l f d e p t h , 132 m e t e r s . I t l i e s between t h e C o a s t P l u t o n i c Complex ( w h i c h i n l a r g e p a r t c o i n c i d e s w i t h the p h y s i o g r a p h i c C o a s t M o u n t a i n s ) and t h e I n s u l a r B e l t ( w h i c h i n c l u d e s V a n c o u v e r I s l a n d and t h e Queen C h a r l o t t e I s l a n d s ) . S e v e r a l h y p o t h e s e s f o r the o r i g i n o f G e o r g i a D e p r e s s i o n have been p r o p o s e d . They a r e : 1. Upper C r e t a c e o u s to e a r l y T e r t i a r y downwarping a n d / o r d o w n f a u l t i n g ( J e l e t z k y 1965; S u t h e r l a n d Brown 1966; M u l l e r and J e l e t z k y 1970) . 1 3. 2. E a s t w a r d t i l t i n g o f t h e V a n c o u v e r I s l a n d b l o c k ( M u l l e r 1 9 7 5 ) . 3. F o u n d e r i n g o f a zone o f l i t h o s p h e r i c w e a k n e s s o v e r a s u t u r e j o i n i n g t h e two p r o v i n c e s . Monger e t a l . (1972) s u g g e s t t h a t t h e I n s u l a r B e l t i s a l l o c h t h o n o u s , a l t h o u g h t h e y f a v o r a s u t u r e a l o n g t h e Y a l a k o m f a u l t s y s t e m . 4. N o r t h e a s t - s o u t h w e s t e x t e n s i o n a s s o c i a t e d w i t h a C r e t a c e o u s o r C e n o z o i c r i f t zone o r rhombochasm ( B o s t r o m 1968; Wynne-Edwards p e r s . comm. 1 9 7 3 ) . 5. S u b s i d e n c e c o m p l e m e n t a r y t o and a c c o m p a n y i n g t h e u p l i f t o f t h e C o a s t M o u n t a i n s ( t h i s s t u d y ) . T h e s e h y p o t h e s e s a r e a d d r e s s e d t o t h r e e i s s u e s : t h e t i m i n g , s t y l e , and u l t i m a t e c a u s e s o f t h e D e p r e s s i o n . I n o r d e r t o e v a l -u a t e them t h i s s t u d y u t i l i z e s b o t h p u b l i s h e d l i t e r a t u r e and t h e r e s u l t s o f a f i e l d i n v e s t i g a t i o n o f t h e I n s u l a r B e l t / C o a s t P l u t o n i c Complex b o u n d a r y c a r r i e d o u t i n t h e summer o f 1974. I t b e g i n s w i t h a summary of p r e s e n t k n o w l e d g e o f t h e h i s t o r y o f G e o r g i a D e p r e s s i o n and t h e two t e c t o n i c p r o v i n c e s t h a t i t s p a n s . The f i e l d p r o j e c t p r o v i d e s f u r t h e r d e t a i l on t h e n a t u r e o f t h e p r o v i n c e b o u n d a r y . The p i c t u r e p r e s e n t e d does n o t c o n f o r m e a s i l y t o e i t h e r a s u t u r e o r a r i f t ; t h e p o s i t i o n and t i m i n g o f t h e D e p r e s s i o n s u g g e s t t h a t i t f o r m e d as a complement t o t h e r i s i n g C o a s t - M o u n t a i n s . L i t e r a t u r e s u r v e y s o f t h e r e s t o f t h e t r o u g h show t h a t t h e o t h e r segments l i k e w i s e s u b s i d e d s y n c h r o n o u s l y w i t h t h e u p l i f t o f t h e a d j a c e n t r a n g e . P o s s i b l e m e c h anisms of c o u p l i n g a r e d i s c u s s e d i n t h e f i n a l c h a p t e r . 4. CHAPTER I; THE HISTORY OF GEORGIA DEPRESSION AND VICINITY The most obvious f e a t u r e of the ge o l o g i c s e t t i n g of Georgia Depression i s that i t l i e s across the Coast P l u t o n i c Complex/ I n s u l a r B e l t province boundary. The h i s t o r y of these two pr o v i n c e s , so f a r as they are known, are summarized below. The o r i g i n of t h e i r present boundary i s of p a r t i c u l a r i n t e r e s t . For c l a r i t y , the two provinces w i l l f i r s t be d i s c u s s e d s e p a r a t e l y . (The Time-Space P l o t , P l a t e 1, and the Fraser River 1 : 1,000,000 Map-Sheet (Roddick, Muller and O k u l i t c h 1973) should be co n s u l t e d during the f o l l o w i n g d i s c u s s i o n ) . i . The I n s u l a r B e l t The o l d e s t s t r a t i g r a p h i c u n i t i n Vancouver I s l a n d i s the Sicker Group which contains s t r a t a from e a r l i e s t Devonian or ol d e r (Muller 1976) to Pennsylvanian-Permian i n age. M u l l e r (1975) has d i v i d e d the S i c k e r Group i n t o three u n i t s . The lowest c o n s i s t s of ande s i t e and d a c i t e p y r o c l a s t i c s and subordinate flows. A t u r b i d i t e sequence, r e s t r i c t e d i n occurrence, o v e r l i e s the S i c k e r v o l c a n i c s . The uppermost u n i t i s a limestone c o n t a i n -ing f u s i l i n i d s dated as Pennsylvanian or Permian. The species present are of A u s t r a l i a n , not North American, a f f i n i t y (Danner pers. comm. 1973). I f t h i s a f f i n i t y has paleogeographic r a t h e r than paleoenvironmental s i g n i f i c a n c e the southern I n s u l a r B e l t must be al l o c h t h o n o u s . The uneven d i s t r i b u t i o n of limestone and c l a s t i c sediments i n Si c k e r Group exposures, as w e l l as the abundance of a n d e s i t e , has l e d Monger et a l (1972) and Mul l e r (1974) to suggest that i t i s the remains of a P a l e o z o i c i s l a n d arc . 5. The S i c k e r Group was metamorphosed to g r e e n s c h i s t f a c i e s , and i n p l a c e s d e v e l o p e d p e n e t r a t i v e c l e a v a g e , b e f o r e the M i d d l e T r i a s s i c , as undeformed a r g i l l i t e s of t h a t age o v e r l i e i t uncon-f o r m a b l y . The a r g i l l i t e s a r e s u c c e e d e d by the Karmutsen Form-a t i o n o f Upper K a r n i a n age. The K a r m u t s e n F o r m a t i o n i s m a i n l y a p i l e of b a s a l t r e m a r k a b l e f o r i t s t h i c k n e s s (up to 8000 m.) and s h o r t e r u p t i o n i n t e r v a l ( p a r t of a f a u n a l z o n e ) . I t embodies an emergent s e q u e n c e ( C a r l i s l e and S u z u k i 1974) r a n g i n g upward from p i l l o w b a s a l t to p i l l o w b r e c c i a and b r e c c i a to a m y g d a l o i d a l f l o w s w i t h s p a r s e d i s c o n t i n u o u s s e d i m e n t a r y i n t e r c a l a t i o n s . Geo-c h e m i c a l l y t h e K armutsen b a s a l t s a p p e a r to r e s e m b l e o c e a n i c X h o l e i i t e s or ocean i s l a n d b a s a l t s more c l o s e l y t h a n i s l a n d a r c t h o l e i i t e s ( K u n i y o s h i 1 972). K u n i y o s h i p o s t u l a t e d t h a t t h e y were e x t r u d e d a t a m i d - o c e a n r i d g e . C o n t a c t r e l a t i o n s between the Karmutsen and the u n d e r l y i n g S i c k e r Group a r e c l e a r l y d e p o s i t -i o n a l and n o t r e l a t e d to a t h r u s t p l a n e ( M u l l e r p e r s . comm. 1975), so K u n i y o s h i ' s i n t e r p r e t a t i o n must be m o d i f i e d a c c o r d i n g l y . M u l l e r et a l (1974) s u g g e s t t h a t the b a s a l t s were formed a t a r i d g e i n an i n t e r a r c b a s i n . Symons (1971b) and I r v i n g and Y o l e (1972) have d e t e r m i n e d p a l e o m a g n e t i c p o l e s f r o m t h e K a r m u t s e n b a s a l t s . The p o l e s l i e about 50° s o u t h of t h e mean Upper T r i a s s i c p o l e f o r N o r t h A m e r i c a . I n o r d e r to r e c o n c i l e t h e p o l e s V a n c o u v e r I s l a n d would have had to l i e i n t h e s o u t h e r n h e m i s p h e r e d u r i n g t h e Upper K a r n i a n . T h i s i s c o m p a t i b l e . 6. with the A u s t r a l i a n a f f i n i t y of Pennsylvanian-Permian f u s i l i n i d s i n the S i c k e r Group. The t h i n l e n s o i d sedimentary i n t e r c a l a t i o n s i n the Karmutsen Formation are probably lagoonal d e p o s i t s . A t y p i c a l lens c o n t a i n s an upward sequence of benthic limestone to p e l a g i c limey shale that C a r l i s l e and Suzuki (1974) i n t e r p r e t as submergent. Gentle subsidence, then, accompanied accumulation of the Karmutsen v o l c a n i c p i l e . The shallow-water r a t h e r than a b y s s a l e n v i r o n -ment l e s s e n s the l i k e l i h o o d that the Karmutsen was formed at an oceanic r i d g e . I t a l s o precludes d i r e c t a n a l o g i e s with modern back-arc b a s i n s , which are deep (a necessary consequence of t h e i r t h i n c r u s t ) . The Karmutsen b a s a l t s are succeeded by upper Karnian to P l i e n s b a c h i a n sediments, which are d i v i d e d i n t o three formations with g r a d a t i o n a l contacts (also i n t e r p r e t e d by C a r l i s l e and Suzuki as a subergent sequence); the upper Karnian Quatsino Formation ( l i m e s t o n e ) ; the Norian Parson Bay Formation, a ser-i e s of thin-bedded c h e r t s , a r g i l l i t e s , and carbonates; and the Lower J u r a s s i c Harbledown Formation, .'.distinguished from the Parson Bay Formation by absence of carbonate and presence of greywacke. V o l c a n i c c l a s t i c sediments i n the Harbledown Formation mark the resurgence of volcanism. The Harbledown sediments are succ-eeded by and are i n part coeval with the Sinemurian to Lower P l i e n s b a c h i a n Bonanza V o l c a n i c s . Dominantly a n d e s i t i c , they are thought to represent a c a l c - a l k a l i n e i s l a n d arc (Monger et a l . 1972; M u l l e r et a l . 1974). Most of the p l u t o n s , and a l l of the l a r g e ones, on V a n c o u v e r I s l a n d a r e of J u r a s s i c age (148 to 181 m.y.: M u l l e r e t a l . 1974). The J u r a s s i c I s l a n d I n t r u s i o n s o c c u p y about t e n p e r c e n t of t h e i s l a n d ' s s u r f a c e a r e a and a r e r o u g h l y c o e v a l w i t h t h e Bonanza V o l c a n i c s . M u t u a l i n t r u s i v e r e l a t i o n s h i p s have been o b s e r v e d : a n d e s i t i c d i k e s i n t h e p l u t o n s and p l u t o n s i n t r u d i n g and m e t a m o r p h o s i n g Bonanza a n d e s i t e s . N o r t h c o t e and M u l l e r (1972) s u g g e s t t h a t t h e y a r e comagmatic. C o n f i r m a t i o n a w a i t s t r a c e - e l e m e n t and i s o t o p i c c o m p a r i s o n s and d a t i n g of p o s s i b l e Bonanza d i k e s i n the p l u t o n s . Symons (1971a) has d e t e r m i n e d a p a l e o m a g n e t i c p o l e f o r t h e I s l a n d I n t r u s i o n s w h i c h , w i t h i n the l i m i t s o f e r r o r , c o i n c i d e s w i t h t h e M i d d l e J u r a s s i c N o r t h A m e r i c a n p o l e . The s o u t h e r n I n s u l a r B e l t must have been i n p l a c e by t h a t t i m e . The d a t e of p o s s i b l e s u t u r i n g i s t h u s c o n s t r a i n e d between upper K a r n i a n ( a p p r o x i m a t e l y 210 m.y.) and 166 m.y., t h e o l d e s t i n t r u s t i o n sampled by Symons. E x c e p t f o r a few Lower C r e t a c e o u s b o d i e s on Texada I s l a n d and t h e n o r t h e a s t s h o u l d e r o f V a n c o u v e r I s l a n d , t h e y o u n g e s t d a t e d I s l a n d I n t r u s t i o n i s 148 m.y. o l d . S i g n i f i c a n t i g n e o u s a c t i v i t y i n t h e s o u t h e r n I n s u l a r B e l t c e a s e d a t t h a t t i m e . I t d i d not resume u n t i l t h e T e r t i a r y , and t h e n i n subdued f o r m . T h i s l a c k o f C r e t a c e o u s i g n e o u s a c t i v i t y i s one -of t h e main d i f f e r e n c e s between t h i s p r o v i n c e and t h e C o a s t P l u t o n i c Complex w h i c h c o n t a i n s an enormous volume of C r e t a c e o u s - E o c e n e i n t r u s i v e m a t e r i a l . D u r i n g and a f t e r emplacement of t h e I s l a n d I n t r u s i o n s t h e a r e a now o c c u p i e d by V a n c o u v e r I s l a n d and G e o r g i a S t r a i t was u p l i f t e d enough to shed a p p r e c i a b l e c l a s t i c d e b r i s , i n c l u d i n g g r a n i t i c m a t e r i a l , westward onto the c o n t i n e n t a l s h e l f ( J e l e t z k y 1965; Muller et a l . 1 9 7 4 ) . A d d i t i o n a l evidence f o r r e g i o n a l u p l i f t above wavebase i n c l u d e s : 1) the absence of P l i e n s b a c h i a n to Santonian s t r a t a on most of Vancouver I s l a n d and 2) the unconformity below the Upper Cretaceous Nanaimo Group which cuts across a l l o l d e r u n i t s and d i s p l a y s over 300 meters of r e l i e f (Ross and Barnes, unpub. ms.). Upper J u r a s s i c and V a l a n g i n i a n to Barremian c l a s t i c marine sediments of the Longarm Formation accumulated on the northern and western edges of the u p l i f t . A nearshore to o f f s h o r e environment i s i n f e r r e d from the Longarm fauna. Conglomerates i n the Longarm Formation c o n t a i n few i f any g r a n i t i c c l a s t s . On northwestern Vancouver I s l a n d the Longarm Formation i s o v e r l a i n by the Aptian to Cenomanian Queen C h a r l o t t e Group. As i n the type area i n the Queen C h a r l o t t e Islands (Sutherland Brown 1968) i t i s p a r t l y non-marine and contains coarse g r a n i t i c d e b r i s , which may r e f l e c t more vigourous u p l i f t of the I n s u l a r B e l t (Muller et a l . 1 9 7 4 ) . The P a c i f i c Rim Complex, p a r t l y coeval with the Longarm Formation, occurs i n fault-bounded blocks along the western margin of Vancouver I s l a n d . It c o n s i s t s mostly of h i g h l y deformed t u r b i d i t e s with subordinate c h e r t , b a s a l t and lime-stone. Page ( 1 9 7 4 ) has d e s c r i b e d o l i s t o s t r o m e s i n the P a c i f i c Rim Complex. Both Page and M u l l e r (1973) term i t a melange. It may be a F r a n c i s c a n e q u i v a l e n t , or at l e a s t an analogue, i n which case the Late J u r a s s i c - E a r l y Cretaceous trench lay somewhat east of the present p l a t e boundary. 9. The S a n t o n i a n to H a e s t r i c h t i a n Nanaimo Group o u t c r o p s i n and a r o u n d G e o r g i a S t r a i t . I t s i n f e r r e d o r i g i n a l d i s t r i b u t i o n ( F i g u r e 2) i m p l i e s a b a s i n c o n f i g u r a t i o n v e r y s i m i l a r to t h e modern G e o r g i a D e p r e s s i o n . The Nanaimo Group i s a complex i n t e r t o n g u i n g s e quence of g r e y w a c k e - s i l t s t o n e r h y t h m i t e s , c o n g -l o m e r a t e l e n s e s , s h a l e , m a s s i v e greywacke, s h a l e and c o a l . I t i s c u r r e n t l y i n t e r p r e t e d as a complex d e l t a / s u b m a r i n e f a n i n a s u b s i d i n g b a s i n (Ross and B a r n e s p e r s . comm. 1973; R i n n e 1973; Simmons 1973; Hudson 1974; S t u r d a v a n t 1974). The s o u r c e of s e d i m e n t f o r t h e Nanaimo Group was the I n s u l a r B e l t u p l i f t : p a l e o c u r r e n t s a r e e a s t e r l y to n o r t h e r l y and c o b b l e s i n c o n g l o -m e r a t e s s u g g e s t a V a n c o u v e r I s l a n d p r o v e n a n c e . The Upper C r e t -a c e ous b a s i n was t h e d i r e c t p r e c u r s o r o f G e o r g i a D e p r e s s i o n , s u p e r i m p o s e d on a p o s t - p l u t o n i c r e g i o n a l u p l i f t of t h e s o u t h e r n I n s u l a r B e l t . I t s p l a c e i n t h e h i s t o r y of t h e C o a s t P l u t o n i c Complex w i l l n e x t be shown. i i . The C o a s t P l u t o n i c Complex The C o a s t M o u n t a i n s a r e d o m i n a t e d by p l u t o n i c and p o l y d e f -ormed metamorphic r o c k s ( s e e F r a s e r R i v e r map-sheet, R o d d i c k , M u l l e r and O k u l i t c h 1973) t h a t have n e a r l y o b l i t e r a t e d t h e s t r a t i g r a p h i c r e c o r d . The e x i s t e n c e of P r e c a m b r i a n basement i n t h e s o u t h e r n C o a s t M o u n t a i n s i s s t i l l u n p r o v e n . The C u s t e r G n e i s s n o r t h of t h e F r a s e r R i v e r ( D a l y 1912) i s c o n s i d e r e d c o r r e l a t i v e w i t h t h e S k a g i t G n e i s s i n the N o r t h C a s c a d e s ( M i s c h 1966) b a s e d on l i t h -o l o g i c s i m i l a r i t y . The S k a g i t G n e i s s has g i v e n z i r c o n ages 1Zt° lit' 10 •Vp-pcr Crctactovs \^ /_J»ftrrut limit af WVcr ~^ -. J Cretacmcs basin-J (present qcagia,'Dtynssipyu Me • fauna th^tlm-rE-rn-nt-trum ^ms .'rticLtyitiffid as lowtx CYllauows,. thus matPHmUmo ttpuvaitM- (jcldxAy fz> .0/. Qrce^i,," t^-t)/ 1 1 . greater than 1650 m.y. OMattl.ns.on 1973). Even assuming that the Custer Gneiss i s autochthonous and not tr a n s p o r t e d from the south by f a u l t i n g , the extent of basement i n the Coast Mountains cannot be guessed a t . Upper P a l e o z o i c (Lower Pennsylvanian to Lower Permian) s t r a t a i n the southern Coast Mountains are co n f i n e d to p a r t l y f a u l t -bounded septa near H a r r i s o n Lake. They are c o r r e l a t i v e with the C h i l l i w a c k Group i n the North Cascades (Monger 1970). L i t h o l o g -i c a l l y the C h i l l i w a c k Group resembles the S i c k e r Group, but i t s s t r a t i g r a p h i c order i s d i f f e r e n t . I t i n c l u d e s v o l c a n i c sandstone, p e l i t e , c h e r t , s i l t s t o n e , limestone, metabasalt and/or meta-an d e s i t e , and v o l c a n i c e l a s t i c s . As f u s i l i n i d s i n the C h i l l -iwack limestones are of North American a f f i n i t y (Monger and Ross 1971), the S i c k e r and C h i l l i w a c k Groups may have been deposited thousands of k i l o m e t e r s a p a r t . If so a suture zone l i e s between them. Does i t f o l l o w Georgia Depression? The T r i a s s i c record sheds some l i g h t on t h i s q u e s t i o n . The Karmutsen Formation i s truncated a b r u p t l y on the east by voluminous i n t r u s i o n s . There are no c e r t a i n Karmutsen e q u i v a l e n t s w i t h i n the Coast P l u t o n i c Complex, although a few have been suggested. P i l l o w b a s a l t s i n J e r v i s I n l e t bear a s t r i k i n g resemblance to the Karmutsen (Woodsworth pers. comm. 1974). Tipper (pers. comm.) a l s o r e p o r t s l i k e l y analogues. These may argue a g a i n s t a p o s t - T r i a s s i c suture between the two p r o v i n c e s . T r i a s s i c s t r a t a i n the southern Coast Mountains between Pemberton and L i l l o o e t are in c l u d e d i n the Fergusson 12. and C a d w a l l a d e r G r o u p s , The F e r g u s s o n Group i s d o m i n a t e d by c h e r t , a r g i l l i t e and b a s a l t w i t h m i n o r l i m e s t o n e . P a r t o f i t has b e e n d a t e d as L a d i n i a n (Cameron and Monger 1 9 7 1 ) , c o e v a l w i t h t h e M i d d l e T r i a s s i c a r g i l l i t e s on V a n c o u v e r I s l a n d . Monger e t a l . (1972) i n t e r p r e t i t as an o c e a n i c a s s e m b l a g e . The Hozomeen G r o u p , l y i n g b e t w e e n t h e Y a l e and Hozameen F a u l t s , has y i e l d e d no f o s s i l s , b u t i s c o r r e l a t e d on l i t h o l o g i c g r o u n d s w i t h t h e F e r g u s s o n Group (Cameron and Monger 1 9 7 1 ) . The C a d w a l l -a d e r Group has y i e l d e d f o s s i l s o f N o r i a n age ( R o d d i c k and H u t c h i s o n 1 9 7 3 ) . I t c o n t a i n s g r e e n s t o n e , c l a s t i c s e d i m e n t s i n c l u d i n g v o l c a n i c c o n g l o m e r a t e w i t h q u a r t z m o n z o n i t e b o u l d e r s , a r g i l l i t e , p y r o c l a s t i c s , c h e r t and l i m e s t o n e . U n l i k e t h e K a r m u t s e n F o r m a t i o n a l l T r i a s s i c u n i t s w i t h i n t h e C o a s t M o u n t a i n s a r e h i g h l y d e f o r m e d . F i g u r e 3 shows f u s i l i n e a f f i n i t i e s and p a l e o m a g n e t i c p o l e s i n s o u t h e r n B r i t i s h C o l u m b i a t o d e m o n s t r a t e p o s s i b l e p o s i t i o n s f o r an Upper T r i a s s i c - Lower J u r a s s i c s u t u r e . Of t h e p o s s i b l e l o c a t i o n s , t h e I n s u l a r B e l t / C o a s t P l u t o n i c Complex b o u n d a r y seems much l e s s l i k e l y t h a n t h e S h u l a p s T e r r a n e , w h i c h f i t s w e l l i n t o t h e p r e c o n c e p t i o n o f s u t u r e . I n i t a r e c h e r t s - " o c e a n b o t t o m s e d i m e n t s " , g r e y w a c k e s - " t r e n c h s e d i m e n t s " , v o l c a n i c s , and u l t r a m a f i t e s , a l l h i g h l y d e f o r m e d and f a u l t e d . The S h u l a p s u l t r a -m a f i c body o v e r l i e s F e r g u s s o n Group r o c k s a l o n g a s h a l l o w l y e a s t -d i p p i n g c o n t a c t , p o s s i b l y a s h e a r zone ( N a g e l p e r s . comm. 1 9 7 5 ) . The u l t r a m a f i c s a r e t h o u g h t t o have b e e n e m p l a c e d i n Upper T r i a s s i c t o Lower J u r a s s i c t i m e , as t h e y a r e f a u l t e d a g a i n s t M i d d l e T r i a -s s i c and N o r i a n r o c k s and d e t r i t a l c h r o m i t e a p p e a r s i n l a t e Lower yolt -positions -from- •• Jjnwns 1371a,, 13/1 b, nju, 1973 Jnryim and JoLc ~19JP~ 14 J u r a s s i c s e d i m e n t s ( L e e c h 1953) . A l t h o u g h s h e a r z o n e s a r e d e s c r i b e d i n t h e e a s t e r n I n s u l a r B e l t ( K u n i y o s h i 1972; M u l l e r p e r s . comm. 1975; C a r l i s l e p e r s . comm. 1975) t h e y a r e n o t o f t h e same s c a l e , n o r a r e u l t r a m a f i t e s p r e s e n t a l o n g them. The most c o m p l e t e J u r a s s i c - C r e t a c e o u s r e c o r d p e r t a i n i n g t o t h e C o a s t P l u t o n i c Complex i s p r e s e r v e d i n a n a r r o w f a u l t - b o u n d e d band ( i n c l u d i n g t h e P a s a y t e n and Methow G r a b e n s ) s o u t h and e a s t o f t h e C o a s t M o u n t a i n s . The s e q u e n c e , r a n g i n g i n age f r o m S i n e m u r i a n t o C e n o m a n i a n , c o n s i s t s o f g r e y w a c k e - s i l t s t o n e r h y t h m i t e s , c o n g l o m e r a t e and p h y l l i t e . A few v o l c a n i c and v o l c a n i c c l a s t i c u n i t s o c c u r e a s t o f Mt. W a d d i n g t o n ( T i p p e r 1 9 6 9 ) . U n t i l r e c e n t l y t h e s e s e d i m e n t s were t h o u g h t t o have been d e p o s i t e d i n a f a i r l y r e s t r i c t e d t r o u g h , t h e T y a u g h t o n T r o u g h . However, s e v e r a l d e t a i l e d f a c i e s and p a l e o c u r r e n t s t u d i e s ( C o l e 1 973; C o a t e s 1974; T e n n y s o n 1974) ha v e shown t h a t t h e " t r o u g h " was open t o t h e w e s t u n t i l A p t i a n t i m e . C u r r e n t d i r e c t i o n s were f r o m t h e e a s t o r l o n g i t u d i n a l , as was s e d i m e n t t r a n s p o r t d e d u c e d f r o m f a c i e s c h a n g e s The p a l e o s l o p e was w e s t e r l y : t h e r e i s no i n d i c a t i o n o f a p r o t o -C o a s t M o u n t a i n s e i t h e r as a p o s i t i v e t o p o g r a p h i c f e a t u r e o r as a s e d i m e n t s o u r c e u n t i l A l b i a n t i m e . J u r a s s i c - L o w e r C r e t a c e o u s s t r a t a w i t h i n t h e C o a s t M o u n t a i n s i n c l u d e 1) t h e Gamb i e r Group and c o r r e l a t i v e s t r a t a , 2) t h e F i r e L a k e G r o u p , and 3) t h e H a r r i s o n L a k e G r o u p . The s e q u e n c e a t H a r r i s o n L a k e i s M i d d l e J u r a s s i c t o Lower C r e t a c e o u s , l a r g e l y c o e v a l w i t h t h e P a s a y t e n G r a b e n s e q u e n c e . The s o u r c e f o r t h e m a r i n e c l a s t i c s e d i m e n t s , i n c l u d i n g g r a n i t i c c o b b l e s , a t H a r r i s o n 15. L a k e i s u n d e t e r m i n e d . I n c o n t r a s t t o t h e P a s a y t e n s t r a t a t h e s e c t i o n i s b r o k e n by a n g u l a r u n c o n f o r m i t i e s and c o n t a i n s a b u n d a n t a n d e s i t e and p y r o c l a s t i c s . The F i r e L a k e Group n o r t h o f H a r r i s o n L a k e i n c l u d e s Upper J u r a s s i c / L o w e r C r e t a c e o u s v o l c a n i c s and m a r i n e s e d i m e n t s , I n c l u d i n g g r a n i t i c d e b r i s ( R o d d i c k 1 9 6 5 ) . The G a m b i e r Group and i t s e q u i v a l e n t s . o u t c r o p i n t e r m i t t e n t l y f o r 150 km. a l o n g t h e s o u t h w e s t e r n s i d e o f t h e C o a s t M o u n t a i n s . They r e s t u n c o n f o r m a b l y on p l u t o n i c r o c k s ( R o d d i c k 1965; C a r o n 1974) b u t a r e i n t r u d e d and m e t a m o r p h o s e d by l a t e r p l u t o n s . A l b i a n ammonites h a v e been f o u n d i n t h e G ambier Group ( J e l e t z k y p e r s . comm. t o Mathews 1 9 7 2 ) : t h e s e a r e t h e y o u n g e s t known m a r i n e s e d i m e n t s i n t h e w e s t e r n C o a s t M o u n t a i n s . The s p a r s e J u r a s s i c and Lower C r e t a c e o u s s e d i m e n t s w i t h i n t h e C o a s t M o u n t a i n s , m o s t l y m a r i n e , s u g g e s t t h a t much of t h e C o a s t P l u t o n i c Complex l a y a t o r b e l o w s e a l e v e l d u r i n g t h a t t i m e . S u f f i c i e n t e r o s i o n o c c u r r e d t o u n r o o f p l u t o n s : b u t some o f t h e g r a n i t i c d e b r i s may have b e e n d e r i v e d f r o m I n s u l a r B e l t and I n t e r m o n t a n e . h i g h l a n d s . C e r t a i n l y no e x t e n s i v e p l u t o n i c / m e t a m o r p h i c t e r r a n e was e x p o s e d . The e a r l i e s t w e s t e r l y -d e r i v e d c o n g l o m e r a t e s i n t h e Methow and P a s a y t e n G r a b e n s c o n t a i n m a i n l y c h e r t and v o l c a n i c c o b b l e s ( T e n n y s o n 1 9 7 4 ) . T h e s e a r e A p t i a n - A l b i a n i n age and p r o v i d e a maximum age f o r t h e s o u t h e r n C o a s t M o u n t a i n s as a h i g h - s t a n d i n g r a n g e . The J u r a s s i c p l u t o n i c h i s t o r y o f t h e C o a s t M o u n t a i n s i s u n d e f i n e d . Two J u r a s s i c d a t e s have been o b t a i n e d so f a r i n t h e s o u t h e r n p a r t o f t h e r a n g e (Mathews 1968; M c K i l l o p 1 9 7 3 ) . G r a n i t i c c o b b l e s i n Upper J u r a s s i c - L o w e r C r e t a c e o u s c o n g l o m e r a t e s 1 6 . i n d i c a t e e a r l y p l u t o n i s m b u t n o t h i n g c a n be s a i d y e t a b o u t i t s i n t e n s i t y o r t i m i n g . Was t h e C o a s t P l u t o n i c Complex a l o c u s o f i n t e n s e magmatism t h e n as i n t h e C r e t a c e o u s , o r was i t s J u r a s s i c i n t r u s i v e h i s t o r y s i m i l a r t o t h a t of t h e I n s u l a r B e l t ? T h i s p r o b l e m may be r e s o l v e d by Rb/Sr and U/Pb d a t i n g . I n t h e E a . r l y C r e t a c e o u s t h e C o a s t P l u t o n i c Complex became t h e s i t e o f a n a r r o w zone o f i n t e n s e p l u t o n i s m . I t r e m a i n e d so u n t i l E o c e n e t i m e . P o t a s s i u m - a r g o n d a t e s i n t h e s o u t h e r n C o a s t m o u n t a i n s g e n e r a l l y d e c r e a s e i n age e a s t w a r d f r o m E a r l y C r e t a c -eous a l o n g t h e w e s t e r n m a r g i n t o E o c e n e i n t h e e a s t . The u l t i m a t e o r i g i n o f t h e p l u t o n i c r o c k s i s d e b a t a b l e . Two s o u r c e s h a v e been p r o p o s e d : g e n e r a t i o n w i t h i n a s u b d u c t e d l i t h o s p h e r i c p l a t e , and c r u s t a l a n a t e x i s , p o s s i b l y i n v o l v i n g r e m e l t i n g of B e n i o f f - z o n e - d e r i v e d r o c k s ( G r i f f i t h s p e r s . comm.). C r u s t a l d e r i v a t i o n i s f a v o r e d by G e o l o g i c a l S u r v e y w o r k e r s who have mapped t h e n o r t h e r n C o a s t M o u n t a i n s ( H u t c h i s o n 1970; R o d d i c k and H u t c h i s o n 1 9 7 2 ) . They h a v e o f f e r e d c o n v i n c i n g a r g u m e n t s . t h a t p l u t o n i c b o d i e s i n t h e P r i n c e R u p e r t a r e a c a n be t r a c e d t o o r i g i n s i n t h e C e n t r a l G n e i s s Complex. A g r a d a t i o n a l s e q u e n c e e x t e n d s f r o m m i g m a t i t i c , h i g h l y f o l i a t e d a u t o c h t h o n o u s b o d i e s , c o n c o r d a n t w i t h s u r r o u n d i n g m i g m a t i t e s , t o u n f o l i a t e d , d i s c o r d -a n t p l u t o n s i n t r u d i n g l o w - g r a d e r o c k s . Some, t e r m e d " p a r a -a u t o c h t h o n o u s p l u t o n s " , embody t h e e n t i r e s e q u e n c e . R o d d i c k and H u t c h i s o n e n v i s a g e p a r t s o f t h e m i g m a t i t i c t e r r a n e b e c o m i n g m o b i l i z e d and m o v i n g by p l a s t i c f l o w i n t o t h e o v e r l y i n g r o c k s . T h i s m o d e l may o r may n o t a p p l y t o t h e s o u t h e r n C o a s t M o u n t a i n s . 17. An i m p o r t a n t c o n s e q u e n c e o f p l a t e - t e c t o n i c t h e o r y i s t h a t calc,".?. > a l k a l i n e magmas may be d e r i v e d by p a r t i a l m e l t i n g a l o n g B e n i o f f z o n e s . By a n a l o g y w i t h t h e Andean a r c , H a m i l t o n (1969) r e a s o n s t h a t l o n g a r c u a t e v o l c a n i c / p l u t o n i c / m e t a m o r p h i c t e r r a n e s s u c h as t h e S i e r r a Nevada a r e t h e d e e p l y e r o d e d r e m n a n t s o f v o l c a n i c a r c s . U s e f u l g u i d e l i n e s f o r r e c o g n i z i n g f o s s i l magmat-i c a r c s i n c l u d e : 1. e l o n g a t e a r c u a t e shape a n d / o r p a r a l l e l i s m t o an i n f e r r e d p l a t e b o u n d a r y ( s u c h as a c o n t i n e n t a l m a r g i n ) ; r e a s o n a b l e p r o x i m i t y t o t h e p l a t e b o u n d a r y , say 100 t o 400 km. 2. p r e d o m i n a n c e o f a n d e s i t e s and t h e i r i n t r u s i v e e q u i v a l e n t s , g r a n o d i o r i t e o r q u a r t z d i o r i t e . 3. h i g h T/P m e t a m o r p h i s m 4. a c o e v a l low T/P m e t a m o r p h i c zone t o w a r d s t h e p l a t e b o u n d a r y ( M i y a s h i r o 1 9 6 1 ; E r n s t 1 9 7 1 ) . 5. y o u n g i n g o f i n t r u s i o n s away f r o m t h e p l a t e b o u n d a r y . 6. i n c r e a s e i n K^O l i k e w i s e . 