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

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CENOZOIC THERMAL AND TECTONIC HISTORY OF THE COAST MOUNTAINS OF BRITISH COLUMBIA AS REVEALED BY FISSION TRACK AND GEOLOGICAL DATA AND QUANTITATIVE THERMAL MODELS by Ran d a l l Richardson P a r r i s h B.A., Middlebury C o l l e g e , 1974 M.Sc, U n i v e r s i t y 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 COLUMBIA We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA January, 1982 © R a n d a l l Richardson P a r r i s h , 1982 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y a v a i l a b l e for reference and study. I further agree that permission for extensive copying of t h i s thesis for s c h o l a r l y purposes may be granted by the head of my department or by his or her representatives. It i s understood that copying or p u b l i c a t i o n of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of Geological Sciences The University of B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date M&*n&r /if. Ml DE-6 (2/79) i i Abstract Fission track dating of zircon and apatite has been used to determine the Cenozoic u p l i f t history of the B r i t i s h Columbia Coast Mountains from 50°-55°N. 115 dates were obtained from rocks of variable geographic location and a l t i t u d e , and the resulting date pattern constrains the movement and deformation of the f i s s i o n track retention isotherms (175°C for zircon, 105°C for apatite) within the crust. Because date-altitude correlations (apparent u p l i f t rates) cannot always be used confidently to estimate actual rates of u p l i f t , a f i n i t e difference numerical scheme was formulated to construct models of heat flow, u p l i f t , denudation, and cooling that s a t i s f y not only f i s s i o n track dates, but also present heat flow, other isotopic dates, geologic considerations, and f i s s i o n track-derived estimates of paleo-geothermal gradient. In most cases, apparent u p l i f t rates derived from apatite date-altitude correlations are very close to modeled rates of u p l i f t . Zircon-derived apparent rates, however, often exceed modeled rates and r e f l e c t post-orogenic cooling a,nd relaxation of isotherms. The relationship of the movement of isotherms to rates of u p l i f t and f i s s i o n track-derived apparent u p l i f t rates i s quantified and discussed. Orogenic rapid cooling and u p l i f t occurred from Cretaceous to Eocene time in most of the Coast Mountains. Rates during orogenic u p l i f t were near 1.0 km/Ma, causing setting of K-Ar clocks in b i o t i t e and hornblende. U p l i f t rates during the middle Cenozoic ranged from 0.2 km/Ma in the a x i a l region of the mountains between 52° and 55°N to less than 0.1 km/Ma south of 52°N. The moderate rates north of 52°N were l i k e l y the result of gradual erosion of crust thickened during Eocene orogeny. A thermal o r i g i n for t h i s northern u p l i f t i s not l i k e l y . Rates of u p l i f t south of 52°N were low despite arc-related volcanic a c t i v i t y during the Oligocene and Miocene. Accelerated u p l i f t in the Late Miocene near Bella Coola-Ocean F a l l s was probably the result of passage of the transverse Anahim Volcanic Belt or hotspot beneath the area about 10 Ma ago, after which u p l i f t slowed. Rapid Pliocene-Recent u p l i f t south of 52°N at rates of up to 0.75 km/Ma elevated a broad region creating the present southern Coast Mountains and deforming 7-10 Ma lavas erupted on the mountains' east flank. It is suggested that t h i s u p l i f t resulted from thermal expansion in the mantle related to a westward jump in the locus of late Neogene arc volcanism. The extent of thi s rapid Pliocene-Recent u p l i f t correlates with the area above the Juan de Fuca-Explorer subducted slab and confirms a re l a t i o n between continental u p l i f t and plate tectonic setting. i v TABLE OF CONTENTS TITLE PAGE . i ABSTRACT i i TABLE OF CONTENTS i v LIST OF TABLES v i LIST OF FIGURES v i i INTRODUCTORY REMARKS 1 CHAPTER 1. FISSION TRACK DATING, APPARENT UPLIFT RATES, AND PATTERNS OF UPLIFT 4 Abs t r a c t • 5 I n t r o d u c t i o n .• 7 G e o l o g i c a l S e t t i n g Of The Coast Mountains 8 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 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 Dates 28 Sea L e v e l A p a t i t e Dates 28 Sea L e v e l Z i r c o n Dates 30 V a r i a t i o n Of Dates With A l t i t u d e And Apparent 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 Area 42 Pemberton Area 44 Spatial-Temporal V a r i a t i o n s Of Apparent U p l i f t Rates ... 45 Estimates Of T o t a l U p l i f t 49 U p l i f t Since 40 Ma 51 U p l i f t Since 10 Ma 52 Miocene Paleogeography 55 Neogene E r o s i o n S urfaces And Deformation 59 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 68 Summary 72 Acknowledgements 74 References 75 CHAPTER 2. HEAT FLOW MODELS, THERMAL EVOLUTION, AND THE CAUSES OF UPLIFT 82 Ab s t r a c t 83 I n t r o d u c t i o n 85 Geology Of The Coast Mountains 86 Thermal Modeling Of Mountain B e l t s 87 The Model 89 D i s c u s s i o n Of Parameters 93 Surface Temperature V a r i a t i o n 93 Heat Production 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 Application To The Coast Mountains 99 Objectives Of Modeling 99 Presentation Of The Models 100 Kemano 104 Northern King Island - Ocean F a l l s 105 Mount Waddington 108 Central Bute Inlet - Mount Raleigh 110 Discussion 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 Its Application 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 Relaxation 152 Discussion 153 Acknowledgements 155 References 156 APPENDIX 1. DESCRIPTION OF THE FORTRAN PROGRAM COASTMTN ...158 v i LIST OF TABLES Table I. Values Of p Determined From Dating The Tuff Of Fish Canyon 23 Table I I . A n a l y t i c a l Data For The Tuff Of Fish Canyon 24 Table I I I . Fi s s i o n Track A n a l y t i c a l Data 25 Table IV. K-Ar An a l y t i c a l Data 49 Table V. Parameters Of Thermal Models 101 LIST OF FIGURES Figure 1. Geographic And Geologic Reference Map 11 Figure 2. K-Ar B i o t i t e Dates In The Coast Mountains 17 Figure 3. Location Of Samples Dated By Fi s s i o n Track Or K-Ar 18 Figure 4. Sea Level Or Low Altitude Fission Track Apatite Dates 29 Figure 5. Sea Level Or Low Altitude Fission Track Zircon Dates 32 Figure 6. Fission Track Date Vs. Altitude For Kemano And Northern King Island-Ocean F a l l s 36 Figure 7. Fis s i o n Track Date Vs. Altitude For Bella Coola Valley And Mount Waddington 39 Figure 8. Fis s i o n Track Date Vs. Altitude For Central Bute Inlet 42 Figure 9. Fission Track Date Vs. Altitude For Mount Bute, Mount Raleigh, And For The Pemberton Area 44 Figure 10. Total U p l i f t Since 40 Ma Ago (Late Eocene) 50 Figure 11. Total U p l i f t Since 10 Ma Ago (Late Miocene) .... 54 Figure 12. Miocene Paleogeography 58 Figure 13. Dist r i b u t i o n Of Late Miocene Lavas And Remnants Of Miocene-Pliocene Erosion Surfaces 62 Figure 14. Smoothed Surface Of Summit Altitudes In The Coast Mountains 66 Figure 15a. Thermal Evolution Diagram For The Kemano Area .102 Figure 15b. Thermal Evolution Diagram For The King Island-Ocean F a l l s Area 107 Figure 15c. Thermal Evolution Diagram For The Mount Waddington Area 110 Figures 15d And 15e. Thermal Evolution Diagram For Mount Bute And Mount Raleigh (d) And Central Bute Inlet (e) ..112 Figure 16. Gravity Anomalies And Crustal Thickness In The Coast Mountains Area 125 Figure 17. Reconstruction Of Past Plate Movements And Orthogonal Convergence Rates 127 Figure 18. Schematic Structural Section Across The Southern Coast Mountains 131 Figure 19. Surface Heat Flow Evolution For Thermal Models .147 Figure 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 This thesis is a study of the Cenozoic tectonics of the Coast Mountains of B r i t i s h Columbia from 50°-55°N. The late Neogene u p l i f t has received special attention because i t s fundamental cause is poorly understood. The f i r s t paper documents the timing and patterns of u p l i f t throughout the area. This was done by means of f i s s i o n track dating of apatite and zircon separated from plutonic rocks of the Coast Mountains. In addition, by u t i l i z i n g f i s s i o n track data, i t was possible to measure paleo-geothermal gradients at two l o c a l i t i e s . Reasonable estimates of these gradients can in turn help to refine the track retention temperature for zircon, which is not precisely known. The f i r s t paper concludes with a discussion of the geological implications and correlations of the f i s s i o n track-derived u p l i f t data, and i t places these observations in a broad regional framework. The second paper of the thesis integrates thermal modeling and the f i s s i o n track data. A f i n i t e difference computer program was written to simulate the flow of heat in the earth's crust while experiencing variable u p l i f t , variable sub-crustal geothermal flux, variable surface temperature, and non-equality of u p l i f t and erosion. The program accepts as input the u p l i f t history derived from f i s s i o n track dating. It produces, by t r i a l and error, a set of values of u p l i f t rate, sub-crustal heat flux, and other 2 parameters, that f i t a l l observed f i s s i o n track dates, paleo-geothermal gradients, and present heat flow measurements. These models are used to analyse mantle processes. It is shown that thermal expansion of the mantle beneath the Coast Mountains has been responsible for the late Neogene u p l i f t . This situation d i f f e r s from most of the world's great mountain systems (Himalaya, European Alps, South Island New Zealand) in that c r u s t a l thickening is not involved. In t h i s respect, the u p l i f t in the B r i t i s h Columbia Coast Mountains is more similar to areas l i k e the Colorado Plateau of the western United States. The possible causes of u p l i f t are discussed in a framework of plate tectonics, and an explanation for the Cenozoic tectonic patterns i s suggested. The t h i r d paper discusses the caution that must be exercised when interpreting f i s s i o n track data. In previous studies, the apparent u p l i f t rates derived from zircon and apatite f i s s i o n track dates (the slope of the date vs. a l t i t u d e curve) has been assumed equal to the true rate of u p l i f t . In the common situ a t i o n where the isotherms are either moving upwards (by u p l i f t ) or downwards (by isotherm relaxation with or without u p l i f t ) , the slope of t h i s curve (the apparent u p l i f t rate) w i l l not be the true rate. The departure of apparent from true rate is quantified using thermal models, and geological situations where the apparent and true rates are grossly d i f f e r e n t are i d e n t i f i e d . Corrections can be applied to real situations i f knowledge 3 of the geological history and probable geothermal gradient is a vailable. This correction has been overlooked in previous studies, and i t emphasizes the importance of u p l i f t and denudation on the upward flow of heat. 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 of B r i t i s h Columbia has been produced by rapid late Neogene u p l i f t ; t his u p l i f t , however, i s not obviously related to the present plate tectonic regime off the west coast of B r i t i s h Columbia. To study this problem, the areal patterns, rates, and t o t a l amounts of Cenozoic u p l i f t from 50° to 55°N have been determined using f i s s i o n track dating of zircon and apatite separated from rocks c o l l e c t e d at high and low alti t u d e s along several traverses through the mountains. Assuming blocking temperatures of 105°C and 175°C for apatite and zircon, respectively, paleo-geothermal gradients have been measured at Kemano (26°+4-6°C/km 35 Ma ago) and near Ocean Falls-King Island (17°±2°C/km 20 Ma ago). Sub-crustal heat flow can be shown to have s i g n i f i c a n t l y changed through time near King Island. During the middle Cenozoic, u p l i f t rates in the ax i a l part of the northern (52°-55°N) Coast Mountains were 0.1-0.2 km/Ma from 25 to 15 Ma ago; thi s u p l i f t was temporally and t e c t o n i c a l l y related to subsidence in the adjacent Queen Charlotte basin. U p l i f t rates increased s l i g h t l y in the Late Miocene to 0.4 km/Ma, probably because of passage of the Anahim 'hot spot' beneath the area. Late Miocene-Recent erosion in the north has reduced summit alt i t u d e s and been more extensive than farther south. Despite t h i s , r e l i c t s of Miocene river valleys and 6 topography are s t i l l occasionally preserved. The area of the present southern (50° to 52°N) Coast Mountains was near sea lev e l and experienced very low u p l i f t rates (<0.1 km/Ma) throughout the middle Cenozoic, despite i t s more active volcanism. Late Miocene bas a l t i c lavas that were erupted onto a mature erosion surface near Taseko Lakes were elevated and warped by rapid Pliocene-Pleistocene u p l i f t (>0.5 km/Ma) of the southern Coast Mountains. The p r o f i l e of thi s u p l i f t was broadly plateau-like, and this u p l i f t led to the inversion of the Miocene topography. Middle Cenozoic u p l i f t from 52° to 55°N was probably the result of denudation and diminishing u p l i f t after the terminal Eocene orogenic episode which grossly thickened the crust. This middle Cenozoic erosion resulted in the restoration of a normal cr u s t a l thickness. Late Neogene u p l i f t in the Coast Mountains was the result of thermal expansion in the mantle caused by a mantle hot spot near 53°N and changes in the geometry of the subducted slab farther south. 7 Introduct ion The Coast Mountains of B r i t i s h Columbia, which r i s e from sea l e v e l to al t i t u d e s of up to 4 km, are a climatic and physiographic barrier over 150 km wide and about 1000 km in length (Figure 1). They merge with the St. E l i a s Mountains of the Yukon Ter r i t o r y and the Cascade Mountains of the northwestern United States in nearly unbroken continuity. The topography i s considerably younger than both the rocks themselves and the terminal orogeny in late Cretaceous to early Tertiary time. Geological evidence indicates rapid Late Miocene-Pliocene u p l i f t (Douglas et a_l 1970). For the most part, the u p l i f t e d region i s continuous despite i t s juxtaposition with both subduction and transform fault plate boundaries off the west coast of B r i t i s h Columbia. The r e l a t i o n of u p l i f t to plate motions i s therefore unclear. Lack of clear c o r r e l a t i o n with volcanic episodes adds to this ambiguity. To explain the late Cenozoic u p l i f t , i t is necessary to quantify i t s areal and temporal v a r i a b i l i t y . Geological mapping of u p l i f t e d Late Miocene lavas in southern B r i t i s h Columbia (Tipper 1963, 1978), paleontological work on Cenozoic deposits (Rouse and Mathews 1979) and study of possible Miocene erosion surfaces (Baer 1973, Culbert 1971, Mathews 1968) has provided some data relevant to this problem, but the magnitude and timing of u p l i f t i s unknown in areas that lack deposits of late Cenozoic age. The technique of f i s s i o n track dating of apatite has 8 recently been applied to chronology of u p l i f t in several areas of the world (Schaer e_t a_l 1975, Wagner and Reimer 1972, Gleadow and Lovering 1978a,b, Naeser 1979). The technique promises quantitative data on u p l i f t in areas where geological data are sparse. Determination of both the timing of u p l i f t (Gleadow and Lovering 1978a) and, where s u f f i c i e n t r e l i e f i s present, u p l i f t rates (Schaer e_t a_l 1975, Wagner et a l 1977) are possible. Studies on batholiths and eroded material from such rocks has also yielded chronologies of plutonic emplacement, u p l i f t and unroofing, and deposition of detritus in adjacent basins (Wagner et a l 1979). This paper uses f i s s i o n track dating of apatite and zircon to quantify the middle and late Cenozoic. u p l i f t history of the Coast Mountains of B r i t i s h Columbia. Detailed u p l i f t h i s t o r i e s are useful for examining the causes of u p l i f t and building paleo-geological reconstructions. Chapter 2 describes the application of the u p l i f t data to models of heat transfer in order to evaluate various geophysical parameters important in explanations of u p l i f t . Geological Sett ing of the Coast Mountains The Coast Mountains, as physiographically defined (Holland 1964), are largely coincident with the Coast Plutonic Complex. The Complex (Roddick and Hutchison 1974) is dominated by Jurassic to Eocene plutonic g r a n i t i c rocks 9 intruded into a varied assemblage of metamorphosed supracrustal rocks. In recent years the interpretation of eugeoclinal s t r a t i f i e d rocks in terms of accreted terranes or tectonostratigraphic assemblages has become popular (Monger 1977, Coney 1980). For instance the greater part of the southern Coast Plutonic Complex in addition to Vancouver Island and part of Queen Charlotte Islands i s thought to consist of a terrane termed Wrangellia in which thick T r i a s s i c pillow lavas are found (Monger and Price 1979, Woodsworth and Tipper 1980). Near Kemano, however, the eastern margin of the Complex is underlain by rocks common to the Intermontane zone, part of the Stikine terrane or S t i k i n i a (Woodsworth and Tipper 1980). West of these rocks and occupying the a x i a l , high-grade core of the complex are g r a n i t i c gneisses and metasediments, best known and exposed between Terrace and Prince Rupert (Figure 1). These rocks are c a l l e d the Central Gneiss Complex and have been extensively studied by H o l l i s t e r (1979) and Hutchison (1970). The p r o t o l i t h age i s largely unknown, although probably Paleozoic or early Mesozoic (Armstrong and Runkle 1979, Woodsworth 1979). These gneisses pinch out southwards in the Bella Coola region. West of the core gneisses in the Prince Rupert area and farther south along s t r i k e are metamorphic rocks that are c o r r e l a t i v e with the early to middle Paleozoic rocks of the Alexander terrane of southeast Alaska (Armstrong and Runkle 1979). Recently discovered Devonian f o s s i l s on M e l v i l l e 10 Island northwest of Prince Rupert confirm this correlation (G.Woodsworth, personal communication 1981). This Paleozoic terrane i s faulted against a portion of Wrangellia near the east edge of Hecate S t r a i t (Woodsworth and Tipper 1980) and perhaps elsewhere on Queen Charlotte Islands (Yorath and Chase 1981). These terranes were accreted to the North American continent and welded together by plutonism and metamorphism during Mesozoic time (Monger and Price 1979). Some of the plutonic, metamorphic, and s t r u c t u r a l features were l i k e l y related to t h i s accretionary, c o l l i s i o n a l process, but a large part of the plutonic and volcanic a c t i v i t y i s due to a magmatic arc above an east-dipping late Mesozoic to early Cenozoic subduction zone (Monger e_t a_l 1972). The plutonic chronology of the Coast Mountains is poorly established; only near Skeena River, Squamish, and in the Bella Coola region are there r e l i a b l e Rb-Sr or U-Pb dates which c l e a r l y define the age of intrusives (Harrison et a l 1979, R.L.Armstrong and R.Parrish unpublished data). K-Ar determinations, which range between 40 and 140 Ma (Figure 2), indicate approximate plutonic ages in a few cases but in most, they give merely the time since f i n a l u p l i f t - r e l a t e d cooling (Roddick and Hutchison 1974, Harrison et a l 1979). There i s a general decrease in both K-Ar dates and the probable ages of plutonic rock from west to east. Plutonism culminated, especially in the northern segment (latitude 53°-60° N), with an intense Eocene plutonic and 11 F i g u r e 1. Geographic and g e o l o g i c r e f e r e n c e map. P l a t e b o u n d a r i e s o f f s h o r e a r e m o d i f i e d from 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 from B e v i e r e t a l ^ (1979) and Berman and Armstr o n g (19 8 0 ) . The dashed and 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 merges 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 Yalakom and F r a s e r f a u l t s . 