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Cenozoic thermal and tectonic history of the Coast Mountains of British Columbia : as revealed by fission.. Parrish, Randall Richardson 1982

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CENOZOIC THERMAL OF  AND TECTONIC HISTORY  THE COAST MOUNTAINS OF BRITISH  COLUMBIA  AS REVEALED BY FISSION TRACK AND GEOLOGICAL DATA AND QUANTITATIVE THERMAL  MODELS  by Randall  Richardson  B.A., M i d d l e b u r y M.Sc,  University  Parrish  C o l l e g e , 1974  of B r i t i s h  Columbia,  1977  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE  REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in  THE  FACULTY OF GRADUATE STUDIES  DEPARTMENT OF GEOLOGICAL SCIENCES UNIVERSITY OF BRITISH  We a c c e p t  this  thesis  to the required  THE  COLUMBIA  as conforming standard  UNIVERSITY OF BRITISH January,  © Randall Richardson  COLUMBIA  1982 Parrish,  1982  In p r e s e n t i n g  this thesis in partial  f u l f i l m e n t of  requirements f o r an advanced degree at the  the  University  o f B r i t i s h Columbia, I agree t h a t the L i b r a r y s h a l l make it  f r e e l y a v a i l a b l e for reference  and  study.  I  further  agree t h a t p e r m i s s i o n f o r e x t e n s i v e copying of t h i s t h e s i s f o r s c h o l a r l y purposes may  be  department o r by h i s o r her  granted by  the head of  representatives.  my  It i s  understood t h a t c o p y i n g o r p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain  s h a l l not  be  allowed without my  permission.  Department of  Geological  Sciences  The U n i v e r s i t y of B r i t i s h 2075 Wesbrook P l a c e Vancouver, Canada V6T 1W5 Date  DE-6  (2/79)  Columbia  M&*n&r /if. Ml  written  ii  Abstract Fission  track  dating  of  z i r c o n and  used to determine the Cenozoic u p l i f t Columbia Coast  Mountains  from  obtained  rocks  variable  from  a l t i t u d e , and movement  the  and  of  resulting  deformation  h i s t o r y of the  50°-55°N.  115  been  British  dates  were  geographic l o c a t i o n and  date  of  a p a t i t e has  pattern  the  constrains  fission  the  track retention  isotherms (175°C f o r z i r c o n , 105°C f o r a p a t i t e )  within  the  crust. Because  (apparent  uplift  r a t e s ) cannot always be used c o n f i d e n t l y to estimate  actual  rates  of  date-altitude  uplift,  formulated  to  denudation, dates,  a f i n i t e d i f f e r e n c e numerical  construct  and  correlations  models  of  heat  flow,  c o o l i n g that s a t i s f y not only  but a l s o present  heat  g e o l o g i c c o n s i d e r a t i o n s , and  flow,  other  fission  derived  very  close  apparent reflect The  from  to  relationship  uplift  and  fission  q u a n t i f i e d and Orogenic  rates  however,  post-orogenic  often  c o o l i n g a,nd  of  of  uplift. exceed  during  dates,  estimates  apparent  uplift  Zircon-derived  modeled r a t e s  relaxation  of  and  isotherms.  the movement of isotherms to r a t e s of  t r a c k - d e r i v e d apparent  uplift  rates  is  discussed. rapid  cooling  and  Cretaceous to Eocene time in most of Rates  track  a p a t i t e d a t e - a l t i t u d e c o r r e l a t i o n s are  modeled  rates,  fission  track-derived  was  uplift,  isotopic  of paleo-geothermal g r a d i e n t . In most cases, rates  scheme  orogenic  uplift the  u p l i f t were near  occurred  Coast 1.0  from  Mountains.  km/Ma, causing  s e t t i n g of K-Ar c l o c k s i n b i o t i t e  and hornblende.  rates  ranged  during  the a x i a l less of  the middle  Cenozoic  Uplift  from 0.2 km/Ma i n  r e g i o n of the mountains between 52° and  55°N  to  than 0.1 km/Ma south of 52°N. The moderate r a t e s north  52°N were l i k e l y the r e s u l t of gradual e r o s i o n  thickened  during  northern u p l i f t  Eocene orogeny. A thermal  origin  i s not l i k e l y . Rates of u p l i f t  were low d e s p i t e a r c - r e l a t e d v o l c a n i c  of  crust  for t h i s  south of 52°N  activity  during the  Oligocene and Miocene. Accelerated  uplift  i n the Late  Miocene  near  Bella  Coola-Ocean F a l l s was probably the r e s u l t of passage of the transverse about  Anahim V o l c a n i c B e l t or hotspot beneath the area  10 Ma ago,  a f t e r which u p l i f t  Rapid Pliocene-Recent up  to  0.75 km/Ma  uplift  elevated  a  slowed.  south of 52°N at r a t e s of broad  region  present southern Coast Mountains and deforming  c r e a t i n g the 7-10 Ma l a v a s  erupted on the mountains' east f l a n k . I t i s suggested this  uplift  resulted  from thermal expansion  that  i n the mantle  r e l a t e d t o a westward jump i n the locus of l a t e Neogene a r c volcanism.  The  extent of t h i s r a p i d Pliocene-Recent  c o r r e l a t e s with the area above subducted uplift  slab  the Juan  de  uplift  Fuca-Explorer  and confirms a r e l a t i o n between c o n t i n e n t a l  and p l a t e t e c t o n i c  setting.  iv  TABLE OF  CONTENTS  T I T L E PAGE . i ABSTRACT i i TABLE OF CONTENTS iv L I S T OF TABLES vi L I S T OF FIGURES v i i INTRODUCTORY REMARKS 1 CHAPTER 1. FISSION TRACK DATING, APPARENT UPLIFT RATES, AND PATTERNS OF UPLIFT 4 Abstract • 5 Introduction .• 7 G e o l o g i c a l S e t t i n g Of The C o a s t M o u n t a i n s 8 F i s s i o n T r a c k D a t i n g And I t s I n t e r p r e t a t i o n 13 Sampling Techniques 16 A n a l y t i c a l Techniques 19 A r e a l V a r i a t i o n Of A p a t i t e And Z i r c o n D a t e s 28 Sea L e v e l A p a t i t e D a t e s 28 Sea L e v e l Z i r c o n D a t e s 30 Variation Of Dates With A l t i t u d e And A p p a r e n t U p l i f t Rates 32 Kemano 33 Northern King Island-Ocean F a l l s 34 B e l l a Coola V a l l e y 38 Mount Waddington 39 C e n t r a l Bute I n l e t 41 Mount Bute-Mount R a l e i g h A r e a 42 Pemberton A r e a 44 S p a t i a l - T e m p o r a l V a r i a t i o n s Of A p p a r e n t U p l i f t R a t e s ... 45 E s t i m a t e s Of T o t a l U p l i f t 49 U p l i f t S i n c e 40 Ma 51 U p l i f t S i n c e 10 Ma 52 Miocene Paleogeography 55 Neogene E r o s i o n S u r f a c e s And D e f o r m a t i o n 59 D i s c u s s i o n And P o s s i b l e C a u s e s Of U p l i f t 68 Summary 72 Acknowledgements 74 References 75 CHAPTER 2. HEAT FLOW MODELS, THERMAL EVOLUTION, AND THE CAUSES OF UPLIFT 82 Abstract 83 Introduction 85 G e o l o g y Of The C o a s t M o u n t a i n s 86 T h e r m a l M o d e l i n g Of M o u n t a i n B e l t s 87 The M o d e l 89 D i s c u s s i o n Of P a r a m e t e r s 93 S u r f a c e Temperature V a r i a t i o n 93 Heat P r o d u c t i o n 94 Scale Height 95 C o n d u c t i v i t y And D i f f u s i v i t y 95 Lapse Rate 96 E r o s i o n - u p l i f t Balance 96 Reduced Heat Flow 97 U p l i f t Rate 98  A p p l i c a t i o n To The Coast Mountains 99 O b j e c t i v e s Of Modeling 99 P r e s e n t a t i o n Of The Models 100 Kemano 104 Northern King I s l a n d - Ocean F a l l s 105 Mount Waddington 108 C e n t r a l Bute I n l e t - Mount R a l e i g h 110 D i s c u s s i o n Of Models 114 Causes Of U p l i f t 116 Orogenic Culmination And U p l i f t 116 Middle Cenozoic U p l i f t 118 Late Neogene U p l i f t 119 U p l i f t In The North (52° To 55°N) 120 U p l i f t In The South (49° To 52°N) 122 Summary 133 Acknowledgements 135 References 136 CHAPTER 3. REFINEMENT OF APPARENT UPLIFT RATES DETERMINED BY FISSION TRACK DATING 141 Abstract 142 Introduction 143 Ways To Generate Apparent U p l i f t Rates 144 A Heat Flow Model And I t s A p p l i c a t i o n 145 Apparent Vs. True U p l i f t Rates 148 Apparent U p l i f t Rates During U p l i f t 149 Apparent Rates During Isotherm R e l a x a t i o n 152 Discussion 153 Acknowledgements 155 References 156 APPENDIX 1. DESCRIPTION OF THE FORTRAN PROGRAM COASTMTN ...158  vi  LIST OF TABLES Table I. Values Of p Determined From Dating The Tuff F i s h Canyon Table I I . A n a l y t i c a l Data For The T u f f Of F i s h Canyon Table I I I . F i s s i o n Track A n a l y t i c a l Data Table IV. K-Ar A n a l y t i c a l Data Table V. Parameters Of Thermal Models  Of  23 24 25 49 101  LIST OF FIGURES F i g u r e 1. Geographic And Geologic Reference Map 11 F i g u r e 2. K-Ar B i o t i t e Dates In The Coast Mountains 17 F i g u r e 3. L o c a t i o n Of Samples Dated By F i s s i o n Track Or K-Ar 18 Figure 4. Sea L e v e l Or Low A l t i t u d e F i s s i o n Track A p a t i t e Dates 29 F i g u r e 5. Sea L e v e l Or Low A l t i t u d e F i s s i o n Track Z i r c o n Dates 32 Figure 6. F i s s i o n Track Date Vs. A l t i t u d e For Kemano And Northern King Island-Ocean F a l l s 36 F i g u r e 7. F i s s i o n Track Date Vs. A l t i t u d e For B e l l a Coola V a l l e y And Mount Waddington 39 Figure 8. F i s s i o n Track Date Vs. A l t i t u d e For C e n t r a l Bute Inlet 42 F i g u r e 9. F i s s i o n Track Date Vs. A l t i t u d e For Mount Bute, Mount R a l e i g h , And For The Pemberton Area 44 F i g u r e 10. T o t a l U p l i f t Since 40 Ma Ago (Late Eocene) 50 F i g u r e 11. T o t a l U p l i f t Since 10 Ma Ago (Late Miocene) .... 54 F i g u r e 12. Miocene Paleogeography 58 Figure 13. D i s t r i b u t i o n Of Late Miocene Lavas And Remnants Of Miocene-Pliocene E r o s i o n Surfaces 62 F i g u r e 14. Smoothed Surface Of Summit A l t i t u d e s In The Coast Mountains 66 F i g u r e 15a. Thermal E v o l u t i o n Diagram For The Kemano Area .102 Figure 15b. Thermal E v o l u t i o n Diagram For The King Island-Ocean F a l l s Area 107 Figure 15c. Thermal E v o l u t i o n Diagram For The Mount Waddington Area 110 Figures 15d And 15e. Thermal E v o l u t i o n Diagram For Mount Bute And Mount R a l e i g h (d) And C e n t r a l Bute I n l e t (e) ..112 F i g u r e 16. G r a v i t y Anomalies And C r u s t a l Thickness In The Coast Mountains Area 125 Figure 17. R e c o n s t r u c t i o n Of Past P l a t e Movements And Orthogonal Convergence Rates 127 F i g u r e 18. Schematic S t r u c t u r a l S e c t i o n Across The Southern Coast Mountains 131 F i g u r e 19. Surface Heat Flow E v o l u t i o n For Thermal Models .147 F i g u r e 20. Apparent U p l i f t Rate Vs. Time Curves For Thermal Models With Short Term U p l i f t 150  1  INTRODUCTORY REMARKS  T h i s t h e s i s i s a study of the Cenozoic t e c t o n i c s of the Coast Mountains of B r i t i s h Neogene  uplift  has  Columbia from 50°-55°N. The  r e c e i v e d s p e c i a l a t t e n t i o n because i t s  fundamental cause i s p o o r l y The  first  understood.  paper documents the timing  uplift  throughout  fission  t r a c k d a t i n g of a p a t i t e and  plutonic  rocks  utilizing  fission  estimates track  the  of  of the  the  known. The f i r s t  observations  zircon  two  and  earth's  variable  from  In a d d i t i o n , by to  localities.  measure  Reasonable  which  the  i s not  paper concludes with a d i s c u s s i o n  uplift  paper the  of  and data,  the  fission  correlations and  of the  i t places  these  thesis  crust  while  accepts  as  experiencing  geothermal  input  integrates  thermal  track data. A f i n i t e d i f f e r e n c e  temperature, and non-equality  fission  separated  for zircon,  implications  sub-crustal  program  of  by means of  possible  computer program was w r i t t e n t o simulate the  done  Mountains.  at  patterns  i n a broad r e g i o n a l framework.  second  modeling  was  i t was  temperature  track-derived  The  and  of these g r a d i e n t s can i n turn help to r e f i n e  geological  fission  This  Coast  gradients  retention  precisely  area.  track data,  paleo-geothermal  late  of  flux, uplift  the flow of heat i n variable  uplift,  variable and  surface  erosion.  The  the u p l i f t h i s t o r y d e r i v e d  from  t r a c k d a t i n g . I t produces, by t r i a l and e r r o r , a set  of values  of u p l i f t  r a t e , s u b - c r u s t a l heat f l u x ,  and  other  2  parameters,  that  fit  paleo-geothermal  beneath  observed  mantle  I t i s shown that thermal expansion of the  mantle  models  Coast  are  Mountains  present  dates,  analyse  These  and  track  flow  the  gradients,  fission  heat  measurements. processes.  all  used  has  to  been r e s p o n s i b l e for the  l a t e Neogene u p l i f t . T h i s s i t u a t i o n d i f f e r s from most of world's great mountain South  I s l a n d New  involved.  In  Columbia  (Himalaya,  respect,  the  uplift  possible  framework  of  causes  plate  of  fission  track  vs. a l t i t u d e curve) has uplift.  the  are  discussed  the  caution  that  departure  r a t e s d e r i v e d from  dates  (the  thermal models, and and  true  Corrections  rates can  f o r the  slope  previous  zircon of  be  and date  been assumed equal to the true  rate  uplift),  rate) w i l l not  apparent  a  the  with or without  of  the  in  must  track data. In  In the common s i t u a t i o n where the  (the apparent u p l i f t  like  States.  isotherms  e i t h e r moving upwards (by u p l i f t ) or downwards (by relaxation  British  and an e x p l a n a t i o n  when i n t e r p r e t i n g f i s s i o n  s t u d i e s , the apparent u p l i f t apatite  Alps,  i s suggested.  t h i r d paper d i s c u s s e s  exercised  of  uplift  tectonics,  Cenozoic t e c t o n i c p a t t e r n s The  in  Mountains i s more s i m i l a r to areas  Colorado P l a t e a u of the western U n i t e d The  European  Zealand) in that c r u s t a l t h i c k e n i n g i s not  this  Coast  systems  the  from  isotherm  the slope of t h i s be the  true  are  rate.  curve The  t r u e rate i s q u a n t i f i e d using  g e o l o g i c a l s i t u a t i o n s where the apparent  are  grossly  different  are  identified.  be a p p l i e d to r e a l s i t u a t i o n s i f knowledge  3  of the g e o l o g i c a l h i s t o r y and probable  geothermal  is  been  available.  This  correction  has  overlooked  previous s t u d i e s , and i t emphasizes the importance and denudation  on the upward flow of heat.  gradient in  of u p l i f t  4  CHAPTER 1 .  CENOZOIC THERMAL EVOLUTION AND TECTONICS  OF THE  COAST MOUNTAINS OF BRITISH COLUMBIA I :  FISSION TRACK DATING, APPARENT UPLIFT RATES,  AND  PATTERNS OF UPLIFT  5  Abstract  The dramatic scenery of the Coast Mountains Columbia  has been  this u p l i f t , present  produced  however,  British  by r a p i d l a t e Neogene u p l i f t ;  i s not o b v i o u s l y  plate tectonic  of  related  to the  regime o f f the west coast of B r i t i s h  Columbia. To study t h i s problem, the a r e a l p a t t e r n s , and  total  amounts of Cenozoic u p l i f t  been determined using f i s s i o n apatite  separated  from  track  rocks  from 50° t o 55°N have  dating  collected  of at  a l t i t u d e s along s e v e r a l t r a v e r s e s through the Assuming  rates,  zircon  and  high and low  mountains.  b l o c k i n g temperatures of 105°C and  175°C f o r  a p a t i t e and z i r c o n , r e s p e c t i v e l y , paleo-geothermal g r a d i e n t s have  been  measured  near  Ocean  Sub-crustal changed  a t Kemano (26°+4-6°C/km  Falls-King  Island  heat  can be shown t o have  flow  (17°±2°C/km  20  Ma  ago).  significantly  through time near King I s l a n d .  During the middle Cenozoic, u p l i f t part  35 Ma ago) and  of  the northern  0.1-0.2 km/Ma temporally  from  and  25  (52°-55°N) to  15  tectonically  adjacent  Queen  Charlotte  slightly  i n the Late Miocene  Ma  Coast  Mountains  ago; t h i s  related  basin.  r a t e s i n the a x i a l were  uplift  to subsidence i n the  Uplift  rates  increased  t o 0.4 km/Ma, probably because  of passage of the Anahim 'hot spot' beneath the a r e a . Miocene-Recent  erosion  altitudes  been  Despite  and this,  i n the n o r t h  more  relicts  was  extensive of Miocene  has than  Late  reduced summit farther  river  south.  valleys  and  6  topography  are s t i l l  o c c a s i o n a l l y preserved.  The area of the present southern Mountains rates  (50° t o  52°N)  Coast  was near sea l e v e l and experienced very low u p l i f t  (<0.1 km/Ma)  throughout  the middle Cenozoic, d e s p i t e  i t s more a c t i v e volcanism. Late Miocene b a s a l t i c  lavas  that  were erupted onto a mature e r o s i o n s u r f a c e near Taseko Lakes were  elevated  uplift  of  this  uplift  rapid Pliocene-Pleistocene Coast  l e d t o the i n v e r s i o n of the Miocene  result  of  Mountains.  The  was broadly p l a t e a u - l i k e , and t h i s  Middle Cenozoic u p l i f t the  by  (>0.5 km/Ma) of the southern  profile uplift  and warped  from 52° t o  topography.  55°N  was  denudation and d i m i n i s h i n g u p l i f t  probably a f t e r the  t e r m i n a l Eocene orogenic episode which g r o s s l y thickened the crust.  This  restoration uplift  middle of  a  Cenozoic normal  crustal  i n the Coast Mountains  expansion  i n the mantle  erosion  was  south.  in  the  t h i c k n e s s . Late Neogene the r e s u l t  of  thermal  caused by a mantle hot spot near  53°N and changes i n the geometry farther  resulted  of  the subducted  slab  7  I n t r o d u c t ion The  Coast  Mountains  of  B r i t i s h Columbia,  from sea l e v e l to a l t i t u d e s of up to 4 km, and p h y s i o g r a p h i c b a r r i e r over in  length  (Figure  1).  They  merge  with  of the Yukon T e r r i t o r y and the  of  northwestern  United  c o n t i n u i t y . The topography the rocks  themselves  Cretaceous  to  early  are  States  the  climatic  in  1000 km  St.  Cascade  Elias  Mountains  nearly  unbroken  i s c o n s i d e r a b l y younger than both  and  the  Tertiary  terminal time.  orogeny  in  Geological  i n d i c a t e s r a p i d Late Miocene-Pliocene u p l i f t 1970).  a  150 km wide and about  Mountains the  which r i s e  For the most p a r t , the u p l i f t e d  late  evidence  (Douglas et a_l  region i s continuous  d e s p i t e i t s j u x t a p o s i t i o n with both subduction and transform fault  plate  Columbia.  The  boundaries relation  off of  the  west  coast  of  to  plate  motions  uplift  t h e r e f o r e u n c l e a r . Lack of c l e a r c o r r e l a t i o n episodes adds to t h i s  i t s areal  and  temporal  (Tipper  1963,  1978),  Cenozoic d e p o s i t s (Rouse and possible  volcanic  i t i s necessary to  variability.  mapping of u p l i f t e d Late Miocene l a v a s i n Columbia  is  ambiguity.  To e x p l a i n the l a t e Cenozoic u p l i f t , quantify  with  British  southern  paleontological  Mathews  Geological  1979)  and  British work  on  study  of  Miocene e r o s i o n s u r f a c e s (Baer 1973, C u l b e r t 1971,  Mathews 1968)  has  problem,  the magnitude and t i m i n g of u p l i f t  but  provided  some  data  relevant  to  this  i s unknown  in areas that lack d e p o s i t s of l a t e Cenozoic age. The  technique of f i s s i o n t r a c k d a t i n g  of  apatite  has  8  recently  been  applied  areas of the world 1972,  Gleadow  to  chronology of u p l i f t  (Schaer e_t a_l  and  Lovering  1978a,b,  technique promises q u a n t i t a t i v e where  g e o l o g i c a l data are  timing of u p l i f t sufficient 1975, and  eroded  material  Wagner  and  Naeser  1979).  on  uplift  1978a)  uplift  such  rocks  has  The areas  of d e t r i t u s in adjacent  (Schaer e_t a_l on  batholiths  and  basins  the  where  also  of p l u t o n i c emplacement, u p l i f t  deposition  in  and,  rates  are p o s s i b l e . Studies from  Reimer  Determination of both  Lovering  present,  Wagner et a l 1977)  chronologies and  is  data  sparse.  (Gleadow and  relief  1975,  in s e v e r a l  yielded  unroofing,  (Wagner et  al  track d a t i n g of a p a t i t e  and  1979). This  paper  z i r c o n to q u a n t i f y  uses the  fission middle  and  late  Cenozoic.  uplift  h i s t o r y of the Coast Mountains of B r i t i s h Columbia. D e t a i l e d uplift  histories  uplift  and  Chapter models  2  are  building describes  of  heat  geophysical  useful  for  examining the causes of  paleo-geological  reconstructions.  the a p p l i c a t i o n of the u p l i f t  transfer  in  order  parameters important  to  data to  evaluate  in explanations  of  various uplift.  G e o l o g i c a l Sett ing of the Coast Mountains The  Coast  (Holland  1964),  Plutonic  Complex.  Mountains, are The  largely  as  physiographically coincident  with  defined  the  Complex (Roddick and Hutchison  i s dominated by J u r a s s i c to Eocene p l u t o n i c  granitic  Coast 1974) rocks  9  intruded  into  a  varied  assemblage  s u p r a c r u s t a l rocks. In recent years eugeoclinal or  stratified  the  1977,  metamorphosed  interpretation  of  rocks i n terms of a c c r e t e d t e r r a n e s  t e c t o n o s t r a t i g r a p h i c assemblages  (Monger  of  has  become  popular  Coney 1980). For i n s t a n c e the g r e a t e r p a r t of  the southern Coast P l u t o n i c Complex i n a d d i t i o n to Vancouver I s l a n d and p a r t of Queen C h a r l o t t e consist  of  a  terrane  termed  T r i a s s i c p i l l o w lavas are Woodsworth and Tipper  Islands  is  thought  Wrangellia  in  which t h i c k  found  (Monger  and  Price  to  1979,  1980).  Near Kemano, however, the e a s t e r n margin of the Complex is  u n d e r l a i n by rocks common to the Intermontane zone, p a r t  of the S t i k i n e terrane or S t i k i n i a 1980).  West  high-grade  of  P r i n c e Rupert  best  and  unknown,  Mesozoic  (Armstrong  have  pinch  although  occupying  the  axial,  and  granitic  been  south  extensively  and and  out  along  probably  southwards the  Paleozoic  by  or  early  Woodsworth 1979). These  i n the B e l l a Coola r e g i o n . Prince  Rupert  area  and  s t r i k e are metamorphic rocks that are P a l e o z o i c rocks of  t e r r a n e of southeast Alaska  1979). Recently  studied  (1970). The p r o t o l i t h age i s  c o r r e l a t i v e with the e a r l y to middle Alexander  gneisses  and exposed between Terrace  and Runkle 1979,  West of the core gneisses i n farther  Tipper  are  (1979) and Hutchison  largely  gneisses  known  and  (Figure 1). These rocks are c a l l e d the C e n t r a l  Complex  Hollister  rocks  core of the complex  metasediments,  Gneiss  these  (Woodsworth  discovered  Devonian  (Armstrong fossils  on  the  and Runkle Melville  10  Island  northwest  of P r i n c e Rupert confirm  (G.Woodsworth, personal communication terrane  this  correlation  1981). T h i s  Paleozoic  i s f a u l t e d a g a i n s t a p o r t i o n of W r a n g e l l i a  east edge of Hecate S t r a i t perhaps  elsewhere  on  (Woodsworth and Tipper  Queen  Charlotte Islands  near the  1980)  and  (Yorath  and  Chase 1981). These t e r r a n e s were continent  and  accreted  welded together  metamorphic,  and  the  by plutonism  d u r i n g Mesozoic time (Monger and plutonic,  to  North and  P r i c e 1979).  metamorphism Some  collisional  l a r g e p a r t of the p l u t o n i c and  volcanic a c t i v i t y  arc  above  Cenozoic subduction The  plutonic  of  the  s t r u c t u r a l f e a t u r e s were l i k e l y  r e l a t e d to t h i s a c c r e t i o n a r y ,  magmatic  American  process,  but  i s due  to a  an e a s t - d i p p i n g l a t e Mesozoic to e a r l y zone (Monger e_t a_l 1972).  chronology  of  the  Coast  Mountains  p o o r l y e s t a b l i s h e d ; only near Skeena R i v e r , Squamish, and the  Bella  Coola  region  are  there r e l i a b l e Rb-Sr or  dates which c l e a r l y d e f i n e the age et  al  K-Ar  1979,  R.L.Armstrong and  determinations,  (Figure cases  but  2),  a  indicate  which  of  intrusives  between  approximate  40  plutonic  in most, they give merely  the  u p l i f t - r e l a t e d c o o l i n g (Roddick and Hutchison  U-Pb  data).  and  140  ages i n a  time  in  (Harrison  R . P a r r i s h unpublished  range  is  since 1974,  Ma few  final  Harrison  et  a l 1979). There i s a general decrease in both K-Ar  dates  and  the probable  east.  Plutonism  ages of p l u t o n i c rock  culminated,  especially  ( l a t i t u d e 53°-60° N), with an  in  intense  from  west  to  the northern Eocene  segment  plutonic  and  11  F i g u r e 1. G e o g r a p h i c a n d g e o l o g i c r e f e r e n c e map. P l a t e boundaries o f f s h o r e a r e m o d i f i e d f r o m R i d d i h o u g h ( 1 9 7 7 ) , and t h e l o c a t i o n o f v o l c a n i c b e l t s i s f r o m B e v i e r e t a l ^ ( 1 9 7 9 ) a n d Berman a n d Armstrong (1980). The d a s h e d a n d d o t t e d l i n e r e f e r s t o t h e p h y s i o g r a p h i c boundary o f t h e Coast Mountains ( H o l l a n d 1964), and i t m e r g e s a l o n g t r e n d f a r t h e r s o u t h w i t h t h e Y a l a k o m a n d Fraser faults.  12  orogenic  event, a f t e r which a c t i v i t y v i r t u a l l y ceased.  in the south, approximately above Fuca  plate,  did  the  Oligocene to Recent  and plutonism continue, a l b e i t on a and Armstrong  a  cessation  slowed  resulted over  reduced  of  rapid  subduction  to  (Berman  at about  40-50 Ma  of  the  Coast  and  Armstrong  north of  south  Mountains  about  (Coney  Volcanic Belt  and  (Bevier et a_l 1979)  by  quiescence e_t a_l 1 972),  in  the  52°N  1978). T h i s  (Monger  igneous events  1980)  likely  subduction along the e n t i r e  the  i n t e r r u p t e d only by s c a t t e r e d (Berman  scale  in a middle Cenozoic p e r i o d of r e l a t i v e  much  de  a r c - r e l a t e d volcanism  coast and a change to transform motion and  Juan  1980).  A major change in p l a t e motion caused  subducting  Only  the  south  Miocene Anahim  i n the B e l l a Coola  region  (Figure 1). Volcanic  activity  east of the Coast Mountains r e v i v e d  in the Late Miocene with the e r u p t i o n of voluminous basalts,  which flowed over the present eastern flank of the  mountains. C l i m a t i c evidence mountainous Mathews  barrier  1979)  suggests  (Mathews  prior  to  initiation  of  this uplift  (Tipper 1963,  uplift.  the Plateau  of 51°N and transform motion significantly It  is  changed possible  and  no  middle  Rouse  Late  1963,  Miocene  Cenozoic Rouse and  or  Pliocene  l a v a s were deformed d u r i n g  Douglas et a l 1970).  The present p l a t e t e c t o n i c  1977).  plateau  regime of  subduction  south  to the north (Figure 1) has not  s i n c e at l e a s t that  the  10 Ma same  ago  (Riddihough  basic  plate  13  configuration  has  Cenozoic as w e l l This  been  tectonic setting to  f o r much  (Atwater 1970,  relatively  Miocene  present  stable  middle  contrasts  Pliocene  Byrne  uplift  1979,  to  sharply  of  late  the middle  Coney  1978).  Cenozoic  with  the  plate  rapid  Late  that gave r i s e to the present  topography.  F i s s i o n Track Dating and i t s I n t e r p r e t a t i o n Both the theory and the i n t e r p r e t i v e a s p e c t s of f i s s i o n track d a t i n g have been t r e a t e d by F l e i s c h e r Naeser  (1979),  and  and  will  Faul  anneal  1969,  a_l  (1975),  Wagner and Reimer (1972). For rocks i n  most g e o l o g i c c o n d i t i o n s , zircon  et  fission  tracks  in  at temperatures of about  Haack  1977,  Wagner  Zimmermann and Gaines 1978) and 175°C  and  apatite  and  100°C  (Naeser  Reimer  1972,  (Harrison e_t a l 1979),  r e s p e c t i v e l y . In t h i s paper a value of 105°C f o r a p a t i t e has been  used.  These  annealing s t u d i e s 1972),  estimates  are  (Naeser and Faul  theoretical  analysis  on  experimental  1969, Wagner  of  Zimmermann and Gaines 1978) and  based  such  perhaps  data  and  Reimer  (Haack  more  1977,  importantly,  s t u d i e s of n a t u r a l a n n e a l i n g i n boreholes (Naeser and Forbes 1976).  Data  on  zircon  e f f e c t i v e annealing between  that  biotite/sphene histories  of  i s much  temperature  of  apatite  fission  track,  well-dated  H a r r i s o n and McDougall  less appears  fission as  plutons  1980).  abundant, well  constrained  track  deduced (Harrison  but i t s  and  from  K-Ar cooling  e_t a_l 1979,  14  The t r a c k representing  retention  temperature  the e f f e c t i v e c l o s u r e  is a  temperature  in a process of i n c r e a s i n g l y e f f i c i e n t a  narrow  simplification  track  (Haack 1977)  retention  over  temperature  interval.  The  effective  closure  temperature  i s a function  of c o o l i n g  r a t e , but f o r  example,  v a r i e s only  from 98°C to 111°C f o r a p a t i t e when c o o l i n g  changes  from  1°C/Ma  to  lO°C/Ma,  1977). Dating z i r c o n and a p a t i t e reveal  average  can be used Cooling  cooling  to  rates  further  respectively  from  (Zimmermann rock  will  r a t e between 175°C and 105°C,  which  define  a  rate  the  f o r the vast m a j o r i t y  over the 175° t o 105°C temperature  single  closure  temperature.  of rocks i n t h i s study  i n t e r v a l v a r i e d only  from  3° to 1U C/Ma, r e s u l t i n g i n date d i f f e r e n c e s between a p a t i t e 0  and z i r c o n of 7 to 25 Ma. For the purposes of t h i s work, the track  retention  constant. This  temperature  and introduces  error  apatite  and  zircon  o  normal  temperatures  (25°C/km)  closure  younger  geothermal  retain fission  i n a p a t i t e . Deeper samples  is  gradients,  uplift  tracks  these  about  4  and e r o s i o n  first  in zircon  w i l l consequently  have  dates than shallower ones at any one l o c a l i t y .  If rocks a r e sampled large  be  temperatures  (105°, 175°C) correspond to depths of  upwards and w i l l  and l a t e r  to  respectively.  and 7 km i n the c r u s t . Rocks e x p e r i e n c i n g pass  considered  l i t t l e a d d i t i o n a l u n c e r t a i n t y . The  estimated at ± 5 - 1 0 C and ±20°C, Given  be  s i m p l i f i c a t i o n f a c i l i t a t e s data treatment and  modelling in  will  relief,  an  at d i f f e r e n t a l t i t u d e s i n areas of  increase  in fission  track date at higher  15  altitudes divided  i s usually present. by  the  fact,  difference  in  altitude  d i f f e r e n c e i n date w i l l be termed,  study, the apparent u p l i f t in  The  however,  is  r a t e . T h i s apparent u p l i f t  the  rate  at  which  the  column.  The  apparent  represent the u p l i f t level,  nor  does  rate  does  not  to  to  sea  n e c e s s a r i l y equal the denudation  rate  with respect to the average s u r f a c e of the land. The connotation  in  the term, apparent u p l i f t  both f o r convenience and because date-altitude  trends  are  consequences  of  uplift  rate, i s retained  the mechanism by which  the  developed i s p r i m a r i l y u p l i f t of  c r u s t a l r o c k s . C o o l i n g , denudation, are  the  necessarily  r a t e of the rocks with respect  it  rate,  critical  isotherms (105°, 175°C) moved downward with respect rock  in t h i s  and  isotherm  movement  the primary u p l i f t . The reader should  c o n t i n u a l l y bear i n mind the true meaning of  this  term;  a  c o n s i s t e n t usage has been maintained throughout t h i s study. The  apparent  uplift-erosion These  uplift  rate w i l l  rate only when s e v e r a l  conditions  are  that  1)  s t r i c t l y equal the true conditions  isotherms  h o r i z o n t a l and u n i n f l u e n c e d by e i t h e r  must  are  met.  have been  surface topography  or  v a r i a b l e thermal c o n d u c t i v i t y , 2) isotherms must remain at a constant uplift  depth  with  r a t e and 3)  respect  uplift  t o the s u r f a c e r e g a r d l e s s of  must  be  equal  to  erosion.  