7. l o w S r 87/86 and Pb 206/204 r a t i o s p a r t i c u l a r l y i n v o l c a n i c s , as an i n d e x of s u b c r u s t a l o o r j g i n . The C r e t a c e o u s / E o c e n e C o a s t P l u t o n i c Complex c o n f o r m s t o 1 ) , 2 ) , 3 ) , 5 ) , and 6 ) ; t h e P a c i f i c Rim Complex as a p o s s i b l e F r a n c i s c a n c o r r e l a t i v e s u g g e s t s t h a t i t c o n f o r m s t o 4 ) , a l t h o u g h g l a u c o p h a n e and l a w s o n i t e h ave n o t as y e t b een f o u n d i n i t . S t r o n t i u m i s o t o p i c r a t i o s i n t h e C o a s t P l u t o n i c C o mplex t e n d t o be l o w . However, t h e y a r e n o t e a s i l y i n t e r p r e t e d ( s e e b e l o w ) , 18. The Coast P l u t o n i c Complex as a magmatic arc i s not incom-p a t i b l e with the d e r i v a t i o n of i t s p l u t o n i c bodies by c r u s t a l m e l t i n g . Greenwood (1975) favors a two-stage h i s t o r y f o r i n t r u s i v e rocks: primary o r i g i n at a B e n i o f f zone and r e m e l t i n g w i t h i n the c r u s t . This model i s favoured by P r e s n a l l and Bateman (1973) f o r the S i e r r a Nevada. Strontium i s o t o p e s are not u s e f u l f o r d i s t i n g u i s h i n g between s u b c r u s t a l and c r u s t a l o r i g i n f o r i n t r u s i o n s i n the Coast Mountains. Although as yet inadequately sampled, they tend to be rudidium poor; t h e i r Sr 87/86 r a t i o s i n c r e a s e very slowly through time (Culbert 1972; and t h i s s t u d y ) . Magmas d e r i v e d from them would have essen-t i a l l y the same S r - i s o t o p i c r a t i o s as B e n i o f f zone melts. Pre-Cretaceous metamorphic events have been deduced by v a r i o u s authors (Tipper 1969; Roddick and Hutchison 1972; Reamsbottom 1973). L i k e pre-Cretaceous plutonism, assessments of degree, extent or timing of these events cannot be made. Extensive r e g i o n a l meta-morphism accompanied the Cretaceous-Eocene plutonism. The p a t t e r n of Cretaceous metamorphism i s complex and not w e l l known due to a lack of d e t a i l e d pressure-temperature s t u d i e s . Although presumably accompanying plutonism i t i s i n many cases u n r e l a t e d to s p e c i f i c plutons ( H o l l i s t er 1969; Reamsbottom 1973; Woodsworth 1974). Metamorphic grade seems to i n c r e a s e towards the e a s t e r n Coast Mountains, p o s s i b l y refleeting :a deeper e r o s i o n l e v e l . The k y a n i t e -s i l l i m a n i t e t r a n s i t i o n i s observed i n s e v e r a l places i n the eastern Coast Mountains ( H o l l i s t e r 1969; Lowes 1972; Pigage 1973). This r e a c t i o n r e q u i r e s a minimum of 3.8 kb p r e s s -19. u r e ( H o l d a w a y 1 9 7 1 ) . W oodsworth ( p e r s . comm.) r e p o r t s t h a t t h e r e may be a p a t t e r n o f m e t a m o r p h i c h i g h s and l o w s i n t h e s o u t h e r n p a r t o f t h e r a n g e . D e s p i t e t h e s k e t c h i n e s s and o b v i o u s i n t r i c a c y o f t h e C r e t a c e o u s s t o r y t h r e e f a i r l y s a f e and s i g n i f i c a n t g e n e r a l i z a t ^ -i o n s c a n be made: 1. The g r o s s c h a r a c t e r i s t i c s o f t h e C o a s t P l u t o n i c Complex -i t s p r e p o n d e r a n c e o f p l u t o n i c r o c k , i t s p r e v a l e n t m o d e r a t e t o h i g h g r a d e o f m e t a m o r p h i s m , i t s n a r r o w n e s s and a b r u p t b o u n d a r i e s , and t h e g r e a t d e p t h o f e r o s i o n a t t a i n e d i n p l a c e s w i t h i n i t - were a c q u i r e d o r ( i n t h e c a s e o f e r o s i o n ) i n i t i a t e d d u r i n g t h e C r e t a c e o u s . 2. The p r e s e n t b o u n d a r i e s o f t h e C o a s t P l u t o n i c Complex a r e e i t h e r i n t r u s i v e c o n t a c t s o r f a u l t s ( B a e r 1 973; R o d d i c k and H u t c h i s o n 1 9 7 4 ) . The w e s t e r n c o n t a c t i s a l i n e a l o n g w h i c h I n s u l a r B e l t s t r a t a a r e t r u n c a t e d by i n t r u s i o n s m o s t l y d a t e d as E a r l y C r e t a c e o u s . The w e s t e r n edge o f t h e p r o v i n c e t h u s was e s t a b l i s h e d i n Lower C r e t a c e o u s t i m e . 3. The c r e a t i o n o f t h e .Coast P l u t o n i c Complex was l i n k e d t o subdue t i o n . The amount o f u p l i f t i s v a r i a b l e : some s e p t a a r e - i n l o w e r g r e e n s c h i s t f a c i e s ; some have been m e t a m o r p h o s e d a t p r e s -s u r e s above t h e a l u m i n o s i l i c a t e t r i p l e p o i n t . G e n e r a l l y t h e e r o s i o n l e v e l d e pens e a s t w a r d , b u t t h i s i s c o m p l i c a t e d by w a r p i n g , f a u l t i n g and s m a l l e r - s c a l e m e t a m o r p h i c c u l m i n a t i o n s . E v i d e n c e f o r t i m i n g o f u p l i f t i s s p a r s e . W e s t e r l y - d e r i v e d A p t i a n - A l b i a n s e d i m e n t s i n t h e P a s a y t e n G r a b e n i n d i c a t e t h a t 20. the e a s t e r n C o a s t M o u n t a i n s began to r i s e i n t h e M i d d l e C r e t a c e o u s . Much of t h e u p l i f t was a c c o m p l i s h e d a l o n g t h e F r a s e r R i v e r F a u l t System. Movement on the Hozameen F a u l t , w h i c h j u x t a p o s e s M i d d l e T r i a s s i c Hozameen Group and J u r a - C r e t -aceous s e d i m e n t s , c e a s e d b e f o r e 84 m.y. ago, t h e d a t e of a s m a l l p l u t o n t h a t c u t s a c r o s s i t and i s not d i s p l a c e d ( C o a t e s 1 9 7 4 ) . The Y a l e F a u l t has b r o u g h t the C u s t e r G n e i s s i n c o n t a c t w i t h Hozameen r o c k s . T i l t e d Eocene c o n g l o m e r a t e a l o n g i t a t t e s t s to T e r t i a r y a c t i v i t y . The C h i l l i w a c k B a t h -o l i t h , 26 m.y. o l d , ( F r a s e r R i v e r map-sheet) c u t s a c r o s s i t s s o u t h e r n e x t e n s i o n , the S t r a i g h t C r e e k F a u l t . I t i s s u g g e s t e d t h a t t h e e a s t e r n C o a s t M o u n t a i n s began to r i s e i n M i d d l e C r e t a c e o u s t i m e and were the w e s t e r n s o u r c e t h a t ( C o a t e s ( 1 9 7 4 ) i n f e r s f o r t h e P a s a y t e n / J a c k a s s M o u n t a i n F o r m a t i o n s . The w e s t e r n p a r t of the r a n g e c o u l d n o t have been a h i g h - s t a n d i n g f e a t u r e u n t i l a t l e a s t t h e L a t e C r e t a c e o u s , as t h e r e a r e A l b i a n m a r i n e s e d i m e n t s i n t h e Gambier Group. F u t h e r -more, had the w e s t e r n C o a s t M o u n t a i n s e x i s t e d i n S a n t o n i a n t i m e , t h e y s u r e l y would have c o n t r i b u t e d s e d i m e n t to t h e p r o t o - G e o r g i a D e p r e s s i o n t h e n f o r m i n g i m m e d i a t e l y to the w e s t . T h e r e i s no e v i d e n c e t h a t t h e y d i d . The P a l e o c e n e (?) Chuckanut F o r m a t i o n has a d o m i n a n t l y e a s t e r n s o u r c e : the C a s c a d e and C o a s t Mount-a i n s must have been e l e v a t e d by t h a t t i m e . The L i o n s Gate Member of Campanian age (Rouse e t a l 1 9 7 5 ) , the o l d e s t Nanaimo c o r r e l a t i v e on t h e e a s t s i d e of G e o r g i a S t r a i t , r e s t s u n c o n f o r m a b l y on p l u t o n i c r o c k s i n C a p i l a n o Canyon. I t 21. contains l e s s than 10% g r a n i t i c c l a s t s ( l e s s even than the Gambier Group) and v i r t u a l l y no high-grade metamorphic or f o l i a t e d p l u t o n i c c l a s t s , r e f l e c t i n g a mostly v o l c a n i c source t e r r a n e without l a r g e amounts of exposed p l u t o n i c / c r y s t a l l i n e rocks as are seen i n the southern Coast Mountains today. T h i s shallow e r o s i o n l e v e l as l a t e as Upper Cretaceous i s another i n d i c a t i o n that strong r e g i o n a l u p l i f t had not occurred before that t ime. In summary, the province boundary that u n d e r l i e s Georgia Depression i s a (roughly) EaTly Cretaceous i n t r u s i v e c o n t a c t . The Depression i t s e l f , however, began to form i n the Santonian: i t w e l l postdates the establishment of t h i s boundary. What evidence there i s suggests that i t accompanied another s i g n i f -i c a n t event i n Coast P l u t o n i c Complex h i s t o r y , the Upper C r e t -aceous-Eocene u p l i f t of the range. i i i Georgia Depression The Upper Cretaceous s t r u c t u r a l d e p ression was not a graben sensu s t r i c t o , as both downfaulting and downwarping were i n v o l v e d . The prevalence of f a u l t s on g e o l o g i c maps i s m i s l e a d i n g : there are s e v e r a l generations of f a u l t s i n the Depression, some of which have nothing to do with i t s o r i g i n . The massive v o l c a n i c u n i t s of Vancouver I s l a n d have tended to undergo b r i t t l e f a i l u r e under compression, y i e l d i n g a map p a t t e r n of f a u l t blocks and open f o l d s . Major f a u l t s on the east s i d e of Georgia S t r a i t have been proposed (Hopkins 1966"; Berry et a l . 1971) but not confirmed. I t i s hoped that f u t u r e s t u d i e s w i l l separate the d i f f e r e n t s t r e s s systems. 22. The p r e s e n t G e o r g i a D e p r e s s i o n i s a m o d i f i e d v e r s i o n of the o r i g i n a l b a s i n . I t s C e n o z o i c h i s t o r y i s d i f f i c u l t to r e -( c o n s t r u c t b e c a u s e , e x c e p t f o r a few o u t c r o p s n e a r V a n c o u v e r , no T e r t i a r y r o c k s a r e exposed above sea l e v e l . The s e d i m e n t o l o g i -c a l r e c o r d of the u p l i f t of the C o a s t M o u n t a i n s i s l i k e w i s e s c a n t y ( E i s a b a c h e r , p e r s . comm. 19 7 5 ) . Nanaimo Group d e p o s i t i o n a p p a r e n t l y c e a s e d i n the M a e s t r i c h t i a n . I t may have c o n t i n u e d i n t o t h e P a l e o c e n e ( M u l l e r and J e l e t z k y . 1970); i t may have e v o l -ved i n t h e s o u t h i n t o c o n t i n e n t a l - d e l t a i c s e d i m e n t a t i o n of t h e Chuckanut F o r m a t i o n (P.Ward, p e r s . comm. 1974). B o t h t h e Chuckanut F o r m a t i o n and the Nanaimo Group were i n v o l v e d i n p r e -M i d d l e Eocene d e f o r m a t i o n . F o l d s and f a u l t s i n t h e C h u c k a n u t F o r m a t i o n a r e o v e r l a i n u n c o n f o r m a b l y by t h e M i d d l e Eocene Hunt-i n g t o n F o r m a t i o n ( M i s c h 1 9 6 6 ) . On S a l t s p r i n g I s l a n d t h e Nanaimo Group i s f o l d e d a b o u t n o r t h w e s t e r l y a x e s . The major f o l d s v e r g e w e s t e r l y . E a s t e r n l i m b s of the - a n t i c l i n e s a r e g e n e r a l l y s t e e p ; some a r e o v e r t u r n e d . F a u l t i n g was more i m p o r t a n t t h a n f o l d i n g on V a n c o u v e r I s l a n d where a t h i n c o v e r of Nanaimo s e d i m e n t s o v e r l i e s v o l c a n i c / p l u t o n i c basement (Ross and Barnes,Ms. u n p u b . ) . B l o c k s bounded by s t e e p f a u l t s d i p g e n t l y e a s t towards G e o r g i a S t r a i t ( M u l l e r 1 9 7 4 ) . I t was p r o b a b l y d u r i n g t h e E a r l y T e r t i a r y e p i s o d e t h a t the Nanaimo Group r o c k s i n Mt. W a s h i n g t o n and t h e F o r b i d d e n P l a t e a u were r a i s e d to t h e i r p r e s e n t e l e v a t i o n a l m o s t 2000 m e t e r s above s e a l e v e l . A t l e a s t p a r t of G e o r g i a D e p r e s s i o n r e m a i n e d a s e d i m e n t a r y b a s i n t h r o u g h o u t T e r t i a r y t i m e . The T e r t i a r y r e c o r d i n Whatcom 23. Basin near Vancouver i s f a i r l y complete. Dating i s mainly based on spores and p o l l e n (Hopkins 1966; Rouse et a l 1975). D e p o s i t i o n i n the::Whatcom . Basin.-was not interrupted', by Ittie." • Eocene deformation. The Lions Gate Member passes without s i g n i f i c a n t break i n t o Eocene d e l t a i c sediments of the K i t -s i l a n o Formation (Hopkins 1966; Blunden 1971). The K i t s i l a n o Formation i s succeeded by an unnamed u n i t of Miocene age seen only i n d r i l l h o l e s (Hopkins 1966). T o t a l t h i c k n e s s of the T e r t i a r y s e c t i o n exceeds 3000 meters. The T e r t i a r y d e p o s i t i o n a l environment was t e r r e s t r i a l d e l t a i c . T i f f i n (1969),using s e i s m i c r e f l e c t i o n p r o f i l e s , traced T e r t i a r y u n i t s part of the way across Georgia S t r a i t . Major T e r t i a r y f a u l t s under the s t r a i t are p o s s i b l e . T i f f i n suggested that the T e r t i a r y sequence i s truncated by a major f a u l t at the base of the eastern slope of Vancouver I s l a n d . The southern part of Georgia Depression was a f f e c t e d by the same Miocene event that produced west-northwest f o l d s i n the Columbian Embayment. The San Juan Islands are the backbone of a Miocene upwarp (Snavely and Wagner 1963),that cuts o b l i q u e l y across the southern end of Georgia S t r a i t . F o l d o r i e n t a t i o n s i n the Nanaimo Group change from northwest to west-northwest between the Gulf Islands and the San Juans; t h i s may p a r t l y be due to the Miocene t e c t o n i c event. Late T e r t i a r y movements have accentuated Georgia Depression; i t has remained low while the f l a n k i n g ranges rose. The lack of marine sediments except i n the l a t e s t P l e i s t o c e n e , however, i n d i c a t e s that i t probably stood above sea l e v e l , t h r o u g h o u t the 24. T e r t i a r y . L a t e C e n o z o i c u p l i f t o f t h e C o a s t M o u n t a i n s amounted t o a b o u t two t h o u s a n d m e t e r s . ( R o d d i c k 1 9 6 6 ) . R e c o g n i z a b l e M i o c e n e and P l i o c e n e e r o s i o n s u r f a c e s p e n e t r a t e deep i n t o t h e r a n g e . M i o c e n e v o l c a n i c s l i e on d e e p l y b e v e l e d s u r f a c e s ( B a e r 1973 and t h e F r a s e r R i v e r m a p - s h e e t ) . Downward movement of t h e D e p r e s s i o n was p r o b a b l y somewhat l e s s t h a n t h e u p l i f t ( B l u n d e n 1 9 7 1 ) . The p r e s e n t s t r a i t owes much o f i t s d e p t h and n a r r o w n e s s t o g l a c i a l e r o s i o n , f i r s t b e c a u s e g l a c i e r s were c h a n n e l l e d b e t w e e n t h e a d j a c e n t r a n g e s and s e c o n d b e c a u s e o f d i f f e r e n t i a l e r o s i o n o f s o f t Nanaimo Group s e d i m e n t s (Mathews 1 9 6 8 ) . The s o u t h e a -s t e r n p a r t o f t h e D e p r e s s i o n i s c u r r e n t l y s u b s i d i n g . The p a t t e r n seems u n r e l a t e d to t h e g e o m e t r y o f t h e F r a s e r D e l t a (Mathews e t . a l . 1970) and t h u s may be p a r t l y t e c t o n i c , i v . Summary and E v a l u a t i o n of R e g i o n a l S y n t h e s i s F i g u r e 4 s u m m a r i z e s t h e t e c t o n i c and p a l e o g e o g r a p h i c e v o l -u t i o n o f t h e I n s u l a r B e l t and t h e C o a s t P l u t o n i c Complex up t o t h e o r i g i n o f G e o r g i a D e p r e s s i o n . T h i s h i s t o r i c a l v i e w b e a r s on t h e h y p o t h e s e s l i s t e d on Page 1. The t i m i n g o f o r i g i n o f t h e D e p r e s s i o n i s c e r t a i n . The S a n t o n i a n - M a e s t r i c h t i a n Nanaimo Group s e d i m e n t s w e re d e p o s i t e d i n a b a s i n whose c o n f i gur a t its tin m i m i c s t h e p r e s e n t one. T h e r e f o r e t h e D e p r e s s i o n was c r e a t e d i n t h e U pper C r e t a c e o u s : C e n o z o i c r i f t i n g , f a u l t i n g and e r o s i o n c a n o n l y have m o d i f i e d a p rfi~--ex i s t i n g t r o u g h . The s t r u c t u r a l n a t u r e o f t h e D e p r e s s i o n i s p o o r l y known. I t i s r e f e r r e d t o as a comb-i n a t i o n d o w n w a r p - d o w n f a u l t i n r e g i o n a l s t u d i e s , b u t d e f i n i t i v e e v i d e n c e as t o t h e i m p o r t a n c e o f f a u l t c o n t r o l has y e t t o be 26. p r e s e n t e d . H o p e f u l l y some of t h e s t u d i e s now i n p r o g r e s s w i l l f i l l t h i s gap . The u l t i m a t e c a u s e of t h e s u b s i d e n c e of G e o r g i a D e p r e s s i o n has been l i n k e d to a s u t u r e , a r i f t zone, or a complement to the u p l i f t of t h e C o a s t M o u n t a i n s . I t i s w o r t h w h i l e b e f o r e e v a l u a t i n g t h e s e h y p o t h e s e s to enumerate the e v i d e n c e t h a t p e r t a i n s to e a c h . C o u p l e d u p l i f t s and s u b s i d e n c e s o c c u r of c o u r s e s i m u l t a n -e o u s l y , making f i e l d e v i d e n c e as to t i m i n g c l e a r l y v a l u a b l e . On t h e o t h e r hand, as no p a r t i c u l a r s t r u c t u r a l s t y l e i s r e q u i r e d , f i e l d e v i d e n c e p e r t a i n i n g to s t r u c t u r e i s not d i a g n o s t i c . G r e a t f a u l t s s u c h as the T e t o n F a u l t may be i n v o l v e d ; or t h e movement may c o n s i s t of g e n t l e t i l t i n g . The c u r r e n t method of i d e n t i f y i n g s u t u r e s and r i f t zones i s by c o m p a r i s o n w i t h a c c e p t e d e x a m p l e s . Examples of s u t u r e s w h i c h have r e c e i v e d g e n e r a l a c c e p t a n c e a r e t h o s e i n the K l a m a t h M o u n t a i n s ( I r w i n 1973; Cashman 1974 ) ; n o r t h e r n New G u i n e a (Dewey and B i r d 1970); and N ewfoundland ( W i l l i a m s e t . a l . 1 9 7 1). Dewey and B i r d (1970) and Le P i c h o n et a l (1973) t r e a t s u t u r i n g and s u t u r e zones i n g e n e r a l . C h a r a c t e r i s t i c s of s u t u r e zones a r e : 1. j u x t a p o s i t i o n of u n r e l a t e d g e o l o g i c p r o v i n c e s . 2. a l i n e of o p h i o l i t e s or a l p i n e u l t r a m a f i t e s , "dismembered o p h i o l i t e s " m a r k i n g th e s u t u r e . 3. b l u e s c h i s t a nd/or melange t e r r a n e between the two p r o v i n c e s , i n d i c a t i n g a f o s s i l s u b d u c t i o n zone. 27. 4. i n t e n s e deformation - i s o c l i n a l f o l d i n g , t h r u s t i n g , obduction. Since there i s no r e c o g n i z a b l e Upper Cretaceous oceanic c r u s t i n Georgia Depression, any r i f t i n g there must have i n v o l v e d a small amount of e x t e n s i o n . There are s e v e r a l w e l l -known modern c o n t i n e n t a l r i f t s of t h i s s o r t , the B a i k a l and Rhine Grabens (Beloussov 1969; Milanovsky 1972), and the East A f r i c a n r i f t system ( G i r d l e r et a l 1969; Baker et a l 1972). Important c h a r a c t e r i s t i c s of c o n t i n e n t a l r i f t zones i n c l u d e : 1. They are narrow l i n e a r complex grabens. F a u l t s e i t h e r dip s t e e p l y inwards or are v e r t i c a l . Dominant f a u l t s p a r a l l e l the s i d e s of the graben; cross f a u l t s are a l s o present. 2. C h a r a c t e r i s t i c t o t a l v e r t i c a l displacement i s 2 to 5 km. 3. High heat flow and vulcanism ( t h o l e i i t i c and a l k a l i c ) accompany r i f t i n g . High temperature metamorphic assemb-lages may provide evidence f o r high heat flow i n deeply eroded f o s s i l r i f t zones. 4. The graben w i l l probably f i l l with sediment, producing a l a r g e negative g r a v i t y anomaly. F a u l t c o n t r o l should be evidenced by coarse, unsorted border f a c i e s . 5. In east A f r i c a a r e g i o n a l negative BoBguer anomaly has been a s c r i b e d to low-density m a t e r i a l i n the upper mantle ( G i r d l e r et a l 1969). The evidence f o r a suture zone between the I n s u l a r B e l t and the Coast P l u t o n i c Complex i s that Vancouver I s l a n d lay i n the southern hemisphere i n the Karnian but was i n place by the Middle J u r a s s i c (166 m.y.). Paleomagnetic data (Symons 1971b; 28. I r v i n g and Yole 1972) r e q u i r e s the suture to l i e between Vancouver I s l a n d and the southern Intermontane B e l t : i t need not c o i n c i d e with the I n s u l a r B e l t / C o a s t P l u t o n i c Complex boundary. Monger et a l . ' s (1972) suggestion that the Upper T r i a s s i c - Lower J u r a s s i c suture l i e s along the Yalakom F a u l t System i s probably the c o r r e c t one, as the T r i a s s i c rocks there are h i g h l y deformed and f a u l t e d and p i e r c e d by Upper T r i a s s i c -Lower J u r a s s i c u l t r a m a f i c bodies. The i m p r o b a b i l i t y of a suture between the I n s u l a r B e l t and the Coast P l u t o n i c Complex i s f u r t h e r demonstrated by the f i e l d study d e s c r i b e d i n the next s e c t i o n . A r i f t zone, i f i t engendered the Depression, must have been a c t i v e i n the Upper Cretaceous. Arguments based on s t r u c t u r a l s t y l e are f u t i l e at t h i s p o i n t . There are no v o l c a n i c s i n the Nanaimo Group. Heat flow at present i n the Georgia S t r a i t area i s low (Hyndman 1976). Past heat flow can only be guessed at from metamorphic assemblages. Metamorphic phases i n the Nanaimo Group i n c l u d e h e u l a n d i t e , laumontite and u n i d e n t i f i e d p h y l l o s i l i c a t e s (Stewart and Page 1974). The absence of high-temperature z e o l i t e s such as w a i r a k i t e might be taken as evidence against high heat flow (and consequently r i f t i n g ) i n the past. At the time of i t s formation Georgia Depression lay between an e v o l v i n g magmatic ar c , the Coast P l u t o n i c Complex, and i t s trench. Because i t lay w i t h i n an a r c - t r e n c h gap, the Cretaceous depression was by d e f i n i t i o n a f o r e a r c b a s i n (Dickinson 1974). It i s important that t h i s term be used without g e n e t i c s i g n i f -i cance. The f o r e a r c basins o f f the coast of C h i l e (Ross and Shor 1965; Coulbourn and Moberly 1975) l i e immediately above t h e t r e n c h - s l o p e b r e a k . They a r e bounded oceanward by a h i g h l y d e formed and t h r u s t e d w e l t o f s e d i m e n t s . G e o r g i a D e p r e s s i o n , by c o n t r a s t , l i e s a hundred k i l o m e t e r s from t h e P a c i f i c / N o r t h A m e r i c a n p l a t e b o u n d a r y and i s basement-c o n t r o l l e d . I t s l o c a t i o n i m m e d i a t e l y west o f t h e w e s t e r n l i m i t o f c r e t a c e o u s p l u t o n i s m and i t s c o i n c i d e n c e w i t h t h e major u p l i f t of t h e C o a s t P l u t o n i c Complex s u g g e s t t h a t i t s s u b s i d e n c e was l i n k e d to the r i s e o f t h e r a n g e , r a t h e r t h a n some s u b d u c t i o n - r e l a t e d p r o c e s s . 30. SECTION I I : FIELD STUDY I n t r o d u c t i o n - G e n e r a l G e o l o g y of the Map A r e a The p o s i t i o n of the C o a s t a l T r o u g h i n B r i t i s h C o l u m b i a o v e r the C o a s t P l u t o n i c C o m p l e x / I n s u l a r B e l t b o u n d a r y ought to have g e n e t i c s i g n i f i c a n c e , w h a t e v e r t h e c a u s e of t h e d e p r e -s s i o n . I u n d e r t o o k a f i e l d s t u d y of a s e c t i o n of t h e b o u n d a r y i n o r d e r to i n v e s t i g a t e i t s n a t u r e and p o s s i b l y i t s r o l e i n e n g e n d e r i n g a s t r u c t u r a l t r o u g h . The map-area c h o s e n f o r t h i s p u r p o s e i n c l u d e s H a r d w i c k e , West T h u r l o w and n e i g h b o r i n g s m a l l e r i s l a n d s , and the c o a s t of V a n c o u v e r I s l a n d n e a r K e l s e y Bay ( F i g u r e 5 ) . The i s l a n d s were p r e v i o u s l y mapped a t 4 m i l e s to the i n c h d u r i n g t h e t e n - y e a r r e c o n n a i s s a n c e of the C o a s t M o u n t a i n s c o n d u c t e d by the G e o l o g i c a l S u r v e y of Canada ( R o d d i c k et a l . 1971 and i n p r e p a r a t i o n ) . K u n i y o s h i (1972) and M u l l e r (1972) mapped the V a n c o u v e r I s l a n d s h o r e a t m i l e and 4 m i l e s to the i n c h r e s p e c t i v e l y . The p r o v i n c e b o u n d a r y has been mapped e l s e w h e r e , a l t h o u g h not w i t h t h e o b j e c t of s t u d y i n g the r e l a t i o n s h i p of t h e two p r o v i n c e s . The work of C a r l i s l e and S u z u k i (1965) on Quadra I s l a n d , A s i h e n e (1970) on Texada I s l a n d , and K u n i y o s h i (1972) on the n o r t h e a s t s h o u l d e r o f V a n c o u v e r I s l a n d , w i l l be d i s c u s s e d i n t h e summary c h a p t e r f o r c o m p a r i s o n . The b o u n d a r y between the I n s u l a r B e l t and t h e C o a s t P l u t o n i c Complex exposed on H a r d w i c k e and West T h u r l o w I s l a n d s i s an i n t r u s i v e c o n t a c t ( G e o l o g i c Map, P l a t e 2 ) . To t h e n o r t h <yiq- 5 Jiardv/kkt/'W- Thurlow islands tCNtlstm 13JC) /northeast shoulder iff yancouvcr JslavcL J iXuniyoshi 19JZ) :->. Oven Hay, QimcLra. JsUncL ° ( Carlisle avid SwLuKi 196(0) H"wada. Jsla-nd/ (tAsihimt lljo ~) jplutcmvc rocks Index wiGLp : (Field area on JrfardwfdU and "West Tkurlow Js lands 0 and other detailed majytinfl oJ the Coast 'Plutonic Complex/'JnsuUr 'delt 'proyvnet boundary 32. and east of i t are p l u t o n i c rocks, dominated by a Late J u r a s s i c quartz d i o r i t e . S t r a t i f i e d u n i t s of the Mesozoic Vancouver Group l i e to the south and west. The southwestern ends of Hardwicke and West Thurlow Islands and the northern coast of Vancouver I s l a n d are u n d e r l a i n by b a s a l t s of the Karmutsen Formation. The Quatsino Formation outcrops on the southern coast of West Thurlow I s l a n d . I t passes upwards i n t o the Parson Bay Formation, which a l s o occurs as septa w i t h i n p l u t o n i c rocks near Eden P o i n t . The Harbledown Formation outcrops east of Carmichael Point on Hardwicke I s l a n d . I t i s succeeded i n l a n d by the Bonanza V o l c a n i c s . Two major west-no r t hwe s t - t rend i n g f a u l t s , the J o h n s t o n e S t r a i t and the Telegraph H i l l f a u l t s , have broken the Vancouver Group i n t o homoclinal b l o c k s . P r i o r to i n t r u s i o n the Vancouver Group underwent p r e h n i t e - p u m p e l l y i t e b u r i a l metamorphism. Contact-metamorphic g r e e n s c h i s t assemblages are developed i n Vancouver Group rocks l e s s than 4 k i l o m e t e r s from the quartz d i o r i t e , and hornblende h o r n f e l s assemblages l e s s than 500 meters from i t . S y nplutonic d u c t i l e deformation a f f e c t e d the Vancouver Group w i t h i n the inner aureole. N o r t h e a s t - t r e n d i n g d i k e s , p e r v a s i v e l y a l t e r e d to c h l o r i t e -e p i d o t e - a l b i t e , cut both the quartz d i o r i t e and the o l d e r rocks. They mark the c l o s e of igneous a c t i v i t y and are the l a s t recorded event i n the map area except f o r P l e i s t o c e n e g l a c i a t i o n . 33. CHAPTER I I : STRATIGRAPHY OF THE VANCOUVER GROUP Every formation i n the e a r l y Mesozoic Vancouver Group i s represented i n the map-area (Figure 6, column 3; a l s o see the Geologic Map, P l a t e 2), although no complete s e c t i o n i s seen. Most u n i t s are bounded by f a u l t s , i n t r u s i v e c o n t a c t s , or water. Probable f a u l t r e p e t i t i o n and, near the i n t r u s i o n s , t e c t o n i c t h i n n i n g , render t h i c k n e s s estimates approximate only. A reasonable sequence may n e v e r t h e l e s s be r e c o n s t r u c t e d by t a k i n g account of l i t h o l o g y and s t r u c t u r e . i . Karmutsen Formation The Karmutsen Formation i s r e a d i l y d i v i d e d i n t o three parts based on l i t h o l o g y . Kuniyoshi (1972) assigned f o r m a t i o n a l names to them. However, to avoid ambiguity, Kuniyoshi's names w i l l not be used here. The lower Karmutsen Formation (Kuniyoshi's Kelsey Bay Formation) i s w e l l exposed as shore c l i f f s near Kelsey Bay. I t i s a monotonous p i l e of p i l l o w b a s a l t r e l i e v e d by minor b r e c c i a s and tongues and flows of massive b a s a l t . The i n t e r i o r s of the p i l l o w s are h y a l o p i l i t i c with small phenocrysts and amygdules ( l e s s than 1 mm. a c r o s s ) . T h e i r o r i g i n a l g l a s s y selvages are a l t e r e d to f i n e - g r a i n e d e p i d o t e - p u m p e l l y i t e - c h l o r i t e mixtures. P i l l o w i n t e r s t i c e s are f i l l e d with quartz, c a l c i t e , epidote and c l i n o z o i s i t e . Only approximate bedding a t t i t u d e s can be obtained from the p i l l o w s . These a t t i t u d e s are somewhat e r r a t i c , perhaps r e f l e c t i n g i n i t i a l sea-bottom topography. 35 . The m i d d l e Karmutsen F o r m a t i o n ( K u n i y o s h i ' s Hkusam F o r m a t i o n ) i s c h a r a c t e r i z e d by a p r e d o m i n a n c e of p i l l o w b r e c c i a s w i t h sub-s i d i a r y f l o w s , aquagene t u f f s and t r u e b r e c c i a s . I t i s exposed i n a h o m o c l i n e a l o n g t h e n o r t h e a s t s h o u l d e r of V a n c o u v e r I s l a n d , e a s t o f Camp P o i n t . The b a s a l c o n t a c t i s a f a u l t ( s e e K u n i y o s h i 1972; M u l l e r 1 9 72). The c o n t a c t w i t h t h e upper Karmutsen F o r m a t i o n i s a g r a d a t i o n a l zone i n w h i c h a m y g d a l o i d a l f l o w s r e p l a c e b r e c c i a upwards. S e v e r a l p a r t i a l s e c t i o n s of the upper Karmutsen F o r m a t i o n ( K u n i y o s h i ' s M e n z i e s F o r m a t i o n ) a r e p r e s e n t i n the n o r t h w e s t e r l y d i p p i n g h o m o c l i n e between th e J o h n s t o n e S t r a i t and T e l e g r a p h H i l l F a u l t s ( s e e C h a p t e r IV). A l l of t h e s e c t i o n s a r e p r e d o m i n a n -t l y composed of a m y g d a l o i d a l f l o w b a s a l t w i t h v a r y i n g p r e c e n t -ages of b r e c c i a , p i l l o w b a s a l t , and i n t e r l a v a s e d i m e n t s . On t h e n o r t h e a s t s h o u l d e r o f V a n c o u v e r i s l a n d a p p r o x i m a t e l y 2000 m e t e r s of upper Karmutsen F o r m a t i o n o v e r l i e the m i d d l e K a r m u t s e n . P i l l o w b a s a l t , p i l l o w b r e c c i a and b r e c c i a form about 10% o f t h e s e c t i o n , w h i l e s e d i m e n t s a r e l a c k i n g . T h i s a s s e m b l a g e i s t y p i c a l of the l o w e r p a r t of the u p p e r K a r m u t s e n . On Helmcken I s l a n d , by c o n t r a s t , o n l y one b r e c c i a - p i l l o w b r e c c i a u n i t i s p r e s e n t , and i s a s s o c i a t e d w i t h an i n t e r l a v a s e d i m e n t a r y c y c l e ( F i g u r e 7 ) . The p r e s e n c e of s e d i m e n t s i s t y p i c a l of t h e top 1000 m e t e r s of t h e Karmut-sen F o r m a t i o n ( K u n i y o s h i 1 9 72). On s o u t h w e s t e r n H a r d w i c k e I s l a n d t h e upper Karmutsen F o r m a t i o n c o n s i s t s e n t i r e l y o f f l o w b a s a l t . S t r a t i g r a p h i c a l l y above i t on Y o r k e I s l a n d a r e b r e c c i a s , p i l l o w b a s a l t s and i n t e r l a v a s e d i m e n t s . A l l of t h e s e C7Y* • 7 Stratx^mykj <rj intcrvolcanic s e d i m e n t a r y cycle, Ji time k m Jslund 'Urcccia. • Coarsms upwards \Mtitd fvnt bncc'i&s nr,d tv.jfs . Graded btddrrru, amyolutt.