1 2 orogenic event, after which a c t i v i t y v i r t u a l l y ceased. Only in the south, approximately above the subducting Juan de Fuca plate, did Oligocene to Recent arc-related volcanism and plutonism continue, a l b e i t on a reduced scale (Berman and Armstrong 1980). A major change in plate motion at about 40-50 Ma l i k e l y caused a cessation of rapid subduction along the entire coast and a change to transform motion north of about 52°N and slowed subduction to the south (Coney 1978). This resulted in a middle Cenozoic period of r e l a t i v e quiescence over much of the Coast Mountains (Monger e_t a_l 1 972), interrupted only by scattered igneous events in the south (Berman and Armstrong 1980) and by the Miocene Anahim Volcanic Belt (Bevier et a_l 1979) in the Bella Coola region (Figure 1). Volcanic a c t i v i t y east of the Coast Mountains revived in the Late Miocene with the eruption of voluminous plateau basalts, which flowed over the present eastern flank of the mountains. Climatic evidence suggests no middle Cenozoic mountainous barrier (Mathews and Rouse 1963, Rouse and Mathews 1979) prior to the Late Miocene or Pliocene i n i t i a t i o n of u p l i f t . Plateau lavas were deformed during th i s u p l i f t (Tipper 1963, Douglas et a l 1970). The present plate tectonic regime of subduction south of 51°N and transform motion to the north (Figure 1) has not s i g n i f i c a n t l y changed since at least 10 Ma ago (Riddihough 1977). It i s possible that the same basic plate 1 3 configuration has been present for much of the middle Cenozoic as well (Atwater 1970, Byrne 1979, Coney 1978). This r e l a t i v e l y stable middle to late Cenozoic plate tectonic setting contrasts sharply with the rapid Late Miocene to Pliocene u p l i f t that gave r i s e to the present topography. Fis s i o n Track Dating and i t s Interpretation Both the theory and the interpretive aspects of f i s s i o n track dating have been treated by Fleischer et a_l (1975), Naeser (1979), and Wagner and Reimer (1972). For rocks in most geologic conditions, f i s s i o n tracks in apatite and zircon w i l l anneal at temperatures of about 100°C (Naeser and Faul 1969, Haack 1977, Wagner and Reimer 1972, Zimmermann and Gaines 1978) and 175°C (Harrison e_t a l 1979), respectively. In this paper a value of 105°C for apatite has been used. These estimates are based on experimental annealing studies (Naeser and Faul 1969, Wagner and Reimer 1972), t h e o r e t i c a l analysis of such data (Haack 1977, Zimmermann and Gaines 1978) and perhaps more importantly, studies of natural annealing in boreholes (Naeser and Forbes 1976). Data on zircon i s much less abundant, but i t s ef f e c t i v e annealing temperature appears well constrained between that of apatite f i s s i o n track and K-Ar biotite/sphene f i s s i o n track, as deduced from cooling h i s t o r i e s of well-dated plutons (Harrison e_t a_l 1979, Harrison and McDougall 1980). 14 The track retention temperature i s a s i m p l i f i c a t i o n representing the e f f e c t i v e closure temperature (Haack 1977) in a process of increasingly e f f i c i e n t track retention over a narrow temperature i n t e r v a l . The e f f e c t i v e closure temperature i s a function of cooling rate, but for example, varies only from 98°C to 111°C for apatite when cooling rate changes from 1°C/Ma to lO°C/Ma, respectively (Zimmermann 1977). Dating zircon and apatite from a single rock w i l l reveal average cooling rate between 175°C and 105°C, which can be used to further define the closure temperature. Cooling rates for the vast majority of rocks in this study over the 175° to 105°C temperature i n t e r v a l varied only from 3° to 1U0C/Ma, resulting in date differences between apatite and zircon of 7 to 25 Ma. For the purposes of this work, the track retention temperature w i l l be considered to be constant. This 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 and introduces l i t t l e additional uncertainty. The error in apatite and zircon closure temperatures is estimated at ±5-10 oC and ±20°C, respectively. Given normal (25°C/km) geothermal gradients, these temperatures (105°, 175°C) correspond to depths of about 4 and 7 km in the crust. Rocks experiencing u p l i f t and erosion pass upwards and w i l l retain f i s s i o n tracks f i r s t in zircon and later in apatite. Deeper samples w i l l consequently have younger dates than shallower ones at any one l o c a l i t y . If rocks are sampled at d i f f e r e n t a l t i t u d e s in areas of large r e l i e f , an increase in f i s s i o n track date at higher 1 5 a l t i t u d e s i s usually present. The difference in a l t i t u d e divided by the difference in date w i l l be termed, in th i s study, the apparent u p l i f t rate. This apparent u p l i f t rate, in fact, however, is the rate at which the c r i t i c a l isotherms (105°, 175°C) moved downward with respect to the rock column. The apparent rate does not necessarily represent the u p l i f t rate of the rocks with respect to sea l e v e l , nor does i t necessarily equal the denudation rate with respect to the average surface of the land. The u p l i f t connotation in the term, apparent u p l i f t rate, i s retained both for convenience and because the mechanism by which the date-altitude trends are developed i s primarily u p l i f t of c r u s t a l rocks. Cooling, denudation, and isotherm movement are consequences of the primary u p l i f t . The reader should continually bear in mind the true meaning of t h i s term; a consistent usage has been maintained throughout th i s study. The apparent u p l i f t rate w i l l s t r i c t l y equal the true u p l i f t - e r o s i o n rate only when several conditions are met. These conditions are that 1) isotherms must have been horizontal and uninfluenced by either surface topography or variable thermal conductivity, 2) isotherms must remain at a constant depth with respect to the surface regardless of u p l i f t rate and 3) u p l i f t must be equal to erosion. A complex i n t e r r e l a t i o n s h i p exists between isotherm migration, u p l i f t , denudation, and cooling, and since these conditions are rarely met, caution in interpreting apparent u p l i f t rates must be exercised. Corrections for these effects are 1 6 discussed in Chapter 3, but for most conditions, f i s s i o n track-derived apparent u p l i f t rates are approximately correct. By dating apatites c o l l e c t e d at various a l t i t u d e s and locations in the Alps, Schaer e_t a_l (1975) and Wagner e_t a_l (1977) were able to document u p l i f t rates, s p a t i a l variation of rates, and deformation of paleoisotherms. No such detailed data are available for a mountain system in North America. Sampling Techniques Fresh g r a n i t i c rock samples were c o l l e c t e d for th i s study along four traverses across the B r i t i s h Columbia Coast Mountains (Figure 3). Because the coastline of the province i s indented by numerous deep fjords immediately adjacent to mountains of 2-4 km topographic r e l i e f , the area i s ideal for sampling both low and high a l t i t u d e rocks across the width of the mountains. In addition to the four traverses, two other areas were sampled in d e t a i l , Powell Peak near Kemano, and Mount Waddington near the head of Knight Inlet (Figures 2,3). The samples in the Bella Coola traverse and the southern traverse were co l l e c t e d by a combination of foot, boat, and helicoptor transportation. The rocks at Powell Peak, Bute Inlet, Mount Raleigh, and Mount Waddington were co l l e c t e d by the Geological Survey of Canada and were made available by Glen Woodsworth. In t o t a l , over 70 rocks were processed resulting in 115 f i s s i o n track dates. 17 Figure 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 only from pre-Oligocene rocks. Contoured from a compilation of R. L. Armstrong which includes data from Wanless et a l (1964-1979), Richards and White (1970), Richards and McTaggart (1976), Nelson (1979), Bartholomew (1979), Carter (1974), Harrison et a l (1979), and unpublished data of R. L. Armstrong.  19 A n a l y t i c a l Techniques Techniques adapted from Naeser (1976) and Gleadow and Lovering (1975) were used to date apatite and zircon in thi s study using the population and external detector methods, respectively. Accessory minerals were abundant and eas i l y separated from nearly a l l rocks c o l l e c t e d . I n i t i a l mineral separation involved crushing and sieving 1 kg samples of rock to retain the -80+170 fr a c t i o n , rapid high volume magnetic separation using a CARPCO separator, washing in water and acetone, and heavy mineral separation using bromoform. Further separation involved a Frantz magnetic separator and methylene iodide. In dating apatite by the population method, two s p l i t s were made, one being annealed at 480°-520°C for at least two hours to remove spontaneous, naturally occuring tracks. This annealed s p l i t was irr a d i a t e d and subsequently mounted, polished, and etched together with the other s p l i t which retained the natural tracks. Apatite etching conditions were 7% n i t r i c acid at 22°-24°C for about 30 seconds. Tracks were counted at 800x. Zircons were mounted and etched according to the procedure of Gleadow et a l (1976) using FEP te f l o n and a eutectic KOH-NaOH etch at 200 0-210°C for about 48 hours (depending on track density). Zircons were etched u n t i l natural tracks were f u l l y exposed. Muscovite detectors were used to record the induced track density and were etched in 48% HF for 12 minutes at 22°-24°C. Tracks in zircon were 20 counted at 1250x or 1600x in o i l . A geometry factor of 0.5 was assumed for the external detector method. Dates were calculated according to the f i s s i o n track age equation, date=ln( 1 + (/>s//>i ) * U 2 3 8alpha/X 2 3 8 f ission) (U 2 35/U 2 3 8 )X 2 35 ) x ( l A 2 3 8alpha) where X.2 3 8alpha and X. 2 3 8 f i s s i o n are U 2 3 8 decay constants for alpha and f i s s i o n , respectively, * i s the thermal neutron dose, u 2 3 5 / U 2 3 8 i s the atomic r a t i o of uranium isotopes, and X 2 3 5 i s the cross-section for neutron f i s s i o n reaction of U 2 3 5 . With appropriate substitution of numerical constants l i s t e d in Table II, the equation becomes, date (Ma ) = 6. 446x1 0 3 In ( 1 +9 . 322x 1 0- 1 8 (ps//>i )*) . Thermal neutron i r r a d i a t i o n was performed in Denver, Colorado at the USGS TRIGA research reactor (United States Geological Survey 1974) under the supervision of C.W.Naeser and D.Rusling. Neutron dose measurements and internal gradients were calibrated by the use of mica detectors on NBS 962 glass (Carpenter and Reimer 1974) and by repeated dating of apatite and zircon standards from the tuff of Fish Canyon provided by C.W.Naeser. Seven d i f f e r e n t i r r a d i a t i o n s over a period of one year were done to arrive at a consistent c a l i b r a t i o n constant, r e l a t i n g thermal neutron dose (#) to induced track density (pi) in muscovite set against the NBS 962 glass according 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 the counted track density r a t i o , ps/pi, for the tuff mineral standards assuming their age as 27.9 Ma (Steven et a l 1967, based on constants l i s t e d in Table I I ) . Substituting 27.9 Ma as the date of the tuff of Fish Canyon and rearranging the above equation, the neutron dose for ir 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 2)=4.653x10 1*/(ps/pi) where ps and />i refer to track densities in minerals of the tuff of Fish Canyon. This method was preferred since the NBS irr a d i a t e d glasses RT-1 and RT-2 (Carpenter and Reimer 1974) were found to be somewhat unreliable as * c a l i b r a t o r s . The NBS 962 glass did, however, continue to be used for the source of induced tracks in adjacent detectors. These Fish Canyon tuff-derived * estimates were consistently close, but s l i g h t l y less than, the independent estimates provided by reactor personnel for each i r r a d i a t i o n . For eight 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 tuff of Fish Canyon for dose c a l i b r a t i o n , the value and standard error of fi were found to be 5.41±0.22 x 109 neutrons/track (8 i r r a d i a t i o n s , Table I ) . This value was then used in turn to determine * using the induced track density, pi, of the NBS 962 muscovite detectors bracketing samples in each i r r a d i a t i o n . * estimates for samples were made by interpolation. Apatite and zircon ages from the tuff of Fish Canyon calculated using t h i s dose, and corrected for gradient (determined as 4%/cm of material for the USGS TRIGA reactor), average 22 27.3±1.0 Ma (standard error) for zircon and 27.3±0.8 Ma for apatite (Table I I ) . The calculated mean and standard deviation of uranium content for the minerals i s 352±42 ppm for zircon and 10.8±1.4 ppm for apatite, in close agreement with values reported by Naeser et a l (1979). If estimates provided by the USGS reactor f a c i l i t y are used and no correction for gradient i s made, the mean and standard error of the tuff ages are 29.4±1.0 and 29.6±1.1 for zircon and apatite, respectively. Necessary flux gradient corrections w i l l lower these values by about 5%, in close agreement with the known age of the minerals determined by Steven e_t a l (1967, new constants). Dating results for zircon and apatite are reported in the format suggested by Naeser et a l (1979), and errors for dates are calculated according to formulae given by Johnson et a l (1979). Instead of assuming Poisson s t a t i s t i c s in the cal c u l a t i o n of the standard deviation of track density (where (standard deviation) 2=mean), standard deviation of the counts was calculated from individual counting s t a t i s t i c s . A standard deviation of 3% on neutron dose estimates was assumed in the cal c u l a t i o n of errors for a l l dates. The uranium decay constants of Steiger and Jager (1977) and Hurford and Gleadow (1977) were used in this study. A n a l y t i c a l results are presented in Table I I I . Another important element in f i s s i o n track dating involves the necessity of eliminating, as far as possible, a l l counting bias on the part of the researcher. In this T a b l e I. V a l u e s of 'fi' D e t e r m i n e d f rom D a t i n g the T u f f of F1sh Canyon ( 2 7 . 9 Ma) 1 I r r a d l a t I o n M i n e r a l pS/p\ „ ' " d e t e c t o r , t / c m ! • ' , 10 1 ' n / c r a ' B'•10'n/track 9 - 7 9 a p a t 1 t e 1 . 47 7 .21 x10* 3 . 17 4 . 4 0 2 . 1 - 8 0 z 1 r c o n ' ' 1 . 38 6 .55 x 1 0 ' 3 . 37 5 . 15 2 . 1 - 8 0 z 1 r c o n 1 . 48 6 . 55 x 10 ' 3 . 14 4 . 79 2 . 1 - 8 0 a p a t 1 t e 1 . 34 6 . 55 x10" 3 .47 5 . 30 3 . . 1 - 8 0 z 1 r c o n 1 . 03 7 . 7 0 x10* 4 .52 5 . 87 3 . 1 - 8 0 a p a t 1 t e 1 . 25 7 . 70 x 1 0 ' 3 .72 4 83 4 . 1 - 8 0 a p a t 1 t e 1 . 45 5 .56 x l O ' 3 .21 5 , , 77 4 . 1 - 8 0 a p a t 1 t e 1 . 60 5 . 56 x 10 ' 2 9 1 5 . . 23 4 . 1 - 8 0 z 1 r c o n 1 . 30 5 .56 x 1 0 ' 3 .58 6 . 44 4 2 - 8 0 z 1 r c o n 1 . 48 6 . 5 0 x 1 0 ' 3 . 14 4 . 83 4 . 2 - 8 0 a p a t 1 t e 1 . 31 6 .50 x 1 0 ' 3 . 55 5 . 46 4 . 2 -BO a p a t 1 t e 1 . 36 6 .50 x 1 0 ' 3 . 42 5 . . 26 6 . 1 - 8 0 a p a t 1 t e 0 . 80 8 .57 x 1 0 ' 5 .82 6 . 79 6 . 1 - 8 0 a p a t 1 t e 0 . 85 8 .57 x 1 0 ' 5 . 47 6 38 8 1 - 8 0 a p a t 1 t e 1 . 85 5 .02 x 1 0 ' 2 52 5 . 02 8 . , 1 - 8 0 z 1 r c o n 1 . 67 5 . .02 x 1 0 ' 2 79 5 . 56 8 . , 2 - 8 0 a p a t 1 t e 1 . 49 5 .83 x10* 3 . 12 5 . . 35 8 . . 2 - 8 0 z 1 r c o n 1 . 39 5 . .83 x 10" 3 . 35 5 75 Mean and s t a n d a r d e r r o r of p U s i n g 1 a v e r a g e v a l u e f rom e a c h I r r a d i a t i o n : fi - 5 . 4 1 + 0 . 2 2 x 10 ' n e u t r o n s / t r a c k Assumed age bf F1sh Canyon T u f f = 2 7 . 9 Ma' 1 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 symbols * For t h e t u f f o f F1sh Canyon , * = 4 . 6 5 3 x 1 0 1 ' / ( p s / p 1 ) ; U c o n s t a n t s l i s t e d i n T a b l e I I . ' fi = * / p ' " ' ; p'" i s the d e n s i t y of the d e t e c t o r on NBS962 g l a s s . 4 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_^ (1967) and r e c a l c u l a t e d w i t h : Ve=0.581 x 1 0 - ' ° / y r : Xb = 4 . 9 6 2 x 1 0 - ' ° / y r ; «°K=0.01167 atom'/. T a b l e I I . F i s s i o n T r a c k A n a l y t i c a l Data f o r the T u f f o f F i s h Canyon I r r . M i n e r a l T r a c k s , s p\  1 T r a c k s , 1 T r a c k s , • Date±S n , ( s/1 ) ' r , S ' * U, (ppm) USGS • e s t . ( 1 0 - t / c m ' ) ( 10' 1 t / c m ! ) ( 10' • ' n / c m ! ) ( 10' ' ' n / c m ' ) 9 - 7 9 a p a t 1 t e 0 . 134 1699 0 . 091 1 157 3 . 16 8673 27 . 9 + 1 . 0 3 0 0 / 3 0 0 4 .4% 10 3 . 15 2 . 1 - 8 0 z 1 r c o n 5 . 84 841 4 . 24 610 3 .32 2171 27 . 4±1 . 5 9 0 . 9 0 414 3 . 44 2 . 1 - 8 0 z 1 r c o n 4 . 45 712 3 . 00 480 3 . 32 2171 29 . 5±1 . 2 10 0 .96 293 3 . 44 2 . 1 - 8 0 a p a t 1 t e 0 . 137 283 0 . 102 648 3 .31 2 171 26 . . 7±1 . 7 4 9 / 1 5 0 3 . 5 % 10 3 . .44 3 . 1 - 8 0 z 1 r c o n 3 . 95 1392 3 . 85 1356 4 .15 2802 2 5 . ,5+0. 8 22 0 .97 322 4 . 30 3 . 1 - 8 0 a p a t 1 t e 0 . 14 356 0 . 112 2 12 4 .13 2802 3 0 . .9 + 2 . 7 . 6 0 / 4 5 1 1 . 1% 9 4 . 30 4 . 1 - 8 0 a p a t 1 t e 0 . 137 526 0 . 095 364 2 .98 5673 25 . 8+1 . 8 6 0 / 6 0 4 . 9 % 10 3 . 44 4 . 1 - 8 0 a p a t 1 t e 0 . 160 51 1 0 . 100 321 2 .98 5673 28 . 6±2 . 1 50/50 5 . 6 % 1 1 3 . 44 4 . 1 - 8 0 z 1 r c o n 4. 20 806 3 . 24 622 3 . 0 0 5673 23 . . 3±1 . 2 12 0 . 78 347 3 . 44 4 . 2 - 8 0 a p a t 1 t e 0 . 121 464 0 . 093 356 3 . 48 5673 27 . . 1±1 . 9 6 0 / 6 0 5 . 2 % 10 3 . 44 4 . 2 - 8 0 a p a t 1 t e 0 . 160 51 1 0 . 1 18 378 3 .48 5673 28 . . 3±2 . 1 50/50 4 .6% 12 3 . 44 4 . 2 - 8 0 z 1 r c o n 5 . 24 838 3 . 53 564 3 .49 5673 31 . . 1 + 1 . 7 10 0 .97 364 3 . , 44 6 . 1 - 8 0 a p a t 1 t e 0 . 120 692 0 . 150 433 4 .64 2055 22 . 3+1 . 7 9 0 / 4 5 5 .2% 12 5 . 16 6 . 1 - 8 0 a p a t 1 t e 0 . 150 630 0 . 176 372 4 .64 2055 2 3 . . 7±1 . 8 100/50 5 .2% 14 5 . . 16 8 . 1 - 8 0 a p a t 1 t e 0 . 150 630 0 . 081 171 2 . 72 2055 30 . 2 + 3 . 1 100/50 8 .9% 10 3 . 44 8 . 1 - 8 0 z 1 r c o n 4 . 43 528 2 . 66 317 2 .73 2055 27 . 3+1 . 1 12 0 . 99 327 3 . . 44 8 . 2 - 8 0 a p a t 1 t e 0 . 150 630 0 . 101 214 3 . 2 0 2055 28 . 5±2 . 2 100/50 5 .7% 1 1 3 . 44 8 . 2 - 8 0 z 1 r c o n 5 . 02 698 3 . 61 502 3 .21 2055 26 . ,8±1 . 0 14 0 .94 394 3 .44 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 ) = 2 7 . 3 ± 2 . 5 , ( 1 . 0 ) M a 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 ) = 2 7 . 3 + 2 . 6 . ( 0 . 8 ) M a 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 = 352+42 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 = 10 .8+1 .4 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 = 2 9 . 4 + 2 . 7 , ( 1 . 0 ) M a 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 = 2 9 . 6 + 3 . 8 , ( 1 . 1 ) M a 1 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\, Induced 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 dose ' S , s t a n d a r d e r r o r 1 number of 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 ' n o t 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 pha) = 1 . 55 125 x 10- 1 0 / y r ; X. " - (a 1 pha ) =9 . 8485 x 1 0 - ' ° / y r o t h e r c o n s t a n t s : U ' 1 • / U ' 1 5 = 1 3 7 . 8 8 ; X ' 3 5 = 5 8 0 x 1 0 - ! * c m ' T a b l e I I I . F i s s i o n Track A n a l y t i c a l D a t a T e r r a c e a r e a TO Tgn1 T2 T3 54° 1 2 ' 3 0 " 129°35'30" <100 a p a t 1 t e 1 . 36 655 3 . 27 784 8 . 76 2 1 .9+1 . 5 30/15 4 . .8% 5 4 ° 2 5 ' 4 0 " 128°52'00" <100 a p a t 1 t e 0 . 107 204 0 . 303 577 8 . . 77 18 .6+1 .8 45/45 5 ,0% 5 4 ° 3 0 ' 2 0 " 128°32'30" <100 a p a t 1 t e 0 .037 70 0 .088 . 168 8 . . 4 1 21 . 2±3 . 0 45/45 9 . 3% 54°4 1 ' 1 5 " 1 2 8 " 2 0 ' 0 0 " < 100 a p a t 1 t e 0 .062 13 1 0 .099 209 8 42 3 1 .6 + 4 .6 50/50 9 . 