A  complex i n t e r r e l a t i o n s h i p e x i s t s between isotherm m i g r a t i o n , uplift, are  denudation, and c o o l i n g , and s i n c e these c o n d i t i o n s  r a r e l y met,  rates  must  caution  in  interpreting  apparent  uplift  be e x e r c i s e d . C o r r e c t i o n s f o r these e f f e c t s are  16  d i s c u s s e d i n Chapter track-derived  3, but  apparent  for  most  uplift  conditions,  rates  are  fission  approximately  correct. By d a t i n g a p a t i t e s c o l l e c t e d at v a r i o u s locations  i n the A l p s , Schaer  rates,  detailed  and data  and  e_t a_l (1975) and Wagner e_t a_l  (1977) were able to document u p l i f t of  altitudes  deformation  of  rates, s p a t i a l  variation  paleoisotherms.  No  are a v a i l a b l e f o r a mountain system  such  i n North  America.  Sampling Fresh g r a n i t i c  Techniques  rock samples  were  collected  for  study along four t r a v e r s e s a c r o s s the B r i t i s h Columbia Mountains is  mountains  of  for sampling of  2-4 km  topographic r e l i e f ,  both low and high  altitude  to  the area i s i d e a l rocks  and  detail,  across  Powell  the  Peak  near  Mount Waddington near the head of Knight  Inlet  ( F i g u r e s 2,3). The samples i n the B e l l a Coola the  adjacent  the mountains. In a d d i t i o n to the four t r a v e r s e s ,  two other areas were sampled i n Kemano,  Coast  ( F i g u r e 3 ) . Because the c o a s t l i n e of the p r o v i n c e  indented by numerous deep f j o r d s immediately  width  this  southern  f o o t , boat, and  traverse  were  helicoptor  traverse  and  c o l l e c t e d by a combination of transportation.  The  rocks  at  Powell Peak, Bute I n l e t , Mount R a l e i g h , and Mount Waddington were  collected  by the G e o l o g i c a l Survey of Canada and were  made a v a i l a b l e by Glen Woodsworth. In t o t a l , over were processed r e s u l t i n g  i n 115 f i s s i o n  70  track dates.  rocks  17  F i g u r e 2. K-Ar b i o t i t e dates i n the Coast ^fountains. Only data from the Coast Mountains are shown, and o n l y from p r e - O l i g o c e n e r o c k s . Contoured from a c o m p i l a t i o n of R. L. Armstrong which i n c l u d e s data from Wanless et a l (1964-1979), Richards and White (1970), Richards and McTaggart (1976), Nelson (1979), Bartholomew (1979), C a r t e r (1974), H a r r i s o n et a l (1979), and unpublished data of R. L. Armstrong.  19  A n a l y t i c a l Techniques Techniques Lovering  adapted  from Naeser (1976) and Gleadow and  (1975) were used to date a p a t i t e and z i r c o n i n t h i s  study using the p o p u l a t i o n respectively. separated  involved  to r e t a i n  magnetic water  minerals  crushing  the  detector  bromoform.  using  acetone,  and  Further  Initial  rapid  high  CARPCO separator,  heavy  mineral  separation  involved  volume  washing i n  separation a Frantz  using  magnetic  and methylene i o d i d e .  In d a t i n g a p a t i t e by the p o p u l a t i o n were made, one being  method, two  split  was  p o l i s h e d , and etched  splits  annealed at 480°-520°C f o r at l e a s t two  hours to remove spontaneous, n a t u r a l l y occuring annealed  mineral  s i e v i n g 1 kg samples of  fraction, a  methods,  were abundant and e a s i l y  and  -80+170  separation  and  separator  external  from n e a r l y a l l rocks c o l l e c t e d .  separation rock  Accessory  and  irradiated together  and  with  tracks. This  subsequently mounted,  the  other  split  which  r e t a i n e d the n a t u r a l t r a c k s . A p a t i t e e t c h i n g c o n d i t i o n s were 7% n i t r i c  a c i d at 22°-24°C f o r about 30 seconds. Tracks were  counted at 800x. Zircons  were  mounted  and  etched  procedure of Gleadow et a l (1976) using eutectic  KOH-NaOH  (depending on t r a c k natural  etch  density).  t r a c k s were f u l l y  used t o r e c o r d the induced 48%  HF  for  12  at  200 -210°C 0  Zircons  according FEP  teflon  to the and  a  f o r about 48 hours were  etched  until  exposed. Muscovite d e t e c t o r s were t r a c k d e n s i t y and were etched  in  minutes at 22°-24°C. Tracks i n z i r c o n were  20  counted at 1250x or 1600x i n o i l .  A geometry f a c t o r  of  0.5  was assumed f o r the e x t e r n a l d e t e c t o r method. Dates  were  calculated  a c c o r d i n g to the f i s s i o n  track  age equation, date=ln( 1 + (/>s//>i ) * U alpha/X 23 8  x (lA  23 8  2 38  f ission) (U /U  23 8  238  alpha and f i s s i o n ,  X  2 3 5  U  2 3 5  2 3 5  /U  is  2 3 8  the  respectively, * i s  the  23 5  thermal  i s the atomic r a t i o of uranium cross-section  neutron  i s o t o p e s , and  f o r neutron f i s s i o n  r e a c t i o n of  numerical  constants  i n Table I I , the equation becomes,  date (Ma ) = 6. 446x1 0  3  Thermal  neutron  In ( 1 +9 . 322x 1 0- (ps//>i )*) . 18  irradiation  was performed  Colorado at the USGS TRIGA r e s e a r c h r e a c t o r Geological and  )X )  decay constants f o r  2 38  . With a p p r o p r i a t e s u b s t i t u t i o n of  listed  2 38  alpha)  where X. a l p h a and X. f i s s i o n are U  dose, u  2 35  Survey  D.Rusling.  gradients  were  i n Denver,  (United  States  1974) under the s u p e r v i s i o n of C.W.Naeser  Neutron  dose  calibrated  measurements  and  internal  by the use of mica d e t e c t o r s on  NBS 962 g l a s s (Carpenter and Reimer 1974)  and  by  repeated  d a t i n g of a p a t i t e and z i r c o n standards from the t u f f of F i s h Canyon  p r o v i d e d by C.W.Naeser. Seven d i f f e r e n t  over a  period  of  one  year  were  c o n s i s t e n t c a l i b r a t i o n constant, dose  (#)  to  induced  track  done  to  irradiations arrive  at  a  r e l a t i n g thermal neutron  density  ( p i ) i n muscovite s e t  a g a i n s t the NBS 962 g l a s s a c c o r d i n g to the r e l a t i o n ,  For the c a l c u l a t i o n , the neutron dose (*) was determined  by  21  back-calculation  using  ps/pi, f o r the t u f f 27.9  Ma  (Steven  the counted  track  density ratio,  mineral standards assuming t h e i r age as  et  al  1967, based  on c o n s t a n t s l i s t e d i n  Table I I ) . S u b s t i t u t i n g 27.9 Ma as the date of the Fish  tuff  of  Canyon and r e a r r a n g i n g the above equation, the neutron  dose f o r i r r a d i a t i o n s and c a l i b r a t i o n was estimated by *(neutron/cm )=4.653x10 */(ps/pi) 2  1  where ps and />i r e f e r to t r a c k d e n s i t i e s i n minerals of the t u f f of F i s h Canyon. T h i s method was p r e f e r r e d s i n c e the NBS i r r a d i a t e d g l a s s e s RT-1 and RT-2 (Carpenter and Reimer 1974) were  found  to be somewhat u n r e l i a b l e as * c a l i b r a t o r s . The  NBS 962 g l a s s d i d , however, continue source  of  to  be  used  f o r the  induced t r a c k s i n adjacent d e t e c t o r s . These F i s h  Canyon t u f f - d e r i v e d * estimates were c o n s i s t e n t l y c l o s e , but slightly  l e s s than, the independent  estimates  provided  by  reactor  personnel f o r each i r r a d i a t i o n . For e i g h t d i f f e r e n t  i r r a d i a t i o n s using minerals from the t u f f of F i s h Canyon f o r dose c a l i b r a t i o n , the value and found  to be 5.41±0.22 x 10  9  standard  error  neutrons/track  of fi were  (8 i r r a d i a t i o n s ,  Table I ) . T h i s value was then used the  induced  track  density,  detectors  bracketing  estimates  f o r samples  and  samples  z i r c o n ages from the  using  this  i n turn t o determine pi, in  *  using  of the NBS 962 muscovite each  irradiation.  *  were made by i n t e r p o l a t i o n . A p a t i t e tuff  of  Fish  Canyon  calculated  dose, and c o r r e c t e d f o r g r a d i e n t (determined as  4%/cm of m a t e r i a l  f o r the USGS  TRIGA  reactor),  average  22  27.3±1.0 Ma apatite  (standard e r r o r ) f o r z i r c o n and 27.3±0.8 Ma f o r  (Table  deviation  I I ) . The  calculated  of uranium content  mean  values  reported  p r o v i d e d by the correction of  the  apatite,  reactor  facility  ages are 29.4±1.0 and 29.6±1.1  r e s p e c t i v e l y . Necessary  lower  the  known  are  agreement  I f estimates used  and  f o r g r a d i e n t i s made, the mean and standard  tuff  will  in close  by Naeser et a l (1979).  USGS  standard  f o r the m i n e r a l s i s 352±42 ppm  f o r z i r c o n and 10.8±1.4 ppm f o r a p a t i t e , with  and  flux  no  error  f o r z i r c o n and  gradient  corrections  these values by about 5%, i n c l o s e agreement with age  of  the m i n e r a l s determined  by Steven  e_t a l  (1967, new c o n s t a n t s ) . Dating r e s u l t s the  format  f o r z i r c o n and a p a t i t e are  suggested  (where the  given by  Johnson  a l (1979). Instead of assuming Poisson s t a t i s t i c s  c a l c u l a t i o n of  the  (standard  counts  statistics.  was A  standard  deviation  of  track  The  deviation) =mean),  standard d e v i a t i o n of  calculated  individual  2  standard  from  deviation  uranium  decay  of  constants  (1977) and H u r f o r d and Gleadow  3%  counting  on neutron  important  element  (1977)  in  were  used  counting  bias  on  all  in  this  i n Table I I I .  fission  track  i n v o l v e s the n e c e s s i t y of e l i m i n a t i n g , as f a r as all  for  dose  of S t e i g e r and Jager  study. A n a l y t i c a l r e s u l t s are p r e s e n t e d Another  i n the density  estimates was assumed i n the c a l c u l a t i o n of e r r o r s dates.  in  by Naeser et a l (1979), and e r r o r s f o r  dates a r e c a l c u l a t e d a c c o r d i n g t o formulae et  reported  dating  possible,  the p a r t of the r e s e a r c h e r . In t h i s  T a b l e I.  Values  IrradlatIon  of  'fi'  Determined  Mineral  from D a t i n g „ ' "  pS/p\  t h e T u f f of  detector,t/cm  !  F 1 s h Canyon • ' , 10 ' n / c r a ' 1  (27.9  Ma)  1  B'•10'n/track  9-79  apat1te  1 . 47  7 .21 x10*  3 . 17  4 .40  2 . 1-80 2 . 1-80 2 . 1-80  z1rcon z1rcon apat1te  ' ' 1 . 38 1 . 48 1 . 34  6 .55 x 1 0 ' 6 . 55 x 1 0 ' 6 . 55 x 1 0 "  3 . 37 3 . 14 3 .47  5 . 15 4 . 79 5 . 30  3 .. 1 - 8 0 3 . 1-80  z1rcon apat1te  1 . 03 1 . 25  7 . 7 0 x10* 7 . 70 x 1 0 '  4 .52 3 .72  5 . 87 4 83  4 . 1-80 4 . 1-80 4 . 1-80  apat1te apat1te z1rcon  1 . 45 1 . 60 1 . 30  5 .56 x l O ' 5 . 56 x 1 0 ' 5 .56 x 1 0 '  3 .21 2 91 3 .58  5 , , 77 5 . . 23 6 ..44  4 2-80 4 . 2-80 4 . 2-BO  z1rcon apat1te apat1te  1 . 48 1 . 31 1 . 36  6 .50 x10' 6 .50 x 1 0 ' 6 .50 x 1 0 '  3 . 14 3 . 55 3 ..42  4 . 83 5 . 46 5 . . 26  6 . 1-80 6 . 1-80  apat1te apat1te  0 . 80 0 . 85  8 .57 x 1 0 ' 8 .57 x 1 0 '  5 .82 5 . 47  6 . 79 6 38  8 1-80 8 ., 1 - 8 0  apat1te z1rcon  1 . 85 1 . 67  5 .02 x 1 0 ' 5 . .02 x 1 0 '  2 52 2 79  5 . 02 5 . 56  8 ., 2 - 8 0 8 .. 2 - 8 0  apat1te z1rcon  1 . 49 1 . 39  5 .83 x10* 5 . .83 x 10"  3 . 12 3 . 35  5 .. 35 5 75  Mean a n d s t a n d a r d e r r o r of  p Using  fi - 5 . 4 1 + 0 . 2 2 x Assumed age bf  1 average 10'  value  from each  Irradiation:  neutrons/track  F 1 s h Canyon T u f f  = 2 7 . 9 Ma'  S e e T a b l e I I f o r e x p l a n a t i o n of s y m b o l s * For the t u f f of F1sh Canyon, * = 4 . 6 5 3 x 1 0 ' / ( p s / 1 ) ; U c o n s t a n t s ' fi = * / p ' " ' ; p'" i s t h e d e n s i t y of t h e d e t e c t o r on NBS962 g l a s s . b a s e d on K - A r d a t i n g of S t e v e n e t a_^ ( 1 9 6 7 ) and r e c a l c u l a t e d w i t h : V e = 0 . 5 8 1 x 1 0 - ' ° / y r : Xb = 4 . 9 6 2 x 1 0 - ' ° / y r ; «°K=0.01167 atom'/. 1  1  p  4  listed  in Table  II.  Table II. Irr.  Mineral  9-79 2.1-80 2.1-80 2. 1-80 3.1-80 3.1-80 4.1-80 4.1-80 4.1-80 4.2-80 4.2-80 4.2-80 6.1-80 6.1-80 8.1-80 8.1-80 8.2-80 8.2-80  apat1te z1rcon z1rcon apat1te z1rcon apat1te apat1te apat1te z1rcon apat 1 te apat1te z1rcon apat1te apat 1 te apat 1 te z1rcon apat1te z1rcon  Tracks,s (10- t/cm')  F i s s i o n Track A n a l y t i c a l  p\  1  Tracks,1  ( 10' t / c m ) 1  0 . 134 5 . 84 4 . 45 0 . 137 3 . 95 0 . 14 0 . 137 0 . 160 4. 20 0 . 121 0 . 160 5 . 24 0 . 120 0 . 150 0 . 150 4 . 43 0 . 150 5 . 02  1699 841 712 283 1392 356 526 51 1 806 464 51 1 838 692 630 630 528 630 698  Data f o r  of  Fish  Canyon  Tracks,•  Date±S  n, (s/1 )'  8673 2171 2171 2 171 2802 2802 5673 5673 5673 5673 5673 5673 2055 2055 2055 2055 2055 2055  27 . 9 + 1 .0 27 . 4±1 . 5 29 . 5±1 . 2 26 .. 7±1 . 7 2 5 . ,5+0. 8 3 0 . .9 + 2 . 7 25 . 8+1 . 8 28 . 6±2 . 1 23 .. 3±1 . 2 27 .. 1±1 . 9 28 .. 3±2 . 1 31 .. 1 + 1 7. 22 . 3+1 . 7 2 3 . . 7±1 . 8 30 . 2 + 3 . 1 27 . 3+1 . 1 28 . 5±2 . 2 26 .,8±1 . 0  300/300 9 10 49/150 22 .60/45 60/60 50/50 12 60/60 50/50 10 90/45 100/50 100/50 12 100/50 14  ( 1 0 ' '•n / c m )  !  0 . 091 4 . 24 3 . 00 0 . 102 3 . 85 0 . 112 0 . 095 0 . 100 3 . 24 0 . 093 0 . 1 18 3 . 53 0 . 150 0 . 176 0 . 081 2 . 66 0 . 101 3 . 61  the Tuff  !  1 157 610 480 648 1356 2 12 364 321 622 356 378 564 433 372 171 317 214 502  3 3 3 3 4 4 2 2 3 3 3 3 4 4 2 2 3 3  Z i r c o n mean ± s t a n d a r d d e v i a t i o n , ( s t a n d a r d e r r o r ) A p a t i t e mean ± s t a n d a r d d e v i a t i o n , ( s t a n d a r d e r r o r ) Z i r c o n mean U (ppm) ± s t a n d a r d d e v i a t i o n A p a t i t e mean U (ppm) + s t a n d a r d d e v i a t i o n Z i r c o n mean d a t e u s i n g USGS * e s t i m a t e A p a t i t e mean d a t e u s i n g USGS * e s t i m a t e  = = = = = =  . 16 .32 . 32 .31 .15 .13 .98 .98 .00 . 48 .48 .49 .64 .64 . 72 .73 .20 .21  r , S ' * U, (ppm) USGS • e s t . ( 10' '' n / c m ' ) 4 .4% 0 .90 0 .96 3 .5% 0 .97 1 1 . 1% 4 .9% 5 .6% 0 . 78 5 .2% 4 .6% 0 .97 5 .2% 5 .2% 8 .9% 0 . 99 5 .7% 0 .94  10 414 293 10 322 9 10 1 1 347 10 12 364 12 14 10 327 1 1 394  27.3±2.5,(1.0)Ma 27.3+2.6.(0.8)Ma 352+42 10.8+1.4 29.4+2.7,(1.0)Ma 29.6+3.8,(1.1)Ma  ps, s p o n t a n e o u s t r a c k d e n s i t y ; p\, I n d u c e d t r a c k d e n s i t y : * , t h e r m a l n e u t r o n d o s e S, standard error number o f g r a i n s o r f i e l d s c o u n t e d : s , s p o n t a n e o u s ; i , i n d u c e d ' r , c o r r e l a t i o n c o e f f i c i e n t ; S ' , r e l a t i v e s t a n d a r d e r r o r of i n d u c e d t r a c k s ' not a d j u s t e d f o r f l u x g r a d i e n t d e c a y c o n s t a n t s : V " ( f 1 s s 1on) = 7 . 0 0 x 1 0 - " / y r ; V " • (a 1 p h a ) = 1 . 55 125 x 1 0 - / y r ; X. " - ( a 1 p h a ) =9 . 8485 x other constants: U' •/U' =137.88; X' =580 x 10- *cm' 1  ' 1  1  1  1 5  3 5  !  0  10-'°/yr  3 . 15 3 . 44 3 . 44 3 ..44 4 . 30 4 . 30 3 . 44 3 . 44 3 . 44 3 . 44 3 . 44 3 . , 44 5 . 16 5 . . 16 3 . 44 3 . . 44 3 . 44 3 .44  Table III.  Terrace  54° 1 2 ' 3 0 " 54°25'40" 54°30'20" 54°4 1 ' 1 5 "  Kemano-Powel1  78 -WU- 342 78 -wu- 342 78 - W U - 343  343 344 344 345 345 347 347 348 348  Peak  53 " 5 1 ' 2 5 " 53 " 5 1 ' 2 5 " 53 ° 5 0 ' 5 5 " 53 " 5 0 ' 5 5 " 53 ° 5 0 ' 0 5 " 53 ° 5 0 ' 0 5 " 53 ° 4 9 ' 5 0 " 53 ° 4 9 ' 5 0 " 53 " 4 9 ' 2 5 " 53 ° 4 9 ' 2 5 " 53 " 4 9 ' 2 5 " 53 ° 4 9 ' 2 5 "  129°35'30" 128°52'00" 128°32'30" 128"20'00"  52 " 2 6 ' 4 0 " 52 " 2 6 ' 4 0 " 52 1 8 ' 2 5 " 52 ° 1 8 ' 2 5 " 52 1 8 ' 2 5 " 52 1 8 ' 2 5 " 52 " 2 4 ' 0 0 " 52 " 2 4 ' 0 0 " 52 o i g ' 4 0 " 52 o 1 8 - 4 0 " 52 o 1 8 ' 4 0 " 52 o 1 8 ' 4 0 " 52 " 1 5 ' 3 5 " 52 1 5 ' 3 5 " 52 1 5 ' 3 5 " 52 1 5 ' 3 5 " 52 ° 2 0 ' 4 5 " 52 ° 2 0 ' 4 5 " 52 ° 2 0 ' 4 5 " 52 " 2 2 ' 3 0 " 52 ° 2 2 ' 3 0 " 52 ° 2 2 ' 3 0 " 52 ° 2 6 ' 2 0 " 52 o 2 6 ' 2 0 " 52 " 2 1 ' 3 0 " 52 " 2 1 ' 3 0 " 0  0  0  0 0 0  <100 <100 <100 < 100  apat1te apat1te apat1te apat1te  1 0 0 0  . 36 . 107 .037 .062  655 204 70 13 1  3 . 27 0 . 303 0 .088 0 .099  784 577 . 168 209  8 . 76 8 .. 77 8 .. 4 1 8 42  2 1 .9+1 . 5 18 .6+1 .8 21 . 2±3 . 0 3 1 .6 + 4 . 6  30/15 45/45 45/45 50/50  4 ..8% 5 ,0% 9 . 3% 9 . 2%  1951 1951 1570 1570 1 189 1 189 945 945 305 305 91 91  apat1te z i rcon apat1te z1rcon apat1te z1rcon apat1te z1rcon apat1te z i rcon apat1te z1rcon  0 . 372 2 . 70 0 . 455 3 . 69 0 . 179 3 . 50 0 . . 336 2 . 94 0 . . 276 2 .95 0 . . 169 4 .80  707 1208 865 1 182 447 67 1 639 1082 524 1744 321 6 14  0 . 213 1 . 18 0 . 257 1 . 70 0 . 111 1 . 51 0 . 190 1 . 27 0 . .213 1 . 36 0 . 126 2 .51 .  405 528 488 546 280 289 362 468 405 806 240 321  3 .. 22 3 .. 24 3 .. 20 3. 2 1 3 .. 18 3 .. 19 3 .. 16 3 17 3 . 14 3 . 15 3 . 12 3 . 13  3 3 , .7 + 2 . 9 44 .,4+1 .6 34 ..0+2 . 7 4 1 .. 7+2 .4 3 0 . . 7±2 . 9 44 . 3 + 3 . 1 33 . 5±3 . 2 43 .9+1 . 8 24 . 4±2 . 2 40 .9±1 . 5 ' 25 . 1+2 . 7 35 .9+1 . 8  45/45 28 45/45 20 59/60 12 45/45 23 45/45 37 45/45 8  6 ..2% 0 . .91 5 ..8% 0 . 79 6 . .2% 0 ..89 7 .6% 0 .91 6 . 6% 0 . 87 7 .6% 0 .65  1052 1052 2085 2085 2085 2085 1920 1920 1868 1868 1868 1868 2023 2023 2023 2023 1579 1579 1579 1237 1237 1237 146 146 274 274  z1rcon apat1te z i rcon apat1te apat1te z1rcon apat1te z1rcon apat1te z1rcon apat1te z1rcon apat1te z1rcon apat1te z1rcon apat1te apat1te z1rcon apat1te z1rcon apat1te apat1te z i rcon apat i te z1rcon  5 . 51 0 . 270 3 . 24 0 . 809 1 . 31 3 . 79 0 . 440 3 . 51 0 . 106 1 . 73 0 . 263 1 . 85 0 . 027 3 . 05 0 . 024 4 . 83 0 . 284 0 . . 151 10. 4 0 . .267 6 76 0 . . 255 0 . 072 2 . 24 0 .601 8 . 34  882 57 1 829 1537 1382 1092 372 786 340 360 824 504 130 684 92 1 159 599 482 1 163 565 1407 8 15 151 574 1524 14G7  2 . 04 0 . 166 1 . 39 0 . 352 0 . 659 1 . 74 0 . 188 1 .66 . 0 . .082 1 .08 0 . . 207 0 ..903 0. 020 1.. 53 0 .020 2 . 20 0 . 351 0 . 145 5 . 18 0 . 309 3 . 49 0 . 307 0 .049 1 . 46 0 . 334 3 . 56  326 351 356 670 668 500 191 372 268 225 66 1 246 95 343 77 528 742 465 580 653 726 982 103 374 860 626  3 . 82 3 ..84 3 ..92 3 .. 90 3 .. 8 6 3 . 88 3 ..94 3 . 96 2 . 87 2 .89 2 . 90 2 .91 . 2 . 93 2 . 94 2 .96 2 .97 3 . 26 3 . 25 3 . 23 3 . 30 3 .31 3 . 28 4 .00 4 .01 4 .05 4 .07  6 1 .. 7±4 , 3 37 .. 4±3. 6 54 . 7±4,. 0 5 3 . 6+3 . 9 4 5 . 9+3. . 9 5 0 . 6+3 . 0 5 5 . 2+6 . 3 5 0 . . 1+3 . 0 22 .. 3+2 .. 5 27 . 8+1 . 0 22 ..1+2 . 8 3 5 . .7+1 . 2 23 ,7±3 . 5 35 . 1±1 . 5 2 1 . 3±3 . 4 39 . 1±1 . 6 15 8±1 . 9 20 . 3 + 2 . 4 38 . 9 + 2 . 4 17 .1 + 1 . 3 38 .4 + 2 . 2 16 . 4+ 1 . 1 35 . 2+4 . 0 36 . 9 + 2 . 2 43 . 6 1 3 . 3 57 .0+4 . 2  10 50/50 16 45/45 25/24 • 18 20/24 14 50/5 1 13 49/50 18 75/75 14 60/60 15 50/50 50/50 7 50/50 13 50/50 50/50 16 60/61 1 1  area 128 " 0 1 ' 1 0 " 128 " 0 1 ' 1 0 " 127 " 5 9 ' 1 5 " 127 ° 5 9 ' 1 5 " 127 " 5 8 ' 4 0 " 127 ° 5 8 ' 4 0 " 127 ° 5 8 ' 3 5 " 127 " 5 8 ' 3 5 " 127 " 5 7 ' 4 0 " 127 " 5 7 ' 4 0 " 127 " 5 7 ' 3 0 " 127 " 5 7 ' 3 0 "  Be11 a B e l 1 a - B e l 1 a C o o l a BBC - 1 BBC - 1 BBC -2A BBC - 2 A BBC - 2 B BBC - 2 B BBC - 3 A BBC - 3 A BBC - 4 A BBC - 4 A BBC - 4 B BBC - 4 B BBC - 5 A BBC - 5 A BBC - 5 B BBC - 5 B BBC - 6 A BBC - 6 B BBC - 6 B BBC - 7 A BBC - 7 A BBC - 7 B BBC - 8 E BBC - 8 E BBC - 9 BBC - 9  Data  area  TO Tgn1 T2 T3  78 - W U 78 - W U 78 - W U 78 - W U 78 - W U 78 - W U 78 - W U 78 - W U 78 - W U -  F i s s i o n Track A n a l y t i c a l  area  125°52'00" 125°52'00" 125°57'10" 125°57'10" 125°57'10" 125°57'10" 126°24'25" 126°24'25" 126°48'00" 126°48'00" 126°48'00" 126°48'00" 127°05'20" 127°05'20" 127°05'20" 127°05'20" 127°18'45" 127°18'45" 127°18'45" 127"42'05" 127"42'05" 127°42'05" 126"23'15" 1 2 6 " 2 3 ' 15" 126"00'45" 126°00'45"  0 . 97 7 . 4% 0 . 98 5. 5 . 1% 0 . 81 9 . .7% 0 . 51 8 . 4% 0 . 97 8 .. 3% 0 97 10 .7% 0 .. 8 5 1 1 .3% 0 .95 8 . 1% 7 .9% O .95 4 .0% 0 . 70 4 .0% 7 .9% . 0 . 77 4 .9"/ ' 0 . 77  Table Sample  Latitude  Lonqltude  BBCBBCBBCBBCBBCBBCBBCBBCBBCBBCBBCBBCBBCBBCBBCBBCBBCBBC-  52 ° 2 2 ' 3 0 " 52 ° 2 2 ' 3 0 " 52 ° 2 3 ' 0 0 " 52 ° 2 3 ' 0 0 " 52 1 8 ' 4 5 " 52 ° 1 8 ' 4 5 " 52 o ' 1 5 " 52 o 2 ' 1 5 " 52 ° 2 3 ' 2 0 " 52 1 5 ' 3 0 " 52 1 5 ' 3 0 " 52 ° 1 4 ' 4 5 " 52 < M 4 ' 4 5 " 52 " 1 0 ' 3 0 " 52 o 1 0 ' 3 0 " 52 o 1 2 ' 2 5 " 52 °21 ' 2 5 " 52 " 2 1 ' 2 5 "  126 " 4 8 ' 0 0 " 126 " 4 8 ' 0 0 " 126 ' 3 3 ' 3 0 " 126 ° 3 3 ' 3 0 " 127 " 0 6 ' 3 0 " 127 " 0 6 ' 3 0 " 127 ° 1 4 ' 0 5 " 127 1 4 ' 0 5 " 127 ° 1 3 ' 0 0 " 127 " 0 6 ' 4 5 " 127 " 0 6 ' 4 5 " 127 ° 4 5 ' 0 0 " 127 ° 4 5 ' 0 0 " 127 ° 5 8 ' 3 5 " 127 " 5 8 ' 3 5 " 127 ° 5 1 ' 1 0 " 127 o 2 ' 1 0 " 127 o ' 1 0 "  1 1S 1 1S 12G 12G 13 13 14 14 15 21 21 22 22 24 24 25 28 28  Mount  10086 10088 10099 10099 20091 20408 30325 30325 30450 30461 30461  2  2  2  0  0  iml  Mineral  continued.  Tracks,s (10"t/cm')  82 866 1 1 19 1433 295 296 724 47 2628 190 1449 34 703 122 272  0.048 0.622 0.606 0.991 0.082 0.065 1.40 0.030 1.40 0.063 2.49 0.022 1.64 0 . 179 0.094  0 . 088 0 . 320 0 . 651 3 . 88 0 . 404 1 .05 . 0 . 779 3 . 95 0 . 483 2 . 39 4 . 58  1 12 406 1 101 497 682 498 988 506 408 1517 586  125° 3 5 ' 0 0 " 125° 3 5 ' 0 0 " 125° 1 6 ' 1 0 " 125" 1 6 ' 1 0 " 125° 1 4 ' 0 0 " 125° 1 4 ' 1 0 " 125" 1 4 ' 1 0 " 125" 2 0 ' 0 0 " 125" 2 0 ' 0 0 " 125° 02 ' 4 5 " 125° 02 ' 4 5 " 125° 2 6 ' 5 0 " 125° 2 6 ' 5 0 " 125° 3 0 ' 0 0 " 125° 0 5 ' 0 0 "  0 0 3960 3960 3800 2900 2900 2210 2210 910 910 1295 1295 760 2290  apat1te zircon apat1te z1rcon apat1te apat1te z1rcon apatite zircon apat1te z1rcon apat1te z1rcon apat1te apat1te  125 " 1 1 ' 1 0 " 125 1 0 ' 1 0 " 125 " 0 4 ' 5 0 " 125 " 0 4 ' 5 0 " 125 1 2 ' 3 5 " 125 ° 1 6 ' 1 0 " 124 " 5 3 ' 0 0 " 124 " 5 3 ' 0 0 " 124 " 5 0 ' 2 0 " 124 " 4 7 ' 1 0 " 124 " 4 7 ' 1 0 "  160O 762 1600 1600 0 0 0 O 1753 2515 2515  apat1te apat1te apat1te z1rcon apat1te apat1te apat1te z1rcon apat1te apat1te z1rcon  2  Data  Tracks,1 (10't/cm')  0 . 039 1 . 95 1 . 27 2 . 42 0 . 141 0 . . 117 3 ..48 0 .019 4 .78 0 .075 7 .55 0 .016 1 .88 0 .058 0 . 125  0 . 246 3.89 0.021 2 .66 0 .. 0 8 0 4 ..21 0 .. 0 1 9 5 .. 2 2 0 .. 0 1 9 0 .. 6 3 5 3 .. 32 0.221 18.7 0.201 8.58 O. 14 1 0 ..111 8 .00  4  Analytical  0 . 264 1 .87 0.015 1 . 57 0 . 146 2 .. 28 O.. 0 4 6 2 ..8 9 0.038 1 . 23 1 . 53 O.. 329 5 .. 9 1 0 .. 205 3 .. 23 O.. 2 13 0 .. 206 3 .. 48  apat1te z1rcon apat1te z1rcon apat1te z1rcon apat1te z1rcon apat1te apat1te z1rcon apat1te z i rcon apat1te z1rcon apat1te apat1te z1rcon  4  Track  944 810 29 510 261 1012 91 752 60 805 797 467 2094 709 1785 297 356 1408  30 30 94 94 0 0 0 0 0 0 0 0 0 0 0 0 10 10  0  Fission  ( 1 0 ' ' n/cm'  )  n.(s/1)  r ,S'  17 . 0±1 . 3 3 8 . 3±2 . 2 34 . 3 ± 1 2 . 2 4 1 . 7+2 . 7 10. 1±1 . 2 34 . 2±2 . 2 8 .. 2±1 . 1 36 .. 0 + 2 . 5 10 . 0 + 1 . 7 10 . 4 + 0 . 8 43 . 5 + 3 . 3 13 . 6 + 1 . 4 64 . 1 + 2 . 9 20 . 1 + 1 . 7 54 . 0 + 4 . 4 13 . 6 + 1 . 1 1 1.2±0.9 47 . 8 + 2 . 1  60/60 13 33/23 12 50/50 15 75/50 9 50/50 30/25 15 50/51 7 55/55 13 50/50 50/50 1 1  5 . 0% 0 . 78 31 . 0% 0 . 94 8 . 3% 0 . 91 7 ,5% . 0 .96 10 . 3% 4 .8% 0 .62 7 .8% 0 .92 6 . 1% 0 . 78 5 .2% 5 .2% 0 .93  Date±S  1014 388 15 301 472 547 146 4 16 122 1301 367 709 662 721 67 1 450 660 6 13  3 . 04 3 . 07 4 . 09 4. 1 1 3 . 08 3 ..09 3 . 32 3 . 33 3 . 34 3 . 36 3 . 35 3 . 38 3 . 39 3 .41 3 .40 3 .43 3 . 46 3 . 47  102 262 536 586 172 164 292 77 769 161 478 47 637 375 198  2.67 2.67 2.75 2.73 2.76 2.77 2.77 2.70 2.70 2.80 2.80 2.69 2.70 2.68 2.79  1 3 . 0±2 . 3 5 0 . 1+3. 4 34 . 5 + 3 . 0 3 9 . 9+2 . 4 . 28 . 5+3. 7 2 9 . 9+4 . 0 4 1 .2+1 . ..8 10 . 3+1 ,, 7 55 .2+2 . 6 20 .0+2 . 8 50 . 8 + 3 . 1 1 1 .7±2 . 9 18 .6+1 . 8 5 . 2+0 . 6 22 . 3±2 . 2  50/50 14 2 1/21 14 50/50 60/60 13 50/50 13 60/60 12 50/50 13 50/50 50/50  1 1 . 0% 0 . 51 5 . 8% 0 . 86 8 . 9% 10. .2% 0 98 10 .0% 0 .94 9 .8% 0 .96 16 .2% 0 .87 6 .TA 6 .2%  39 11 1 368 1 18 169 21 1 435 127 96 726 107  2 . 90 2 . 91 2 . 98 2 . 99 2 .87 . 2 ..92 3 .08 3 .09 3 .07 3 .06 3 .06  49 . 3 + 1 0 . 0 63 .. 3±1 1 . 7 66 . 7 ± 7 . 0 75. 2+5.4 86 . 5±7 . 6 9 0 . 1±8 . 5 41 .. 9±3 . 7 73 . 4 + 4 . 9 77 . 7 + 1 0 . 0 76 . 2 ± 6 . 2 99 .•9±3 . 8  30/30 30/30 40/50 8 40/50 32/35 30/30 8 20/20 15/30 8  15. 3% 14 . 3% 7 . 9% O. 62 7 . 4% 8 . 4% 6 . 0% 0 . 92 9 .3% 4 .5% 0 .96  Waddington  19041 19041 19148 19148 19151 19154 19154 46125 46125 56035 56035 9901 1 9901 1 99014 991 17 Bute  0  Alt.  III.  Inlet  51 ° 0 4 ' 4 0 " 51 " 0 4 ' 4 0 " 51 ° 2 2 ' 3 0 " 51 " 2 2 ' 3 0 " 51 ° 2 3 ' 3 0 " 51 ° 2 4 ' 2 0 " 51 ° 2 4 ' 2 0 " 51 ° 1 9 ' 3 5 " 51 " 1 9 ' 3 5 " 51 » 2 7 ' 3 0 " 51 ° 2 7 ' 3 0 " 51 • 1 6 ' 0 0 " 51 ° 1 6 ' 0 0 " 51 • 1 3 ' 4 0 " 51 " 2 5 ' 3 0 " area 50° 2 7 ' 3 4 " 50° 2 6 ' 4 2 " 50° 3 8 ' 4 4 " 50° 3 8 ' 4 4 " 50° 1 7 ' 5 0 " 50" 2 , 2 8 " 50° 4 8 ' 5 4 " 50° 4 8 ' 5 4 " 50° 4 8 ' 2 2 " 50° 4 7 ' 1 8 " 50" 4 7 ' 1 8 " 7  0  0  031 088 174 922 080 203 343 993 1 14 573 836  Table Sample  Latltude  Longi tude  50°54 '28" 124" 17 '25" 30503 50°54 '28" 124° 17'25" 30503 50°55 '22" 124°42 '25" 3601 1 50°55 '22" 124°42 '25" 3601 1 40170, 171 50 "56 '58" 124°22 '35" 50°54 ' 10" 124M4 '40" 46094 50°54 '50" 124"38 '05" 46099 50°54 '50" 124°38 '05" 46099 50°35 '00" 124°57 ' 40" 50364 50°35 '00" 124°57 '40" 50364  A l t . Mineral imj.  III,  continued.  (10"t/cm')  F i s s i o n Track  Tracks,s  Analytical  ( 10'' t / c m ' )  Data  Tracks,1  •  Date±S  n, ( s / i )  r,S'  ( 10' n/cm') 1  2789 2789 2286 2286 152 1295 610 610 0 O  apat i t e z1rcon apat1te z i rcon apat1te apat1te apat1te z i rcon apat1te z i rcon  67 1 . 3 .99 0. 925 2 .83 0. 056 0. 167 0. 561 4 .50 0. 436 8 .69  1 130 51 1 1 172 407 61 352 71 1 648 737 556  0. 888 1 . 56 0. 3 13 0. 944 0. 137 0. 093 0. 255 1 . 44 0. 175 1 . 53  600 200 662 136 151 235 431 207 369 98  3. 18 3. 19 3- 15 3. 16 3. 17 3. 12 3. 1 1 3 .10 2 .97 2 .94  35.8+2 48.8±2 55.7+3 56.7+3 7.8+1 33.6+3 41.0±2 58.0+4 44.3+3 99.6±4  16/16 8 30/50 9 11 50/60 30/40 9 40/50 4  5 . 4% 0.89 4 .7% 0. 74 0. 53  1417 1417 0 0 305 1722 823 198 198 2134 518 2179 655 655 625 625 76 76 820  apat1te apat i te apat i te z1rcon apat i t e apat1te apat1te apat1te z1rcon apat1te apat1te apat1te apat1te z1rcon apat1te z1rcon apat 1 t e z i rcon apat1te  0. 047 2 . 30 0,.019 1 1, .2 0..014 0.. 229 0.. 349 0.. 560 4 .. 34 0..704 0 . 125 0 . 124 O .066 6 . 13 0 .054 5 . 73 3 .71 8 . 43 O . 343  60 438 25 895 35 387 590 7 10 833 595 271 263 84 392 1 14 916 1019 944 725  0. 015 1 .13 0. 030 2 :99 . 0..008 0 . 144 0.. 196 0.. 349 1 .67 0 . 543 0 .077 0..090 0 .065 5 . 58 0 . 04 1 2 . 38 1 . 35 2 . 70 0 . 106  19 262 39 239 21 243 332 442 320 459 162 190 124 357 86 381 7 15 302 224  4 .05 4. 1 1 4 .. 15 4 .. 17 4 . 19 4 .. 24 4.. 28 4 32 4 . 34 4 . 36 4 . 40 4 . 45 4 .49 4 .47 4 . 53 4 .51 4 .07 4 .09 4 .57  75.8+21.2 50.1+7.7 15.8+4.7 93.2+4.9 43.9+13.1 40.4±5.5 45.6±5.. 2 4 1 . 5±5 .5. 67.4+4..0 33.9+3..6 42.8+4..6 36.7+4.4 27.3±4.8 29.4+1 35.8±5. 64.9+3. 66.9±5. 76.3+4. 88.3+8.  30/30 9/11 30/30 5 60/60 40/40 4O/40 30/30 12 40/40 50/50 50/50 30/45 4 50/50 10 13/25 7 50/50  23 . 4% 8 . 6% 18.9% 0. 15 26.2% 10.5% 8.6% 8 . 8% 0.96 7 . 3% 7 . 7% 8 .9% 9.9% 0.94 10.6% 0.97 5.1% 0.99 6 . 9%  17= 3%  91 5% 99  Vancouver-L111ooet area VL- 1 VL- 3 VL- 4 VL- 4 VL- 5 VI- 6 VL- 8 VL- 9 VL- 9 VL- 10 VL- 13 VL- 14 VL- 15 VL- 15 VL- 17 VL- 17 VL- 18 VL- 18 MC- 1  49°26' 50" 49°40' 05" 49°40' 30" 49°40' 30" 49°55' 30" 50°20' 25" 50°19' 10" 50°18' 40" 50°18' 40" 50°23' 55" 50°39' 15" 50°45' 45" 50°47' 05 " 50°47' 05" 50°10' 30" 50°10' 30" 49°25' 35" 49°25' 35" 50°53' 51 "  123° 12'05" 123°06 '25" 123°09 '35" 123»09 '35" 123°09 '45" 122°36 '00" 122°34 '35" 122°36 '00" 122*36 '00" 122°26 '40" 122°24 '40" 122° 10'05" 122°13 '20" 122°13 '20" 122°53 '00" 122°53 '00" 123° 13'50" 123°13 '50" 121°47 '11"  N o t e : s e e T a b l e II f o r e x p l a n a t i o n o f symbols and decay c o n s t a n t s . S a m p l e 40170,40171 was d a t e d by t h e e x t e r n a l d e t e c t o r m e t h o d .  28  study, a l l sample numbers were were  objectively  i d e n t i t y . The dates  from  counted  masked, without  knowledge  a  batch  (10-20)  were  complete.  reduce  the  samples  of  sample all  This "blind"  possibility  of  bias.  A r e a l V a r i a t i o n of A p a t i t e and  Sea  thus,  sample i d e n t i t i e s were r e v e a l e d only when  counting method should g r e a t l y counting  and  level apatite  Z i r c o n Dates  dates  F i g u r e 4 shows the d i s t r i b u t i o n and contours of a l l sea l e v e l - l o w a l t i t u d e a p a t i t e dates. The dates range from a low of  about 5 Ma  the western have  near Mount Waddington to more than 80 Ma  margin of the southern  Coast  Mountains.  been contoured at v a l u e s of 10, 20,  there are v i r t u a l l y no exceptions error  in  the  pattern,  35, and  outside  of  which i s remarkably  near Dates  50 Ma,  and  experimental c o n s i s t e n t and  r e g u l a r . A l l dates, with the p o s s i b l e e x c e p t i o n of VL-14,-15 (Figure 3) are from pre-Oligocene uplift  and  deformation  f i n a l c o o l i n g , and of the  105°C  rocks and c l e a r l y  r e v e a l a c l e a r p a t t e r n of the  isotherm.  z i r c o n dates from VL-14,-15 average zircon  are  Oligocene  concordant) granitic  reflect  and  The  31.114.9 Ma  probably  intrusive  two  near  apatite  and  (apatite  and  reflect uplift Lillooet,  of an  British  Columbia. In  general,  the  axial  region of the mountains or the  area to i t s immediate west g i v e s the  youngest  dates  at  a  30  given  latitude,  r e g i o n . The west,  and dates  date g r a d i e n t  i n c r e a s e east and  i s steeper  i n the  west from t h i s east  which p a r t l y r e l a t e s to the younger age  high l e v e l ,  than  of  the  relatively  r a p i d l y cooled p l u t o n s on the east side  of  the  mountains. North-south VL-14 is  v a r i a t i o n s are present  and VL-15, none of the dates less  than  about 40 Ma.  sample beneath Mount R a l e i g h 7.8  Ma,  and  isotherm.  e x t e n s i v e l y in the  farther  in the southern  In the Bute I n l e t t r a v e r s e , (40170,40171)  north, dates  northern  refers  to  northwest  Bella  the  Coola-Ocean  Coast  that of  Mountains  part  between  northern  the  Falls  young,  area,  part  which  (Bella  Coola  10  52°  and  and  Skeena  River  displaced  topography and  and  49°  altitude.  local  west  relief.  r e f l e c t more r a p i d e r o s i o n on the windward s i d e  mountains,  of  55°N,  p a r t , from  average  clearly  Ma  i n t h i s paper  southern  summits and  and  characteristic  latitudes  Mount Waddington. The  the a x i a l area of highest  T h i s may  more  area,  t r a v e r s e s ) , the youngest dates are from  very  over a width of n e a r l y 40 km are  to 52°N, however, has higher In  is  one  F a r t h e r north near Mount Waddington,  or l e s s . Younger dates are c l e a r l y the  traverse  i n d i c a t e s i t s recent passage upwards above the  105°C c r u s t a l and  as w e l l ; except for  of  i s not a f e a t u r e produced by a thermal  high.  Sea  l e v e l z i r c o n dates F i g u r e 5 shows the d i s t r i b u t i o n of  sea  level  or  low  31  altitude  zircon  dates and  i s a h y b r i d having and a  contours.  In general  the p a t t e r n  s i m i l a r i t i e s to both the a p a t i t e  the p a t t e r n of K-Ar  b i o t i t e dates  pattern  (Figure 2), which have  b l o c k i n g temperature of about 250°C (Harrison e_t a_l  H a r r i s o n and McDougall rapid  cooling  areas  u p l i f t and northern  in  (Hutchison  the  northern  giving  1970,  uniformly  axial  region c l e a r l y  K-Ar  dates,  Zircon  young  K-Ar  In  decreasing  sequential unroofing  i n t r u s i v e ages, as  calculation  4 of  and  in the  age  as  north  traverses, deduced  by  of  the  east,  plutonism  reflecting and  broadly  from west to e a s t .  5  do  uplift  paleogeothermal gradient r a t e , low and spaced  dates  i n the a p a t i t e p a t t e r n , are  V a r i a t i o n of Dates with A l t i t u d e and Figures  the  Terrace,  biotite  o l d e r in the southern  s i g n i f i c a n t l y o l d e r in the west than eastward  reflect  40 Ma.  young z i r c o n dates mimic t h i s p a t t e r n  dates.  but  H a r r i s o n et a l 1979).  local  closely  especially  subject to Eocene metamorphic r e s e t t i n g and  remain more e r r a t i c and  of u p l i f t  reflect  west margins of the Coast Mountains,  c o r r e l a t i n g more with  both  dates  Coast Mountains between P r i n c e Rupert and  rapid u p l i f t  but  zircon  c o o l i n g , many being younger than  a l a r g e area was  The  Some  of r e l a t i v e l y high l e v e l p l u t o n s ,  along the east and large  1980).  