• I^trfvyj ce-yrrynoru. OMy.ti Umditur- and. oyiajszvt -tyi&thtrfaiq black chert- fPtltcyyods and-awrncmibU. ( QSC # • fbisty-wtedkvfWia blactn ohirt with, smm amounts opjyyriU a.nd chulcpj'jrilt ^tacl and jvrtt/n churt, smt w(th CcUcmrtous /traded nrc^niu-Hkck. Chert, semi vdwtous . yAmmvriibs coy oftd inttryd C4 m 0 'YlSitititr basnit. 37. s e c t i o n s are i n seeming s t r u e t u r a l c o n t i n u i t y . I f they are combined, however, the aggregate t h i c k n e s s , 10,000 meters, g r e a t l y exceeds a l l estimates f o r the upper Karmutsen Formation. A n o r t h e a s t e r l y c r o s s f a u l t i s hypothesised between Helmcken Is l a n d and Bendickson Harbor. I t d i v i d e s the homocline i n t o two packages of reasonable t h i c k n e s s , and places Helmcken I s l a n d w i t h i n the uppermb'st-iarmutseni: Joimation . The Upper Karmutsen Formation a l s o occurs on southern West Thurlow I s l a n d where i t i s p a r t l y o v e r l a i n by the Quatsino Formation, and p a r t l y truncated by i n t r u s i v e r o c k s . The s e c t i o n contains f i v e separate sedimentary i n t e r c a l a t i o n s and s e v e r a l t h i c k p i l l o w and/or p i l l o w b r e c c i a u n i t s . Flows i n the upper Karmutsen Formation are t y p i c a l l y c o a r s e l y amygdaloidal at t h e i r bases, s p a r s e l y amygdaloidal i n t h e i r c e n t e r s , and c o n t a i n a densely amygdaloidal h o r i z o n near t h e i r tops. Pipe v e s i c l e s occur near flow bases. The amygdules tend to i r r e g u l a r amoeboid shapes. They are up to a centimeter i n diameter, i n d i c a t i n g d e p o s i t i o n i n water much shallower than that i n which the lower Karmutsen Formation was d e p o s i t e d . The i n t e r l a v a sediment lenses w i t h i n the upper Karmutsen Formation range from one to seven meters t h i c k . They are never t r a c e a b l e f o r more than a hundred meters or so. C a r l i s l e and Suzuki (1974) a t t r i b u t e d t h e i r l a c k of c o n t i n u i t y to d e p o s i t i o n i n small basins and lagoons. Three b a s i c l i t h o l -ogies are present i n v a r y i n g amounts: limestone, thin-bedded 38. carbonaceous c h e r t , and f i n e - g r a i n e d green greywacke i n t e r -preted by C a r l i s l e and Suzuki as l o c a l b a s a l t - w e a t h e r i n g products. The i n t e r l a v a sediments on Helmcken I s l a n d f o l l o w a t y p i c a l sequence (Figure 7). An ammonoid c o l l e c t e d at t h i s l o c a l i t y was examined by E.T. Tozer (see Appendix). He sets i t s age as Upper T r i a s s i c , a r e a s s u r i n g but u n r e v e a l i n g d e t e r m i n a t i o n . I t has been observed before ( C a r l i s l e and Suzuki 1974) that these sedimentary i n t e r v a l s are s c a t t e r e d throughout the uppermost Karmutsen Formation. The only i n t e r v a l which seems to represent a c o n s i s t e n t time h o r i z o n i s the I n t e r v o l c -anic Limestone (Muller et a l . 1974) which l i e s near the top of the Karmutsen :•. Formation. This limestone i s seen i n two places on West Thurlow I s l a n d , on Yorke I s l a n d , and p o s s i b l y on Fanny I s l a n d . I t c o n s i s t s of a s e r i e s of l e n s o i d bodies 3 to 7 meters t h i c k . L i t h o l o g i c a l l y i t i s i d e n t i c a l to the Quatsino Formation, i i i The Vancouver Group Sediments: Quatsino, Parson Bay and Harbledown Formations The Quatsino Formation succeeds the "upper Karmutsen Formation on southern West Thurlow I s l a n d . Large t h i c k n e s s v a r i a t i o n s suggest a l e n s o i d shape. Above Miner's Bay approximately 150 meters of massive marble i s truncated by the i n t r u s i o n . East of V a n s i t t a r t Point the Quatsino i n c l u d e s 5 meters of pure marble o v e r l a i n by 15 meters of marble with c h a o t i c a l l y f o l d e d chert i n t e r b e d s . The lower contact i s obscured by a p o s t - p l u t o n i c d i k e . I t may be a minor shear zone. The primary f e a t u r e s of 39. Quatsino limestone d e s c r i b e d by other workers - s t y l o t i t i c bedding, cross-bedding, a l g a l s t r u c t u r e s - are absent due to metamorphic r e c r y s t a i l i z a t i o n . Near Miner's Bay the Quatsino Formation i s a massive l i g h t grey marble with very sparse 2 to 5 centimeter cherty i n t e r l a y e r s . East of V a n s i t t a r t Point the percentage of chert l a y e r s i n c r e a s e s upwards to pass i n t o the Parson Bay Formation. The lower boundary of the Parson Bay Formation i s the poin t of which black limestone, shal e , and s i l t s t o n e predominate over pure limestone (Muller et a l . 1974). This t r a n s i t i o n i s observed east of V a n s i t t a r t P o i n t . Above i t f i n e b l ack s i l i c e o u s s i l t s t o n e predominates. Marble u n i t s i n the Parson Bay Formation are l e n t -i c u l a r or small p i l l o w - l i k e lenses g e n e r a l l y l e s s than a meter t h i c k . In the upper part of the s e c t i o n there are a few thoroughly r e c r y s t a l l i z e d greywackes which preserve primary f e a t u r e s such as load c a s t s , graded bedding, cross-bedding, and c l a s t i c d i k e s . Near Eden Point on western West Thurlow I s l a n d the Parson Bay Form-a t i o n i s 75% s i l t s t o n e and 25% marble. The s i l t s t o n e l a y e r s are black to yellow. Some, c o n t a i n i n g c h a l c o p y r i t e and p y r i t e , weather to a r u s t c o l o r . One package of thin-bedded black s i l i c e o u s i t s c ontains pelecypods i d e n t i f i e d as Monotis s u b c i r c u l a r i s Gabb of upper Norian age (E.T. Tozer pers. comm. see Appendix). T h i s age agrees with that of the Parson Bay Formation elsewhere. The sediments between Carmichael Po i n t and P a t t e r s o n Bay on Hardwicke I s l a n d y i e l d e d no f o s s i l s ; however l i t h o l o g i c a l l y they resemble the Harbledown Formation which i s d i s t i n g u i s h e d 40. from the P a r s o n Bay F o r m a t i o n by i t s l a c k of l i m e s t o n e . The H a r d w i c k e I s l a n d e x p o s u r e c o n s i s t s of f i n e b l a c k s i l i c e o u s s i l t s t o n e , some of i t s l i g h t l y c a l c a r e o u s . I t i s o v e r l a i n by v o l c a n i c b r e c c i a and m a s s i v e s i l t s t o n e o f the Bonanze V o l c a n i c s . i i i . Bonanza V o l c a n i c s The Bonanza V o l c a n i c s a r e most e x t e n s i v e i n l a n d on H a r d -w i c k e I s l a n d where e x p o s u r e s a r e p o o r . They a l s o o c c u r i n s e p t a e a s t of P a t t e r s o n Bay bounded to the n o r t h by q u a r t z d i o r i t e and t o t h e s o u t h by J o h n s t o n e S t r a i t . I d e n t i f i c a t i o n o f the Bonanza i s ba s e d on l i t h o l o g i c and m i n e r a l o g i c c r i t e r i a . The i n t e r b e d d i n g o f v o l c a n i c b r e c c i a and m a s s i v e s i l t s t o n e f i t s t he Bonanza f a r b e t t e r t h a n t h e Karmutsen F o r m a t i o n . The b r e c c i a s c o n t a i n s m a l l f l a t t e n e d c l a s t s 1 to 10 cm. i n d i a m e t e r , e i t h e r p r i m a r i l y or t e c t o n i c a l l y f l a t t e n e d ( o r b o t h ) p a r a l l e l to b e d d i n g . They a r e t r u e b r e c c i a s , n ot p i l l o w b r e c c i a s . M i n e r a l o g i c a l l y t h e y a r e a n d e s i t e s , n ot b a s a l t s . They c o n t a i n abundant meta-m o r p h i c q u a r t z and b i o t i t e ; p l a g i o c l a s e a n o r t h i t e c o n t e n t s v a r y but i n s e v e r a l c a s e s a r e much l e s s t h a n i n K a r m u t s e n r o c k s a t e q u i v a l e n t g r a d e . F i g u r e 6 shows t h a t t h e c o m p o s i t e s t r a t i g r a p h i c s e c t i o n i n t h i s map a r e a c l o s e l y r e s e m b l e s the V a n c o u v e r Group e l s e w h e r e on V a n c o u v e r I s l a n d , i n c l u d i n g t h e t h r e e s u b d i v i s i o n s of t h e Karmutsen F o r m a t i o n , the l o w e r p a r t o f t h e Bonanza V o l c a n i c s , and the i n t e r v e n i n g s e d i m e n t a r y f o r m a t i o n s . The r e g i o n a l h i s t o r y f o r V a n c o u v e r I s l a n d i n the e a r l y M e s o z o i c o u t l i n e d i n C h a p t e r I a p p l i e s e q u a l l y h e r e . A l t h o u g h i n t h i s f i e l d s t u d y s t r a t i g r a p h y 41. has mainly been used to d e f i n e s t r u c t u r e i t also has i n t e r e s t i n g i m p l i c a t i o n s f o r d e p o s i t i q n a l environments. S e v e r a l workers have noted that the Vancouver Group sediments o f f e r no evidence f o r strong c u r r e n t s , steep s l o p e s , or nearby sediment sources (Asihene 1970; Kuniyoshi 1972; C a r l i s l e and Suzuki 1974). This i s even true i n the easternmost exposures such as the present study area. An Upper T r i a s s i c - L o w e r J u r a s s i c s u t u r i n g event ought to generate a high-energy d e p o s i t i o n a l environment. Yet the Upper T r i a s s i c - L o w e r J u r a s s i c Vancouver Group shows no evidence f o r i t . This argues a g a i n s t a suture near the province boundary. i v . S t r a t i g r a p h y and Major S t r u c t u r e Three homoclinal domains, based on bedding a t t i t u d e s and s t r a t i g r a p h i c s u c c e s s i o n s , are i n f e r r e d . Domain I i n c l u d e s West Thurlow I s l a n d and Hardwicke I s l a n d northeast of Telegraph H i l l . The average bedding a t t i t u d e i n i t i s 85/40 N. (Figure 8). The lowest part of the s e c t i o n seen i n t h i s domain i s the upper Karmutsen Formation on southern West Thurlow I s l a n d succeeded by the Quatsino Formation at Miner's Bay and east of V a n s i t t a r t P o i n t ; and the Parson Bay Formation east of V a n s i t t a r t Point and at Eden Point (where a tongue of the i n t r u s i o n has o b l i t e r a t e d the Quatsino l i m e s t o n e ) . In the same domain, and i n proper s t r a t i -graphic order, are the Harbledown and Bonanza Formations on Hardwicke I s l a n d . Domain II i s the narrow northwest - t r e n d i n g homocline extending from Yorke I s l a n d to the northeast shoulder of Vancouver I s l a n d . I t i s separated from Domain I by the 1 43. Telegraph H i l l F a u l t . Average bedding i n i t i s 44/32 N.W. (Figure 9). A c r o s s - f a u l t i s i n f e r r e d south of Bendickson Harbor to give reasonable t h i c k n e s s e s of the upper Karmutsen formation i n Domain I I . Domain I I I encompasses the Vancouver I s l a n d coast west of Camp P o i n t , southwest of the Johnstone S t r a i t F a u l t . I t c o n s i s t s of n e a r - h o r i z o n t a l lower Karmutsen Formation. The r e l a t i o n s h i p between these domains and the major f a u l t s that separate them are d i s c u s s e d i n Chapter IV. i 45. CHAPTER I I I : PLUTONIC ROCKS One p r o m i n e n t f e a t u r e of t h e I n s u l a r B e l t / C o a s t P l u t o n i c Complex b o u n d a r y i s t h e v i r t u a l d i s a p p e a r a n c e o f t h e V a n c o u v e r Group e a s t of i t . In some i n s t a n c e s the I n s u l a r B e l t s t r a t a a r e s h a r p l y t r u n c a t e d by C o a s t M o u n t a i n s i n t r u s i o n s ; i n o t h e r s t h e r e seems to be a c o n t i n u u m between m e t a v o l c a n i c s and p l u t o n i c r o c k s ( R o d d i c k and H u t c h i s o n , 1974,and C a r l i s l e , p e r s . comm. 1975). T h i s c h a p t e r documents the n a t u r e of t h e p l u t o n i c b o d i e s on H a r d w i c k e and West T h u r l o w I s l a n d s and t h e i r r e l a t i o n to the V a n c o u v e r Group. T h e r e a r e a t l e a s t t h r e e d i s t i n c t p l u t o n i c p h a s e s i n t h e map a r e a ( s e e G e o l o g i c Map, P l a t e 2 ) . The y o u n g e s t i s q u a r t z d i o r i t e w i t h a h o r n b l e n d e gabbro b o r d e r phase t h a t o c c u p i e s most of t h e s u r f a c e a r e a of the i s l a n d s . I t i s r e s p o n s i b l e f o r t h e s e c o n d - p h a s e s t r u c t u r e s d e s c r i b e d i n C h a p t e r TV and t h e c o n t a c t metamorphism d e s c r i b e d i n C h a p t e r V.. By Rb/Sr i s o c h r o n s i t i s a p p r o x i m a t e l y 154 m.y. o l d ; by K/Ar i t i s between 147 and 158 m.y. o l d . The o l d e r p l u t o n s a r e t r u n c a t e d and meta-morphosed by i t . B e c a u s e r e s e t t i n g was e x p e c t e d t h e y were n o t r a d i o m e t r i c a l l y d a t e d . T h e i r c o n t a c t a u r e o l e s a r e not s e p a r a b l e from t h a t of the main i n t r u s i o n , i . O l d e r B o d i e s One of t h e o l d e r b o d i e s i s a gabbro w h i c h i n t r u d e s t h e u p p e r r.mutsen F o r m a t i o n west o f M i n e r ' s Bay. I t s t e x t u r e i s - d i s t i n c t -i v e ( F i g u r e 1 0 a ) , c o n s i s t i n g of l a r g e s q u a r i s h e u h e d r a l p l a g -i o c l a s e s u r r o u n d e d by i n t e r s t i t i a l c l i n o p y r o x e n e , h o r n b l e n d e and A MINERS BAY GABBR.0 "WITH BA5ALT/C DIKE B . W E L L B O R E C H A N N E L G N E I S S I C Q U A R T Z . D I 0 R . I T E 4 7 . magnetite. The Miner's Bay body i s r i d d l e d with small i r r e g -u l a r northwest-trending b a s a l t i c i n l i e r s , which commonly i n c l u d e l a r g e square p l a g i o c l a s e c r y s t a l s . These, along with the o b s c u r i t y of the contact r e l a t i o n s , suggested an :. o r i g i n of the body by d i o r i t i z a t i o n to the G e o l o g i c a l Survey r e c o n a i -ssance team (Roddick and Hutchison, i n . p r e p a r a t i o n ) . An i n t r u s i v e o r i g i n i s favored here. M i c r o s c o p i c a l l y the b a s a l t i c i n l i e r s are c l e a r l y d i k e s . If they were Karmutsen, the l a r g e p l a g i o c l a s e s i n them should o v e r p r i n t any f o l i a t i o n . On the c o n t r a r y , the i n l i e r s are p i l o t a x i t i c ; the o r i e n t a t i o n of small p l a g i o c l a s e and hornblende l a t h s bends around the l a r g e p l a g i o -c l a s e s (Figure 1 0 a ) . In a d d i t i o n the t e x t u r e of the Miner's Bay gabbro with i t s square o s c i l l a t o r y - z o n e d p l a g i o c l a s e and i n t e r s t i t i a l mafics bears no resemblance to that of Karmutsen metabasalts, even at the highest grades. The contact between t h i s body and the younger quartz d i o r i t e i s an i n t r u s i v e b r e c c i a . Comb-layered v e i n s or dikes i n the Miner's Bay body are probably a s s o c i a t e d with the younger i n t r u s i o n . The other o l d e r p l u t o n i s a g n e i s s i c quartz d i o r i t e exposed on Hardwicke I s l a n d next to Wellbore Channel. In t h i n s e c t i o n i t s prominent n o r t h e a s t e r l y f o l i a t i o n appears to be the r e s u l t of deformation and r e e r y s t a l i i z a t i o n a f t e r i n i t i a l c o o l i n g , (Figure 1 0 b ) . The f o l i a t i o n i s l a r g e l y due to t r a i n s of f i n e - g r a i n e d b i o t i t e and c h l o r i t e which bend around l a r g e 4 8 . p l a g i o c l a s e and h o r n b l e n d e c r y s t a l s . B e n t t w i n s a r e common i n p l a g i o c l a s e . Some p l a g i o c l a s e s a r e b r o k e n , t h e f r a c t u r e s h e a l e d by r e c r y s t a l l i z a t i o n o f s m a l l e n h e d r a l new p l a g i o c l a s e . S u t u r e d b o r d e r s s u r r o u n d e d by new c r y s t a l s a r e t y p i c a l . As no s e v e r e r e g i o n a l d e f o r m a t i o n has been i n f e r r e d f o r t h e a r e a , and as t h e g n e i s s i c q u a r t z d i o r i t e o c c u r s v e r y n e a r t o t h e y o u n g -e s t i n t r u s i o n , i t s d e f o r m a t i o n and l o w t e m p e r a t u r e a l t e r a t i o n may be c o n t a c t e f f e c t s s i m i l a r t o t h o s e s e e n i n t h e B o n a n z a F o r m a t i o n . The y o u n g e r q u a r t z d i o r i t e t r u n c a t e s i t . i i . Q u a r t z D i o r i t e and A s s o c i a t e d H o r n b l e n d e G a b b r o s The y o u n g e s t and l a r g e s t i n t r u s i o n t r e n d s w e s t - n o r t h w e s t , p a r a l l e l t o t h e m a j o r f a u l t s and t o t h e n o r t h e a s t c o a s t o f V a n c o u v e r I s l a n d . I t c o n s i s t s o f two p h a s e s : an i n c o m p l e t e s h e a t h o f h o r n b l e n d e g a b b r o ( o f i n s i g n i f i c a n t v o l u m e ) , and an i n n e r q u a r t z d i o r i t e . I t i s i n t e r p r e t e d h e r e as a l i q u i d o r m o s t l y l i q u i d i n t r u s i o n t h a t d i f f e r e n t i a t e d b e l o w i t s p r e s e n t e x p o s u r e s t o g i v e two p h a s e s . The q u a r t z d i o r i t e p h a s e i s t e x t u r a l l y and c o m p o s i t i o n a l l y homogeneous e x c e p t f o r v a r i a t i o n s i n d e g r e e o f f o l i a t i o n . M i c r o -s c o p i c a l l y i t i s c h a r a c t e r i z e d by h o r n b l e n d e l a t h s and l a r g e , c o m p l e x l y z o n e d and t w i n n e d p l a g i o c l a s e s ( F i g u r e 1 1 ) . L i k e most C o a s t M o u n t a i n s p l u t o n s t h i s one c o n t a i n s r o u n d e d f i n e - g r a i n e d b a s a l t i c i n c l u s i o n s . M i c r o s c o p i c a l l y t h e i n c l u s i o n s d i s p l a y a n o n - d i r e c t i o n a l p o l y g o n a l t e x t u r e v e r y l i k e K a r m u t s e n m e t a b a s a l t s i n t h e h o r n b l e n d e - p l a g i o c l a s e z o n e . They d i f f e r f r o m K a r m u t s e n v o l c a n i c s i n c o n t a i n i n g l a r g e o s c i l l a t o r y z oned p l a g i o c l a s e p o r p h -y r o b l a s t s , p r o b a b l y a r e s u l t o f i n t e r d i f f u s i o n and m i n e r o l o g i c a l 50. e q u i l i b r i u m w i t h t h e e n c l o s i n g q u a r t z d i o r i t e o v e r a c o n s i d e r a b l e p e r i o d . The q u a r t z d i o r i t e t r u n c a t e s b o t h t h e V a n c o u v e r Group and t h e e a r l i e r h o r n b l e n d e g a b b r o s a l o n g s h a r p c o n t a c t s . D i k e s on a l l s c a l e s p e n e t r a t e t h e c o u n t r y r o c k s , e x t e n d i n g h u n d r e d s o f m e t e r s f r o m t h e p a r e n t body. I n t r u s i v e b r e c c i a s , b o t h mono-and p o l y l i t h o l o g i c , a r e a common c o n t a c t f e a t u r e . The b l o c k s i n them a r e a n g u l a r and c a n measure 2 m e t e r s a c r o s s ( F i g u r e 1 2 ) . These f e a t u r e s a r e more i n d i c a t i v e o f l i q u i d i n t r u s i o n t h a n o f d i a p i r e m p lacement o r p a s s i v e d i o r i t i z a t i o n . They s u g g e s t f o r c i b l e i n t r u s i o n and a l a r g e v i s c o s i t y c o n t r a s t b e t w e e n p l u t o n and c o u n t r y r o c k . The f o l i a t i o n i n t h e q u a r t z d i o r i t e i s b e s t d e f i n e d by f l a t -t e n e d / s t r e t c h e d i n c l u s i o n s ; b i o t i t e and h o r n b l e n d e a r e o n l y w e a k l y o r i e n t a t e d . The f o l i a t i o n g e n e r a l l y t r e n d s w e s t - n o r t h w e s t b u t v a r i e s c o n s i d e r a b l y ( F i g u r e 1 3 ) . Near t h e o u t s i d e o f t h e body i t t e n d s t o f o l l o w l o c a l c o n t a c t s . The d e f o r m a t i o n o f t h e i n c l u s -i o n s s u g g e s t s a r a t h e r s m a l l v i s c o s i t y ( o r c o m p e t e n c y ) c o n t r a s t b e t w e e n them and t h e s u r r o u n d i n g q u a r t z d i o r i t e ( G.Woodsworth, p e r s . comm. 1 9 7 5 ) . The f o l i a t i o n may t h u s h a v e f o r m e d a t a l a t e s t a g e o f c r y s t a l l i z a t i o n . Low t e m p e r a t u r e a l t e r a t i o n o f t h e q u a r t z d i o r i t e i s u b i q -u i t o u s b u t m i n o r e x c e p t i n n o r t h e a s t - t r e n d i n g f r a c t u r e z o n e s ( C h a p t e r V I ) and n e a r c o n t a c t s . The f o l l o w i n g s e c o n d a r y p h a s e s a r e p r e s e n t : p a l e g r e e n h o r n b l e n d e , c h l o r i t e and p r e h n i t e i n b i o t i t e , s a u s s u r i t e i n p l a g i o c l a s e , and e p i d o t e . 51. 5 2 . The h o r n b l e n d e g a b b r o s a r e g e n e r a l l y p e r i p h e r a l t o t h e q u a r t z d i o r i t e . They t e n d t o be more e x t e n s i v e n e x t t o s e d i m e n t s t h a n n e x t t o m e t a v o l c a n i c s ( s e e G e o l o g i c Map, P l a t e 2 ) . ( S e v e r a l s m a l l g a b b r o i c b o d i e s w i t h i n t h e q u a r t z d i o r i t e may o r may n o t be r e l a t e d t o t h e b o r d e r p h a s e . ) P e r i p h e r a l g a b b r o s b e t w e e n p l u t o n s and t h e V a n c o u v e r Group have been i n t e r p r e t e d as d i o r i t i z e d and m o b i l i z e d K a r m u t s e n ( K u n i y o s h i 1972; C a r l i s l e p e r s . comm. 1 9 7 5 ) . I n t h i s s t u d y a r e a t h e y a r e p r o b a b l y i n t r u s i v e . T h e r e a r e c l e a r t e x t u r a l d i s t i n c t i o n s b e t w e e n them and t h e m e t a v o l c a n i c s . I n a d d i t i o n , c o n t a c t s w i t h t h e V a n c o u v e r Group a r e s h a r p and i n t r u s i v e b r e c c i a s and d i k e s a r e common, p o i n t i n g t o l i q u i d e m p l a c e m e n t . The l a t e r q u a r t z d i o r i t e s h a r p l y i n t r u d e s t h e h o r n b l e n d e g a b b r o : c o n t a c t s a r e n e v e r t r a n s i t i o n a l , a l t h o u g h a s u c c e s s i o n of p r o g r e s s i v e l y more l e u c o c r a t i c p h a s e s may i n t e r v e n e , f o r e x a m p l e s o u t h o f Eden P o i n t . I n c o n t r a s t t o t h e q u a r t z d i o r i t e f o l i a t i o n i s a b s e n t ; i n c l u s i o n s i n t h e h o r n b l e n d e g a b b r o a r e n e v e r s t r e t c h e d o r f l a t t e n e d . I n s t r i k i n g c o n t r a s t t o t h e h o m o g e n e i t y o f t h e q u a r t z d i o r i t e , t h e h o r n b l e n d e g a b b r o s a r e h i g h l y v a r i a b l e i n g r a i n s i z e , t e x t u r e and c o m p o s i t i o n . P r o p o r t i o n s o f t h e m a i n p h a s e s , h o r n -b l e n d e and p l a g i o c l a s e , v a r y b o t h a c r o s s c o n t a c t s and g r a d a t -i o n a l l y w i t h i n u n i t s . Numerous h o r n b l e n d e h a b i t s - l a t h s , n e e d -l e s , a n h e d r a , p o i k i l i t i c g l o b s o r s q u a r e s , c l u m p s , v e i n s , s t r i n g s and l a y e r s - and s i z e s ( f r o m l e s s t h a n 1 mm. t o 5 cm.) p r i n c i p a l l y 5 3 . i n f o r m the t e x t u r e s ( F i g u r e 1 4 ) . P l a g i o c l a s e m a i n l y o c c u r s as s m a l l l a t h s w i t h s t r o n g n o r m a l z o n i n g (An n to An ) . j U o u C l i n o p y r o x e n e r e l i c s o c cupy th e c e n t e r s of some h o r n b l e n d e s . M a g n e t i t e i s u b i q u i t o u s . The h o r n b l e n d e g a b b r o s p o s s e s s s e v e r a l t e x t u r e s t y p i c a l o f the S i e r r a n c o m b - l a y e r e d b o d i e s d e s c r i b e d by Moore and L o c k -wood ( 1 9 7 3 ) . These a r e : 1) o r b i c u l e s , 2) c o m b - l a y e r e d v e i n s or d i k e s i n t h e M i n e r ' s Bay body n e x t to the h o r n b l e n d e g a b b r o s , and 3) h o r n b l e n d e - p l a g i o c l a s e v e i n s i n t h e g a b b r o . Most gabbr.os have been p e r v a s i v e l y a l t e r e d . They c o n t a i n s e v e r a l p h a s e s w h i c h a r e r a r e or a b s e n t i n the q u a r t z d i o r i t e : p y r i t e , sphene, l a r g e a p a t i t e i n c l u d e d i n h o r n b l e n d e , c a l c i t e and p u m p e l l y i t e , as w e l l as t h e more common a l t e r a t i o n p r o d u c t s . T h i s m i n e r o l o g y i s a l s o t y p i c a l of the S i e r r a n c o m b - l a y e r e d i n t r u s i o n s . Moore and L o c k -wood (1973) b e l i e v e c o m b - l a y e r i n g to form i n the p r e s e n c e of a s e p a r a t e f l u i d p h a s e . L o f g r e n and D o n a l d s e n (1975) have shown e x p e r i m e n t a l l y t h a t comb t e x t u r e s can r e s u l t from s u p e r c o o l i n g and r a p i d c r y s t a l l i z a t i o n r a t e s s u c h as m i g h t o b t a i n n e a r th e m a r g i n o f a p l u t o n i c body. The t e x t u r e s i n t h e h o r n b l e n d e g a b b r o s h e r e do n o t s u p p o r t a f i r m c h o i c e between the two m o d e l s . However the h e t e r o g e n e i t y o f t h e r o c k s may be e x p l a i n e d by l a r g e v a r i a t i o n s i n f l u i d p r e s s u r e . T h a t t h e y c r y s t a l l i z e d i n an a c t -i v e e n v i r o n m e n t i s shown by t h e d e f o r m a t i o n of the c o u n t r y r o c k s , by r e p e a t e d c r o s s - c u t t i n g and b r e c c i a t i o n w i t h i n t h e g a b b r o s , and by v e r t i c a l t r u n c a t i o n s and c h a n n e l s ( F i g u r e 15b, 17b, 18b) c) diarmlokmAtte, pla/jioclast-a) Typical qabbn •• hornblende yoikiliik to fadusitms (jruthcmwrhuriewJ}) d) Orbicular diktr-, CLarcvtctr-Jsltmd skeletal ; Jpiaqwclase euhedral^ strantLy ' worwiLl zoned . x 50 55. Table 1 - Comparison between l a y e r i n g i n h o r n b l e n d e  gabbros, cumulate and s c h l i e r e n l a y e r i n g . Hornblende Gabbros m i n e r a l Hornblende i n v a r i a b l y morphology a n h e d r a l - p o i k i l i t i c , clumped; i n some cases i n t e r s t i t i a l - s k e l e t a l . P l a g i o c l a s e e u h e d r a l l a t h s to e n h e d r a l . S c h l i e r e n L a y e r i n g ( W i l s h i r e 1969) M o s t l y e u h e d r a l , Cumulate L a y e r i n g (Wager and Brown 1968) E u h e d r a l (cumulate phases) p l u s o v e r -growths. I n t e r -cumulus m i n e r a l s may be p o i k i l i t i c a n h e d r a l . s k e l e t a l . m i n e r a l o r i e n t a t i o n P l a g i o c l a s e may be s u b p a r a l l e l to l a y e r s . Hornblende never o r i e n t e d . I d e a l l y l o n g axes i n p l a n t of low p o i n t i n g i n rlow d i r e c t i o n . Long axes commonly i n p l a n e of l a y e r i n g but not i n v a r i a b l y . C r e s c u m u l a t e t e x t u r e s form n e a r l y p e r p e n d i c u l a r to l a y e r i n g . shape of l a y e r s F i n e , c o n t i n u o u s . Bands or l e n s e s , i f c r y s t a l s e g r e g a t i o n s . Wispy and d i s c o n t i n -uous i f due to "smear-i n g " of i n h o m o g e n e i t i e s F i n e , c o n t i n u o u s . l a y e r o r i e n t a t i o n G e n e r a l l y v e r t i c a l or near v e r t i c a l ( F i g u r e 15b). Any o r i e n t a t i o n . Low d i p s ( g r a v i t y c o n t r o l l e d ) . s t r u c t u r e s T r u n c a t i o n s , c h a n n e l s , c r o s s - b e d s , " c l a s t i c " dikes,, g r a d i n g , i n t e r - " m i x i n g and c r o s s c u l c i n g of massive gabbros. B i f u r c a t i o n s ,.. t r u n c -a t i o n s , , c r o s s - b e d s , p l a s t i c ^ d e f o r m a t i o n , g r a d i n g , i n t e r m i x i n g w i t h massive phases. T r u n c a t i o n s , chan-n e l s , c r o s s - b e d s , g r a d i n g m o s t l y one way . *. b o t h ways. 56. b) Layered hornblende-rich gabbro truncated by Lighter imlayercd •phtast—'. 'ZNote the -near-vcrtfcat attitvdt of the layerina. - Clarence Jslund .. 57. As Lofgren and Donaldsen poin t out, d i s t u r b a n c e s could cause r a p i d changes i n f l u i d p r e s s u r e . The hornblende gabbros are best developed next to or near sedimentary i n t e r v a l s : the high water content of the sediments may have been important i n t h e i r g e n e s i s . D e l i c a t e c o m p o s i t i o n a l l a y e r i n g , due to rhythmic v a r i a t i o n i n p l a g i o c l a s e / h o r n b l e n d e r a t i o s , i s seen i n gabbros on Clarence I s l a n d , northern Yorke I s l a n d , and on the southern coast of West Thurlow I s l a n d . P l a g i o c l a s e s i n the l a y e r e d gabbros are g e n e r a l l y euhedral and are much smaller than the anhedral horn-blendes, which range up to 1 centimeter i n diameter (Figure 15a). The l a r g e r hornblendes p o i k i l i t i c a l l y enclose p l a g i o c l a s e : they are l a t e r than p l a g i o c l a s e i n the paragenetic sequence.- The r e v e r s e , p l a g i o c l a s e e n c l o s i n g hornblende, i s never seen. The l a y e r i n g shows c e r t a i n s i m i l a r i t i e s to s c h l i e r e n l a y e r i n g and to cumulate l a y e r i n g but d i f f e r s from both of them i n s i g n i f i c a n t r e s p e c t s (Table 1 and F i g u r e s 15 - 18). I t does not appear to be due to alignment and f l o w - s o r t i n g of p l a g i o c l a s e and hornblende phenocrysts: the hornblende i s anything but euhedral. In appe-arance the l a y e r i n g resembles accumulate l a y e r i n g , but the a t t i -tudes of the l a y e r s are n e a r l y v e r t i c a l . (Most l a y e r i n g of t h i s type seen i n the Coast Mountains i s s u b v e r t i c a l - Hutchison, pers. comm. 1975). P o s t c r y s t a l l i z a t i o n r o t a t i o n from n e a r - h o r i z o n t a l to n e a r - v e r t i c a l i s i m p l a u s i b l e , as bedding j u s t east of Clarence I s l a n d dips l e s s than 40°. I t i s thus c e r t a i n l a y e r i n g formed p a r a l l e l to the p l u t o n w a l l r a t h e r than to i t s f l o o r . 