2% K e m a n o - P o w e l 1 Peak a r e a 78 -WU- 342 53 " 5 1 ' 2 5 " 128 " 0 1 ' 1 0 " 1951 a p a t 1 t e 0 . 372 707 78 -wu- 342 53 " 5 1 ' 2 5 " 128 " 0 1 ' 1 0 " 1951 z i r c o n 2 . 70 1208 78 -WU- 343 53 ° 5 0 ' 5 5 " 127 " 5 9 ' 1 5 " 1570 a p a t 1 t e 0 . 455 865 78 -WU- 343 53 " 5 0 ' 5 5 " 127 °59 '15" 1570 z 1 r c o n 3 . 69 1 182 78 -WU- 344 53 °50'05 " 127 " 5 8 ' 4 0 " 1 189 a p a t 1 t e 0 . 179 447 78 -WU- 344 53 ° 5 0 ' 0 5 " 127 °58 '40" 1 189 z 1 r c o n 3 . 50 67 1 78 -WU- 345 53 ° 4 9 ' 5 0 " 127 °58 '35" 945 a p a t 1 t e 0 . . 336 639 78 -WU- 345 53 ° 4 9 ' 5 0 " 127 " 5 8 ' 3 5 " 945 z 1 r c o n 2 . 94 1082 78 -WU- 347 53 " 4 9 ' 2 5 " 127 " 5 7 ' 4 0 " 305 a p a t 1 t e 0 . . 276 524 78 -WU- 347 53 ° 4 9 ' 2 5 " 127 " 5 7 ' 4 0 " 305 z i r c o n 2 . 95 1744 78 -WU- 348 53 " 4 9 ' 2 5 " 127 " 5 7 ' 3 0 " 91 a p a t 1 t e 0 . . 169 321 78 -WU- 348 53 ° 4 9 ' 2 5 " 127 " 5 7 ' 3 0 " 91 z 1 r c o n 4 . 80 6 14 0 . 213 405 3 . . 22 3 3 , .7 + 2 . 9 45/45 6 . .2% 1 . 18 528 3 . . 24 44 . ,4+1 .6 28 0 . .91 0 . 257 488 3 . . 20 34 . .0+2 . 7 45/45 5 . .8% 1 . 70 546 3 . 2 1 4 1 . . 7+2 .4 20 0 . 79 0 . 111 280 3 . . 18 30. . 7±2 . 9 59/60 6. .2% 1 . 51 289 3 . . 19 44 . 3 + 3 . 1 12 0 . .89 0 . 190 362 3 . . 16 33 . 5±3 . 2 45/45 7 .6% 1 . 27 468 3 17 43 .9+1 . 8 23 0 .91 0 . .213 405 3 . 14 24 . 4±2 . 2 45/45 6 . 6% 1 . 36 806 3 . 15 40 .9±1 . 5 ' 37 0 . 87 0 . 126 240 3 . 12 25 . 1+2 . 7 45/45 7 .6% 2 . 51 321 3 . 13 35 .9+1 . 8 8 0 . 6 5 Be11 a B e l 1 a - B e l 1 a C o o l a a r e a BBC - 1 52 " 2 6 ' 4 0 " 125°52'00" 1052 z 1 r c o n 5 . 51 BBC - 1 52 " 2 6 ' 4 0 " 125°52'00" 1052 a p a t 1 t e 0 . 270 BBC -2A 52 0 1 8 ' 2 5 " 125°57'10" 2085 z i r c o n 3 . 24 BBC -2A 52 ° 1 8 ' 2 5 " 125°57'10" 2085 a p a t 1 t e 0 . 809 BBC - 2 B 52 0 1 8 ' 2 5 " 125°57'10" 2085 a p a t 1 t e 1 . 31 BBC - 2 B 52 0 1 8 ' 2 5 " 125°57'10" 2085 z 1 r c o n 3 . 79 BBC -3A 52 " 2 4 ' 0 0 " 126°24'25" 1920 a p a t 1 t e 0 . 440 BBC -3A 52 " 2 4 ' 0 0 " 126°24'25" 1920 z 1 r c o n 3 . 51 BBC -4A 52 o i g ' 4 0 " 126°48'00" 1868 a p a t 1 t e 0 . 106 BBC - 4 A 52 o 1 8 - 4 0 " 126°48'00" 1868 z 1 r c o n 1 . 73 BBC - 4 B 52 o 1 8 ' 4 0 " 126°48'00" 1868 a p a t 1 t e 0 . 263 BBC - 4 B 52 o 1 8 ' 4 0 " 126°48'00" 1868 z 1 r c o n 1 . 85 BBC -5A 52 " 1 5 ' 3 5 " 127°05'20" 2023 a p a t 1 t e 0 . 027 BBC -5A 52 0 1 5 ' 3 5 " 127°05'20" 2023 z 1 r c o n 3 . 05 BBC - 5 B 52 0 1 5 ' 3 5 " 127°05'20" 2023 a p a t 1 t e 0 . 024 BBC - 5 B 52 0 1 5 ' 3 5 " 127°05'20" 2023 z 1 r c o n 4 . 83 BBC -6A 52 ° 2 0 ' 4 5 " 127°18'45" 1579 a p a t 1 t e 0 . 284 BBC - 6 B 52 ° 2 0 ' 4 5 " 127°18'45" 1579 a p a t 1 t e 0 . . 151 BBC - 6 B 52 °20 '45" 127°18'45" 1579 z 1 r c o n 10. 4 BBC - 7 A 52 " 2 2 ' 3 0 " 1 2 7 " 4 2 ' 0 5 " 1237 a p a t 1 t e 0 . .267 BBC - 7 A 52 °22 '30" 1 2 7 " 4 2 ' 0 5 " 1237 z 1 r c o n 6 76 BBC - 7 B 52 °22 '30" 127°42'05" 1237 a p a t 1 t e 0 . . 255 BBC - 8 E 52 ° 2 6 ' 2 0 " 1 2 6 " 2 3 ' 1 5 " 146 a p a t 1 t e 0 . 072 BBC - 8 E 52 o 2 6 ' 2 0 " 1 2 6 " 2 3 ' 15" 146 z i r c o n 2 . 24 BBC - 9 52 " 2 1 ' 3 0 " 1 2 6 " 0 0 ' 4 5 " 274 apat i t e 0 .601 BBC - 9 52 " 2 1 ' 3 0 " 126°00'45" 274 z 1 r c o n 8 . 34 882 2 . 04 326 3 . 82 6 1 . . 7±4 , 3 10 0 . 97 57 1 0 . 166 351 3 . .84 37 . . 4±3. 6 50/50 7 . 4% 829 1 . 39 356 3 . .92 54 . 7±4, . 0 16 0 . 98 1537 0 . 352 670 3 . . 90 5 3 . 6+3 .9 45/45 5 . 1382 0 . 659 668 3 . .86 4 5 . 9+3. .9 25/24 • 5 . 1% 1092 1 . 74 500 3 . 88 5 0 . 6+3 . 0 18 0 . 81 372 0 . 188 191 3 . .94 5 5 . 2+6 . 3 20/24 9 . .7% 786 1 . .66 372 3 . 96 5 0 . . 1+3 . 0 14 0 . 51 340 0 . .082 268 2 . 87 22 . . 3+2 . . 5 50/5 1 8 . 4% 360 1 .08 225 2 .89 27 . 8+1 . 0 13 0 . 97 824 0 . . 207 66 1 2 . 90 22 . .1+2 .8 49/50 8 . . 3% 504 0 . .903 246 2 .91 . 35 . .7+1 . 2 18 0 97 130 0 . 020 95 2 . 93 23 ,7±3 . 5 75/75 10 .7% 684 1. . 53 343 2 . 94 35 . 1±1 . 5 14 0 . . 8 5 92 0 .020 77 2 .96 2 1 . 3±3 .4 6 0 / 6 0 1 1 . 3 % 1 159 2 . 20 528 2 .97 39 . 1±1 . 6 15 0 . 9 5 599 0 . 351 742 3 . 26 15 8±1 . 9 5 0 / 5 0 8 . 1% 482 0 . 145 465 3 . 25 20 . 3 + 2 .4 50/50 7 . 9 % 1 163 5 . 18 580 3 . 23 38 .9 + 2 . 4 7 O . 9 5 565 0 . 309 653 3 . 30 17 .1 + 1 . 3 50/50 4 .0% 1407 3 . 49 726 3 .31 38 .4 + 2 . 2 13 0 . 70 8 15 0 . 307 982 3 . 28 16 . 4+ 1 . 1 50/50 4 .0% 151 0 .049 103 4 . 0 0 35 . 2+4 . 0 50/50 7 . 9% 574 1 . 46 374 4 .01 36 . 9 + 2 . 2 16 0 . 77 1524 0 . 334 860 4 . 0 5 43 .613 . 3 60/61 4 .9"/ 14G7 3 . 56 626 4 .07 57 .0+4 . 2 1 1 ' 0 . 77 T a b l e I I I . c o n t i n u e d . F i s s i o n T r a c k A n a l y t i c a l D a t a Sample L a t i t u d e L o n q l t u d e A l t . M i n e r a l iml B B C - 1 1S 52 ° 2 2 ' 3 0 " 126 " 4 8 ' 0 0 " 30 a p a t 1 t e B B C - 1 1S 52 °22 '30" 126 " 4 8 ' 0 0 " 30 z 1 r c o n B B C - 12G 52 ° 2 3 ' 0 0 " 126 ' 3 3 ' 3 0 " 94 a p a t 1 t e B B C - 12G 52 ° 2 3 ' 0 0 " 126 °33'30" 94 z 1 r c o n B B C - 13 52 0 1 8 ' 4 5 " 127 " 0 6 ' 3 0 " 0 a p a t 1 t e B B C - 13 52 ° 1 8 ' 4 5 " 127 " 0 6 ' 3 0 " 0 z 1 r c o n B B C - 14 52 o 2 2 ' 1 5 " 127 °14'05" 0 a p a t 1 t e B B C - 14 52 o 2 2 ' 1 5 " 127 0 1 4 ' 0 5 " 0 z 1 r c o n B B C - 15 52 ° 2 3 ' 2 0 " 127 °13'00" 0 a p a t 1 t e B B C - 21 52 0 1 5 ' 3 0 " 127 " 0 6 ' 4 5 " 0 a p a t 1 t e B B C - 21 52 0 1 5 ' 3 0 " 127 " 0 6 ' 4 5 " 0 z 1 r c o n B B C - 22 52 ° 1 4 ' 4 5 " 127 °45'00" 0 a p a t 1 t e BBC - 22 52 <M4'45" 127 °45'00" 0 z i r c o n BBC- 24 52 " 1 0 ' 3 0 " 127 °58'35" 0 a p a t 1 t e BBC- 24 52 o 1 0 ' 3 0 " 127 " 5 8 ' 3 5 " 0 z 1 r c o n BBC- 25 52 o 1 2 ' 2 5 " 127 °51 '10" 0 a p a t 1 t e BBC- 28 52 °21 ' 2 5 " 127 o 4 2 ' 1 0 " 10 a p a t 1 t e BBC- 28 52 " 2 1 ' 2 5 " 127 o 4 2 ' 1 0 " 10 z 1 r c o n ( 1 0 " t / c m ' ) 0 . 246 3 . 8 9 0 . 0 2 1 .66 . 0 8 0 .21 .019 .22 .019 .635 . 32 0 . 2 2 1 18 .7 0 . 2 0 1 8 . 5 8 14 1 .111 8 . 0 0 2 0 . 4 0 . 5 . 0 . 0 . 3 . O. 0 . T r a c k s , s 944 810 29 510 261 1012 91 752 60 805 797 467 2094 709 1785 297 356 1408 ( 1 0 ' t / c m ' ) 0 . 264 1 .87 0 . 0 1 5 1 . 57 0 . 146 . 28 .046 .89 0 . 0 3 8 1 . 23 . 53 . 329 .91 . 205 . 23 .2 13 . 206 . 48 T r a c k s , 1 Date±S n . ( s / 1 ) r , S ' 2 . O. 2 . 1 O. 5 . 0 . 3 . O. 0 . 3 . ( 10' ' n/cm' ) 1014 3 . 04 17 . 0±1 . 3 6 0 / 6 0 5 . 0% 388 3 . 07 3 8 . 3±2 .2 13 0 . 78 15 4 . 09 34 . 3±12.2 33/23 31 . 0% 301 4 . 1 1 4 1 . 7+2 . 7 12 0 . 94 472 3 . 08 10. 1±1 . 2 50/50 8 . 3% 547 3 . .09 34 . 2±2 . 2 15 0 . 91 146 3 . 32 8 . . 2±1 . 1 75/50 7 . ,5% 4 16 3 . 33 36 . . 0 + 2 . 5 9 0 .96 122 3 . 34 10 .0+1 .7 50/50 10 . 3% 1301 3 . 36 10 . 4 + 0 . 8 30/25 4 .8% 367 3 . 35 43 . 5 + 3 . 3 15 0 .62 709 3 . 38 13 .6+1 .4 50/51 7 .8% 662 3 . 39 64 . 1 + 2 . 9 7 0 .92 721 3 .41 20 .1+1 .7 55/55 6 . 1% 67 1 3 . 4 0 54 .0+4 .4 13 0 . 78 450 3 .43 13 . 6 + 1 . 1 50/50 5 .2% 660 3 . 46 1 1 . 2 ± 0 . 9 50/50 5 .2% 6 13 3 . 47 47 .8 + 2 . 1 1 1 0 . 9 3 Mount W a d d i n g t o n 19041 51 ° 0 4 ' 4 0 " 125° 3 5 ' 0 0 " 19041 51 " 0 4 ' 4 0 " 125° 3 5 ' 0 0 " 19148 51 ° 2 2 ' 3 0 " 125° 1 6 ' 1 0 " 19148 51 " 2 2 ' 3 0 " 125" 1 6 ' 1 0 " 19151 51 ° 2 3 ' 3 0 " 125° 1 4 ' 0 0 " 19154 51 ° 2 4 ' 2 0 " 125° 1 4 ' 1 0 " 19154 51 ° 2 4 ' 2 0 " 125" 1 4 ' 1 0 " 46125 51 ° 1 9 ' 3 5 " 125" 2 0 ' 0 0 " 46125 51 " 1 9 ' 3 5 " 125" 2 0 ' 0 0 " 56035 51 » 2 7 ' 3 0 " 125° 02 ' 4 5 " 56035 51 ° 2 7 ' 3 0 " 125° 02 ' 4 5 " 9901 1 51 • 1 6 ' 0 0 " 125° 2 6 ' 5 0 " 9901 1 51 ° 1 6 ' 0 0 " 125° 2 6 ' 5 0 " 99014 51 • 1 3 ' 4 0 " 125° 3 0 ' 0 0 " 991 17 51 " 2 5 ' 3 0 " 125° 0 5 ' 0 0 " 0 a p a t 1 t e 0 . 039 82 0 z i r c o n 1 . 95 866 3960 a p a t 1 t e 1 . 27 1 1 19 3960 z 1 r c o n 2 . 42 1433 3800 a p a t 1 t e 0 . 141 295 2900 a p a t 1 t e 0 . . 117 296 2900 z 1 r c o n 3 . .48 724 2210 a p a t i t e 0 .019 47 2210 z i r c o n 4 .78 2628 910 a p a t 1 t e 0 .075 190 910 z 1 r c o n 7 .55 1449 1295 a p a t 1 t e 0 .016 34 1295 z 1 r c o n 1 .88 703 760 a p a t 1 t e 0 .058 122 2290 a p a t 1 t e 0 . 125 272 0 . 0 4 8 102 2 . 6 7 0 . 6 2 2 262 2 . 6 7 0 . 6 0 6 536 2 . 7 5 0 . 9 9 1 586 2 . 7 3 0 . 0 8 2 172 2 . 7 6 0 . 0 6 5 164 2 . 7 7 1 .40 292 2 . 7 7 0 . 0 3 0 77 2 . 7 0 1 . 4 0 769 2 . 7 0 0 . 0 6 3 161 2 . 8 0 2 . 4 9 478 2 . 8 0 0 . 0 2 2 47 2 . 6 9 1.64 637 2 . 7 0 0 . 179 375 2 . 6 8 0 . 0 9 4 198 2 . 7 9 13. 0±2 . 3 50/50 1 1 . 0% 5 0 . 1+3. 4 14 0 . 51 34 . 5 + 3 . 0 2 1/21 5 . 8% 3 9 . 9+2 . 4 . 14 0 . 86 28 . 5+3. 7 5 0 / 5 0 8 . 9% 2 9 . 9+4 . 0 60/60 10. .2% 4 1 . .2+1 . .8 13 0 98 10 . 3+1 , , 7 50/50 10 .0% 55 .2+2 . 6 13 0 .94 20 .0+2 . 8 60/60 9 .8% 50 .8 + 3 . 1 12 0 .96 1 1 .7±2 . 9 50/50 16 .2% 18 .6+1 . 8 13 0 .87 5 . 2+0 .6 5 0 / 5 0 6 .TA 22 . 3±2 . 2 50/50 6 .2% B u t e I n l e t a r e a 10086 50° 2 7 ' 3 4 " 125 " 1 1 ' 1 0 " 160O a p a t 1 t e 0 . 088 10088 50° 2 6 ' 4 2 " 125 0 1 0 ' 1 0 " 762 a p a t 1 t e 0 . 320 10099 50° 3 8 ' 4 4 " 125 " 0 4 ' 5 0 " 1600 a p a t 1 t e 0 . 651 10099 50° 3 8 ' 4 4 " 125 " 0 4 ' 5 0 " 1600 z 1 r c o n 3 . 88 20091 50° 1 7 ' 5 0 " 125 0 1 2 ' 3 5 " 0 a p a t 1 t e 0 . 404 20408 5 0 " 2 7 , 2 8 " 125 °16'10" 0 a p a t 1 t e 1 . .05 30325 50° 4 8 ' 5 4 " 124 " 5 3 ' 0 0 " 0 a p a t 1 t e 0 . 779 30325 50° 4 8 ' 5 4 " 124 " 5 3 ' 0 0 " O z 1 r c o n 3 . 95 30450 50° 4 8 ' 2 2 " 124 " 5 0 ' 2 0 " 1753 a p a t 1 t e 0 . 483 30461 50° 4 7 ' 1 8 " 124 " 4 7 ' 1 0 " 2515 a p a t 1 t e 2 . 39 30461 5 0 " 4 7 ' 1 8 " 124 " 4 7 ' 1 0 " 2515 z 1 r c o n 4 . 58 1 12 406 1 101 497 682 498 988 506 408 1517 586 031 088 174 922 080 203 343 993 1 14 573 836 39 1 1 1 368 1 18 169 21 1 435 127 96 726 107 2 . 90 49 . 3 + 1 0 . 0 30/30 15. 3% 2 . 91 63 . . 3±1 1 .7 30/30 14 . 3% 2 . 98 66 . 7 ± 7 . 0 40/50 7 . 9% 2 . 99 7 5 . 2+5.4 8 O. 62 2 . 87 86 . 5±7 . 6 40/50 7 . 4% 2 . 92 9 0 . 1±8 . 5 32/35 8 . 4% 3 .08 41 . . 9±3 . 7 30/30 6 . 0% 3 .09 73 . 4 + 4 . 9 8 0 . 92 3 .07 77 . 7 + 1 0 . 0 20/20 9 .3% 3 .06 76 . 2±6.2 15/30 4 . 5 % 3 .06 99 .•9±3 . 8 8 0 .96 T a b l e I I I , c o n t i n u e d . F i s s i o n T rack A n a l y t i c a l D a t a Sample L a t l t u d e L o n g i tude A l t . M i n e r a l 30503 30503 3601 1 3601 1 40170, 46094 46099 46099 50364 50364 171 50°54 50°54 50°55 50°55 50 "56 50°54 50°54 50°54 50°35 50°35 '28" '28" '22" '22" '58" ' 10" '50" '50" '00" '00" 124" 17 124° 17 124°42 124°42 124°22 124M4 124"38 124°38 124°57 124°57 '25" '25" '25" '25" '35" '40" '05" '05" ' 40" '40" 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' 49°40' 49°40' 49°40' 49°55' 50°20' 50°19' 50°18' 50°18' 50°23' 50°39' 50°45' 50°47' 50°47' 50°10' 50°10' 49°25' 49°25' 50°53' 50" 05" 30" 30" 30" 25" 10" 40" 40" 55" 15" 45" 05 " 05" 30" 30" 35" 35" 51 " 123° 12 123°06 123°09 123»09 123°09 122°36 122°34 122°36 122*36 122°26 122°24 122° 10 122°13 122°13 122°53 122°53 123° 13 123°13 121°47 '05" '25" '35" '35" '45" '00" '35" '00" '00" '40" '40" '05" '20" '20" '00" '00" '50" '50" '11" imj. 2789 2789 2286 2286 152 1295 610 610 0 O 1417 1417 0 0 305 1722 823 198 198 2134 518 2179 655 655 625 625 76 76 820 (10" t/cm' ) T r a c k s , s T r a c k s , 1 ( 10'' t / c m ' ) ( 10' • 1 n/cm') Date±S n , ( s / i ) r , S ' apat i t e 1 . 67 1 130 0. 888 600 3. 18 z1 r c o n 3 . 99 51 1 1 . 56 200 3. 19 a p a t1 t e 0. 925 1 172 0. 3 13 662 3- 15 z i r c o n 2 . 83 407 0. 944 136 3. 16 a p a t1 t e 0. 056 61 0. 137 151 3. 17 a p a t1 t e 0. 167 352 0. 093 235 3. 12 a p a t1 t e 0. 561 71 1 0. 255 431 3. 1 1 z i r c o n 4 . 50 648 1 . 44 207 3 . 10 a p a t1 t e 0. 436 737 0. 175 369 2 . 97 z i r c o n 8 . 69 556 1 . 53 98 2 . 94 a p a t1 t e 0. 047 60 0. 015 19 4 . 05 apat i t e 2 . 30 438 1 . 13 262 4 . 1 1 apat i t e 0, .019 25 0. 030 39 4 . 15 z1 r c o n 1 1 . , 2 895 2 . :99 239 4 . 17 apat i t e 0. .014 35 0. .008 21 4 . 19 a p a t1 t e 0. . 229 387 0 . 144 243 4 . 24 a p a t1 t e 0. . 349 590 0. . 196 332 4. . 28 a p a t1 t e 0. . 560 7 10 0. . 349 442 4 32 z1 r c o n 4 . 34 833 1 .67 320 4 . 34 a p a t1 t e 0. .704 595 0 . 543 459 4 . 36 a p a t1 t e 0 . 125 271 0 .077 162 4 . 40 a p a t1 t e 0 . 124 263 0. .090 190 4 . 45 a p a t1 t e O .066 84 0 .065 124 4 .49 z1 r c o n 6 . 13 392 5 . 58 357 4 .47 a p a t1 t e 0 .054 1 14 0 . 04 1 86 4 . 53 z1 r c o n 5 . 73 916 2 . 38 381 4 .51 apat 1 t e 3 .71 1019 1 . 35 7 15 4 .07 z i r c o n 8 . 43 944 2 . 70 302 4 .09 a p a t1 t e O . 343 725 0 . 106 224 4 .57 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 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. 4 1 . 5±5 . 67.4+4. 33.9+3. 42.8+4. . 2 .5 .0 .6 .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. 16/16 8 30/50 9 1 1 50/60 30/40 9 40/50 4 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 5 . 4% 0.89 4 .7% 0. 74 0. 53 17= 3% 91 5% 99 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% N o t e : s e e T a b l e I I f o r e x p l a n a t i o n o f symbo ls and decay c o n s t a n t s . Sample 40170,40171 was d a t e d by the e x t e r n a l d e t e c t o r method. 28 study, a l l sample numbers were masked, and thus, samples were objectively counted without knowledge of sample ide n t i t y . The sample i d e n t i t i e s were revealed only when a l l dates from a batch (10-20) were complete. This "blind" counting method should greatly reduce the p o s s i b i l i t y of counting bias. Areal Variation of Apatite and Zircon Dates Sea l e v e l apatite dates Figure 4 shows the d i s t r i b u t i o n and contours of a l l sea level-low a l t i t u d e apatite dates. The dates range from a low of about 5 Ma near Mount Waddington to more than 80 Ma near the western margin of the southern Coast Mountains. Dates have been contoured at values of 10, 20, 35, and 50 Ma, and there are v i r t u a l l y no exceptions outside of experimental error in the pattern, which i s remarkably consistent and regular. A l l dates, with the possible exception of VL-14,-15 (Figure 3) are from pre-Oligocene rocks and c l e a r l y r e f l e c t u p l i f t and f i n a l cooling, and reveal a clear pattern of the deformation of the 105°C isotherm. The two apatite and zircon dates from VL-14,-15 average 31.114.9 Ma (apatite and zircon are concordant) and probably r e f l e c t u p l i f t of an Oligocene g r a n i t i c intrusive near L i l l o o e t , B r i t i s h Columbia. In general, the a x i a l region of the mountains or the area to i t s immediate west gives the youngest dates at a  30 given l a t i t u d e , and dates increase east and west from this region. The date gradient i s steeper in the east than the west, which partly relates to the younger age of r e l a t i v e l y high l e v e l , rapidly cooled plutons on the east side of the mountains. North-south variations are present as well; except for VL-14 and VL-15, none of the dates in the southern traverse is less than about 40 Ma. In the Bute Inlet traverse, one sample beneath Mount Raleigh (40170,40171) i s very young, 7.8 Ma, and indicates i t s recent passage upwards above the 105°C c r u s t a l isotherm. Farther north near Mount Waddington, and extensively in the Bella Coola-Ocean F a l l s area, and farther north, dates over a width of nearly 40 km are 10 Ma or l e s s . Younger dates are c l e a r l y more c h a r a c t e r i s t i c of the northern Coast Mountains area, which in t h i s paper refers to that part between latitudes 52° and 55°N, northwest of Mount Waddington. The southern part, from 49° to 52°N, however, has higher summits and average a l t i t u d e . In the northern part (Bella Coola and Skeena River traverses), the youngest dates are c l e a r l y displaced west from the a x i a l area of highest topography and l o c a l r e l i e f . This may r e f l e c t more rapid erosion on the windward side of the mountains, and i s not a feature produced by a thermal high. Sea l e v e l zircon dates Figure 5 shows the d i s t r i b u t i o n of sea l e v e l or low 31 a l t i t u d e zircon dates and contours. In general the pattern is a hybrid having s i m i l a r i t i e s to both the apatite pattern and the pattern of K-Ar b i o t i t e dates (Figure 2), which have a blocking temperature of about 250°C (Harrison e_t a_l 1979, Harrison and McDougall 1980). Some zircon dates r e f l e c t rapid cooling of r e l a t i v e l y high l e v e l plutons, especially along the east and west margins of the Coast Mountains, but large areas in the northern a x i a l region c l e a r l y r e f l e c t u p l i f t and cooling, many being younger than 40 Ma. In the northern Coast Mountains between Prince Rupert and Terrace, a large area was subject to Eocene metamorphic resetting and rapid u p l i f t giving uniformly young K-Ar b i o t i t e dates (Hutchison 1970, Harrison et a l 1979). The young zircon dates mimic th i s pattern in the north but remain more e r r a t i c and older in the southern traverses, c o r r e l a t i n g more with l o c a l intrusive ages, as deduced by K-Ar dates. Zircon dates, as in the apatite pattern, are s i g n i f i c a n t l y older in the west than the east, r e f l e c t i n g both eastward decreasing age of plutonism and broadly sequential unroofing from west to east. Variation of Dates with Altitude and Apparent U p l i f t Rates Figures 4 and 5 do not in themselves allow a c a l c u l a t i o n of u p l i f t rate unless an assumption of paleogeothermal gradient is made. To arrive at an estimate of u p l i f t rate, low and high a l t i t u d e samples and samples at clo s e l y spaced alt i t u d e intervals were dated. The location  33 of medium or high a l t i t u d e samples i s shown in Figure 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 co l l e c t e d at various alt i t u d e s on Powell Peak (Figure 1) by Glen Woodsworth of the Geological Survey of Canada. A t o t a l of twelve apatite and zircon dates were obtained and are shown in Figure 6a. Zircon and apatite dates vary from 44 to 36 Ma and 34 to 24 Ma, respectively, co r r e l a t i n g well with a l t i t u d e . Regressing the zircon data leads to a slope of about 0.25-0.30 km/Ma for the period 35-45 Ma. The 30-35 Ma slope on apatite data was assumed to be the same, but must fl a t t e n somewhat to accomodate the low al t i t u d e apatite data. The probable minimum slope i s 0.1 km/Ma for the period 15-25 Ma, after which the rate increases to 0.15-0.20 km/Ma. The present depth at which the apatites have a date of zero ( i . e . at 105°C) was estimated according to the formula, T ( Z ) = ( Q * Z / K ) + (D 2AO/K)(1-exp(-Z/D)) + a where T(Z) i s the temperature at depth Z, below the average surface a l t i t u d e , Q* i s the reduced heat flow, K i s the conductivity, D i s the scale height, Ao i s the surface heat production, and a i s the approximate mean atmospheric temperature at the average a l t i t u d e . An exponentially downward decreasing heat production i s assumed in thi s model 34 c a l c u l a t i o n . Values of Q*, K, ho, and D of 50 kW/km2, 2.5 kW/km°C (5.98X10- 3 cal/sec-cm-°C), 1.0 kW/km3 (T.Lewis, personal communication 1980), and 10 km, respectively, were used. The average a l t i t u d e , in the v i c i n i t y of Powell Peak, is 1.0 km and the probable mean annual temperature at that a l t i t u d e i s approximately 5°C, yielding a depth of -3.3 km for the 105°C apatite annealing isotherm. The effect of recent u p l i f t w i l l be to decrease this figure somewhat so that i t should represent a maximum estimate of the zero-age depth. At Kemano, the high a l t i t u d e apatite curve and the low alt i t u d e zircon curve nearly overlap at 35 Ma. Using the rela t i o n of Parrish (1980), the difference in blocking temperature (175°-105° = 70° ) divided by the actual or projected a l t i t u d e difference (2.9±0.6 km; error indicates the l i k e l y range) yie l d s a paleogeothermal gradient of 26°+4°-6°/km at 35 Ma ago. The gradient of 26°/km is similar to the present value of 27°/km for the Stewart area to the northwest (Mathews 1972b). The Kemano data thus indicate rather low- apparent u p l i f t rates with a small increase in the last 10 or 15 Ma. Northern King Island-Ocean F a l l s F i s s i o n track dates are remarkably uniform in the northern King Island-Ocean F a l l s region (Figure 4). Included in t h i s analysis 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 apatite and zircon dates from this area are shown