1979,  not rate  in  Apparent U p l i f t themselves  unless  an  allow  a  assumption  of  i s made. To a r r i v e at  an  high a l t i t u d e samples and  altitude  Rates  i n t e r v a l s were dated.  estimate samples at  The l o c a t i o n  33  of medium or high a l t i t u d e samples i s shown i n F i g u r e 3,  4,  and 5.  Kemano Six  samples  of f o l i a t e d quartz d i o r i t e were c o l l e c t e d  at v a r i o u s a l t i t u d e s on Woodsworth twelve  of  Powell  Peak  (Figure  34  about  to  24 Ma,  respectively,  from 44 t o 36 Ma  correlating  the z i r c o n data leads  slope  with of  0.25-0.30 km/Ma f o r the p e r i o d 35-45 Ma. The 30-35 Ma  flatten  somewhat  to accomodate  d a t a . The probable  but must  the low a l t i t u d e a p a t i t e  minimum slope i s 0.1 km/Ma f o r the p e r i o d  Ma, a f t e r which the r a t e i n c r e a s e s t o 0.15-0.20 km/Ma. The  present  depth a t which the a p a t i t e s have a date  ( i . e . a t 105°C) was estimated  T(Z)=(Q*Z/K)  where  well  to a  slope on a p a t i t e data was assumed t o be the same,  zero  Glen  a p a t i t e and z i r c o n dates were obtained and are shown  a l t i t u d e . Regressing  15-25  by  the G e o l o g i c a l Survey of Canada. A t o t a l of  in F i g u r e 6a. Z i r c o n and a p a t i t e dates vary and  1)  T(Z)  2  i s the temperature a t depth Z, below the average  conductivity,  temperature  a c c o r d i n g t o the formula,  + (D AO/K)(1-exp(-Z/D)) + a  s u r f a c e a l t i t u d e , Q* i s the reduced  production,  of  heat  flow,  K  i s the  D i s the s c a l e h e i g h t , Ao i s the surface heat and a at  i s the approximate  the average  downward d e c r e a s i n g heat  altitude.  production  mean An  atmospheric exponentially  i s assumed i n t h i s model  34  c a l c u l a t i o n . Values of  Q*,  2.5 kW/km°C  cal/sec-cm-°C),  (5.98X10-  p e r s o n a l communication used. is  3  K,  1980),  The average a l t i t u d e ,  ho, and D  of  50 kW/km , 2  1.0 kW/km  (T.Lewis,  3  and 10 km, r e s p e c t i v e l y , i n the v i c i n i t y  of Powell Peak,  1.0 km and the probable mean annual temperature  altitude  that  at  that  i s approximately 5°C, y i e l d i n g a depth of -3.3 km  for the 105°C a p a t i t e recent  were  uplift  annealing  will  isotherm.  The e f f e c t  of  be t o decrease t h i s f i g u r e somewhat so  i t should represent a maximum estimate of the  zero-age  depth. At  Kemano, the h i g h a l t i t u d e a p a t i t e curve and the low  a l t i t u d e z i r c o n curve n e a r l y o v e r l a p relation  of  temperature projected  Parrish  (1980),  the l i k e l y  range)  difference yields  a  35 Ma.  the d i f f e r e n c e  (175°-105° = 70° ) d i v i d e d altitude  at  by  the  Using the in blocking actual  (2.9±0.6 km; e r r o r paleogeothermal  or  indicates  g r a d i e n t of  26°+4°-6°/km at 35 Ma ago. The g r a d i e n t of 26°/km i s s i m i l a r to  the present value of 27°/km f o r the Stewart area t o the  northwest rather the l a s t  (Mathews 1972b). The Kemano  low- apparent  uplift  Fission  track  dates  northern King Island-Ocean this  thus  indicate  r a t e s with a small i n c r e a s e i n  10 or 15 Ma.  Northern King Island-Ocean  in  data  Falls a r e remarkably Falls  uniform  i n the  region (Figure 4 ) . Included  a n a l y s i s are a l l dates from samples east of BBC-22  (Figure 3) and west of samples BBC-4 and BBC-11, a width  of  35  about 25 km. A l l a p a t i t e and z i r c o n dates from t h i s area are shown  on  Figure  representing  6b.  Ten  apatite  and  six  z i r c o n dates  a v e r t i c a l a l t i t u d e d i f f e r e n c e of  2.1 km  vary  from 8 to 24 Ma and 34 to 39 Ma, r e s p e c t i v e l y , and c o r r e l a t e very  well  with  area,  r e c a l c u l a t e d with constants  from  58  to  altitude.  89  Ma  The K-Ar b i o t i t e dates f o r t h i s  (Figure  listed  i n Table IV,  range  2 ) , except f o r the d i s t i n c t l y  younger King I s l a n d s y e n i t e , which  is  12-13 Ma  old  (Baer  1973). It  could  responsible  be  for  argued the  dates  that  young are  this  apatite very  syenite  was  dates;  however,  the  do not vary  with  fission  track  distance  to the s y e n i t e , and at sea l e v e l are 3-4 Ma younger  than the age of the s y e n i t e . heat  flow  introduced at  2  model  of  an  A  two-dimensional  infinitely  long  conductive  dyke  6 km wide  at 750°C i n t o rock at 0°C with i t s top and bottom  and  formulated  10 km  below  according  the  to  3  surface,  Carslaw  c o n d u c t i v i t y f o r the model was (5.98x10-  regular,  partly  respectively,  and  assumed  cal/sec-cm-°C). The nearest  Jaeger  (1959). The  to  2.5 kW/km°C  be  fission  track  i s 9 km from the n e a r l y s t r a i g h t margin of the s y e n i t e 1973), and the maximum r i s e 3-4 km below the surface after  3 Ma.  The  sample (Baer  i n temperature f o r such a sample  i s about 8°C a f t e r 1  Ma  and  15°C  fission  t r a c k samples are 20 km  away, and t h e i r temperature increase  r e l a t e d to i n t r u s i o n i s  only about 5°C. convection,  farthest  was  If  the  syenite  cooled  the e f f e c t on the f i s s i o n track  predominantly  by  samples would be  KEMANO 0.25-0.3 km/Ma  I  I  1  1  1  1  0  10  20  30  40  50  Date (Ma)  NORTHERN KING ISLAND-OCEAN FALLS  i  i  i  i  1  1  0  10  20  30  40  50  Date (Ma)  F i g u r e 6. F i s s i o n t r a c k date v s . a l t i t u d e f o r samples from Kemano (a) and n o r t h e r n King Island-Ocean F a l l s (b) a r e a s . Sample numbers and a n a l y t i c a l data a r e l i s t e d i n Table I I I . The z e r o - d a t e p o i n t below sea l e v e l was c a l c u l a t e d a c c o r d i n g t o a procedure d i s cussed i n t h e t e x t .  37  even l e s s . Since they preserve the same f i s s i o n the  effect  although  of  the i n t r u s i o n  i t cannot  i s considered  calculated  except  very minor,  be ignored completely.  The present depths of the 105° and 175° been  t r a c k dates,  using  the same  that the average  surface  isotherms  have  parameters as at Kemano, altitude  i s 0.6 km. The  depths are -3.7 km f o r a p a t i t e and -6.6 km f o r z i r c o n . Using both  fission  t r a c k data and these estimated depths of zero  date, the apparent 30-40 Ma  ago  uplift  (zircon  rates  dates),  ( a p a t i t e d a t e s ) , and about (Figure  a r e about  0.16 km/Ma f o r 12-24  0.4 km/Ma  6b). E x t r a p o l a t i o n  0.4 km/Ma f o r  of  Ma ago  i n the l a s t  10 Ma  the z i r c o n curve y i e l d s an  a l t i t u d e d i f f e r e n c e and approximate e r r o r of 4.2±0.6 km f o r the  zircon-apatite  overlap  at  rather low paleo-geothermal present  heat  gradient  flow data obtained  (T.Lewis,  R.Hyndman  2  Burke, and Bentinck Channels, gradients  than  2  was used  temperatures, about  of  17°±2°C/km.  The  with  1.0 kW/km  (T.Lewis  The apparent  uplift  3  Physics  communication obtained  1980).  i n Dean,  i n d i c a t e somewhat  f o r 20 Ma  ago  s u r f a c e value of heat  incalculating  along  were  the E a r t h  and probably  obtained  f i s s i o n t r a c k d a t a . An average 60 kW/km  by  personal  Uncorrected v a l u e s of 34-75 kW/km  higher  ago. T h i s y i e l d s a  flow can be roughly estimated from p r e l i m i n a r y  ocean probe heat Branch  20 Ma  the depths  of  by using flow of zero-age  known s u r f a c e heat p r o d u c t i o n of unpublished data, rate  values  near  1980). northern  King  I s l a n d i n d i c a t e that r a t e s changed from 0.16 t o 0.4 km/Ma at  38  about  10 Ma  ago. The  latter  rate  i s n e a r l y double the  p r o j e c t e d r a t e of Kemano f o r the same time  interval.  B e l l a Coola V a l l e y Nearly a l l dates east of BBC-15 (Figure 3) are p l o t t e d in  Figure  7a. The  data  fall  i n t o two groups: those  North Bentinck Arm (samples BBC-4 and BBC-11) and the The  North  Bentinck  Arm  apatite  dates  t r a n s i t i o n a l between the samples t o the west vary  from  an apparent present.  similiar,  rest.  are c l e a r l y and  east and  17-22 Ma over an a l t i t u d e range of 1.9 km, g i v i n g uplift The  scattered,  from  rate of 0.2-0.25 km/Ma from 20 Ma t o the  data  and  from  the z i r c o n  probably  Bella  Coola  and  apatite  indicating  c o o l i n g , with c o n s i d e r a b l e  Valley  a r e more  dates  a r e very  r e l a t i v e l y high l e v e l  experimental  scatter.  rapid  Apparent  uplift  rates  vary from 0.07 t o 0.18 km/Ma f o r a p a t i t e . The-  zircon  dates  lack  altitude. -3.5  clearcut  with  decreasing  The a p a t i t e zero age depth v a r i e s from -2.8 km to  model.  Miocene cannot  An  increase  parameters  i n the u p l i f t  as  i n the  rate i n the Late  be documented, but c o u l d be' accomodated i f ,  instance,  the curves  actually  10 Ma, and subsequently steepen below  younging  km, assuming the same geothermal  Kemano  for  a  present  interpretat ion.  Mount Waddington  sea l e v e l  could  f l a t t e n between 30 and  again.  Only  establish  samples this  from  alternate  BELLA COOLA VALLEY North  Bentinck  Eastern Bella Coola Valley  Arm  3h 0.2-0.25 km/Ma  2  Altitude (km)  1  h h  105  a)  Lines refer to apatite dates only -J  10  I  i  i  20  30  40  l  _  50  60  Date (Ma)  MOUNT WADDINGTON  Altitude (km) E  b)  10  20  30  Easternmost sample  W  Westernmost sample  •  Apatite  •  Zircon  40  50  60  Date (Ma) F i g u r e 7. F i s s i o n t r a c k date v s . a l t i t u d e f o r samples from B e l l a Coola V a l l e y (a) and Mount Waddington ( b ) . The E and W symbols i n (b) r e f e r t o easternmost and westernmost samples, r e s p e c t i v e l y . They a r e not c o n s i d e r e d when drawing t h e best f i t c u r v e through a p a t i t e d a t a .  40  Samples from Mount Waddington, c o l l e c t e d by Survey  Geological  of Canada p e r s o n e l from 1970-1979, range i n a l t i t u d e  from sea l e v e l t o the summit a t n e a r l y 4 km. F i g u r e 7b shows 9 a p a t i t e and 6 z i r c o n  dates  f o r the a r e a .  Because  the  samples  were c o l l e c t e d a t d i s t a n c e s of up to 40 km from the  summit,  there  account  f o r . Dates  easternmost clearly  and  fall  dates form a altitude,  i s clearly  some  marked  E  westernmost  geographic or  W  samples,  variation  to  i n Figure 7b a r e the respectively,  and  o f f the main p a t t e r n . The remainder of a p a t i t e clear  pattern  ranging  from  of  35  younging  with  decreasing  t o 5 Ma over a 3.3 km a l t i t u d e  range. The z i r c o n data, however, a r e much more s c a t t e r e d and difficult from  t o i n t e r p r e t . The apparent u p l i f t  apatite  0.09  . data  km/Ma. T h i s slope  average  altitude  of  f o r the p e r i o d appears  to  the area  rate  calculated  30-10 Ma  persist  to  i s about 5 Ma. The  i s 1.7 km, and assuming a  reduced heat flow, Q*, of 40 kW/km as w e l l as the remainder 2  of  the Kemano geothermal parameters, the zero-age  about  -2.5 km.  Assuming  this  c o r r e c t , the apparent u p l i f t about  0.6 km/Ma.  r e l i e f . This u p l i f t young  Armstrong  and  t o be approximately  rate from 5 Ma  uplift  history  (6.8 Ma)  G l a c i e r complex,  is  to present i s  These data c l e a r l y document the e f f e c t of  l a t e s t Miocene-Recent  of  depth  depth  which  produced  the present  i s c o n s i s t e n t with the u n r o o f i n g  sub-volcanic  plutons  of the F r a n k l i n  10 km southwest of Mount Waddington, (R.L. J.G. Souther,  P l i o c e n e and P l e i s t o c e n e .  unpublished  data) during the  41  C e n t r a l Bute I n l e t Data from Francis  Bute  Drake  Inlet  include  and Cosmos  rocks  Heights  from  (Figure  1)  Mount S i r and are  summarized i n Figure 8. A p a t i t e dates vary from 76 to and  yield  an apparent  uplift  r a t e of 0.063 km/Ma f o r 75 t o  40 Ma, and probably younger. The zero-age about  -4.8 km.  except  that the assumed Q*  30 kW/km  2  and  105°C isotherm  1.1 km,  and  the average  respectively.  This  flow c o n s i s t e n t with  a l t i t u d e are results  r a t e between 40 Ma and present  0.063 km/Ma  about  r e q u i r e d by the data, t h i s  of  uplift  i s not c o n s t r a i n e d by f i s s i o n  i s drawn, assuming  until  in a  the data  (1976) f o r the Bute I n l e t a r e a . The apparent  t r a c k data and  is  I t was d e r i v e d using the Kemano parameters  present low s u r f a c e heat Hyndman  42 Ma  an  unchanged  7-8 Ma.  Although  cooling  history  rate of  clearly  not  i s consistent  with the Late Miocene t o Recent a c c e l e r a t e d u p l i f t  indicated  by the Mount Waddington, Ocean F a l l s , and Mount R a l e i g h data and  the present  physiography.  Mount Bute-Mount R a l e i g h area East from the head of Bute I n l e t , samples were analyzed from  Mount Bute  are shown on areas.  The  p a t t e r n than altitude  and Mount R a l e i g h (Figure 1). These dates  Figure apatite zircon,  range  of  9a, d i f f e r e n t i a t e d  between  dates,  a more c o n s i s t e n t  vary  which from  form 56 Ma  t o 8 Ma  the two  over  an  n e a r l y 3 km. Separate a p a t i t e l i n e s a r e  CENTRAL BUTE INLET  F i g u r e 8. F i s s i o n t r a c k date v s . a l t i t u d e f o r samples from c e n t r a l Bute I n l Only a p a t i t e data were used to d e f i n e the c u r v e .  43  shown on F i g u r e 9a f o r Mount R a l e i g h and f o r Mount Bute. The Mount R a l e i g h apparent  uplift  r a t e v a r i e s from 0.1 km/Ma f o r  the p e r i o d 40-10  Ma, and i n c r e a s e s t o 0.4 km/Ma s i n c e  The  data  40-10  Mount Bute  Ma, although  Since  these  probably  areas  very  Southgate since  rate  of 0.04 km/Ma from well-constrained.  are n e a r l y along s t r i k e and about  from each other, t h e i r  uplift  h i s t o r i e s are  s i m i l i a r , suggesting that the best apparent  r a t e value probably The  a  i t i s not p a r t i c u l a r l y  two  30 km d i s t a n t  indicate  young,  8 Ma.  i s an average 7.8 Ma,  date  of the two  curves  uplift shown.  below Mount R a l e i g h i n the  River v a l l e y probably dates the approximate  inception  of  time  recent r a p i d u p l i f t , and i s c o n s i s t e n t  with very young, low a l t i t u d e  dates  on  Mount Waddington.  Zero-age depths were c a l c u l a t e d assuming Q* of 50 kW/km , an 2  average  altitude  of 1.7 km, and other parameters as i n the  Kemano example. The >3 km r e l i e f  i n the area  suggests  very  recent u p l i f t and e r o s i o n , c o n s i s t e n t with the f i s s i o n t r a c k data.  Pemberton area In oldest,  the southern t r a v e r s e , f i s s i o n  track dates are the  none  40 Ma  being  Consequently,  less  than  i t i s more  about  difficult  (Figure 4 ) .  to document  d i f f e r e n c e s i n dates between samples a t d i f f e r e n t Samples from the over  the Spetch Creek Pluton  Pemberton an  area  altitude  (Woodsworth  small  altitudes. 1977) i n  (VL-6,-8,-9,) range from about 40-46 Ma difference  of  1.6 km,  and  show  no  MOUNT BUTE-MOUNT RALEIGH AREA  Altitude (km) •  Apatite  +  Zircon  B  Mount Bute samples  R  Mount Raleigh samples  a) 10  20  30  40  50  60  70  Date (Ma)  PEMBERTON 0.25 km/Ma  0.1 km/Ma  •  Apatite  4  Zircon  F i g u r e 9. F i s s i o n t r a c k date v s . a l t i t u d e f o r samples from Mount Bute (B) and Mount R a l e i g h (R), shown i n ( a ) , and Pemberton (b) a r e a s . Only a p a t i t e data were used to d e f i n e t h e c u r v e s .  45  demonstrable  correlation  with  altitude  (Figure  9b). The  -3 km  assuming  present zero-age depth f o r a p a t i t e i s about Q*  of  50 kW/km  2  and an average a l t i t u d e of about  Since Late Miocene-Recent on  geologic  curve  than  and physiographic grounds, the apparent  has been  consequence  u p l i f t must be g r e a t e r  1-1.2 km.  drawn  to take  this  into  2 km uplift  account.  i s a very low (0.02 km/Ma) apparent u p l i f t Ma. The low a l t i t u d e  close  the b i o t i t e and hornblende K-Ar dates of 86±3 and  77±4 r e s p e c t i v e l y , and i n d i c a t e s f a i r l y Spetch Creek P l u t o n . The c l u s t e r indicate  a  period  of  of  moderately  Eocene. T h i s Eocene u p l i f t  date,  rate  from 35-10 to  zircon  A  67.4 Ma, i s  r a p i d c o o l i n g of the  40-45 Ma  dates  could  r a p i d u p l i f t during the  i s a l s o suggested by the 35-45 Ma  low a l t i t u d e dates that span n e a r l y the e n t i r e width of the southern Coast Mountains  (Figure 4 ) .  The presence of t h i c k Eocene nonmarine in  the F r a s e r  Washington  Lowland  (Frizzell  support t o t h i s  clastic  (Rouse et a_l 1975) and i n northwest  1979, Johnson  1981),  adds  additional  possibility.  S p a t i a l and Temporal V a r i a t i o n s of Apparent U p l i f t The uplift uplift  assumptions  involved  have  been  Rates  the apparent  b r i e f l y discussed e a r l i e r  paper. The assumption most l i k e l y is  i n equating  r a t e s d e r i v e d from f i s s i o n t r a c k s t u d i e s with rates  deposits  t o cause l a r g e  actual in this  deviations  the s t a b i l i t y , with respect t o the s u r f a c e , of isotherms  during u p l i f t .  I t i s shown i n Chapter 3 that t h i s  assumption  46  can  r a r e l y be met. In a d d i t i o n , making s p e c i f i c  to  apparent  uplift  r a t e s depends not only on the r a t e s of  u p l i f t and e r o s i o n but a l s o heat  flow,  the heat  by  e x p l a i n the higher  uplift  rapid u p l i f t  apparent r a t e s i n the Mount  and Ocean  Despite  these  data,  middle  cautions  Waddington,  activity.  in interpreting  the  geologic  from 30 t o 15 Ma ago.  Falls-Bella  Coola  i n the  Maximum r a t e s  axial  region  during  near  Ocean  and were up t o 0.2 km/Ma. I t appears that  these smaller middle Cenozoic r a t e s were maintained 15 Ma  i n the  information.  a l l areas i n d i c a t e a r a t h e r slow u p l i f t  Cenozoic  for at  i n most areas. The r a t e s i n the southern part  (Vancouver and Bute I n l e t t r a v e r s e s ) were (maximum  fission  there are s p a t i a l and temporal p a t t e r n s  t h i s time were i n the northern  least  can be  F a l l s areas ( F i g u r e s 6a, 6b, 7b) during  apparent r a t e s which c o n t a i n v a l u a b l e Nearly  rates  and e r o s i o n . T h i s may  the Eocene, immediately f o l l o w i n g orogenic  track  reduced  a downward r e l a x a t i o n of isotherms f o l l o w i n g a  p e r i o d of high heat flow,  Kemano,  production,  and on the g e o l o g i c h i s t o r y of the sample. For  example, f i c t i t o u s p o s i t i v e apparent produced  corrections  0.1 km/Ma)  and may  have  definitely  been  lower  virtually  zero  (Pemberton a r e a ) . The  more r a p i d middle Cenozoic u p l i f t  i n the north  synchronous with the s i n k i n g and consequent adjacent  Queen  Charlotte  Miocene time (Shouldice the  is  i n f i l l i n g of the  b a s i n , which developed mainly i n  1971). T h i s  basin  north t i p of Vancouver I s l a n d , adjacent  terminates  near  t o the southern  47  Coast Mountains  where middle Cenozoic u p l i f t  The middle Cenozoic basin  subsidence  Coast north  Mountains of  51°N  was  uplift  low or n i l .  and  therefore  adjacent  appear  to be  r a t e s are  more  farther  north  related. The Late Miocene-Recent rapid  in  (<0.4  the  south  apparent  (>0.6  km/Ma). In g e n e r a l , the  modest These  evidence  of  differences  physiography, topography Other histories  in  late  km/Ma)  than  northern  samples  show  Cenozoic a c c e l e r a t i o n of  appear that  uplift  to  the  correlate  well  comes  from  Coast  evidence the  Mountains.  In  for  Mountains.  contrasting  relation  of  the  the  Taseko  Lakes  1963,1978) the b a s a l t s were extruded on an of  uplift  Late Miocene flank  area (Tipper  erosion  surface  3,500' (1.1  on the p l a t e a u to g r e a t e r than 8,000' (2.4 km) Taseko  Lakes.  essentially  Farther  southeast  f l a t l y i n g at 4,500'-5,000' (1.4-1.5 km) beneath  peaks  p r e s e n t l y r i s e above 7,500' (2.3 km)(Baer 1973). The the  north  postdate  km) of  north near B e l l a Coola,' the b a s a l t s  flowed i n t o v a l l e y s a l r e a d y developed  in  of  low to moderate r e l i e f and subsequently t i l t e d up i n the  west. T h e i r b a s a l a l t i t u d e v a r i e s from about  are  the  higher summits and more rugged  p l a t e a u b a s a l t s to physiography along the e a s t e r n the  uplift.  with  are concentrated i n the southern Coast supporting  only  and  which basalts  the development of the mountainous  physiography, whereas i n the south they predate i t . S e v e r a l samples basal,  of  columnar-jointed  the flows  basalt and  were  collected  were dated by the  from K-Ar  48  whole rock method. Most of these samples are from side  of  the  basalt  cover  -7  and BC-6.  labelled  -4,  listed  i n Table IV. The ages of the flows sampled range  Mountains  and  and  CC-2,  6.0-9.9 Ma.  -6,  demonstrate  -6)  range  along  the  eastern  Mountains. A b a s a l t date of 14 Ma (Farquharson is  from  in  and S t i p p 1969) conflict  The  the  physiography  three  synchronous  difference  in  total  uplift  (Figure 11) have been different  of s t r a t i g r a p h i c  depth  40 Ma  isotherms  based  on  using  and  as  Mathews  existing  Late  Miocene  south.  Uplift  since  40 Ma by  (Figure 10)  and  composite  of  a  and  altitudes  of  consisting projected  e r o s i o n s u r f a c e s , 2) by c a l c u l a t i n g and an  10 Ma assumed  e s t i m a t i n g the depth of the isotherm  Coast  e r u p t i o n of p l a t e a u lavas  estimated  thicknesses and  the  Rouse  methods: 1) by g e o l o g i c c o n t r o l  unconformities of  the  i s of u n c e r t a i n s i g n i f i c a n c e  between north and  of  of  from the Taseko Lakes area  Estimates of T o t a l  10 Ma  and  with numerous dates on p l a t e a u b a s a l t s  broadly  emphasizes  Values  Coast  7.6-9.9 Ma  margin  (Bevier and Armstrong unpublished data; 1979).  from  the e s s e n t i a l l y contemporaneous e r u p t i o n of the  plateau basalts  it  as  A n a l y t i c a l data are  Basal flows along the e a s t e r n flank of the (CC-4,-5,  west  near the e a s t e r n margin of the  Coast Mountains and are shown i n F i g u r e 3 -5,  the  175°C  105°C  apatite  25°C/km 40 Ma  thermal models (Chapter  annealing  g r a d i e n t , and zircon  2) and  the  3)  annealing  subsequently  T a b l e IV. Sample BC-G' CC-2' CC-4' CC-5' CC-6« CC-7'  1  ' 1  • * " 7  ' ' 1 0  1  Latitude  Lonqltude  52"05'30" 52°05'30" 52°25'30" 52°31'10" 52"32'50" 51°26'15"  123°23'18" 123°23'18" 123°38'05" 125"49'35" 125°42'30" 123°39'10"  %K<  0 . 337 0 . .333 0 . .995 0 . .901 0 . . 784 0 . 355  K-Ar  Analytical  Ar'"*(x10-'cc/gm)" 0.0787 0 . 0 8 15 O.3067 0.3478 0.2541 O. 1051  Data %rad.Ar*° 24 . 8 17.2 59 . 0 6 1.3 65.5 30. 3  Date±s(Ma)' 6 ,0±0. 2 6 . 3±0. 3 7 .9+0.3 9.9+0.3 8 . 3+0.3 7.6+0.3  w h o l e r o c k b a s a l t . - 3 0 + 5 0 mesh c o l l e c t e d by M . L . B e v l e r f r o m t o p o f s e c t i o n a t B u l l C a n y o n on t h e C h i l c o t i n R i v e r b o t t o m o f s e c t i o n , same l o c a l i t y a s B C - 6 c o l l e c t e d f r o m t h e P r e c i p i c e on t h e H o t n a r k o R i v e r , b a s a l f l o w c o l l e c t e d n e a r mouth o f n o r t h b r a n c h of Young C r e e k ; c o n t a i n s p l a g i o c l a s e m e g a c r y s t s (5 cm) e r r a t i c b o u l d e r f r o m n e a r b y b e d r o c k a t Heckman P a s s ; c o n t a i n s s m a l l e r p l a g i o c l a s e m e g a c r y s t s c o l l e c t e d near bottom of s e c t i o n of Taseko R i v e r b a s a l t s %K d e t e r m i n e d by a t o m i c a b s o r p t i o n by K. S c o t t , U n i v e r s i t y o f B r i t i s h C o l u m b i a Ar i s o t o p l c c o m p o s i t i o n a n d c o n c e n t r a t i o n d e t e r m i n e d by J . H a r a k a l , U n i v e r s i t y of B r i t i s h Co c o n s t a n t s ; Kb = 4 . 9 6 2 x 1 0 - ' ° / y r : Ve = 0 . 5 8 1 x 1 0 - ' ° / y r : «°K = O . 0 1 1 6 7 a t o m %  F i g u r e 10. T o t a l u p l i f t , w i t h r e s p e c t to sea l e v e l , s i n c e 40 Ma ago ( L a t e Eocene) . The i n t e r p r e t a t i o n i s made assuming a sea l e v e l l a n d s u r f a c e 40 Ma ago.  51  g e n e r a l i z i n g over the a r e a . Geological control subsidence 1971) of  data  from  comprises  1968), the  or Late Miocene (10 Ma)  of the Coast Mountains, and  accordant  basal  altitude  20°C and  age  10°C,  40 Ma  the  eastern  i n p l a c e s the a l t i t u d e s of  (Rouse and Mathews 1979)  r e s p e c t i v e l y , the depth to the  and  10 Ma  sea l e v e l  uplift  gradient  is  of  about  25°C/km  even  105°C isotherm  when  assumed.  orogeny i n the north, g r a d i e n t s were much gradient  of  a p a t i t e dates g i v e s a f a i r l y  a c c u r a t e i n d i c a t i o n of t o t a l  uniform  on  summits.  Using s u r f a c e temperatures  for  and  Miocene p l a t e a u b a s a l t s or other v o l c a n i c rocks of  Eocene (40 Ma) side  thickness  the Queen C h a r l o t t e b a s i n (Shouldice  and Whatcom basin (Hopkins  Late  sediment  a  uniform  F o l l o w i n g Eocene steeper,  and  should be assumed when c a l c u l a t i n g  no  depths  to the 175°C z i r c o n a n n e a l i n g isotherm. Heat flow models can correct  f o r thermal  r e l a x a t i o n and provide u p l i f t  where g e o l o g i c c o n t r o l  Uplift  s i n c e 40  The  rocks  sedimentary,  (see Chapter  2).  Ma  presence  stratified  i s absent  estimates  of at  indicates  Eocene-Early sea net  level,  Oligocene whether  subsidence  since  terrestrial volcanic 40 Ma.  areas i n c l u d e the northern Queen C h a r l o t t e I s l a n d s and of  Hecate S t r a i t  which may  u n d e r l a i n by the Masset Formation,  be as o l d as Late Eocene (Sutherland  Brown  or Such part  part of 1968,  Young 1981), the Whatcom b a s i n near Vancouver which c o n t a i n s  52  Late  Cretaceous,  (Hopkins  1966, Rouse et a l  western  Interior  sedimentary at  Eocene,  and  younger  Cenozoic  1975,  Johnson  1981),  Plateau  where  Eocene  rocks are preserved (Ewing  1980,  and  the  volcanic  and  Souther  1977)  0-1 km a l t i t u d e . No such s t r a t a are preserved w i t h i n the  Coast  Mountains. The v a l u e s of u p l i f t  apatite  contours  respectively  are  (Figure  derived  from  approximately 10).  40 Ma 5-6 km  Subsidence  Figure  10,  Cenozoic  are  based  sediments  01igocene-Miocene  and  Strait,  and  3.5 km, in  the  shown i n  on the cumulative t h i c k n e s s of l a t e  (Shouldice Masset  1971)  Formation  and  the u n d e r l y i n g  (Sutherland Brown 1968,  Young 1981). Because the b a s a l age of the be  zircon  estimates  northern Queen C h a r l o t t e I s l a n d s and Hecate  may  strata  Masset  volcanics  both t i m e - t r a n s g r e s s i v e and younger than 40 Ma, the  estimate of u p l i f t  since 40 Ma may be somewhat i n e r r o r . New  i n f o r m a t i o n on s t r u c t u r e and subsidence  in  Charlotte  1981)  basin  (Yorath  and  Chase  southern may  Queen require  m o d i f i c a t i o n s to the p a t t e r n shown i n F i g u r e 10. There (latitude southern  is  clear  52°  to  Coast  difference  evidence 55°N)  Mountains  would  since  The  the  been  e l e v a t e d more than the  since  Late  northern  Eocene  (see H o l l i s t e r  portion  time.  be even more dramatic i f Paleocene  was taken i n t o account  Uplift  has  that  The uplift  1979).  10 Ma  total uplift  since  10 Ma i s shown i n F i g u r e 11. The  53  3.5  km  contour  represents ago.  follows  the  10 Ma  apatite  the approximate depth of the  Values  of  uplift  determined  contour  and  105°C isotherm  using  10  thermal  Ma  models  (Chapter 2) are a l s o shown. Subsidence in the Queen C h a r l o t t e basin below  is  the  sea l e v e l of the Miocene-Pliocene boundary  1971). T h i s may involved  (Shouldice  be an underestimate because t h i s  (6 Ma)  i s somewhat l e s s than 10 Ma.  depth  time  span  Estimates  for  the westernmost Queen C h a r l o t t e I s l a n d s and Vancouver I s l a n d are based upon the a l t i t u d e s of summits which may a  former e r o s i o n s u r f a c e of Miocene age  represent  (Mathews 1968).  presence of Late Miocene to P l i o c e n e v o l c a n i c rocks et a l 1974)  and  sediments (Cox  Vancouver  Island  basin was  probably  subsidence  broadly a  during  1962)  Neogene  v o l c a n i c rocks of Middle and  provide  estimates  around the perimeter  site  of  (Hopkins  deposition 1968).  Late Miocene age  f r i n g e of the Coast Mountains and Belt  (Muller  i n the  in the Pemberton  of the a l t i t u d e of the  al  (see Table 1979)  IV), unroofed 8 Ma  and  erosional  eastern Volcanic  pre-eruptive  (Woodsworth 1977,  Baer  Mathews et. a_l 1981)  1973,  p l u t o n i c rocks  unfaulted Berman  w i t h i n the Pemberton and  10  Ma  (Wanless et  remnants and  and  Finally,  land s u r f a c e . These -include p l a t e a u b a s a l t s l e s s than old  of  supports t h i s c o n c l u s i o n . Whatcom  continuous  the  The  of  lavas  Armstrong  1980,  Anahim V o l c a n i c  Belts. The  area  of  maximum l a t e Neogene u p l i f t  c e n t r a l part of the Coast Mountains from the  occupies  head  of  the Bute  54  F i g u r e 11. T o t a l u p l i f t , w i t h r e s p e c t to sea l e v e l , s i n c e 10 Ma ago (Late Miocene) . The i n t e r p r e t a t i o n i s made assuming a sea l e v e l l a n d s u r f a c e 10 Ma ago. T h i s assumption i s probably reasonable i n t h e southern Coast Mountains, but must be c o n s i d e r e d w i t h c a u t i o n i n t h e n o r t h where Late Miocene p a l e o - r e l i e f and a l t i t u d e were c o n s i d e r a b l e . The u p l i f t f i g u r e s i n t h e n o r t h , t h e r e f o r e , a r e maximum v a l u e s .  55  Inlet  to  Hawkesbury  amounts of u p l i f t area  and  (about  the present r e l i e f (Holland  1964).  uplift  in  whereas  considerably  higher  in  T h i s i n d i c a t e s that the age of  the  in  is  north the  is  predominately  south  P l i o c e n e - P l e i s t o c e n e . The area Bella  total  the King Island-Kemano area are comparable  most of the Miocene  the  1). Though the  Inlet  south  between  (Figure  Mount Waddington-Bute  3.0-3.5 km), the  Island  of  it  maximum  Late  is  mostly  uplift  in  the  Coola-Kemano p o r t i o n of the Coast Mountains i s o f f s e t  westwards at l e a s t 20 km relief.  This  may  from  the  highest  altitudes  and  be a r e s u l t of more r a p i d e r o s i o n on the  windward s i d e of the mountains, as w e l l as p r o x i m i t y to base l e v e l . Since both average Island since  area 10 Ma  are  and  summit a l t i t u d e s i n  considerably less  (3.5 km),  i t i s c l e a r that  pace with u p l i f t . T h i s s i t u a t i o n where  the  summit  altitudes  a l t i t u d e of the 10 Ma recent  uplift  that topographic  are  (<2  relief  erosion  (>3 km)  km may  is  King uplift  keeping  i n the  south  are approximately the  very apparent,  of >3  than the  i s not yet met  s u r f a c e . There, still  km)  the  be  effects  of  and t h i s  suggests  preserved  only  mountain systems that are but a few m i l l i o n years o l d .  