9. a) deformed layermj truncated by ftimar layers ; iaryt euhtdrcd horn blendes alonj *'unconformity " * Clarence Js b) layered jcebbro on left intruded by jabbro dike; dfkdtts cut xcross and between layers. J4oie dive-rymj Layers at br/er Itft. ~ CLurence JsUncL. b) ChsmnU , Clarinet Js. 6 1 . F i g u r e 19 shows a p o s s i b l e model f o r t h e f o r m a t i o n of h o r n b l e n d e - p l a g i o c l a s e l a y e r s a l o n g th e w a l l of a magma chamber. In F i g u r e 19a the h o r n b l e n d e - p l a g i o c l a s e c o t e c t i c i n a r e a l s y s t e m i s s c h e m a t i c a l l y r e p r e s e n t e d by t h e p l a g i o c l a s e - d i o p s i d e c o t e c t i c i n the w a t e r - f r e e t e r n a r y s y s t e m D i - Ab - An. A h y p o t h e t i c a l m e l t whose b u l k c o m p o s i t i o n i s shown a t A c o n t a i n s numerous s m a l l p l a g i o c l a s e c r y s t a l s and under n o r m a l c r y s t a l l i -z a t i o n i s w h o l l y i n t h e p l a g i o c l a s e f i e l d . I t comes to r e s t a g a i n s t a c o o l w a l l ( w a l l t e m p e r a t u r e , W, shown on t e r n a r y d i a -gram). I f no c o n v e c t i o n o c c u r s , ' s u p e r c o o l i n g w i l l i n d u c e r a p i d a c c r e t i o n to p l a g i o c l a s e n u c l e i n e x t to t h e w a l l . I f t h e r a t e . o f c r y s t a l l i z a t i o n of p l a g i o c l a s e o u t s t r i p s d i f f u s i o n r a t e s i n t h e m e l t , the m e l t c o m p o s i t i o n n e a r the w a l l w i l l change from A to B on the c o t e c t i c . P l a g i o c l a s e i n t h e l a y e r e d g a b b r o s i s s t r o n g l y n o r m a l zoned. A t B h o r n b l e n d e b e g i n s to c r y s t a l l i z e a l o n g w i t h p l a g i o c l a s e . The c o m p o s i t i o n a l g r a d i e n t w h i c h d e v e l o p s between the m e l t a t t h e w a l l and the b u l k m e l t i s , as shown i n F i g u r e 19b, a l s o a l i q u i d u s - t e m p e r a t u r e g r a d i e n t . F i g u r e 19b shows a s e r i e s o f l i q u i d u s p r o f i l e s d e v e l o p e d a t s u c c e s s i v e t i m e s i n f r o n t of a moving c r y s t a l - m e l t i n t e r f a c e . From time t ^ to .6 the r a t e of c r y s t a l growth i s g r e a t e r t h a n d i f f u s i o n . I t i s f a s t e s t on t h e c r y s t a l - m e l t i n t e r f a c e b e c a u s e t h e d e g r e e of s u p e r c o o l i n g i s g r e a t e s t t h e r e ; however c r y s t a l l i z a t i o n of p l a g i o c l a s e o c c u r s t h r o u g h o u t t h e b o u n d a r y l a y e r . Up to time t ^ p l a g i o c l a s e a l o n e c r y s t a l l i z e s . At t 1 h o r n b l e n d e can n u c l e a t e Oho- ij a. Vatk of milt composition in a. supercooled boundary layer (Ternary approximation of a. jjobbroic nilt) DISTANCE. FROM MOVINQ INTERFACE ^Apt* leftrtn xM Dt-nilAsim 197? 63. n e a r t h e i n t e r f a c e . As t h e l i q u i d u s t e m p e r a t u r e s i n t h e b o u n d a r y l a y e r p r o g r e s s i v e l y a p p r o a c h t h e a c t u a l t e m p e r a t u r e s , t h e r a t e o f c r y s t a l g r o w t h s l o w s . A t t ^ t h e r a t e o f c r y s t a l l -i z a t i o n a t t h e i n t e r f a c e i s e q u a l t o t h e r a t e o f p l a g i o c l a s e c r y s t a l l i z a t i o n i n t h e o u t e r b o u n d a r y l a y e r . Rapdid c r y s t a l l -i z a t i o n a t t h e i n t e r f a c e no l o n g e r d o m i n a t e s ; t h e w h o l e b o u n d a r y l a y e r c r y s t a l l i z e s more s l o w l y as a u n i t . A new c r y s t a l - m e l t i n t e r f a c e i s t h e n e s t a b l i s h e d t o t h e r i g h t o f t h e s e c o n d p l a g i o -c l a s e l a y e r ( t i m e t ^ ) and t h e s e q u e n c e i s r e p e a t e d . E a c h c y c l e c o n s i s t s o f t h r e e l a y e r s ; p l a g i o c l a s e - r i c h , h o r n b l e n d e - r i c h , p l a g -i o c l a s e - r i c h . S u c c e s s i v e l y y o u n g e r l a y e r s t h u s f o r m p a r a l l e l t o t h e i n i t i a l w a l l o f t h e p l u t o n and t h e l o c u s o f c r y s t a l l i z a t i o n moves p r o g r e s s i v e l y i n w a r d . T h i s m o d e l i s s i m i l a r t o t h o s e p r e s e n t e d by L o f g r e n and D o n a l d s o n ( 1 9 7 5 ) and S i b l e y e t a l . ( 1 9 7 6 ) t o a c c o u n t , r e s p e c t i v e l y , f o r c o m b - l a y e r i n g i n i g n e o u s r o c k s and f o r o s c i l l a t o r y z o n i n g o f p l a g i o c l a s e . I t s h o u l d be n o t e d t h a t t h e w a l l t e m p e r a t u r e W ( F i g u r e 19a) l i e s above t h e g a b b r o l i q u i d u s . The l a y e r e d g a b b r o s p r o b a b l y were n o t c o m p e t e l y s o l i d when t h e y f i r s t f o r m e d ; r a t h e r t h e y were a f r a m e w o r k c o n t a i n i n g some i n t e r s t i t i a l m e l t . T h i s i n t e r -s t i t i a l m e l t w o u l d p r o d u c e c l a s t i c d i k e s ( F i g u r e 18a) and a i d i n d e f o r m a t i o n o f t h e l a y e r s ( F i g u r e 1 7 a ) . The p r i n c i p a l d i f f e r e n c e s b e t w e e n t h e q u a r t z d i o r i t e and t h e h o r n b l e n d e g a b b r o s a r e 1) h o m o g e n e i t y v s . h e t e r o g e n e i t y , 2) f o l i a t i o n v s . l a c k o f d i r e c t i o n a l f a b r i c , 3) m i n o r v s . p e r v a s i v e a l t e r a t i o n , 4) p l a g i o c l a s e i n t h e q u a r t z d i o r i t e i s i n t r i c a t e l y 64. zoned and twinned while i n the gabbros i t i s s t r o n g l y normal zoned and simply twinned, and 5) hornblende i n the quartz d i o r i t e i s lath-shaped while i t s most common h a b i t i n the gabbro i s anhedral p o i k i l i t i c . Some of these d i f f e r e n c e s have been a t t r i b u t e d to the r o l e of the f l u i d phase. Throughout the main p a r t of the i n t r u s i o n i t would evolve (or d i f f u s e i n) slowly; but when the e a r l y gabbros encountered wet sediments l a r g e amounts of f l u i d would be a v a i l a b l e . The l a c k of f o l i a -t i o n and strong p l a g i o c l a s e zoning, may r e f l e c t r a p i d c o o l i n g . The gabbro t e x t u r e s would thus be due to c h i l l i n g of an e a r l y phase of the i n t r u s i o n . The h i s t o r y of the main i n t r u s i o n can be summarized as f o l l o w s : 1. The e a r l y gabbros rose from a magma chamber at depth along deep c r u s t a l f r a c t u r e s p a r a l l e l to the Telegraph H i l l F a u l t . 2. D i f f e r e n t i a t i o n ( c r y s t a l f r a c t i o n a t i o n ? ) gave r i s e to the quartz d i o r i t e which rose along the same f r a c t u r e s . It cut i n t o and b r e c c i a t e d the gabbros as w e l l as the Vancouver Group. At t h i s p o i n t the country rocks began to deform. 3. Expansion of the quartz d i o r i t e a f t e r emplacement due to continued i n t r o d u c t i o n of magma created the f o l i a t i o n w i t h i n i t and p e r s i s t e n t deformation out-s i d e i t . Two p o s s i b i l i t i e s f o r t h e l a t e c o o l i n g h i s t o r y and a l t e r a t i o n o f t h e q u a r t z d i o r i t e and t h e d i k e s i n i t w i l l be d i s c u s s e d i n t h e n e x t s e c t i o n s . I t may h a v e c o o l e d t o an a m b i e n t t e m p e r a t u r e w i t h i n t h e p r e h n i t e - p u m p e l l y i t e f a c i e s , o r t h e a l t e r a t i o n may have been due t o h e a t r e m a i n i n g w i t h i n t h e i n t r u s i o n i t s e l f . 66. i i i . R a d i o m e t r i c D a t i n g : Q u a r t z D i o r i t e A b s o l u t e ages o b t a i n e d 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 y o u n g e s t i n t r u s i o n on H a r d w i c k e and West T h u r l o w I s l a n d s ( F i g u r e s 20 and 21; T a b l e 2) l i e b e t w e e n 147 and 158 m.y. B o t h p o t a s s i u m - a r g o n m i n e r a l d a t e s and r u b i d i u m - s t r o n t i u m b i o t i t e i s o c h r o n s a r e c o n c o r d a n t w i t h i n e x p e r i m e n t a l e r r o r e x c e p t t h e b i o t i t e - h o r n b 1 e n d e p a i r f r o m s i t e 1 ( d i s c u s s e d b e l o w ) . The a p p a r e n t l y s i m u l t a n e o u s s e t t i n g o f t h e v a r i o u s r a d i o m e t r i c c l o c k s i m p l i e s a r a p i d t e m p e r a t u r e d r o p t h r o u g h t h e c r i t i c a l i s o t h e r m s . One i n t e r p r e t a t i o n w h i c h r e a d i l y e x p l a i n s t h e c o n c o r d a n c e i s t h a t t h e c l o c k s w e r e -set d u r i n g c r y s t a l l i z a t i o n o f t h e p l u t o n and n o t d i s t u r b e d s i n c e . The d i k e s w h i c h c u t t h e q u a r t z d i o r i t e h a v e been m e t a m o r p h o s e d a t t e m p e r a t u r e s above 200°C ( s e e C h a p t e r V I ) . H a r t ( 1 9 6 4 ) showed t h a t s i g n i f i c a n t d i f f u s i o n o f a r g o n and s t r o n t i u m f r o m b i o t i t e can o c c u r down t o 200°C. Thus i t i s p o s s i b l e t h a t t h e i n t r u s i o n c o o l e d t o an a m b i e n t t e m p e r a t u r e above t h a t r e q u i r e d f o r a r g o n and s t r o n t i u m r e t e n t i o n i n b i o t i t e and ( b e c a u s e o f t h e o b s e r v e d c o n c o r d a n c e ) above a r g o n r e t e n t i o n i n h o r n b l e n d e as w e l l . I f s o , t h e d a t e s o b t a i n e d do n o t r e c o r d i n t r u s i o n ; r a t h e r t h e y mark t h e end Jladtomctric ages a n d uncertainties r no • \J0 • 160 Lion • 150 • Jf 0 . '30 i b i 7w. 2 1 ry <Rb 87 / Sr 86 T a b l e 2 • R a d i o m e t r i c ARC Data From Quart's D i o r i t e sample no. m a t e r i a l Rb(ppra) whole rock b i o t i t e whole r o c k b i o t i t e 29.8 179.1 29.4 200.2 sample no. m a t e r i a l b i o t i t e h o r n b l e n d e b i o t i t e h o r n b l e n d e b i o t i t e h o r n b l e n d e sample no. f i e l d no. 5.28.1 5.16.1 8.14.1 Rb-Sr Sr(ppm) 3 6 6 . 6 3 2 . 0 7 384.9 24.02 K-Ar K. 6. 26 .584 5.59 .336 5. 80 .359 Rb/Sr-1 . 2 3 5 1 r . 0 0 4 7 0 1 6 . 2 0 8 3 - . 3 2 4 1 7 .22097.0042 24.2308-.4846 moles K40/gm 1.908x10"' l . 7 7 7 x l 0 " B 1.799x10 1.023x10 1.765x10" 1.093x10" -7 -8 Sr87/Sr86-1 s l o p e , 7 0 4 4 2 r . 0 0 1 6 . 7 3 9 4 8 - . 0 0 1 3 . 70425-.00054 . 75688-. 00046 .0021949 .0021920 moles Ar40/gm ( r o d ) 2.016x10 1 .509x10" 1 . 52 2x10 9 .446x10' 1.703x10 1.008x10" 10 11 10 i n t . .7039 + .0002 . 7037 +.0005 ap p a r e n t age(my) 1 5 4 ± 7 1 5 4 ^ 4 a p p a r e n t . a g e (my) 17 2j6 140-5 147^5 152-5 158±6 151-5 S i t e d a t a and l i t h o l o g i c d e s c r i p t i o n s f o r a n e - d a t l n R s a m p l e s . l a t i t u d e 50°26'15"N 50°25'11"N 50°23'5"N l o n g i t u d e 125°54'9"W 125 51'51"W 125°40*40"W l i t h o l o g y c o a r s e g r a i n e d e q u i -g r a n u l a r s l i g h t l y f o l i a t e d b i o t i t e -h o r n b l e n d e q u a r t s d i o r i t e . . same same A n a l y t i c a l Methods; P o t a s s i u m - P e r k l n - E l m e r 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 e r by Dr. R. L. A r m s t r o n g Argon "»•« s p e c t r o m e t r y , by Mr. Joe H a r a k a l R u b i d i u m / S t r o n t i u m - X-ray f l u o r e s c e n c e f o l l o w i n g t he method d e v e l o p e d by 3. Ryan (1973) S r 8 7 / S r 8 6 and t o t a l S t r o n t i u m Dr. P. L e C o u t e u r . mass s p e c t r o m e t r y , by SAMPLE PURITY Sample P e r c e n t I m p u r l t l e B hb b i c h l sphene t o t a l 1 hb 1.6 .6 2.2 1 b i 4.5 1.4 5.9 2 hb 1 . 2 1 2.2 2 b i .2 2.4 .2 2.8 3 hb .2 .3 3.2 3.7 3 b l 1.2 1.2 2.4 t o t a l 500 c o u n t a 70. o f a l o w - g r a d e m e t a m o r p h i c e p i s o d e o f unknown d u r a t i o n . C a r s o n ( p e r s . comm. 1974) c o n s i d e r s t h e y o u n g e r d a t e s f r o m t h e M i d d l e J u r a s s i c I s l a n d I n t r u s i o n s ( o f w h i c h t h i s may be one) t o r e c o r d m e t a m o r p h i s m a f t e r e m p l a c e m e n t . I n t h i s c a s e u p l i f t w o u l d have had t o be v e r y r a p i d i n o r d e r t o s e t a l l c l o c k s s i m u l t a n e o u s l y . ' A p r e f e r a b l e e x p l a n a t i o n , c o n s i s t e n t w i t h t h e o b s e r v e d c o n c o r d a n c e , i s t h a t t h e d i k e s were i n t r u d e d and s u b s e q u e n t l y m e t a m o r p h o s e d w h i l e t h e q u a r t z d i o r i t e was s o l i d i f i e d b u t s t i l l h o t ( s e e C h a p t e r V I ) . The 172 m.y. ( K / A r ) b i o t i t e and t h e 140 m.y. ( K / A r ) h o r n b l e n d e f r o m s i t e 1 a r e n o t c o n c o r d a n t . Does t h i s b i o t i t e p r e s e r v e an o r i g i n a l c o o l i n g a g e , w h i l e a l l o t h e r c l o c k s have been r e s e t by l o w - t e m p e r a t u r e metamorphism? Or has i t a c q u i r e d e x c e s s .argon e i t h e r i n t h e p a r e n t r o c k o r d u r i n g a n a l y s i s ? The l a t t e r p o s s i b i l i t y i s more p r o b a b l e . C a s e s o f K/Ar d a t e s e x c e e d i n g Rb/Sr d a t e s f r o m t h e same b i o t i t e a r e uncommon ( Y o r k and F a r q u h a r 1 9 7 2 ) . The d i s c o r d a n c e b e t w e e n K/Ar d a t e s on b i o t i t e and h o r n b l e n d e i s a l s o o p p o s i t e f r o m t h e t e m p e r a t u r e -r e s e t t i n g p r e d i c t e d by H a r t ' s d i a g r a m (1964) and n o r m a l l y o b s e r v e d i n r o c k s . W h e t h e r t h e Upper J u r a s s i c age o f t h e q u a r t z d i o r i t e i s t a k e n as c r y s t a l l i z a t i o n o r as p o s t - m e t a m o r p h i c c o o l i n g i t i s s t i l l one o f t h e o l d e s t ages so f a r o b t a i n e d w i t h i n t h e s o u t h e r n C o a s t P l u t o n i c Complex. I n f a c t , s i n c e C o a s t M o u n t a i n s i n t r u s -i o n s a r e g e n e r a l l y E a r l y C r e t a c e o u s t o Eocene and t h e I s l a n d I n t r u s i o n s M i d d l e J u r a s s i c , one m i g h t p r e f e r t o a s s i g n t h e q u a r t z 71. d i o r i t e on H a r d w i c k e and West T h u r l o w I s l a n d s to t h e I n s u l a r B e l t on the b a s i s of i t s age r a t h e r t h a n to the C o a s t P l u t o n i c Complex b e c a u s e of i t s p o s i t i o n . The i s s u e i n v o l v e d i s w h e t h e r the w e s t e r n b o u n d a r y of the C o a s t P l u t o n i c Complex s h o u l d be drawn a l o n g the w e s t e r n l i m i t o f C r e t a c e o u s i n t r u s i o n s or a l o n g the w e s t e r n l i m i t of p r e d o m i n a n t l y p l u t o n i c t e r r a n e . The l a t t e r i n t e r p r e t a t i o n i s f o l l o w e d h e r e , s i n c e the l a c k o f J u r a s s i c d a t e s i n t h e w e s t e r n C o a s t M o u n t a i n s may p a r t l y r e f l e c t p r e f e r -e n t i a l s a m p l i n g o f u n a l t e r e d b o d i e s (Woodsworth, p e r s . comm. 19 7 4 ) . T h i s p a r t of t h e p r o v i n c e b o u n d a r y , t h e n , i s a t l e a s t 148 m.y. o l d . Of c o u r s e th e p r e s e n t s t r o n g d i s t i n c t i o n between the two p r o v i n c e s d i d not e x i s t a t t h a t t i m e . The i n i t i a l S r 8 7 / S r 8 6 r a t i o s c a l c u l a t e d f r o m th e i s o c h r o n s a r e .7037 and .7039 ( T a b l e 2 ) . They a r e among the most " p r i m i t i v e " o b s e r v e d i n c o n t i n e n t a l i n t r u s i v e r o c k s ; t h e y c l o s e l y r e s e m b l e the r a t i o s f o u n d i n c i r c u m - P a c i f i c a n d e s i t e s . Such low r a d i o -g e n i c s t r o n t i u m r u l e s out b o t h th e C e n t r a l G n e i s s Complex and o l d e r s i a l i c c r u s t as p o s s i b l e p a r e n t s . I t does n o t , however, p r o h i b i t c r u s t a l o r i g i n . C u l b e r t (1970) o b s e r v e s t h a t t h e Rb/Sr r a t i o i n t y p i c a l C o a s t M o u n t a i n s i n t r u s i o n s i s so low t h a t S r 8 7 / S r 8 6 o n l y changes by .001 e v e r y 550 m.y. Thus two-s t a g e models f o r t h e o r i g i n o f p l u t o n s s u c h as t h i s one a r e j u s t as a c c e p t a b l e i s o t o p i c a l l y as s i n g l e - s t a g e d e r i v a t i o n from the u p p e r m a n t l e o r a s u b d u c t e d s l a b . The most c o n v i n c -i n g u ses of s t r o n t i u m i s o t o p e s as i n d i c a t o r s of c r u s t a l or sub-c r u s t a l o r i g i n depend on p a t t e r n s f r o m l a r g e a r e a s r a t h e r t h a n s i n g l e v a l u e s ( e . g . C h u r c h and T i l t o n 1973; K i s t l e r and P eterman 1973). Such a s y n t h e s i s i s n o t y e t p o s s i b l e f o r t h e C o a s t Mount-72. a i n s . L e C o u t e u r and T e m p e l m a n - K l u i t (1976)' have a s s e m b l e d t h e a v a i l a b l e s t r o n t i u m i s o t o p i c d a t a f r o m w e s t e r n B r i t i s h C o l u m b i a T h e i r summary shows c l e a r l y t h e p r e s e n t p a u c i t y o f d a t a i n t h e C o a s t M o u n t a i n s and t h e v a s t o p p o r t u n i t i e s f o r f u t u r e i s o t o p i c w o r k . 73. CHAPTER IV PRE-AND SYN-PLUTONIC STRUCTURE AND DIKES The t e c t o n i c h i s t o r y of t h i s a r e a i n i t s s i m p l i c i t y and m i l d n e s s i s more a k i n to t h e I n s u l a r B e l t t h a n t o t h e C o a s t P l u t o n i c Complex. The f i r s t phase o f d e f o r m a t i o n i n v o l v e d p r e -p l u t o n i c f a u l t i n g ; the s e c o n d , s y n p l u t o n i c d e f o r m a t i o n a t t r i b u t e d to f o r c e f u l i n t r u s i o n ; and t h e t h i r d , f r a c t u r i n g and minor f a u l t i n g w h i c h a c c o m p a n i e d p o s t - p l u t o n i c d i k e emplacement ( C h a p t e r VI) . i . P r e - P l u t o n i c D i k e s T h e r e a r e two s e t s of p r e - p l u t o n i c d i k e s i n t h e map a r e a . The f i r s t i n c l u d e s b a s a l t , a n d e s i t e and gabbro d i k e s and s i l l s p r o b a b l y r e l a t e d to t h e Karmutsen v o l c a n i c s u i t e . Wherever c r o s s -c u t t i n g r e l a t i o n s a r e seen t h e s e a r e t r a n s e c t e d by " g r e y " d i k e s of the s e c o n d s e t w h i c h i n c l u d e s a n d e s i t e s , b a s a l t i c a n d e s i t e s and h o r n b l e n d e p o r p h y r y d i k e s r e s e m b l i n g Bonanza f e e d e r s on V a n c o u v e r I s l a n d . T h e s e " g r e y " d i k e s a r e e q u a l l y abundant i n Karmutsen and Bonanza r o c k s . They a r e c o n t a c t metamorphosed to the same g r a d e as t h e i r s u r r o u n d i n g s . They a r e i n v o l v e d i n sy n - m e t a m o r p h i c f o l d i n g ; n e a r the i n t r u s i o n t h e y may be f o l i a t e d . The " g r e y " d i k e s a r e a l l s t e e p l y d i p p i n g to v e r t i c a l . They show no c l e a r l y p r e f e r r e d s t r i k e o r i e n t a t i o n ( F i g u r e 2 2 ) . I t i s r e a s -o n a b l e to i n f e r from t h i s t h a t no d i r e c t e d t e c t o n i c s t r e s s p r e v a i l e d d u r i n g t h e i r emplacement. I f t h e y a r e Bonanza d i k e s , t h e n the b e g i n n i n g of the f i r s t d e f o r m a t i o n a l e p i s o d e p o s t d a t e d Bonanza v o l c a n i s m . total -points 75. i i • Phase One ( P r e - P l u t o n i c ) : The Major F a u l t s There are two steep to v e r t i c a l f a u l t s of l a r g e d i s p l a c e -the north and south of Helmcken I s l a n d , and l i e mostly under water. The Johnstone S t r a i t F a u l t (Kuniyoshi 1972) forms a prominent topographic depression and slope break on Vancouver I s l a n d . The middle and upper Karmutsen Formations l i e n o r t h -east of i t (Domain I I ) ; the lower Karmutsen Formation l i e s to the southwest of i t (Domain I I I ) . The maximum p o s s i b l e s t r a t i g r a p h i c s e p a r a t i o n across i t i s 7,000 meters. The Telegraph H i l l F a u l t juxtaposes Bonanza and upper Karmutsen rocks along the northeast s i d e of Telegraph H i l l on Hardwicke I s l a n d . The maximum p o s s i b l e s t r a t i g r a p h i c s e p a r a t i o n across i t i s 5,000 meters. In Figure 23 the r o t a t i o n a l r e l a t i o n s h i p s between bedding i n adjacent domains are shown. The Johnstone S t r a i t / T e l e g r a p h H i l l f a u l t planes are assumed to be v e r t i c a l . The fi . pole r e l a t i n g Domains II and I I I i s 44/0. The r o t a t i o n path i s ne a r l y i n the f a u l t plane, as r e q u i r e d i n r o t a t i o n f a u l t i n g (Ragan 1968). The p . pole r e l a t i n g Domains I and II i s 314/32, l y i n g n e a r l y i n the f a u l t plane, as i f i t were an a x i a l plane. This type of motion, with a r o t a t i o n path p e r p e n d i c u l a r to the f a u l t plane, i s not t y p i c a l of s l i p along a plan a r f a u l t . The f a u l t s u r f a c e might curve at depth to become convex towards Domain I. A l t e r n a t i v e l y the Telegraph H i l l F a u l t may be on a x i a l plane, the broken 'back of a l a r g e f o l d with planar limbs. The l a t t e r ment i n the area. They trend approximately 1 2 0 ° , p a s s i n g to zJlofational relations kips between kmociinai domains yoles to tnxvor jauLls and shw ^ ant 77. p o s s i b i l i t y i s l i k e l y . The ^ . p o l e d e t e r m i n e d f o r t h e f a u l t c o i n c i d e s w i t h t h e axes o f the m e s o s c o p i c f o l d s . I n s t e a d o f f o l d i n g under c o m p r e s s i o n the V a n c o u v e r Group v o l c a n i c s f r a c t u r e d . R e v e r s e f a u l t i n g a t t h e h i n g e would a c c o u n t f o r t h e v e r t i c a l s e p a r a t i o n . S t e e p m i n o r f a u l t s on Helmcken and H a r d w i c k e I s l a n d s p a r a l l e l t h e major f a u l t s and a r e p r e s u m a b l y : r e l a t e d to them. Movement on t h e T e l e g r a p h H i l l F a u l t c e a s e d b e f o r e emplace-ment, o r a t l e a s t c o o l i n g , o f the main q u a r t z d i o r i t e : i t i s t r u n c a t e d by an i n t r u s i v e c o n t a c t on t h e r i d g e s o u t h w e s t of Clam Bay. The p a r a l l e l i s m between the major f a u l t s and the s t r i k e of t h e p l u t o n i c c o n t a c t s u g g e s t s t h a t the i n t r u s i o n r o s e a l o n g deep f a u l t s o f t h e same s y s t e m . S e v e r a l I n s u l a r B e l t p l u t o n s have u t i l i z e d p r e - e x i s t i n g f a u l t s and a r e c o n s e q u e n t l y e l o n g a t e p a r a l l e l to them ( S u t h e r l a n d Brown 1966). i i i . Phase Two: S y n - P l u t o n i c D e f o r m a t i o n The s e c o n d phase of d e f o r m a t i o n had a v a r i e t y of e x p r e s s -i o n s d e p e n d i n g on t h e s t r a i n - r e p o n s e c h a r a c t e r i s t i c s o f t h e v a r i o u s u n i t s . V a n c o u v e r Group v o l c a n i c s i n g r e e n s c h i s t f a c i e s ( o u t e r a u r e o l e ) were not s i g n i f i c a n t l y f o l d e d . I t i s l i k e l y t h a t t h e y a r e c u t by numerous s y n p l u t o n i c f a u l t s and f r a c t u r e s ; however t h e s e w o u l d have the same t r e n d as phase-one s t r u c t u r e s and thus c a n n o t be s e p a r a t e d from them. The c h e r t y i n t e r l a v a l e n s e s i n t h e upper Karmutsen F o r m a t i o n and t h i n - b e d d e d H a r b l e d o w n s e d i m e n t s d e v e l o p e d c o n s i s t e n t m e s o s c o p i c f o l d s . W i t h i n 75 m e t e r s o f t h e 78. o f t h e q u a r t z d i o r i t e / g a b b r o i n t r u s i o n c h a o t i c f o l d i n g a f f e c t e d t h e Q u a t s i n o and P a r s o n Bay F o r m a t i o n s . The B o n a n z a m e t a v o l -c a n i c s i n t h e i n n e r a u r e o l e u n d e r w e n t d u c t i l e d e f o r m a t i o n w h i c h p r o d u c e d a s t r o n g f o l i a t i o n . A l l o f t h e s e f e a t u r e s ( e x c e p t t h e c h a o t i c f o l d i n g ) a r e n o r t h w e s t t o w e s t - n o r t h w e s t t r e n d i n g ; most a r e d e m o n s t r a b l y s y n - m e t a m o r p h i c . E p i d o t e s t r i n g e r s a r e a b u n d a n t i n K a r m u t s e n v o l c a n i c s i n t h e o u t e r a u r e o l e o f t h e m a i n i n t r u s i o n . They f i l l s m a l l f r a c t u r e s and f a u l t s w h i c h g e n e r a l l y s t r i k e b e t w e e n 90° and 150° ( F i g u r e 2 4 ) . The e p i d o t e s e g r e g a t i o n i s p r o b a b l y l i n k e d t o g r e e n s c h i s t c o n t a c t m e t a m o r p h i s m ( C h a p t e r V) ; b u t many o f t h e f r a c t u r e s may be p r e -p l u t o n i c . M i n o r f o l d o r i e n t a t i o n s i n Domains I and I I ( F i g u r e 25) a r e c o n s i s t e n t e x c e p t i n v e r y t h i n i n t e r l a v a l e n s e s and i n s e d i m e n t s n e a r t h e i n t r u s i v e c o n t a c t . V e r g e n c e s o f t h e m e s o s c o p i c f o l d s i n b o t h d o mains a r e u n i f o r m l y s o u t h w a r d , i n d i c a t i n g s t r u c t u r a l c u l -m i n a t i o n t o w a r d s V a n c o u v e r I s l a n d . The a v e r a g e a x i a l p l a n e i s 123/58 NE., c o n s i d e r a b l y s t e e p e r t h a n b e d d i n g b u t n o t as s t e e p as t h e f o l i a t i o n o r m a j o r f a u l t s . The m i n o r f o l d s w e r e p r o b a b l y g e n e r a t e d by s h e a r i n g b e t w e e n t h e m a s s i v e v o l c a n i c s t h a t e n c l o s e t h e s e d i m e n t s ( F i g u r e 2 6 ) . A s e c o n d p h a s e o f d e f o r m a t i o n a f f e c t i n g t h e m e s o s c o p i c f o l d o r i e n t a t i o n s i s n o t a p p a r e n t on F i g u r e 25. O r i e n t a t i o n s o f t h e few f o l d s o b s e t v e d i n Domain I I c o i n c i d e w i t h t h o s e i n Domain I . 'total points 133 80. 81. rMinor Jolds in interlava sediments^ uyptr JCarmwtstn formation, 'West Thurlow Js. (Romania, C?) cUki cuts stdvmmts on left. Cliff vn uyyir lift background ts JCurtnutstYi fUm oytr^ lyfnq jfdiments . 82. T h i s would imply that r o t a t i o n on the Telegraph H i l l F a u l t had ceased before development of the minor f o l d s . The s t y l e of the f o l d s ( F igure 26), not t y p i c a l of b r i t t l e deformation, may i n d i c a t e that they formed during metamorphism. Within 75 meters of the quartz d i o r i t e i n t r u s i o n f o l d o r i e n t -a t i o n s l o s e c o n s i s t e n c y . A dramatic i n s t a n c e of c h a o t i c f o l d i n g i s seen i n the upper Quatsino and lower Parson Bay Formations east of V a n s i t t a r t P o i n t . Thin cherty beds are c h a o t i c a l l y f o l d e d and boudinaged; r o o t l e s s f o l d s of a l l o r i e n t a t i o n s swim i n a marble matrix. W o l l a s t o n i t e l a t h s a l i g n e d i n the a x i a l plane of at l e a s t one f o l d suggest that deformation occured at the peak of contact metamorphism. Two-centimeter to three-meter gabbroic dikes are f o l d e d and boudinaged. The i n c r e a s i n g i n t e n s i t y of deformation towards the i n t r u s i o n and the c o i n c i d e n c e of deformation with contact metamorphism suggest;; that f o r c e f u l i n t r u s i o n may have caused the deformation. If so, f o l d o r i e n t a t i o n s can show whether the motion between pl u t o n and country rock was e s s e n t i a l l y v e r t i c a l , p a r a l l e l to the c o n t a c t , or whether i t was compressional. F i g u r e 27a i s a p l o t of a x i a l planes and f o l d axes from t h i s exposure. F i g u r e 27b, shown f o r comparison, i s a t h e o r e t i c a l diagram a f t e r Hansen (1971) showing the f o l d axis/vergence d i s t r i b u t i o n s given the f o l l o w i n g c o n d i t i o n s : 1) the i n t r u s i v e contact i s v e r t i c a l and trends 110° and 2) that f o l d i n g i s completely due to shear between the pluton and the country rock, the p l u t o n moving r e l a t i v e l y upwards. In such a s i t u a t i o n the mesoscopic f o l d s 83. 