on Figure 6b. Ten apatite and six zircon dates representing a v e r t i c a l a l t i t u d e difference of 2.1 km vary from 8 to 24 Ma and 34 to 39 Ma, respectively, and correlate very well with a l t i t u d e . The K-Ar b i o t i t e dates for this area, recalculated with constants l i s t e d in Table IV, range from 58 to 89 Ma (Figure 2), except for the d i s t i n c t l y younger King Island syenite, which i s 12-13 Ma old (Baer 1973). It could be argued that t h i s syenite was partly responsible for the young apatite dates; however, the f i s s i o n track dates are very regular, do not vary with distance to the syenite, and at sea le v e l are 3-4 Ma younger than the age of the syenite. A two-dimensional conductive heat flow model of an i n f i n i t e l y long dyke 6 km wide introduced at 750°C into rock at 0°C with i t s top and bottom at 2 and 10 km below the surface, respectively, was formulated according to Carslaw and Jaeger (1959). The conductivity for the model was assumed to be 2.5 kW/km°C (5.98x10-3 cal/sec-cm-°C). The nearest f i s s i o n track sample i s 9 km from the nearly straight margin of the syenite (Baer 1973), and the maximum r i s e in temperature for such a sample 3-4 km below the surface i s about 8°C after 1 Ma and 15°C after 3 Ma. The farthest f i s s i o n track samples are 20 km away, and their temperature increase related to intrusion is only about 5°C. If the syenite cooled predominantly by convection, the effect on the f i s s i o n track 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) Figure 6. F i s s i o n track date vs. a l t i t u d e for samples from Kemano (a) and northern King Island-Ocean F a l l s (b) areas. Sample numbers and a n a l y t i c a l data are l i s t e d i n Table I I I . The zero-date point below sea l e v e l was calculated according to a procedure d i s - cussed i n the text. 37 even le s s . Since they preserve the same f i s s i o n track dates, the effect of the intrusion i s considered very minor, although i t cannot be ignored completely. The present depths of the 105° and 175° isotherms have been calculated using the same parameters as at Kemano, except that the average surface a l t i t u d e i s 0.6 km. The depths are -3.7 km for apatite and -6.6 km for zircon. Using both f i s s i o n track data and these estimated depths of zero date, the apparent u p l i f t rates are about 0.4 km/Ma for 30-40 Ma ago (zircon dates), 0.16 km/Ma for 12-24 Ma ago (apatite dates), and about 0.4 km/Ma in the last 10 Ma (Figure 6b). Extrapolation of the zircon curve yi e l d s an alti t u d e difference and approximate error of 4.2±0.6 km for the zircon-apatite overlap at 20 Ma ago. This y i e l d s a rather low paleo-geothermal gradient of 17°±2°C/km. The present heat flow can be roughly estimated from preliminary ocean probe heat flow data obtained by the Earth Physics Branch (T.Lewis, R.Hyndman personal communication 1980). Uncorrected values of 34-75 kW/km2 were obtained in Dean, Burke, and Bentinck Channels, and probably indicate somewhat higher gradients than obtained for 20 Ma ago by using f i s s i o n track data. An average surface value of heat flow of 60 kW/km2 was used in cal c u l a t i n g the depths of zero-age temperatures, along with known surface heat production of about 1.0 kW/km3 (T.Lewis unpublished data, 1980). The apparent u p l i f t rate values near northern King Island indicate that rates changed from 0.16 to 0.4 km/Ma at 38 about 10 Ma ago. The l a t t e r rate i s nearly double the projected rate of Kemano for the same time i n t e r v a l . Bella Coola Valley Nearly a l l dates east of BBC-15 (Figure 3) are plotted in Figure 7a. The data f a l l into two groups: those from North Bentinck Arm (samples BBC-4 and BBC-11) and the rest. The North Bentinck Arm apatite dates are c l e a r l y t r a n s i t i o n a l between the samples to the west and east and vary from 17-22 Ma over an al t i t u d e range of 1.9 km, giving an apparent u p l i f t rate of 0.2-0.25 km/Ma from 20 Ma to the present. The data from Bella Coola Valley are more scattered, and the zircon and apatite dates are very s i m i l i a r , probably indicating r e l a t i v e l y high l e v e l rapid cooling, with considerable experimental scatter. Apparent u p l i f t rates vary from 0.07 to 0.18 km/Ma for apatite. The- zircon dates lack a clearcut younging with decreasing a l t i t u d e . The apatite zero age depth varies from -2.8 km to -3.5 km, assuming the same geothermal parameters as in the Kemano model. An increase in the u p l i f t rate in the Late Miocene cannot be documented, but could be' accomodated i f , for instance, the curves actually f l a t t e n between 30 and 10 Ma, and subsequently steepen again. Only samples from below present sea lev e l could establish t h i s alternate interpretat ion. Mount Waddington BELLA COOLA VALLEY a) Altitude (km) b) 3 h 2 h 1 h Altitude (km) N o r t h B e n t i n c k A r m Eastern Bella Coola Valley 0.2-0.25 km/Ma 105 Lines refer to apatite dates only - J I i i l _ 10 20 30 40 50 60 Date (Ma) MOUNT WADDINGTON E Easternmost sample W Westernmost sample • Apatite • Zircon 10 20 30 40 50 60 Date (Ma) Figure 7. F i s s i o n track date vs. a l t i t u d e for samples from B e l l a Coola Valley (a) and Mount Waddington (b). The E and W symbols i n (b) r e f e r to easternmost and western- most samples, r e s p e c t i v e l y . They are not considered when drawing the best f i t curve through a p a t i t e data. 40 Samples from Mount Waddington, co l l e c t e d by Geological Survey of Canada personel from 1970-1979, range in al t i t u d e from sea le v e l to the summit at nearly 4 km. Figure 7b shows 9 apatite and 6 zircon dates for the area. Because the samples were coll e c t e d at distances of up to 40 km from the summit, there is c l e a r l y some geographic variation to account for. Dates marked E or W in Figure 7b are the easternmost and westernmost samples, respectively, and c l e a r l y f a l l off the main pattern. The remainder of apatite dates form a clear pattern of younging with decreasing a l t i t u d e , ranging from 35 to 5 Ma over a 3.3 km al t i t u d e range. The zircon data, however, are much more scattered and d i f f i c u l t to interpret. The apparent u p l i f t rate calculated from apatite . data for the period 30-10 Ma i s about 0.09 km/Ma. This slope appears to persist to 5 Ma. The average a l t i t u d e of the area i s 1.7 km, and assuming a reduced heat flow, Q*, of 40 kW/km2 as well as the remainder of the Kemano geothermal parameters, the zero-age depth i s about -2.5 km. Assuming t h i s depth to be approximately correct, the apparent u p l i f t rate from 5 Ma to present i s about 0.6 km/Ma. These data c l e a r l y document the effect of latest Miocene-Recent u p l i f t which produced the present r e l i e f . This u p l i f t history is consistent with the unroofing of young (6.8 Ma) sub-volcanic plutons of the Franklin Glacier complex, 10 km southwest of Mount Waddington, (R.L. Armstrong and J.G. Souther, unpublished data) during the Pliocene and Pleistocene. 41 Central Bute Inlet Data from Bute Inlet include rocks from Mount Sir Francis Drake and Cosmos Heights (Figure 1) and are summarized in Figure 8. Apatite dates vary from 76 to 42 Ma and y i e l d an apparent u p l i f t rate of 0.063 km/Ma for 75 to 40 Ma, and probably younger. The zero-age 105°C isotherm i s about -4.8 km. It was derived using the Kemano parameters except that the assumed Q* and the average a l t i t u d e are 30 kW/km2 and 1.1 km, respectively. This results in a present low surface heat flow consistent with the data of Hyndman (1976) for the Bute Inlet area. The apparent u p l i f t rate between 40 Ma and present i s not constrained by f i s s i o n track data and i s drawn, assuming an unchanged rate of 0.063 km/Ma u n t i l about 7-8 Ma. Although c l e a r l y not required by the data, t h i s cooling history i s consistent with the Late Miocene to Recent accelerated u p l i f t indicated by the Mount Waddington, Ocean F a l l s , and Mount Raleigh data and the present physiography. Mount Bute-Mount Raleigh area East from the head of Bute Inlet, samples were analyzed from Mount Bute and Mount Raleigh (Figure 1). These dates are shown on Figure 9a, d i f f e r e n t i a t e d between the two areas. The apatite dates, which form a more consistent pattern than zircon, vary from 56 Ma to 8 Ma over an al t i t u d e range of nearly 3 km. Separate apatite lines are CENTRAL BUTE INLET Figure 8. F i s s i o n track date vs. a l t i t u d e for samples from c e n t r a l Bute I n l Only apatite data were used to define the curve. 43 shown on Figure 9a for Mount Raleigh and for Mount Bute. The Mount Raleigh apparent u p l i f t rate varies from 0.1 km/Ma for the period 40-10 Ma, and increases to 0.4 km/Ma since 8 Ma. The Mount Bute data indicate a rate of 0.04 km/Ma from 40-10 Ma, although i t i s not p a r t i c u l a r l y well-constrained. Since these two areas are nearly along s t r i k e and about 30 km distant from each other, their u p l i f t h i s t o r i e s are probably s i m i l i a r , suggesting that the best apparent u p l i f t rate value probably is an average of the two curves shown. The very young, 7.8 Ma, date below Mount Raleigh in the Southgate River valley probably dates the approximate time since inception of recent rapid u p l i f t , and i s consistent with very young, low al t i t u d e dates on Mount Waddington. Zero-age depths were calculated assuming Q* of 50 kW/km2, an average a l t i t u d e of 1.7 km, and other parameters as in the Kemano example. The >3 km r e l i e f in the area suggests very recent u p l i f t and erosion, consistent with the f i s s i o n track data. Pemberton area In the southern traverse, f i s s i o n track dates are the oldest, none being less than about 40 Ma (Figure 4). Consequently, i t i s more d i f f i c u l t to document small differences in dates between samples at dif f e r e n t a l t i t u d e s . Samples from the Spetch Creek Pluton (Woodsworth 1977) in the Pemberton area (VL-6,-8,-9,) range from about 40-46 Ma over an al t i t u d e difference of 1.6 km, and show no MOUNT BUTE-MOUNT RALEIGH AREA Altitude (km) a) 10 • Apatite + Zircon B Mount Bute samples R Mount Raleigh samples 20 30 40 50 Date (Ma) PEMBERTON 60 70 0.25 km/Ma 0.1 km/Ma • Apatite 4 Zircon Figure 9. F i s s i o n track date vs. a l t i t u d e f o r samples from Mount Bute (B) and Mount Raleigh (R), shown i n (a), and Pemberton (b) areas. Only a p a t i t e data were used to define the curves. 45 demonstrable cor r e l a t i o n with a l t i t u d e (Figure 9b). The present zero-age depth for apatite i s about -3 km assuming Q* of 50 kW/km2 and an average a l t i t u d e of about 1-1.2 km. Since Late Miocene-Recent u p l i f t must be greater than 2 km on geologic and physiographic grounds, the apparent u p l i f t curve has been drawn to take th i s into account. A consequence is a very low (0.02 km/Ma) apparent u p l i f t rate from 35-10 Ma. The low alt i t u d e zircon date, 67.4 Ma, i s close to the b i o t i t e and hornblende K-Ar dates of 86±3 and 77±4 respectively, and indicates f a i r l y rapid cooling of the Spetch Creek Pluton. The cluster of 40-45 Ma dates could indicate a period of moderately rapid u p l i f t during the Eocene. This Eocene u p l i f t is also suggested by the 35-45 Ma low a l t i t u d e dates that span nearly the entire width of the southern Coast Mountains (Figure 4). The presence of thick Eocene nonmarine c l a s t i c deposits in the Fraser Lowland (Rouse et a_l 1975) and in northwest Washington ( F r i z z e l l 1979, Johnson 1981), adds additional support to thi s p o s s i b i l i t y . Spatial and Temporal Variations of Apparent U p l i f t Rates The assumptions involved in equating the apparent u p l i f t rates derived from f i s s i o n track studies with actual u p l i f t rates have been b r i e f l y discussed e a r l i e r in this paper. The assumption most l i k e l y to cause large deviations i s the s t a b i l i t y , with respect to the surface, of isotherms during u p l i f t . It is shown in Chapter 3 that t h i s assumption 46 can rarely be met. In addition, making s p e c i f i c corrections to apparent u p l i f t rates depends not only on the rates of u p l i f t and erosion but also the heat production, reduced heat flow, and on the geologic history of the sample. For example, f i c t i t o u s p o s i t i v e apparent u p l i f t rates can be produced by a downward relaxation of isotherms following a period of high heat flow, rapid u p l i f t and erosion. This may explain the higher apparent rates in the Mount Waddington, Kemano, and Ocean F a l l s areas (Figures 6a, 6b, 7b) during the Eocene, immediately following orogenic a c t i v i t y . Despite these cautions in interpreting the f i s s i o n track data, there are sp a t i a l and temporal patterns in the apparent rates which contain valuable geologic information. Nearly a l l areas indicate a rather slow u p l i f t in the middle Cenozoic from 30 to 15 Ma ago. Maximum rates during th i s time were in the northern a x i a l region near Ocean F a l l s - B e l l a Coola and were up to 0.2 km/Ma. It appears that these smaller middle Cenozoic rates were maintained for at least 15 Ma in most areas. The rates in the southern part (Vancouver and Bute Inlet traverses) were d e f i n i t e l y lower (maximum 0.1 km/Ma) and may have been v i r t u a l l y zero (Pemberton area). The more rapid middle Cenozoic u p l i f t in the north i s synchronous with the sinking and consequent i n f i l l i n g of the adjacent Queen Charlotte basin, which developed mainly in Miocene time (Shouldice 1971). This basin terminates near the north t i p of Vancouver Island, adjacent to the southern 47 Coast Mountains where middle Cenozoic u p l i f t was low or n i l . The middle Cenozoic Coast Mountains u p l i f t and adjacent basin subsidence north of 51°N therefore appear to be related. The Late Miocene-Recent apparent u p l i f t rates are more rapid in the south (>0.6 km/Ma) than farther north (<0.4 km/Ma). In general, the northern samples show only modest evidence of late Cenozoic acceleration of u p l i f t . These differences appear to correlate well with the physiography, in that the higher summits and more rugged topography are concentrated in the southern Coast Mountains. Other supporting evidence for contrasting u p l i f t h i s t o r i e s comes from the r e l a t i o n of the Late Miocene plateau basalts to physiography along the eastern flank of the Coast Mountains. In the Taseko Lakes area (Tipper 1963,1978) the basalts were extruded on an erosion surface of low to moderate r e l i e f and subsequently t i l t e d up in the west. Their basal a l t i t u d e varies from about 3,500' (1.1 km) on the plateau to greater than 8,000' (2.4 km) southeast of Taseko Lakes. Farther north near Bella Coola,' the basalts are e s s e n t i a l l y f l a t lying at 4,500'-5,000' (1.4-1.5 km) and flowed into valleys already developed beneath peaks which presently r i s e above 7,500' (2.3 km)(Baer 1973). The basalts in the north postdate the development of the mountainous physiography, whereas in the south they predate i t . Several samples of the basalt were c o l l e c t e d from basal, columnar-jointed flows and were dated by the K-Ar 48 whole rock method. Most of these samples are from the west side of the basalt cover near the eastern margin of the Coast Mountains and are shown in Figure 3 and labelled as CC-2, -4, -5, -6, and -7 and BC-6. Ana l y t i c a l data are l i s t e d in Table IV. The ages of the flows sampled range from 6.0-9.9 Ma. Basal flows along the eastern flank of the Coast Mountains (CC-4,-5, -6) range from 7.6-9.9 Ma and demonstrate the e s s e n t i a l l y contemporaneous eruption of the plateau basalts along the eastern margin of the Coast Mountains. A basalt date of 14 Ma from the Taseko Lakes area (Farquharson and Stipp 1969) i s of uncertain significance as i t i s in c o n f l i c t with numerous dates on plateau basalts (Bevier and Armstrong unpublished data; Rouse and Mathews 1979). The broadly synchronous eruption of plateau lavas emphasizes the difference in existing Late Miocene physiography between north and south. Estimates of Total U p l i f t Values of t o t a l u p l i f t since 40 Ma (Figure 10) and 10 Ma (Figure 11) have been estimated by a composite of three d i f f e r e n t methods: 1) by geologic control consisting of stratigraphic thicknesses and a l t i t u d e s of projected unconformities and erosion surfaces, 2) by c a l c u l a t i n g the depth of the 40 Ma and 10 Ma 105°C apatite annealing isotherms based on an assumed 25°C/km gradient, and 3) estimating the depth of the 175°C 40 Ma zircon annealing isotherm using thermal models (Chapter 2) and subsequently T a b l e IV. K - A r A n a l y t i c a l D a t a S a m p l e 1 L a t i t u d e L o n q l t u d e %K< B C - G ' 5 2 " 0 5 ' 3 0 " 123°23'18" 0 . 337 C C - 2 ' 52°05'30" 123°23'18" 0 . .333 C C - 4 ' 52°25'30" 123°38'05" 0 . .995 C C - 5 ' 52°31'10" 1 2 5 " 4 9 ' 3 5 " 0 . .901 C C - 6 « 5 2 " 3 2 ' 5 0 " 125°42'30" 0 . . 784 C C - 7 ' 51°26'15" 123°39'10" 0 . 355 A r ' " * ( x 1 0 - ' c c / g m ) " % r a d . A r * ° D a t e ± s ( M a ) ' 0 . 0 7 8 7 0 . 0 8 15 O .3067 0 . 3 4 7 8 0 . 2 5 4 1 O. 1051 24 . 8 17 .2 59 . 0 6 1.3 6 5 . 5 3 0 . 3 6 , 0 ± 0 . 2 6 . 3±0. 3 7 . 9 + 0 . 3 9 . 9 + 0 . 3 8 . 3+0.3 7 . 6 + 0 . 