Miocene Paleogeography  the more  in  56  Evidence  from  sedimentary  combined with data f o r u p l i f t Miocene  The  i s shown in Figure  shows  pre-Pliocene  been  features  0.2  and  of  a  Pliocene  time;  considerably with  received  reconstructed.  This  Charlotte  all  Ma  Miocene  paleogeography  One  (52°  to  55°N)  to 52°N) Coast Mountains. Higher  uplift  between the northern  in  the  north  resulted  Mountains  therefore, than  may  I s l a n d s moved Chase  Adjacent  from both the  Late  data  Miocene  to  and  of  or  probably basin  adjacent  been  active,  northwesterly  with  B e l t probably  as  and Queen the  some  rifting  Queen  rotation associated  contributed  to  the  that  the  Vancouver I s l a n d  was  of the r e g i o n . It seems l i k e l y  c o a s t a l region northwards from northern  that  mountains  I s l a n d s . Both the Sandspit have  relief  c l i m a t i c influence  the Queen C h a r l o t t e  1981). Volcanism and  with the Anahim V o l c a n i c tectonics  until  the  summit a l t i t u d e s must have been  today.  t h i s u p l i f t was  little  in km  (Miocene Coast Mountains). The  Coast  less  ago,  post-Eocene,  mountain system of perhaps 1.5  faults  and  15-20  main  the Queen C h a r l o t t e  unstable  the  the  sediment d e r i v e d  Charlotte  of  already.  Mathews (1979) r e q u i r e  of the a n c e s t r a l  (Yorath  allow  of  km/Ma)  through the Cenozoic  and  amount  rocks.  discussed  (49°  to  maintenance  linked  distribution  i s the c o n t r a s t  (up  Rouse  and  12.  features of Middle  briefly  southern  rates  the  igneous  Many of the  and  rate  r e c o n s t r u c t i o n d e p i c t s events about  also  have  v o l c a n i c d e p o s i t s when  paleogeography to be a c c u r a t e l y  reconstruction  but  and  57  bounded on the west, as i t i s today, by a and  transform  fault  system  not  present  southern Coast Mountains (Riddihough The  southern  Juan de Fuca  plate,  Coast  the  hot-spot  adjacent to the  1977).  Mountains,  were  complex  adjacent  site  to  the  of low topography  and  s c a t t e r e d v o l c a n i c a c t i v i t y of the Pemberton V o l c a n i c  Belt.  The  1968), but  i t is  margins of  this  Whatcom  basin  was  s u b s i d i n g (Hopkins  not c l e a r where the northwest  and  basin were, as Miocene sediments the  Strait  of  Georgia.  southwest  are only l o c a l l y present i n  Miocene sediments  coast of Vancouver I s l a n d i n d i c a t e south, Vancouver I s l a n d was  that  at  low and p a r t i a l l y  on the southern least  in  the  submerged  (Cox  1962). The  of  the  Interior  extruded between 6  and  10 Ma  Mathews  bulk  1979,  M.L.  Bevier  d a t a ) . These b a s a l t s f i l l e d  ago  1979)  and  (Table  and R.L.  IV;  Armstrong  Rouse  covered  i t s tributaries  l a r g e areas of low  and  sites  (Rouse and relief  w i t h i n the I n t e r i o r P l a t e a u and along the e a s t e r n the  was  unpublished  i n v a l l e y s which were the  of the a n c e s t r a l F r a s e r R i v e r and Mathews  Plateau flood basalts  both  flank  of  Coast Mountains i n the Taseko Lakes area. F a r t h e r north  in the B e l l a Coola area, these  into  broad  v a l l e y s adjacent to the Miocene Coast Mountains (Baer  1973).  The  contemporaneous  basalts  flowed  p e r a l k a l i n e v o l c a n i c s of the  Anahim V o l c a n i c B e l t were superimposed  upon  building  (Bevier e_t a_l 1979).  Little  s e v e r a l large s h i e l d volcanoes evidence of p a l e o - r e l i e f  this  east-west setting,  i s preserved i n the present  58  F i g u r e 12. Miocene paleogeography. Included a r e a l l Miocene igneous r o c k s , a r e a s o f M i o c e n e s e d i m e n t a t i o n , and e s t i m a t e s o f a v e r a g e a l t i t u d e and p a l e o - r e l i e f i n t h e n o r t h e r n p a r t .  59  southern  Coast  remnants  of  Mountains  because  (> 2 km).  fault,  s e v e r a l more s i l i c i c  active  (Berman and Armstrong of  a  few  unfaulted  Miocene l a v a remain. Where preserved, they are  g e n e r a l l y at high a l t i t u d e s  because  only  East  of  the  Fraser  Miocene e r u p t i v e complexes were 1980,  Mathews et a l 1981),  l e s s subsequent u p l i f t ,  v o l c a n i c rocks of  and these  complexes are r e l a t i v e l y w e l l - p r e s e r v e d . Perhaps the most tectonic  setting  surprising  aspect  of  this  i s the negative c o r r e l a t i o n between  ( c o n f i n e d to the north) and c a l c - a l k a l i n e volcanism to  subduction  be mutually entirely  Miocene uplift related  (the Pemberton V o l c a n i c B e l t ) . They appear to  exclusive.  post-dated  The  these  uplift  in  the  south  almost  Middle and Late Miocene  igneous  rocks.  Neogene E r o s i o n Surfaces and During occurred,  the  middle  especially  up to 4 km of rock was this  period  of  river  landscape  valleys.  considerable  s t r i p p e d . In  the  resulted  interrupted Plateau  in by  Plateau,  a very subdued land resistant flowed  hills  and  over  this  that extensions of  this  to preserve i t s c h a r a c t e r i s t i c s .  erosion  present Coast surface  Interior  basalts  V a r i o u s authors have suggested Interior  erosion  i n the northern Coast Mountains, where  erosion  surface o c c a s i o n a l l y broad  Cenozoic  Deformation  surface  Mountains.  consist  of  existed Arguments  across in  the area of the  favor  of  such  a  rock shoulders i n the mountains at the  60  approximate l e v e l accordance  of  of  summits  mountains (Culbert Milbanke plutonic flat  1971,  strandflat rocks  the  surface  throughout  large  Holland  which  (Baer  1964),  contains  1973, Holland  The  fission  g e o l o g i c evidence  (Tipper  track  data  (Baer  the  low  1964), and the r e l a t i v e l y of the p l a t e a u  the physiographic  (1.5-1.8 km),  sea  level  have  exceeded  a p a t i t e dates  s u r f a c e upon which 8-10 Ma b a s a l t s were The surface  suggestion or  plateau  to  to  extruded. of  an  erosion  this further, aerial  indeed  photographs  of  f l a n k of the Coast Mountains from l a t i t u d e 51°  there  i n l a n d - s l o p i n g upland 5000'  regional  60°C/km  to 53°N were examined. In t h i s t r a n s i t i o n between and  maximum  s u r f a c e s w i t h i n the Coast Mountains does  eastern  Coola  1.5 km beneath a  that there are remnants  have m e r i t . To explore the  at  then  and  simple.  (1973) suggests i n the B e l l a  5,000'-6,000'  8-10 Ma  lavas  1978). and  thermal g r a d i e n t s would have to explain  relief  v o l c a n i c and  area, the e r o s i o n s u r f a c e c r o s s e s the mountains of  of the  i n d i c a t e that t h i s concept i s too  i n s t a n c e , i f as Baer  altitudes  1973),  areas  Miocene  g e n t l y e a s t - d i p p i n g basal contact  in the Taseko Lakes area  For  Interior  8000'  mountains  are many examples of low r e l i e f , surfaces  ranging  gently  in altitude  from  (1.5 to 2.4 km). These s u r f a c e s c u t across  lithology  i n d i s c r i m i n a n t l y , and are  cirques  and  other  d i s t r i b u t i o n of these  recent  variably  erosional  dissected features.  s u r f a c e s i s shown i n F i g u r e  with approximate a l t i t u d e s .  by The  13b, along  61 In the southern  area  near Taseko Lakes, these  are l a r g e l y c o n s t r u c t e d on p l a t e a u b a s a l t s and clearly  controlled  surface  gradually  merges these  by  their  declines  l a y e r i n g . The  surfaces  in places  are  a l t i t u d e of  the  northeastwards  and  smoothly  with the modern I n t e r i o r P l a t e a u . For the most p a r t , s u r f a c e s postdate  As  these  headwaters  of  the b a s a l t s .  surfaces the  are  traced  northwards  K l i n a k l i n i River, their  change. They are no longer b u i l t f a c t , the b a s a l t i s present  on Late Miocene b a s a l t ;  only on the p l a t e a u to the  present  topography  Interior  Plateau.  These  l i e well  upland  above  surfaces  in  east,  examples of  developed on g r a n i t i c  west of T a t l a Lake and  the  characteristics  s e v e r a l thousand feet lower in a l t i t u d e . Elegant "biscuit-board"  near  rocks  are  the  present  do not  smoothly  merge with the I n t e r i o r P l a t e a u as i n the Taseko Lakes area, and  as they are higher  the  basalts,  older  erosion  basalts.  Their  in a l t i t u d e and  some of these surface, age,  would  project  above  s u r f a c e s must be remnants of an  one  that  predates  the  however, remains unclear as  plateau apparently  no g e o l o g i c d e p o s i t s o v e r l i e them. F a r t h e r north, near the there  is  an  1.5  been km  eroded  above  extensions  Bella  Coola  e r o s i o n s u r f a c e of Late Miocene age  u n d e r l i e s the b a s a l t and have  eastern  this  extends westwards where  (Baer  that both the  lavas  1973). Mountains which r i s e up to  surface  represent  monadnocks  of the Miocene Coast Mountains i n t o the  P l a t e a u . These easternmost  Valley,  mountains  are  rounded  and  Interior and  in  62  F i g u r e 13. D i s t r i b u t i o n o f L a t e M i o c e n e l a v a s ( a ) and r e m n a n t s o f M i o c e n e - P l i o c e n e e r o s i o n s u r f a c e s (b) . F i g u r e ( a ) i s c o m p i l e d f r o m T i p p e r ( 1 9 6 9 , 1978) a n d 1:250,000 t o p o g r a p h i c maps. Figure (b) h a s b e e n c o n s t r u c t e d f r o m a e r i a l p h o t o g r a p h s a n d t o p o g r a p h i c maps. G e n t l y s l o p i n g s m o o t h t o p o g r a p h y , shown i n s o l i d b l a c k , i s o n l y shown o n t h e m o u n t a i n f r i n g e a n d n o t o n t h e I n t e r i o r Plateau area.  63  places  have  pediment-like lower  relatively  flat  sub-basalt  e r o s i o n s u r f a c e which l i e s about 5,000' (1.5  in  altitude  Mountains  penetrates and  has  Miocene-Pliocene Miocene  sub-basalt  slopes that merge with the  erosion  the peaks i n the  surface.  deeply been  uplift.  summits  in  (20 km)  little  this  area  into  the  the  modified  km)  Coast  by  Late  The westward extension of the  Late  s u r f a c e must pass w e l l over the summits of northern  King  probably at an a l t i t u d e of 3-3.5 of  In  Island-Ocean km.  Falls  C l e a r l y , the  area,  accordance  t h i s area must have a d i f f e r e n t e x p l a n a t i o n  than simply r e p r e s e n t i n g r e l i c t s  of a  single  Late  Miocene  erosion surface. The d i s t r i b u t i o n of Late Miocene b a s a l t s and upland  surfaces  Except  for  Anahim  minor  can  be  compared  faulting  Volcanic Belt  (Baer  probably 1973)  low  relief  i n F i g u r e s 13a and associated  13b.  with  the  the p l a t e a u b a s a l t s i n the  north are e s s e n t i a l l y  f l a t l y i n g at 4,500'±500' (1.4±0.2  and p a r t l y surrounded  by  older  higher  are  distinctly  bent  contains  the topography  low  relief  surface,  south and  up towards the southwest at the  edge of the Coast Mountains. The  in  that  remnant s u r f a c e s . In c o n t r a s t , the l a v a s i n the  are l a r g e l y coextensive with the they  topography  km)  6 to  10 Ma  lavas  predate  i n the south whereas they postdate much of i t  the n o r t h . In t h i s respect they demonstrate the c o n t r a s t  in post-Late Miocene t e c t o n i c h i s t o r y  from north to south.  By c o n t o u r i n g the a l t i t u d e of the base of (Figure  13a),  the  basalts  i t i s p o s s i b l e to e v a l u a t e the gross s t r u c t u r e  64  of  the  late  Neogene  deformation  in  the  southern Coast  Mountains.  Assuming the b a s a l l a v a s were extruded on a  or  horizontal  less  surface,  estimated to be about 8000'  (2.4 km)  gradient  with  southwest  in  1.3 a  minor  within  at  structural  lavas r i s e  smooth,  amplitude  to  about  than  undulations.  Farther  Miocene  7,500'  volcanic  (2.3 km).  rocks  This  (Woodsworth  suggests that  land s u r f a c e on  which  the  Georgia-Whatcom  perhaps  be  basin a  area.  gentle  Thus  the  monocline  on  the form  the  a l s o the west s i d e . As yet there i s l i t t l e  suggest that f a u l t i n g has played  an  important  the  rocks  extruded f l a t t e n s over the southern Coast Mountains  deformation may  to  more  is  a p p a r e n t l y unbroken  a high p l a t e a u - l i k e manner before dropping i n t o of  relief  the Pemberton V o l c a n i c B e l t near B r a l o r n e  pre-Late Miocene (>10 Ma) were  The  fairly low  and L i l l o o e t Lake are 1977)  km.  the  more  in  Strait of  the  east  and  evidence role  in  this Pliocene-Pleistocene u p l i f t . That the Neogene u p l i f t is  both P l i o c e n e - P l e i s t o c e n e i n age and s i m i l i a r  plateau-like uplift of  i n the southern Coast  the a r e a . The  Mountains to a broad  i s supported by the g e n e r a l physiography Coast  Mountains  have been contoured and are shown i n F i g u r e 14 to  illustrate  this  point.  A  summits over especially relatively summit  summit a l t i t u d e s  broad  2-3 on  km  the  >100 in  km  in  wide  altitude  southwest  in  the  area i n the south has  with  side.  recent, i t i s more l i k e l y  altitudes  the  south  smooth  gradients,  Because the u p l i f t i s that  (49°  the  surface  of  to 52°N) r e f l e c t s a  65  pre-uplift 55°N)  e r o s i o n s u r f a c e than  area.  in  the  same a l t i t u d e as t h i s  and  contrast  the  summit s u r f a c e  portion  of the northern  of  main  embayments  fjords  which  and  may  along  in  Figure of  of  channels  coincide  part  related  by  be  later  this  ancestry  Skeena  suggestion.  A  River  Both  the  The  summit  of r e l i e f  and  surface. E s s e n t i a l l y  (1974) and  suggested  more  surface  uplift  with  broad  The  the l o c a t i o n  pattern  support  embayed,  suggests in the  and  greater  north.  Volcanic  British  Belt,  described  Geological  s u r f a c e and  are c o n s i s t e n t with  Columbia,  low  Survey lie  on  reflect  relief rocks  subduction  (Keen  and  land of  by M u l l e r et a_l of  Canada  and  a f a i r l y smooth  t h i s c o n c l u s i o n . Seismic  on Vancouver I s l a n d tectonic,  probably  f l a t lying Pliocene volcanic  of  a  main  paleo-drainage  irregular,  of a r e l a t i v e l y  University  suggest  several  i s shown d i a g r a m a t i c a l l y in  drainage p a t t e r n s  dated by the  g r a v i t y data  small  Queen C h a r l o t t e I s l a n d s and Vancouver I s l a n d  Pliocene-Pleistocene  Bay  lower  to Miocene to  represent  have r e l a t i v e l y smooth summit s u r f a c e s and  Alert  the  a  12. There i s , however, no d i r e c t evidence i n  g e n e r a l l y lower  the  at  of  and  glaciation.  Douglas Channel may  the  14). Only  l i e s above 2 km,  of the a n c e s t r a l Skeena R i v e r . pattern  l i e roughly  i s considerably  (Figure  region  P l i o c e n e e r o s i o n , modified embayment  to  surface.  more i r r e g u l a r north of 51°N  the  (52°  S i g n i f i c a n t l y , the u n f a u l t e d v o l c a n i c rocks  the Pemberton V o l c a n i c B e l t , though few,  In  northern  Hyndman  and 1979)  r e l a t e d , mechanism f o r the  66  F i g u r e 14. Smoothed s u r f a c e of summit a l t i t u d e s i n the Coast Mountains. Recent v o l c a n i c summits a r e excluded. The map was c o n s t r u c t e d u s i n g 1:500,000 topographic maps by measuring about 25-30 summits per l°xl° a r e a .  67  recent and c o n t i n u i n g u p l i f t Culbert  (1971)  physiography  in  concluded  that  presented  the  computer-assisted  there.  Coast  contouring a  of  boundary,  to  the  west.  interpretation  Mountains  based  summit  termed  separated areas of block u p l i f t areas  an  This a x i a l  axial  east  1),  and  a and  fracture,  from  f r a c t u r e was  j o i n the heads of Howe Sound, J e r v i s I n l e t (Figure  on  altitudes  the  to the  of  subsided  p o s t u l a t e d to  and  Bute  Inlet  i t c o i n c i d e s with the l o c a t i o n of s e v e r a l  hot s p r i n g s . He a l s o suggested  that  the  transverse  Bella  Coola V a l l e y bounded an u p l i f t e d block to the south. None of these  features  can  be  easily  recognized  on  Figure  14.  U n f o r t u n a t e l y no d i r e c t comparison of summit contour maps i s p o s s i b l e because  no  corresponding  map  was  Culbert  (1971). The c o i n c i d e n c e of thermal  linear  "axial  of  steepest  f r a c t u r e " may thermal  topographic  relief,  trends,  a  in  hotsprings 1979),  in  although  contribute  such  s p r i n g s with  resulting  from  l a r g e u p l i f t , and northwest  manner New  similiar  Zealand's  higher  (Hyndman  to  the  Southern  regional 1976).  by this  be r e l a t e d to i t being an area  gradients  c o n v e c t i v e hydrothermal steep heat  presented  Block  t r a n s p o r t of  flow faulting  heat  structural  development Alps  heat  maximum  to  of  ( A l l i s e_t a l may  also  could  aid  explain  the  flow g r a d i e n t of Lewis and Hyndman (1980), but  fault(s)  has  yet  been  recognized,  no  either  p h y s i o g r a p h i c a l l y or g e o l o g i c a l l y . In  summary, the c o n t r a s t s between the northern  (52°  to  68  55°N)  and  southern  (49° to 52°N) Coast Mountains,  in the f i s s i o n t r a c k data, are borne evidence.  A  surface  out  by  apparent  physiographic  p r e s e n t l y higher a l t i t u d e , p l a t e a u - l i k e summit  with  remnants  characteristic  of  the  of  Late  southern  region.  northern r e g i o n was  never  continuous  during the middle  uplift  Miocene  plateau-like;  is  In c o n t r a s t , the it  and  lavas  has  undergone  l a t e Cenozoic,  and  probably c o n t a i n s e r o s i o n s u r f a c e s that are of s e v e r a l ages. Embayments i n the northern summit s u r f a c e may  indicate  the  Miocene p o s i t i o n of l a r g e v a l l e y s .  D i s c u s s i o n and P o s s i b l e Causes of U p l i f t There  are  three  Cenozoic  uplift  Mountains of B r i t i s h Columbia which The  first  the  was  central  an and  eastern  Coast  uplift from  stage was  Alaska  to  from  the  Coast  Mountains  region  (>3  km)  giving  Cenozoic extending  the Mount Waddington area. T h i s  Coast  kilometers  Mountains  of the Queen C h a r l o t t e  t h i r d stage, of l a t e Neogene age, Recent u p l i f t  Hollister  the l e s s r a p i d middle  northern  complementary subsidence  extending  by  r e s p o n s i b l e f o r the e r o s i o n of s e v e r a l  material  broad  P l u t o n i c Complex  i t i s reviewed  i n the a x i a l part of the  stage was of  second  southeast  distinguished.  D i s c u s s i o n of t h i s major event i s  beyond the scope of t h i s paper; The  be  intense orogenic to post-orogenic u p l i f t i n  from Alaska southeastward.  (1979).  can  stages i n the Coast  and  basin.  the The  i n v o l v e d r a p i d P l i o c e n e to  of the southern Coast Mountains over a rise  to the present topography.  This  69  l a t e Neogene u p l i f t  was  manifest  to a l e s s e r degree  in  the  northern area where i t probably began i n the Late Miocene. Explanations  for the Eocene orogenic event  i n t e r a c t i o n of the destructive  America  margin.  and  The  Farallon  intense  i n v o l v e the  plates  Eocene  along  event  in  n o r t h - c e n t r a l Coast Mountains of B r i t i s h Columbia 52°-56°N)  a the  (latitude  i n v o l v e d voluminous p r o d u c t i o n of magma and  rapid  c r u s t a l t h i c k e n i n g r e l a t e d to r e g i o n a l s h o r t e n i n g d u r i n g the f i n a l p e r i o d of  rapid  convergence.  amounted  to  > 10 km,  and  (Hollister  1979), the c r u s t  Since  perhaps  must  Eocene  erosion  significantly  have  been  more  exceptionally  thickened. The  ensuing middle Cenozoic  the Eocene event related.  uplift  i n the Coast P l u t o n i c Complex  Assuming  the  formation  of the  lack  (Berry  such  a  thick  crust  i n d i c a t e s that i t has been eroded. erosion,  d i m i n i s h i n g u p l i f t , and  would l i k e l y  may  be  Eocene,  The  and  the  present  Forsyth  linked  1975)  process  of  resultant crustal thinning  take s e v e r a l tens of m i l l i o n years to e l i m i n a t e  such a r o o t . T h i s process the middle Cenozoic Coast  and  a l a r g e c r u s t a l root  s u p p o r t i n g high mountains d u r i n g of  i s c o e x t e n s i v e with  Mountains.  uplift The  i s thought  to be  responsible  stage i n the northern and  lack  of  gradients  suggests that the u p l i f t does not have a deep-seated The  approximately  what  linear would  arch-like be  central  a thermal high manifest i n  e i t h e r v o l c a n i c a c t i v i t y or steep paleogeothermal  explanation.  for  form  expected  thermal  of the u p l i f t  from  an  is  isostatic  70  recovery mechanism. Other continued middle  explanations l e s s intense  Cenozoic.  are oblique  However,  v o l c a n i c expression except this p o s s i b i l i t y  possible  is  no  in  the  late  middle  Neogene  Features  that must be e x p l a i n e d a r e : 1)  south,  Pliocene-Pleistocene  uplift  the  Mountains  Coast  Neogene u p l i f t  throughout  the  Cenozoic and  seems u n l i k e l y .  especially  to  involve  f o r the Anahim V o l c a n i c B e l t ,  uplift,  southwards  could  subduction  there  An e x p l a n a t i o n for the  southern  and  the  is  acceleration  of  much more e l u s i v e . >3  km  of  probably  over n e a r l y the e n t i r e width of from  about  F r a s e r V a l l e y , and  in the northern Coast  latitude  52°N  2) the moderate l a t e  Mountains  axial  zone,  western Queen C h a r l o t t e I s l a n d s , and Vancouver I s l a n d . Observations h i a t u s in  which  calc-alkaline  should  be  volcanic  considered activity  a c t i v i t y of the Pemberton V o l c a n i c B e l t and in  the  Garibaldi  Volcanic Belt  orientation plates  of  convergence  (Riddihough  broadest  and  1977), and  highest  >7  Ma  (Bevier et a_l 1979,  Berman  in the  development  the  of  the  Bay V o l c a n i c B e l t ,  relative  between 4)  between  events  'plate edge', P l i o c e n e A l e r t  3) the changes at about 5 Ma  1) the  the <2 Ma  and Armstrong 1980), 2) the subsequent transverse,  are  Explorer  velocity and  coincidence  and  America of  the  l a t e Neogene u p l i f t with the area  on  the mainland opposite the convergent boundary. The  production  maintaining  of  2-3  km  of  crustal  uplift  while  i s o s t a t i c e q u i l i b r i u m can be produced by s e v e r a l  71  mechanisms: I s l a n d New and  crustal Zealand  Tapponier  t h i c k e n i n g by compression,  (Walcott  1978)  or  the  as i n South  Himalaya  (Molnar  1977); by u n d e r p l a t i n g a l a r g e amount of  low  d e n s i t y a r c - r e l a t e d magmatic m a t e r i a l ; or by t h i n n i n g of the l i t h o s p h e r e as i n western  the  Basin  and  Range  province  of  United S t a t e s .  The  most  significant  c o r r e l a t i o n s with the u p l i f t  the southern Coast Mountains are the r e o r i e n t a t i o n de  Fuca-Explorer-America  plate  of  7  and  2 Ma.  During  h i a t u 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 migrated the  Garibaldi  response jump  Volcanic  (Figure  to steepening of the subducted  and this  westward  1), probably i n  slab. This  westward  would lead to warming of the formerly c o o l l i t h o s p h e r e  causing thermal  expansion  Mountains.  subduction  crust  Belt  of  Juan  motions about 5 Ma ago  the v o l c a n i c arc h i a t u s between  to  the  The  (Riddihough  1977)  and  uplift  of  very  would c e r t a i n l y  beneath hot  the  (0-15 Ma)  Coast oceanic  contribute  to  the  high a l t i t u d e of the o v e r r i d i n g p l a t e e s p e c i a l l y r e l a t i v e to the nonconvergent area to the north (Stacey The northern  l e s s r a p i d predominately part  Late Miocene u p l i f t  of the Coast Mountains i s enigmatic  i s adjacent to a transform boundary and has with  volcanic  activity.  g r a d i e n t data and unpublished  present  data)  p r e s e n t l y than 20 Ma to  1974).  Fission-track heat  indicate ago,  flow higher  no  since i t  association  paleogeothermal  estimates regional  although t h i s may  in the  (T. Lewis heat  be more  flow  related  the Anahim V o l c a n i c B e l t than a r e g i o n a l p a t t e r n . Whether  72  the thermal expansion caused by t h i s hot spot c o u l d induce a longitudinal  uplift  in  and deserves f u r t h e r  the north (52° to 55°N) i s unclear  study.  Summary G e o l o g i c a l and p h y s i o g r a p h i c Coast  Mountains  result  of  of  B r i t i s h Columbia  vigorous  examination  of  late  this  Neogene  uplift  fission-track  dating  collected  various  at  data  of  indicate  been  apatite  the  are predominately the uplift.  has  that  and  Quantitative  made  possible  zircon  l o c a t i o n s and a l t i t u d e s  from  by  rocks  i n the Coast  Mountains. Uplift estimates  rates of  have  total  uplift  e n t i r e r e g i o n . These Cenozoic  uplift which was  adjacent  Queen  paleogeography distribution  the  track  in  flow may  Explanations older  middle  to  55°N)  subsidence  data  in  the  shows  Coast of  that  the  the  o p p o s i t e to the south;  g r a d i e n t s using d e t a i l e d f i s s i o n  altitude profiles  reduced heat  significant  average  both areas, however, were lower. Estimates of  paleo-geothermal vs.  and  f o r the  in the Miocene was  present s i t u a t i o n of higher r e l i e f altitudes  time,  calculated  (52° to  and  b a s i n . R e c o n s t r u c t i o n of Miocene  fission  relief  been  document  closely related  using  space  northern  Charlotte  of  in  have  results  in  Mountains  varied  (Eocene and  t r a c k date  i n d i c a t e that the r e g i o n a l p a t t e r n  of  have v a r i e d c o n s i d e r a b l y with time.  for  the  middle  uplift Cenozoic)  are c l e a r e r stages  f o r the  than  for  two the  73  youngest  (Late  Miocene  crustal thickening latitude  in  52°-56°N  to  the  was  Recent). Eocene orogenesis and northern  followed  by  Coast a  Mountains  transition  convergent t o a s t r i k e - s l i p p l a t e t e c t o n i c regime. but  steady  e r o s i o n and  isostatic  uplift  present  thickness  is  similar  to  from a Moderate  i n the Miocene and  i n t o the l a t e Neogene g r a d u a l l y thinned the its  from  crust  stable  so  that  continental  crust. During the middle Cenozoic the southern Coast were low i n a l t i t u d e uplift,  despite  and  relief  subduction  and and  subject  or  h i a t u s i n the Coast Mountains, of  minimal volcanic  plate.  In  late  e a r l y P l i o c e n e time, d u r i n g a r e o r g a n i z a t i o n of  Juan de Fuca - E x p l o r e r p l a t e geometry and  width  to  intermittent  a c t i v i t y above the subducting Juan de Fuca Miocene  Mountains  the  southern  Coast  uplift  a  volcanic  arc  a c c e l e r a t e d a c r o s s the  Mountains,  leading  present dramatic topography. T h i s southern u p l i f t  to the  i s thought  to be r e l a t e d to the westward m i g r a t i o n of the v o l c a n i c  arc  f r o n t and the ensuing thermal expansion i n the formerly c o o l l i t h o s p h e r e beneath most of the southern Coast  Mountains.  74  Acknowledgements The support, The  author  i s indebted to R.L.Armstrong f o r continuous  a d v i c e , and ideas d u r i n g the course  of t h i s  study.  f i n a n c i a l help provided by a P r e - d o c t o r a l F e l l o w s h i p at  the U n i v e r s i t y of B r i t i s h Columbia, the N a t u r a l Sciences and E n g i n e e r i n g Research C o u n c i l as a and  grant  to  R.L.Armstrong,  a g r a n t - i n - a i d from the G e o l o g i c a l S o c i e t y of America i s  gratefully  acknowledged.  helped d e f r a y the c o s t s discussion,  the  G.K.C.Clarke,  drafting University  Geological  K-Ar  Survey of Canada  dating.  For  greatly  and  and  T.Lewis;  of  Survey.  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Geochronology and thermal h i s t o r y of the Coast P l u t o n i c Complex, near P r i n c e Rupert, British Columbia. Canadian Journal of Earth Sciences, 16, pp.400-410. H a r r i s o n , T.M., and McDougall, I. 1980. I n v e s t i g a t i o n s of an intrusive contact, northwest Nelson, New Zealand-I. Thermal, chronological, and isotopic constraints. Geochimica et Cosmochimica Acta, 44, pp.1985-2004. H o l l a n d , S.S. 1964. Landforms of B r i t i s h Columbia. British Columbia Department of Mines and Petroleum Resources, B u l l e t i n 48, 138p. Hollister, L.S. displacements: pp.3-8.  1979. Metamorphism and crustal New insights. Episodes, 1979, no.3,  Hopkins, W.S. 1968. Subsurface Miocene rocks, British Columbia-Washington, a palynological investigation. G e o l o g i c a l S o c i e t y of America B u l l e t i n , 50_, pp.763-768. H u r f o r d , A . J . , and Gleadow, fission track dating D e t e c t i o n , J_, pp.41-48.  A.J.W. 1977. C a l i b r a t i o n of parameters. Nuclear Track  Hutchison, W.W. 1970. Metamorphic framework and p l u t o n i c styles i n the P r i n c e Rupert region of the c e n t r a l Coast Mountains. Canadian Journal of Earth Sciences, 7, pp.376-405. Hyndman, R.D. 1976. Heat flow measurements i n the i n l e t s of southwestern B r i t i s h Columbia. Journal of Geophysical Research, 8_J_, pp.337-349. Johnson, N.M., McGee, V.E., and Naeser, C.W. 1979. A p r a c t i c a l method of e s t i m a t i n g standard e r r o r of age i n the fission track dating method. Nuclear Tracks, 3, pp.93-99. Johnson, S.Y. Chuckanut  1981. Sedimentation and t e c t o n i c s of the Formation on Bellingham Bay, Washington.  78  G e o l o g i c a l A s s o c i a t i o n of Canada Programme and A b s t r a c t s , p.25.  Cordilleran  Section,  Keen, C.A. and Hyndman, R.D. 1979. Geophysical review of the continental margins of eastern and western Canada. Canadian J o u r n a l of E a r t h Sciences, ]_6 pp.712-747. r  Lewis, T . J . and Hyndman, R.D. 1981. Heat flow in Jervis Inlet, southwestern B r i t i s h Columbia. EOS, v.62, no.6, p.59. Mathews, W.H. 1968. Geomorphology, southwestern British Columbia. I_n Guidebook for geological f i e l d trips in southwestern British Columbia, Mathews, W.H., ed. Department of G e o l o g y , U n i v e r s i t y of B r i t i s h Columbia, Report no.6, pp.18-25. Mathews, W.H. 1972b. Geothermal data from the Granduc area, northern Coast Mountains of B r i t i s h Columbia. Canadian J o u r n a l of E a r t h Sciences, 9, pp.1333-1337. Mathews, W.H., and Rouse, G.E. 1963. Late T e r t i a r y v o l c a n i c rocks and p l a n t - b e a r i n g d e p o s i t s i n B r i t i s h Columbia. G e o l o g i c a l S o c i e t y of America B u l l e t i n , 7_4, pp.55-60. Mathews, W.H., Berman, R.G., and Harakal, J . E . 