84. w i l l be drag f o l d s v e r g i n g away from the pl u t o n . Since the sense of vergence i s by convention measured l o o k i n g down the f o l d a x i s , there are two po p u l a t i o n s of opposite vergences l y i n g i n the s l i p plane, separated by a v e r t i c a l s l i p v e c t o r . C l e a r l y the d i s t r i b u t i o n i n Figure 27a does not represent a simple case of v e r t i c a l shear. There are not two d i s t i n c t p o p u l a t i o n s of clockwise and counter-clockwise vergence, nor do the f o l d axes l i e i n the s l i p plane. Instead they c l u s t e r i n a s u b v e r t i c a l a r r a y . Shearing between p l u t o n and country rock may thus have been minor with compression p e r p e n d i c u l a r to the contact producing the c h a o t i c f o l d s . On the other hand, the steepness of the f o l d axes suggests extensive flow i n a v e r t i c a l d i r e c t i o n . Near Edwards' Homestead on Hardwicke I s l a n d , going u p s e c t i o n from Harbledown to Bonanza and towards the quartz d i o r i t e , bedding steepens to v e r t i c a l and the moderately d i p p i n g a x i a l planes i n the sediments are re p l a c e d by b e d d i n g - p a r a l l e l f o l i a t i o n i n the metavo l c a n i c s . F o l i a t i o n i s e x t e n s i v e l y developed i n Bonanza v o l c a n i c s and sediments i n the inner a u r e o l e , i n marbles, and i n places i n the Karmutsen w i t h i n a few meters of i n t r u s i v e cont-a c t s . At Miner's Bay, 500 meters from the i n t r u s i o n , m ineral f o l i a t i o n i n the marble i s c o n s i d e r a b l y steeper than bedding; by c o n t r a s t the inner aureole ( l e s s than 300 - 500 meters from the i n t r u s i o n ) f o l i a t i o n ( d e f ined by mineral o r i e n t a t i o n ; f l a t t e n i n g of b r e c c i a c l a s t s ) and bedding (defined by g r a i n s i z e and t e x t u r a l v a r i a t i o n s ) are p a r a l l e l and n e a r l y v e r t i c a l . The s t r i k e of the f o l i a t i o n ranges from 70° to 160° (Figure 28). Somewhat greater 86. s t r i k e c o n s i s t e n c y i s se e n i n t h e f o l l o w i n g a r e a s : s o u t h e r n West T h u r l o w I s l a n d ; H a r d w i c k e I s l a n d e a s t of Edwards' Homestead; between Edwards' Homestead and Pump C r e e k ; and n e a r Pump C r e e k . The f o l i a t i o n i n t h e s e a r e a s r o u g h l y p a r a l l e l s t h e l o c a l t r e n d o f t h e c o n t a c t ( i n s e t F i g u r e 2 8 ) . T h i s p a t t e r n r e s e m b l e s t h e accommodation o f an i n c o m p e t e n t m a t r i x to competent members d u r i n g d e f o r m a t i o n . However the newly i n t r u d e d p l u t o n w h i l e i t s u p p l i e d h e a t to t h e r e c r y s t a l l i z i n g m e t a v o l c a n i c s was a n y t h i n g but c o mpetent; i t was s t i l l o n l y p a r t l y s o l i d i f i e d ( s e e page 111)70-The p a r a l l e l i s m o f f o l i a t i o n and i n t r u s i v e c o n t a c t s may be b e s t e x p l a i n e d by c o n s i d e r i n g t h e i n t r u s i o n i t s e l f to be the s o u r c e of t h e d e f o r m a t i o n . S i n c e t h e c h a o t i c f o l d s may be i n t e r p r e t e d as r e f l e c t i n g c o m p r e s s i o n r a t h e r t h a n s h e a r between p l u t o n and c o u n t r y r o c k s , t h e d e f o r m a t i o n may be due to e x p a n s i o n of t h e i n t r u s i o n p e r p e n d i c u l a r to i t s w a l l s , i v . S ummary The s e c o n d phase s t r u c t u r e s have been l i n k e d i n t i m i n g and o r i e n t a t i o n to t h e main q u a r t z d i o r i t e i n t r u s i o n . The r e l a t i o n -s h i p s a r e shown i n t h e s t r u c t u r a l c r o s s - s e c t i o n , F i g u r e 29. P r e - p l u t o n i c d i k e s were emplaced i n an u n s t r e s s e d e n v i r o n m e n t , w h i l e c o n t a c t metamorphism p r o c e e d e d under d i r e c t e d s t r e s s . E a s t of V a n s i t t a r t P o i n t , d i k e s i n v o l v e d i n c h a o t i c f o l d i n g a r e t r a c e -a b l e to t h e e a r l y g a b b r o i c phase of the i n t r u s i o n ( C h a p t e r I I I ) . The i n t e n s i t y of d e f o r m a t i o n i n c r e a s e s towards t h e p l u t o n . The deve l o p m e n t of n e a r - v e r t i c a l f o l i a t i o n i n m e t a v o l c a n i c s and m a r b l e s 87. 88. may h a v e been c a u s e d by o u t w a r d e x p a n s i o n o f t h e p l u t o n . C l e a r l y , much o f t h e d e f o r m a t i o n i n t h e map a r e a i s t h e r e s u l t o f f o r c i b l e i n t r u s i o n . 89. CHAPTER V: METAMORPHISM Three metamorphic episodes can be d i s t i n g u i s h e d i n the map. area. The f i r s t , i n the Vancouver Group, i s due to b u r i a l ; the second i s contact metamorphism r e s u l t i n g from emplacement of the quartz d i o r i t e ; the t h i r d , d i s c u s s e d i n Chapter VI;', encompases low-temperature a l t e r a t i o n of the i n t r u s i v e rocks and p o s t - p l u t -onic dikes as w e l l as the ol d e r rocks. One of the ob j e c t s of t h i s study has been to determine the maximum depth of b u r i a l of the I n s u l a r B e l t - C o a s t P l u t o n i c Complex boundary, and the maximum metamorphic temperatures achieved near i t . Contact assemblages, i n both b a s i c and impure carbonate rocks, have been used. Carbonates are r e s t r i c t e d to the innermost aureole of the quartz d i o r i t e where t h e i r parageneses are u s e f u l i n e s t a b l i s h i n g maximum contact temperatures. Bas i c rocks are u b i q u i t o u s . They i n c l u d e Karmutsen b a s a l t s , Bonanza i n t e r m e d i a t e v o l c a n i c s , and p r e - p l u t o n i c dikes of probable Bonanza a f f i n i t y . Three metamorphic zones are recognized i n them: a l b i t e - e p i d o t e -a c t i n o l i t e ( g r e e n s c h i s t ) , p l a g i o c l a s e - a m p h i b o l e - e p i d o t e - c h l o r i t e (greensihist-hornblende hornf e l s - t r a n s i t i o n a l ) , and p l a g i o c l a s e -hornblende (hornblende h o r n f e l s ) , i . B u r i a l Metamorphism B u r i a l metamorphism has been o v e r p r i n t e d by contact e f f e c t s except i n the area around Kelsey Bay which l i e s o u t s i d e or n e a r l y o u t s i d e the au r e o l e . P i l l o w s i n the Kelsey Bay Formation there show, m i c r o s c o p i c a l l y , p e r f e c t p r e s e r v a t i o n of igneous t e x t u r e s . 90. S e c o n d a r y m i n e r a l s g e n e r a l l y do n o t t r a n s g r e s s g r a i n b o u n d a r i e s o f t h e o r i g i n a l p h a s e s , C l i n o p y r o x e n e p h e n o c r y s t s a r e u n a l t e r e d . P l a g i o c l a s e p h e n o c r y s t s a r e t h o r o u g h l y a l b i t i z e d and a r e f u l l o f c h l o r i t e and p u m p e l l y i t e m i c r o l i t e s . T h e s e s e c o n d a r y f e r r o m a g n e s -ium m i n e r a l s , w i t h i n c r y s t a l s more t h a n two m i l l i m e t e r s a c r o s s , i n d i c a t e c o n s i d e r a b l e i r o n and magnesium m o b i l i t y and a l s o t h a t t h e a l b i t i z a t i o n p r o c e s s was more c o m p l e x t h a n C a A l o S i o 0 o + N a + + S i 4 + = N a A l S i o 0 o + C a 2 + + A l 3 + 2. 2. o 3 o ( s e e K u n i y o s h i 1 972, 1972b; Surdam 1 9 7 3 ) . V o i d s b e t w e e n p i l l o w s a r e f i l l e d w i t h e p i d o t e , f e r r i a n c l i n o z o i s i t e , q u a r t z , c a l c i t e , and p u m p e l l y i t e . The t i n y i r r e g u l a r v e s i c l e s c o n t a i n c h l o r i t e . E p i d o t e - c h l o r i t e - q u a r t z v e i n l e t s , w h i l e common, a r e f i n e and n o t as p r o m i n e n t as t h e v e i n - n e t w o r k s of t h e a l b i t e - e p i d o t e - a c t i n o l i t e zone ( o u t e r a u r e o l e ) . The b u r i a l m e t a m o r p h i c a s s e m b l a g e n e a r K e l s e y Bay i s e p i d o t e -a l b i t e - c h l o r i t e-pump e l l y i t e - q u a r t z - c a l c i t e - f e r r i a n c l i n o z o i s i t e . The a b s e n c e o f a c t i o n o l i t e d e f i n e s i t as s u b - g r e e n s c h i s t . A c c o r d i n g t o K u n i y o s h i (1972) a c t i n o l i t e f o r m s a t a p p r o x i m a t e l y 400°C . The minimum t e m p e r a t u r e f o r t h i s a s s e m b l a g e i s d e f i n e d by t h e r e a c t i o n a n a l c i m e + q u a r t z = a l b i t e + H^O w h i c h p a s s e s t h r o u g h 200°, 2 kb. -P H 2 0 and 196°, 3 k b . P H 2 0 ( L i o u 1 9 7 1 ) . K u n i y o s h i ( 1 9 7 2 ) e s t i m a t e s t h e t e m p e r a t u r e o f r e g i o n a l m e t a m o r p h i s m i n t h i s t o be a p p r o x -i m a t e l y 350°, n e a r t h e h i g h - t e m p e r a t u r e l i m i t o f t h e p r e h n i t e -p u m p e l l y i t e f a c i e s . F u r t h e r w i t h i n t h e a u r e o l e t h e e f f e c t s o f b u r i a l m e t a m o r p h i s m a r e o b s c u r e d e x c e p t t h a t f e r r o m a g n e s i a n i n c l u s i o n s i n o r i g i n a l p l a g i o c l a s e p h e n o c r y s t s p e r s i s t t o t h e h i g h e s t t e m p e r a t u r e s . T h e s e 91. m i c r o l i t e s , u p g r a d e d t o a m p h i b o l e , s e r v e t o i d e n t i f y t h e i r h o s t f e l d s p a r s as p r i m a r y . i i . C o n t a c t M e t a m o r p h i s m a) A s s e m b l a g e s i n B a s i c R o c k s M i n e r a l a s s e m b l a g e s o f t h e g r e e n s c h i s t f a c i e s a r e f o u n d on H e l m c k e n I s l a n d and on H a r d w i c k e I s l a n d n e a r B e n d i c k s o n H a r b o r ( F i g u r e 3 0 ) . A l b i t e , e p i d o t e , " . a c t i n o l i t e , c h l o r i t e and q u a r t z c o n s t i t u t e t h e e s s e n t i a l m a t r i x a s s e m b l a g e i n t h i s z o n e ; i n a d d i t i o n m a g n e t i t e , s p h e n e , p y r i t e and c a l c i t e may be p r e s e n t . V e s i c l e s c o n t a i n e p i d o t e , q u a r t z and c h l o r i t e ( F i g u r e 3 1 a ) . The c h l o r i t e , p r o b a b l y p e n n i n i t e , i s n e a r l y i s o t r o p i c w i t h s t r o n g a n o m a l o u s b l u e b i r e f r i n g e n c e . E p i d o t e v e i n s and zo n e s o f e p i d o t e e n r i c h m e n t a r e p r o m i n e n t f e a t u r e s o f t h i s z o n e . I n t h i n s e c t i o n many a r e s e e n t o be g r a n u l a r c o n c e n t r a t i o n s t h a t r e p l a c e and g r a d e i n t o t h e s u r r o u n d i n g r o c k . Some v e i n s f o l l o w s m a l l f a u l t s w h i c h d i s p l a c e f l o w - t o p s o r d i k e s . A l t h o u g h v e i n o r i e n t a t i o n s a r e v a r i a b l e t h e y t e n d t o f o l l o w t h e w e s t - n o r t h w e s t s t r u c t u r a l g r a i n . Most a r e v e r t i c a l o r n e a r l y s o . The a r e a s o f e p i d o t e e n r i c h m e n t e i t h e r a r e i n pods and p a t c h e s o r f o l l o w t h e h i g h l y a m y g d a l o i d a l z o n e s n e a r f l o w - t o p s . M i c r o s c o p i c a l l y t h e e p i d o t e e n r i c h m e n t c o n s i s t s o f two e f f e c t s - r i d d l i n g w i t h e p i d o t e v e i n l e t s and t h e g r o w t h o f up t o 50% g r a n u l a r e p i d o t e i n t h e m a t r i x . A l b i t e and opaque m i n e r a l s a r e e n t i r e l y l a c k i n g i n t h e s e z o n e s . A c t i n o l i t e and c h l o r i t e p e r s i s t i n s m a l l q u a n t i t i e s . A m y g d u l e s c o n t a i n q u a r t z , . rX'\hmtUri b) GrruMsth'ist - hornblende homfels transition (n amyadulb-in uyyptr Xcurmuisen^ "West ThurtowJskrui t*5~0~j . VlajiocUse OMi ctifjsiie rtpl&ct efiiote mi. ipmurtt,. Tbt spcctUtd. Zf*ie a.rou*A -the rfm cumsHts of fine-jrtfnei. t-j>icLot( surriiuntlid by spUjftclastr: I'ht Clear splajfotlasej m Oit centre art a.fter lp*xrtz. Cmjiare efiiite habits witt O^yvrt Jia.. J • £ • 3 94. e p i d o t e and m i n o r c a l c i t e and p y r i t e . C h l o r i t e , an i m p o r t a n t a m y g d u l a r phase e l s e w h e r e , i s a b s e n t . E p i d o t e - r i c h zones g r a d e i n t o n o r m a l a l b i t e - e p i d o t e - a c t i n o l i t e - c h l o r i t e g r e e n s c h i s t . The c o n c e n t r a t i o n o f e p i d o t e i n f a u l t s , f r a c t u r e s and f l o w t o p s s u g g e s t s a m e t a s o m a t i c p r o c e s s . V e r t i c a l f a u l t s and f r a c t u r e s form l i k e l y passageways f o r m i g r a t i n g f l u i d s d u r i n g d e h y d r a t i o n . The h i g h l y a m y g d a l o i d a l f l o w t o p s must have been r e l a t i v e l y p e r m e a b l e , a c t i n g as c a t c h m e n t zones s e a l e d above by d e n s e , n e a r l y u n v e s i c u l a r f l o w b o t t o m s . J o l l y and S m i t h (1972) have s t u d i e d a s i m i l a r p a t t e r n o f e p i d o t e s e g r a t i o n i n t h e l o w e r P o r t a g e Lake b a s a l t s of n o r t h e r n M i c h i g a n . As i n the p r e s e n t a r e a , t h e y f o u n d a m y g d a l o i d a l f l o w -t o p s and f r a c t u r e s to be so e n r i c h e d i n e p i d o t e as to form " e l p i d -o t e metadomains". J o l l y and Smith model f l o w t o p s and f r a c t u r e s as p e r m e a b l e z o n e s . D u r i n g d e h y d r a t i o n w a t e r would e s c a p e from them more r e a d i l y t h a n f r o m m a s s i v e b a s a l t . I n t h e e p i d o t e meta-domains the c o n d i t i o n P T T n < P, , would p r e v a i l , t e n d i n g to H^O l o a d s t a b l i z e t h e r e l a t i v e l y a n h y d r o u s m i n e r a l e p i d o t e . J o l l y and S m i t h ' s a n a l y s e s f o r w a t e r c o n t e n t s t e n d to s u b s t a n t i a t e t h i s m odel. Un-a l t e r e d P o r t a g e Lake b a s a l t s c o n t a i n an a v e r a g e of 3.8 wt.% w a t e r ; a l b i t e b a s a l t 2.7% and e p i d o t e metadomains 1.1%. The p o s s i b i l i t y t h a t f l u i d p r e s s u r e was l e s s t h a n l o a d p r e s s u r e i n p a r t s of t h e K armutsen b a s a l t s d u r i n g g r e e n s c h i s t metamorphism a f f e c t s t h e a p p l i c a t i o n of e x p e r i m e n t a l d a t a to r e a c t i o n s o c c u r r i n g i n them as a means of d e t e r m i n i n g d e p t h o f b u r i a l . A more d e t a i l e d c o m p a r i s o n between c o n d i t i o n s i n t h e 95. Portage Lake S e r i e s and the Karmutsen Group i s t h e r e f o r e a p p r o p r i a t e . Epidote metadomains i n the Portage Lake b a s a l t s . are a s s o c i a t e d with uppermost z e o l i t e assemblages. In the Karmut-sen they occur with the g r e e n s c h i s t assemblage alb i t e-epido t e-a c t i n o l i t e - c h l o r i t e - m a g n e t i t e - s p h e n e - q u a r t z which i s i t s e l f r e l -a t i v e l y anhydrous. L i o u et a l . (1974) obtained a water content of 1.61 wt. % from a g r e e n s c h i s t c o n t a i n i n g 24% c h l o r i t e . Green-s c h i s t s from Helmcken and southern Hardwicke Islands g e n e r a l l y c o n t a i n l e s s than 10% c h l o r i t e ; t h e i r water contents may approach those of the epidote metadomains. I f , as J o l l y and Smith propose, present water contents are r e l i a b l e i n d i c a t o r s of r e l a t i v e PTT „, H 2 O / P, , i t can be argued that d i f f e r e n c e s between f l u i d and load load b pressure i n Karmutsen b a s a l t s were s m a l l . N e v e r t h e l e s s , water-pressure g r a d i e n t s provide an a t t r a c t i v e e x p l a n a t i o n f o r epidote s e g r e g a t i o n . I n t e r p r e t a t i o n of parageneses i s t h e r e f o r e s u b j e c t to a dual u n c e r t a i n t y : the exact value of PTT n/P, , i n a p a r t i c -H 2 O load u l a r rock, and the e f f e c t of t h i s r a t i o on the r e a c t i o n s observed i n i t . Less than 1250 meters from the quartz d i o r i t e , g r e e n s c h i s t assemblages give way to g r e e n s c h i s t - a m p h i b o l i t e t r a n s i t i o n a l assem-blages •(Figure 30). At the outer l i m i t of the t r a n s i t i o n a n o r t h i t e contents i n p l a g i o c l a s e i n c r e a s e sharply from near zero to g r e a t e r than 20%. Epidote, quartz, and c h l o r i t e are r e p l a c e d by i d i o b l a s t i c amphibole and new p l a g i o c l a s e (Figure 31b). Blue-green hornblende r e p l a c e s or grows s i d e - b y - s i d e with pale green a c t i n o l i t e . Quartz i n amygdules decreases and f i n a l l y d i s a p p e a r s . The inner l i m i t 96. o f t h e t r a n s i t i o n zone, a p p r o x i m a t e l y 500 m e t e r s from th e i n t r u s -i o n , i s marked by t h e d i s a p p e a r a n c e o f e p i d o t e and c h l o r i t e . The s i g n i f i c a n t t r a n s i t i o n a l a s s e m b l a g e s o b s e r v e d i n r o c k s f r o m H a r d w i c k e and West T h u r l o w I s l a n d s a r e the f o l l o w i n g : ( s e e F i g u r e 30 f o r l o c a t i o n s ) T^ : p l a g ( A n ^ g ) - a c t - h b - m t - e p - q z - a p - p y - s p h e n e - c h l ( a n d e s i t e d i k e ) : p l a g ( A n ^ Q ) - a c t i n o 1 i t i c h o r n b l e n d e - c h l - b i ( 5 % ) - m t - p y - q z (Bonanza) T^ : p l a g ( A n 2 0 ) - h b - c h l - e p - q z - m t - p y (Karmutsen) T^ : p l a g ( A n 2 0 ) - h b - a c t - e p - m t (Karmutsen) T^ : p l a g ( A n 3 0 ) - h b - a c t - c h l - e p - m t - i l m ; i n v e i n , e p - q z - g a (Karmutsen). ( p l a g = p l a g i o c l a s e , a c t = a c t i n o 1 i t e , hb=hornb1ende, m t = m a g n e t i t e , e p = e p i d o t e , q z = q u a r t z , a p = a p i t e , p y = p y r i t e , c h l = c h l o r i t e , b i = b i o t i t e , i l m = i l m e n i t e , g a = g a r n e t t ) The t r a n s i t i o n from g r e e n s c h i s t to a m p h i b o l i t e i n Karmutsen b a s a l t s has been i n v e s t i g a t e d e x p e r i m e n t a l l y by L i o u et a l . (1974) a t PJJ2Q, = ^ t o t a l a n ^ oxygen f u g a c i t i e s d e f i n e d by t h e q u a r t z - f a y -a l i t e - m a g n e t i t e b u f f e r . They c o n s i d e r t h e t r a n s i t i o n to be bounded by two r e a c t i o n s : 1) c h l + ep + ab + qz = o l i g + t s c h e r m a k i t e + Fe^O^ + H^O and 2. c h l + sphene + qz = A l - a m p h i b o l e + i l m + H^O ( u p p e r ) . The f i r s t i n v o l v e s a d e c r e a s e i n c h l o r i t e w i t h r i s i n g t e m p e r a t u r e ; th e s e c o n d i t s e l i m i n a t i o n . T h e s e r e a c t i o n s a r e shown s c h e m a t i c a l l y i n F i g u r e 32 a l o n g w i t h t h e r e a c t i o n ep + qz = g r a n d i t e ^ + an + mt ( L i o u 1973) w h i c h may o c c u r i n e p i d o t e - q u a r t z v e i n s . F i e l d 1 i s g r e e n s c h i s t ; F i e l d 2 i s a m p h i b o l i t e . The t r a n s i t i o n a l f i e l d s 3 to 6 a r e of g r e a t e s t i n t e r e s t . F i e l d 3 i s a l b i t e - e p i d o t e amphi-98. b o l i t e . In F i e l d 4 t r a n s i t i o n a l assemblages and e p i d o t e - q u a r t z segregations c o e x i s t ; i n F i e l d 5 amphibolites may c o n t a i n e p i d o t e -quartz v e i n s . F i e l d 6 i n c l u d e s c h l o r i t e - b e a r i n g assemblages i n which epid o t e - q u a r t z i s u n s t a b l e . Because of the d i f f e r i n g slopes of the c h l o r i t e - d e c r e a s i n g and c h l o r i t e - o u t curves g r e e n s c h i s t - a m p h i b o l i t e t r a n s i t i o n a l assemblages can be used as pressure i n d i c a t o r s . On Hardwicke and West Thurlow I s l a n d s a l b i t e amphibolite i s conspicuously absent; p l a g i o c l a s e r e p l a c e s a l b i t e at the outer border of the t r a n s i t i o n zone. Pressure c e r t a i n l y d i d not exceed, that at point A on F i g u r e 32. Epidote + quartz i s a s t a b l e v e i n and v e s i c l e assemblage i n T ^  and T^. They t h e r e f o r e l i e i n F i e l d 4. The r e a c t i o n ep + qz = gr = an + mt + H„0 i s i n f e r r e d i n T,. , as garnet occurs ss 2 5 i n contact with epidote and quartz i n the wider v e i n s . ( I n v e i n l e t s l e s s than 1 mm. across amphibole has r e p l a c e d ep-qz, i n d i c a t i n g exchange with the matrix over short d i s t a n c e s . ) Epidote and c h l o r i t e i n the matrix of t h i s .rock are sparse and t e x t u r a l l y u n s t a b l e . Epidote i n v e s i c l e s i s embayed and engulfed by amph-i b o l e . Small patches of c h l o r i t e are t r a n s e c t e d by i d i o b l a s t i c amphibole c r y s t a l s . and T^ are more d i f f i c u l t to i n t e r p r e t . T^ i s a Bonanza b a s a l t i c andesite c o n t a i n i n g no e p i d o t e . I t might f o r that reason f i t i n F i e l d 6. However the ACF diagram approximating assemblages i n F i e l d 4 shows that epidote may be e l i m i n a t e d e a r l y i n A l - r i c h , Ca-poor rocks. T y p i c a l Bonanza v o l c a n i c s c o n t a i n f a r l e s s calcium 99. t h a n K a r m u t s e n b a s a l t s ; some c o n t a i n a p p r e c i a b l y more a l u m i n u m ( M u l l e r e t a l 1974 T a b l e s 2 and 4 ) . The p o s i t i o n of i s u n -c e r t a i n . I t i s shown i n b o t h F i e l d s 2 and 6. T^ c o n t a i n s e p i d -o t e b u t no c h l o r i t e . T h e s e c h a r a c t e r i s t i c s p l a c e i t i n F i e l d 5. However T^ i s h i g h l y a m y g d a l o i d a l and c o n t a i n s enough e p i d o t e t o be an e p i d o t e m e t a d o m a i n , w h i c h as d e s c r i b e d above c o n t a i n l i t t l e o r no c h l o r i t e e v e n i n g r e e n s c h i s t f a c i e s . The c l o s e s p a t i a l a s s o c i a t i o n o f T^ and T^ s u g g e s t . t h a t T^ b e l o n g s i n F i e l d 4. The a b o v e a s s e m b l a g e s a l l l i e b e l o w p o i n t A. T,. and p o s s i b l y i n d i c a t e p r e s s u r e s b e l o w p o i n t B. P u t t i n g a n u m e r i c a l v a l u e on t h e p r e s s u r e s a t p o i n t s A and B r e q u i r e s a k n o w l e d g e o f t h e o x y g e n f u g a c i t i e s i n t h e r o c k s d u r i n g t h e i r m e t a m o r p h i s m . The p o s i t i o n s o f t h e c u r v e s , w h i c h i n v o l v e o x i d a t i o n - r e d u c t i o n r e a c t -i o n s , and t h u s o f t h e i r i n t e r s e c t i o n s , a r e v e r y s e n s i t i v e t o o x y g e n f u g a c i t y . The d i s p l a c e m e n t v e c t o r s shown on F i g u r e 32 f o r i n c r e a s i n g f g s h i f t t h e i n t e r s e c t i o n p o i n t s t o l o w e r p r e s s -u r e ( L i o u e t a l 1 9 7 4 ) . The v a l u e o f f . i n K a r m u t s e n m e t a b a s a l t s 0 2 i s u n c e r t a i n . L i o u e t a l t a k e t h e u b i q u i t y o f m a g n e t i t e + q u a r t z i n many a u r e o l e s t o s i g n i f y f_ h i g h e r t h a n t h a t d e f i n e d by t h e °2 QFM b u f f e r . T h i s a s s o c i a t i o n i s commonly o b s e r v e d h e r e . However, s m a l l amounts o f t i t a n i u m i n m a g n e t i t e t e n d t o s t a b i l i z e i t r e l a t i v e t o f a y a l i t e ( I . D u n c a n , p e r s . comm. 1974) r e n d e r i n g q u a r t z - m a g n e t i t e an u n r e l i a b l e i n d i c a t o r o f o x y g e n f u g a c i t y . The a p p l i c a t i o n o f t h e m a g n e t i t e - i l m e n i t e g e o t h e r m o m e t e r ( B u d d i n g t o n and L i n d s l e y 1964) t o K a r m u t s e n m e t a b a s a l t s w o u l d p r o v e a u s e f u l s t u d y . 100. While f grea t e r than QFM cannot be demonstrated, f,. l e s s (J 2 U 2 than QFM i s u n l i k e l y . In n a t u r a l systems oxygen f u g a c i t y i s c o n t r o l l e d by the phases and r e a c t i o n s present. The breakdown 3+ 2+ of epidote i n v o l v e s r e d u c t i o n of Fe to Fe . I t has a p o s i t i v e slope on a temperature-F diagram (Liou 1973): as temperature u2 inc r e a s e d the r e a c t i o n would move through i n c r e a s i n g oxygen f u g a c i t i e s ( Figure 33). The c h l o r i t e - d e c r e a s i n g r e a c t i o n would behave s i m i l a r l y . I t i s p o s s i b l e , then, that the g r e e n s c h i s t -amphibolite t r a n s i t i o n i t s e l f acted as an oxygen b u f f e r and that f i n c r e a s e d with r i s i n g temperature. With the QFM b u f f e r °2 p o i n t B l i e s s l i g h t l y above 3 kb (Liou et a l 1974). This provides a maximum estimate f o r load pressure ( h y d r o s t a t i c ) during contact metamorphism. The inner l i m i t of the t r a n s i t i o n i s about 500 meters from the quartz d i o r i t e . The temperature here, assuming QFM c o n d i t i o n s , was 550° (L i o u et a l 1974). Basi c assemblages t y p i c a l of the hornblende-hornfels f a c i e s are developed i n Karmutsen and Bonanza v o l c a n i c s l e s s than 500 meters from the i n t r u s i o n . Textures i n the Karmutsen are t y p i c -a l l y granoblas t i c , or s u b l e p i d o b l a s t i c i f dominated by hornblende. This hornblende lac k s the ragged (001) terminations of g r e e n s c h i s t and t r a n s i t i o n zone amphiboles. I t tends to form s t r a i g h t , r a t i o n -a l boundaries, p a r t i c u l a r i l y the prism f a c e s , although polygonal g r a i n s with 120° i n t e r s e c t i o n s are not uncommon. Hornblende c o l o r s vary from brownish green to brown. Rims and new c r y s t a l s of prograde texture are browner than the amphibole which they super-cede. Within a few centimeters of the quartz dior-ite the horn-blende i n the Karmutsen i s green, as i s that i n the i n t r u s i o n T °C 102 ( F i g u r e 34). C o m p o s i t i o n a l e q u i l i b r i u m between magma and c o u n t r y r o c k p r o b a b l y o b t a i n e d o v e r a s h o r t d i s t a n c e . P o i k i l i t -i c h o r n b l e n d e p r o p h y r o b l a s t s a r e d e v e l o p e d i n t h e i n n e r m o s t a u r e o l e . I n t e r g r o w n m a g n e t i t e and i l m e n i t e o c c u r as s c a t t e r e d round, g r a i n s o r clumps i n s i d e h o r n b l e n d e . P a t t e r n s of opaque m i c r o l i t e s i n h o r n b l e n d e may be r e l i c s o f t h e d e s t r u c t i o n of i g n e o u s c l i n o p y r o x e n e . P o i k i l i t i c m e t amorphic c l i n o p y r o x e n e o c c u r s i n some p a r a g e n e s e s . I t seems to c o n c e n t r a t e i n and a r o u n d amygdules.. I t a p p a r e n t l y r e f l e c t s e x c e p t i o n a l l y c a l c i c c o m p o s i t i o n due to l o c a l abundance of e p i d o t e . B i o t i t e and c u m m i n g t o n i t e p o r p h y r o b l a s t s have d e v e l o p e d i n t h e K armutsen m e t a b a s a l t s o n l y a d j a c e n t to i n t r u s i v e c o n t a c t s ( F i g u r e 34). L i k e g r e e n h o r n b l e n d e t h e y a r e p r o b a b l y the r e s u l t of s m a l l -s c a l e metasomatism. P l a g i o c l a s e c o m p o s i t i o n s i n K armutsen r o c k s o f t h i s zone, d e t e r m i n e d from e x t i n c t i o n a n g l e s , a r e An to A n o n . T h e i r z o n i n 4U o U p a t t e r n s a r e n o r m a l , p a t c h y , o r i r r e g u l a r . E x c e p t r a r e l y i n amyg d u l e s p l a g i o c l a s e does not f o r m p o r p h y r o b l a s t s . On the c o n t r a r y , p h e n o c r y s t s t e n d to r e c r y s t a l l i z e to s m a l l , x e n o b l a s t i c g r a i n s . New c r y s t a l s c r e a t e a p o l y g o n a l g r a n o b l a s t i c f a b r i c w h i c h b e a r s no r e s e m b l a n c e e i t h e r to the mat of p l a g i o c l a s e l a t h s i n the o r i g i n a l r o c k o r to t h e e u h e d r a l to s u b h e d r a l s t r o n g l y n o r m a l -zoned p l a g i o c l a s e s i n t h e p e r i p h e r a l gabbro phase of the main i n t r u s i o n . P o l y g o n a l f a b r i c i s r e t a i n e d even w i t h i n 2 mm. of i n t r u s i v e c o n t a c t s . P l a g i o c l a s e c o m p o s i t i o n s n e x t to the q u a r t z d i o r i t e , r a n g i n g from An^ f i to A n / R i n s t r o n g l y n o r m a l - z o n e d 10 3 . 0*8? Jf X JO Contact between Loctt- Jurassic quartz diorxtt and XarnuLtstn hornfels^ Jfardwicke JslcmcL, (the iarji ylajwclases at the -toy of the yhotojrayh cure ycurt of~~" •tht o/mrtz cLiorittr. CNoti thejnen korribliwcLe a.nd the L&rtjt skiUtai biotite- [UbeUecC) jrvwmj nnm.net small yiajiocU^s in tht hcrrytfUs.Hint harnjiLs has d yobjCjtmO'l tstytun dnd a fai-nt pliatfcm nnat -visible-in the yhfftojrayk • ^ PLa^iocLaj-t is strcmtjUy normal z-cnecL~ . 104. c r y s t a l s , a r e e q u i v a l e n t t o and p r o b a b l y i n e q u i l i b r i u m w i t h c o m p o s i t i o n s i n t h e i n t r u s i o n . B o n a n z a m e t a v o 1 c a n i c s , u n l i k e 'Karmutsen r o c k s , a r e f o l i a t e d e v e r y w h e r e w i t h i n t h e h o r n b l e n d e - p l a g i o c l a s e z o n e . B o n a n z a h o r n b l e n d e i s g r e e n . I f a c i c u l a r i t l i e s i n t h e f o l i a t i o n ( F i g u r e . 3 5 a ) . I t s most common h a b i t i s x e n o b l a s t i c t o sub i d i o b l a s t i c and e q u a n t . Such g r a i n s , a l t h o u g h l y i n g i n c o m p o s i t i o n a l b a n d s , a r e n o t i n d i v i d u a l l y o r i e n t e d . As i n t h e K a r m u t s e n , l a r g e p l a g i o c l a s e g r a i n s a r e b l a s t o -p o r p h y r i t i c . I n t h i n s e c t i o n t h e y a r e s e e n t o c o n t a i n p a t c h e s o r s y m m e t r i c a l z o n e s o f a m p h i b o l e m i c r o l i t e s , a p r o b a b l e r e l i c o f b u r i a l m e t a m o r p h i s m . The p l a g i o c l a s e s a r e r a n d o m l y o r i e n t e d and commonly b r o k e n and r e e r y s t a l l i z e d . The f o l i a t i o n bends a r o u n d them ( F i g u r e 3 5 a ) . I n t h e o u t e r p a r t o f t h e h o r n b l e n d e - p l a g i o c l a s e zone some a r e s t i l l o s c i 1 l a t o r y - z o n e d . T h i s i s a r e l i c t , n o t a p r o g r a d e f e a t u r e , a b s e n t c l o s e r t o t h e i n t r u s i o n . S t a b l e m e t a -m o r p h i c z o n i n g modes a r e n o r m a l , p a t c h y , o r i r r e g u l a r . A n o r t h i t e c o n t e n t s o f m e t a m o r p h i c p l a g i o c l a s e r a n g e f r o m An 0 t o An . T h i s 2. o / U 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 d i f f e r e n c e s i n b u l k c o m p o s i t i o n ; t h e r e i s no c o r r e l a t i o n w i t h d i s t a n c e f r o m t h e i n t r u s i o n . New p l a g i o c l a s e s a r e s m a l l r o u n d o r p o l y g o n a l g r a i n s , o r sub i d i o b 1 a s t i c l a t h s l y i n g i n t h e f o l i a t i o n . W i t h i n 70 m e t e r s o f p l u t o n i c c o n t a c t s t h e g r o w t h o f l a r g e p o i k i l o b l a s t i c to s k e l e t a l b i o t i t e , h o r n b l e n d e , and c u m m i n g t o n i t e p a r t l y o b s c u r e s t h e f o l i a t i o n ( F i g u r e 3 5 b ) . C u m m i n g t o n i t e i s n o t e w o r t h y i n t h a t i t h a s n o t b e e n p r e v i o u s l y r e p o r t e d f r o m I n s u -l a r B e l t c o n t a c t a u r e o l e s . I t i s c o l o r l e s s t o s l i g h t l y g r e e n 105 d) 3onanzii tuff C V in hornbltndb hornfcLs z.<yntr ( X fo) 7^3 A Hht foUntie-n, defined by bittitt fl-trf nch'noUU^ bends sharply ground. plajfaclase yorphyroclastsy <yyh/'ch^ ttncL -to lit With -thitr Unj qjis in -tht faliatfon • Tht larjt plaafoclait t*\t centre show rcUcr-aiefUafrry 2.CK!HJ . ^Atfi'mUtt needles JYM A all -pUji'itltjt ptrphyrteiasts • 0~h*y cerncintrate Tnt-arrt of- Chi tr*!- at th tvj> of tte ph.otcjfT.ph, when albrtfLatfrn wuld. hayt free-n iwtstr severe. b) 'Bonanza, hor/nfcLs ( x S'o) Larje bfotfbCj hornblende a,nd tu^mfnyto^ike jr*» across a foUtutity, defined by yiajroclase Laths. Hv. larot ^ Utjioanses kxn embayed borders a-ncL art fx tiu process of /ricrys+a.LLT2.r*a to maJL ycAobtastfc jrafwi. 