3 1 w h o l e r o c k b a s a l t . -30+50 mesh ' c o l l e c t e d by M . L . B e v l e r f rom top of s e c t i o n a t B u l l Canyon on the C h i l c o t i n R i v e r 1 b o t t o m of s e c t i o n , same l o c a l i t y as B C - 6 • c o l l e c t e d f rom 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 near mouth of 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 rom 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 7 c o l l e c t e d near b o t t o m o f s e c t i o n o f 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 and 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 1 0 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 .01167atom % Figure 10. Total u p l i f t , with respect to sea l e v e l , since 40 Ma ago (Late Eocene) . The interpretation i s made assuming a sea l e v e l land surface 40 Ma ago. 51 generalizing over the area. Geological control comprises sediment thickness and subsidence data from the Queen Charlotte basin (Shouldice 1971) and Whatcom basin (Hopkins 1968), the basal al t i t u d e of Late Miocene plateau basalts or other volcanic rocks of Eocene (40 Ma) or Late Miocene (10 Ma) age on the eastern side of the Coast Mountains, and in places the a l t i t u d e s of accordant summits. Using surface temperatures (Rouse and Mathews 1979) of 20°C and 10°C, respectively, the depth to the 105°C isotherm for 40 Ma and 10 Ma sea l e v e l apatite dates gives a f a i r l y accurate indication of t o t a l u p l i f t even when a uniform gradient of about 25°C/km is assumed. Following Eocene orogeny in the north, gradients were much steeper, and no uniform gradient should be assumed when ca l c u l a t i n g depths to the 175°C zircon annealing isotherm. Heat flow models can correct for thermal relaxation and provide u p l i f t estimates where geologic control i s absent (see Chapter 2). U p l i f t since 40 Ma The presence of Eocene-Early Oligocene t e r r e s t r i a l s t r a t i f i e d rocks at sea l e v e l , whether volcanic or sedimentary, indicates net subsidence since 40 Ma. Such areas include the northern Queen Charlotte Islands and part of Hecate S t r a i t underlain by the Masset Formation, part of which may be as old as Late Eocene (Sutherland Brown 1968, Young 1981), the Whatcom basin near Vancouver which contains 52 Late Cretaceous, Eocene, and younger Cenozoic strata (Hopkins 1966, Rouse et a l 1975, Johnson 1981), and the western Interior Plateau where Eocene volcanic and sedimentary rocks are preserved (Ewing 1980, Souther 1977) at 0-1 km a l t i t u d e . No such strata are preserved within the Coast Mountains. The values of u p l i f t derived from 40 Ma zircon and apatite contours are approximately 5-6 km and 3.5 km, respectively (Figure 10). Subsidence estimates in the northern Queen Charlotte Islands and Hecate S t r a i t , shown in Figure 10, are based on the cumulative thickness of late Cenozoic sediments (Shouldice 1971) and the underlying 01igocene-Miocene Masset Formation (Sutherland Brown 1968, Young 1981). Because the basal age of the Masset volcanics may be both time-transgressive and younger than 40 Ma, the estimate of u p l i f t since 40 Ma may be somewhat in error. New information on structure and subsidence in southern Queen Charlotte basin (Yorath and Chase 1981) may require modifications to the pattern shown in Figure 10. There i s clear evidence that the northern portion (latitude 52° to 55°N) has been elevated more than the southern Coast Mountains since Late Eocene time. The difference would be even more dramatic i f Paleocene u p l i f t was taken into account (see H o l l i s t e r 1979). U p l i f t since 10 Ma The t o t a l u p l i f t since 10 Ma i s shown in Figure 11. The 53 3.5 km contour follows the 10 Ma apatite contour and represents the approximate depth of the 105°C isotherm 10 Ma ago. Values of u p l i f t determined using thermal models (Chapter 2) are also shown. Subsidence in the Queen Charlotte basin i s the depth below sea l e v e l of the Miocene-Pliocene boundary (Shouldice 1971). This may be an underestimate because t h i s time span involved (6 Ma) is somewhat less than 10 Ma. Estimates for the westernmost Queen Charlotte Islands and Vancouver Island are based upon the a l t i t u d e s of summits which may represent a former erosion surface of Miocene age (Mathews 1968). The presence of Late Miocene to Pliocene volcanic rocks (Muller et a l 1974) and sediments (Cox 1962) around the perimeter of Vancouver Island broadly supports t h i s conclusion. Whatcom basin was probably a continuous s i t e of deposition and subsidence during the Neogene (Hopkins 1968). F i n a l l y , volcanic rocks of Middle and Late Miocene age in the eastern fringe of the Coast Mountains and in the Pemberton Volcanic Belt provide estimates of the a l t i t u d e of the pre-eruptive land surface. These -include plateau basalts less than 10 Ma old (see Table IV), unroofed 8 Ma plutonic rocks (Wanless et a l 1979) and erosional unfaulted remnants of lavas (Woodsworth 1977, Baer 1973, Berman and Armstrong 1980, Mathews et. a_l 1981) within the Pemberton and Anahim Volcanic Belts. The area of maximum late Neogene u p l i f t occupies the central part of the Coast Mountains from the head of Bute 54 Figure 11. Total u p l i f t , with respect to sea l e v e l , since 10 Ma ago (Late Miocene) . The in t e r p r e t a t i o n i s made assuming a sea l e v e l land surface 10 Ma ago. This assumption i s probably reasonable i n the southern Coast Mountains, but must be considered with caution i n the north where Late Miocene p a l e o - r e l i e f and a l t i t u d e were considerable. The u p l i f t figures in the north, therefore, are maximum values. 55 Inlet to Hawkesbury Island (Figure 1). Though the t o t a l amounts of u p l i f t between the Mount Waddington-Bute Inlet area and the King Island-Kemano area are comparable (about 3.0-3.5 km), the present r e l i e f is considerably higher in the south (Holland 1964). This indicates that the age of most of the u p l i f t in the north is predominately Late Miocene whereas in the south i t i s mostly Pliocene-Pleistocene. The area of maximum u p l i f t in the Bella Coola-Kemano portion of the Coast Mountains i s offset westwards at least 20 km from the highest alt i t u d e s and r e l i e f . This may be a result of more rapid erosion on the windward side of the mountains, as well as proximity to base l e v e l . Since both average and summit a l t i t u d e s in the King Island area are considerably less (<2 km) than the u p l i f t since 10 Ma (3.5 km), i t i s clear that erosion i s keeping pace with u p l i f t . This situation i s not yet met in the south where the summit al t i t u d e s (>3 km) are approximately the a l t i t u d e of the 10 Ma surface. There, the e f f e c t s of more recent u p l i f t are s t i l l very apparent, and t h i s suggests that topographic r e l i e f of >3 km may be preserved only in mountain systems that are but a few m i l l i o n years old. Miocene Paleogeography 56 Evidence from sedimentary and volcanic deposits when combined with data for u p l i f t rate and amount allow the Miocene paleogeography to be accurately reconstructed. This reconstruction is shown in Figure 12. The reconstruction depicts events about 15-20 Ma ago, but also shows the d i s t r i b u t i o n of a l l post-Eocene, pre-Pliocene igneous rocks. Many of the features of Middle Miocene paleogeography have been b r i e f l y discussed already. One of the main features is the contrast between the northern (52° to 55°N) and southern (49° to 52°N) Coast Mountains. Higher u p l i f t rates (up to 0.2 km/Ma) in the north resulted in the maintenance of a mountain system of perhaps 1.5 km r e l i e f through the Cenozoic (Miocene Coast Mountains). The data of Rouse and Mathews (1979) require l i t t l e c l imatic influence of the ancestral Coast Mountains u n t i l Late Miocene or Pliocene time; therefore, summit a l t i t u d e s must have been considerably less than today. Adjacent to and probably linked with t h i s u p l i f t was the Queen Charlotte basin that received sediment derived from both the adjacent mountains and the Queen Charlotte Islands. Both the Sandspit and Queen Charlotte f a u l t s may have been active, as the Queen Charlotte Islands moved northwesterly with some rotation (Yorath and Chase 1981). Volcanism and r i f t i n g associated with the Anahim Volcanic Belt probably contributed to the unstable tectonics of the region. It seems l i k e l y that the coastal region northwards from northern Vancouver Island was 57 bounded on the west, as i t is today, by a complex hot-spot and transform fault system not present adjacent to the southern Coast Mountains (Riddihough 1977). The southern Coast Mountains, adjacent to the Juan de Fuca plate, were the s i t e of low topography and scattered volcanic a c t i v i t y of the Pemberton Volcanic Belt. The Whatcom basin was subsiding (Hopkins 1968), but i t is not clear where the northwest and southwest margins of this basin were, as Miocene sediments are only l o c a l l y present in the S t r a i t of Georgia. Miocene sediments on the southern coast of Vancouver Island indicate that at least in the south, Vancouver Island was low and p a r t i a l l y submerged (Cox 1962). The bulk of the Interior Plateau flood basalts was extruded between 6 and 10 Ma ago (Table IV; Rouse and Mathews 1979, M.L. Bevier and R.L. Armstrong unpublished data). These basalts f i l l e d in valleys which were the s i t e s of the ancestral Fraser River and i t s t r i b u t a r i e s (Rouse and Mathews 1979) and covered large areas of low r e l i e f both within the Interior Plateau and along the eastern flank of the Coast Mountains in the Taseko Lakes area. Farther north in the Bella Coola area, these basalts flowed into broad valleys adjacent to the Miocene Coast Mountains (Baer 1973). The contemporaneous peralkaline volcanics of the east-west Anahim Volcanic Belt were superimposed upon t h i s setting, building several large shield volcanoes (Bevier e_t a_l 1979). L i t t l e evidence of pa l e o - r e l i e f i s preserved in the present 5 8 F i g u r e 12. Miocene paleogeography. I n c l u d e d a r e a l l Miocene igneous r o c k s , a r e a s of Miocene 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 the n o r t h e r n p a r t . 59 southern Coast Mountains because only a few unfaulted remnants of Miocene lava remain. Where preserved, they are generally at high alt i t u d e s (> 2 km). East of the Fraser f a u l t , several more s i l i c i c Miocene eruptive complexes were active (Berman and Armstrong 1980, Mathews et a l 1981), and because of less subsequent u p l i f t , volcanic rocks of these complexes are r e l a t i v e l y well-preserved. Perhaps the most surprising aspect of this Miocene tectonic setting i s the negative correlation between u p l i f t (confined to the north) and c a l c - a l k a l i n e volcanism related to subduction (the Pemberton Volcanic B e l t ) . They appear to be mutually exclusive. The u p l i f t in the south almost e n t i r e l y post-dated these Middle and Late Miocene igneous rocks. Neogene Erosion Surfaces and Deformation During the middle Cenozoic considerable erosion occurred, especially in the northern Coast Mountains, where up to 4 km of rock was stripped. In the Interior Plateau, th i s period of erosion resulted in a very subdued land surface occasionally interrupted by resistant h i l l s and broad r i v e r valleys. Plateau basalts flowed over this landscape to preserve i t s c h a r a c t e r i s t i c s . Various authors have suggested that extensions of t h i s Interior erosion surface existed across the area of the present Coast Mountains. Arguments in favor of such a surface consist of rock shoulders in the mountains at the 60 approximate l e v e l of the Interior surface (Baer 1973), accordance of summits throughout large areas of the mountains (Culbert 1971, Holland 1964), the low r e l i e f Milbanke strandflat which contains Miocene volcanic and plutonic rocks (Baer 1973, Holland 1964), and the r e l a t i v e l y f l a t gently east-dipping basal contact of the plateau lavas in the Taseko Lakes area (Tipper 1978). The f i s s i o n track data and the physiographic and geologic evidence indicate that t h i s concept is too simple. For instance, i f as Baer (1973) suggests in the Bella Coola area, the erosion surface crosses the mountains at maximum alti t u d e s of 5,000'-6,000' (1.5-1.8 km), then regional thermal gradients would have to have exceeded 60°C/km to explain 8-10 Ma sea le v e l apatite dates 1.5 km beneath a surface upon which 8-10 Ma basalts were extruded. The suggestion that there are remnants of an erosion surface or surfaces within the Coast Mountains does indeed have merit. To explore t h i s further, a e r i a l photographs of the eastern flank of the Coast Mountains from lat i t u d e 51° to 53°N were examined. In this t r a n s i t i o n between mountains and plateau there are many examples of low r e l i e f , gently inland-sloping upland surfaces ranging in al t i t u d e from 5000' to 8000' (1.5 to 2.4 km). These surfaces cut across lithology indiscriminantly, and are variably dissected by cirques and other recent erosional features. The d i s t r i b u t i o n of these surfaces is shown in Figure 13b, along with approximate a l t i t u d e s . 61 In the southern area near Taseko Lakes, these surfaces are largely constructed on plateau basalts and in places are c l e a r l y controlled by their layering. The a l t i t u d e of the surface gradually declines northeastwards and smoothly merges with the modern Interior Plateau. For the most part, these surfaces postdate the basalts. As these surfaces are traced northwards near the headwaters of the K l i n a k l i n i River, their c h a r a c t e r i s t i c s change. They are no longer b u i l t on Late Miocene basalt; in fact, the basalt is present only on the plateau to the east, several thousand feet lower in a l t i t u d e . Elegant examples of "biscuit-board" topography developed on g r a n i t i c rocks are present west of Tatla Lake and l i e well above the present Interior Plateau. These upland surfaces do not smoothly merge with the Interior Plateau as in the Taseko Lakes area, and as they are higher in a l t i t u d e and would project above the basalts, some of these surfaces must be remnants of an older erosion surface, one that predates the plateau basalts. Their age, however, remains unclear as apparently no geologic deposits o v e r l i e them. Farther north, near the eastern Bella Coola Valley, there is an erosion surface of Late Miocene age that both underlies the basalt and extends westwards where the lavas have been eroded (Baer 1973). Mountains which r i s e up to 1.5 km above th i s surface represent monadnocks and extensions of the Miocene Coast Mountains into the Interior Plateau. These easternmost mountains are rounded 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 Miocene l a v a s (a) and remnants 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 from T i p p e r (1969, 1978) and 1:250,000 t o p o g r a p h i c maps. F i g u r e (b) has been c o n s t r u c t e d from a e r i a l photographs and 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 smooth topography, shown i n s o l i d b l a c k , i s o n l y shown on t h e mountain f r i n g e and not on t h e I n t e r i o r P l a t e a u a r e a . 63 places have pediment-like lower slopes that merge with the r e l a t i v e l y f l a t sub-basalt surface. In t h i s area the sub-basalt erosion surface which l i e s about 5,000' (1.5 km) in a l t i t u d e penetrates deeply (20 km) into the Coast Mountains and has been l i t t l e modified by Late Miocene-Pliocene u p l i f t . The westward extension of the Late Miocene erosion surface must pass well over the summits of the peaks in the northern King Island-Ocean F a l l s area, probably at an a l t i t u d e of 3-3.5 km. Clearly, the accordance of summits in this area must have a d i f f e r e n t explanation than simply representing 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 basalts and low r e l i e f upland surfaces can be compared in Figures 13a and 13b. Except for minor fa u l t i n g probably associated with the Anahim Volcanic Belt (Baer 1973) the plateau basalts in the north are e s s e n t i a l l y f l a t lying at 4,500'±500' (1.4±0.2 km) and partly surrounded by higher topography that contains older remnant surfaces. In contrast, the lavas in the south are largely coextensive with the low r e l i e f surface, and they are d i s t i n c t l y bent up towards the southwest at the edge of the Coast Mountains. The 6 to 10 Ma lavas predate the topography in the south whereas they postdate much of i t in the north. In this respect they demonstrate the contrast in post-Late Miocene tectonic history from north to south. By contouring the a l t i t u d e of the base of the basalts (Figure 13a), i t i s possible to evaluate the gross structure 64 of the late Neogene deformation in the southern Coast Mountains. Assuming the basal lavas were extruded on a more or less horizontal surface, the st r u c t u r a l r e l i e f i s estimated to be about 1.3 km. The lavas r i s e to more than 8000' (2.4 km) in a f a i r l y smooth, apparently unbroken gradient with minor low amplitude undulations. Farther southwest within the Pemberton Volcanic Belt near Bralorne and L i l l o o e t Lake are Miocene volcanic rocks (Woodsworth 1977) at about 7,500' (2.3 km). This suggests that the pre-Late Miocene (>10 Ma) land surface on which the rocks were extruded flattens over the southern Coast Mountains in a high plateau-like manner before dropping into the S t r a i t of Georgia-Whatcom basin area. Thus the form of the deformation may be a gentle monocline on the east and perhaps also the west side. As yet there i s l i t t l e evidence to suggest that f a u l t i n g has played an important role in this Pliocene-Pleistocene u p l i f t . That the Neogene u p l i f t in the southern Coast Mountains is both Pliocene-Pleistocene in age and s i m i l i a r to a broad plateau-like u p l i f t is supported by the general physiography of the area. The summit al t i t u d e s in the Coast Mountains have been contoured and are shown in Figure 14 to i l l u s t r a t e t his point. A broad >100 km wide area in the south has summits over 2-3 km in a l t i t u d e with smooth gradients, especially on the southwest side. Because the u p l i f t i s r e l a t i v e l y recent, i t i s more l i k e l y that the surface of summit a l t i t u d e s in the south (49° to 52°N) r e f l e c t s a 65 p r e - u p l i f t erosion surface than in the northern (52° to 55°N) area. S i g n i f i c a n t l y , the unfaulted volcanic rocks of the Pemberton Volcanic Belt, though few, l i e roughly at the same a l t i t u d e as this surface. In contrast the summit surface i s considerably lower and more irregular north of 51°N (Figure 14). Only a small portion of the northern region l i e s above 2 km, and several of the main fjords and channels coincide with broad embayments which may in part be related to Miocene to Pliocene erosion, modified by later g l a c i a t i o n . The main embayment along Douglas Channel may represent the location of the ancestral Skeena River. A suggested paleo-drainage pattern of the Skeena River i s shown diagramatically in Figure 12. There i s , however, no d i r e c t evidence in support of t h i s suggestion. The more irr e g u l a r , embayed, and generally lower summit surface pattern suggests greater ancestry of r e l i e f and drainage patterns in the north. Both the Queen Charlotte Islands and Vancouver Island have r e l a t i v e l y smooth summit surfaces and probably r e f l e c t Pliocene-Pleistocene u p l i f t of a r e l a t i v e l y low r e l i e f land surface. E s s e n t i a l l y f l a t lying Pliocene volcanic rocks of the Alert Bay Volcanic Belt, described by Muller et a_l (1974) and dated by the Geological Survey of Canada and University of B r i t i s h Columbia, l i e on a f a i r l y smooth surface and are consistent with t h i s conclusion. Seismic and gravity data on Vancouver Island (Keen and Hyndman 1979) suggest a tectonic, subduction related, mechanism for the 66 Figure 14. Smoothed surface of summit a l t i t u d e s in the Coast Mountains. Recent volcanic summits are excluded. The map was constructed using 1:500,000 topographic maps by measuring about 25-30 summits per l°xl° area. 67 recent and continuing u p l i f t there. Culbert (1971) presented an interpretation of physiography in the Coast Mountains based on a computer-assisted contouring of summit altit u d e s and concluded that a boundary, termed the a x i a l fracture, separated areas of block u p l i f t to the east from subsided areas to the west. This a x i a l fracture was postulated to join the heads of Howe Sound, J e r v i s Inlet and Bute Inlet (Figure 1), and i t coincides with the location of several hot springs. He also suggested that the transverse Bella Coola Valley bounded an u p l i f t e d block to the south. None of these features can be e a s i l y recognized on Figure 14. Unfortunately no direct comparison of summit contour maps i s possible because no corresponding map was presented by Culbert (1971). The coincidence of thermal springs with t h i s linear " a x i a l fracture" may be related to i t being an area of steepest thermal gradients resulting from maximum topographic r e l i e f , large u p l i f t , and northwest s t r u c t u r a l trends, in a manner s i m i l i a r to the development of hotsprings in New Zealand's Southern Alps ( A l l i s e_t a l 1979), although higher regional heat flow may also contribute (Hyndman 1976). Block f a u l t i n g could aid convective hydrothermal transport of heat to explain the steep heat flow gradient of Lewis and Hyndman (1980), but no such f a u l t ( s ) has yet been recognized, either physiographically or geologically. In summary, the contrasts between the northern (52° to 68 55°N) and southern (49° to 52°N) Coast Mountains, apparent in the f i s s i o n track data, are borne out by physiographic evidence. A presently higher a l t i t u d e , plateau-like summit surface with remnants of Late Miocene lavas is c h a r a c t e r i s t i c of the southern region. In contrast, the northern region was never plateau-like; i t has undergone continuous u p l i f t during the middle and late Cenozoic, and probably contains erosion surfaces that are of several ages. Embayments in the northern summit surface may indicate the Miocene position of large v a l