1981. Mid-Tertiary v o l c a n i c rocks of the Cascade Mountains, southwestern B r i t i s h Columbia, ages and c o r r e l a t i o n s . Canadian J o u r n a l of E a r t h Sciences, j_8, pp.662-664. Molnar, P., and Tapponier, P. 1977. R e l a t i o n of the t e c t o n i c s of eastern China to the India-Eurasia collision: A p p l i c a t i o n of s l i p - l i n e field theory t o large-scale continental tectonics. Geology, 5, pp.21 2-216. Monger, J.W.H. 1977. Upper P a l e o z o i c rocks of the western Canadian C o r d i l l e r a and t h e i r bearing on C o r d i l l e r a n evolution. Canadian Journal of E a r t h S c i e n c e s , 14, pp.1832-1859. Monger, J.W.H., Souther, J.G., and G a b r i e l s e , H. 1972. Evolution of the Canadian C o r d i l l e r a : A p l a t e t e c t o n i c model. American J o u r n a l of Science, 272, pp.577-602. Monger, J.W.H., and P r i c e , R.A. 1979. Geodynamic e v o l u t i o n of Canadian C o r d i l l e r a - progress and problems. Canadian J o u r n a l of E a r t h S c i e n c e s , j_6, pp.770-791. Muller, J . E . , Northcote, K.E., and C a r l i s l e , D. 1974. Geology and mineral deposits of Alert-Cape Scott map-area, Vancouver I s l a n d , B r i t i s h Columbia. G e o l o g i c a l Survey of Canada Paper 74-8, 77p.  79  Naeser, C.W. 1976. F i s s i o n track d a t i n g . United States G e o l o g i c a l Survey, O p e n - f i l e Report 76-190. Naeser, C.W. 1979. F i s s i o n track dating and geologic annealing of f i s s i o n tracks. I_n L e c t u r e s i n isotope geology, ed. By Jager, E., and Hunziker, J.C. S p r i n g e r - V e r l a g , pp.154-169. Naeser, C.W., and F a u l , H. 1969. F i s s i o n track annealing i n a p a t i t e and sphene. J o u r n a l of G e o p h y s i c a l Research, 74, pp.705-710. Naeser, C.W., and Forbes, R.B. 1976. V a r i a t i o n of fission track ages with depth i n two deep d r i l l holes ( a b s t r a c t ) . EOS, 57, p.363. Naeser, C.W., Gleadow, A.J.W., and Wagner, G.A. 1979. Standardization of f i s s i o n t r a c k data r e p o r t s . Nuclear Tracks, 3, pp.133-136. Nelson, J.A. 1979. The western margin of the Coast Plutonic Complex on Hardwicke and West Thurlow I s l a n d s , B r i t i s h Columbia. Canadian Journal of E a r t h Sciences, 16, pp.1166-1175. Parrish, R.R. 1980. F i s s i o n track e s t i m a t i o n of paleo-heat flow, Coast Mountains of B r i t i s h Columbia, Canada. EOS, 6J_, p. 1 1 31 . Richards, T., and White, W.H. 1970. K-Ar ages of p l u t o n i c rocks between Hope, British Columbia, and the 49th parallel. Canadian Journal of E a r t h Sciences, 1_, pp. 1203-1207. Richards, T., and McTaggart, K.C. 1976. G r a n i t i c rocks of the southern Coast P l u t o n i c Complex and northern Cascades of B r i t i s h Columbia. G e o l o g i c a l S o c i e t y of America B u l l e t i n , 87, pp.935-953. Riddihough, R.P. 1977. A model f o r recent p l a t e i n t e r a c t i o n s off Canada's west c o a s t . Canadian Journal of E a r t h Sciences, \±, pp. 384-396. Roddick, J.A., and Hutchison, W.W. 1974. S e t t i n g Coast Plutonic Complex, B r i t i s h Columbia. Geology, 8, pp.91-108.  of the Pacific  Rouse, G.E., Mathews, W.H., and Blunden, R.H. 1975. The Lions Gate member: A new Late Cretaceous sedimentary s u b d i v i s i o n i n the Vancouver area of B r i t i s h Columbia. Canadian J o u r n a l of E a r t h S c i e n c e s , J_2, pp.464-471. Rouse, G.E., and palynology of  Mathews, W.H. the Quesnel  1979. T e r t i a r y geology and area, British Columbia.  80  Bulletin  of Canadian Petroleum Geology, 27, pp.418-445.  Schaer, J.P., Reimer, G.M., and Wagner, G.A. 1975. A c t u a l and ancient u p l i f t rate i n the Gotthard r e g i o n , Swiss A l p s : A comparison between p r e c i s e l e v e l i n g and f i s s i o n track a p a t i t e age. T e c t o n o p h y s i c s , 29, pp.293-300. Shouldice, D.A. 1971. Geology of the western continental s h e l f . Canadian Petroleum Geology J_9, pp.405-436.  Canadian Bulletin,  Souther, J.G. 1977. Volcanism and t e c t o n i c environments i n the Canadian C o r d i l l e r a - A second look. I_n Baragar, W.R.A., and o t h e r s , eds., V o l c a n i c regimes i n Canada. Geological Association of Canada S p e c i a l Paper 16, pp. 3-24. Stacey, R.A. 1974. lithosphere in pp.133-134.  Plate tectonics, volcanism, British Columbia. Nature,  and 250,  Steiger, R.H., and Jager, E. 1977. Subcommission on geochronolgy: Convention on the use of decay c o n s t a n t s in geo- and cosmochronology. E a r t h and P l a n e t a r y Science L e t t e r s , 3_6, pp.359-362. Steven, T.A., Mehnert, H.H., and Obradovich, J.D. 1967. Age of volcanic activity i n the San Juan Mountains, Colorado. United States G e o l o g i c a l Survey, Professional Paper 575-D, pp. 47-55. Sutherland Brown, A. 1968. Geology of the Queen C h a r l o t t e I s l a n d s , B r i t i s h Columbia. B r i t i s h Columbia Department of Mines and Petroleum Resources, B u l l e t i n 54, 226p. Tipper, H.W. 1963. Geology, Taseko Lakes, B r i t i s h G e o l o g i c a l Survey of Canada, Map 29-1963.  Columbia.  T i p p e r , H.W. 1969. Anahim Lake. G e o l o g i c a l Survey of Map 1202A.  Canada  Tipper, H.W. 1978. Taseko Lakes (920) map-area. G e o l o g i c a l Survey of Canada, open f i l e map. U n i t e d S t a t e s G e o l o g i c a l Survey. 1974. Utilization of the Geological Survey TRIGA r e a c t o r f a c i l i t y . U n i t e d S t a t e s G e o l o g i c a l Survey, informal r e p o r t . Wagner, G.A., and Reimer, G.M. 1972. Fission track tectonics: The t e c t o n i c i n t e r p r e t a t i o n of f i s s i o n track a p a t i t e ages. E a r t h and P l a n e t a r y Science L e t t e r s , 14, pp.263-268. Wagner,  G.A.,  Reimer, G.M.,  and Jager, E. 1977. The  cooling  81  ages d e r i v e d by a p a t i t e f i s s i o n t r a c k , mica Rb-Sr, and K-Ar dating: The uplift and cooling h i s t o r y of the C e n t r a l A l p s . Memoir of the I n s t i t u t e of Geology and Mineralogy, University of Padova, Padova, I t a l y , XXX, 27p. Wagner, G.A., M i l l e r , D.A., and Jager, E. 1979. Fission track ages on a p a t i t e of B e r g e l l rocks from C e n t r a l Alps and B e r g e l l boulders in Oligocene sediments. E a r t h and P l a n e t a r y Science L e t t e r s , _45, pp.355-360. Walcott, R.I. 1978. Present t e c t o n i c s and Late Cenozoic evolution of New Zealand. Geophysical J o u r n a l of the Royal Astronomical S o c i e t y , 5_2, pp. 137-164. Wanless, R.K., Stevens, R.D., Lachance, G.R., and Delabio, R.N. 1964-1979. Age d e t e r m i n a t i o n s and g e o l o g i c s t u d i e s . G e o l o g i c a l Survey of Canada, Papers 64-17, 65-17, 66-17, 67-2A, 69-2A, 71-2, 73-2, 74-2, 77-2, and 79-2. Woodsworth, G.J. 1977. Pemberton (92J) map-area. G e o l o g i c a l Survey of Canada, Open f i l e map 482. Woodsworth, G.J. 1979. Geology of W h i t e s a i l Lake map area, British Columbia. I_n Current research, Part A, G e o l o g i c a l Survey of Canada, Paper 79-1A, pp.25-29. Woodsworth, G.J., and Tipper, H.W. 1980. Stratigraphic framework of the.Coast P l u t o n i c Complex, western B r i t i s h Columbia. G e o l o g i c a l A s s o c i a t i o n of Canada C o r d i l l e r a n S e c t i o n , Programme and A b s t r a c t s , pp.32-34. Yorath, C.J., and Chase, R.L. 1981. Tectonic h i s t o r y of allochthonous terranes: Northern Canadian Pacific c o n t i n e n t a l margin. G e o l o g i c a l Association of Canada C o r d i l l e r a n S e c t i o n , Programme and A b s t r a c t s , pp. 42-43. Young, I. 1981. S t r u c t u r e of the western margin of the Queen Charlotte basin, British Columbia. M.Sc.thesis, U n i v e r s i t y of B r i t i s h Columbia, 380 p. Zimmermann, R.A. 1977. The i n t e r p r e t a t i o n of a p a t i t e f i s s i o n t r a c k ages with an a p p l i c a t i o n to the study of uplift since the Cretaceous in eastern North America. Unpublished Ph.D. T h e s i s , U n i v e r s i t y of Pennsylvania, 146 p. Zimmermann, R.A., and Gaines, A.M. 1978. A new approach to the study of fission track f a d i n g . United States G e o l o g i c a l Survey, Open F i l e Report 78-701, pp.467-468.  CHAPTER 2.  CENOZOIC TECTONICS AND THERMAL EVOLUTION  OF THE  COAST MOUNTAINS OF BRITISH COLUMBIA I I :  HEAT FLOW MODELS, THERMAL EVOLUTION,  AND  THE CAUSES OF UPLIFT  83  Abstract  The  thermal and  t e c t o n i c e v o l u t i o n of the c r u s t in the  Coast Mountains of B r i t i s h Columbia has a  quantitative  migration  and  program was erosion  fission  lagging  geothermal  and  model,  heat  allow  f l u x v a r i a t i o n s and  a  of  rate,  temperature with  r a t e , surface temperature, heat  determination  flow  sub-crustal  production  heat flow, when used as  isotherm  uplift  changing  surface  using  1). A heat  variable  uplift,  fluctuating  h i s t o r y of  (Chapter  incorporate  decreasing  uplift  present  evolution  behind  flux,  exponentially Estimated  track-derived  topographic  w r i t t e n to  been examined  input  for  and depth.  production, the  thermal  the s u b - c r u s t a l geothermal  refinement  of  Cenozoic  uplift  and  denudation h i s t o r y . Early  Cenozoic  orogenesis  i n the c e n t r a l and  Coast Mountains (52°-55°N) r e s u l t e d in thickening development  and of  (0.1-0.2 km/Ma)  transform  erosion  to  elimination  acceleration  plate  of thickened of  of  the  uplift  isostatic  was  lithosphere  s u r f a c e heat  flux.  and  caused  root.  probably  changes  The  in  the spot' in  the  moderate  rates in t h i s  c r u s t . T h i s process crustal  crustal  f o l l o w e d by  uplift  s u b - c r u s t a l passage of the Anahim 'hot the  was  margin.  middle Cenozoic u p l i f t  were the r e s u l t of gradual of  substantial  r a p i d Eocene u p l i f t , and a  northern  region  consequence  eventually led Late result which  Miocene of  the  thinned  sub-crustal  and  84  The  southern  characterized (<0.1  by  km/Ma) and  Miocene-Recent (>0.4  Coast very  Mountains low middle  uplift  area.  The  i n 2 to  environment.  early  As  a  warmed  Latest  of  uplift  over  a  thickening  for this  uplift  is  reorganization  Pliocene,  the mantle,  was  the v o l c a n i c  resulting  and a p l a t e a u - l i k e u p l i f t . Steepened  in a  fore-arc,  r e s u l t of downgoing s l a b  westwards; u p w e l l i n g of asthenosphere zone  rates  3 km  Mountains  steepening and p l a t e motion or  uplift  i n the upper mantle. During the Miocene,  most of the southern Coast  Miocene  Cenozoic  lack of s e i s m i c i t y and c r u s t a l  expansion  low-heat-flow  were  a r c volcanism.  s t r o n g l y suggests that the mechanism thermal  52°N)  i n the south, by c o n t r a s t , was r a p i d  km/Ma) and r e s u l t e d  broad  Miocene  sporadic  (49° t o  above  in  the  belt  latest  migrated  the subduction  in lithospheric  thinning  subduction a l s o l e d t o  t h i c k e n i n g of the l i t h o s p h e r e west of the P l i o c e n e t o Recent magmatic  front  and consequent  subsidence i n the S t r a i t of  Georgia. Warming of a proposed c u t - o f f Late Miocene  shallow  d i p p i n g s l a b induced phase changes and thermal expansion and resulted of  British  i n moderate (1 km) u p l i f t Columbia.  of the southern  Interior  85  Introduction Heat t r a n s f e r models have been the  thermal  uplift  evolution  of  developed  to  mountain b e l t s which  simulate experience  and e r o s i o n and the consequent c o o l i n g of rocks. Such  models can be c o n s t r u c t e d to s a t i s f y record  the  dates  which  time when the b l o c k i n g temperature isotherms of  the v a r i o u s geochronologic (Harrison and Clarke inverted  isotopic  to  systems moved through  the  rocks  1979). In some cases, the models can be  determine  g e o l o g i c and g e o p h y s i c a l parameters  which may be p o o r l y known. Three accurate rates  parameters and  necessary  realistic  models  to are  the  formulation  1) u p l i f t  and e r o s i o n  ( f u n c t i o n of space and time), 2) s u b - c r u s t a l (reduced)  geothermal f l u x thermal  ( f u n c t i o n of space and  effect  time),  and  3)  without  uplift  i n t r u s i v e a c t i v i t y , the i n t r u s i v e p e r t u r b a t i o n s  ignored.  In  can  a d d i t i o n , i f l a t e r a l v a r i a t i o n s i n thermal  c o n d u c t i v i t y , reduced one-dimensional  the  of i n t r u s i o n s ( f u n c t i o n of s i z e , shape, and  temperature c o n t r a s t ) . In areas that are e x p e r i e n c i n g  be  of  heat  model  flow and u p l i f t  will  suffice  r a t e are s m a l l , a  to  simulate  heat  transfer. In  this  paper  a  numerical  model i s presented  simulates heat  flow i n a column of the earth's  exponentially  d e c l i n i n g heat production downwards, which i s  e x p e r i e n c i n g time-dependent u p l i f t , the  sub-crustal  temperatures,  geothermal  and non-equality  temporal  flux, between  crust,  which with  variations in  changing erosion  and  surface uplift  86  giving  rise  to an e l e v a t e d  land s u r f a c e . The  model i s then  a p p l i e d to v a r i o u s p a r t s of the Coast Mountains Columbia  to  simulate  (Cretaceous-early followed  by  of  British  p o s t - i n t r u s i v e syn- to l a t e - o r o g e n i c  Tertiary)  post-orogenic  rapid slow  cooling uplift  and  uplift  and most r e c e n t l y ,  l a t e Neogene r a p i d u p l i f t g i v i n g r i s e to the  present  Coast  Mountains. Estimates  of p a l e o - u p l i f t r a t e s have been d e r i v e d  g e o l o g i c a l data and 1).  Under  values  of  geochronometry  conditions  T e c t o n i c and track  evaluating regions,  and  dating  especially  (Chapter  track data  gradients.  can  Other  m a t e r i a l p r o p e r t i e s are a v a i l a b l e unpublished  sources.  geothermal i n f e r e n c e s and  geophysical  large-scale  fission  paleo-geothermal  from v a r i o u s published and  ^fission  track  s p e c i a l circumstances the  a l s o provide boundary  fission  from  thermal  processes with  derived  from  both  modeling are u s e f u l in in  regard  to  tectonically the  active  mechanisms  of  uplift.  Geology of the Coast Mountains The and  Coast Mountains of B r i t i s h  Hutchison  1974,  Monger  et  Columbia al  1972,  (see  Chapter 1) are  l a r g e l y c o i n c i d e n t with  the Coast P l u t o n i c Complex,  of  rocks  granitic  plutonic  of  Jurassic  Roddick  to  a  belt  Eocene  age  superimposed on a v a r i e d framework of s t r a t i g r a p h i c rocks. Though some of the c r y s t a l l i n e to  crustal  shortening,  the  bulk  rocks owe  their  of p l u t o n i c rock  origin i s the  87  r e s u l t of e p i s o d i c roots  of  a  intrusion  west-facing  e a r l y Cenozoic metamorphism  age  plutons  magmatic  (Monger  culminated  Coast Mountains,  of  et  that  Plutonism  and  the Eocene i n the northern  a f t e r which orogenic a c t i v i t y n e a r l y ceased  a l t o g e t h e r . During the middle Cenozoic, the 51°-52°N  the  arc of l a t e Mesozoic to  a_l 1972).  during  formed  probably  Queen  C h a r l o t t e transform f a u l t , whereas the region to  the  south  relatively  slow  offshore  of  the  continuous,  bounded  north  by  had  was  area  subduction  magmatic a c t i v i t y  through the remainder  much  fashion  the  Armstrong all  of  same  1980, Souther  Plateau  age.  (Figure  Neogene r a p i d u p l i f t the  present  today  (Berman  f o r a few  volcanism  coincided  in  temporally  of the Coast Mountains,  mountains  (Mathews  the  Interior  with  the  which  produced  began with the Jaeger  (1964),  study  thermal of  Lovering  models  thermal  Belts to g e o l o g i c problems  effects  not  of  intrusions.  (1935), and others have presented  both theory and examples of t h i s type. T h i s itself  late  paper.  Modeling of Mountain of  late  1968). C h a r a c t e r i s i n g the  Neogene event are the main goals of t h i s  Applications  and  are of  thermal e v o l u t i o n and understanding the causes of t h i s  Thermal  in  localities,  rocks i n the Coast Mountains  Basaltic  1)  of the Cenozoic,  observed  1977). Except  the c r y s t a l l i n e  pre-Oligocene  as  and sporadic  paper  concerns  with g e o l o g i c a l s i t u a t i o n s where igneous i n t r u s i o n i s important,  and  consequently  the  flow  of  heat  is  88  c o n t r o l l e d predominatly by conduction upwards and by u p l i f t  and  erosion.  S i t u a t i o n s where u p l i f t described (1959,  by  Clark  solution  and to  metamorphic  used i n  constant  Jager  (1969)  be  presented  had  a  uplift  and  initial  simple mathematical  conjunction  with  pressure and temperature  isotopic  an  problem  the  form.  dating,  e s t i m a t e s , and present  heat flow values to p l a c e c o n s t r a i n t s on the u p l i f t in  can  s o l u t i o n of Carslaw and Jaeger  assumed uniform  distribution  T h i s model was  assumed  the one-dimensional  where heat p r o d u c t i o n was temperature  is  the a n a l y t i c a l  p.388).  analytical  influenced  history  the Swiss A l p s . L a t e r C l a r k (1979) r e f i n e d t h i s model to  i n c o r p o r a t e an e x p o n e n t i a l d i s t r i b u t i o n of r a d i o a c t i v e sources  in  the  crust,  u p l i f t . Woodhouse and model and a p p l i e d surface  heat  but  Birch  retained (1980)  i t to the apparent  a  heat  constant rate of  presented  a  similiar  l i n e a r r e l a t i o n between  p r o d u c t i o n and s u r f a c e heat flow  (Lachenbruch  1968) . Numerical methods s u f f e r from decreased accuracy of the s o l u t i o n but have the advantage  that they can  accomodate  wide range of space- and time- dependent parameters more  that are  realistic. H a r r i s o n and C l a r k e (1979) used a two-dimensional  that  a  model  i n c o r p o r a t e d the e f f e c t s of both igneous i n t r u s i o n  uplift,  and they  techniques  utilized  a  variety  of  isotopic  and  dating  to t e s t t h e i r model. They d i d , however, r e t a i n a  f i x e d s u b c r u s t a l geothermal  f l u x as w e l l as constant s u r f a c e  89  temperature and uniformly d i s t r i b u t e d heat et  al  (1980)  have  r e c e n t l y developed  topography  and  lateral  Lee  a finite difference  scheme to model, i n two dimensions, the of  production.  additional  inhomogeneities  effects  in  material  propert i e s .  The  Model  Many p r e v i o u s l y p u b l i s h e d models more  incorporate  i n f l e x i b l e parameters ( i e . constant  cause some divergence uplift  rate (Chapter  from  realistic  uplift  one  rate) which  situations.  Because  1), average s u r f a c e temperature  and Mathews 1979), and s u b - c r u s t a l geothermal f l u x 1980) for  are a l l f u n c t i o n s of time, and reasonably the  Coast  one-dimensional allowed  Mountains finite  of  British  (Rouse  (Parrish  w e l l known,  Columbia,  d i f f e r e n c e model was developed  a that  temporal v a r i a t i o n s of these parameters.  The model n u m e r i c a l l y s o l v e s the d i f f e r e n t i a l  6 V 6Z  W 6V c 6Z  2  Z  1 6V = a 6t  surface,  W  diffusivity,  is  uplift  equation,  Ao exp(-(Z-Wt)/D) ic  where V i s temperature, Z i s depth below  the  average  the  t i s time, Ao i s the i n i t i a l  exponential  land  r a t e with respect to sea l e v e l ,  a is  heat p r o d u c t i o n of  surface rocks, K i s the c o n d u c t i v i t y , D i s the s c a l e of  or  height  d i s t r i b u t i o n of heat p r o d u c t i o n , and 6  represents the p a r t i a l d e r i v a t i v e . The model i n c o r p o r a t e s an exponentially  downwards  decreasing  distribution  of  heat  90  production  (Lachenbruch  1970)  given by  A(Z,t) = Ao exp(-(Z-Wt)/D)  and an i n i t i a l  temperature d i s t r i b u t i o n d e s c r i b e d by  V(Z,0) = a + (Q*Z/ic) +  where  a  is  reduced heat material are  the  initial  flow  at  ( D A O / K ) ( 1-exp(-Z/D) ) 2  s u r f a c e temperature and Q*  depth.  p r o p e r t i e s and  Lateral  flow.  In  the  models  heterogeneities  in geothermal and u p l i f t  assumed to have a n e g l i g i b l e presented  i s the  effect  on  parameters  vertical  i n t h i s paper  in  heat  K and a are  assumed constant throughout the f i n i t e d i f f e r e n c e g r i d ; more complex models with s p a t i a l v a r i a t i o n  i n these two  material  p r o p e r t i e s would  r e q u i r e data not p r e s e n t l y a v a i l a b l e . Other  parameters  allowed to vary with time (W,a,Q*) or space  (Ao),  are  a c c o r d i n g to data a v a i l a b l e  f o r each l o c a l i t y  The method of Crank-Nicolson i s used with the form  of  Carnahan is  the  finite  difference  as  et a l (1969, p.440-451). A system of M-1  developed  for  each  s u r f a c e boundary  condition,  = surface  a lower boundary  implicit  outlined  by  equations  time step where M i s the number of  depth g r i d p o i n t s . The depth  a(t)  equation  modeled.  temperature  condition,  Q*(t) = reduced heat flow  grid  has  a  fixed  size  and  91  and  uplift variation, W(t)  The  = uplift  r a t e , with  respect  to sea  level.  system of equations r e s u l t i n g from each time step forms  a t r i d i a g o n a l c o e f f i c i e n t matrix  which i s r e a d i l y solved  by  Gaussian e l i m i n a t i o n . In with was  practice  incremental  Q*  and W are s p e c i f i e d as step  changes whereas a, the surface  chosen as a l i n e a r  finite  difference  f u n c t i o n of time. The  grid  was  functions  temperature,  depth  chosen as 40 km,  of  approximately  the t h i c k n e s s of normal c o n t i n e n t a l c r u s t . With usual of the s c a l e height, D,  t h i s assures  the base of the g r i d i s <2% c o n s i s t e n t with  the very  thought to e x i s t The  that heat production  low  elevated  geologic  past.  average a l t i t u d e and  to n e g l i g i b l e  lagged  This  a  according  behind  has caused an  decrease  in  the  consequence of lagging e r o s i o n  i s simulated  which  temperature  material  passes  uplift  in  increase average  and  the  land  surface  the  in the surface  to the value of the atmospheric of  surface  heat • production  landscape of the Coast Mountains  r a t e . In the model, e l e v a t i o n  the  at  in the lowermost c r u s t or uppermost mantle.  present  temperature  values  that of the s u r f a c e , a c o n d i t i o n  i s evidence that e r o s i o n has recent  the  lapse as  a  by a decrease i n  by a decrease i n the r a t e at  through  d i f f e r e n c e g r i d , which represents  the  top  of  the  finite  the land s u r f a c e . When the  l a n d s u r f a c e r i s e s from l a g g i n g e r o s i o n , the denudation r a t e at  the land s u r f a c e  (with  respect  to  (average a l t i t u d e ) i s the u p l i f t sea  level),  mutiplied  rate, W  by the r a t i o of  92  denudation be v a r i e d onset (in  model  realistic  conditions.  e r o s i o n i s g i v e n by  has  order  situations  to u p l i f t  The  can  late  Neogene  parameter  TNONEQ  f o r Q*  and  reasonably  heat  W cause  approximate  unrealistic  the u p l i f t can  considering  This the  appears  tectonic  that  short  The  correspond transfer  a  term  step second  changes  but in  Q*  to  situations  beneath  t h e base o f  reasonable  environment  lateral  parameters  history.  only  almost  Incremental  geothermal  where c o n v e c t i o n d o m i n a t e s h e a t crust.  incorporating  transfer.  in  the base of the g r i d  of  w i t h the requirement  discontinuities  the  the  the c a p a b i l i t y  transfer<<vertical  functions  at  of d e n u d a t i o n  before present).  The  heat  This ratio  to simulate r e a l i s t i c  of l a g g i n g  Ma  all  to u p l i f t .  approximation,  f o r the Coast  Mountains  region. The against  numerical the  solution  analytical  5% and  3%,  lower  for  part given than  (1-2% by  40  km  heat  finite  f o r a 40  t h e base o f t h e  Woodhouse  50 Ma  of t h e r m a l  km  depth  grid  size  80 Ma  of  t o 40  solution  g r i d ) of  and  and  the  is  accuracy and  evolution,  approximately heat  flow  because heat  finite  x 50 Ma;  difference  excluded  time  and  grid depth  are  a small  production  i s at depths  thus  Birch  greater by  t h e r m a l g r a d i e n t a t t h e base o f  the km  within  temperature  difference  of a s p e c i f i e d  The by  Both  for  of  flow v a l u e s agree  respectively. the  After  tested  the e x p o n e n t i a l d i s t r i b u t i o n  condition grid.  and  been  solution  (1980) f o r u n i f o r m u p l i f t . temperatures  has  varies  the the from  increments  93  of  1%  were  chosen  f o r both models. Fewer mesh steps cause  i n c r e a s i n g d e v i a t i o n s from the significant  improvement was  analytic  solution,  and  no  o b t a i n e d by d e c r e a s i n g the mesh  s i z e . The FORTRAN program 'COASTMTN' i s l i s t e d  in  Appendix  1 .  D i s c u s s i o n of  Surface temperature  Parameters  variation  Paleoc1imatic  and  geochronologic  and Mathews (1979) i n the Quesnel area Columbia  has  throughout  the Cenozoic  decrease  established  with  mean  which  in  such  absent.  In  central  indicate  a  Rouse British  variations  grossly  linear  time. T h i s g e n e r a l p a t t e r n can be documented  as  the Coast Mountains the  paleotemperatures about  by  temperature  where Cenozoic d e p o s i t s e x i s t but can only areas  work  models decrease  be  about  mean 25°C  annual  50 Ma ago to  5-10°C at p r e s e n t . T h i s decrease r e s u l t s i n middle  l a t e Cenozoic paleotemperature  for  where such d e p o s i t s are  presented, from  assumed  and  e s t i m a t e s s i m i l a r to those of  Rouse and Mathews (1979), although the r e t e n t i o n of a l i n e a r decrease  requires  paleotemperatures  early to  be  Cenozoic  and  late  Mesozoic  somewhat higher (up to 32°C) than  would be expected. The o v e r a l l e f f e c t of t h i s higher s u r f a c e temperature,  however, w i l l be very  fluctuations  i n paleotemperature  and Mathews 1979),  small.  Though  Cenozoic  are well documented (Rouse  the o v e r a l l p a t t e r n of a l i n e a r  decrease  94  is  a  good  approximation  and i s l i k e l y a p p l i c a b l e f o r the  e n t i r e region of the present Coast  Mountains.  Heat p r o d u c t i o n , Ao The e x p o n e n t i a l downward decrease chosen  for this  model  i s based  on  p r o d u c t i o n o b s e r v a t i o n s (Lachenbruch and  a  theoretical  d i f f e r e n t i a l erosion heat  production  of  heat  production  both heat  1968, Roy e_t a_l, 1968)  a n a l y s i s of such a r e l a t i o n (Lachenbruch  measurements  flow-heat  1970).  i n areas of  Average  surface  f o r the Coast Mountains are  p r o v i d e d by Lewis  (1976 and unpublished data) and Lewis  Souther  and cover  this  (1978),  study.  production  Prior  most of the areas d i s c u s s e d i n  to Cenozoic  values, at least  erosion,  3  and standard  deviation  of  150 measurements)  0.1 t o 3.4 kW/km  3  and  Souther  British  range and  from 5.5  Mountains great,  Columbia  (Lewis  3  production  with 70% f a l l i n g  1976).  from Lewis  heat p r o d u c t i o n of p l u t o n i c  t o the east i s 2.6 kW/km  where the depth of  heat  data,  Zone and Omineca C r y s t a l l i n e  3  3  50° t o 55°N  3  1.0 t o 8.0 kW/km kW/km  from  i s 0.8±0.5 kW/km and range  The mean  rocks i n the Intermontane of  a t p r e s e n t . The  (Lewis 1976 and unpublished  1978).  heat  p r e s e n t l y a v a i l a b l e heat  p r o d u c t i o n data f o r the Coast Mountains (about  surface  f o r the models, a r e r e q u i r e d t o  have been g r e a t e r (up t o 5.6 kW/km ) than mean  and  Belt  but values  between  2.0  In many areas of the Coast  erosion  i s not  i s g e n e r a l l y l e s s than  The models have been designed so that the f i n a l  particularly 1.5 kW/km . 3  s u r f a c e heat  95  production  i s approximately the average value f o r the Coast  Mountains  (0.8-0.9 kW/km ). T h i s r e q u i r e s i n i t i a l Ao values 3  to be between 1.5 and 5.6 kW/km , depending  on the amount of  3  uplift  modeled.  Scale h e i g h t , D The absence Coast Mountains from  the  would  heat  1968).  t o be  California  p r e c l u d e s e s t i m a t i n g the s c a l e  linear  (Lachenbruch assumed  of s u f f i c i e n t heat flow measurements i n the  flow - heat  Consequently,  10 km,  like  (Roy e_t a l 1968).  greatly  vary  from  height,  production  this  value  D,  relation has  been  that f o r the S i e r r a Nevada of I t i s u n l i k e l y that t h i s  place  value  to place  w i t h i n any one  not been  systematically  geologic province.  C o n d u c t i v i t y , K, and d i f f u s i v i t y , a Values of c o n d u c t i v i t y measured  have  f o r t h i s study. Rather, s i n c e the rock composition  in the Coast Mountains granodiorite  i s rather uniform  (Roddick  and Hutchison  quartz-diorite  1974), and since both  f i s s i o n t r a c k dates (Chapter 1) and modeling have been for  areas  composed  rocks, a s i n g l e  or  done  c h i e f l y of these intermediate g r a n i t i c  average  value  has been  applied  to  a l l  models. A l a r g e number of c o n d u c t i v i t y measurements would be required  t o j u s t i f y a more d e t a i l e d approach. Values of 2.5  kW/km°C ( 5 . 9 8 X 1 0 - cal/sec-cm-°C) and 3  cm /sec) 2  have been chosen  32.0  km /Ma 2  (0.0101  f o r c o n d u c t i v i t y and d i f f u s i v i t y ,  96  r e s p e c t i v e l y . These estimates are i d e n t i c a l to those for  the  Sierra  conductivity  Nevada  model  (2.5 kW/km°C)  measured  rocks  from  Columbia  (2.7±0.3  kW/km°C;  1978,  Harrison  et  of  Lachenbruch  i s c l o s e to  the  diffusivity,  a,  values f o r  the Coast P l u t o n i c Complex of B r i t i s h Lewis a_l  1976, 1979)  Lewis  and  Souther  and  for  quartz  Clark  1966).  equals K/PC where p i s 2.7 g/cm  heat c a p a c i t y , i s 0.22 cal/g°C  Lapse  (1968); the  average  d i o r i t e - g r a n o d i o r i t e i n general (3.1 kW/km°C; The  chosen  (2.9 x 1 0 -  1 3  3  and c,  W Ma/kg°C).  rate An atmospheric lapse rate of -7°C/km  This  value  saturated  falls  between  adiabatic  respectively  rates  (Strahler  the of  dry  has  been  chosen.  (no condensation) and  -lO°C/km  and  -5.5°C/km,  1973) and i s probably a p p r o p r i a t e to  t h i s f a i r l y humid r e g i o n .  E r o s i o n - u p l i f t balance Averaged over most of the Cenozoic, e r o s i o n and have  uplift  been e s s e n t i a l l y balanced. U n t i l the late-Neogene when  vigorous u p l i f t  i n the Coast Mountains  1970),  and a l t i t u d e were g e n e r a l l y low. Only i n the  last  relief  s e v e r a l m i l l i o n years has r a p i d u p l i f t caused a  increase  in  the  average  altitude  denudation. The onset of l a g g i n g accelerated u p l i f t , to  began (Douglas ejt a_l  be  resulting  erosion  as  marked  from lagging a  result  of  termed TNONEQ i n the model, i s estimated  from 5 to 10 Ma years ago. U p l i f t  rates derived  from  97  fission  t r a c k data  present  average  (Chapter altitude  1) when combined with values of  of  d i f f e r e n t areas of the Coast  Mountains, d i c t a t e the r a t i o of e r o s i o n / u p l i f t , a c c o r d i n g to the  relation,  W x TNONEQ x ( 1 - e r o s i o n / u p l i f t ) = average a l t i t u d e .  The  range of t h i s r a t i o  average  altitudes  of  i s 0.53 to 0.86 which  results  0.6 t o 1.8 km f o r the s p e c i f i c  d i s c u s s e d i n t h i s paper. The values of mean  points  within  a  model was designed identical The  15 km  were spaced  r a d i u s of each modeled l o c a l e . The  to have  the "model"  average  t o the a c t u a l value f o r each area  r e s u l t i n g s u r f a c e temperature i s lowered,  the s e c u l a r decrease,  areas  altitude  c a l c u l a t e d by averaging a l t i t u d e s of about 30 e q u a l l y  in  by an  amount  equal  altitude  (see Table V ) . i n a d d i t i o n to  t o the average  a l t i t u d e m u l t i p l i e d by the lapse rate (-7°C/km).  