7-19 • r 1 0 6 . w i t h numerous l a m e l l a r t w i n s . Two s e t s of g r e e n p l e o c h r o i c l a m e l l a e , p r e s u m a b l y o f c a l c i c a m p h i b o l e , a r e d i s c e r n i b l e . The c o m b i n a t i o n of l a r g e p o s i t i v e 2V and r e l i e f s i m i l a r to h o r n b l e n d e s e r v e to d i s t i n g u i s h c u m m i n g t o n i t e from c l i n o p y r o x e n e . S h i d o (1958) n o t e s the p r e s e n c e of c u m m i n g t o n i t e i n b a s i c r o c k s f r o m c o n t a c t a u r e o l e s i n t h e Abukuma P l a t e a u . She c o n s i d e r s i t a low p r e s s u r e phase f o r two r e a s o n s . The f i r s t i s i t s a s s o c i a t i o n w i t h o t h e r l o w - p r e s s u r e i n d i c a t o r s , a n d a l u s i t e i n p e l i t i c h o r n f e l s e s and the a c t i n o l i t e - p l a g i o c l a s e t r a n s i t i o n a l a s s e m b l a g e i n m e t a v o l c a n i c r o c k s . S e c o n d l y , a l i k e l y r e a c t i o n f o r i t s f o r m a t i o n i s : 7 t s c h e r + 10 qz = 14 an + 3 cumm + 4 ^ 0 T h e r e i s a c o n s i d e r a b l e volume i n c r e a s e f r o m l e f t to r i g h t (2044 v s . 2221 c c . f o r the s o l i d p h a s e s ) . (Note t h a t abundant f r e e q u a r t z f a v o r s c u m m i n g t o n i t e d e v e l o p m e n t , a c c o u n t i n g f o r i t s p r e s e n c e i n Bonanza but not Karmutsen h o r n f e l s e s . ) The p r e s e n c e of cumming-t o n i t e c o r r o b o r a t e s the l o w - p r e s s u r e c o n d i t i o n i n f e r r e d above from g r e e n s c h i s t - a m p h i b o 1 i t e t r a n s i t i o n a l p a r a g e n e s e s . S e v e r a l t e x t u r a l f e a t u r e s of K armutsen and Bonanza meta-v o l c a n i c s i n the h o r n b l e n d e - p l a g i o c l a s e zone d e s e r v e r e i t e r a t i o n as t h e y b e a r on t h e o r i g i n of t h e p l u t o n i c r o c k s , p a r t i c u l a r i l y the p e r i p h e r a l g a b b r o s . Rocks i n t h i s zone a l l have p o l y g o n a l g r a n o b l a s t i c or l e p i d o b l a s t i c f a b r i c s ( F i g u r e s 34, 3 5 ) . Near t h e p l u t o n i c c o n t a c t s a m p h i b o l e s and b i o t i t e d e v e l o p as l a r g e p o i k i l o b l a s t i c g r a i n s w h i c h a p p e a r to p o s t d a t e t h e f o l i a t i o n . 107. P l a g i o c l a s e e x h i b i t s no such tendency. I t t y p i c a l l y forms as small x e n o b l a s t i c to s u b i d i o b l a s t i c g r a i n s with simple zoning and twinning p a t t e r n s . I t i s d i s t i n g u i s h a b l e from the p l a g i o c l a s e i n the p e r i p h e r a l gabbros, and bears no resemblance to that i n the quartz d i o r i t e . Convincing arguments f o r d i o r i t i z a t i o n based on t r a n s i t i o n a l m e t a v o l c a n i c - p l u t o n i c textures cannot be made . b) Carbonate Metamorphic Assemblages Impure carbonate l a y e r s are common i n the i n t r a v o l c a n i c s e d i -ments .of: the upperKarmutseh Formation and i n the Quatsino and Parson Bay Formations on West Thurlow I s l a n d . Carbonates a l s o occur on Yorke I s l a n d and Hardwicke I s l a n d . Fanny I s l a n d i s an i s o l a t e d marble block surrounded by gabbro. Metamorphic minerals i n the impure carbonates i n c l u d e c a l c i t e , quartz, w o l l a s t o n i t e , c l i n o z o i s i t e , e p i d o t e , garnet, d i o p s i d e , i d o c r a s e , sphene, and p l a g i o c l a s e . A number of carbonate parageneseswere found to be u s e f u l i n determining temperature and P^ ,_ /P .. . They can be CO2 t o t a l approximated by parageneses i n the system CaO - Al^O^ - SiO^ - H^O - CO^ i n v e s t i g a t e d by Gordon and Greenwood (1971) and St o r r e and N i t s c h (1972). They are l i s t e d i n Table 3 and p l o t t e d on a s i m p l i f i e d v e r s i o n of Gordon and Greenwood's phase diagram ( F i g -ure 36). T o t a l pressure i n t h e i r experiments was 2 kb., c l o s e to the estimated t o t a l pressure i n the aureole. I t w i l l be noted that s e v e r a l p l o t t e d assemblages e i t h e r l i e on or bracket r e a c t i o n boundaries. The most s i g n i f i c a n t of these i s the r e a c t i o n g r o s s u l a r + quartz = a n o r t h i t e + w o l l a s t o n i t e . As 108. T a b l e 3 = Carbonate Parageneses Cj_. p l a g - ga - qz p l a g = A n 6 5 ga = g r o s ^ a n d r 5 0 (d±-sphene-ap) ( r e f r a c t i v e , index.) C 2- ga - p l a g (An 28-36) - qz; ga - ep ( l a y e r e d ) C.j. ga - wo - p l a g (sphene - py - d i ) . C 4- p l a g ( A n 6 8 ) - wo - ga (sphene - py - d i ) C^. cc - wo - qz Cg. cc - wo - qz - ga (ga & qz not t o u c h i n g . q z i n wo) C^. cc - q z - wo - ep - ga C g . cc - c z t - p l a g ; cc - wo - ga ( i d o c r a s e - d i ) ( l a y e r e d ) Cg. wo - ga ( i d o c r a s e - d i ) not d i a g n o s t i c C 1 Q . cc - wo - ga ( d i ) A b r e v i a t i o n s p l a g p l a g i o c l a s e ga g a r n e t qz q u a r t z d i d i o p s i d e ap a p a t i t e ep e p i d o t e wo w o l l a s t o n i t e y p y r i t e cc c a l c i t e c z t c l i n o z o i s i t e 109. J00 r ice 500 art a/nr J^zP 0 zorp^ T _ : x c o z 5Vacc -VT- Zkb. 5 Carbonate assemblages plotted in tht system CaO - M z 0 5 - S i O L - C O z - • H t O 400 .2, X co. LO i COL Qoriom and Qrtmwod 19 71 an-1- ca <• Xt0 jro-m Storrt und. Kitsch 1372. 110. o n l y s o l i d p h a s e s a r e i n v o l v e d t h e p o s i t i o n o f t h e c u r v e does n o t change w i t h f l u d i c o m p o s i t i o n : t h e t e m p e r a t u r e of t h e r e -a c t i o n i s a f u n c t i o n o f t o t a l p r e s s u r e a l o n e . The a s s e m b l a g e s w h i c h b r a c k e t t h i s r e a c t i o n were a l l c o l l e c t e d w i t h 30 m e t e r s o f t h e m a i n i n t r u s i o n ; t h e r e f o r e i t c a n be u s e d t o e s t i m a t e t h e maximum t e m p e r a t u r e i n t h e c o u n t r y r o c k n e x t t o t h e c o n t a c t . On G o r d o n and Greenwood's d i a g r a m f o r 2 kb. t o t a l p r e s s u r e t h e r e a c t i o n t e m p e r a t u r e i s s l i g h t l y l e s s t h a n 600°C. The r e a c t i o n b o u n d a r i e s d e p i c t e d on F i g u r e 36 a r e n o t q u a n t i t a t i v e l y a p p l i c a b l e t o t h e a s s e m b l a g e s a t h a n d . I t i s e v i d e n t f r o m t h e p h a s e s p r e s e n t ( i . e . d i o p s i d e , e p i d o t e , i d o c r a s e , p l a g i o c l a s e i n s t e a d o f a n o r t -h i t e , and g r a n d i t e so 1 i d - s o 1 u t i o n i n s t e a d o f g r o s s u l a r ) t h a t F e , Mg, and Na a r e p r e s e n t i n a d d i t i o n t o t h e f i v e c o m p o n e n t s o f G o r d o n and Greenwood's s y s t e m . To be u s e f u l , t h e p o s i t i o n s o f t h e r e a c t i o n s must be a d j u s t e d t o a c c o u n t f o r m i x i n g i n t h e p a r t -i c i p a t i n g p h a s e s . The d i s p l a c e m e n t s o f t h e l i n e g r o s + gz = an + wo due t o and-r a d i t e i n t h e g a r n e t and a l b i t e i n t h e p l a g i o c l a s e c a n be c a l c u l -a t e d f r o m t h e e q u i l i b r i u m c o n s t a n t i f i d e a l m i x i n g i s assumed (Greenwood l e c t u r e 1 9 7 4 ) . The b a l a n c e d r e a c t i o n i s : g r o s + qz = an + 2 wo. 2 The e q u i l b r i u m c o n s t a n t K = ( a ) ( a ) an wo (a , ( a ) g r o s ) q z 7 The e q u a t i o n f o r t h i s c o n s t a n t , u s i n g t h e d a t a o f Newton ( 1 9 6 6 ) , i s : 111. l o g K = 3492.2 + 4.4464 - .17060 (P - 1)' T T K o r T° = 3492.2 + .17060 ( P - 1 ) / T, , (P i s i n b a r s ) 4.4464 - l o g 1 Q K I n t h e a s s e m b l a g e C. a = .65 and a = .25 t h e r e f o r e log,„K = 1 an g r o s 6 1 0 .415. The r e a c t i o n p r o c e e d s a t 656.7°C. a t 1.5 k b . ; 677.8 ° a t 2 kb. ; and 720.2° a t 3 kb. T e m p e r a t u r e s a t t h e c o n t a c t , t h e n , l a y b e t w e e n 650 and 720°C. S i O I f c o n s i d e r a b l e Fe 0 i s added t o t h e s y s t e m CaO - A l 0 2 o 2 3 2 - H 20 - C 0 2 t h e l o w - t e m p e r a t u r e l i m i t o f g a r n e t s t a b i l i t y a t v e r y low i s d e f i n e d by: 2 ep + 2 c c + 3 wo = g r a n d i t e + 2 CO + IH 0 ( C h a t t e r j e e S S I I 1966) r a t h e r t h a n b y : 2 zo + 5 cc + 3 qz = 3 g r o s + 5 C 0 2 + 2 H^O. C h a t t e r j e e ' s n u m e r i c a l d a t a f o r g r a n d i t e b r e a k d o w n a r e n o t a p p l i c a b l e b e c a u s e f . was u n b u f f e r e d i n h i s r u n s and P ., was 0 2 t o t a l h e l d a t 1 k b . The g e o m e t r i e s o f h i s r e a c t i o n s a r e , h o w e v e r , i n t e r -e s t i n g . They a r e r e p r o d u c e d on F i g u r e 37 w h i c h shows s c h e m a t i c a l l y t h e e f f e c t o f f e r r i c i r o n on G o r d o n and Greenwood's s y s t e m . I n t h e i r o n - f r e e s y s t e m z o i s i t e and w o l l a s t o n i t e c a n n o t c o e x i s t ( K e r r i c k e t a l 1 9 7 3 ) . E p i d o t e and w o l l a s t o n i t e c a n , and a r e o b s e r v e d h e r e . The a s s e m b l a g e c c - q z - w o - e p - g a f a l l s on an i n v a r i a n t p o i n t on C h a t t e r j e e ' s d i a g r a m . C a l c i t e and q u a r t z i n i t a r e i n t h e p r o c e s s o f r e a c t i n g t o f o r m w o l l a s t o n i t e , b u f f e r i n g X n t o t h e C 0 2 .112. 1 1 3 i n v a r i a n t p o i n t . t h e r e a c t i o n s cp + c c + wo = Gr + C0„ + H„0 s s 2 2 and ep + wo .= 8 r s s + gz + R^O w o u l d t h e n have p r o c e e d e d s i m u l t -a n e o u s l y u n t i l one r e a c t a n t was u s e d up. T h i s p athway i n d i c a t e s an i n i t i a l l y v e r y w a t e r - r i c h f l u i d b e c o m i n g somewhat l e s s so w i t h r i s i n g t e m p e r a t u r e . H i g h X _ i s a l s o e v i d e n t i n t h e o t h e r H 2 U h y d r o u s p h a s e s p r e s e n t i n t h e a u r e o l e - c l i n o z o i s i t e and i d o c r a s e -and i n t h e p r e s e n c e o f g a r n e t w h i c h r e q u i r e s l o w X__. ( F i g u r e 3 6 ) . C 0 2 A h y d r o u s f l u i d p h a s e f a v o u r s t h e f o r m a t i o n o f w o l l a s t o n i t e a t r e l a t i v e l y l o w t e m p e r a t u r e s ; t h e f i r s t a p p e a r a n c e o f w o l l a s t o n -i t e h e r e ( ) i s a p p r o x i m a t e l y 500 m e t e r s f r o m t h e i n t r u s i o n w here t h e t e m p e r a t u r e was n e a r 550°C. The o c c u r r e n c e o f t h e a s s e m b l a g e cc-qz-wo a t s e v e r a l d i s t a n c e s f r o m t h e i n t r u s i o n s u g g -e s t s p o s s i b l e b u f f e r i n g o f X „ N by t h i s r e a c t i o n . W i t h a l l t h e s e L. (J 2 i n d i c a t o r s i t i s s a f e t o s a y t h a t X ^ Q i n most c a r b o n a t e s was a t l e a s t 0.8. c. C o n c l u s i o n s T h i s s t u d y o f c o n t a c t m e t a m o r p h i c a s s e m b l a g e s has p r o d u c e d f i v e q u a n t i t a t i v e r e s u l t s . T h e y a r e as f o l l o w s : F . -j ^ 3 kb. t o t a l X _ „ i n c a r b o n a t e s £0.2 L U 2 T e m p e r a t u r e s d u r i n g c o n t a c t m e t a m o r p h i s m ( m a x i m a ) : b e t w e e n 200° and 400°, p r o b a b l y c l o s e t o 350° ( K u n i y o s h i 1972) * 6 km f r o m t h e m a i n i n t r u s i o n 550° 500 m e t e r s f r o m t h e i n t r u s i o n ( u p p e r l i m i t o f g r e e n -s c h i s t - a m p h i b o l i t e t r a n s i t i o n , QFM c o n d i t i o n s ) 650 - 72 0° a t t h e c o n t a c t ( g r + qz = a n + w o ) 114 . Since these r e s u l t s w i l l be used to make important i n f e r e n c e s a caveat should be o f f e r e d before proceeding. They are sub j e c t to s e v e r a l u n c e r t a i n t i e s : oxygen f u g a c i t y , phase compositions, the assumption of i d e a l mixi'ng, P„ _/P, . i n the metavolcanics and load experimental u n c e r t a i n t i e s . The f i r s t two could be e l i m i n a t e d by extensive microprobe work. The three temperatures determined i n the aureole d e f i n e a curve (Figure 38) l i k e those computed by Jaeger (1959). A f u r t h e r r e s t r i c t i o n oh pressure can be drawn from t h i s temperature-d i s t a n c e curve. The g r e e n s c h i s t - a m p h i b o l i t e t r a n s i t i o n commences 1.3 km. from the i n t r u s i o n . The temperature at that p o i n t a c c o r d i n to F i g u r e 38 i s l e s s than or equal to 500°C . For QFM c o n d i t i o n s such a s i t u a t i o n o btains only i f P ., 2.5 kb. (Liou et a l 1974) t o t a l The maximum s t r a t i g r a p h i c load at the top of the Karmutsen Formation on Vancouver I s l a n d i s 1.5 kb. I f the maximum pressure i n the present area d i d not exceed 2.5 kb. i t i s reasonably c e r t -a i n that we are d e a l i n g with the Vancouver I s l a n d s t r a t i g r a p h i c column. Therefore here, at the edge of the Coast P l u t o n i c Complex, n e i t h e r depth of b u r i a l or depth of e r o s i o n have s i g n i f i c a n t l y exceeded c o n d i t i o n s anywhere i n the I n s u l a r B e l t . Jaeger's c a l c u l a t i o n s allow the i n i t i a l temperature of an i n t r u s i o n to be i n f e r r e d from the temperature at i t s c o n t a c t s . It i s assumed that i n t r u s i o n i s instantaneous. I f f u r t h e r i t i s assumed that there i s no l a t e n t heat of c r y s t a l l i z a t i o n then the simple r e l a t i o n ( T - T . . . . . ) = h ( T. contact i n i t i a l i n t r u s i o n -T •. . . , ) holds, where the i n i t i a l temperature i s that of i n i t i a l 0. 650 (bOO 550 1 ( Z k b ) max. ^emperaturer ys. Distance from the Contact-Jnmr Limitojimnntion (assuming QTM) Outer Ifmltraj grans chist-« transitional asswiblLti. .2 fr0 ( D * Width of intrusion. = Wdtk/of awriolt (?) ^5hnJ 1 1 6 . the country rocks before emplacement of the p l u t o n . T i s ^ contact at most 700°C. The ambient temperature, using Kuniyoshi's estimate (1972), i s 350°. T. ^ . i s then 1050°. I f l a t e n t i n t r u s i o n heat i s allowed f o r i t i s approximately .1000°. d) S t r u c t u r e and Metamorphic Textures Pervasive deformation i n the a l b i t e - e p i d o t e - a c t i n o l i t e zone i s c onfined to close-spaced shear-planes i n the f a u l t zones. Stress-indueed r e e r y s t a l 1 i z a t i o n has not o c c u r r e d . V e s i c l e s are equant; r e l i c t phenocrysts, even i f bent or broken, r e t a i n : t h e i r o r i g i n a l form. A c t i n o l i t e needles form a n o n - d i r e c t i o n a l matte. These t e x t u r e s i n d i c a t e s l u g g i s h r e c r y s t a l l i z a t i o n . Under g r e e n s c h i s t metamorphism the Karmutsen Group behaved as competent; and b r i t t l e b l o c k s . I t s response to t e c t o n i c s t r e s s was l i m i t e d to f a u l t i n g , as d e s c r i b e d i n the l a s t chapter. F o l i a t i o n i s r a r e l y observed i n Karmutsen metabasalts. Bonanza rocks, by c o n t r a s t , a c q u i r e a f o l i a t i o n i n the innermost greenschist-amphibo1ite t r a n s i t i o n zone which c h a r a c t e r i s e s them throughout the inner aureole. This planar f a b r i c can be a t t r i b u to three f e a t u r e s : the f l a t t e n i n g of b r e c c i a c l a s t s , alignment of b i o t i t e and a c i c u l a r hornblende, and the a c c e n t u a t i o n of d e l i c a t e primary m i n e r o l o g i c a l / t e x t u r a l l a y e r i n g i n t u f f a c e o u s u n i t s by r e c r y s t a 1 1 i z a t i o n . Why Bonanza rocks were able to develop f o l i a t i o n i n response to s t r e s s while Karmutsen rocks g e n e r a l l y were not may be explained i n s e v e r a l ways. 1. B i o t i t e , which r e a d i l y assumes a planar o r i e n t a t i o n , i s abundant i n Bonanza metavolcanics. 117. 2. Q u a r t z i s a l s o a b u n d a n t . I t d e f o r m s w i t h e a s e a t much l o w e r t e m p e r a t u r e s t h a n h o r n b l e n d e and p l a g i o -c l a s e , t h e p r i n c i p a l c o n s t i t u e n t s o f K a r m u t s e n h o r n f e l s e s . 3. Some o f t h e " f o l i a t i o n " o b s e r v e d i n t h e f i e l d may a c t u a l l y be b e d d i n g f e a t u r e s t r a n s p o s e d p a r a l l e l t o t h e b i o t i t e s c h i s t o s i t y . A c c o r d i n g t o s t r u c t u r a l e v i d e n c e ( C h a p t e r I V ) t h i s f o l i a t i o n r e s u l t e d f r o m f o r c i b l e e m p lacement o f t h e q u a r t z d i o r i t e . T e x t u r a l r e l a t i o n s h i p s show t h a t i t a c c o m p a n i e d c o n t a c t metamor-p h i s m . Q u a r t z - b i o t i t e - p l a g i o c l a s e and h o r n b l e n d e v e i n s a r e f o l d e d w i t h a x i a l p l a n e s p a r a l l e l t o t h e f o l i a t i o n . S m a l l b i o t i t e p l a t e s a r e o r i e n t e d . I n one e x a m p l e o f r e f o l d i n g i n a s i l t s t o n e l e n s a s e c o n d a x i a l - p l a n e a l i g n m e n t s u p e r c e d e s b i o t i t e b e d d i n g -p a r a l l e l a l i g n m e n t . E x c e p t i m m e d i a t e l y a d j a c e n t t o t h e q u a r t z d i o r i t e s t a t i c r e c r y s t a l l i z a t i o n has n o t o v e r p r i n t e d t h e f o l i a t i o n . T h i s s u g g e s t s t h a t d e f o r m a t i o n : a f f e c t e d t h e m e t a v o l c a n i c s a t t h e maximum t e m p e r a t u r e s a c h i e v e d . A c c o r d i n g t o J a e g e r ' s d a t a (1957 F i g u r e 1 ) , t h e t e m p e r a t u r e maximum a t t h e o u t e r l i m i t o f d e v e l o p e d f o l i a t i o n , 500 m e t e r s f r o m t h e i n t r u s i o n , p e r s i s t e d t o a b o u t 320,000 y e a r s a f t e r i t s e m p l a c e m e n t . D u r i n g t h i s t i m e - i n t e r v a l , o f c o u r s e , t h e p l u t o n i t s e l f was c o o l i n g . The d i s t a n c e o f t h e p l a n e o f s o l i d i f i c a t i o n f r o m t h e c o n t a c t 320,000 y e a r s a f t e r i n t r u s i o n i s a b o u t 750 m e t e r s ( f r o m J a e g e r 1957 e q u a t i o n ( 9 ) ) . I t i s t h u s p r o b a b l e t h a t s o l i d u s o r n e a r - s o l i d u s c o n d i t i o n s p r e v a i l e d i n t h e o u t e r p a r t o f t h e i n t r u s i o n w h i l e t h e c o u n t r y r o c k s were s t i l l d e f o r m i n g . T h i s i s s u b s t a n t i a t e d by t h e d e f o r m -118. a t i o n o f i n c l u s i o n s i n t h e q u a r t z d i o r i t e ( C h a p t e r I I I ) , w h i c h p r o b a b l y o c c u r r e d a t a l a t e s t a g e o f c r y s t a l l i z a t i o n . The s i m u l t a n e o u s d e v e l o p m e n t o f f o l i a t i o n i n t h e q u a r t z d i o r i t e and i t s c o u n t r y r o c k s may have r e s u l t e d f r o m c o n t i n u e d u p r i s e o f magma i n t o t h e c e n t e r o f t h e i n t r u s i o n , c a u s i n g i t t o e x p a n d o u t w a r d s . 119. CHAPTER V I : POST-PLUTONIC STRUCTURE AND DIKES F r a c t u r i n g , f a u l t i n g d i k e emplacement, and l o w - t e m p e r a t u r e metamorphism f o l l o w e d c r y s t a l l i z a t i o n o f t h e q u a r t z d i o r i t e . T h e s e p o s t - p l u t o n i c s t r u c t u r e s t r e n d n o r t h e a s t e r l y , n e a r l y p e r p e n d i c u l a r to the e a r l i e r s t r u c t u r a l g r a i n . N o r t h e a s t - t r e n d i n g j o i n t s e t s a r e w i d e s p r e a d b o t h i n t h e p l u t o n and i n t h e V a n c o u v e r Group. P o s t - p l u t o n i c d i k e s t e n d to s t r i k e p a r a l l e l to l o c a l j o i n t s e t s ( F i g u r e 39). A p l o t of d i k e o r i e n t a t i o n s ( F i g u r e 40) has a w e l l -d e f i n e d maximum n e a r N 45 E. The p a t t e r n of s t r a i g h t e a s t - n o r t h -e a s t v a l l e y s on H a r d w i c k e I s l a n d f o l l o w s a s y s t e m of c h l o r i t i z e d f r a c t u r e z o n e s . The b o r d e r o f t h e q u a r t z d i o r i t e i s n o t s i g n i f -i c a n t l y o f f s e t a c r o s s t h e s e l i n e a r s . F a u l t i n g was g e n e r a l l y l e s s i m p o r t a n t t h a n f r a c t u r i n g . The o n l y major n o r t h e a s t t r e n d i n g f a u l t i n t h e a r e a o f f s e t s t h e i n t r u s i v e c o n t a c t 200 m e t e r s i n a r i g h t -l a t e r a l s e n s e and j u x t a p o s e s Q u a t s i n o and upper K a r m u t s e n Form-a t i o n s i n M i n e r ' s Bay; i t s p r o b a b l e e x t e n s i o n s o u t h of J o h n s t o n e S t r a i t j u x t a p o s e s upper and m i d d l e Karmutsen F o r m a t i o n s ( K u n i y -o s h i 1972). P o s t - p l u t o n i c d i k e s a r e d i s r u p t e d i n a number of l o c a l i t i e s , s u g g e s t i n g s m a l l - s c a l e movements. P o s t - p l u t o n i c d i k e s c o n s t i t u t e f i v e p e r c e n t o f o u t c r o p volume a l o n g t h e c o a s t between P a t t e r s o n Bay and the s o u t h e a s t e r n t i p of H a r d w i c k e I s l a n d : t h e i r emplacement i n v o l v e d a f i v e p e r c e n t n o r t h w e s t - s o u t h e a s t c r u s t a l e x t e n s i o n . B e c a u s e of p e r v a s i v e e p i d o t e - c h l o r i t e a l t e r a t i o n the d i k e s a r e t y p i c a l l y g r e e n . They c r o s s c u t each o t h e r a t many l o c a l i t i e s ( F i g u r e 41) but i n no a p p a r e n t p e t r o l o g i c s e q u e n c e . They r a n g e from h o r n -b l e n d e b a s a l t to d a c i t e w i t h a n d e s i t e p r e d o m i n a n t ( T a b l e 4 ) . . 1 . 2 1 . O2?^ ±0 J Strike orientations aj'yostylutmic dikes CN ^ 7 total points : f f J Cross- cultinj post-plutonic dikes Chilled, mora in visible on Inter cLiktr: T a b l e 4 3 P e t r o g r a p h i c Data = Post P l u t o n i c Dikes Sample Number //1 Primary: c l i n o p y r o x e n e h o r n b l e n d e p l a g i o c l a s e 1 0 10(An54) //2 //3 5 0 40(An60-63) 3 0 10 //5 0 0 0 86 0 40 0 Secondary: a l b i t e ( + s a u s s u r i t e + s e r i c i t e ) e p i d o t e c h l o r i t e p r e h n i t e p u m p e l l y i t e c a l c i t e s p h e n c 55 1 15 0 0 2 1 50 5 30 0 1 0 1 25 2 15 0 0 0 0 60 5 10 2 0 0 1 7 5 2 5 3 0 t r 0 35 10 10 0 0 0 0 E i t h e r : q u a r t z m a g n e t i t e i l m e n i t e 5 10 0 2 1 10 3 10 0 1 5 ( p h e n o c r y s t s ) 2 1 3 0 ; 0 Percentage of secondary mineralai 74 87 42 78 85 Rock name: a n d e s i t e a n d e s i t e a n d e s i t e a n d e s i t e d a c i t e h o r n b l e n d e b a s a l t 124. The l a r g e p r o p o r t i o n s o f s e c o n d a r y p h a s e s shown i n T a b l e 4 a t t e s t t o t h e h i g h d e g r e e o f a l t e r a t i o n . By c o n t r a s t t h e q u a r t z d i o r i t e and m e t a v o l c a n i c s i n t r u d e d by t h e s e d i k e s a r e g e n e r a l l y q u i t e f r e s h . They c o n t a i n o n l y s m a l l q u a n t i t i e s o f l o w - t e m p e r a t u r e p h a s e s , i n c l u d i n g c h l o r i t e i n b i o t i t e and h o r n b l e n d e , p r e h n i t e i n b i o t i t e , s a u s s u r i t e , e p i d o t e , and p u m p e l l y i t e . S i g n i f i c a n t l o w - t e m p e r a t u r e a l t e r a t -i o n i s c o n f i n e d t o j o i n t s u r f a c e s ( m a i n l y e p i d o t e ) and t o t h e e a s t - n o r t h e a s t f r a c t u r e z o n e s ( c h l o / t / i t e - a l b i t e , q u a r t z - a l b i t e -e p i d o t e - c h l o r i t e ) . T h u s , a l t h o u g h t h e e n t i r e a r e a has been s u b j . e c t e d t o l a t e p r e h n i t e - p u m p e l l y i t e m e t a m o r p h i s m , p e r v a s i v e a l t e r a t i o n has o c c u r r e d o n l y i n f r a c t u r e s and i n t h e d i k e s w h i c h f i l l them. T h i s a s s o c i a t i o n o f a b u n d a n t l o w - t e m p e r a t u r e p h a s e s w i t h f r a c t u r e s s u g g e s t s t h a t f l u i d c i r c u l a t i o n i n them e x e r t e d a c o n t r o l l i n g i n f l u e n c e on t h e a l t e r a t i o n p r o c e s s . P r e h n i t e - p u m p e l l y i t e f a c i e s m e t a m o r p h i s m a t l o w p r e s s u r e r e q u i r e s t e m p e r a t u r e s b e t w e e n 200°C ( a n a l c i m e + qz = a l b i t e + H 20 - L i o u 1971) and 400°C ( p r e h n i t e b r e a k d o w n - L i o u 1 9 7 0 ) . I f t h e f r a c t u r e s were f o r m e d and t h e d i k e s e m p l a c e d s h o r t l y a f t e r c r y s t a l l i z a t i o n o f t h e p l u t o n t h e r e m a i n i n g h e a t w o u l d have been s u f f i c i e n t t o metamorphose them. T h i s m o d e l i s somewhat p r e f e r a b l e , t o t h a t o f a p o s t - p l u t o n i c b u r i a l - m e t a m o r p h i c e p i s o d e b e c a u s e o f t h e c o n c o r d a n c e o f t h e r a d i o m e t r i c d a t e s ( s e e C h a p t e r I I I ) . The n o r t h e a s t f r a c t u r i n g and d i k e emplacement a r e t h e l a t e s t t e c t o n i c e v e n t s r e c o r d e d i n t h e map a r e a . As t h e y a r e p r o b a b l y r e f l e c t e d i n t h e r a d i o m e t r i c d a t e s on t h e q u a r t z d i o r i t e , no p o s t -125. J u r a s s i c t e c t o n i c e v e n t s o f any s i g n i f i c a n c e a r e s e e n a t t h i s p a r t o f t h e p r o v i n c e b o u n d a r y . I n p a r t i c u l a r t h e r e a r e no n o r t h w e s t - t r e n d i n g f a u l t s o r d i k e swarms t h a t m i g h t be a s s o c i a t e d w i t h a n o r t h w e s t - t r e n d i n g U p p e r C r e t a c e o u s r i f t z o n e . T h i s o f c o u r s e does n o t e l i m i n a t e t h e p o s s i b i l i t y , i t m e r e l y f a i l s t o s u p p o r t i t . 126. SUMMARY OF FIELD RESULTS The h i s t o r y of the map area began with the d e p o s i t i o n of the Vancouver Group - the Karmutsen b a s a l t s , the Quatsino limestone, the limestones and f i n e s i l t s t o n e s of the Parson Bay and Harbledown Formations,and l a s t the v o l c a n i c b r e c c i a s and s i l t s t o n e s of the Bonanza Formation. E x t e n s i v e d i k i n g accompanied both Karmutsen and Bonanza volcanism. F o l l o w i n g t h e i r d e p o s i t i o n , rocks of the Vancouver Group were subjected to p r e h n i t e - p u m p e l l y i t e b u r i a l metamorphism at pressures l e s s than 3 kb. Two e a r l y p l u t o n i c phases have been i d e n t i f i e d at Miner's Bay and on eastern Hardwicke I s l a n d . The main i n t r u s -i o n , a homogeneous quartz d i o r i t e with a gabbroic rim, dated as Eater J u r a s s i c , was probably emplaced along deep west-northwest f r a c t u r e s p a r a l l e l to the Telegraph H i l l and J o h n s t o n e S t r a i t f a u l t s . The Telegraph H i l l F a u l t i s demonstrably p r e - p l u t o n i c . The e x i s t e n c e of these l a r g e f a u l t s provides an e x p l a n a t i o n f o r the anomalous west-northwest trend of the province boundary along J o h n s t o n e S t r a i t . The quartz d i o r i t e produced a thermal aureole about 4 km. wide; maximum temperatures at the contact reached 650°C. I t a l s o generated a l l the important s t r u c t u r e s i n the area except the monoclines between the major f a u l t s and the n o r t h e a s t - t r e n d i n g f r a c t u r e system, which i s p o s t - p l u t o n i c P o s t - p l u t o n i c dikes i n d i c a t e 5% northwest-southeast e x t e n s i o n , i . e . p a r a l l e l to the r e g i o n a l t e c t o n i c t r e n d . The dikes have been metamorphosed i n p r e h n i t e - p u m p e l l y i t e f a c i e s . They probably predate the 150 m.y. r a d i o m e t r i c age of the quartz d i o r i t e . 127. No s u p p o r t was f o u n d i n t h e a r e a f o r a Lower J u r a s s i c s u t u r e b e t w e e n t h e two p r o v i n c e s ; i n f a c t a r g u m e n t s c a n be made a g a i n s t i t . No s t r o n g p r e - p l u t o n i c d e f o r m a t i o n c a n be demonst-r a t e d and no " t r e n c h " s e d i m e n t s o c c u r i n t h e V a n c o u v e r G r o u p . B o n a n z a b r e c c i a s a r e m o n o l i t h o l o g i c and p u r e l y v o l c a n i c ; t h e f i n e g r e y w a c k e s i n t h e H a r b l e d o w n F o r m a t i o n a r e most l i k e l y a c c u m u l a t i o n s o f l o c a l b a s a l t w e a t h e r i n g d e b r i s ( C a r l i s l e and S u z u k i 1 9 7 4 ) . No e v i d e n c e was f o u n d e i t h e r t o s u p p o r t o r r e f u t e t h e e x i s t e n c e o f a n o r t h w e s t - t r e n d i n g r i f t z o n e . The T e l e g r a p h H i l l F a u l t w i t h a maximum s t r a t i g r a p h i c s e p a r a t i o n o f 5000 m e t e r s , and t h e J o h n s t o n e S t r a i t F a u l t w i t h a maximum o f 7000 m e t e r s , a r e t h e o n l y m a j o r t e c t o n i c f e a t u r e s i n t h e a r e a n o t d i r e c t l y r e l a t e d t o t h e i n t r u s i o n . F a u l t s o f s i m i l a r m a g n i t u d e a r e common on V a n c o u v e r I s l a n d . They c a n n o t be s a i d t o c o n s t i t u t e a m a j o r t e c t o n i c b r e a k b e t w e e n t h e I n s u l a r B e l t and t h e C o a s t P l u t o n i c C omplex. I n f a c t t h e most s i g n i f i c a n t f e a t u r e o f t h e b o u n d a r y h e r e i s an i n t r u s i v e c o n t a c t . The p r o v i n c e b o u n d a r y has a l s o been mapped on Quadra I s l a n d by C a r l i s l e and S u z u k i (1965) and on t h e n o r t h e a s t s h o u l d e r o f V a n c o u v e r I s l a n d by K u n i y o s h i (1972) ( F i g u r e 5 ) , who s i m i l a r i l y d e s c r i b e i t as an i n t r u s i v e c o n t a c t . The p l u t o n i c r o c k s i n v o l v e d a r e g e n e r a l l y E a r l y C r e t a c e o u s . M e t a m o r p h i c c o n d i t i o n s r e s e m b l e t h o s e f o u n d h e r e : p r e h n i t e - p u m p e l l y i t e r e g i o n a l m e t a m o r p h i s m p r e c e d e d c o n t a c t m e t a m o r p h i s m on n o r t h e a s t e r n 128. V a n c o u v e r I s l a n d ( K u n i y o s h i 1972) and j u s t w e s t o f t h e p r o v i n c e b o u n d a r y on T e x a d a I s l a n d ( A s i h e n e 1 9 7 0 ) . K u n i y o s h i d e t e r m i n e d a t o t a l p r e s s u r e o f a p p r o x i m a t e l y 2 kb. ; A s i h e n e , a s s u m i n g 2 kb . , e s t i m a t e d a maximum c o n t a c t m e t a m o r p h i c t e m p e r a t u r e o f 700°C. On Q u a d r a I s l a n d i n t e n s e p r e - p l u t o n i c d e f o r m a t i o n - f o l d i n g o f t h e l i m e s t o n e s and f o l i a t i o n i n t h e m e t a v o l c a n i c s - was i n t e r p r e t e d by C a r l i s l e as due to r e g i o n a l d e f o r m a t i o n i n a s h e a r zone e x t e n d i n g o n t o V a n c o u v e r I s l a n d ( K u n i y o s h i 1 9 7 2 ) . On H a r d w i c k e and West T h u r l o w I s l a n d s t h e f o l i a t i o n and i n t e n s e f o l d i n g a r e i n t e r p r e t e d as due t o f o r c i b l e i n t r u s i o n . No n o r t h -w e s t r e g i o n a l d e f o r m a t i o n i s e v i d e n t p r i o r t o t h e w e s t - n o r t h w e s t s y n p l u t o n i c p h a s e . The c a s e o f H a r d w i c k e and West T h u r l o w I s l a n d s p o i n t s o u t t h e i m p o r t a n c e o f i n t r u s i o n - r e l a t e d d e f o r m a t i o n , w h i c h may be o f s i g n i f i c a n c e e l s e w h e r e a l o n g t h e p r o v i n c e b o u n d a r y . T h i s f i e l d s t u d y has v e r i f i e d on a s m a l l s c a l e what i s a p p a r e n t on l a r g e - s c a l e g e o l o g i c maps, t h a t t h e b o u n d a r y b e t w e e n t h e I n s u l a r B e l t and t h e C o a s t P l u t o n i c Complex i s s i m p l y t h e w e s t e r n l i m i t o f v o l u m i n o u s i n t r u s i o n . W h i l e t h i s l i m i t may i n p a r t be c o n t r o l l e d by f a u l t s , i t i s e s s e n t i a l l y D i c k i n s o n ' s m a g m a t i c f r o n t ( 1 9 7 1 ; 1 9 7 3 ) . I t has shown so f a r t h a t G e o r g i a D e p r e s s i o n l i e s w e s t o f a m a i n l y E a r l y C r e t a c e o u s m a g m a t i c f r o n t , and t h a t t h e d e p r e s s i o n f o r m e d i n t h e . U p p e r C r e t a c e o u s , t e n t a t i v e l y as a complement t o t h e u p l i f t o f t h e C r e t a c e o u s a r c . I t has been p r o p o s e d t h a t t h e w h o l e C o a s t a l T r o u g h has had a common mode o f o r i g i n . I f s o , i t t o o s h o u l d l i e . . i m m e d i a t e l y w e s t o f m a g m a t i c a r c s and 129. f o r m a t t h e same t i m e t h a t t h e y r o s e . I n t h e f o l l o w i n g s e c t i o n s on H e c a t e D e p r e s s i o n , t h e P u g e t and W i l l i a m e t t e L o w l a n d s and t h e G r e a t V a l l e y t h e s e r e l a t i o n s h i p s w i l l be e s t a b l i s h e d . 