l e y s . Discussion and Possible Causes of U p l i f t There are three Cenozoic u p l i f t stages in the Coast Mountains of B r i t i s h Columbia which can be distinguished. The f i r s t was an intense orogenic to post-orogenic u p l i f t in the central and eastern Coast Plutonic Complex extending from Alaska southeastward. Discussion of this major event is beyond the scope of t h i s paper; i t i s reviewed by H o l l i s t e r (1979). The second stage was the less rapid middle Cenozoic u p l i f t in the a x i a l part of the Coast Mountains extending from southeast Alaska to the Mount Waddington area. This stage was responsible for the erosion of several kilometers of material from the northern Coast Mountains and the complementary subsidence of the Queen Charlotte basin. The t h i r d stage, of late Neogene age, involved rapid Pliocene to Recent u p l i f t (>3 km) of the southern Coast Mountains over a broad region giving r i s e to the present topography. This 69 late Neogene u p l i f t was manifest to a lesser degree in the northern area where i t probably began in the Late Miocene. Explanations for the Eocene orogenic event involve the interaction of the America and Farallon plates along a destructive margin. The intense Eocene event in the north-central Coast Mountains of B r i t i s h Columbia (latitude 52°-56°N) involved voluminous production of magma and rapid c r u s t a l thickening related to regional shortening during the f i n a l period of rapid convergence. Since Eocene erosion amounted to > 10 km, and perhaps s i g n i f i c a n t l y more (H o l l i s t e r 1979), the crust must have been exceptionally thickened. The ensuing middle Cenozoic u p l i f t is coextensive with the Eocene event in the Coast Plutonic Complex and may be related. Assuming the formation of a large c r u s t a l root supporting high mountains during the Eocene, the present lack of such a thick crust (Berry and Forsyth 1975) indicates that i t has been eroded. The linked process of erosion, diminishing u p l i f t , and resultant c r u s t a l thinning would l i k e l y take several tens of m i l l i o n years to eliminate such a root. This process i s thought to be responsible for the middle Cenozoic u p l i f t stage in the northern and central Coast Mountains. The lack of a thermal high manifest in either volcanic a c t i v i t y or steep paleogeothermal gradients suggests that the u p l i f t does not have a deep-seated thermal explanation. The linear arch-like form of the u p l i f t is approximately what would be expected from an i s o s t a t i c 70 recovery mechanism. Other explanations are possible and could involve continued less intense oblique subduction throughout the middle Cenozoic. However, there is no middle Cenozoic volcanic expression except for the Anahim Volcanic Belt, and this p o s s i b i l i t y seems unlikel y . An explanation for the late Neogene acceleration of u p l i f t , e s pecially in the south, i s much more elusive. Features that must be explained are: 1) >3 km of probably Pliocene-Pleistocene u p l i f t over nearly the entire width of the southern Coast Mountains from about latitude 52°N southwards to the Fraser Valley, and 2) the moderate late Neogene u p l i f t in the northern Coast Mountains a x i a l zone, western Queen Charlotte Islands, and Vancouver Island. Observations which should be considered are 1) the hiatus in c a l c - a l k a l i n e volcanic a c t i v i t y between >7 Ma a c t i v i t y of the Pemberton Volcanic Belt and the <2 Ma events in the Garibaldi Volcanic Belt (Bevier et a_l 1979, Berman and Armstrong 1980), 2) the subsequent development of the transverse, 'plate edge', Pliocene Alert Bay Volcanic Belt, 3) the changes at about 5 Ma in the r e l a t i v e v e l o c i t y and orientation of convergence between Explorer and America plates (Riddihough 1977), and 4) the coincidence of the broadest and highest late Neogene u p l i f t with the area on the mainland opposite the convergent boundary. The production of 2-3 km of c r u s t a l u p l i f t while maintaining i s o s t a t i c equilibrium can be produced by several 71 mechanisms: c r u s t a l thickening by compression, as in South Island New Zealand (Walcott 1978) or the Himalaya (Molnar and Tapponier 1977); by underplating a large amount of low density arc-related magmatic material; or by thinning of the lithosphere as in the Basin and Range province of the western United States. The most s i g n i f i c a n t correlations with the u p l i f t of the southern Coast Mountains are the reorientation of Juan de Fuca-Explorer-America plate motions about 5 Ma ago and the volcanic arc hiatus between 7 and 2 Ma. During this hiatus, the position of the volcanic front migrated westward to the Garibaldi Volcanic Belt (Figure 1), probably in response to steepening of the subducted slab. This westward jump would lead to warming of the formerly cool lithosphere causing thermal expansion and u p l i f t beneath the Coast Mountains. The subduction of very hot (0-15 Ma) oceanic crust (Riddihough 1977) would c e r t a i n l y contribute to the high a l t i t u d e of the overriding plate 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 1974). The less rapid predominately Late Miocene u p l i f t in the northern part of the Coast Mountains is enigmatic since i t is adjacent to a transform boundary and has no association with volcanic a c t i v i t y . Fission-track paleogeothermal gradient data and present heat flow estimates (T. Lewis unpublished data) indicate higher regional heat flow presently than 20 Ma ago, although t h i s may be more related to the Anahim Volcanic Belt than a regional pattern. Whether 72 the thermal expansion caused by t h i s hot spot could induce a longitudinal u p l i f t in the north (52° to 55°N) i s unclear and deserves further study. Summary Geological and physiographic data indicate that the Coast Mountains of B r i t i s h Columbia are predominately the result of vigorous late Neogene u p l i f t . Quantitative examination of this u p l i f t has been made possible by fi s s i o n - t r a c k dating of apatite and zircon from rocks co l l e c t e d at various locations and alti t u d e s in the Coast Mountains. U p l i f t rates have varied in space and time, and estimates of t o t a l u p l i f t have been calculated for the entire region. These results document s i g n i f i c a n t middle Cenozoic u p l i f t in the northern (52° to 55°N) Coast Mountains which was clo s e l y related to subsidence of the adjacent Queen Charlotte basin. Reconstruction of Miocene paleogeography using f i s s i o n track data shows that the d i s t r i b u t i o n of r e l i e f in the Miocene was opposite to the present s i t u a t i o n of higher r e l i e f in the south; average al t i t u d e s in both areas, however, were lower. Estimates of paleo-geothermal gradients using detailed f i s s i o n track date vs. a l t i t u d e p r o f i l e s indicate that the regional pattern of reduced heat flow may have varied considerably with time. Explanations for the u p l i f t are clearer for the two older (Eocene and middle Cenozoic) stages than for the 73 youngest (Late Miocene to Recent). Eocene orogenesis and crustal thickening in the northern Coast Mountains from latitude 52°-56°N was followed by a t r a n s i t i o n from a convergent to a s t r i k e - s l i p plate tectonic regime. Moderate but steady erosion and i s o s t a t i c u p l i f t in the Miocene and into the late Neogene gradually thinned the crust so that i t s present thickness i s similar to stable continental crust. During the middle Cenozoic the southern Coast Mountains were low in a l t i t u d e and r e l i e f and subject to minimal u p l i f t , despite subduction and intermittent volcanic a c t i v i t y above the subducting Juan de Fuca plate. In late Miocene or early Pliocene time, during a reorganization of Juan de Fuca - Explorer plate geometry and a volcanic arc hiatus in the Coast Mountains, u p l i f t accelerated across the width of the southern Coast Mountains, leading to the present dramatic topography. This southern u p l i f t is thought to be related to the westward migration of the volcanic arc front and the ensuing thermal expansion in the formerly cool lithosphere beneath most of the southern Coast Mountains. 74 Acknowledgements The author is indebted to R.L.Armstrong for continuous support, advice, and ideas during the course of this study. The f i n a n c i a l help provided by a Pre-doctoral Fellowship at the University of B r i t i s h Columbia, the Natural Sciences and Engineering Research Council as a grant to R.L.Armstrong, and a grant-in-aid from the Geological Society of America i s gra t e f u l l y acknowledged. The Geological Survey of Canada helped defray the costs of K-Ar dating. For stimulating discussion, the author thanks W.H.Mathews, J.K.Mortensen, G.K.C.Clarke, G.Woodsworth, and T.Lewis; for help with i r r a d i a t i o n s , I thank C.W.Naeser and D.Rusling of the United States Geological Survey. L. Gilmore and K. 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Tertiary geology and palynology of the Quesnel area, B r i t i s h Columbia. 80 B u l l e t i n of Canadian Petroleum Geology, 27, pp.418-445. Schaer, J.P., Reimer, G.M., and Wagner, G.A. 1975. Actual and ancient u p l i f t rate in the Gotthard region, Swiss Alps: A comparison between precise leveling and f i s s i o n track apatite age. Tectonophysics, 29, pp.293-300. Shouldice, D.A. 1971. Geology of the western Canadian continental shelf. Canadian Petroleum Geology B u l l e t i n , J_9, pp.405-436. Souther, J.G. 1977. Volcanism and tectonic environments in the Canadian C o r d i l l e r a - A second look. I_n Baragar, W.R.A., and others, eds., Volcanic regimes in Canada. Geological Association of Canada Special Paper 16, pp. 3-24. Stacey, R.A. 1974. Plate tectonics, volcanism, and lithosphere in B r i t i s h Columbia. Nature, 250, pp.133-134. Steiger, R.H., and Jager, E. 1977. Subcommission on geochronolgy: Convention on the use of decay constants in geo- and cosmochronology. 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Geological 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. Geological Survey of Canada, Open f i l e map 482. Woodsworth, G.J. 1979. Geology of Whitesail Lake map area, B r i t i s h Columbia. I_n Current research, Part A, Geological Survey of Canada, Paper 79-1A, pp.25-29. Woodsworth, G.J., and Tipper, H.W. 1980. Stratigraphic framework of the.Coast Plutonic Complex, western B r i t i s h Columbia. Geological Association of Canada Cordilleran Section, Programme and Abstracts, pp.32-34. Yorath, C.J., and Chase, R.L. 1981. Tectonic history of allochthonous terranes: Northern Canadian P a c i f i c continental margin. Geological Association of Canada Cordilleran Section, Programme and Abstracts, pp. 42-43. Young, I. 1981. Structure of the western margin of the Queen Charlotte basin, B r i t i s h Columbia. M.Sc.thesis, University of B r i t i s h Columbia, 380 p. Zimmermann, R.A. 1977. The interpretation of apatite f i s s i o n track ages with an application to the study of u p l i f t since the Cretaceous in eastern North America. Unpublished Ph.D. Thesis, University of Pennsylvania, 146 p. 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 Geological 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 II: HEAT FLOW MODELS, THERMAL EVOLUTION, AND THE CAUSES OF UPLIFT 83 Abstract The thermal and tectonic evolution of the crust in the Coast Mountains of B r i t i s h Columbia has been examined using a quantitative f i s s i o n track-derived history of isotherm migration and topographic evolution (Chapter 1). A heat flow program was written to incorporate variable u p l i f t rate, erosion lagging behind u p l i f t , changing sub-crustal geothermal flux, fluctuating surface temperature and exponentially decreasing heat production with depth. Estimated u p l i f t rate, surface temperature, heat production, and present heat flow, when used as input for the thermal model, allow determination of the sub-crustal geothermal flux variations and a refinement of Cenozoic u p l i f t and denudation history. Early Cenozoic orogenesis in the central and northern Coast Mountains (52°-55°N) resulted in substantial c r u s t a l thickening and rapid Eocene u p l i f t , and was followed by the development of a transform plate margin. The moderate (0.1-0.2 km/Ma) middle Cenozoic u p l i f t rates in t h i s region were the result of gradual i s o s t a t i c u p l i f t in consequence of erosion of thickened crust. This process eventually led to elimination of the c r u s t a l root. Late Miocene acceleration of u p l i f t was probably the result of the sub-crustal passage of the Anahim 'hot spot' which thinned the lithosphere and caused changes in sub-crustal and surface heat flux. 84 The southern Coast Mountains (49° to 52°N) were characterized by very low middle Cenozoic u p l i f t rates (<0.1 km/Ma) and sporadic M i o c e n e arc volcanism. Latest Miocene-Recent u p l i f t i n the south, by contrast, was rapid (>0.4 km/Ma) and resulted i n 2 to 3 km of u p l i f t over a broad area. The lack of seismicity and crus t a l thickening strongly suggests that the mechanism for thi s u p l i f t i s thermal expansion i n the upper mantle. During the Miocene, most of the southern Coast Mountains was in a fore-arc, low-heat-flow environment. As a result of downgoing slab steepening and plate motion reorganization i n the la t e s t Miocene or early Pliocene, the volcanic belt migrated westwards; upwelling of asthenosphere above the subduction zone warmed the mantle, resulting in lithospheric thinning and a plateau-like u p l i f t . Steepened subduction also led to thickening of the lithosphere west of the Pliocene to Recent magmatic front and consequent subsidence in the S t r a i t of Georgia. Warming of a proposed cut-off Late Miocene shallow dipping slab induced phase changes and thermal expansion and resulted in moderate (1 km) u p l i f t of the southern Interior of B r i t i s h Columbia. 85 Introduction Heat transfer models have been developed to simulate the thermal evolution of mountain belts which experience u p l i f t and erosion and the consequent cooling of rocks. Such models can be constructed to s a t i s f y isotopic dates which record the time when the blocking temperature isotherms of the various geochronologic systems moved through the rocks (Harrison and Clarke 1979). In some cases, the models can be inverted to determine geologic and geophysical parameters which may be poorly known. Three parameters necessary to the formulation of accurate and r e a l i s t i c models are 1) u p l i f t and erosion rates (function of space and time), 2) sub-crustal (reduced) geothermal flux (function of space and time), and 3) the thermal e f f e c t of intrusions (function of size, shape, and temperature contrast). In areas that are experiencing u p l i f t without intrusive a c t i v i t y , the intrusive perturbations can be ignored. In addition, i f l a t e r a l variations in thermal conductivity, reduced heat flow and u p l i f t rate are small, a one-dimensional model w i l l s u f f i c e to simulate heat transfer. In t h i s paper a numerical model is presented which simulates heat flow in a column of the earth's crust, with exponentially declining heat production downwards, which i s experiencing time-dependent u p l i f t , temporal variations in the sub-crustal geothermal flux, changing surface temperatures, and non-equality between erosion and u p l i f t 86 giving r i s e to an elevated land surface. The model is then applied to various parts of the Coast Mountains of B r i t i s h Columbia to simulate post-intrusive syn- to late-orogenic (Cretaceous-early Tertiary) rapid cooling and u p l i f t followed by post-orogenic slow u p l i f t and most recently, late Neogene rapid u p l i f t giving r i s e to the present Coast Mountains. Estimates of paleo-uplift rates have been derived from geological data and f i s s i o n track geochronometry (Chapter 1). Under special circumstances the f i s s i o n track data can also provide values of paleo-geothermal gradients. Other boundary conditions and material properties are available from various published and unpublished sources. Tectonic and geothermal inferences derived from both ^fission track dating and thermal modeling are useful in evaluating geophysical processes in t e c t o n i c a l l y active regions, especially with regard to the mechanisms of large-scale u p l i f t . Geology of the Coast Mountains The Coast Mountains of B r i t i s h Columbia (see Roddick and Hutchison 1974, Monger et a l 1972, Chapter 1) are largely coincident with the Coast Plutonic Complex, a belt of g r a n i t i c plutonic rocks of Jurassic to Eocene age superimposed on a varied framework of stratigraphic rocks. Though some of the c r y s t a l l i n e rocks owe their o r i g i n to c r u s t a l shortening, the bulk of plutonic rock i s the 87 result of episodic intrusion of plutons that formed the roots of a west-facing magmatic arc of late Mesozoic to early Cenozoic age (Monger et a_l 1972). Plutonism and metamorphism culminated during the Eocene in the northern Coast Mountains, after which orogenic a c t i v i t y nearly ceased altogether. During the middle Cenozoic, the area north of 51°-52°N was probably bounded offshore by the Queen Charlotte transform f a u l t , whereas the region to the south had continuous, r e l a t i v e l y slow subduction and sporadic magmatic a c t i v i t y through the remainder of the Cenozoic, in much the same fashion as observed today (Berman and Armstrong 1980, Souther 1977). Except for a few l o c a l i t i e s , a l l of the c r y s t a l l i n e rocks in the Coast Mountains are of pre-Oligocene age. Basaltic volcanism in the Interior Plateau (Figure 1) coincided temporally with the late Neogene rapid u p l i f t of the Coast Mountains, which produced the present mountains (Mathews 1968). Characterising the thermal evolution and understanding the causes of this late Neogene event are the main goals of thi s paper. Thermal Modeling of Mountain Belts Applications of thermal models to geologic problems began with the study of thermal effects of intrusions. Jaeger (1964), Lovering (1935), and others have presented both theory and examples of this type. This paper concerns i t s e l f with geological situations where igneous intrusion i s not important, and consequently the flow of heat is 88 controlled predominatly by conduction upwards and influenced by u p l i f t and erosion. Situations where u p l i f t i s assumed constant can be described by the a n a l y t i c a l solution of Carslaw and Jaeger (1959, p.388). Clark and Jager (1969) presented an a n a l y t i c a l solution to the one-dimensional u p l i f t problem where heat production was assumed uniform and the i n i t i a l temperature d i s t r i b u t i o n had a simple mathematical form. This model was used in conjunction with isotopic dating, metamorphic pressure and temperature estimates, and present heat flow values to place constraints on the u p l i f t history in the Swiss Alps. Later Clark (1979) refined t h i s model to incorporate an exponential d i s t r i b u t i o n of radioactive heat sources in the crust, but retained a constant rate of u p l i f t . Woodhouse and Birch (1980) presented a s i m i l i a r model and applied i t to the apparent linear r e l a t i o n between surface heat production and surface heat flow (Lachenbruch 1968) . Numerical methods suffer from decreased accuracy of the solution but have the advantage that they can accomodate a wide range of space- and time- dependent parameters that are more r e a l i s t i c . Harrison and Clarke (1979) used a two-dimensional model that incorporated the effects of both igneous intrusion and u p l i f t , and they u t i l i z e d a variety of isotopic dating techniques to test their model. They did, however, retain a fixed subcrustal geothermal flux as well as constant surface 89 