Reduced heat Values heat  flow, Q* of  reduced  heat  flow Q*, corresponding  f l u x e n t e r i n g the base of the f i n i t e  vary  i n models  from  25  values and  subduction  2  zones  grid, fall  flow i n the a r c - t r e n c h (<20 kW/km ) 2  to  high  i n areas of thinned l i t h o s p h e r e and a c t i v e v o l c a n i c  tectonic a c t i v i t y  Range  difference  t o 60 kW/km . These values  between the extremes of very low heat gap above a c t i v e  to the  province  of  (>50 kW/km ) such 2  western  United  as  the Basin and  States  (see B l a c k w e l l  98  1978). Q* f o r the models i s g e n e r a l l y assigned of  about  50  post-orogenic flux  kW/km  2  i n the syn-orogenic  p e r i o d t o simulate  both the higher  Q*  and u p l i f t  rates  were  a good f i t to f i s s i o n t r a c k dates,  uplift  Uplift  of  stored  1), as w e l l as present  heat flow  adjusted  to  r e l i a b l e apparent  r a t e s , and paleo-geothermal g r a d i e n t s  Chapter  ( P a r r i s h 1980,  (Hyndman 1976).  rate, W  Approximate track d a t i n g  values  of u p l i f t  Chapter  conditions  rate derived  f o r v a r i o u s areas of the Coast 1. As d i s c u s s e d  rates  contemporary  surface  apparent u p l i f t  true u p l i f t balance  only  when  despite u p l i f t ;  otherwise,  track data a r e accurate  r a t e since e r o s i o n and u p l i f t  during  i s that the  r a t e s can be produced. Sustained  fission  this  will  uplift several  with respect  low ( l e s s than 0.3 km/Ma) apparent  apatite  fission  Mountains are  a r e met. The most c r u c i a l c o n d i t i o n  isotherms must remain at f i x e d d i s t a n c e  relatively  from  i n Chapter 3, apparent  r a t e s are equal t o a c t u a l u p l i f t  from  than normal  thermal energy caused by deformation, i n t r u s i o n or  both. In g e n e r a l , obtain  value  to immediately  from the mantle as w e l l as the d i s s i p a t i o n  orogenic  from  a high  t o the  misleading (10-20 Ma),  uplift  rates  i n d i c a t o r s of likely  time p e r i o d , whereas higher  be i n  values of  z i r c o n - d e r i v e d apparent r a t e s are not. The z i r c o n data overestimate the true  uplift  rate  and  r e l a x a t i o n of isotherms f o l l o w i n g orogenic Late  Neogene  rapid  uplift  both  reflect  often  downward  activity.  elevated  the  land  99  surface and i n c r e a s e d leading uplift  to  higher  the  downward  movement  apparent u p l i f t  of  uplift  will  lag  however,  with  f o r the be  fission  reasonable  initial  within  a f t e r only a b r i e f apatite  respect  period.  to  track  estimates  of  5 Ma)  data middle  rates  will,  of surface denudation (Chapter  thus  and  the  s i n c e denudation  Apatite  period  can  apparent  underestimate  to sea l e v e l ,  25% of the value  (3  isotherms  r a t e s . Derived  r a t e s i n such a case w i l l probably  rate  of  3), and  be used t o provide  late  Cenozoic  uplift  r a t e s f o r use i n thermal models. When erosion  elevation  of  i s simulated,  the  land  surface  the model a d j u s t s the value of surface  denudation to be l e s s than W, the u p l i f t to  sea  level.  denudation elevated. occurs,  is In  due t o lagging  rate  with  respect  The d i f f e r e n c e i n r a t e between W and s u r f a c e the  rate  addition,  at  which  the  land  surface  when e l e v a t i o n of the land  the s u r f a c e temperature i s decreased  as  is  surface  discussed  under e r o s i o n - u p l i f t balance.  A p p l i c a t i o n t o the Coast Mountains  Objectives The  of modeling o b j e c t i v e of thermal modeling i s a s e l f - c o n s i s t e n t  q u a n t i t a t i v e t h e r m a l - u p l i f t model that f i t s u p l i f t fission  track  and  other  isotopic  dates,  paleogeothermal  g r a d i e n t measurements, and  other  observations.  i s a p p l i e d to f i v e  The  method  geologic  history,  and  geomorphic  areas of the  100  Coast Mountains where f i s s i o n  t r a c k , g e o l o g i c data, and  heat  flow estimates are a v a i l a b l e . These areas are, from north to south, Kemano, northern Waddington,  central  King  Bute  I s l a n d - Ocean  Falls,  Mount  I n l e t , and Mount R a l e i g h (Figure  1).  P r e s e n t a t i o n of the models The parameters of the models are along  with  their  tmax, t, Zmax, and the  maximum  SI  units.  depth,  r a t e (W1,  W2,  and tW1,tW2, and tW3, In  the  interest  positive  and depth  W3,  heat  W4)  of  V,  parameters step  size,  flow  (Q*1, Q*2,  The  Q*3)  and  are changed at times tQ*1,tQ*2  clarity,  ( i n Ma  uplift  before rates  present).  are shown as  numbers.  isotopic  data  relevant  fission  track  f o r each area are presented  15a. The  average  computational  Table  step s i z e , r e s p e c t i v e l y .  respectively  The models and the  Figure  in  Z are the maximum time, time  s e q u e n t i a l values of reduced uplift  The  listed  uplift  altitude  at  vs. time c u r v e . Abrupt  path each  of  rocks  locality  and  i n the format  presently  at  i s shown by the  changes i n u p l i f t  K-Ar  rate  are  of the  depth  clearly  e v i d e n t . When combined i n the model, the v a r i a t i o n s of W and Q*  produce  surface  heat  flow v a r i a t i o n s shown i n the top  part of F i g u r e 15a. The value about  6 km  6 km deep estimate  depth Q  of  (Q)  at  i s shown i n F i g u r e 15a by short bars.  The  values  are  paleo-heat  of  shown flow  paleo-heat  for  derived  flow  comparison from  with  the  f i s s i o n track  T a b l e V. Kemano  Parameter model: I n i t i a l Ao a it D a da/dt tmax dt Zmax dz  A (B.C.D)* 3 0 32 0 2 5 10 0 24 0 -O 28 50 0 0 5 40 0 0 4 50 0 0*1 45 0 t0*1 35 0 0*2 15 0 t0*2 50 0 0*3 W1 1 2 45 0 tW1-2 W2 0 18 35 0 tW2-3 W3 0 08 tW3-4 10 0 W4 0 24 -7 0 lapse rate eroslon/upl1ft 0 58 TNONEQ 10 0 1 0 surf.alt. 1 0 actual s u r f . a l t . » f i n a l Ao' 0 98 1  * * * ' '  P a r a m e t e r s of  N . K I n q I s 1 a n d - O c . F a 1 Is A (B,C,D)+ 2 5 32 0 2 5 10 0 24 0 - 0 28 50 0 0 5 40 0 O 4 35 0 45 0 25 0 20 0 60 0 0 20 30 0 0 14 10 0 0 62 6 0 0 30 -7 0 0 86 10 0 0 6 0 6 0 87  Therma1 Mode 1s  M t . Wadd 1 nrjton 3 32 2 10 25 -0 55 0 40 0 50 45 35 20 50 0 45 0 35 0 5 0 -7 0 5 1 1 0  8 0 5 0 0 28 0 55 0 4 0 0 0 0 0 9 0 2 0 1 1 0 75 0 53 0 8 8 75  Central  Bute 1 32 2 10 31 -0 75 0 40 0 30  --  Inlet  5 0 5 0 0 28 0 75 0 4 0  30 0  --  30 0 0 065  --  0 065  -  0 5 0 -7 0 5 1 1 0  065 0 47 0 53 0 10 08 84  M o d e l B : same p a r a m e t e r s as A e x c e p t 0*1=0*2=0*3=40.0 kW/knv M o d e l C : same p a r a m e t e r s a s A e x c e p t u p l i f t r a t e s W1.W2.W3, and W4 a r e i n c r e a s e d by 20% M o d e l D: same p a r a m e t e r s a s A e x c e p t u p l i f t r a t e s W1.W2.W3. a n d W4 a r e d e c r e a s e d by 20% o u t p u t from the model; a l l other parameters are input m e a s u r e d f r o m a l t i t u d e s of 30 e q u a l l y - s p a c e d p o i n t s i n a 15 km r a d i u s of e a c h l o c a l i t y  Mt.  Raleigh 5 32 2 10 32 -0 80 0 40 0 50 60 35 20 50 1 70 0 50 0 7 0 -7 0 7 1 1 0  6 0 5 0 0 28 0 8 0 4 0 0 0 0 0 0 0 2 0 06 0 6 0 6 0 73 7 86  um  ts  kW/km' km'/Ma kW/km" km °C °C/Ma Ma Ma km km l-W/km' Ma bp kW/km' Ma bp kW/km' km/Ma Ma bp km/Ma Ma bp km/Ma Ma bp km/Ma C/km c  Ma bp km km kW/km'  KEMANO Fi88ion  track estimate ot paleo-heat flow  I  Surface Heat Flow.Q (kW/km ) 2  T  Model A Model B Heat flow at 6 km  8 0  60  Depth (km) Model A Model B • — Sea level curve ^ ^ - ^ 2.0 km curve  300|  Temperature  Zircon Apatite —• • - Sea level dates 2.0 km dates  2001-  (°C)  100 50  a)  40  30  20  10  0  Time Before Present (Ma) Model C Model D  Surface  ' * — '  100  i  Heat flow at 6 km  Heat Flow.Q  (kW/km )eo 2  Depth (km) Model C Model D Sea level curve 2.0 km curve  Temperature <°C)  Symbols 50  40  30  20  10  same as above  0  Time Before Present (Ma) F i g u r e 15a. Thermal e v o l u t i o n diagram f o r t h e Kemano a r e a . Shown a r e s u r f a c e heat f l o w (Q), depth, and temperat u r e v s . time curves which a r e d e r i v e d from models w i t h parameters l i s t e d i n Table V. D i s c o n t i n u i t i e s i n Q v s . time c u r v e r e s u l t from increments i n u p l i f t rate. F i s s i o n t r a c k d a t e s and e s t i m a t e s o f p a l e o heat f l o w a r e from Chapter 1.  1 03  dating  (Parrish  1980,  Chapter  1), which corresponds to about  that same depth. In the temperature  vs. time graph, two main curves  are  shown. The  top curve r e p r e s e n t s the c o o l i n g curve of samples  presently  at  sea  level;  the  curve f o r rocks that are now cooling  curve  for  zircon),  curves to t h e i r consistent fission  km a l t i t u d e .  track  temperature  the  of u p l i f t  data.  Other  blocking  (105°C  for  e_t  the  al  cooling a  r a t e s , reduced heat flow, and isotopic  temperatures  1979).  apatite,  track dates r e p r e s e n t s  dates  can  also  .and  All  fission  track  Chapter  1 and other i s o t o p i c dates are from  Survey  of Canada (Woodsworth 1979,  Baer  their  be used, when  a v a i l a b l e , to p l a c e f u r t h e r c o n s t r a i n t s on the Harrison  Since  f i t of each of these two  respective f i s s i o n  modeling  respective  at 2.0  must pass through the f i s s i o n track dates at  the a p p r o p r i a t e b l o c k i n g 175°C  bottom curve r e p r e s e n t s the  models  (see  data are from the  1973,  Geological  Wanless et a l  1979) . In order to assess the variations  in  on F i g u r e 15a  sensitivity  the parameters,  model  A  except  to  that  Q*  is  at 40 kW/km . Though the c o o l i n g curves of model B 2  the  fission  from Kemano, they do f i t the dates w i t h i n  represent  model  three other models are shown  f a l l c l e a r l y o f f the c e n t r o i d of  In  the  i n a d d i t i o n to the best f i t of model A.  Model B i s i d e n t i c a l to constant  of  the  lower  portion  i d e n t i c a l parameters  of  track  dates  error.  F i g u r e 15a, models C and D  as model A except  that  they  1 04  have u p l i f t model  r a t e s u n i f o r m l y i n c r e a s e d and d e c r e a s e d  A. I t i s a p p a r e n t  that although  the heat  flow data at  6 km d e p t h  i s c o n s i s t e n t w i t h e i t h e r model C o r  of  models  the  can  satisfy  the  fission  P r e s e n t a t i o n o f e a c h o f m o d e l s A, B, C, that u p l i f t and  still  in  Ao  D,  neither  track D  data.  illustrates  r a t e v a r i a t i o n s o f a t most ±15% c a n be t o l e r a t e d  remain c o n s i s t e n t w i t h f i s s i o n  will  have a s m a l l e f f e c t  long as those present  and  20% from  of  Ao.  considerable effect  Changes  Changes  on t h e r e s u l t a n t c u r v e s a s  changes a r e not l a r g e  values  track data.  and in  K  inconsistent  with  and  have  c  will  on t h e m o d e l s , b u t s i n c e t h e v a r i a b i l i t y  of t h i s p a r a m e t e r  i s limited  such  n o t s t r o n g l y a f f e c t c o n c l u s i o n s drawn  the  changes w i l l  i n quartz  diorite-granodiorite, from  models.  Kemano D a t e d r o c k s f r o m Kemano a r e o f f o l i a t e d q u a r t z at  Powell  are  similiar  gneiss  Peak, o f Eocene or p r o b a b l y to  rocks  present  o l d e r a g e . The r o c k s within  the  complex of t h e n o r t h e r n Coast Mountains  Hutchison  1 9 7 4 ) . No K-Ar d a t e s  vicinity  of  complex  farther  spread  other  over  Kemano,  are available  although  northwest  most  dates  central  ( R o d d i c k and  i n t h e immedite in  a r e Eocene. F i s s i o n  t h e 2 km o f a l t i t u d e  diorite  o f P o w e l l Peak  the  gneiss  track  dates  range  from  24  t o 34 Ma f o r a p a t i t e a n d 36 t o 44 Ma f o r z i r c o n  (Chapter  1).  An e s t i m a t e o f p a l e o g e o t h e r m a l  ago  26+4/-6°C ( P a r r i s h  1980).  gradient  35 Ma  is  1 05  Model  A of Figure 15a i s the best thermal model w i t h i n  the c o n s t r a i n t s of the data, although solution.  I t incorporates  it  i s not a  variations  i n Q* from  unique  35 to 50  kW/km and u p l i f t rate v a r i a t i o n s from 0.08 t o 1.2 km/Ma. 2  Rapid u p l i f t and high Q* i n the Eocene f o l l o w i n g Eocene orogenic a c t i v i t y uplift  and Q*  (Hollister  i n the middle  tectonic s t a b i l i t y . U p l i f t l a t e Neogene a t about track  data,  relatively Coast  1979)  Mountains  Cenozoic  d u r i n g a p e r i o d of  r a t e s and Q* are i n c r e a s e d i n the  10 Ma, c o n s i s t e n t  physiographic  high heat  gave way t o low r a t e s of  evolution  flow along  (Hyndman  with  both  fission  (Chapter  1) and  the eastern  1976, Mathews  part  of the  1972b,  T. Lewis  unpublished d a t a ) . On the b a s i s of t h i s model, K-Ar dates  near  Kemano  assuming a b l o c k i n g (Harrison  would be expected temperature  and McDougall  1979).  p r o d u c t i o n of 0.98 kW/km is  virtually  (T.  Lewis unpublished d a t a ) .  Northern The described  by  of  280±40°C  The present s u r f a c e heat  57-87 Ma.  plutonic  rocks  Table  V) 3  this  falls region  (Figure  1) has  been  Baer (1973). The rocks c o n s i s t of plutons and  subordinate metamorphic rocks which y i e l d K-Ar of  for biotite  t o the measured value of 1.0 kW/km  King I s l a n d - Ocean geology  t o be about 45-50 Ma,  given by the model (see  3  identical  of  biotite  Preliminary suggests  Rb~Sr they  (R.L.Armstrong and R.R.Parrish  work  on  biotite  some  of  a r e mid-Cretaceous unpublished  data).  dates these in age  Fission  106  track  dates  spread  over  2 km  24 Ma f o r a p a t i t e and 34 t o  of a l t i t u d e vary from 8 t o  39 Ma  for zircon.  t r a c k - d e r i v e d value of paleogeothermal 17±2°C/km about  (Chapter  43  fission  g r a d i e n t 20 Ma ago i s  1) which r e p r e s e n t s a paleo-heat  kW/km ,  assuming  2  cal/sec-cm-°C).  A  This  K=2.5 kW/km°C  is significantly  lower  flow of  (5.98x10-  3  than the value  d e r i v e d f o r the Kemano area f o r 35 Ma ago. Models A, B, C, and D of F i g u r e format  to those  apparent  fission the  that  that Q* i s a constant 40 kW/km . I t 2  model  B  i s c o n s i s t e n t with n e i t h e r the  t r a c k d e r i v e d value of paleo-heat  fission  track  data.  Thus,  of  flow  i t appears,  suggests, t h a t Q* was s i g n i f i c a n t l y most  in  of F i g u r e 2a. Model B i s i d e n t i c a l t o the  best f i t model A except is  15b a r e s i m i l i a r  nor a l l as  of  model A  l e s s than 40 kW/km f o r 2  the middle Cenozoic. Models C and D of F i g u r e 15b,  have u p l i f t  rates  of  ±20% of  consistent  with f i s s i o n  model  A,  but n e i t h e r i s  t r a c k d a t a . T h e i r heat  however, are very s i m i l a r and f a l l  w i t h i n the e r r o r  of  the f i s s i o n  The  f i n a l present s u r f a c e heat p r o d u c t i o n from  0.87 kW/km , 3  t r a c k - d e r i v e d paleo-heat  similar  t o present  flow curves, limits  flow d e t e r m i n a t i o n . model  A is  values of 0.9-1.0 kW/km  3  T.Lewis, unpublished d a t a ) . Model A, which f i t s a l l of the data very w e l l , i n v o l v e s uplift from  r a t e s that are very s i m i l a r t o those d e r i v e d d i r e c t l y  fission  moderate  track  through  dating  of  apatite.  the middle Cenozoic  These  rates are  (0.14-0.2 km/Ma) but  i n c r e a s e i n the Late Miocene. Geomorphic arguments  (Chapter  NORTHERN KING ISLAND OCEAN FALLS AREA Model A Model B Heat flow at 7 km  Surface Heat Flow, Q (kW/km ) 2  Fission track estimate of paleo-heat flow  6 0  Depth (km) Sea level curve 2.0 km curve  300  Temperature |E=:  Sea level dates 2.0 km dates  200  (°C)  100 40  30  20  10  0  Time Before Present (Ma) Model C Model D Heat flow at 7 km  Surface Heat Flow, Q (kW/km ) 2  5 0  Depth(km)  Sea level curve  Temperature 200  (°C) 100  b)  2.0  r-~^ 1  km curve  Symbols same as above  h 40  30  20  10  0  Time Before Present (Ma)  F i g u r e 15b. Thermal e v o l u t i o n diagram f o r t h e K i n g Ocean F a l l s a r e a .  Island-  108  1)  suggest  that the most r a p i d l a t e Neogene u p l i f t  in this  region was i n the Late Miocene, f o l l o w e d by slower to  Pliocene  Recent r a t e s . T h i s slowing of r a t e has been i n c o r p o r a t e d  in models f o r t h i s region (see Table V ) .  Mount Waddington Mount Waddington, with an highest  area  i n the Coast  g r a n i t o i d g n e i s s , migmatite, (G.  Woodsworth,  Paleocene. Table  IV) range  dates  zircon  (Chapter  data)  55 Ma  at  least  as  f o r hornblende about  localities  range from 6 to 32 Ma f o r  is  these  rock  o l d as  listed  t o 48 Ma f o r  rocks.  Fission  and  dates  19  to  clearly  52 Ma f o r  show a l t i t u d e  c o r r e l a t i o n but z i r c o n dates do not. A r e l i a b l e estimate paleogeothermal present heat 90 kW/km  2  heat  of  g r a d i e n t has been impossible t o o b t a i n , but  flow, uncorrected f o r u p l i f t ,  i s probably 70 to  (shown i n F i g u r e 15c). T h i s value of Q was d e r i v e d  by adding a 24% u p l i f t from  in  spread over 3 km i n a l t i t u d e  apatite,  1). A p a t i t e  the  I t i s composed of  ( c a l c u l a t e d with constants  from  from  4 km,  and more massive p l u t o n i c  b i o t i t e . L i t t l e e l s e i s known track  of  Mountains.  unpublished  K-Ar dates  altitude  flow  correction,  measurements  by  originally Hyndman  subtracted  (1976),  (about 63 kW/km ) f o r the higher  average  Q value  region  of the Coast Mountains, of which Mount Waddington i s  3  heat  to the flow  part. The best f i t thermal model, shown i n F i g u r e been  constructed  by  e a r l y Cenozoic  uplift  15c, has  r a t e s which f i t  109  the K-Ar and z i r c o n present,  apatite  fission data  constraints  on  (0.9 km/Ma)  in early  t r a c k data. From  are accurate  the u p l i f t  rates.  Cenozoic  r a p i d c o o l i n g of the rocks. (0.11 by  km/Ma) i s s i m i l i a r fission  track  data  (post-5 Ma) r a p i d u p l i f t to  f i t apatite  physiography and  50  fission  been  uplift  2  to s a t i s f y  adjusted  to  slow  uplift  r a t e s documented  km/Ma).  Late  km/Ma) i s r e q u i r e d  of the r e g i o n . Q* has been  kW/km  are rapid  Cenozoic not only  t r a c k data but a l s o to e x p l a i n the  imprecise, of present heat has  Cenozoic  (0.09  (0.75  rates  (55-45Ma) r e s u l t i n g i n  Middle  to apparent  to the  enough t o p l a c e good The  time  35 Ma  varied  between  dates and the estimate,  flow.  result  The  albeit  erosion/uplift  i n an  average  35  ratio  a l t i t u d e of  1.8 km. The model suggests that r a p i d the  Paleocene-Eocene  was  orogenic  followed  by  uplift  very  Cenozoic  r a t e s implying low r e l i e f and a l t i t u d e  Neogene  uplift  appears  during  low  middle  then.  t o have been i n i t i a t e d very  Late  recently  producing the present mountains.  C e n t r a l Bute I n l e t - Mount R a l e i g h The  last  two areas modeled are from the Bute I n l e t  area  of the southern Coast Mountains ( F i g u r e 1). I t w i l l be shown that t h e i r r e s p e c t i v e  geochronology,  thermal  t e c t o n i c h i s t o r i e s are q u i t e d i f f e r e n t and have  models, and implications  for the orogenic h i s t o r y of t h i s r e g i o n . K-Ar  b i o t i t e dates from the dominantly  plutonic Central  MT. WADDINGTON 120  50  40  30  20  10  0  Sea Level Curve — •— 2.0 km Curve 4.0 km Curve  Time Before Present (Ma)  Figure 15c. Thermal evolution diagram for the Mount Waddington region. K-Ar dates are from Wanless et a l (1979). The present heat flow i s modified from Hyndman (1976).  111  Bute  I n l e t are 90-100 Ma (Roddick  and Woodsworth 1977), and  f i s s i o n t r a c k dates range from 42 t o 75 Ma f o r  a p a t i t e and  73 to 100 Ma f o r z i r c o n , depending on l o c a t i o n and a l t i t u d e . The  present heat  is  about  low heat  40  flow i n Bute I n l e t , uncorrected f o r u p l i f t ,  t o 75 kW/km  2  flow r e s u l t s from  descending  oceanic  (Hyndman 1976, the heat  plate  during  F i g u r e 15e). T h i s  sink  effect  subduction,  of the  and s i n c e  subduction has probably continued, a p p a r e n t l y u n i n t e r r u p t e d , since the l a t e Cretaceous  (Monger e t a l 1972), t h i s low heat  flow has l i k e l y p e r s i s t e d  for  rates 0.1  derived  from  some  fission  time.  track  km/Ma s i n c e 75 Ma ago except  Apparent  data  uplift  are less  than  f o r the Neogene, when r a t e s  increased t o about 0.4 km/Ma (Chapter 1). In.the model (Figure 15e) level  and 2.5 km f i t the a p a t i t e f i s s i o n  i s a constant those  derived  30 kW/km .  Uplift  2  from  fission  p r e d i c t e d present heat in  the c o o l i n g  rates  curves  for  sea  t r a c k data when Q* are  approximately  t r a c k d a t i n g . In a d d i t i o n , the  flow i s i d e n t i c a l t o values  Bute I n l e t by Hyndman (1976), once the u p l i f t  measured correction  i s added. In t h i s example, r e s u l t a n t s u r f a c e heat p r o d u c t i o n and  s u r f a c e a l t i t u d e are  0.84 and  1.10 km,  respectively,  very c l o s e t o a c t u a l values of about 1.0 kW/km and 1.08 km. 3  The  rocks  of  t h i s area have been u p l i f t e d only about 5 km  s i n c e 75 Ma ago. The Mount R a l e i g h area the  previous  example,  p l u t o n i c rocks of mostly  (Figure 1), about 30 km east  i s underlain Mesozoic  by  of  metamorphic and  age. Plutons  of  Late  MOUNT RALEIGH AREA Surface Heat Flow, Q  1 2 0  Present  heat  based  1 0 0  flow  on data  estimate o f H y n d m a n (1976)  (kW/km )eo 2  Depth (km)  2-4  Estimate  Kb  from Data  from  Woodsworth  of depth Woodsworth  (1979).  (1979)  — —  S e a level  curve  2.8 k m c u r v e (2.7) A l t i t u d e  of  •  Apatite  •  Zircon  •  Biotite K - A r  date  fission fission  track track  •  Muscovite K - A r  •  HornblendeK - A r  (0.2)  d)  CENTRAL BUTE INLET Surface so Heat Flow.• 60  ^  Present from  heat  flow  estimate  H y n d m a n (1976)  (kW/km ) F 2  Depth (km)  40  1 # —O"  e)  S e a level 2.5  km  curve  curve  a n d apatite  a n dapatite  date  date  Time Before Present (Ma)  Figures 15d and 15e. Thermal evolution diagrams for the Mount Raleigh (d) and central Bute Inlet (e) areas.  11 3  Cretaceous-Paleocene  age  intrude  rocks  previously  metamorphosed during the Late Cretaceous, about 70-90 Ma ago (Woodsworth 1979). A 55-60 Ma  plutonic  River  at  Pluton,  was  emplaced  suite,  the  Bishop  pressures of about 2-3 kb  (200-300 MPa) f o l l o w i n g metamorphism t o p r e s s u r e s of 5-6 kb (500-600 MPa) dates  at  least  80 Ma  ago (Woodsworth 1979). K-Ar  ( c a l c u l a t e d with constants  metamorphic  listed  IV)  i n Table  rocks range from 71 Ma (hornblende)  on  t o 66-68 Ma  (muscovite). A K-Ar b i o t i t e date on the Mount G i l b e r t p l u t o n i s 71 Ma. N e i t h e r heat p r o d u c t i o n nor heat available  but values of heat  of 70-90 kW/km  2  from  2.8 km  fission  altitude.  Cenozoic  uplift  rate  i n the region  t r a c k date a v a i l a b l e  Apatite  a l t i t u d e s are 36 Ma and 7.8 Ma, apparent  of  which i n c r e a s e d  data a r e  flow, u n c o r r e c t e d f o r u p l i f t ,  would be expected  1976). The only z i r c o n  flow  dates  t o over  i s 49 Ma  from 2.8 and 0.2 km  respectively,  about  (Hyndman  0.1 km/Ma 0.4 km/Ma  implying i n the  an  middle  i n the  late  Neogene (Chapter 1). As  i n the Mount Waddington model, the 75-50 Ma u p l i f t  r a t e s were chosen t o f i t the K-Ar and f i s s i o n t r a c k dates i n a d d i t i o n t o the metamorphic pressure estimates  (Figure 15d).  Rates of 1 km/Ma or more a r e r e q u i r e d t o f i t t h i s data. lower  rates  i n the middle  c l o s e t o those d e r i v e d from  Cenozoic  (0.06-0.2 km/Ma) a r e  f i s s i o n t r a c k d a t i n g and f i t the  a p a t i t e dates c l o s e l y . The values of Q* a r e c o n s i s t e n t estimates of present heat In  contrast  The  with  flow.  t o C e n t r a l Bute I n l e t , the Mount R a l e i g h  .114  area has experienced the  two  areas  differential  15 km of u p l i f t  are only  uplift  about  must  have  since  occurred  and Paleocene.  plutonism  i n the two areas i s c l e a r l y  to  east  across  in  the  Late  The c u l m i n a t i o n of deformation and  and u p l i f t  the Coast  d i f f e r e n t and i m p l i e s  have progressed  Plutonic  (Roddick  from  Complex.  c o n s i s t e n t with p a t t e r n s of K-Ar dates, g e o l o g i c age of p l u t o n s elsewhere  Because  30 km apart, c o n s i d e r a b l e  Cretaceous  that orogenic a c t i v i t y  75 Ma.  and Hutchison  west  This i s data,  and  1974).  D i s c u s s i o n of Models The  heat  flow  models i n t e g r a t e u p l i f t  thermal h i s t o r y , heat p r o d u c t i o n , parameters directly  such  as  available  incorporated  into  of  about  or suspect (Mount  inferred  data  syn-orogenic  This  value  uplift  f o r rocks i s and  r a t e s can be  rate  Mount  The  data a r e  Raleigh,  t o post-orogenic u p l i f t with  (1 km/Ma)  petrology i s close  c o n s i d e r e d a p p l i c a b l e f o r the Alps (Schaer et a l other a c t i v e mountain  when  (as f o r z i r c o n d a t e s ) . In  Waddington,  1 km/Ma are c o n s i s t e n t  geochronometry.  and  flow  geochronometry,  track-derived  not a v a i l a b l e  initial  rate  observed  models, estimates of u p l i f t  s e v e r a l of the models Kemano),  Cooling  through  d e r i v e d when f i s s i o n either  Q*.  heat  rate,  rates  and K-Ar t o that 1975) and  belts.  K-Ar and f i s s i o n  t r a c k data and the thermal models  i n d i c a t e that s e q u e n t i a l p l u t o n i c emplacement, c o o l i n g , subsequent r a p i d u p l i f t  and  progressed broadly from west t o east  11 5  across Kemano  the Coast P l u t o n i c Complex. The Alaska border t o the area  was  characterized  by  Eocene  orogenic  culmination. The  modeling  difference  of  heat  flow  and u p l i f t  i n middle Cenozoic u p l i f t  (0.1-0.2 km/yr  from  r a t e s between  52° to 55°N) and southern  from 50° t o 52°N) Coast Mountains  a l s o found t o be reasonable estimates As  indicated  northern  (0-0.1 km/yr  (Chapter 1).  The values of a p a t i t e - d e r i v e d apparent  models.  confirms the  i n Chapter  uplift  r a t e s are  f o r use i n thermal 3  the use of  zircon  d a t e - a l t i t u d e r e l a t i o n s can overestimate r a t e s of u p l i f t and c o n s i d e r a b l e c a u t i o n must be used The  reduced  geothermal combined remain  heat  flow  parameters, fission  consistent  in their  Q*,  the most  has been  track-heat with  between 25  and 60 kW/km  Falls-King  Island  modeling track  example  of  utilizing approach.  data  f o r a l l areas.  2  elusive  estimated  flow  fission  interpretation.  Q* In  a To  must l i e  the Ocean  (Figure 15b, Table V) Q* can be  shown t o have i n c r e a s e d by a f a c t o r of 1.5 from 20 Ma ago t o the p r e s e n t . Low s u r f a c e  heat  flow  20 Ma  p e r i o d of r e l a t i v e t e c t o n i c s t a b i l i t y , Miocene  due  to the s u b - c r u s t a l  r e l a t e d t o the Anahim v o l c a n i c b e l t Considerable this  lithospheric  thinning  a  i n c r e a s e d i n the Late  passage (Bevier  of a 'hot spot' et a_l 1979).  may have r e s u l t e d  from  event. Values of 25-60 kW/km  2  of  ago, d u r i n g  f o r Q* f a l l  between the extremes  subduction r e l a t e d a r c - t r e n c h gap low heat flow (about 20  11 6  kW/km ) and e x t e n s i o n - r e l a t e d high  heat  2  like  the present  Basin  and Range  apatite  and z i r c o n  fission  2  track  dates  and t h e i r d e r i v e d apparent  uplift  r a t e s , and estimates of paleogeothermal very  (60 kW/km )  (Roy et a_l 1968). The  c o n s i s t e n c y between heat flow models, for  flow  gradients  i s thus  satisfying.  Causes of U p l i f t Three main u p l i f t  events i n the Coast Mountains  must be  addressed. They are the syn-orogenic to l a t e - o r o g e n i c  uplift  of  middle  Late  Cretaceous-Eocene  age, the subsequent  Cenozoic low u p l i f t  r a t e s and t h e i r s p a t i a l v a r i a t i o n s ,  lastly,  Neogene r a p i d u p l i f t  for  the l a t e  the present  and  largely responsible  mountains.  Orogenic c u l m i n a t i o n and u p l i f t Orogenic  plutonic  activity  i n the Coast  Mountains  occurred from middle Cretaceous t o Eocene time. The r e l e v a n t geological by Monger Woodsworth Hollister represents  data  and t e c t o n i c  et a l (1972), (1979),  Roddick  Woodsworth  (1979). In g e n e r a l an  eroded  i n t e r p r e t a t i o n s are d i s c u s s e d and Hutchison and Tipper  the Coast  (1974),  (1980),  Plutonic  and  Complex  v o l c a n i c arc above a subduction zone  which consumed the F a r a l l o n p l a t e . The c u l m i n a t i o n of events in the Eocene from Alaska south t o the Kemano area large  uplift,  the development  high-grade a x i a l metamorphic  of a  belt,  wide  involved  and u n u s u a l l y  and voluminous  Eocene  1 17  intrusive  rocks,  and  resulted  i n synorogenic  t h i c k e n i n g . P l u t o n i c u n d e r p l a t i n g may a l s o have to  this  episode.  approximately surface  Present c r u s t a l  30 km (Berry  rocks  were  crustal  kb  Forsyth  tectonically  thickened  isostatically  by  during  has  not  been  since  the Eocene Hollister  would  uplift  to  1979),  modeled  have  responded  as  erosion  proceded,  this  Eocene  orogenic  and  c o o l i n g of r o c k s . Though the core of belt  and  must have exceeded 50 km. Such a crust  rapid  1975),  (600-900 MPa,  thickness  contributed  t h i c k n e s s i n the area i s  metamorphosed  pressures of up t o 6-9 Eocene  and  crustal  i n t h i s study, areas near the  p e r i p h e r y of the Eocene metamorphic core zone such as Kemano and  f a r t h e r south near Mount Waddington  uplift likely have  r a t e s of about  been  several 1979),  experienced  times  more  this  rate  comparable  i n the southern  (perhaps  >10 km/Ma,  rates  have  may  presently (Wellman  c o n t r i b u t e d to  t h i c k e n i n g episode.  which  lacked  Eocene deformation southern  felt  Eocene  i n the southern  plutonism  British  Columbia  were  Following  the Eocene,  mainland  was  nearly  quiescent  concentrated Belt  a l l of until  Coast  and l a r g e - s c a l e  and metamorphism. Eocene orogenic  Intermontane Zone and Omineca C r y s t a l l i n e  Columbia  the b e l t  Alps of New Zealand  T h i s Eocene event was l i t t l e Mountains  of  to  1979). P l u t o n i c u n d e r p l a t i n g may a l s o this crustal  Eocene  1 km/Ma; t h e i r t e c t o n i c s i t u a t i o n was  r e l a t e d . U p l i f t r a t e s i n the core  Hollister  in  experienced  (Ewing the  events i n the 1980). British  the l a t e Neogene  118  (Monger et a l 1972).  