130. SECTION I I I : THE COASTAL TROUGH CHAPTER V I I : GEOLOGIC SETTING OF THE COASTAL TROUGH i . H e c a t e D e p r e s s i o n H e c a t e D e p r e s s i o n i s t h e n o r t h e r n m o s t s u b - b a s i n o f t h e C o a s t a l T r o u g h . L i k e G e o r g i a D e p r e s s i o n i t l i e s a c r o s s t h e C o a s t P l u t o n i c C o m p l e x / I n s u l a r B e l t b o u n d a r y . The e x a c t p o s i t i o n o f t h e p r o v i n c e b o u n d a r y u n d e r H e c a t e S t r a i t i s u n c e r t a i n . T r i a s s i c and J u r a s s i c v o l c a n i c s , i n c l u d i n g p r o b a b l e K a r m u t s e n e q u i v a l e n t s , o c c u r n e a r P r i n c e R u p e r t ( S k e e n a R i v e r 1 : 1,000,000 map s h e e t H u t c h i s o n e t a l . 1 9 7 3 ) . They may r e p r e s e n t t h e e a s t e r n m a r g i n o f t h e I n s u l a r B e l t , i n w h i c h c a s e i t e x t e n d s c o n s i d e r a b l y e a s t o f t h e a x i s o f t h e d e p r e s s i o n , as i t does u n d e r G e o r g i a S t r a i t . The p a l e o g e o g r a p h i c e v o l u t i o n o f H e c a t e D e p r e s s i o n i s n o t w e l l known. Most o f i t l i e s u n d e r w a t e r and i s o n l y a c c e s s i b l e t o d r i l l i n g and s e i s m i c m e t h o d s . The a d j a c e n t C o a s t M o u n t a i n s have b e e n mapped on a r e g i o n a l s c a l e by t h e G e o l o g i c a l S u r v e y ( H u t c h i s o n 1970; B a e r 1 973; R o d d i c k and H u t c h i s o n 1 9 7 4 ) . However s t r a t i f i e d i n l i e r s a r e t o o s c a r c e t o s u p p o r t a d e t a i l e d h i s t o r y . The Queen C h a r l o t t e I s l a n d s h a v e b e e n mapped on a r e g i o n a l s c a l e by S u t h e r l a n d Brown ( 1 9 6 8 ) . The T r i a s s i c to m i d - C r e t a c e o u s h i s t o r y o f t h e Queen C h a r l o t t e s r e s e m b l e s t h a t o f t h e s o u t h e r n I n s u l a r B e l t ( F i g u r e 4 2 ) . The Lower t o M i d d l e C r e t a c e o u s c l a s t i c s e q u e n c e s - t h e Longarm F o r m a t i o n and Queen C h a r l o t t e Group -a r e c h r o n o l o g i c a l l y and l i t h o l o g i c a l l y e q u i v a l e n t t o t h o s e on 1>lw-?kJs1ouni J Toctnt CrUacams J Turassic Iriassic-Qtrni i an yancouvcr is. (Alert lay) NANAIMO Grip-^UEEN_^CttA1J.L0TTE Grp. Qtmsylyimcin RM T^n. ^BONANZA Tjn. B A U S O N B*Y/QlMT5mo middle Triassic nrq. QH.OVV .1.31 'MASS-ET Tn, basalt/Thy oUtc Q U E E N C H A R L O T T E Q-rp. JjOHQAHhA <T*m. T ^ K O U N T W . /ytlcimcs//)>olc-std-Sutherland Iraym i%% •Shouldiu 13J1 I D yUulUrttal 1974 Comparative Jnsular mt Stratigraphies 132. V a n c o u v e r I s l a n d , b u t w h i l e t h e Queen C h a r l o t t e Group on V a n c o u v e r I s l a n d seems t o be d e r i v e d f r o m an e a r l y I n s u l a r B e l t u p l i f t ( M u l l e r a t a l - 1 9 7 4 ) , s e d i m e n t p a t t e r n s i n t h e Queen C h a r l o t t e s a r e more c o m p l e x and i n s u f f i c i e n t l y s t u d i e d ( S u t h e r l a n d Brown 1 9 6 8 ) . The p r e s e n c e o f M i d d l e C r e t a c e o u s l a n d i n t h e p l a c e o f H e c a t e D e p r e s s -i o n has y e t t o be e s t a b l i s h e d . H e c a t e D e p r e s s i o n p r o b a b l y d i d n o t d e v e l o p a t t h e same t i m e as G e o r g i a D e p r e s s i o n . T h e r e i s n o t known e q u i v a l e n t o f t h e Nanaimo Group e i t h e r on t h e Queen C h a r l o t t e s o r b e l o w H e c a t e S t r a i t , so f a r as c a n be a s c e r t a i n e d f r o m w e l l and s e i s m i c r e f l -e c t i o n d a t a ( S h o u l d i c e 1 9 7 1 ) , a l t h o u g h t h e S k i d e g a t e F o r m a t i o n ( u p p e r Queen C h a r l o t t e Group) may e x t e n d i n t o t h e U p p e r C r e t a c e o u s ( M u l l e r p e r s comm. 1 9 7 6 ) . A p r e - T e r t i a r y u n c o n f o r m i t y s e p a r a t e s t h e P a l e o c e n e - E o c e n e M a s s e t V o l c a n i c s f r o m a l l o t h e r f o r m a t i o n s . The M a s s e t V o l c a n i c s t h e m s e l v e s a r e p r e d o m i n a n t l y c o n t i n e n t a l . On t h e e a s t e r n Queen C h a r l o t t e s and u n d e r H e c a t e S t r a i t t h e y a r e o v e r l a i n , a g a i n u n c o n f o r m a b l y , by Upper M i o c e n e and P l i o c e n e m a r i n e and n o n - m a r i n e s e d i m e n t s o f t h e Skonun F o r m a t i o n , t h e f i r s t c l e a r i n d i c a t i o n o f a c o a s t a l t r o u g h i n n o r t h e r n B r i t i s h C o l u m b i a ( F i g u r e 4 3 ) . The M a s s e t v o l c a n i c s and t h e e r o s i o n s u r f -a c e s on t o p o f them d i p . g e n t l y i n t o H e c a t e D e p r e s s i o n . The o v e r -l y i n g Skonun F o r m a t i o n t h i c k e n s b a s i n w a r d s ; i t c h a n g e s f r o m d o m i n a n t l y c o n t i n e n t a l t o d o m i n a n t l y m a r i n e i n a s e r i e s o f w e l l s f r o m n o r t h e r n Graham I s l a n d s o u t h e a s t i n t o H e c a t e S t r a i t ( S h o u l d i c e 1 9 7 1 ) . S u t h e r l a n d Brown (1968) d e s c r i b e s t h e M i o - P l i o c e n e b a s i n as " . . . m o d e r a t e l y r a p i d l y s i n k i n g . . . p o s s i b l y f a u l t c o n t r o l l e d a t i t s e a s t e r n and n o r t h e r n m a r g i n s . . . f i l l e d t o c a p a c i t y w i t h d e t r i t -us f r o m t h e p r o t o - Q u e e n C h a r l o t t e I s l a n d s and p r o b a b l y t h e M a i n l a n d . " 133, distribution oj thtr^ 5konunc7m . •o • wtM Locutions after ShouUfct 0 ' ° O' .o 134. Two e v e n t s i n t h e h i s t o r y o f t h e C o a s t P l u t o n i c C o mplex nee d t o be e m p h a s i z e d h e r e - t h e d a t e s o f i n t r u s i o n and t h e t i m i n g o f u p l i f t . P o t a s s i u m - a r g o n d a t e s f o l l o w an e a s t w a r d y o u n g i n g t r e n d f r o m E a r l y C r e t a c e o u s t o E o c e n e (Woodsworth p e r s . comm.), l i k e t h o s e i n t h e s o u t h e r n C o a s t M o u n t a i n s . E i s b a c h e r (1974) a t t r i b u t e s p r e - C e n o m a n i a n d e f o r m a t i o n o f t h e Bowser Group and t h e a d v e n t o f w e s t e r l y - d e r i v e d e l a s t i c s i n t o t h e S u s t u t B a s i n 54 m.y. ago t o p u l s e s o f u p l i f t i n t h e n o r t h e r n C o a s t P l u t o n i c C o mplex. The l a t e s t p h a s e has i n v o l v e d as much as 1000 m e t e r s of u p l i f t o f M i o c e n e e r o s i o n s u r f a c e s ( B a e r 1 9 7 3 ) ; i t i s r o u g h l y c o e v a l w i t h t h e d e p o s i t i o n o f t h e Skonun F o r m a t i o n . H e c a t e and G e o r g i a D e p r e s s i o n s , w h i l e o c c u p y i n g s i m i l a r p o s i t i o n s w i t h r e s p e c t t o t h e t e c t o n i c b e l t s , a r e n o t t i m e -e q u i v a l e n t . G e o r g i a D e p r e s s i o n f o r m e d i n t h e Upp e r C r e t a c e o u s w h i l e t h e C o a s t P l u t o n i c Complex was an a c t i v e m a g m a t i c a r c . H e c a t e D e p r e s s i o n f o r m e d i n t h e Upper M i o c e n e , l o n g a f t e r p l u t -o n i s m had c e a s e d . Y e t b o t h a c c o m p a n i e d s i g n i f i c a n t p u l s e s o f u p l i f t o f t h e C o a s t P l u t o n i c Complex'. i i . The P u g e t and W i l l i a m e t t e D e p r e s s i o n s The f i r s t e v i d e n c e f o r a c o a s t a l t r o u g h i n w e s t e r n W a s h i n g t o n and O r e g o n i s i n t h e Upper M i o c e n e , c o n t e m p o r a r y w i t h t h e b e g i n n i n g of u p l i f t o f t h e C a s c a d e s . The p r e s e n t t r o u g h c o m p r i s e s two e l o n g a t e d e p r e s s i o n s , t h e P u g e t and W i l l i a m e t t e L o w l a n d s ( F i g u r e 4 4 ) . They a r e bounded on t h e e a s t by t h e C a s c a d e Range and on t h e w e s t by t h e C o a s t Ranges and O l y m p i c M o u n t a i n s . T h e s e f l a n k -i n g r a n g e s b e l o n g t o two d i s t i n c t g e o t e c t o n i c p r o v i n c e s ( F i g u r e s 135. 1 3 6 . 45 and 4 6 ) . The C o a s t Range P r o v i n c e i n c l u d e s b o t h t h e C o a s t Ranges and most o f t h e C o a s t a l T r o u g h . The C a s c a d e P r o v i n c e c o i n c i d e s w i t h the C a s c a d e Range, an u p l i f t e d C e n o z o i c magmatic a r c . The backbone of t h e C o a s t Ranges i s a n o r t h - s o u t h t r e n d i n g c h a i n of Eocene v o l c a n i c c e n t e r s w h i c h p r o d u c e d the v o l u m i n o u s t h o l e i i t i c and a l k a l i c b a s a l t s of the S i l e t z R i v e r , C r e s c e n t , M e t c h o s i n and Umpqua F o r m a t i o n s . C h e m i c a l l y t h e y r e s e m b l e m i d - o c e a n r i d g e and o c e a n i s l a n d b a s a l t s ( S n a v e l y and Wagner 1968). The Eocene b a s a l t s i n t e r t o n g u e w i t h and a r e o v e r l a i n by m i d d l e Eocene to P l e i s t o c e n e c l a s t i c s e d i m e n t s . The s e d i m e n t s a r e d e r i v e d b o t h from w i t h i n t h e C o l u m b i a n Embayment and from i t s p r e - C e n o z o i c m a r g i n s , t h e K l a m a t h M o u n t a i n s and t h e I n s u l a r B e l t . T u f f s and t u f f a c e o u s s e d i m e n t s from t h e C a s c a d e P r o v i n c e form an i m p o r t a n t p a r t of t h e s e c t i o n , p a r t i c u l a r i l y t h e O l i g o -cene ( S n a v e l y and Wagner 1963). A f t e r t h e E o c e n e , f l o w s and i n t r u s i o n s w i t h i n t h e C o a s t Range P r o v i n c e become r a r e . The C a s c a d e P r o v i n c e i s a n o r t h - s o u t h t r e n d i n g l i n e a r v o l c -a n i c b e l t a p p r o x i m a t e l y 150 km. wide l y i n g a bout 200 to 250 km. from t h e w e s t e r n edge of the N o r t h A m e r i c a n p l a t e . I t may be s u p e r i m p o s e d on Eocene C o a s t Range P r o v i n c e r o c k s ( S n a v e l y and Wagner 1963; B a l d w i n 1964). I t s n o r t h e r n end l a p s onto t h e p r e -T e r t i a r y N o r t h C a s c a d e s , i t s s o u t h e r n end onto t h e K l a m a t h s . W h i l e C a s c a d e v u l c a n i s m spans the e n t i r e C e n o z o i c e r a s i n c e t h e m i d d l e E o c e n e , i t has been d i s t i n c t l y e p i s o d i c . ( S u t t e r and Mc-B i r n e y 1974; M c B i r n e y et a l 1 9 7 4 ) . A n d e s i t e i s most abundant w i t h l e s s e r amounts of b a s a l t , d a c i t e and r h y o d a c i t e . C h u r c h and C O A S T H A N O I P R O V I N C E C T t c j 4 f ?|iOC(H( leant ~ ~ 12. ••iVCWiii T i l TWm Hivir T m . v y y v y v y v ' « Cv« t«« lTFin. w v " * ' • y " . £incoin Cruh. O'm. y y y y ^ v * C w«nf? » i .v * » v y v y y V V V V V „ r v y „ „ ;Vr~v:.-=».*.-;'*;-=f-: •  t — V 4 V V <S * » V y y ft <5> «» » 4 <) XtllKttvi AnAlllit " v , <•• Scrtts o • " o * * a •« • • ' 1 5 ' ' ic y v. v* - ' ^ » t y t • >' '•'.'•(Juye 'Pin.'". * / ":V SouRCf J : Tabor CH7i) 1Uu CU67) » y < •t 1 + • (• o.a-M r | 1 ^ I + OT«»»««i«'ir7»>'. w .. ' y y y s/ lithologic chart = western "Wash.. l* jyrvilistits 4*. • SlOilout ^= Anil/ Jinutc I -C0AS1 HANOI - syiiuiAMmi ^njyiNcE — on. Central coai\id tmx Southern CMitOtna: y/illkmiHi ixlUy CiiUdt 'TUyai QltiillttM * •..:#.*.• T-l . » 1 y v < v ^ v • v y'8grm(|""Uwvv'yvv v-'E-nipire T=w-^:;" ^. / •••• ;/Tr(>uti*U.7rtu. ; . * « •/Caftan ~r • v y ' ^ v v y v V V -ns 37 5S „ Sihlf.'Kpk- V , V/V Vf v v v, * v—*—* • v  v"v V v "V V V V X i t h o l o g i c c h a r t = w e s t e r n Oregon «• V Vv, u p. d o t s ™fc 140, T i l t o n (1972) have used lead i s o t o p i c r a t i o s and low, r e l a t i v e l y constant Sr87/Sr86 (.7037 ave.) i n P l e i s t o c e n e Cascade v o l c a n i c s to support a s u b c r u s t a l o r i g i n . In a d d i t i o n to v o l c a n i c s , Miocene i n t r u s i o n s ( g r a n o d i o r i t e with subordinate d i o r i t e , quartz monzonite, g r a n i t e , etc.) are exposed i n the Washington Cascades. The boundary between the Cascade and Coast Range Provinces ( i . e . the western l i m i t of Cascade volcanism) l i e s w i t h i n the C o a s t a l Trough. I t s exact p o s i t i o n i s l a r g e l y obscured by d r i f t and a l l u v i u m . In places i t i s expressed as a vol c a n i c / s e d i m e n -tar y f a c i e s change near the eastern s i d e of the trough. In King County, Washington, T e r t i a r y v o l c a n i c rocks, mostly andesite b r e c c i a s and flows, extend westward to the Cascade f o o t h i l l s ( L i v i n g s t o n 1971). T e r t i a r y sediments, arkose, v o l c a n i c sand, t u f f , c l a y and c o a l , predominate i n the Puget Lowland. The t r a n s i t i o n ( f o r example the zone of i n t e r f i n g e r i n g between Eocene and Puget Group sediments and the Keechelus V o l c a n i c S e r i e s ) occurs near the eastern margin^of the lowland and i n the Cascade f o o t h i l l s . The p o s i t i o n of the v o l c a n i c f r o n t i n the Miocene i s not as w e l l bracketed as the Eocene. Cascade s t r a t -igraphy i s not yet p r e c i s e enough to d e f i n e the western l i m i t of Miocene v o l c a n i c s ; however i t must have l a i n between the westernmost exposure of Miocene Snoqualmie G r a n o d i o r i t e f i v e k i l o m e t e r s east of North Bend and sediments of the Miocene Hammer B l u f f Formation (Glover 1936) along the Green River south of Black Diamond. 1 4 1 -The O l i g o c e n e v o l c a n i c f r o n t i n O r egon i s e x p o s e d on t h e e a s t e r n s i d e o f t h e W i l l i a m e t t e L o w l a n d . The Eugene F o r m a t i o n ( M i d d l e O l i g o c e n e ) and t h e B u t t e C r e e k beds ( U p p e r O l i g o c e n e ) , o u t c r o p p i n g w i t h i n t h e L o w l a n d , a r e p r e d o m i n a n t l y m a r i n e v o l c a n i c s a n d s t o n e and w a t e r l a i n t u f f . They i n t e r f i n g e r a l o n g i t s e a s t e r n s i d e w i t h t h e Mehama V o l c a n i c s , a s e r i e s o f a n d e s i t i c and d a c i t i c t u f f s , b r e c c i a s , and f l o w s ( B a l d w i n 1964; B e a u l i e u 1 9 7 1 ) . T h e s e e x a m p l e s s h o u l d n o t be t a k e n t o i m p l y t h a t t h e w e s t e r n l i m i t o f C a s c a d e v o l c a n i s m r e m a i n e d f i x e d a t t h e e a s t e r n edge o f t h e p r e s e n t C o a s t a l T r o u g h . I t i s e v i d e n t f r o m p a l e o g e o -g r a p h i c r e c o n s t r u c t i o n s ( S n a v e l y and Wagner 1963) t h a t t h e p o s i t i o n o f t h e v o l c a n i c f r o n t v a r i e d w i t h t i m e . However o n l y i n f r e q u e n t l y and o n l y i n p l a c e s d i d i t a d v a n c e w e s t o f t h e f r o n t o f t h e C a s c a d e Range, w h i c h a c c o r d i n g l y c o r r e s p o n d s t o a s i g n i f i c a n t d e c r e a s e i n v o l c a n i c v o l u m e . The l o c a t i o n o f t h e C o a s t a l T r o u g h , t h e n , i s l i n k e d t o t h e d i s t r i b u t i o n o f C a s c a d e v o l c a n i c s . B u t w h i l e C a s c a d e v o l c a n i s m b egan i n t h e M i d d l e E o c e n e , t h e f i r s t e v i d e n c e f o r a n o r t h - s o u t h t r e n d i n g d e p r e s s i o n on t h e s i t e o f t h e p r e s e n t t r o u g h i s shown by t h e d i s t r i b u t i o n o f t h e C o l u m b i a B a s a l t w e s t o f t h e C a s c a d e s and t h e f l u v i a l - l a c u s t r i n e W i l k e s F o r m a t i o n ( F i g -u r e 4 7 ) . The C o l u m b i a B a s a l t s a r e d a t e d as 13-16 m.y. ( M c B i r n e y e t a l . 1 9 7 4 ) ; t h e W i l k e s F o r m a t i o n i s ftpper M i o c e n e and p o s s i b l y P l i o c e n e ( R o b e r t s 1958) . S i n k i n g o f t h e t r o u g h a c c o m p a n i e d u p l i f t o f t h e C a s c a d e s . As t h e C a s c a d e s r o s e t h e y became a b a r r i e r t o m o i s t u r e f r o m t h e P a c i f i c Ocean. The p r o g r e s s i v e l o w e r i n g o f r a i n f a l l e a s t o f tr CMiocenc -paleoaeoaraj tht Columbian Ir/ibavmcvui: -Jj'nav&iy and, Wayncr 1J)63 143. the Cascades caused an e v o l u t i o n of the f l o r a from moist-to-dry c l i m a t e assemblages. The f i r s t m a n i f e s t a t i o n of t h i s trend i s i n the Upper Miocene M a s c a l l f l o r a , which shows a decrease i n abundance of Metasequoia from e a r l i e r f l o r a s (Chaney 1925). The x e r i c trend culminated i n the middle P l i o c e n e ( D e t l i n g 1968). D i s t r i b u t i o n of the P l i o c e n e T r o u t d a l e Formation provides and estimate of . the amount of d i f f e r e n t i a l movement between the Oregon Cascades and the W i l l i a m e t t e Lowland i n the l a t e P l i o c e n e -P l e i s t o c e n e . F l u v i a l i n o r i g i n , i t i s i n t e r p r e t e d as the remains of a P l i o c e n e Columbia River a l l u v i a l p l a i n which extended from the P o r t l a n d Basin eastward to The D a l l e s (Baldwin 1964). Near P o r t l a n d the Tr o u t d a l e Formation outcrops a few hundred f e e t above sea l e v e l ; at Nesmith Point above the Columbia Gorge i t has been el e v a t e d to 700 meters ( A l l e n 1958). The Oregon Coast Ranges have a l s o r i s e n markedly i n the l a t e Cenozoic. P l e i s t o c e n e beach t e r r a c e s l i e as much as 300 meters above sea l e v e l (Baldwin 1964) . P 1 i o - P l e i s t o c e n e u p l i f t of the Cascades i n c e n t r a l Washington exceeds 1500 meters (McKee 1972). The great t h i c k n e s s of Quat-ernary sediments near S e a t t l e , more than 500 meters (Rogers 1970), a t t e s t s to c o n s i d e r a b l e s i n k i n g of the Puget Lowland. The ques t i o n of warping versus f a u l t i n g i s not e a s i l y r e s o l v e d : examples of both can be found. The Columbia B a s a l t s along the Columbia River Gorge form a simple upwarp of 1000 meters amplitude, somewhat modified by open f o l d s and minor f a u l t s . The g e o l o g i c p r o f i l e across northwestern Oregon by Wells and Peck (1961) shows the Coast Range-Williamette Lowland-Cascade Range as upwarp-down-144. w a r p - u p w a r p ; however t h e d i s c o n t i n u i t y o f u n i t s a c r o s s t h e L o w l a n d may o b s c u r e s i g n i f i c a n t f a u l t s . B r o m e r y and S n a v e l y ( 1 9 6 4 ) i n f e r t h e p r e s e n c e o f a m a j o r n o r m a l f a u l t f r o m s t e e p g r a v i t y g r a d i e n t s on t h e w e s t e r n s i d e o f t h e W i l l i a m e t t e L o w l a n d . B e a u -l i e u ( 1971) p o i n t s o u t t h a t l a r g e f a u l t s a r e p o s s i b l e 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 L o w l a n d . N e a r S e a t t l e t h e c a s e i s c l e a r e r f o r f a u l t c o n t r o l . The w e s t f a c e of Mount S i i s a f a u l t s c a r p o f c o n s i d e r a b l e m a g n i t u d e ( R o g e r s 1 9 7 0 ) ; t h e Mount S i f a u l t c a n be t r a c e d n o r t h t o t h e San J u a n s . E a r t h q u a k e s a l o n g i t demon-s t r a t e c o n t i n u e d m o t i o n ( D a n n e r p e r s . comm. 1 9 7 5 ) . A c c o r d i n g t o Danes e t a l . ( 1 9 6 5 ) , t h e Hood C a n a l L i n e a m e n t i s a m a j o r f a u l t b e t w e e n t h e P u g e t L o w l a n d and t h e O l y m p i c M o u n t a i n s . R o g e r s (1970) i n t e r p r e t s t h e B o u g u e r g r a v i t y f i e l d i n t h e P u g e t L o w l a n d as t h e e x p r e s s i o n o f l a r g e f a u l t b l o c k s c o n c e a l e d by a l l u v i u m and d r i f t . He c o u l d n o t d e t e r m i n e wwhether t h e s e d i m e n t s w e re i n v o l v e d i n f a u l t i n g o r m e r e l y d r a p e d o v e r basement b l o c k s . S i n c e h i s work s e v e r a l P l e i s t o c e n e - R e c e n t f a u l t s c a r p s have b e e n i d e n t i f i e d n e a r t h e Hood C a n a l L i n e a m e n t and n e a r S e a t t l e ( L i v i n g s t o n , Gower 1 9 7 5 ) . The P u g e t and W i l l i a m e t t e L o w l a n d s f i t t h e m o del f o r t h e C o a s t a l T r o u g h s u g g e s t e d by G e o r g i a and H e c a t e D e p r e s s i o n s . They l i e i m m e d i a t e l y w e s t o f t h e C a s c a d e v o l c a n i c f r o n t . They have f o r m e d s i m u l t a n e o u s l y w i t h t h e u p l i f t o f t h e C a s c a d e Range. The C o a s t Range a l s o r o s e a t t h e same t i m e , m a k i n g t h e t r o u g h an a r e a o f s u b s i d e n c e b e t w e e n two a c t i v e u p l i f t s . I t i s s i g n i f i c a n t t o n o t e t h a t b o t h f a u l t i n g and w a r p i n g were i n v o l v e d . v 1 4 5 . i i i . The G r e a t V a l l e y The G r e a t V a l l e y of C a l i f o r n i a i s a g e n t l e s y c l i n o r i u m o f U pper M e s o z o i c and C e n o z o i c s e d i m e n t s . On t h e e a s t t h e y o n l a p o n t o t h e S i e r r a N e v a d a , an u p l i f t e d J u r a - C r e t a c e o u s c o m p o s i t e m a g m atic a r c . On t h e w e s t t h e y a r e i n p r o b a b l e f a u l t c o n t a c t w i t h t h e F r a n c i s c a n t e r r a n e , an 1 a r c h t y p i c a l t r e n c h c o m p l e x (Hsu 1971; E r n s t 1 9 7 1 ) . The C r e t a c e o u s G r e a t V a l l e y i s one o f t h e e x a m p l e s o f a r c - t r e n c h gaps c i t e d by D i c k i n s o n ( 1 9 7 1 ) , a zone of t e c t o n i c q u i e t b e t w e e n t h e a r c and t h e t r e n c h . The b a s a l u n i t of t h e G r e a t V a l l e y S e q u e n c e i s t h e T i t h o n i a n K n o x v i l l e F o r m a t i o n , w h i c h marks t h e b e g i n n i n g o f s u b s i d e n c e . A t t h e same t i m e t h e G r e a t V a l l e y a l s o became an a r c - t r e n c h gap ( F i g u r e 4 8 ) . T h e r e i s e v i d e n c e f o r a d i f f e r e n t a r c - t r e n c h c o n f i g u r a t i o n p r i o r t o t h a t t i m e . V o l c a n i c f l o w s , p y r o c l a s t i c d e b r i s and m a r i n e v o l -c a n i c s e d i m e n t s r a n g i n g i n age f r o m M i d d l e T r i a s s i c t o K i m m e r i d -g i a n a r e e x p o s e d i n t h e K l a m a t h M o u n t a i n s and t h e S i e r r a N e v a d a . They were s e v e r e l y d e f o r m e d and m e t a m o r p h o s e d i n K i m m e r i d g i a n -T i t h o n i a n t i m e . A peak i n p l u t o n i c d a t e s , 158-132 m.y. ( L a n p h e r e and Reed 1 9 7 3 ) , c o i n c i d e s w i t h t h i s l a t e J u r a s s i c t e c t o n i c e p i s o d e . M o o r e s (1972) i n t e r p r e t s t h e e v e n t as t h e r e s u l t o f an a r c - a r c c o l l i s i o n . S c h w e i c k e r t and Cowan(1974) e n v i s a g e a T r i a s s i c - J u r a s s i c t r e n c h i m m e d i a t e l y w e s t o f t h e S i e r r a n f o o t h i l l s w h i c h became c h o k e d and s t e p p e d o u t w a r d t o t h e F r a n c i s a n a b o u t 150 m i l l i o n y e a r s ago. The b e g i n n i n g of s u b s i d e n c e of t h e G r e a t V a l l e y a l s o c o i n c i d e d w i t h t h e i n i t i a l u p l i f t o f t h e S i e r r a n a r c above w a v e b a s e : t h e y o u n g e s t m a r i n e s e d i m e n t s i n t h e S i e r r a < T i j 4 8 Upper Jurassic. ~ Lower Cretaceous events * western California COAST R A N G E S S I E R R A S G R E A T VALLEY loo my. no no 130 HO y -Aptkn fdarrtm\an Jfautcrivian 'Valanqinian 73trriKSian Titiwnian cKimmmdflian, 150-oidat flimxptwn*-Oxjordianr l 6 0 Cailovicuv ijo —subsidtvet btflivs 1</Ar ditts (Armtlrna u\i Svr-'e 0 \ii7s) 4 NEVADAN OROGENY i l -jfwvfltit jfasffs • tvuttrn tniiamoryhh belt—• «0' ,<1 s* 1 147. a r e K i m m e r i d g i a n ( C l a r k ( 1 9 6 4 ) ; t h e f i r s t s e d i m e n t s i n t h e G r e a t V a l l e y , d e r i v e d f r o m t h e S i e r r a n / K l a m a t h u p l i f t , a r e T i t h o n i a n . The u p l i f t c o n t i n u e d as t h e G r e a t V a l l e y s u b s i d e d . The l a r g e v o l u m e o f s e d i m e n t s i n t h e G r e a t V a l l e y , as much as 12-15 km. on t h e w e s t e r n s i d e ( R e p e n n i n g 1960; B a i l e y e t a l . 1964) r a i s e s t h e p o s s i b i l i t y t h a t s u b s i d e n c e m i g h t have b e e n due s o l e l y t o s e d i m e n t l o a d i n g . A l t h o u g h s i g n i f i c a n t , i t was n o t t h e s o l e c a u s e . O j a k a n g a s (1968) r e p o r t s a p r o g r e s s i o n f r o m s h a l l o w - w a t e r t o b a t h y a l d e p o s i t i o n i n t h e w e s t e r n p a r t of t h e G r e a t V a l l e y S e quence b e t w e e n t h e l a t e J u r a s s i c and t h e m i d d l e C r e t a c e o u s , s h o w i n g t h a t s u b s i d e n c e e x c e e d e d s e d i m e n t l o a d i n g d u r i n g t h a t t i m e . I n W a l c o t t ' s (1972) m o d e l f o r c r u s t a l down-w a r p i n g u n d e r s e d i m e n t a r y p r i s m s t h e two p r o c e s s e s keep p a c e . The s t r u c t u r e o f t h e G r e a t V a l l e y i s g e n e r a l l y s i m p l e . The e a s t e r n l i m b of t h e s y n c l i n o r i u m i s u n d e r l a i n by g e n t l y s l o p i n g S i e r r a n b a s e m e n t . The w e s t e r n l i m b , a g a i n s t t h e T e s l a - O r t i g a l i t a F a u l t , i s s t e e p . F a u l t i n g w i t h i n t h e G r e a t V a l l e y i s c o n s i d e r -ed u n i m p o r t a n t , a l t h o u g h Brown and R i c h (1967) and B e r r y (1973) p o s t u l a t e b u r i e d f a u l t s . One u n r e s o l v e d i s s u e c o n c e r n s t h e g e o m e t r y o f t h e G r e a t V a l l e y i n t h e C r e t a c e o u s : was i t a t r o u g h , o r a s h e l f and s l o p e ? The p r e s e n t C o a s t Range i s o f E o c e n e o r i g i n . Was t h e r e an a p p r e c i a b l e b asement u p l i f t w e s t o f t h e G r e a t V a l l e y b e f o r e t h a t t i m e ? D i c k i n s o n ( 1 9 7 1 , 1 9 7 2 ) has a r g u e d f o r a t r o u g h c o n f i g u r a t i o n b a s e d on l o n g i t u d i n a l p a l e o c u r r e n t s i n t h e G r e a t V a l l e y S e q u e n c e . O j a k a n g a s ( 1 9 6 8 ) , on t h e o t h e r h a n d , e x p l a i n s t h e c u r r e n t p a t t e r n by p o s t u l a t i n g a s u b m a r i n e f a n a t t h e n o r t h -e r n end of t h e G r e a t V a l l e y . B a i l e y e t a l . (1964) f a v o r a r i d g e 148. s e p a r a t i n g t h e G r e a t V a l l e y and F r a n c i s c a n d e p o s i t i o n a l e n v i r o n -ments b e c a u s e o f t h e c o m p a r a t i v e l a c k o f d e t r i t a l p o t a s s i u m f e l d -s p a r i n F r a n c i s c a n g r e y w a c k e s , b u t B a r b a t (1971) c o n s i d e r s t h i s t o be a d i a g e n e t i c / m e t a m o r p h i c e f f e c t . W e s t e r l y - d e r i v e d s a n d u n i t s h a v e been n o t e d i n a few p l a c e s i n t h e G r e a t V a l l e y S e q u e n c e , as has d e t r i t a l s e r p e n t i n i t e ( H a c k e l 1966; Raney 1 9 7 3 ; B e r r y 1 9 7 3 ) . T h e s e need n o t have come f r o m a m a j o r basement r i d g e : s e i s m i c p r o f i l i n g o f modern t r e n c h e s has shown a c c u m u l a t i o n s o f d e f o r m e d s e d i m e n t s p i l e d a g a i n s t t h e l a n d w a r d s i d e (Le P i c h o n e t a l . 1 9 7 3 ) . S u c h a w e l t c o u l d s u p p l y m i n o r amounts o f s e d i m e n t t o t h e G r e a t V a l l e y s e q u e n c e . The G r e a t V a l l e y i n t h e C r e t a c e o u s was a zone o f s u b s i d -e nce b o r d e r i n g t h e r i s i n g S i e r r a n m agmatic a r c . I t may o r may n o t have b e e n a t r o u g h a t t h a t t i m e . I t s s y n c l i n a l s t r u c t u r e was c r e a t e d a f t e r c e s s a t i o n o f s u b d u c t i o n by movement on t h e T e s l a - 0 r t i g a l i t a F a u l t . I n t i m i n g i t c o r r e s p o n d s t o t h e o t h e r s e c t i o n s o f t h e t r o u g h , t h e n , b u t i t s s t r u c t u r e i n t h e C r e t a c e o u s may have b e e n open t o t h e w e s t . 