temperature and uniformly d i s t r i b u t e d heat production. Lee et a l (1980) have recently developed a f i n i t e difference scheme to model, in two dimensions, the additional effects of topography and l a t e r a l inhomogeneities in material propert ies . The Model Many previously published models incorporate one or more i n f l e x i b l e parameters ( i e . constant u p l i f t rate) which cause some divergence from r e a l i s t i c situations. Because u p l i f t rate (Chapter 1), average surface temperature (Rouse and Mathews 1979), and sub-crustal geothermal flux (Parrish 1980) are a l l functions of time, and reasonably well known, for the Coast Mountains of B r i t i s h Columbia, a one-dimensional f i n i t e difference model was developed that allowed temporal variations of these parameters. The model numerically solves the d i f f e r e n t i a l equation, 6 2V W 6V 1 6V = Ao exp(-(Z-Wt)/D) 6Z Z c 6Z a 6t ic where V i s temperature, Z i s depth below the average land surface, W i s u p l i f t rate with respect to sea l e v e l , a i s d i f f u s i v i t y , t i s time, Ao i s the i n i t i a l heat production of surface rocks, K is the conductivity, D i s the scale height of the exponential d i s t r i b u t i o n of heat production, and 6 represents the p a r t i a l derivative. The model incorporates an exponentially downwards decreasing d i s t r i b u t i o n 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 described by V(Z,0) = a + (Q*Z/ic) + ( D 2 A O / K ) ( 1-exp(-Z/D) ) where a is the i n i t i a l surface temperature and Q* is the reduced heat flow at depth. Lateral heterogeneities in material properties and in geothermal and u p l i f t parameters are assumed to have a negl i g i b l e e f f e c t on v e r t i c a l heat flow. In the models presented in this paper K and a are assumed constant throughout the f i n i t e difference grid; more complex models with s p a t i a l v a r i a t i o n in these two material properties would require data not presently available. Other parameters are allowed to vary with time (W,a,Q*) or space (Ao), according to data available for each l o c a l i t y modeled. The method of Crank-Nicolson i s used with the i m p l i c i t form of the f i n i t e difference equation as outlined by Carnahan et a l (1969, p.440-451). A system of M-1 equations is developed for each time step where M i s the number of depth grid points. The depth gri d has a fixed size and surface boundary condition, a(t) = surface temperature a lower boundary condition, Q*(t) = reduced heat flow 91 and u p l i f t v ariation, W(t) = u p l i f t rate, with respect to sea l e v e l . The system of equations re s u l t i n g from each time step forms a tridiagonal c o e f f i c i e n t matrix which i s readily solved by Gaussian elimination. In practice Q* and W are specified as step functions with incremental changes whereas a, the surface temperature, was chosen as a linear function of time. The depth of the f i n i t e difference grid was chosen as 40 km, approximately the thickness of normal continental crust. With usual values of the scale height, D, t h i s assures that heat production at the base of the grid i s <2% that of the surface, a condition consistent with the very low to n e g l i g i b l e heat • production thought to exist in the lowermost crust or uppermost mantle. The present elevated landscape of the Coast Mountains i s evidence that erosion has lagged behind u p l i f t in the recent geologic past. This has caused an increase in the average a l t i t u d e and a decrease in the average surface temperature according to the value of the atmospheric lapse rate. In the model, elevation of the land surface as a consequence of lagging erosion i s simulated by a decrease in the surface temperature and by a decrease in the rate at which material passes through the top of the f i n i t e difference grid, which represents the land surface. When the land surface r i s e s from lagging erosion, the denudation rate at the land surface (average altitude) i s the u p l i f t rate, W (with respect to sea l e v e l ) , mutiplied by the r a t i o of 92 denudation to u p l i f t . T h i s r a t i o of denudation to u p l i f t can be v a r i e d to simulate r e a l i s t i c c o n d i t i o n s . The l a t e Neogene onset of l a g g i n g e r o s i o n i s given by the parameter TNONEQ (i n Ma before p r e s e n t ) . The model has the c a p a b i l i t y of i n c o r p o r a t i n g almost a l l r e a l i s t i c s i t u a t i o n s with the requirement that l a t e r a l heat t r a n s f e r < < v e r t i c a l heat t r a n s f e r . Incremental step f u n c t i o n s f o r Q* and W cause u n r e a l i s t i c short term second order d i s c o n t i n u i t i e s i n geothermal parameters but reasonably approximate the u p l i f t h i s t o r y . The changes i n Q* at the base of the g r i d can only correspond to s i t u a t i o n s where c o n v e c t i o n dominates heat t r a n s f e r beneath the base of the c r u s t . T h i s appears a reasonable approximation, c o n s i d e r i n g the t e c t o n i c environment f o r the Coast Mountains r e g i o n . The numerical s o l u t i o n has been t e s t e d f o r accuracy a g a i n s t the a n a l y t i c a l s o l u t i o n of Woodhouse and B i r c h (1980) f o r uniform u p l i f t . A f t e r 50 Ma of thermal e v o l u t i o n , temperatures and heat flow values agree w i t h i n approximately 5% and 3%, r e s p e c t i v e l y . Both temperature and heat flow are lower f o r the f i n i t e d i f f e r e n c e s o l u t i o n because a small p a r t (1-2% f o r a 40 km depth g r i d ) of the heat p r o d u c t i o n given by the e x p o n e n t i a l d i s t r i b u t i o n i s at depths g r e a t e r than the base of the g r i d and i s thus excluded by the c o n d i t i o n of a s p e c i f i e d thermal g r a d i e n t at the base of the g r i d . The s i z e of the f i n i t e d i f f e r e n c e g r i d v a r i e s from 40 km by 80 Ma to 40 km x 50 Ma; time and depth increments 93 of 1% were chosen for both models. Fewer mesh steps cause increasing deviations from the analytic solution, and no s i g n i f i c a n t improvement was obtained by decreasing the mesh size . The FORTRAN program 'COASTMTN' i s l i s t e d in Appendix 1 . Discussion of Parameters Surface temperature variation Paleoc1imatic and geochronologic work by Rouse and Mathews (1979) in the Quesnel area in central B r i t i s h Columbia has established mean temperature variations throughout the Cenozoic which indicate a grossly linear decrease with time. This general pattern can be documented where Cenozoic deposits exist but can only be assumed for areas such as the Coast Mountains where such deposits are absent. In the models presented, mean annual paleotemperatures decrease from about 25°C 50 Ma ago to about 5-10°C at present. This decrease results in middle and late Cenozoic paleotemperature estimates similar to those of Rouse and Mathews (1979), although the retention of a linear decrease requires early Cenozoic and late Mesozoic paleotemperatures to be 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 surface temperature, however, w i l l be very small. Though Cenozoic fluctuations in paleotemperature are well documented (Rouse and Mathews 1979), the overal l pattern of a linear decrease 94 i s a good approximation and i s l i k e l y applicable for the entire region of the present Coast Mountains. Heat production, Ao The exponential downward decrease of heat production chosen for this model is based on both heat flow-heat production observations (Lachenbruch 1968, Roy e_t a_l, 1968) and a theoretical analysis of such a rela t i o n in areas of d i f f e r e n t i a l erosion (Lachenbruch 1970). Average surface heat production measurements for the Coast Mountains are provided by Lewis (1976 and unpublished data) and Lewis and Souther (1978), and cover most of the areas discussed in this study. Prior to Cenozoic erosion, surface heat production values, at least for the models, are required to have been greater (up to 5.6 kW/km3) than at present. The mean and standard deviation of presently available heat production data for the Coast Mountains from 50° to 55°N (about 150 measurements) i s 0.8±0.5 kW/km3 and range from 0.1 to 3.4 kW/km3 (Lewis 1976 and unpublished data, Lewis and Souther 1978). The mean heat production of plutonic rocks in the Intermontane Zone and Omineca C r y s t a l l i n e Belt of B r i t i s h Columbia to the east i s 2.6 kW/km3 but values range from 1.0 to 8.0 kW/km3 with 70% f a l l i n g between 2.0 and 5.5 kW/km3 (Lewis 1976). In many areas of the Coast Mountains where the depth of erosion i s not p a r t i c u l a r l y great, heat production i s generally less than 1.5 kW/km3. The models have been designed so that the f i n a l surface heat 95 production i s approximately the average value for the Coast Mountains (0.8-0.9 kW/km3). This requires i n i t i a l Ao values to be between 1.5 and 5.6 kW/km3, depending on the amount of u p l i f t modeled. Scale height, D The absence of s u f f i c i e n t heat flow measurements in the Coast Mountains precludes estimating the scale height, D, from the linear heat flow - heat production r e l a t i o n (Lachenbruch 1968). Consequently, t h i s value has been assumed to be 10 km, l i k e that for the Sierra Nevada of Ca l i f o r n i a (Roy e_t a l 1968). It is unlikely that t h i s value would greatly vary from place to place within any one geologic province. Conductivity, K, and d i f f u s i v i t y , a Values of conductivity have not been systematically measured for this study. Rather, since the rock composition in the Coast Mountains i s rather uniform quartz-diorite or granodiorite (Roddick and Hutchison 1974), and since both f i s s i o n track dates (Chapter 1) and modeling have been done for areas composed c h i e f l y of these intermediate g r a n i t i c rocks, a single average value has been applied to a l l models. A large number of conductivity measurements would be required to j u s t i f y a more detailed approach. Values of 2.5 kW/km°C (5.98X10- 3 cal/sec-cm-°C) and 32.0 km2/Ma (0.0101 cm 2/sec) have been chosen for conductivity and d i f f u s i v i t y , 96 respectively. These estimates are i d e n t i c a l to those chosen for the Sierra Nevada model of Lachenbruch (1968); the conductivity (2.5 kW/km°C) i s close to average values for measured rocks from the Coast Plutonic Complex of B r i t i s h Columbia (2.7±0.3 kW/km°C; Lewis 1976, Lewis and Souther 1978, Harrison et a_l 1979) and for quartz diorite-granodiorite in general (3.1 kW/km°C; Clark 1966). The d i f f u s i v i t y , a, equals K/PC where p i s 2.7 g/cm3 and c, the heat capacity, i s 0.22 cal/g°C (2.9 x 10- 1 3 W Ma/kg°C). Lapse rate An atmospheric lapse rate of -7°C/km has been chosen. This value f a l l s between the dry (no condensation) and saturated adiabatic rates of -lO°C/km and -5.5°C/km, respectively (Strahler 1973) and is probably appropriate to this f a i r l y humid region. E r o s i o n - u p l i f t balance Averaged over most of the Cenozoic, erosion and u p l i f t have 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 in the Coast Mountains began (Douglas ejt a_l 1970), r e l i e f and al t i t u d e were generally low. Only in the last several m i l l i o n years has rapid u p l i f t caused a marked increase in the average a l t i t u d e resulting from lagging denudation. The onset of lagging erosion as a result of accelerated u p l i f t , termed TNONEQ in the model, i s estimated to be from 5 to 10 Ma years ago. U p l i f t rates derived from 97 f i s s i o n track data (Chapter 1) when combined with values of present average a l t i t u d e of dif f e r e n t areas of the Coast Mountains, dictate the ra t i o of er o s i o n / u p l i f t , according to the r e l a t i o n , W x TNONEQ x (1-erosion/uplift) = average a l t i t u d e . The range of this r a t i o i s 0.53 to 0.86 which results in average al t i t u d e s of 0.6 to 1.8 km for the s p e c i f i c areas discussed in thi s paper. The values of mean al t i t u d e were calculated by averaging a l t i t u d e s of about 30 equally spaced points within a 15 km radius of each modeled l o c a l e . The model was designed to have the "model" average alt i t u d e i d e n t i c a l to the actual value for each area (see Table V). The resulting surface temperature i s lowered, in addition to the secular decrease, by an amount equal to the average a l t i t u d e multiplied by the lapse rate (-7°C/km). Reduced heat flow, Q* Values of reduced heat flow Q*, corresponding to the heat flux entering the base of the f i n i t e difference grid, vary in models from 25 to 60 kW/km2. These values f a l l between the extremes of very low heat flow in the arc-trench gap above active subduction zones (<20 kW/km2) to high values in areas of thinned lithosphere and active volcanic and tectonic a c t i v i t y (>50 kW/km2) such as the Basin and Range province of western United States (see Blackwell 98 1978). Q* for the models i s generally assigned a high value of about 50 kW/km2 in the syn-orogenic to immediately post-orogenic period to simulate both the higher than normal flux from the mantle as well as the dissipation of stored orogenic thermal energy caused by deformation, intrusion or both. In general, Q* and u p l i f t rates were adjusted to obtain a good f i t to f i s s i o n track dates, r e l i a b l e apparent u p l i f t rates, and paleo-geothermal gradients (Parrish 1980, Chapter 1), as well as present heat flow (Hyndman 1976). U p l i f t rate, W Approximate values of u p l i f t rate derived from f i s s i o n track dating for various areas of the Coast Mountains are from Chapter 1. As discussed in Chapter 3, apparent u p l i f t rates are equal to actual u p l i f t rates only when several conditions are met. The most c r u c i a l condition i s that the isotherms must remain at fixed distance with respect to the contemporary surface despite u p l i f t ; otherwise, misleading apparent u p l i f t rates can be produced. Sustained (10-20 Ma), r e l a t i v e l y low (less than 0.3 km/Ma) apparent u p l i f t rates from apatite f i s s i o n track data are accurate indicators of true u p l i f t rate since erosion and u p l i f t w i l l l i k e l y be in balance during t h i s time period, whereas higher values of zircon-derived apparent rates are not. The zircon data often overestimate the true u p l i f t rate and r e f l e c t downward relaxation of isotherms following orogenic a c t i v i t y . Late Neogene rapid u p l i f t both elevated the land 99 surface and increased the downward movement of isotherms leading to higher apparent u p l i f t rates. Derived apparent u p l i f t rates in such a case w i l l probably underestimate the rate of u p l i f t with respect to sea l e v e l , since denudation w i l l lag for the i n i t i a l period. Apatite rates w i l l , however, be within 25% of the value of surface denudation after only a brief (3 to 5 Ma) period (Chapter 3), and apatite f i s s i o n track data can thus be used to provide reasonable estimates of middle and late Cenozoic u p l i f t rates for use in thermal models. When elevation of the land surface due to lagging erosion is simulated, the model adjusts the value of surface denudation to be less than W, the u p l i f t rate with respect to sea l e v e l . The difference in rate between W and surface denudation i s the rate at which the land surface is elevated. In addition, when elevation of the land surface occurs, the surface temperature i s decreased as discussed under e r o s i o n - u p l i f t balance. Application to the Coast Mountains Objectives of modeling The objective of thermal modeling is a self-consistent quantitative thermal-uplift model that f i t s u p l i f t history, f i s s i o n track and other isotopic dates, paleogeothermal gradient measurements, and other geologic and geomorphic observations. The method i s applied to fi v e areas of the 100 Coast Mountains where f i s s i o n track, geologic data, and heat flow estimates are available. These areas are, from north to south, Kemano, northern King Island - Ocean F a l l s , Mount Waddington, central Bute Inlet, and Mount Raleigh (Figure 1). Presentation of the models The parameters of the models are l i s t e d in Table V, along with their SI units. The computational parameters tmax, t, Zmax, and Z are the maximum time, time step size, the maximum depth, and depth step size, respectively. The sequential values of reduced heat flow (Q*1, Q*2, Q*3) and u p l i f t rate (W1, W2, W3, W4) are changed at times tQ*1,tQ*2 and tW1,tW2, and tW3, respectively (in Ma before present). In the interest of c l a r i t y , u p l i f t rates are shown as positive numbers. The models and the relevant f i s s i o n track and K-Ar isotopic data for each area are presented in the format of Figure 15a. The u p l i f t path of rocks presently at the average a l t i t u d e at each l o c a l i t y is shown by the depth vs. time curve. Abrupt changes in u p l i f t rate are c l e a r l y evident. When combined in the model, the variations of W and Q* produce surface heat flow variations shown in the top part of Figure 15a. The value of paleo-heat flow (Q) at about 6 km depth i s shown in Figure 15a by short bars. The 6 km deep Q values are shown for comparison with the estimate of paleo-heat flow derived from f i s s i o n track T a b l e V. P a r a m e t e r s of Therma1 Mode 1s P a r a m e t e r Kemano N . K I n q I s 1 a n d - O c . F a 1 Is Mt . Wadd 1 nrjton C e n t r a l B u t e I n l e t Mt . R a l e i g h um t s m o d e l : A ( B . C . D ) * A ( B , C , D ) + kW/km' I n i t i a l Ao 3 0 2 5 3 8 1 5 5 6 a 32 0 32 0 32 0 32 0 32 0 km'/Ma it 2 5 2 5 2 5 2 5 2 5 kW/km" D 10 0 10 0 10 0 10 0 10 0 km a 24 0 24 0 25 0 31 0 32 0 °C d a / d t -O 28 - 0 28 - 0 28 - 0 28 - 0 28 °C/Ma tmax 50 0 50 0 55 0 75 0 80 0 Ma d t 0 5 0 5 0 55 0 75 0 8 Ma Zmax 40 0 40 0 40 0 40 0 40 0 km dz 0 4 O 4 0 4 0 4 0 4 km 0*1 50 0 35 0 50 0 30 0 50 0 l-W/km' t 0 * 1 45 0 45 0 45 0 -- 60 0 Ma bp 0*2 35 0 25 0 35 0 30 0 35 0 kW/km' t 0 * 2 15 0 20 0 20 0 -- 20 0 Ma bp 0*3 50 0 60 0 50 0 30 0 50 0 kW/km' W1 1 2 0 20 0 9 0 065 1 0 km/Ma tW1-2 45 0 30 0 45 0 -- 70 0 Ma bp W2 0 18 0 14 0 2 0 065 0 2 km/Ma t W 2 - 3 35 0 10 0 35 0 - 50 0 Ma bp W3 0 08 0 62 0 1 1 0 065 0 06 km/Ma tW3-4 10 0 6 0 5 0 5 0 7 0 Ma bp W4 0 24 0 30 0 75 0 47 0 6 km/Ma l a p s e r a t e - 7 0 - 7 0 - 7 0 - 7 0 - 7 0 c C/km e r o s l o n / u p l 1 f t 0 58 0 86 0 53 0 53 0 6 TNONEQ 10 0 10 0 5 0 5 0 7 0 Ma bp s u r f . a l t . 