Middle Cenozoic The a x i a l  uplift region of the Coast Mountains  52°-55°N experienced u p l i f t the  period  from  30  to  were  no  10 Ma  (Chapter  1). In c o n t r a s t ,  i n d i c a t e that r a t e s  i n the  more than 0.1 km/Ma, and perhaps near  There i s evidence  that paleogeothermal  the region near Ocean F a l l s representative  latitude  at r a t e s of 0.1-0.2 km/Ma d u r i n g  f i s s i o n t r a c k data and modeling south  from  of  g r a d i e n t s were low i n  (l7°/km), which  the northern  may  have  been  (52° to 55°N) region as a  whole. Thus, the more r a p i d u p l i f t not r e l a t e d t o the expansion  zero.  i n the north was probably  of c r u s t or mantle. The igneous  a c t i v i t y of the Anahim V o l c a n i c B e l t l a r g e l y postdates middle Cenozoic The uplift (Chapter  p e r i o d of u p l i f t .  coincidence rates 1)  this  of most r a p i d  and maximum  middle  (0.1-0.2 km/Ma) Miocene Cenozoic  total  uplift  with the Eocene orogenic a x i a l zone i s thought  to be s i g n i f i c a n t . A t h i c k e n e d Eocene c r u s t have  i n the northern  area  would  induced r a p i d u p l i f t during orogenesis and d i m i n i s h i n g  uplift  afterwards.  significant  There  crustal  root  would  likely  have  been  a  supporting a mountain system f o r  some time a f t e r c u l m i n a t i o n of t e c t o n i c a c t i v i t y . Since the uplift the  r a t e s i n the region were probably q u i t e steady  Ocean F a l l s - K i n g I s l a n d a r e a ) , i t i s suggested  middle  Cenozoic  uplift  was simply  related  to  (as i n that the  the gradual  1 19  erosion  of  Erosion  the  mountains  bouyed  by such a c r u s t a l  i n t h i s case induces a d d i t i o n a l u p l i f t at  decreasing  rate i n the absence  slowly  of any thermal anomaly. Over  a time of s e v e r a l tens of m i l l i o n s of would  a  years,  such  e s s e n t i a l l y d i s a p p e a r . A mature mountain  a  was  of  10 Ma  basalts  probably the same  throughout  the  topography  the  1 9 7 3 ) , and  this  that  persisted  i n the c e n t r a l and northern  uplift  rates  in  the  southern  Coast  (50° to 52°N) d u r i n g the middle Cenozoic were low  and l i t t l e beneath  of  to  (52° to 55°N).  contrast,  Mountains  kind  middle Cenozoic  Coast Mountains In  (Chapter 1, Baer  relief  is  preserved  on  the  erosion  surface  b a s a l t s of the same Late Miocene age. F i s s i o n  dates p l a c e t i g h t c o n s t r a i n t s on the t o t a l amount of Cenozoic  uplift  and  southern area was had  root  physiography  has been documented i n the B e l l a Coola region p r i o r eruption  root.  little  erosion.  It  seems  very s t a b l e , l a c k e d a  relief  during  likely  crustal  track middle  that the root,  and  most of the Cenozoic, except i n  areas near a c t i v e volcanoes (Chapter 1 ) .  Late Neogene u p l i f t Following  the  prolonged  middle  relative  tectonic  especially  i n the south, e l e v a t e d  Uplift the l a s t suggests  rates 5 Ma total  in  quiescence,  late the  the south exceeded  (Chapter uplift  1),  and  of 2-3  km.  Cenozoic Neogene crust 0.5  up  of  uplift, to  3  km.  km/Ma f o r at l e a s t  geologic The  period  evidence  alone  southern segment  has  120  been above an a c t i v e subduction zone f o r the e n t i r e and probably s i n c e The  100 Ma ago.  northern  segment  of  the Coast  52-55°N had an average a l t i t u d e and l o c a l 1.0  km  and  1.5 km,  10 Ma b a s a l t s models  (Chapter  of about  the north  Neogene u p l i f t somewhat  fault  plate  opposite  setting of  may  uplift  Uplift  about  r e s p e c t i v e l y , p r i o r to the e r u p t i o n of 1). F i s s i o n  track data and heat  occured mainly  flow  i s lower  as  i n the Late Miocene  less  a  rather  than  region, i s  i n the south. Since the northern  (north of 52°N) a r e adjacent t o a  boundary  this  i s the case i n the south. The l a t e  i n the north, o u t s i d e the a x i a l  Coast Mountains  are  of  from  than i n the south, i t i s suggested that  the P l i o c e n e ,  thus  relief  2-3 km. Since the average a l t i t u d e  additional uplift than  Mountains  f o r the a x i a l zone suggest a d d i t i o n a l l a t e Neogene  uplift in  Neogene  whereas the southern Coast  subduction  zone,  i t appears  transform Mountains  that  plate  have played an important r o l e i n the mechanism  i n the two r e g i o n s .  i n the north (52° t o 55°N)  In p a r t , the l a t e Neogene u p l i f t continuation  of  i n the north  is a  the middle Cenozoic u p l i f t p a t t e r n and as  such r e q u i r e s no s p e c i a l e x p l a n a t i o n . An a c c e l e r a t i o n of the r a t e from 0.2 km/Ma to 0.4 km/Ma i n the B e l l a (Chapter  1)  may  be r e l a t e d  region  i n part t o the Anahim v o l c a n i c  b e l t , or hot spot, which passed beneath from  Coola  the Coast  Mountains  14-8 Ma ago (Bevier et a l 1979). T h i s thermal anomaly,  121  which may  have  junction  coincided  between  with  Vancouver  the  Early  Island  Islands (Yorath and Chase 1981), was asthenospheric lithosphere.  u p w e l l i n g that Yorath  thermally-induced,  and  southern  beneath  Queen  Charlotte  l i k e l y a broad zone  of  Chase  (1981)  suggested  that  c r u s t a l - p e r v a s i v e f a u l t s i n i t i a t e d basin Queen  Charlotte  Islands  C h a r l o t t e B a s i n . As the hotspot passed  the c r y s t a l l i n e  thickness  Queen  triple  i n t e r a c t e d with the o v e r l y i n g  development and volcanism in both and  and  Miocene  rocks of the  Coast  Mountains,  of the g r a n i t i c c r u s t probably i n h i b i t e d  f a u l t i n g and the t h i n n i n g of the  lithosphere  the  similiar  was  probably  accomplished e n t i r e l y w i t h i n the l i t h o s p h e r i c mantle  beneath  the c r u s t . If the E a r l y Miocene (20 Ma) 100 km about by  corresponding to an approximate  s u r f a c e heat flow of  40 kW/km , then t h i n n i n g of the l i t h o s p h e r e  the Anahim hot spot would induce about expansion.  coefficient for  40  km  1 km of u p l i f t  by  2  thermal  this  years,  about  calculation  (an average  500°C.  accomplished  uplift Moreover,  rates an  15b).  over a p e r i o d of a few would  be  increase  thermal  million  approximately  those  i n s u b - c r u s t a l heat flow  i s r e q u i r e d by f i s s i o n  present heat flow data, and Figure  6  the  over the 60 km of heated l i t h o s p h e r e was  15 Ma ago  Skinner  i s 30 x 10" /°C  that  r i s e in  the  in  assumes  to  1966), and the average  were  observed.  This  of thermal expansion  forsterite  temperature If  l i t h o s p h e r i c t h i c k n e s s was  track dates, past and models  (Table  V  and  Whether the t r a n s v e r s e path of the Anahim hot  1 22  spot c o u l d induce a l o n g i t u d i n a l NW-trending u p l i f t northern any  Coast  Mountains  in  i s an open q u e s t i o n ; i f there  remanent Eocene thermal s t r u c t u r e the hot spot may  enhanced  i t . At any r a t e there i s c l e a r heat  the was have  flow evidence  that a thermal p e r t u r b a t i o n o c c u r r e d in the B e l l a Coola area of  the northern Coast Mountains  at about  the  same  time  as  somewhat i n c r e a s e d r e g i o n a l u p l i f t , and the two are probably related.  Uplift  i n the south (49° to 52°N)  The in i t s e l f systems  size  of the mountains i n western  suggests analogy to such  as the Himalaya,  and the Southern Alps of New however,  the  the  origin  of  European  the u p l i f t  Most great mountain systems owe  examples  cited,  the  1978,  uplift  s c a l e h o r i z o n t a l motions  mountain  several  reasons,  i n the Coast  Mountains  systems. t h e i r high e l e v a t i o n to LeFort  is a direct  of  great  A l p s , Alaska Range,  Zealand. For  (Walcott  Columbia  world's  cannot be the same as i n these other  c r u s t a l thickening  British  crustal  1975).  In  the  response to l a r g e  plates,  and  present  p l a t e boundaries l i e w i t h i n the mountain b e l t s as deduced by s e i s m i c i t y and d i s t r i b u t i o n of a c t i v e In et  the  Coast Mountains,  a l 1978), absence  1975)  faults.  the lack of s e i s m i c i t y  of t h i c k e n e d c r u s t  and present s t r u c t u r a l  (Berry  and  (Milne Forsyth  i n t e g r i t y s t r o n g l y suggest  the cause of the u p l i f t must l i e w i t h i n the upper mantle that  the  uplift  is  not  the  result  of  ongoing  that and  crustal  1 23  shortening. The and  p a t t e r n of u p l i f t  geophysical  the southern Mathews  f e a t u r e s . The  (50°  1968)  to  is  52°N)  late  Neogene  Washington de  Fuca  plate. is  youngest  volcanic  B e l t on the east (Berman  is  the  h i a t u s i n arc volcanism  and  l a t e Neogene u p l i f t Mountains  less  the  (Chapter  plate  intense  uplift  and  probably  1979). The  i n the Late  Miocene  1980),  and  westward migration  and  mantle opposite different  from  geophysical  the  the  of the  and  farther  the  (Bevier et front  to  a marked change in the  data  i n d i c a t e that the upper  north  plate adjacent wave  boundary  of  8.1  km/sec  to  more  normal  f a r t h e r north  Cumming et a_l 1979). Heat flow data and  northeast  of  Vancouver  velocities the  continental  (Berry and  Forsyth  in J e r v i s and  indicate  is  to the Queen  km/sec in both the southern Coast Mountains and  velocities  Inlets  8 Ma  oldest within  f a u l t . Upper mantle Pn  southern I n t e r i o r in c o n t r a s t  1975,  Pliocene.  about  magmatic  subduct i o n - r e l a t e d  that  C h a r l o t t e transform 7.8  important  pattern.  Thermal  are  southern  event w i t h i n the Pemberton V o l c a n i c  produce the G a r i b a l d i b e l t r e p r e s e n t s volcanic  pattern  began during a pronounced  G a r i b a l d i V o l c a n i c B e l t to the west i s about 2 Ma al  1,  boundary  in  s i d e of the Coast Mountains i s  Armstrong  in  to the main part of the Juan  interesting  that  geological  i s o c c u r r i n g . However, the  uplift  One  km  opposite  and Oregon adjacent  correlation  The  2-3 Coast  directly  o f f s h o r e where subduction of  i s r e l a t e d to v a r i o u s  a  Bute sharp  1 24  transition  from  back-arc values  low f o r e - a r c  (<40 kW/km ) to high arc and 2  (>60 kW/km ) near the present v o l c a n i c  (Hyndman 1976, Jessop and Judge 1971). Heat unfortunately Columbia.  sparse  The heat  velocities  in central flow  data  (7.8 km/sec)  high  temperature,  flow  2  southern  British  and  British  moderately  probably  data are  and northern  (>60 kW/km )  in  i n d i c a t e an upper mantle with fairly  seismic Columbia  low d e n s i t y  corresponding  beneath  a  layer  (MLVL)  i s clearly  can  The  be  considerably  to be  more  (>100 km)  Crustal thickness  from  Mountains  the southern  50-80km, beneath  (Wickens  (Figure  and ray t r a c i n g ,  beneath  Forsyth  Coast  about  I n t e r i o r and Coast Mountains  40 km  and  lies  (wickens  l i t h o s p h e r i c t h i c k n e s s i n the southern I n t e r i o r  inferred  refraction  mantle  t h i n mantle l i d (Wickens 1977). A well-developed  MLVL i s not observed beneath the 1977).  present  and  to hot,  s o l i d p e r i d o t i t e . In the southern I n t e r i o r a d i s t i n c t low v e l o c i t y  front  2  but i t may be  both  the northern  1977).  16), d e r i v e d  from  seismic  i n c r e a s e s eastwards from 25 t o Coast  Mountains  (Berry and  1975, Wickens 1977) and a c r o s s the southern I n t e r i o r  about  33 km  j u s t east of the Coast Mountains to over  40 km near the Rocky Mountain Trench (Cumming et a_l 1979). The  crustal  clearly in  thickness  reflected  Figure  gradient  i n the Coast Mountains i s  i n the g r a d i e n t of Bouguer g r a v i t y ,  16. The c l o s e  correlation  t h i c k n e s s , r e g i o n a l a l t i t u d e , and Bouguer lack  of  appreciable  seismicity  between gravity  i n the Coast  shown crustal  and the Mountains  125  F i g u r e 16. G r a v i t y anomalies and c r u s t a l t h i c k n e s s i n the Coast Mountains a r e a . G r a v i t y anomalies a r e bouguer on l a n d and f r e e a i r at sea, and are from t h e E a r t h P h y s i c s Branch (1980). C r u s t a l t h i c k n e s s i s from Berry and F o r s y t h (1975).  1 26  strongly  suggests  compensated  that  (Stacey  of Berry and F o r s y t h must  decrease  the  topography  is  isostatically  1973). If the c r u s t a l t h i c k n e s s values (1975) are c o r r e c t , the mantle  southwards  density  to e x p l a i n the lower g r a v i t y  higher average a l t i t u d e of the southern (50° to 52°N) Mountains part  which  have  equally  Coast  t h i c k c r u s t as the northern  (52° to 55°N). Changes i n the geometry  deduced with that  and  and motion of oceanic p l a t e s as  from marine g e o p h y s i c a l data can  the  uplift  history.  also  be  compared  Riddihough (1977) has suggested  important changes occurred  about  4-5 Ma  ago  in  the  i n t e r a c t i o n between the American, Juan de Fuca, and E x p l o r e r plates.  Prior  to  about 5 Ma,  subducted beneath the f u l l Mountains  of  British  the E x p l o r e r p l a t e was being  extent  Columbia  of  more  today  active  off  and  central  Pacific-Explorer-America  moved  the Nootka  Island for  probably  Miocene.  of  the  Island.  at  In  least  convergence,  orthogonal  margin,  constant  the  continental  zone,  The  10 Ma  addition  geographic changes, the r a t e s of to  fault  j u n c t i o n has remained near  the north end of Vancouver most  Coast  northwards to i t s p o s i t i o n  Vancouver triple  southern  ( F i g u r e 17a). Subsequently,  the E x p l o r e r - J u a n de Fuca boundary, became  the  as  to  and these  measured  have not remained  ( F i g u r e 17b). Riddihough (1977) notes a decrease i n  Explorer-America convergence from 4 cm/yr to 1.5 cm/yr  from  3 to 5 Ma ago while Juan de Fuca-America  from  4 to 3 cm/yr.  rates declined  127  Figures 17a, 17b, and 17c. Reconstruction of past plate movements (a), orthogonal convergence rates (b), and the location of the northern edge of the subducted slab ( c ) , modified from Riddihough (1977).  1 28  R e c o n s t r u c t i o n of t h i s geometry i n d i c a t e s that p r i o r to 5 Ma,  rapid  subduction  of a s i n g l e Explorer  southern B r i t i s h Columbia was subducted  plate  slowed at 4 Ma  migrated northwards, plate  was  the  Juan  de  and  slow  progressively  (3-4cm/yr)  occurring.  moving  rapidly  moving  Explorer  p l a t e , and  Fuca  Explorer  of the p l a t e s i n c e that  the  17c  Thus,  under  shows the  e x p r e s s i o n of these  Miocene.  changes.  most  probably r e l a t e d to the  The  the  the  Juan  seems  What p l a t e responsible  tectonic  volcanism recent  Garibaldi  locus of and  may  period  Volcanic  be  thermal  lithosphere, base  by  in mantle d e n s i t y  either  beneath  the  of  Belt, is the  suggested  processes  could  be  resides  in the  mantle,  must be suspected.  caused  by  This  heating  of  the  i n t e r n a l l y by magma t r a n s f e r or  at  its  asthenospheric  induced by h e a t i n g . mainly  expansion  an  documented by t h i s study?  Since the cause of u p l i f t most l i k e l y  could  be  volcanism.  mantle  f o r the abrupt u p l i f t  a gross r e d u c t i o n  de  reasonable  renewed more r a p i d subduction of  the westward jump in the  a  then a slowed  Juan de Fuca p l a t e , probably at a steeper angle as by  Coast first  moving  It  in  in  zone  t r a c e of the northern edge  discontinuity  localized  this  Explorer  subducted s l a b was  p l a t e p r i o r to 5 Ma,  the Late  observed  volcanism,  (<2cm/yr)  f i n a l l y a more r a p i d l y  p l a t e . Figure  motion of  by a more r a p i d l y moving  plate.  Mountains north of Vancouver, the  beneath  as the Nootka f a u l t  replaced  Fuca  The  plate  u p w e l l i n g , or due  to phase changes  This change i n mantle d e n s i t y Coast  Mountains  as  must occur  opposed  to  the  1 29  Interior  in  order  to  e x p l a i n the documented  differential  uplift. The  f o l l o w i n g sequence of events  explanation.  Since  concentrated  in  Mountains  region  Pemberton  the  volcanic  southeastern  (Figure  is  1),  suggested  as  an  b e l t volcanism  was  corner  and  of  the  temporally o v e r l a p p i n g  b a s a l t i c volcanism occurred i n the I n t e r i o r P l a t e a u et  a l 1979), the Late Miocene subducted  fairly and  region  flow  less  (Bevier  have had a  upper  mantle  west of the Late Miocene Pemberton arc  would have been in the low heat heat  s l a b may  shallow d i p , perhaps l e s s than 30°. The  crustal  Coast  than  flow  40 kW/km .  regime  In  2  with  t h i s western  surface region,  l i t h o s p h e r i c t h i c k n e s s ( i n c l u d i n g the America p l a t e and subducted  slab)  would  have l i k e l y been 100-150 km  the  (Figure  18) . During Late  the r e o r g a n i z a t i o n of p l a t e  Miocene-Pliocene,  as  motion  during  the Juan de Fuca p l a t e r e p l a c e d  the E x p l o r e r p l a t e , the subduction zone steepened a  westward  shift  in  the  locus  of  s h a l l o w - d i p p i n g , Late Miocene, subducted off  and  remained  suggested  in  place  by Thompson and  P l a t e a u r e g i o n . Subduction as  shown  in  the  beneath  Zoback  volcanism.  The  s l a b probably  broke  the  (1979)  for  then proceeded  to produce  arc i n a manner the  Colorado  at a steeper angle  F i g u r e 18. The h i a t u s i n v o l c a n i c a c t i v i t y  the Late Miocene and P l i o c e n e probably slowed  Explorer  the new  zone of steeper  resulted  from  in  both  convergence r a t e s as w e l l as i n i t i a t i o n of subduction.  1 30  The  material  Volcanic flow  Belts  between  the Pemberton  that was e x p e r i e n c i n g  ( F i g u r e 18) would have  heating  resulting  asthenospheric  and  Late Miocene low heat  experienced  relatively  from steepened subduction  upwelling  Garibaldi  sudden  and a s s o c i a t e d  and magmatism. The warming of  l i t h o s p h e r i c m a t e r i a l above the s l a b would r e s u l t  this  i n thermal  expansion and u p l i f t . A (<40  >50% increase  kW/km ) t o arc  high  2  average  i n s u r f a c e heat flow (>60 kW/km )  corresponds  2  coefficient  thickness of  of  about  100-150km.  6  I f the  thermal expansion f o r p e r i d o t i t e i s assumed  to be 30 x 1 0 - / C , then t h i s temperature  increase  o  results  a d e n s i t y decrease of 1.5%. Mass balance i m p l i e s  of u p l i f t  by t h i s process  Additional uplift of  t o an  temperature i n c r e a s e of about 500° f o r the f o r e - a r c  lithospheric  in  from f o r e - a r c low  eclogite  alone.  may have o c c u r r e d  crust  Assuming 5 km stagnant  slab  or more  of  increase  x  5 km  uplift  asthenospheric subduction-related  have  likely  eclogite  when  additional uplift decrease  by  the  conversion  to gabbro i n the heated mantle region  18). E c l o g i t i c m a t e r i a l c o u l d lowermost  1.8 km  composed  part  of the  the c u t - o f f shallow  converted  the heat  (Figure  t o gabbro  in  even  Any  more.  material melting  that heat can be t r a n s p o r t e d  this  f l u x suddenly i n c r e a s e d , an  of 0.5 km would have r e s u l t e d (10% thickness).  slab.  partial  Convective  above  the  melting  density would  upwelling  of  region  of  i s e s s e n t i a l t o these models so rapidly.  The u p l i f t  of the  Coast Mountains  F i g u r e 18. Schematic s t r u c t u r a l s e c t i o n a c r o s s t h e s o u t h e r n Coast Mountains. P o s t u l a t e d l i t h o s p h e r e - a s t h e n o s p h e r e boundaries a r e shown w i t h the approximate p o s i t i o n o f t h e subducted p l a t e i n Late Miocene and Recent time; c o n t i n e n t a l and o c e a n i c c r u s t , and the predominant v o l c a n i c b e l t s a r e a l s o o u t l i n e d . The area of Miocene low heat flow that was heated as a consequence o f s t e e p e r subduction i s shown by h o r i z o n t a l l i n e s . The a d j a c e n t area to the west where l i t h o s p h e r i c t h i c k e n i n g r e s u l t e d from steeper subduction i s shown by v e r t i c a l l i n e s .  1 32  southern  Coast  Mountains  is  thus r e l a t e d to h e a t i n g of a  formerly c o o l f o r e - a r c upper mantle of  magma  or  subduction The shown  zone.  in  Figure  18,  c o o l i n g and t h i c k e n i n g subduction  in  would,  contrast,  by  subsidence.  0.4  the  The  increase  material,  upwelling  hot asthenospheric m a t e r i a l above a steepened  region j u s t west  density  region by the  might  of  this  as  direct  a  This  of  steepened  c o o l i n g and  thickening  contraction  thickening, resulting  1-1.5%  of  to  result  thermal  lithospheric  20  region,  have experienced l i t h o s p h e r i c  induce  be  mantle  would  Pliocene.  of  heated  former  30 km,  and from a  asthenospheric  and c o u l d induce 0.2  to  km of s u r f a c e subsidence, approximately that observed i n  the S t r a i t of G e o r g i a . The heat  southern I n t e r i o r was  flow  the l o c u s  of  times; l a t e Neogene u p l i f t  1.0  southern  Holland high  in  the  1964)  heat  cannot  flow.  Interior  be e x p l a i n e d  The  the  eclogite  warming,  induced expansion of the f l a t t e r , s l a b that may  by  et a l the  to  c o u l d be the  to 1970,  region's  g e n e r a l l y e l e v a t e d southern 1.0-1.5 km)  gradual  (about 0.5  (Douglas  simply  (average a l t i t u d e about  Interior  result  of  gabbro c o n v e r s i o n , and  c u t - o f f , Miocene  subducted  have been present beneath much of the southern  I n t e r i o r . T h i s e x p l a n a t i o n i s s i m i l a r to and  back-arc  and asthenospheric upwelling i n both Miocene and  P l i o c e n e to Recent km)  high  Zoback (1979) f o r the u p l i f t  that  of  Thompson  of the Colorado P l a t e a u .  1 33  Summary Heat  flow  models  time-dependent u p l i f t is  subjected  changing  to  have  been  presented that simulate  i n a column of the e a r t h ' s c r u s t which  variable  sub-crustal  s u r f a c e temperature,  geothermal  flux,  e r o s i o n l a g g i n g behind  uplift,  and an e x p o n e n t i a l l y downward d e c l i n i n g d i s t r i b u t i o n of heat production.  An  d e r i v e d from  fission  has  been  uplift  history  of  the  Coast  t r a c k and g e o l o g i c a l data  (Chapter  used with estimates of present and past heat  (Hyndman 1976, fission  Chapter  track  1) to d e r i v e models  and  other  isotopic  data,  self-consistent  models  reinforce  h i s t o r y drawn from Chapter rapid u p l i f t  Mountains the  (53°  periphery  metamorphic  to of  core  orogenic a c t i v i t y crust  which,  in  zone  the  central  Eocene with  the  and  on  uplift  northern  high-grade rates  in  a  middle  thinned by denudation-induced of  These  1 .  resulted  during  flow  data.  56°N) during the Eocene was the  with  heat  conclusions  1) flow  consistent  measurements, and g e o l o g i c a l and p h y s i o g r a p h i c  The  Mountains,  up  Coast  felt  around  plutonic  to  1 km/Ma.  substantially Cenozoic,  and This  thickened  was g r a d u a l l y  u p l i f t . A Late Miocene episode  somewhat a c c e l e r a t e d u p l i f t  was  probably  related  to  the  passage of the Anahim hot spot beneath the a r e a . The  southern Coast Mountains (50° to 52°N) experienced  orogenic p l u t o n i c and metamorphic a c t i v i t y mainly d u r i n g the Cretaceous the  and was  tectonically  Cenozoic, except  stable  throughout  most  of  f o r s c a t t e r e d volcanism. Late Neogene,  1 34  probably P l i o c e n e to resulted  Recent  uplift  i n 2 to 3 km of u p l i f t  in  the  south,  which  of a broad a r e a , was  caused  by P l i o c e n e steepening and r e o r g a n i z a t i o n of subducted  plate  geometry and westward m i g r a t i o n of the magmatic f r o n t .  This  m i g r a t i o n caused abrupt h e a t i n g of the formerly c o o l Miocene f o r e - a r c mantle and consequent uplift.  Steepened  subduction  t h i c k e n i n g west of Volcanic  Belt  thermal expansion,  the  and  also  magmatic  subsidence  front of  the  Warming of a proposed  c u t - o f f Miocene  under  Interior  the  southern  moderate u p l i f t expansion. Neogene were  The  by  thermal  vertical  all  of  and  changes i n the p l a t e t e c t o n i c  to of  lithospheric the  Strait  Garibaldi of Georgia.  shallow-dipping  B r i t i s h Columbia possibly  movements  thermally  led  l e a d i n g to  induced regime.  phase  slab  induced transiton  documented i n the Late and  were  caused  by  135  Acknowledgements During' supported British  the  course  of  this  study,  the  author  by a P r e - d o c t o r a l f e l l o w s h i p at the U n i v e r s i t y of Columbia.  Financial  support  was  provided  by  N a t u r a l Sciences and Engineering Research C o u n c i l of grant  awarded  Society  of  constant course  to  America  source  Grant-in-Aid.  of  support,  the  through  Perkins,  and  W.H. Mathews,  Hyndman,  and  G.K.C.  processes.  cheerful  field  R.L. Armstrong  Canada  advice  Clarke  aspects G.K.C.  Clarke  G.T.  Nixon,  T.  provided  Gilmore  assistance,  were  of  a  and  greatly and E.H.  Lewis,  R.D.  ideas which improved  and my understanding L.  was  a d v i c e , and ideas during the  computing  improved  both the manuscript  a  R.L. Armstrong, and through a G e o l o g i c a l  of t h i s study. The  thermal  was  of g e o p h y s i c a l  and  a s s i s t e d with d r a f t i n g and K.  Parrish  aided  with  manuscript  p r e p a r a t i o n and provided encouragement, f o r which  the author  i s grateful.  1 36  References Baer, A . J . 1973. B e l l a Coola-Laredo Sound map-areas, B r i t i s h Columbia. 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American J o u r n a l of S c i e n c e , 272, pp.577-602. Monger, J.W.H., and P r i c e , R.A. 1981. E v o l u t i o n of Canadian C o r d i l l e r a . G e o l o g i c a l A s s o c i a t i o n of Canada, A b s t r a c t s , 6, p.A-47. P a r r i s h , R.R. 1980. F i s s i o n t r a c k e s t i m a t i o n of paleo-heat flow, Coast Mountains of B r i t i s h Columbia, Canada. EOS, 61., p. 1 1 31 . Riddihough, R.P. 1977. A model f o r the recent plate interactions o f f Canada's west c o a s t . Canadian J o u r n a l of E a r t h S c i e n c e s , _1_4, pp. 384-396. Roddick, J.A., and Hutchison, W.W. 1974. S e t t i n g Coast Plutonic Complex, British Columbia. Geology, 8, pp.91-108. Roddick, J.A.,  and Woodsworth, G.J.  1977. Bute  Inlet  of the Pacific (92K)  1 39  map-area. 480.  Geological  Survey  of  Canada, Open f i l e  map  Rouse, G.E., and Mathews, W.H. 1979. Tertiary geology and palynology of the Quesnel area, B r i t i s h Columbia. B u l l e t i n of Canadian Petroleum Geology, 27, pp.418-445. Roy,  R.F., B l a c k w e l l , D.D., and Birch, F. 1968. Heat generation of p l u t o n i c rocks and c o n t i n e n t a l heat-flow provinces. E a r t h and P l a n e t a r y Science L e t t e r s , 5, pp.1 - 12.  Schaer, J.P., Reimer, G.M., and Wagner, G.A. 1975. A c t u a l and a n c i e n t u p l i f t r a t e i n the Gotthard region, Swiss A l p s : A comparison between p r e c i s e l e v e l i n g and f i s s i o n t r a c k a p a t i t e age. T e c t o n o p h y s i c s , 29^, pp. 293-300. Skinner, B.J. 1966. Thermal expansion. I_n Handbook of physical c o n s t a n t s . C l a r k , S.P., ed. G e o l o g i c a l S o c i e t y of America Memoir 97, pp.75-96. Souther, J.G. 1977. Volcanism and t e c t o n i c environments in the Canadian C o r d i l l e r a - A second look. I_n V o l c a n i c regimes i n Canada. Baragar, W.R.A., and others, eds., Geological Association of Canada S p e c i a l Paper 16, pp. 3-24. Stacey, R.A. 1973. G r a v i t y anomalies, c r u s t a l s t r u c t u r e , and p l a t e t e c t o n i c s i n the Canadian Cordillera. Canadian J o u r n a l of E a r t h S c i e n c e s , j_0, pp.615-628. S t r a h l e r , A.N. 1973. I n t r o d u c t i o n to p h y s i c a l geography, 3rd ed. John Wiley and Sons, Inc. New York. Thompson, G.A., and Zoback, M.L. 1979 Regional geophysics of the Colorado P l a t e a u . Tectonophysics, 6J_, pp. 149-181. Walcott, R.I. 1978. Present tectonics and Late Cenozoic e v o l u t i o n of New Zealand. Geophysical J o u r n a l of the Royal Astronomical S o c i e t y , 5_2, pp.137-164. Wanless, R.K., Stevens, R.D., Lachance, G.R., and D e l a b i o , R.N. 1979. Age d e t e r m i n a t i o n s and geologic studies. G e o l o g i c a l Survey of Canada, Paper 79-2. Wellman, H.W. 1979. An u p l i f t map f o r the South I s l a n d of New Zealand, and a model for u p l i f t of the Southern Alps. Royal S o c i e t y of New Zealand, Bulletin 18, pp.13-20. Wickens, A.J. 1977. The upper mantle Columbia. Canadian J o u r n a l of pp.1100-1115.  of southern British Earth Sciences, 14,  1 40  Woodhouse, J.H., and B i r c h , F. 1980. Comment on 'Erosion, uplift, exponential heat source d i s t r i b u t i o n , and t r a n s i e n t heat f l u x ' by T.-C.Lee. J o u r n a l of Geophysical Research, 85, pp.2691-2693. Woodsworth, G.J. 1979. Metamorphism, deformation, and plutonism in the Mount R a l e i g h pendant, Coast Mountains, British Columbia. G e o l o g i c a l Survey of Canada, B u l l e t i n 295, 58p. Woodsworth, G.J., and Tipper, H.W. 1980. Stratigraphic framework of the Coast P l u t o n i c Complex, western B r i t i s h Columbia. G e o l o g i c a l A s s o c i a t i o n of Canada C o r d i l l e r a n S e c t i o n , Programme and A b s t r a c t s , pp.32-34. Yorath, C.J., and Chase, R.L. 1981. Tectonic history of allochthonous terranes: Northern Canadian Pacific c o n t i n e n t a l margin. G e o l o g i c a l Association of Canada C o r d i l l e r a n S e c t i o n , Programme and A b s t r a c t s , pp. 42-43.  CHAPTER 3.  REFINEMENT OF APPARENT UPLIFT RATES  DETERMINED BY  FISSION TRACK DATING  Abstract Fission  track  dating  of a p a t i t e and z i r c o n i s widely  used to i n f e r c o o l i n g and u p l i f t minerals  from  i n date with a l t i t u d e  an apparent u p l i f t  true u p l i f t remain  These  r a t e only  fixed  heat flow  with  conditions  uplift  respect  confidence  equals  translates  uplift,  equal  isotherms  to the s u r f a c e , and when l a t e r a l  compared  to  vertical  heat  flow.  are r a r e l y met; a heat flow model i s used  corrections rates  which  r a t e . T h i s apparent rate w i l l  i f erosion  i s negligible  to d e r i v e  Dating  rocks c o l l e c t e d at v a r i o u s a l t i t u d e s u s u a l l y  r e v e a l s an increase into  h i s t o r i e s of rocks.  so  to  that  fission  they  can  track-derived be  used  apparent  with  greater  i n teconic i n t e r p r e t a t i o n s .  A p a t i t e - d e r i v e d apparent r a t e s w i l l c l o s e l y approximate true u p l i f t  r a t e s when  regional  geothermal  gradients  are  r e l a t i v e l y high and when the dates record u p l i f t  following 3  to  of  5 Ma  after  Apparent u p l i f t with c a u t i o n from  prolonged  initiation  or  cessation  uplift.  r a t e s d e r i v e d from z i r c o n dates must be used  since excessive  gradual  initiation.  the  isotherm  or d e f i c i e n t  accompanied  elements concentrated  can  result  r e l a x a t i o n or slow response to u p l i f t  S l i g h t l y excessive uplift  rates  r a t e s can  also  result  by e r o s i o n of heat  from  producing  near the s u r f a c e of the c r u s t .  1 43 Introduct ion F i s s i o n track numerous  dating  workers  b e l t s . The  to  histories  geologic  evidence has  In  the  (1972), Zeitler track  been u s e f u l  where  little  et  or  with  al  (1977),  represented  rocks.  