149. CHAPTER V I I I : DEEP STRUCTURE OF THE COASTAL TROUGH R e f r a c t i o n seismology and g r a v i t y data can a i d i n i l l u m i n -a t i n g the deep c r u s t a l and upper mantle s t r u c t u r e under the C o a s t a l Trough. Although these s t u d i e s are s t i l l i n e a r l y stages, they give s e v e r a l important p i e c e s of i n f o r m a t i o n . S e i s -mic work i n B r i t i s h Columbia and C a l i f o r n i a i n d i c a t e s the poss-i b i l i t y of thinned c r u s t under the C o a s t a l Trough. Hecate Depression and the Great V a l l e y of C a l i f o r n i a are c l o s e to i s o -s t a t i c , while the Puget Lowland has one of the l a r g e s t negative f r e e - a i r and i s o s t a t i c anomalies i n the world, i . B r i t i s h Columbia The s t r u c t u r e of the l i t h o s p h e r e under northern and south-ern B r i t i s h Columbia d i f f e r s g r e a t l y . F i g u r e 49a o u t l i n e s the paths of the seismic p r o f i l e s used by Berry and F o r s y t h (1975) i n t h e i r r e g i o n a l a n a l y s i s . The northern p r o f i l e ( F o r s y t h et a l . 1974) shows a Moho depth of 26 km. under Graham I s l a n d t h i c k e n i n g eastward to 35 km. under the eastern Coast Mountains. Upper mantle v e l o c i t y (P ) i s 8.0 km/sec. The c e n t r a l p r o f i l e (Johnson et a l . 1972) a l s o shows P^ = 8.0 km/sec. and c r u s t a l t h i c k n e s s i n c r e a s i n g eastward from 23 km. at the shot p o i n t on the west side of the Coast Mountains. The average f r e e - a i r anomaly over Hecate Depression i s -8 m g a l , i n d i c a t i n g approximate i s o s t a t i c e q u i l i b r i u m ( c a l c u l a t e d from G r a v i t y data f o r B.C., compiled by G r a v i t y D i v i s i o n , EMR). The southern p r o f i l e (White and Savage 1965; Tseng 1968; Berry and F o r s y t h 1975) i s c h a r a c t e r i z e d by anomalously low P„ under Vancouver I s l a n d and the Coast Mountains, N c m 43 a) J hot points j recording stations t and ujpyer mantLt yelotitfes for reversed seismic yrojilis in -western "B.C. km 15, NTTTNAT WSULAH BELT Str. of Georgia COAST PLUTONIC COMPLEX Coost -?-8.03 GREEN8USH \ OMINECA CRYSTALLINE BELT Hozameerr-Posoyten Foult Zone HiNTERLANO BELT b) Crustal "profile- across southwestern 'B.C. "berry and Cforsjtk 19/5" 151. The c r u s t u n d e r t h e C o a s t M o u n t a i n s r e s e m b l e s t h a t i n t h e n o r t h e r n and c e n t r a l p r o f i l e s ; b u t t h e c r u s t u n d e r V a n c o u v e r I s l a n d may be 40 t o 50 km. t h i c k ( F i g u r e 4 9 b ) . The p a u c i t y o f s t a t i o n s w i t h i n G e o r g i a and H e c a t e D e p r e s s i o n s makes d e t a i l e d r e c o n s t r u c t i o n s u n c e r t a i n . T h e r e i s some i n d i c a t i o n o f c r u s t a l t h i n n i n g f r o m t h e I n s u l a r h i g h l a n d s i n t o them ( F i g u r e s 49b and 5 0 a ) . The s e i s m i c s c a t t e r i n g zone u n d e r e a s t e r n G e o r g i a S t r a i t shown i n F i g u r e 49b'v i s i n t e r p r e t e d by B e r r y and F o r s y t h (1975) as e i t h e r a m a j o r f a u l t o r an a b r u p t d i s c o n t i n u i t y i n c r u s t a l s t r u c t u r e . The marked d i f f e r e n c e b e t w e e n V a n c o u v e r I s l a n d and C o a s t M o u n t a i n s l i t h o s -p h e r e has b een documented by t h e g r a v i t y s t u d y of S t a c e y ( 1 9 7 3 ) . The l i t h o s p h e r e u n d e r V a n c o u v e r I s l a n d i s u n u s u a l l y d e n s e ; t h e l i t h o s p h e r e u n d e r t h e C o a s t M o u n t a i n s i s u n u s u a l l y l i g h t . F i g u r e 49b shows t h a t G e o r g i a D e p r e s s i o n o v e r l i e s a p r o f o u n d change i n l i t h o s p h e r i c s t r u c t u r e , b u t does n o t d i s c r i m i n a t e b e t w e e n t h e d e p r e s s i o n and t h e r e s t of t h e I n s u l a r B e l t . B e r r y and F o r s y t h ( 1975) have c a l c u l a t e d ^ ( c r u s t a l d e n s i t y ) u s i n g t h e t h i c k n e s s e s shown on F i g u r e 50a. They assume i s o s t a s y ; and t h a t d i f f e r e n c e s b e t w e e n o b s e r v e d B o u g u e r a n o m a l i e s and c a l c u l a t e d r o o t a n o m a l i e s ( u s i n g a c o n s t a n t c r u s t / m a n t l e d e n s i t y c o n t r a s t ) a r e due s o l e l y t o v a r i a t i o n s i n . The c r u s t a l - d e n s i t y map d e t e r m i n e d t h i s way shows a s t r o n g i n c r e a s e i n w e s t w a r d a c r o s s t h e C o a s t Moun-t a i n s w i t h a maximum i n t h e C o a s t a l T r o u g h ( F i g u r e 5 0 b ) . ' T h i s may be r e a l , o r i t may be due t o o t h e r v a r i a t i o n s i n t h e l i t h o s p h e r e , f o r i n s t a n c e i n c r e a s e o f m a n t l e d e n s i t y u n d e r t h e t r o u g h o r s i g n -i f i c a n t d e c r e a s e i n c r u s t a l t h i c k n e s s . F u r t h e r s e i s m i c s t u d i e s i n t h e t r o u g h w o u l d a i d i n e s t a b l i s h i n g w h i c h o f t h e s e m o d e l s i s 152. Variations in crustal thickness ^British Columbia 153. most a p p r o p r i a t e . Most i m p o r t a n t l y , t h e s e p r e l i m i n a r y d a t a s u g g e s t t h a t t h e C o a s t a l T r o u g h i n B r i t i s h C o l u m b i a i s i n some way i s o s t a t i c a l l y c o m p e n s a t e d . The n e a r z e r o a v e r a g e f r e e a i r anomaly o v e r H e c a t e D e p r e s s i o n f u r t h e r s u g g e s t s i s o s t a t i c e q u i l i b r i u m . i i . W a s h i n g t o n and O r e g o n S e i s m i c s t u d i e s i n W a s h i n g t o n and O r e g o n a r e i n d e x e d on F i g u r e 5 1 . L i k e t h e C o a s t M o u n t a i n s o f B r i t i s h C o l u m b i a , t h e W a s h i n g t o n C a s c a d e s have no deep r o o t ( J o h n s o n and Couch 1970);. t h e Moho s l o p e s u n d e r them f r o m 28 km. e a s t o f S e a t t l e t o 31 km. a t 119.5°W. ( F i g u r e 5 2 ) , w h i c h a l s o shows o t h e r s e i s -m i c a l l y d e t e r m i n e d d e p t h s t o t h e Moho f o r w e s t e r n W a s h i n g t o n p r o j e c t e d o n t o J o h n s o n and C o u c h ' s p r o f i l e . The c r u s t a l s t r u c t u r e d e p i c t e d on F i g u r e 52, t h i c k e n i n g f r o m t h e c o a s t to t h e e a s t e r n s i d e o f t h e C a s c a d e s , r e s e m b l e s t h e c r u s t i n n o r t h e r n B.C. P^ i s l o w u n d e r t h e e n t i r e p r o f i l e ; p u b l i s h e d e s t i m a t e s a r e l i s t e d on t h e same f i g u r e . The s e i s m i c w o r k , as i n B r i t i s h C o l u m b i a , has n o t d e l i m i t e d Moho b e h a v i o u r u n d e r t h e C o a s t a l T r o u g h . G r a v i t y s t u d i e s p r o v i d e some c o n s t r a i n t s . T h i r -u v a t h u k a l (1968) and T h i r u v a t h u k a l e t a l . (1970) have p r e s e n t e d i n t e r p r e t a t i o n s o f t h e B o u g u e r and f r e e - a i r g r a v i t y f i e l d s o f O r e g o n . B o t h B o u g u e r and f r e e - a i r h i g h s p r e v a i l o v e r t h e C o a s t Ranges o f s o u t h e r n W a s h i n g t o n and O r e g o n . The B o u g u e r a n o m a l i e s p a s s t h r o u g h z e r o o v e r t h e C o a s t a l T r o u g h and become l a r g e n e g a t i v e o v e r t h e C a s c a d e s . T h i s t r e n d i s due e i t h e r t o c r u s t a l CTia- J l Jndtx mctp^ Crustal seismic studies 120 n5' U3 ' Coast Hangt Stqtilt 1>N = «-oy C7ij• 52-km bltow S-t-10 4 *5H % ; upper wtmUt *>* * 7-96 jo-Crustal thicknesses beneath western 'M/ash. (projected, onto tht 2A5° refraction Live of 'Johnson nvd Couch 13JO) 1 5 6 . t h i c k e n i n g , t o l i g h t e n i n g o f t h e l i t h o s p h e r e , o r b o t h . The f r e e a i r a n o m a l i e s show an a p p a r e n t mass e x c e s s a s s o c i a t e d w i t h t h e C o a s t Ranges w h i c h T h i r u v a t h u k a l e t a l . (1970) a t t r i b u t e e i t h e r t o d e n s e n e a r - s u r f a c e r o c k s o r s h a l l o w m a n t l e o r b o t h . The W i l l i a m e t t e D e p r e s s i o n and t h e C a s c a d e s l i e i n a zone o f n e g a t i v e f r e e - a i r a n o m a l i e s w h i c h may i n d i c a t e l i g h t c r u s t , l i g h t m a n t l e , o r a mass d e f i c i e n c y i n t h e t o t a l c o l u m n . The P u g e t L o w l a n d i s r e m a r k a b l e i n s e v e r a l r e s p e c t s . I t l i e s e a s t o f t h e O l y m p i c s , t h e h i g h e s t and most d e f o r m e d s e c t i o n o f t h e C o a s t R a n g e s ; i t i s u n d e r l a i n by as much as 10 km. o f s e d i m e n t s , and i t i s c h a r a c t e r i z e d by l a r g e d e l a y s i n s e i s m i c a r r i v a l s and one o f t h e l a r g e s t B o u g u e r / f r e e - a i r l o w s i n t h e w o r l d ( W o o l l a r d 1 9 6 9 ) . F i g u r e 5 3, a f t e r Danes ( 1 9 6 9 ) , shows t h i s l o w f l a n k e d by f r e e - a i r h i g h s o v e r t h e C o a s t Ranges and t h e C a s c a d e s . Danes b e l i e v e s t h a t t h e a n o m a l i e s a r e m a i n t a i n e d by h y d r o d y n a m i c s t r e s s e s w h i c h c o u n t e r t h e t e n d e n c y of P u g e t L o w l a n d t o r i s e and t h e a d j a c e n t r a n g e s t o s i n k . i i i . C a l i f o r n i a The c r u s t a l s t r u c t u r e o f w e s t e r n C a l i f o r n i a i s s t i l l s u b j e c t t o m a j o r u n c e r t a i n t i e s . ' E a t o n ( 1 9 6 6 ) , b a s e d on p r o f i l e s a l o n g and a c r o s s s t r i k e , has d e t e r m i n e d a 50 km. r o o t u n d e r t h e S i e r r a N evada. C a r d e r ( 1 9 7 0 , 1973) i n t e r p r e t e d e a r l y a r r i v a l t i m e s a c r o s s t h e S i e r r a s f r o m t h e Nevada T e s t S i t e as e v i d e n c e a g a i n s t a deep r o o t . P a k i s e r . (1974) has r e i n t e r p r e t e d C a r d e r ' s d a t a t o be c o m p a t i b l e w i t h t h i c k e n e d c r u s t u n d e r t h e S i e r r a s . 157 . ria.55 Jsostatlc anomaly map "^Vashifwtoru •~L)an£S Wi Tia 2. . i n f B » » C I S C O I H O t P O U T Chjjl 158 F » u o a I H O T r o i m iHoil moi mini a.. Seismic profile across western California. b.ZModd of tht crust under '" California, from 73oujuer momaliis - Chapman 1%G \w-R£GIONAL GRAVITY ANOMALY TOTAL GRAVITY ANOMALY -rpr-in 0 S1CRAUENT0 S H E L F , C C * i S T | V A L L E Y i S I E R P A | B A S I N A N O R A N G E | R A N « £ S ] I N E V A 0 4 * . ! ! ! / * /"2-zr j ~r -400, -loo o too 0 I S T A . 1 C C i * K M 159. F i g u r e 54a f r o m E a t o n ( 1 966) shows two a l t e r n a t e i n t e r p r e t a t i o n s o f e a r l y P^ a r r i v a l s a c r o s s t h e G r e a t V a l l e y . One i n v o l v e s t h i n n i n g o f t h e w h o l e c r u s t ; t h e o t h e r a t a p e r i n g o u t o f t h e , : g r a n i t i c " l a y e r t o w a r d s t h e w e s t e r n s i d e o f t h e v a l l e y . B o u g u e r d a t a ( F i g u r e 54b) i n d i c a t e c r u s t a l t h i c k n e s s e s u n d e r t h e G r e a t V a l l e y t h a t a r e c o m p a t i b l e w i t h t h e s e i s m i c d a t a . The l o w o v e r t h e w e s t e r n s i d e o f t h e v a l l e y and t h e h i g h o v e r t h e e a s t a r e p r o b a b l y t h e r e s u l t o f v a r i a t i o n s i n t h e u p p e r c r u s t . The B o u g u e r f i e l d does n o t p o i n t t o m a j o r d e p a r t u r e s f r o m i s o s t a s y s u c h as t h o s e o b s e r v e d o v e r t h e P u g e t L o w l a n d . i v . Summary The w e s t e r n s e a b o a r d of t h e U n i t e d S t a t e s and Canada i s g e n e r a l l y "anomalous i n l i t h o s p h e r i c s t r u c t u r e . Upper m a n t l e v e l o c i t i e s t e n d t o be l o w - 7.7 t o 7.9. The c r u s t i n many a r e a s i s much t h i n n e r t h a n a v e r a g e ( W o o l l a r d ' s s t a n d a r d s e a - l e v e l c r u s t i s 33 km. t h i c k ) . T h e r e a r e no r o o t s u n d e r t h e C a s c a d e s o r C o a s t M o u n t a i n s . The C o a s t a l T r o u g h g e n e r a l l y seems compen-s a t e d i n B r i t i s h C o l u m b i a and C a l i f o r n i a : c r u s t a l t h i n n i n g may be r e s p o n s i b l e . N e g a t i v e f r e e - a i r a n o m a l i e s o v e r t h e t r o u g h i n W a s h i n g t o n and O r e g o n may i n d i c a t e t h a t i t i s u n d e r c o m p e n s a t e d , i . e . t h a t a mass d e f i c i e n c y u n d e r l i e s i t . The p o s s i b l e i m p l i c -a t i o n s o f t h e s e d a t a a r e d i s c u s s e d i n t h e f i n a l c h a p t e r . 160. CHAPTER I X : SYNTHESIS AND CONCLUSIONS T h r e e a s p e c t s o f t h e o r i g i n o f G e o r g i a D e p r e s s i o n - t i m i n g , s t r u c t u r a l s t y l e , and c a u s e - have been c o n s i d e r e d i n t h i s s t u d y . Of t h e t h r e e , o n l y t i m i n g i s w e l l - k n o w n . G e o r g i a D e p r e s s i o n b egan t o s u b s i d e i n t h e Upper C r e t a c e o u s , as i s shown by t h e d i s t r i b u t -i o n o f t h e Nanaimo G r o u p . The s t r u c t u r e s t h a t g o v e r n e d t h e Upper C r e t a c e o u s s u b s i d e n c e have so f a r been i m p o s s i b l e t o s e p a r a t e f r o m p o s t - N a n a i m o Group s t r u c t u r e s .(£.!• M u l l e r and J e l e t z k y 1972 ; Rimme 1973; Simmons 1973; Hudson 1974; S t u r d a v a n t 1 9 7 5 ) ; t h e r e f o r e t h e s t r u c t u r a l s t y l e o f t h e o r i g i n a l d e p r e s s i o n r e m a i n s o b s c u r e . T h i s s t u d y has f o c u s s e d on t h e c a u s e o f t h e U p p e r C r e t a c e o u s d e p r e s s i o n and i t s r e l a t i o n t o t h e t e c t o n i c h i s t o r y o f s o u t h w e s t e r n B r i t i s h C o l u m b i a . H y p o t h e s e s f o r i t s o r i g i n i n c l u d e r i f t i n g , l i t h o s p h e r i c c o l l a p s e o v e r a p a l e o s u t u r e , and s u b s i d e n c e c o u p l e d t o t h e u p l i f t o f t h e C o a s t P l u t o n i c C omplex. T h e r e i s no e v i d -ence f o r a r i f t zone b e l o w G e o r g i a D e p r e s s i o n . H e a t f l o w i n t h e G e o r g i a S t r a i t a r e a i s s u b n o r m a l t o n o r m a l a t p r e s e n t (Hyndman 1 9 7 6 ) ; m e t a m o r p h i c a s s e m b l a g e s i n Upper C r e t a c e o u s - T e r t i a r y r o c k s a r e n o t i n d i c a t i v e o f h i g h g e o t h e r m a l g r a d i e n t s . V o l c a n i c s f o r m an i n s i g n i f i c a n t p a r t o f t h e Upper C r e t a c e o u s - T e r t i a r y s e c -t i o n i n G e o r g i a D e p r e s s i o n . The m a j o r argument f o r a s u t u r e n e a r t h e I n s u l a r B e l t / C o a s t P l u t o n i c Complex p r o v i n c e b o u n d a r y i s t h a t t h e P a l e o z o i c and T r i a s s i c o f V a n c o u v e r I s l a n d seem t o be a l l o c h t h o n o u s w i t h r e s p e c t t o N o r t h A m e r i c a . H owever, s u c h a s u t u r e more p r o b a b l y 1 6 1 . l i e s i n t h e S h u l a p s T e r r a n e , w h i c h i s c h a r a c t e r i z e d by s t r o n g d e f o r m a t i o n , j u x t a p o s i t i o n o f " o c e a n i c " and t e r r i g e n o u s s e d i m -e n t s , and Upper T r i a s s i c / L o w e r J u r a s s i c u l t r a m a f i t e e m p l a c e m e n t , r a t h e r t h a n i n t h e e a s t e r n I n s u l a r B e l t . The b o u n d a r y b e t w e e n t h e I n s u l a r B e l t and t h e C o a s t P l u t o n i c Complex on H a r d w i c k e and West T h u r l o w I s l a n d s shows no Upper T r i a s s i c - L o w e r J u r a s s i c t e r r i g e n o u s s e d i m e n t s o r s t r o n g d e f o r m a t i o n a s c r i b a b l e t o a p r e - p l u t o n i c s u t u r i n g e v e n t . I n s t e a d , i t i s a s t r a i g h t f o r w a r d " and r e l a t i v e l y u n d i s t u r b e d i n t r u s i v e c o n t a c t . I n g e n e r a l t h e b o u n d a r y b e t w e e n t h e two p r o v i n c e s i s t h e m a g m a t i c f r o n t o f t h e C o a s t P l u t o n i c C omplex. G e o r g i a D e p r e s s i o n l i e s i m m e d i a t e l y w e s t o f t h e w e s t e r n l i m i t o f d o m i n a n t l y p l u t o n i c t e r r a n e . I t b e g a n t o s u b s i d e i n t h e Upper C r e t a c e o u s , a p p r o x i m a t e l y a t t h e same t i m e t h a t t h e s o u t h e r n C o a s t P l u t o n i c Complex b e g a n t o r i s e , as shown by l a c k o f p o s t - A l b i a n m a r i n e s e d i m e n t s w i t h i n i t and w e s t e r l y -d e r i v e d A l b i a n e l a s t i c s i n t h e T y a u g h t o n T r o u g h . P r i o r t o t h e Upper C r e t a c e o u s t h e s i t e o f G e o r g i a D e p r e s s i o n was a g e n e r a l l y p o s i t i v e a r e a u n d e r g o i n g e r o s i o n . The c o i n c i d e n c e o f o p p o s i t e v e r t i c a l movements i n G e o r g i a D e p r e s s i o n and t h e C o a s t P l u t o n i c Complex i s s t r o n g e v i d e n c e t h a t t h e y w e re c o u p l e d . T h i s c o n c l u s i o n a l s o f i t s t h e o t h e r s e g ments o f t h e C o a s t a l T r o u g h - H e c a t e D e p r e s s i o n , t h e P u g e t - W i l l i a m e t t e L o w l a n d s , and t h e G r e a t V a l l e y . The C o a s t a l T r o u g h l i e s b e t w e e n a s e r i e s o f c o l i n e a r m a g m a t i c a r c s - t h e C o a s t P l u t o n i c C o m p l e x , t h e C a s c a d e P r o v i n c e , and t h e S i e r r a Nevada - and t h e w e s t e r n edge 162. o f t h e N o r t h A m e r i c a n P l a t e . I t s v a r i o u s s e g m e n t s do n o t ha v e a common s t r u c t u r a l s t y l e - t h e P u g e t L o w l a n d i s l a r g e l y f a u l t c o n t r o l l e d , w h i l e o n l y g e n t l e t i l t i n g a f f e c t e d t h e e a s t e r n G r e a t V a l l e y . N e i t h e r f a u l t i n g o r w a r p i n g emerges as t h e d o m i n a n t m e c h a n i s m o f s u b s i d e n c e . The s u b - b a s i n s o f t h e C o a s t a l T r o u g h o r i g i n a t e d a t d i f f e r e n t t i m e s , t h e G r e a t V a l l e y i n t h e Upper J u r a s s i c - Lower C r e t a c e o u s , G e o r g i a D e p r e s s i o n i n t h e Upper C r e t a c e o u s , and H e c a t e D e p r e s s i o n and t h e P u g e t - W i l l i a m e t t e L o w l a n d s i n t h e L a t e C e n o z o i c . Y e t i n e a c h c a s e t h e b e g i n n i n g o f s u b s i d e n c e r o u g h l y c o i n c i d e d w i t h t h e u p l i f t o f t h e a r c i m m e d i a t e l y t o t h e e a s t . T h i s c o i n c i d e n c e i s b e s t e x p l a i n e d by c o n p l e d v e r t i c a l movements o f a r c and t r o u g h . The C e n t r a l G r a b e n o f C h i l e , l y i n g w e s t o f t h e An d e a n - a r c on P r e c a m b r i a n - P a l e o z o i c b a s e m e n t , may have a s i m i l a r o r i g i n . The most o b v i o u s mechanism o f v e r t i c a l movement c o u p l i n g i s h o r s t - a n d g r a b e n t e c t o n i c s , as e x e m p l i f i e d by t h e T e t o n s and J a c k s o n H o l e . The C o a s t a l T r o u g h i s n o t a c o n v i n c i n g g r a b e n , h o w e v e r . A l t h o u g h m a j o r f a u l t s a r e p o s s i b l e , d o w n w a r p i n g has p l a y e d an i m p o r t a n t r o l e i n i t s f o r m a t i o n , a t l e a s t i n O r e g o n and C a l i f o r n i a . Thus a more c o m p l i c a t e d m echanism o f i n t e r a c t i o n i s l i k e l y . One p o s s i b i l i t y i s s u g g e s t e d by t h e e x p e r i m e n t s o f Ramberg ( 1 9 6 7 ) . I n h i s c e n t r i f u g e d m o d e l s , domes f o r m i n g f r o m a f l a t s o u r c e r e g i o n a r e a c c o m p a n i e d by m a r g i n a l s y n c l i n e s ( F i g u r e 5 5 ) . The r i s e o f m a t e r i a l i n t o t h e dome c r e a t e s a p r e s s u r e d e f i c i e n c y i n i t s v i c i n i t y i n t o w h i c h t h e o v e r l y i n g l a y e r c o l l a p s e s . A r i s i n g m o u n t a i n s y s t e m c o u l d c r e a t e a s i m i l a r 164. p r e s s u r e d e f i c i e n c y a r o u n d i t s e l f . I t i s p r o p o s e d h e r e t h a t t h e s u b s i d e n c e o f t h e C o a s t a l T r o u g h was i n i t i a t e d by l i t h o s p h -e r i c c o l l a p s e i n t o t h e " h o l e " g e n e r a t e d d u r i n g t h e u p l i f t o f t h e a d j a c e n t a r c . Such a d e p r e s s i o n , s i n c e i t i s d r i v e n by a p r e s s u r e / mass d e f i c i e n c y , i s n o t i s o s t a t i c . N e g a t i v e f r e e - a i r and i s o s t a t i c a n o m a l i e s s h o u l d be a s s o c i a t e d w i t h i t . I t w i l l be r e c a l l e d t h a t n e g a t i v e f r e e - a i r / i s o s t a t i c a n o m a l i e s a r e o b s e r v e d o v e r t h e P u g e t and W i l l i a m e t t e L o w l a n d s , t h e y o u n g e s t s e c t i o n s o f t h e t r o u g h . T h i s a n i s o s t a t i c c o n d i t i o n s h o u l d be e l i m i n a t e d i n t i m e by c o n t i n u e d m a n t l e f l o w . Y e t t h e G r e a t V a l l e y and G e o r g i a D e p r e s s i o n have been i n e x i s t e n c e s i n c e t h e C r e t a c e o u s . M o r e o v e r H e c a t e D e p r e s s i o n and p o s s i b l y t h e G r e a t V a l l e y a r e a p p r o x i m a t e l y i n i s o s t a t i c e q u i l i b r i u m : some me c h a n i s m o f c o m p e n s a t i o n must p e r -p e t u a t e t h e t r o u g h once i t i s c r e a t e d . B e s i d e s s e d i m e n t l o a d i n g , two s u c h m echanisms a r e p o s s i b l e : c r u s t a l t h i n n i n g e i t h e r by e r o s i o n a t t h e b a s e o r by e x t e n s i o n ( A r t e m j e v and A r t y u s h k o v 1 9 7 1 ) , and p h a s e c h a n g e s , f o r ex a m p l e t h e g a b b r o - e c l o g i t e t r a n s f o r m a t i o n ( G r e e n and R i n g w o o d 1967; C o l l e t t e 1 9 6 8 ) . The s e i s m i c e v i d e n c e so f a r a v a i l a b l e f r o m B r i t i s h C o l u m b i a i s c o m p a t i b l e w i t h e i t h e r o f b o t h o f t h e s e c o m p e n s a t i o n m o d e l s . E l o n g a t e b a s i n s a r e a c h a r a c t e r i s t i c f e a t u r e o f a r c - t r e n c h g a p s . So f a r , v e r t i c a l movements, u p l i f t s , and d e p r e s s i o n s i n a r c - t r e n c h gaps have been e x p l a i n e d i n r e l a t i o n t o t h e p r o c e s s o f s u b d u c t i o n ( 6 . 1 . L o m n i t z 1969; F y f e and M c B i r n e y 1 9 7 5 ) . T h i s s t u d y o f t h e C o a s t a l T r o u g h has shown t h a t some f o r e a r c b a s i n s a r e g e n e r a t e d , n o t by s u b d u c t i o n , b u t by e v e n t s w i t h i n t h e o v e r -165 . r i d i n g p l a t e i t s e l f . A b e t t e r u n d e r s t a n d i n g o f t h e r e l a t i o n s h i p b e t w e e n a r c magmatism and u p l i f t and o f t h e mechanisms o f a r c u p l i f t w i l l d o u b t l e s s s h e d more l i g h t on t h e g e n e s i s o f t h e C o a s t a l T r o u g h . 166 . BIBLIOGRAPHY A l l e n , J . E . , 1958. C o l u m b i a R i v e r Gorge - P o r t l a n d t o t h e D a l l e s , G u i d e b o o k f o r F i e l d T r i p s . G e o l . S o c . Am. C o r d i l l e r a n S e c t i o n , E u g ene, O r e g o n , p.4. A r m s t r o n g , R.L., J . Suppe, 1973. P o t a s s i u m - a r g o n g e o c h r o n o m e t r y of M e s o z o i c i g n e o u s r o c k s i n N e v a d a , U t a h and s o u t h e r n C a l i f o r -n i a . G e o l . Soc. Am. B i i i l l . 8 4 , p . 1 3 7 5 . A r t e m j e v , M.E., E.V. Artyushkov-j.. 1 9 7 1 . S t r u c t u r e and i s o s t a s y o f t h e B a i k a l R i f t and t h e m echanism o f r i f t i n g . J o u r . Geophys. Res. 76, p.1197. A s i h e n e , K.A.B., 1970. The T e x a d a F o r m a t i o n o f B.C. and i t s a s s o c i a t e d m a g n e t i t e c o n c e n t r a t i o n s . unpub. PhD - UCLA. B a e r , A . J . , 1973. B e l l a C o o l a - L a r e d o Sound m a p - a r e a s , B.C. G e o l . S u r v . Can. Memoir 372. B a i l e y , E.H., W.P. I r w i n , D.L. J o n e s , 1964. F r a n c i s c a n and r e l a t e d r o c k s , and t h e i r s i g n i f i c a n c e i n t h e g e o l o g y o f w e s t e r n C a l i f o r n i a . C a l i f . D i v . M i n e s G e o l . B u l l . 183. B a k e r , B.H., P.A. Mohr, L . A . J . W i l l i a m s , 1972. G e o l o g y of t h e e a s t e r n r i f t s y s t e m of A f r i c a . G e o l . Soc. Am. S p e c i a l P a p e r 136, p.67. B a l d w i n , E w a r t , 1964. G e o l o g y o f O r e g o n . 2nd e d i t i o n . J.W. E d w a r d s , pub. B a r b a t , W.F., 1 9 7 1 . M e g a t e c t o n i c s o f t h e C o a s t R a n g e s , C a l i f . G e o l . Soc. Am. B u l l . 82, p. 1541. B e a u l i e u , J.D., 1 9 7 1 . G e o l o g i c f o r m a t i o n s o f w e s t e r n O r e g o n . O r e . D e p t . G e o l . M i n . Res. B u l l . 70. B e l o u s s o v , V.V., 1969. C o n t i n e n t a l r i f t s i n P . J . H a r t , e d . , The E a r t h ' s C r u s t and U p p e r M a n t l e . Amer. Geophys. Un. monograph 13. p. 539. B e r g , J.W., L y n n T r e m b l y , D.A. E m i l i a , J.R. H u l t , J.M. K i n g , L.T. L o n g , W.R. M c K n i g h t , S.K. Sarmah, R. S o n d e r s , J.V. T h i r u v a t -h u k a l , D.A. V a s s l e r , 1966. C r u s t a l r e f r a c t i o n p r o f i l e , O r e g o n C o a s t Range. S e i s . S o c . Am. B u l l . 56, p.1357. B e r r y , F.A., 1973. H i g h f l u i d p o t e n t i a l s i n t h e C a l i f . C o a s t Ranges a n d . t h e i r t e c t o n i c s i g n i f i c a n c e . A.A.P.G. B u l l . 5 7 , p .1219. B e r r y , M.J., W.R. J a c o b y , E.R. N i b l e t t , R.A. S t a c e y , 1 9 7 1 . A r e v i e w o f g e o p h y s i c a l s t u d i e s i n t h e C a n a d i a n C o r d i l l e r a . Can. J o u r . E a r t h S c i . 8,p.788. 167 . B e r r y , M.J., D.A. F o r s y t h , 1975. S t r u c t u r e o f t h e C a n a d i a n C o r d i l l e r a f r o m s e i s m i c r e f r a c t i o n and o t h e r d a t a . Can. J o u r . E a r t h S c i . 1 2 , p.2. B l u n d e n , R.H. V a n c o u v e r ' s Downtown ( G o a l ) P e n i n s u l a . Unpub. B.Sc. UBC 1971 B o s t r o m , R.C., 1968. 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G e o l o g i c map of O r e g o n w e s t of t h e 121 s t . m e r i d i a n ^ 3 t a t e o f O r e g o n D e p t . G e o l . M i n e r a l R e s . M i s c . G e o l . I n v e s t i g a t i o n s Map 1-325. W h i t e , W.R.H., J.C. S a v a g e , 1965. A s e i s m i c r e f r a c t i o n and g r a v i t y s t u d y of t h e e a r t h ' s c r u s t i n B.C. S e i s . Soc. Am. B u l l . 55, p . 4 6 3 . 178 . W i l l i a m s , H a r o l d , M.J. K e n n e d y , E.R.W. N e a l e , 1972. The A p p a l a c h -i a n S t r u c t u r a l P r o v i n c e , i n : V a r i a t i o n s i n T e c t o n i c S t y l e s i n  Canada . G e o l . A s s . Can. S p e c . P a p e r 11, p. 118. W i l s h i r e , H.G., 1969. M i n e r a l l a y e r i n g i n t h e T w i n L a k e s Q r a n o d i o r i t e , C o l o r a d o . G e o l . S o c . Am. Memoir 115, p . 2 3 5 . Woodsworth, G., 1974. G e o l o g y o f t h e Mount R a l e i g h a r e a , B r i t i s h C o l u m b i a . unpub. PhD, P r i n c e t o n U n i v e r s i t y . W o o l l a r d , G. P. 1969. R e g i o n a l v a r i a t i o n s i n g r a v i t y . A.G.U. memo. 13, p. 3 2 0. Y o r k e , D. and F a r g u h a r , R.M., 1972. The E a r t h ' s Age and G e o c h r o n o -l o g y . Pergamon P r e s s , O x f o r d . CPldtfr WAtstriChttav: QamQa " - a > 5ar\.cn\a.^ 1 suro-nm M riy\C-Y>\Q.r j i Allot cun (Jtxrrtmx > <?irr',a.<;ia.r' Crj{nd-\an. a.uyv'tan M<o(t&* r/<J4rc ?<i»i ')ivi('HKir'a.'Y) Nor ian "Vancouver Jsia.nct -> Qtor^CL Strtutr' cc Coast /Mown\a.ins Oia»am!i? G r p mc / cc o f t p t i tNN5YL\A NJAN mc 4-Qimbiir <Brclun bade Jf vii, •yi/Yintsu,L0L.r)rYnS. and • wc A 'yiutmk' dates lnwanza. 'Volcanics hlmd 1 niru^ioms -orbit down Tin ch ,JwP n 4< O'trsew 'dwHbn Ck} IS* , Quatsino r/-w t-5 JKar'm H.t S tm* f'W. bos Junt Ins wc jiitihlwtCx ru ft rhm, rtciio Jsltnd'y-m . wc MA /biUti .sand sieve 5icktr Group bas/xnd m • (5 th nrtmi- space plot Jor -ike Upper Paleozoic -CMcsozoic of-'Vancouver J stand and the Coast Chits.} 4 9 0 - 5 2 0 ^  • dfozjinnitrt I ' - ^ Cr?a$ayU'Y) Qr-p Cc Cad^allacUr Qrv Chrqvsson tyre-ChP arq •phyilibt mc 7adU3S5 O^oo^nloan. Qrp /mc cDcnd-my CreiK Orp ladder Qrp mc-t'mi boundary cf Uncertain a^fr- 1 MMtpnfarml I y -fault boundary diruticn of jidr/nertr lithol mbois yyiC warm; clastic sed mints CC continental clastic iidwi"its and i-»Ante und acidic yoicxmt rocks ^rictim>'Viayil bos tasic yoteamc rottis ^rredoyniYiarihr ts [fmtttfit Ch chtrt <J>later x 50*22' 126* 00' V"«8a F A N N Y I S L A N D C O A S T M O U N T A I N S Striitiaraphic units HtNANZA TM volcanic breccia, twn btuatu. to massivt Jim Hack, grvywacki uHTB AR.8LEDOWN TM thi>< ntOJed btaclt durt amyinc firiyv/ackl TARSON 3A V TM black, la »n nutt a, siliceous limstctu, bead chert, any SfmisitM QI/ATSINO TM massivi iiglit'arty limestone with sparse siliceous iincfbtds uTIKu u%Km ' ftow'5 Sparse sediment breccia ftnses 5 UPPER- basalt pillow basalt £ MiDDIE - broken-yilltw breccia, | aqua/jent t*ff, breccia. * tcvvEft jtfllm basalt Jntrusivc units JURASSIC o j M a r i UIOIUTZ weak. Jjliuti?n defined (nclteisnSj *najtc ybases Jfomblivde qabbros Ynainly peripheral to younqist quartz aimtt : 'tviaU textural/ ctnupositi/nai variability 'vVtUbfri Channel autistic quartz, dioritf 'Miners 'Bay aabbro Larai cquant jHaqtoelasi, interstitial Hnaf[c phases rToj>ojra,phic Ltyitawnt rg) hm'itmtal bedding _JV- ctrtunud bidding . — J J U L V C U (rninorjold.) axial plant, (minorplA) joliation Vancouver Orp ^> h^iation yluttmic rocks ZMctcrs 1000 SO' 22 I26"00' I25"42' G E O L O G Y of J-CARDWICKZ and WEST THURLOW ISLANDS, B.C. J.ZNchon 19 74 

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