slope  on  the  of  Po  r a t e s , but  Bergell  Plain  and  and or  rate  Reimer  a l (1975), in  rate graph  (Chapter 1).  and  fission  i n f e r r e d that  Other  workers  Sharma et a_l (1980)  paleo-geothermal track  dating  various  altitudes  Canada, and  only  of  the the  will  be  In  an  the  position  including have  Coast  in c o n j u n c t i o n history.  of  Gleadow  determined  the  the and the  minerals.(zircon,  Chapter and  of  inferred u p l i f t  r a t e s by making an  apatite  documents a Cenozoic u p l i f t  sediments  different  gradient.  of  from  in  pre-erosion  inferred u p l i f t  fission  Columbia,  and  uplift  documented not  r a t e s of rocks by d a t i n g  the  uplift  increase  the  intrusives  and  clasts.  Brooks (1979) and  apatite)  i n mountains  rocks from d i f f e r e n t a l t i t u d e s  a l s o the o r i g i n a l  transported  cooling  by  a p p l i c a t i o n to the B e r g e l l M a s s i f , Wagner e_t aJL  clasts  adjacent  et  altitude-date  (1979) dated jjn s i t u p l u t o n i c  of  Wagner  Schaer  r e f e r r e d to as the apparent u p l i f t interesting  used  stratigraphic  i n c r e a s i n g a l t i t u d e and  slope  This  been  infer  no  system,  altitude-date  and  to  ( i n p r e p a r a t i o n ) documented an date  has  been a v a i l a b l e .  Alpine-Himalaya  Wagner  apatite  i n f e r p a l e o - u p l i f t rates  technique has  cooling  of  1  assumption summarizes  z i r c o n c o l l e c t e d at  Mountains  of  British  with heat flow models,  144  Fission widespread  track  dating  application  relatively  recent  of  and  uplift  zircon  importance  apatite  to  the  has  study  and r e l a t e d t e c t o n i c processes,  the assumption of equating  apparent u p l i f t  r a t e s i s s i m p l i s t i c and v a l i d only when are  and  s a t i s f i e d . These are that  r a t e s with  several  but  actual  conditions  1) e r o s i o n must equal  uplift,  2) isotherms must have remained h o r i z o n t a l , u n i n f l u e n c e d topography  or h e t e r o g e n e i t i e s  s u r f a c e r e g a r d l e s s of the r a t e will  rarely  by  i n c o n d u c t i v i t y or geothermal  f l u x , and 3) isotherms must remain f i x e d with  assumptions  of  of  uplift.  respect  to the  Clearly,  these  be met. I t i s important to assess  the e f f e c t of v i o l a t i o n s of these assumptions on the d e r i v e d apparent u p l i f t  r a t e s i n order  to c o n f i d e n t l y  use  them  in  tectonic interpretation.  Ways to Generate Apparent U p l i f t If  a rock mass i s suddenly u p l i f t e d and  denuded  such  altitude, respect  that  then  the  in  surface  isotherms  to the s u r f a c e .  increase  surface  will  This heat  that  downward will  the through  initially  that  be  at  carried  effect  a  constant  upwards  produces  an  with  initial uplift  track s t u d i e s i s i n essence the  track  retention  isotherm  the rock column, the apparent u p l i f t  be l e s s than the true u p l i f t  Eventually, condition  critical  remains  simultaneously  flow. Since the apparent  r a t e determined from f i s s i o n rate  Rates  with uplift  continued equal  uplift,  erosion,  moves rate  rate. and  with  the  isotherms can e i t h e r  1 45  stabilize,  continue slowly r i s i n g ,  on the v e r t i c a l  or slowly f a l l ,  depending  d i s t r i b u t i o n of heat producing elements  Woodhouse and  Birch  uplift  will  rates  1980).  In  equal,  exceed the t r u e u p l i f t  this  be  situation,  slightly  r a t e . E i t h e r way,  apparent  below, or  slightly  they w i l l  c l o s e to the a c t u a l r a t e , once the i n i t i a l  (see  be  transient  very  l a g has  decayed away. Upon  the  cessation  of  uplift  or a f t e r the i n f l u x of  heat has ceased i n an area s u b j e c t to e i t h e r r a p i d u p l i f t higher  than  sometimes  normal  rapidly.  heat  flow,  This  isotherms  thermal  relaxation  uplift  occurred and w i l l  l e a d to m i s l e a d i n g i n f e r e n c e s of  possibilities way  one  must  even  distinguish  when i n t e r p r e t i n g  to e v a l u a t e  this  is  when  fission  through  relax,  will  p o s i t i v e apparent  Clearly  rates,  will  no  or  produce  uplift  has  uplift.  between  these  track data. The best  quantitative  heat  flow  models.  A Heat Flow Model and A  one-dimensional  developed to erosion  study  rates,  fluctuating exponential  f i n i t e d i f f e r e n c e thermal model  the  effects  changing  of  sub-crustal  s u r f a c e temperature downwards  i t s Application  variable  geothermal  in areas of  flux  and and  on a column of c r u s t with an  decreasing  distribution  p r o d u c t i o n . T h i s model, d e s c r i b e d i n Chapter realistic  uplift  was  of  heat  2, a l l o w s f o r a  approximation of heat flow and isotherm m i g r a t i o n active  uplift  where  lateral  heat  flow  and  1 46  igneous  i n t r u s i o n are not important.  To  test  isotherms clocks,  the e f f e c t  of  uplift  and the consequent eight  r a t e s have been temperature  on  setting  the movement of  of  fission  track  models r e p r e s e n t i n g a wide v a r i e t y of u p l i f t run. The models  incorporate  an  initial  distribution,  V(Z) = ( Q * Z / K ) + ( D A O / K ) ( 1 - e x p ( - Z / D ) ) + a 2  where  V ( Z ) i s the temperature  Q* i s the reduced heat scale  height,  Ao  D is  i s the s u r f a c e heat p r o d u c t i o n , and a i s  K=2.5 kW/km°C (5.98x102  Z below the s u r f a c e ,  flow, K i s the c o n d u c t i v i t y ,  the mean s u r f a c e temperature.  Q*=25 kW/km ,  a t depth  3  In  the models  cal/sec-cm-°C),  and u p l i f t  i s assumed  that  follow,  D=10 km, a=0°C, and t o be  matched  by  erosion. Four of the models have Ao=1.0 kW/km  3  of  and u p l i f t  rates  1.0, 0.6, 0.3, and 0.1 km/Ma, and are shown as the d o t t e d  curves A, B, C, and D of F i g u r e 19a, r e s p e c t i v e l y . U p l i f t i s allowed  to continue  f o r 20 Ma, a f t e r which i t ceases and  isotherms a r e allowed t o r e l a x . A l s o shown i n F i g u r e 19a are the curves of "steady is if  the value of heat uplift  fully  s t a t e " heat  was stopped at any time and isotherms  relax.  heat  case.  This  flow, Q, that would e v e n t u a l l y r e s u l t  Increasing  rates  "steady s t a t e " e q u i l i b r i u m heat the  flow f o r each  of  uplift  flow values  allowed  to  result  i n lower  since  more of  producing elements a r e removed by e r o s i o n . F i g u r e  147  es  A = 1 . 0 (kW/km ) 3  Q  eo  Surface Heat Flow, Q (kW/km )  Q = 2 5 (kW/km ) 2  R  65  Observed Heat Flow  60  "Steady State" Heat Flow 45  2  40 35 30 25  a)  10  20  30  40  Time (Ma)  A= 3.0 (kW/km ) o Q= 25 (kW/km ) 3  2  Surface  Observed Heat Flow  Heat Flow, Q  "Steady State" Heat Flow  (kW/km ) 2  b)  10  20  30  Time (Ma) Figure 19. Surface heat flow vs. time curve for thermal models described i n the text. U p l i f t rates are 1.0, 0.6, 0.3, and 0.1 km/Ma for models A, B, C, and D, respectively. The u p l i f t rates are continued for 20 Ma but are zero from 20-40 Ma while thermal relaxation occurs. "Steady state" heat flow i s that which would r e s u l t i f , at any time, u p l i f t stopped and the i s o therms were allowed to f u l l y relax.  1 48  19b  shows curves for the same four rates of u p l i f t  A, B, C,  and  D with the c o n d i t i o n that Ao=3.0 kW/km . "Steady s t a t e "  heat  flow values  3  Ao=1.0 kW/km ,  and tend  2  value and  i n t h i s case have decreased more than  of Q*. F i g u r e  with l a r g e u p l i f t  19 c l e a r l y  t o approach the  shows the e f f e c t  e r o s i o n have on the surface heat  when  that  uplift  flux.  Apparent vs. True U p l i f t Rates Estimates  of  apparent  uplift  a p a t i t e and z i r c o n by noteing specific  depth  rocks  at  the time  successively  critical  Subtraction rate  greater  isotherms  t o the  the four constant  Figure  from  1.0  20. Note  considerable true u p l i f t  surface)  uplift  effective 1973,  depths f o r each of the  thus  be  (105°C and constructed.  gives the apparent  r a t e have  been  uplift  r a t e s and f o r the cases when Ao 3  i n Figure  difference  uplift  constructed  t o 3.0 kW/km . These curves are 20  between  that  there  apparent  can  uplift  shown i n be  a  rate and  rate.  Though t r a c k s are temperature  at a  retention  of the slope of t h i s curve from the given  (with respect  changes  rock  f o r the two can  r a t e . Curves of apparent u p l i f t for  a  u p l i f t - r e l a t e d c o o l i n g . The same i s done  e i g h t models. A depth-time curve 175°C)  that  passes below i t s r e s p e c t i v e t r a c k  temperature d u r i n g for  r a t e have been made f o r  (Naeser closure  Haack 1977)  i n c r e a s i n g l y r e t a i n e d over a range of  and F a u l  1969),  temperature s i n c e a l l rocks  the concept  of an  i s a p p l i c a b l e here  (Dodson  will  pass  this  closure  1 49  range  in a  similar  f a s h i o n . In t h i s c a l c u l a t i o n , c l o s u r e  temperatures of 105°C and (Zimmerman zircon  and Gaines  of  conclusions  were  chosen  1978, Naeser  (Harrison e_t a l  choice  175°C  1979),  closure  and Forbes 1976) and  respectively.  temperature  for apatite  will  The  specific  not a f f e c t  the  of t h i s paper.  Apparent U p l i f t Rates during U p l i f t Because movement far  inception  isotherms,  of  causes  upward  the i n i t i a l apparent r a t e s w i l l be  few m i l l i o n years  apparent  of u p l i f t  upwards.  r a t e s behind a c t u a l r a t e s i s longer  The dashed  It  apparent  a  for z i r c o n  greater  distance  (75%,  90%, 25%) of the a c t u a l  or  rate.  i s evident  g r a d i e n t s are  move  lag  l i n e s show the time when the apparent  rate i s a f i x e d p r o p o r t i o n former u p l i f t  i n the  (Figure 20), and the  than a p a t i t e s i n c e the isotherms  from  initially  rates  Figure  higher,  approach  20  that when geothermal  as when  Ao=3.0 kW/km , the 3  true r a t e s more r a p i d l y s i n c e the  105°C and 175°C isotherms are and  uplift  below the true r a t e . T h i s i s e s p e c i a l l y important  first of  of  sudden  initially  nearer  the s u r f a c e  are d i s p l a c e d upwards by a smaller amount. Rocks passing  upwards  in this  case move by isotherms more r a p i d l y . T h i s  same e f f e c t would occur When Ao=1.0 kW/km  3  gradient  i f Q* was h i g h e r . and Q*=25 kW/km , 2  the geothermal  i s a rather low 14°C/km. U p l i f t proceeding  a regime w i l l  result  in a p a t i t e apparent u p l i f t  rates  i n such being  A Q = 1.0 k W / k m Apatite Fission Track "Apparent" Uplift Rate (km/Ma) Actual Uplift Rate (km/Ma) Zircon Fission Track "Apparent" Uplift Rate (km/Ma)  A = 3.0  3  Q  (75%...  Actual Uplift Rate 0.8  0.8  0.4  ! 90%  75%.. B ;i.  A \'\25%  Actual Rate During Uplift  c D  •\ 25%  0.0  A  A  1.0  B  B  0.5  C D  0.0  Apparent Uplift Rate  0.2  r.'P..  0.5  C D  0.0 10L  1.0  0.6  the ratio:  0.6 0.4  :  1.0  0.8  Percentages represent  i •' 90% :  0.2 k 0.0  3  1.0L  1.0  0.6  kW/km  0.8  ',90%;  ..V  0.6  ',75%  ' 90% •75%i  i  0.4  I  0.4  ...v  B  0.2 0.0  "  . 25%  I I  0.2  ic.  0.0  -•/"  10  20  Time (Ma)  30  40  C  25%  D 10  30  20  Time  40  (Ma)  F i g u r e 20. A p a t i t e and z i r c o n apparent u p l i f t r a t e v s . time curves f o r models d e s c r i b e d i n t h e t e x t and shown i n F i g u r e 19. Rates o f u p l i f t f o r c u r v e s A, B, C, and D a r e shown by t h e middle diagram ( a c t u a l u p l i f t r a t e v s . t i m e ) . The apparent u p l i f t r a t e r e p r e s e n t s t h e r a t e a t which t h e c r i t i c a l isotherms move downward through t h e r o c k column.  151  within  25% of  the true value a f t e r a p e r i o d of 3 to 5 Ma  (Figure 20). Since most mountain systems by  higher  geothermal  are characterized  g r a d i e n t s , t h i s 5 Ma l a g p e r i o d i s a  maximum; i n most cases, i t w i l l be l e s s and apparent will  be  rates  very c l o s e t o true r a t e s w i t h i n only a few m i l l i o n  y e a r s . Consequently,  i n most  apatite  c l o s e l y approach  rates  will  cases  of  uplift,  apparent  t r u e r a t e s and may be  used as such. The corresponding l a g a f t e r which 75%  of  true  than  commences (Figure 20). I t apatite  r a t e s are s u b j e c t to r a p i d The u p l i f t elements  rates are  r a t e s i s g r e a t e r f o r z i r c o n , and may be up t o  10 Ma a f t e r u p l i f t responsive  apparent  for uplift  less  s t u d i e s , where u p l i f t  fluctuation.  rate and the denudation  are  i s thus  competing  processes  of  heat  which  producing  determine  the  movement of isotherms and v a r i a t i o n s i n heat flow. With a l l models,  the  initiating rate  first  uplift  stage  i s a rise  is sufficiently  i n thermal  evolution  i n isotherms and Q.  high  after  If  uplift  (>0.2 km/Ma f o r Ao=1.0 kW/km , 3  >0.6 km/Ma f o r Ao=3.0 kW/km , >1.2 km/Ma f o r Ao=6.0 kW/km ), 3  3  heat flow w i l l continue t o i n c r e a s e , d e s p i t e the removal upwards-concentrated in  continued  consequence  upward that  heat  of  producing elements. T h i s r e s u l t s  migration  apparent u p l i f t  than the t r u e rate of u p l i f t .  of  isotherms  with  r a t e s w i l l always  If u p l i f t  rate  the  be l e s s  i s less  than  these estimates f o r a given Ao, isotherms and r e s u l t a n t  heat  flow  heat  will  begin  to d e c l i n e ,  reflecting  reduced  1 52  production  in  the  apparent u p l i f t for  long  upper  crust.  When  isotherms  r a t e s w i l l exceed true rates of u p l i f t . Even  (and sometimes u n r e a l i s t i c ) periods  apparent r a t e s f o r a p a t i t e and z i r c o n w i l l true  decline,  of u p l i f t , the  not  exceed  the  r a t e by more than about 20% so that, t h i s e f f e c t i s not  very  important. A f t e r d e c l i n i n g f o r a c o n s i d e r a b l e  time as a  r e s u l t of removal of heat producing elements, isotherms w i l l begin to slowly  r i s e again. T h i s w i l l cause apparent  r a t e s to be s l i g h t l y once has  the i n i t i a l  is  l e s s than true r a t e s . C l e a r l y , however,  3 to 5 Ma p e r i o d s i n c e i n c e p t i o n of u p l i f t  passed, apparent u p l i f t  very  close  rates f o r apatite  will  and e r o s i o n are not i n balance,  but w i l l average out over long u p l i f t  Apparent Rates during Misleading  apparent  periods.  Isotherm  uplift  Relaxation  rates  are  produced  isotherms r e l a x through e i t h e r a c e s s a t i o n of u p l i f t or  through  apatite,  the  the  cooling  fictitious  downward-moving  isotherms  of  longer  therefore,  can  drop  for zircon. be  an  intrusive  apparent  former rate w i t h i n a few m i l l i o n times  strongly  to  rates less  years,  Zircon  data can be m i s l e a d i n g . and  isotherm  when  (Figure  body. For  produced than  by  25% of the  take  several  date-altitude  trends,  affected  but  by  r e l a x a t i o n and the i n t e r p r e t a t i o n of u p l i f t  uplift  remain  (±20%) to the true r a t e of u p l i f t . T h i s s i t u a t i o n  more complex when u p l i f t  20)  uplift  this  thermal  from such z i r c o n  C l e a r l y a combination of d i m i n i s h i n g relaxation  will  produce  an  effect  153  between the extremes d e s c r i b e d Inspection constant  of Figure  uplift  apatite  20 r e v e a l s that  continues  p e r i o d of time, then the apparent  for  a  if  a  r a t e s preserved  the  true  relatively  significant  f i s s i o n track-derived  uplift  c l o s e l y approximate  here.  uplift  (10-20  Ma)  estimates  of  in the  rocks  history.  A  will  similar  observation  for z i r c o n i s not p o s s i b l e since the p o s i t i o n of  the  isotherm  175°C  i s s h i f t e d much more during  thermal r e l a x a t i o n . The where  fission  approach, d e s c r i b e d  track-derived  uplift  in  examples  at  in  accurate  Chapters  significantly consistent  an  1  in excess of  with  uplift  and  thermal  Chapter  2,  evaluated  the best method  history.  In  several  2, z i r c o n apparent r a t e s  the  most  likely  models and  apparent r a t e s , on the other  and  h i s t o r i e s are  and/or v e r i f i e d by thermal modeling provides of a r r i v i n g  uplift  hand,  i n d i c a t o r s of the a c t u a l r a t e s of  uplift  geologic  are  data.  usually  are  rates Apatite  very  good  uplift.  Discussion A  combined  approach of heat flow modeling and  track d a t i n g of a p a t i t e and documentation  the  provide  excellent i n mountain  b e l t s . Thermal modeling does show, however, that  i n areas of  geothermal gradient  both)  the  uplift  can  h i s t o r y of rocks  low  of  zircon  fission  (produced by  corrections  track-derived modeling should  apparent  that uplift  must rates  low be can  Q*,  low  made be  be done to f i n d the a c t u a l u p l i f t  to  Ao,  or  fission  large,  and  rates  that  1 54 will  produce  the  observed  fission  track  date-altitude  trends. The less  fission  than  the  dates preserved  track-derived  r a t e s can  more  or  true  r a t e depending on whether  the  were frozen  beginning of an u p l i f t uplift  uplift in  the  be e i t h e r  rocks  at  the  end  or  episode. A knowledge of the timing  from g e o l o g i c a l data can  of  help to choose between these  alternat ives. If gradient  a  of  Ao,  Q*,  of  evaluating  samples from d i f f e r e n t Figure  20 can  altitudes  geothermal track  is  widely  used  dating  done,  in  a  rate. manner  to that of Wagner et a l (1979), Schaer et a l (1975)  proceed  accompanies  the  the extension with analysis  caution of  of  the  method  unless zircon  to  thermal data.  zircon modeling  Geothermal  i n f e r e n c e s gained from such work (such as c o n s t r a i n t s on can  the  g u i d e l i n e in  the p o s s i b l e e r r o r in c a l c u l a t e d apparent  in Chapter 1, but should  average  be used as a general  A p a t i t e can continue to be similar  or  i s a v a i l a b l e f o r an area where f i s s i o n  s t u d i e s on curves  knowledge  provide  u s e f u l i n s i g h t on the causes of  uplift.  Q*)  155  Acknowledgements This  paper has b e n e f i t e d  from d i s c u s s i o n s on heat  with G.R.C.Clarke. W.H.Mathews and T.J.Lewis to  quantify  isotherms,  the  relationship  and f i s s i o n  encouraged  flow me  between u p l i f t , movement of  t r a c k d e r i v e d apparent u p l i f t  rates.  T h e i r comments, and those of R.L.Armstrong, are a p p r e c i a t e d . This  work  Engineering  was  supported  by  the N a t u r a l  Sciences  Research C o u n c i l of Canada as a grant  R.L.Armstrong.  and  awarded to  1 56  References Dodson, M. 1973. Closure temperature in cooling geochronologic and p e t r o l o g i c systems. C o n t r i b u t i o n s to Mineralogy and Petrology, 40, pp.259-274. Gleadow, A.J.W., and Brooks, C.K. 1979. F i s s i o n track dating, thermal h i s t o r i e s and t e c t o n i c s of igneous intrusions in east Greenland. Contributions to Mineralogy and Petrology, 7J_, pp.45-60. Haack, U. 1977. The c l o s i n g temperature f o r f i s s i o n track r e t e n t i o n in m i n e r a l s . American J o u r n a l of Science, 277, pp.459-464. H a r r i s o n , T.M. Armstrong, R.L., Naeser, C.W., and Harakal, J.E. 1979. Geochronology and thermal h i s t o r y of the coast p l u t o n i c complex, near Prince Rupert, British Columbia. Canadian Journal of Earth Sciences, 16, pp.400-410. Naeser, C.W., and F a u l , H. 1969. F i s s i o n track annealing a p a t i t e and sphene. J o u r n a l of Geophysical Research, pp.705-710. Naeser, C.W., and Forbes, R.B. track ages with depth in ( a b s t r a c t ) . EOS, 57, p.363.  1976. two  V a r i a t i o n of deep drill  in 74,  fission holes  Schaer, J.P., Reimer, G.M., and Wagner, G.A. 1975. A c t u a l and ancient u p l i f t r a t e in the Gotthard region, Swiss Alps: A comparison between p r e c i s e l e v e l i n g and f i s s i o n track a p a t i t e age. Tectonophysics, 2j3, pp.293-300. Sharma, K.K., B a l , K.D., Parshad, R., L a i , N., and Nagpaul, K.K. 1980. P a l e o - u p l i f t and c o o l i n g r a t e s from v a r i o u s orogenic b e l t s of India, as revealed by radiometric ages. Tectonophysics, 70, pp.135-158. Wagner, G.A., and Reimer, G.M. 1972. F i s s i o n track t e c t o n i c s : The t e c t o n i c i n t e r p r e t a t i o n of f i s s i o n track apatite ages. Earth and P l a n e t a r y Science L e t t e r s , 14, pp.263-268. Wagner, G.A., Reimer, G.M., and Jager, E. 1977. The cooling ages d e r i v e d by a p a t i t e f i s s i o n t r a c k , mica Rb-Sr, and K-Ar d a t i n g : The uplift and c o o l i n g h i s t o r y of the Central Alps. Memoir of the I n s t i t u t e of Geology and Mineralogy, U n i v e r s i t y of Padova, Padova, Italy, XXX, 27p. Wagner, G.A., Miller, D.A., and Jager, E. 1979. F i s s i o n track ages on a p a t i t e of B e r g e l l rocks from C e n t r a l Alps and B e r g e l l boulders i n Oligocene sediments. E a r t h and  1 57  P l a n e t a r y Science L e t t e r s , 45, pp.355-360. Woodhouse, J.H., and B i r c h , F. 1980. Comment on ' E r o s i o n , uplift, e x p o n e n t i a l heat source distribution, and t r a n s i e n t heat f l u x ' by T.-C.Lee. J o u r n a l of Geophysical Research, 85, pp.2691-2693. Zimmermann, R.A., and Gaines, A.M. 1978. A new approach to the study of f i s s i o n track fading. United States G e o l o g i c a l Survey, Open f i l e Report 78-701, pp.467-468.  APPENDIX 1.  COASTMTN, A FINITE DIFFERENCE COMPUTER PROGRAM FOR USE IN HEAT FLOW MODELING  1 59  C C C C C C C C C C C C C C  FINITE DIFFERENCE SOLUTION TO THE 1-DIMENSIONAL HEAT FLOW PROBLEM WITH VARIABLE UPLIFT, SURFACE TEMPERATURE, REDUCED FLOW, AND EXPONENTIALLY DECREASING DISTRIBUTION OF HEAT PRODUCTION THE PROGRAM IS DESIGNED FOR THE CENOZOIC HISTORY OF THE BRITISH COLUMBIA COAST MOUNTAINS. THE SURFACE TEMPERATURE CAN BE CHANGED TO SIMULATE CONDITIONS WHEN UPLIFT AND EROSION ARE NOT IN BALANCE. THE PROGRAM IS DESIGNED FOR A MAXIMUM GRID OF 200X200. TO BE REALISTIC, THE HEAT PRODUCTION AT THE BASE OF THE GRID SHOULD BE LESS THAN 2% THAT OF THE SURFACE. INTEGER TT REAL LAM,K,LAPSE DIMENSION A(210),B(210),C(210),DD(210),V(210),QR(210) DIMENSION TSURF(210),U(210,210),W(210,5),P(210),PP(210) DIMENSION TOTUP(210),UPL(210),AQSURF(210) DIMENSION UU(210,210), SURFEL(210) DIMENSION A0BASE(210),A0SURF(210),AQT(210),AQB(210) WRITE (6,800) READ (5,900) AO,K,DIF,D,Q0 WRITE (6,810) READ (5,910) TSURF0,DTSURF WRITE (6,820) READ (5,920) ZMAX,TMAX WRITE (6,830) READ (5,930) DZ,DT  C C C C C C  SET LOWER BOUNDARY CONDITIONS-VARIABLE FLUX; 2 CHANGES IN REDUCED HEAT FLUX ARE ALLOWED AT TIMES TQR1 AND TQR2; THE THREE HEAT FLUX VALUES-OLDEST TO YOUNGEST-ARE QR1,QR2,AND QR4.  C C C C C  SET UPLIFT RATE CONDITIONS-3 CHANGES IN UPLIFT RATE ARE PERMITTED AT TIMES TUPL1, TUPL2, TUPL3; THE 4 RATES ARE UPL1,UPL2,UPL3, AND UPL4.  C C C  WRITE (6,840) READ (5,940) QR1,TQR1,QR2,TQR2,QR3  WRITE (6,850) READ (5,950) TUPL1,TUPL2,TUPL3,UPL1,UPL2,UPL3,UPL4 SET CONDITIONS WHEN UPLIFT .NE. EROSION WRITE (6,1100) READ (5,1110) LAPSE WRITE (6,1120) READ (5,1130) AAA WRITE (6,1140) READ (5,1150) BBB WRITE (6,1160)  1 60  READ (5,1170) TNONEQ C C C  CALCULATE UPL VECTOR, WITH RESPECT TO SURFACE  20 25  30 35  X1=TUPLl/DT+0.1 X2=TUPL2/DT+0.1 X3=TUPL3/DT+0.1 X4=TMAX/DT+0.1 JI01 = IFIX(X1) J102=IFIX(X2) JI03=IFIX(X3) J104=IFIX(X4) J101A=J101+1 J102A=J102+1 J103A=J103+1 J104A=J104+1 DO 20 JV=1,JI01 UPL(JV)=UPL1 CONTINUE DO 25 JV=J101 A,J102 UPL(JV)=UPL2 CONTINUE TN=(TNONEQ/DT)+1.0 DO 30 JV=J102A,J103 RJV=FLOAT(JV) UPL(JV)=UPL3 IF (RJV.GT.TN) UPL(JV)=UPL3*BBB CONTINUE DO 35 JV=J103A,J104 UPL(JV)=UPL4*BBB CONTINUE  C C C  CALCULATE QR VECTOR  C C C  CALCULATE TSURF AND SURFEL VECTORS  X5=TQRl/DT+0.1 X6=TQR2/DT+0. 1 J1 05 = IFIX(X5) J106=IFIX(X6) J105A=J105+1 J106A=J106+1 DO 40 JQ=1,JI05 QR(JQ)=QR1 40 CONTINUE DO 45 JQ=J105A,J106 QR(JQ)=QR2 45 CONTINUE DO 50 JQ=J106A,J104A QR(JQ)=QR3 50 CONTINUE  ALTINC=0.0 ALPHA=0.0  161  DO 60 JS=1,J104A TSURF(JS)=TSURF0+DTSURF*FLOAT(JS-1)*DT TN=(TNONEQ/DT)+1.0 RJS=FLOAT(JS) IF (RJS.LT.TN) SURFEL(JS)=0.0 IF (RJS.EQ.TN) SURFEL(JS)=0.0 IF (RJS.GT.TN) ALPHA=DT*UPL(JS-1)*LAPSE*AAA .*(1.0-BBB)/BBB+ALPHA IF (RJS.GT.TN) TSURF(JS)=TSURF(JS)+ALPHA IF (RJS.GT.TN) ALTINC=DT*(1.0-BBB)*UPL(JS-1) .*AAA/BBB+ALTINC IF (RJS.GT.TN) SURFEL(JS)=ALTINC 60 CONTINUE LAM=(DIF*DT)/(DZ*DZ) C C SET INITIAL CONDITIONS C X7=ZMAX/DZ+0.1 J107=IFIX(X7) J107A=J107-1 DO 300 JZ=1,JI07 Z=FLOAT(JZ)*DZ T0=TSURF0+Q0*Z/K T0=T0+(D*D*A0/K)*(1.0-EXP(-Z/D)) U(1,JZ)=T0 300 CONTINUE TOTUPL=0.0 C C PERFORM TIME INCREMENT C DO 100 TT=2,J104A T=(FLOAT(TT)-1.0)*DT GAM=((UPL(TT-1)+UPL(TT))*DT)/(4.0*DZ) C C CALCULATE TOTAL SURFACE DENUDATION VECTOR C TOTUPL=DT*UPL(TT-1)+TOTUPL TOTUP(TT-1)=TOTUPL C C CALCULATE HEAT PRODUCTION VALUES AT BASE AND SURFACE C A0BASE(1)=A0*EXP(-ZMAX/D) A0BASE(TT)=A0*EXP(-(ZMAX-TOTUP(TT-1))/D) A0SURF(1)=A0 A0SURF(TT)=A0*EXP(TOTUP(TT-1)/D) C C PERFORM DEPTH INCREMENT C LAST=IFIX(X7)-1 DO 120 JZ=1,LAST Z=FLOAT(JZ)*DZ A(JZ)=LAM+GAM B(JZ)=-(2.0*LAM+2.0) IF (JZ.EQ.LAST) B(JZ)=-(LAM+2.0+GAM)  162  C C C C C C  C  C  C C C C  C C C  C(JZ)=LAM-GAM SET ASTAR (HEAT PRODUCTION TERM) ASTAR1=-(Z-TOTUP(TT-1))/D ASTAR=(2.0*DIF*DT*A0/K)*EXP(ASTAR1) SET RIGHT HAND SIDE VECTOR DD IF (JZ.EQ.LAST) GO TO 350 IF (JZ.EQ.1) GO TO 360 DD(JZ)=-A(JZ)*U(TT-1,JZ-1)+(2.0*LAM~2.0)*U(TT-1,JZ) DD(JZ)=DD(JZ)-C(JZ)*U(TT-1,JZ+1)-ASTAR GO TO 120 350  DD(LAST)=-A(JZ)*U(TT-1,LAST-1)+(LAM-2.0+GAM)*U(TT~1,LAST) DD(LAST)=DD(LAST)-C(JZ)*(QR(TT-1)+QR(TT))*DZ/K-ASTAR GO TO 120  360  DD(1)=(2.0*LAM-2.0)*U(TT-1,1)-C(1)*U(TT-1,2) DD(1)=DD(1)-A(1)*(TSURF(TT)+TSURF(TT-1))-ASTAR 120 CONTINUE COMPUTE NEW TEMPERATURES U(TT,JZ) USING SUBROUTINE TRIDAG CALL TRIDAG (1,LAST,A,B,C,DD,V)  DO 500 JZ=1,LAST U(TT,JZ)=V(JZ) 500 CONTINUE U(TT,LAST+1)=U(TT,LAST)+DT*QR(TT-1)/K SET HEAT FLOW MATRIX I=TT-1 IJ=I+1 W(I,1)=T W(I,2) = ((U(I,1 )-TSURF(U) )/DZ)*K X8=ZMAX/(10.0*DZ) JR1=IFIX(X8) K10=JR1+1 W(I,3)=(U(I,K10)-U(I,JR1))*K/DZ X10=ZMAX/(5.0*DZ) JR2=IFIX(X10) K11=JR2+1 W(l,4)=(U(l,K11)-U(I,JR2))*K/DZ W(I 5)=TOTUP(l) r  C C C C  100 CONTINUE PRINT PARAMETERS WRITE (7,1001) WRITE (7,1003) A0,K,DIF,D,Q0,TSURF0  1 63  WRITE WRITE WRITE WRITE WRITE WRITE WRITE WRITE WRITE WRITE WRITE C C C  C C C  C C C  C C C C  (7,1005) (7,1007) (7,1009) (7,1300) (7,1310) (7,1320) (7,1330) (7,1340) (7,1350) (7,1050) (7,1015)  DTSURF,ZMAX,TMAX,DZ,DT QR1,TQR1,QR2,TQR2,QR3 UPL1,TUPL1,UPL2,TUPL2,UPL3,TUPL3,UPL4 LAPSE AAA BBB TNONEQ  PRINT TIME HEADINGS DO 600 J6=1,21 P(J6) = (TMAX/2 0.0)*FLOAT(J6)-TMAX/20.0 600 CONTINUE WRITE (7,1011)(P(J6), J6=1,21) WRITE (7,1015) PRINT SURFACE TEMPERATURES DO 610 J7=1,21 ZDT=TMAX/DT+0.1 J8=(lFIX(ZDT)/20)*J7-(lFIX(ZDT)/20)+1 PP(J7)=TSURF(J8) 610 CONTINUE WRITE (7,1013)(PP(J7), J7=1,21) PRINT TEMPERATURE MATRIX DO 650 J2=1,50 ZDZ=ZMAX/DZ+0.1 J50=J2*IFIX(ZDZ)/50 DO 660 JT=1,20 J51=(IFIX(ZDT)/20)*JT+1 UU(JT,J50)=U(J51,J50) 660 CONTINUE WRITE (7,1014) U(1,J50),(UU(JT,J50),JT=1,20) 650 CONTINUE WRITE (7,1015) WRITE (7,1015) PRINT HEAT FLOW-TOTAL UPLIFT MATRIX X12=DZ/2.0 X13=DZ*(2.0*FLOAT(JR1)+1,0)/2.0 X14=DZ*(2.0*FLOAT(JR2)+1.0)/2.0 X15=ZMAX/50.0 WRITE (7,1020) WRITE (7,1021 ) X12 WRITE (7,1022) X13 WRITE (7,1023) X14  164  WRITE (7,1024) WRITE (7,1015)  C C c  DO 680 JB=1,20 J9=(IFIX(ZDT)/20)*JB WRITE (7,1030) (W(J9,J10), J10=1,5) 6 8 0 CONTINUE WRITE (7,1015) WRITE (7,1040) X15 IZDT=IFIX(ZDT) IZDZ=IFIX(ZDZ) WRITE (7,1042) ZDT, IZDT WRITE (7,1043) ZDZ, IZDZ WRITE (7,1015) WRITE (7, 1015) WRITE (7,1070) WRITE (7,1050) WRITE (7,1011)(P(J6),J6=1 21 ) WRITE (7,1015) PRINT VALUES OF HEAT PRODUCTION AT SURFACE AND BASE DO 700 J20=1,21 J21=J20*IFIX(ZDT)/20-IFIX(ZDT)/20+1 AQT(J20)=A0SURF(J21) AQB(J20)=A0BASE(J21) AQSURF( J20)=SURFELfJ21 ) 7 00 CONTINUE WRITE (7,1080)(AQT(J20),J20=1,21) WRITE (7,1080)(AQB(J20),J20=1,21) WRITE (7,1015) WRITE (7, 1180) WRITE (7,1015)  C C C  PRINT SURFACE ALTITUDE VALUES  C C C  FORMAT STATEMENTS  WRITE (7,1190)  800 900 810 910 820 920 830 930 840 940 850 950 1001  FORMAT FORMAT FORMAT FORMAT FORMAT FORMAT FORMAT FORMAT FORMAT FORMAT FORMAT FORMAT FORMAT  (AQSURF(J20),J20=1,21)  'A0,K,DIF,D,Q0-DECIMAL SEPARATED BY COMMA') 5F10.3) 'SURFACE TEMP PARAMETERS, TSURF0,DTSURF(DEG/MA)') 2F10.3) 'GRID PARAMETERS: ZMAX,TMAX--REAL') 2F10.0) 'DEPTH STEP(DZ),TIME STEP(DT)--REAL') 2F10.3) ' QR1 ,TQR1,QR2,TQR2,QR3--REAL' ) 5F10.3) 'TUPL1,TUPL2,TUPL3,UPL1,UPL2,UPL3,UPL4--REAL') 7F10.3)  ' r)  1 65 ORMAT ('SURFACE HEAT PROD. A0= ',F5.2,12X, 1003 F 'CONDUCTIVITY= *,F5.2, /, 'DIFFUSIVITY= ',F5.2,22X,SCALE HEIGHT= ',F6.1,/, 'INITIAL Q(REDUCED)= ',F5.115X,'INITIAL TSURFACE= F5.1,/) MAT ('DTSURF(DEG/MA)= ',F5.2,19X,'ZMAX=.F5.0,/, 1 005FOR 'TMAX(MA)= ' ,F5. 1 ,25X,'DZ(KM) = ',F5.2,/, 'DT(MA)= ',F5.2,//) F O R AT ('REDUCED HEAT FLOW PARAMETERS: QR •1 F5.=1,/, 1 007 'M TQR1(MA)= ',F5.1,25X,'QR2= ',F5.1,/, 'TQR2(MA)= ',F5.1,25X,'QR3= ',F5.1,//) F O R AT ('UPLIFT PARAMETERS: UPL1= ',F5.3,/, 1009 'M TUPL1(MA)= ',F5.1,24X,'UPL2= ,F5 3,/, 'TUPL2(MA)= ',F5.1,24X,'UPL3= ,F5 3,/, 'TUPL3(MA)= ',F5.1,24X,'UPL4= ,F5 3,///) 1011 FORMAT (1X,50F5.1) 1013 FORMAT (1X,50F5.1) 1014 FORMAT (1X,50F5.0) 1015 FORMAT ( ) 1 020FORMAT (COL.1=TIME(MA) ') 1 021FORMAT COL.2=Q AT DEPTH= F5.2,'KM' 1022 FORMAT COL.3=Q AT DEPTH= ,F5, 2,'KM' 1023 FORMAT COL.4=Q AT DEPTH= ,F5, 2,'KM' 1024 FORMAT COL,5=TOTAL DENUDATIONAT SURFACE(AVE.ALT),(KM)') 1 030FORMAT ( 1X,5F10.3) 1 040FORMAT ('DEPTH PRINTING INTERVAL = ',F' 5.2,'KM') 1 042FORMAT ( 'ZDT= ' ,F10.2,5X,'IFIX(ZDT) I3)= 1 043FORMAT ('ZDZ= ',F10.2,5X,'IFIX(ZDZ)= ,13) 1 050FORMAT (50X,'TIME(MA)') 1070 FORMAT CHEAT PRODUCTION AT SURFACE AND BASE OF GRID',/) 1080 FORMAT ( 1X,50F5.2) 1 1 0F0ORMAT ('ATMOSPHERIC LAPSE RATE: -DEG/KM') 1110 FORMAT ( 1F5. 1 ) 1 1 2F0ORMAT ( 'ERO.LT.UPL.,TYPE(-0);UPL.LT.ERO.(1.0) 1 (O.O)IF.EQ. ) 1 1 3F0ORMAT (1F50) 1 1 4F0ORMAT ('ERO/UPL WHICHEVER, APPLIES,-FROM .0.0-1.0') O R U P L / E R O , 1 1 5F0ORMAT (1F5.2) RMAT ('TIME OF 1 1.6F0O ONSET OF NON-EQUILIBRIUM BETWEEN UPL AND ERO' ) RMAT 1 1 7F 0O ORMAT (1F5.0) ('ALTITUDE(KM)OF SURFACE ABOVE SEA LEVEL WHEN 1 18. 0F UPL.NE E RO' ) ORMAT (2X,50F5.2) .1 190F ORMAT (LAPSE RATE= ',F5.1) 1 300F F RMAT (IF EROSION IS LESS THAN UPLIFT, 1310 FO AAA= -1.0' ) O 1 320 RMAT (IF UPLIFT IS LESS THAN EROSION, AAA= 1 . ' AAA=,F5.1 ) ' ,1 OX, 1 330FORMAT (IF EROSION IS EQUAL TO UPLIFT,AAA= 00 . )=',F5.2) 1340 FORMAT (RATIO OF EROSION/UPLIFT OR UPLIFT/EROS0 I' ON 1 350FORMAT (TIME OF ONSET WHEN EROSION .NEQ. UPLIFT= ', •F5.1,//) R  166  C C  STOP END  r>  C C C C C C C C  C C C  C C C  C  SUBROUTINE FOR SOLVING A SYSTEM OF LINEAR SIMULTANEOUS EQUATIONS HAVING A TRIDIAGONAL COEFFICIENT MATRIX. THE EQUATIONS ARE NUMBERED FROM 1 TO LAST, AND THEIR SUBDIAGONAL, DIAGONAL, AND SUPER-DIAGONAL COEFFICIENTS ARE STORED IN THE ARRAYS A, B, AND C. THE COMPUTED SOLUTION VECTOR U(TT,1)...U(TT,LAST) IS STORED IN THE ARRAY V. SUBROUTINE TRIDAG (IFIRST,LAST,A,B,C,DD,V) DIMENSION A(210),B(210),C(210),DD(210),V(210) DIMENSION BETA(210),GAMMA(210) COMPUTE BETA AND GAMMA ARRAYS BETA(IFIRST)=B(1) GAMMA(IFIRST)=DD(1)/B(1) LL=IFIRST+1 DO 400 L=LL,LAST BETA(L)=B(L)-(A(L)*C(L-1))/BETA(L-1) GAMMA(L)=(DD(L)-A(L)*GAMMA(L-1))/BETA(L) 400 CONTINUE COMPUTE FINAL SOLUTION VECTOR V V(LAST)=GAMMA(LAST) LK=LAST-IFIRST DO 410 K=1,LK JKL=LAST-K V(JKL)=GAMMA(JKL)-(C(JKL)*V(JKL+1))/BETA(JKL) 410 CONTINUE RETURN END  

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