1 1 0 0 6 1 8 1 10 1 73 km a c t u a l s u r f . a l t . » 1 0 0 6 1 8 1 08 1 7 km f i n a l A o ' 0 98 0 87 0 75 0 84 0 86 kW/km' * Model 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 * Model C : same p a r a m e t e r s as 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% * Model D: same p a r a m e t e r s as 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 f rom the m o d e l ; a l l o t h e r p a r a m e t e r s a r e i n p u t ' measured 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 KEMANO Surface Heat Flow.Q (kW/km2) 8 0 60 Depth (km) 300| Temperature 2001- (°C) 100 I Fi88 T ot ion track estimate paleo-heat flow Model A Model B Heat flow at 6 km Model A Model B • — Sea level curve ^ ^ - ^ 2.0 km curve Zircon Apatite —• •- Sea level dates 2.0 km dates a) 50 40 30 20 10 0 Time Before Present (Ma) Surface 100 Heat Flow.Q (kW/km 2)eo Depth (km) Temperature <°C) Model C Model D ' * — ' i Heat flow at 6 km Model C Model D Sea level curve 2.0 km curve Symbols same as above 50 40 30 20 10 0 Time Before Present (Ma) Figure 15a. Thermal evolution diagram for the Kemano area. Shown are surface heat flow (Q), depth, and tempera- ture vs. time curves which are derived from models with 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 vs. time curve r e s u l t from increments i n u p l i f t rate. F i s s i o n track dates and estimates of paleo- heat flow are 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 represents the cooling curve of samples presently at sea l e v e l ; the bottom curve represents the curve for rocks that are now at 2.0 km a l t i t u d e . Since the cooling curve must pass through the f i s s i o n track dates at the appropriate blocking temperature (105°C for apatite, 175°C for zircon), the f i t of each of these two cooling curves to their respective f i s s i o n track dates represents a consistent modeling of u p l i f t rates, reduced heat flow, and f i s s i o n track data. Other isotopic dates .and their respective blocking temperatures can also be used, when available, to place further constraints on the models (see Harrison e_t a l 1979). A l l f i s s i o n track data are from Chapter 1 and other isotopic dates are from the Geological Survey of Canada (Woodsworth 1979, Baer 1973, Wanless et a l 1979) . In order to assess the s e n s i t i v i t y of the model to variations in the parameters, three other models are shown on Figure 15a in addition to the best f i t of model A. Model B i s i d e n t i c a l to model A except that Q* i s constant at 40 kW/km2. Though the cooling curves of model B f a l l c l e a r l y off the centroid of the f i s s i o n track dates from Kemano, they do f i t the dates within error. In the lower portion of Figure 15a, models C and D represent i d e n t i c a l parameters as model A except that they 1 04 have u p l i f t r a t e s u n i f o r m l y i n c r e a s e d and de c r e a s e d 20% from model A. I t i s apparent t h a t a l t h o u g h the heat f l o w d a t a a t 6 km depth i s c o n s i s t e n t w i t h e i t h e r model C or D, n e i t h e r of the models can s a t i s f y the f i s s i o n t r a c k d a t a . P r e s e n t a t i o n of each of models A, B, C, and D i l l u s t r a t e s t h a t u p l i f t r a t e v a r i a t i o n s of a t most ±15% can be t o l e r a t e d and s t i l l remain c o n s i s t e n t w i t h f i s s i o n t r a c k d a t a . Changes i n Ao w i l l have a s m a l l e f f e c t on the r e s u l t a n t c u r v e s as l o n g as those changes are not l a r g e and i n c o n s i s t e n t w i t h p r e s e n t v a l u e s of Ao. Changes i n K and c w i l l have c o n s i d e r a b l e e f f e c t on the models, but s i n c e the v a r i a b i l i t y of t h i s parameter i s l i m i t e d i n q u a r t z d i o r i t e - g r a n o d i o r i t e , such changes w i l l not 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 from the models. Kemano Dated r o c k s from Kemano a r e of f o l i a t e d q u a r t z d i o r i t e a t P o w e l l Peak, of Eocene or p r o b a b l y o l d e r age. The r o c k s a r e s i m i l i a r t o o t h e r r o c k s p r e s e n t w i t h i n the c e n t r a l g n e i s s complex of the n o r t h e r n Coast Mountains (Roddick and H u t c h i s o n 1974). No K-Ar da t e s a r e a v a i l a b l e i n the immedite v i c i n i t y of Kemano, a l t h o u g h most d a t e s i n the g n e i s s complex f a r t h e r northwest a r e Eocene. F i s s i o n t r a c k d a t e s s p r e a d over the 2 km of a l t i t u d e of P o w e l l Peak range from 24 t o 34 Ma f o r a p a t i t e and 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 of p a l e o g e o t h e r m a l g r a d i e n t 35 Ma ago i s 26+4/-6°C ( P a r r i s h 1980). 1 05 Model A of Figure 15a i s the best thermal model within the constraints of the data, although i t i s not a unique solution. It incorporates variations in Q* from 35 to 50 kW/km2 and u p l i f t rate variations from 0.08 to 1.2 km/Ma. Rapid u p l i f t and high Q* in the Eocene following Eocene orogenic a c t i v i t y ( H o l l i s t e r 1979) gave way to low rates of u p l i f t and Q* in the middle Cenozoic during a period of tectonic s t a b i l i t y . U p l i f t rates and Q* are increased in the late Neogene at about 10 Ma, consistent with both f i s s i o n track data, physiographic evolution (Chapter 1) and r e l a t i v e l y high heat flow along the eastern part of the Coast Mountains (Hyndman 1976, Mathews 1972b, T. Lewis unpublished data). On the basis of this model, K-Ar b i o t i t e dates near Kemano would be expected to be about 45-50 Ma, assuming a blocking temperature of 280±40°C for b i o t i t e (Harrison and McDougall 1979). The present surface heat production of 0.98 kW/km3 given by the model (see Table V) is v i r t u a l l y i d e n t i c a l to the measured value of 1.0 kW/km3 (T. Lewis unpublished data). Northern King Island - Ocean f a l l s The geology of th i s region (Figure 1) has been described by Baer (1973). The rocks consist of plutons and subordinate metamorphic rocks which y i e l d K-Ar b i o t i t e dates of 57-87 Ma. Preliminary Rb~Sr work on some of these plutonic rocks suggests they are mid-Cretaceous in age (R.L.Armstrong and R.R.Parrish unpublished data). Fission 106 track dates spread over 2 km of alt i t u d e vary from 8 to 24 Ma for apatite and 34 to 39 Ma for zircon. A f i s s i o n track-derived value of paleogeothermal gradient 20 Ma ago i s 17±2°C/km (Chapter 1) which represents a paleo-heat flow of about 43 kW/km2, assuming K=2.5 kW/km°C (5.98x10-3 cal/sec-cm-°C). This is s i g n i f i c a n t l y lower than the value derived for the Kemano area for 35 Ma ago. Models A, B, C, and D of Figure 15b are s i m i l i a r in format to those of Figure 2a. Model B i s id e n t i c a l to the best f i t model A except that Q* i s a constant 40 kW/km2. It is apparent that model B i s consistent with neither the fi s s i o n track derived value of paleo-heat flow nor a l l of the f i s s i o n track data. Thus, i t appears, as model A suggests, that Q* was s i g n i f i c a n t l y less than 40 kW/km2 for most of the middle Cenozoic. Models C and D of Figure 15b, have u p l i f t rates of ±20% of model A, but neither i s consistent with f i s s i o n track data. Their heat flow curves, however, are very similar and f a l l within the error l i m i t s of the f i s s i o n track-derived paleo-heat flow determination. The f i n a l present surface heat production from model A i s 0.87 kW/km3, similar to present values of 0.9-1.0 kW/km3 T.Lewis, unpublished data). Model A, which f i t s a l l of the data very well, involves u p l i f t rates that are very similar to those derived d i r e c t l y from f i s s i o n track dating of apatite. These rates are moderate through the middle Cenozoic (0.14-0.2 km/Ma) but increase in the Late Miocene. Geomorphic arguments (Chapter NORTHERN KING ISLAND - OCEAN FALLS AREA Model A Model B Surface Heat Flow, Q (kW/km2) 6 0 Depth (km) 300 Temperature |E=: 200 (°C) 100 Heat flow at 7 km Fission track estimate of paleo-heat flow Sea level curve 2.0 km curve Sea level dates 2.0 km dates 40 30 20 10 0 Time Before Present (Ma) Model C Model D Surface Heat Flow, Q (kW/km2) 5 0 Depth(km) Temperature r - ~ ^ 200 1 (°C) 100 h Heat flow at 7 km Sea level curve 2.0 km curve Symbols same as above b) 40 30 20 10 0 Time Before Present (Ma) Figure 15b. Thermal evolution diagram for the King Island- Ocean F a l l s area. 108 1) suggest that the most rapid late Neogene u p l i f t in this region was in the Late Miocene, followed by slower Pliocene to Recent rates. This slowing of rate has been incorporated in models for t h i s region (see Table V). Mount Waddington Mount Waddington, with an al t i t u d e of 4 km, is the highest area in the Coast Mountains. It is composed of granitoid gneiss, migmatite, and more massive plutonic rock (G. Woodsworth, unpublished data) at least as old as Paleocene. K-Ar dates (calculated with constants l i s t e d in Table IV) range from 55 Ma for hornblende to 48 Ma for b i o t i t e . L i t t l e else is known about these rocks. Fission track dates from l o c a l i t i e s spread over 3 km in a l t i t u d e range from 6 to 32 Ma for apatite, and 19 to 52 Ma for zircon (Chapter 1). Apatite dates c l e a r l y show alti t u d e c o r r e l a t i o n but zircon dates do not. A r e l i a b l e estimate of paleogeothermal gradient has been impossible to obtain, but present heat flow, uncorrected for u p l i f t , is probably 70 to 90 kW/km2 (shown in Figure 15c). This value of Q was derived by adding a 24% u p l i f t correction, o r i g i n a l l y subtracted from heat flow measurements by Hyndman (1976), to the average Q value (about 63 kW/km3) for the higher heat flow region of the Coast Mountains, of which Mount Waddington is part. The best f i t thermal model, shown in Figure 15c, has been constructed by early Cenozoic u p l i f t rates which f i t 109 the K-Ar and zircon f i s s i o n track data. From 35 Ma to the present, apatite data are accurate enough to place good constraints on the u p l i f t rates. The rates are rapid (0.9 km/Ma) in early Cenozoic time (55-45Ma) resulting in rapid cooling of the rocks. Middle Cenozoic slow u p l i f t (0.11 km/Ma) i s s i m i l i a r to apparent u p l i f t rates documented by f i s s i o n track data (0.09 km/Ma). Late Cenozoic (post-5 Ma) rapid u p l i f t (0.75 km/Ma) i s required not only to f i t apatite f i s s i o n track data but also to explain the physiography of the region. Q* has been varied between 35 and 50 kW/km2 to s a t i s f y dates and the estimate, a l b e i t imprecise, of present heat flow. The eros i o n / u p l i f t r a t i o has been adjusted to result in an average a l t i t u d e of 1.8 km. The model suggests that rapid orogenic u p l i f t during the Paleocene-Eocene was followed by very low middle Cenozoic rates implying low r e l i e f and alt i t u d e then. Late Neogene u p l i f t appears to have been i n i t i a t e d very recently producing the present mountains. Central Bute Inlet - Mount Raleigh The l a s t two areas modeled are from the Bute Inlet area of the southern Coast Mountains (Figure 1). It w i l l be shown that their respective geochronology, thermal models, and tectonic h i s t o r i e s are quite d i f f e r e n t and have implications for the orogenic history of this region. 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 is modified from Hyndman (1976). 111 Bute Inlet are 90-100 Ma (Roddick and Woodsworth 1977), and fi s s i o n track dates range from 42 to 75 Ma for apatite and 73 to 100 Ma for zircon, depending on location and a l t i t u d e . The present heat flow in Bute Inlet, uncorrected for u p l i f t , is about 40 to 75 kW/km2 (Hyndman 1976, Figure 15e). This low heat flow results from the heat sink effect of the descending oceanic plate during subduction, and since subduction has probably continued, apparently uninterrupted, since the late Cretaceous (Monger et a l 1972), th i s low heat flow has l i k e l y persisted for some time. Apparent u p l i f t rates derived from f i s s i o n track data are less than 0.1 km/Ma since 75 Ma ago except for the Neogene, when rates increased to about 0.4 km/Ma (Chapter 1). In.the model (Figure 15e) the cooling curves for sea leve l and 2.5 km f i t the apatite f i s s i o n track data when Q* is a constant 30 kW/km2. U p l i f t rates are approximately those derived from f i s s i o n track dating. In addition, the predicted present heat flow i s i d e n t i c a l to values measured in Bute Inlet by Hyndman (1976), once the u p l i f t correction is added. In th i s example, resultant surface heat production and surface a l t i t u d e are 0.84 and 1.10 km, respectively, very close to actual values of about 1.0 kW/km3 and 1.08 km. The rocks of thi s area have been u p l i f t e d only about 5 km since 75 Ma ago. The Mount Raleigh area (Figure 1), about 30 km east of the previous example, i s underlain by metamorphic and plutonic rocks of mostly Mesozoic age. Plutons of Late MOUNT RALEIGH AREA Surface 1 2 0 Heat Flow, Q 1 0 0 (kW/km2)eo Depth (km) P r e s e n t h e a t f l o w e s t i m a t e b a s e d o n d a t a o f H y n d m a n (1976) 2 - 4 K b D a t a f r o m W o o d s w o r t h (1979) E s t i m a t e o f d e p t h f r o m W o o d s w o r t h (1979). d) — — S e a l e v e l c u r v e 2.8 k m c u r v e (2.7) A l t i t u d e o f d a t e • A p a t i t e f i s s i o n t r a c k • Z i r c o n f i s s i o n t r a c k • B i o t i t e K - A r • M u s c o v i t e K - A r • H o r n b l e n d e K - A r (0.2) CENTRAL BUTE INLET Surface so Heat Flow. • 60 (kW/km2)40F Depth (km) 1 ^ P r e s e n t h e a t f l o w e s t i m a t e f r o m H y n d m a n (1976) # S e a l e v e l c u r v e a n d a p a t i t e d a t e —O" 2.5 km c u r v e a n d a p a t i t e d a t e e) Time Before Present (Ma) Figures 15d and 15e. Thermal evolution diagrams for the Mount Raleigh (d) and central Bute Inlet (e) areas. 1 1 3 Cretaceous-Paleocene age intrude rocks previously metamorphosed during the Late Cretaceous, about 70-90 Ma ago (Woodsworth 1979). A 55-60 Ma plutonic suite, the Bishop River Pluton, was emplaced at pressures of about 2-3 kb (200-300 MPa) following metamorphism to pressures of 5-6 kb (500-600 MPa) at least 80 Ma ago (Woodsworth 1979). K-Ar dates (calculated with constants l i s t e d in Table IV) on metamorphic rocks range from 71 Ma (hornblende) to 66-68 Ma (muscovite). A K-Ar b i o t i t e date on the Mount Gi l b e r t pluton is 71 Ma. Neither heat production nor heat flow data are available but values of heat flow, uncorrected for u p l i f t , of 70-90 kW/km2 would be expected in the region (Hyndman 1976). The only zircon f i s s i o n track date available i s 49 Ma from 2.8 km a l t i t u d e . Apatite dates from 2.8 and 0.2 km alt i t u d e s are 36 Ma and 7.8 Ma, respectively, implying an apparent u p l i f t rate of about 0.1 km/Ma in the middle Cenozoic which increased to over 0.4 km/Ma in the late Neogene (Chapter 1). As in the Mount Waddington model, the 75-50 Ma u p l i f t rates were chosen to f i t the K-Ar and f i s s i o n track dates in addition to the metamorphic pressure estimates (Figure 15d). Rates of 1 km/Ma or more are required to f i t t h i s data. The lower rates in the middle Cenozoic (0.06-0.2 km/Ma) are close to those derived from f i s s i o n track dating and f i t the apatite dates c l o s e l y . The values of Q* are consistent with estimates of present heat flow. In contrast to Central Bute Inlet, the Mount Raleigh .114 area has experienced 15 km of u p l i f t since 75 Ma. Because the two areas are only about 30 km apart, considerable d i f f e r e n t i a l u p l i f t must have occurred in the Late Cretaceous and Paleocene. The culmination of deformation and plutonism in the two areas is c l e a r l y d i f f e r e n t and implies that orogenic a c t i v i t y and u p l i f t have progressed from west to east across the Coast Plutonic Complex. This is consistent with patterns of K-Ar dates, geologic data, and age of plutons elsewhere (Roddick and Hutchison 1974). Discussion of Models The heat flow models integrate u p l i f t rate, observed thermal history, heat production, heat flow and inferred parameters such as Q*. Cooling rate data for rocks is d i r e c t l y available through geochronometry, and when incorporated into models, estimates of u p l i f t rates can be derived when f i s s i o n track-derived u p l i f t rate data are either not available or suspect (as for zircon dates). In several of the models (Mount Waddington, Mount Raleigh, Kemano), i n i t i a l syn-orogenic to post-orogenic u p l i f t rates of about 1 km/Ma are consistent with petrology and K-Ar geochronometry. This value (1 km/Ma) is close to that considered applicable for the Alps (Schaer et a l 1975) and other active mountain be l t s . The K-Ar and f i s s i o n track data and the thermal models indicate that sequential plutonic emplacement, cooling, and subsequent rapid u p l i f t progressed broadly from west to east 1 1 5 across the Coast Plutonic Complex. The Alaska border to the Kemano area was characterized by Eocene orogenic culmination. The modeling of heat flow and u p l i f t confirms the difference in middle Cenozoic u p l i f t rates between northern (0.1-0.2 km/yr from 52° to 55°N) and southern (0-0.1 km/yr from 50° to 52°N) Coast Mountains (Chapter 1). The values of apatite-derived apparent u p l i f t rates are also found to be reasonable estimates for use in thermal models. As indicated in Chapter 3 the use of zircon date-altitude relations can overestimate rates of u p l i f t and considerable caution must be used in their interpretation. The reduced heat flow Q*, the most elusive of geothermal parameters, has been estimated u t i l i z i n g a combined f i s s i o n track-heat flow modeling approach. To remain consistent with f i s s i o n track data Q* must l i e between 25 and 60 kW/km2 for a l l areas. In the Ocean Falls-King Island example (Figure 15b, Table V) Q* can be shown to have increased by a factor of 1.5 from 20 Ma ago to the present. Low surface heat flow 20 Ma ago, during a period of re l a t i v e tectonic s t a b i l i t y , increased in the Late Miocene due to the sub-crustal passage of a 'hot spot' related to the Anahim volcanic belt (Bevier et a_l 1979). Considerable lithospheric thinning may have resulted from this event. Values of 25-60 kW/km2 for Q* f a l l between the extremes of subduction related arc-trench gap low heat flow (about 20 1 1 6 kW/km2) and extension-related high heat flow (60 kW/km2) li k e the present Basin and Range (Roy et a_l 1968). The consistency between heat flow models, f i s s i o n track dates for apatite and zircon and their derived apparent u p l i f t rates, and estimates of paleogeothermal gradients is thus very s a t i s f y i n g . Causes of U p l i f t Three main u p l i f t events in the Coast Mountains must be addressed. They are the syn-orogenic to late-orogenic u p l i f t of Late Cretaceous-Eocene age, the subsequent middle Cenozoic low u p l i f t rates and their s p a t i a l variations, and l a s t l y , the late Neogene rapid u p l i f t largely responsible for the present mountains. Orogenic culmination and u p l i f t Orogenic plutonic a c t i v i t y in the Coast Mountains occurred from middle Cretaceous to Eocene time. The relevant geological data and tectonic interpretations are discussed by Monger et a l (1972), Roddick and Hutchison (1974), Woodsworth (1979), Woodsworth and Tipper (1980), and H o l l i s t e r (1979). In general the Coast Plutonic Complex represents an eroded volcanic arc above a subduction zone which consumed the Farallon plate. The culmination of events in the Eocene from Alaska south to the Kemano area involved large u p l i f t , the development of a wide and unusually high-grade a x i a l metamorphic belt, and voluminous Eocene 1 17 intrusive rocks, and resulted in synorogenic crustal thickening. Plutonic underplating may also have contributed to t h i s episode. Present c r u s t a l thickness in the area is approximately 30 km (Berry and Forsyth 1975), and since surface rocks were metamorphosed during the Eocene to pressures of up to 6-9 kb (600-900 MPa, H o l l i s t e r 1979), Eocene c r u s t a l thickness must have exceeded 50 km. Such a te c t o n i c a l l y thickened crust would have responded i s o s t a t i c a l l y by rapid u p l i f t and as erosion proceded, cooling of rocks. Though the core of thi s Eocene orogenic belt has not been modeled in this study, areas near the periphery of the Eocene metamorphic core zone such as Kemano and farther south near Mount Waddington experienced Eocene u p l i f t rates of about 1 km/Ma; their tectonic s i t u a t i o n was l i k e l y related. U p l i f t rates in the core of the belt may have been several times t h i s rate (perhaps >10 km/Ma, H o l l i s t e r 1979), more comparable to rates presently experienced in the southern Alps of New Zealand (Wellman 1979). Plutonic underplating may also have contributed to this c r u s t a l thickening episode. This Eocene event was l i t t l e f e l t in the southern Coast Mountains which lacked Eocene plutonism and large-scale Eocene deformation and metamorphism. Eocene orogenic events in southern B r i t i s h Columbia were concentrated in the Intermontane Zone and Omineca C r y s t a l l i n e Belt (Ewing 1980). Following the Eocene, nearly a l l of the B r i t i s h Columbia mainland was quiescent u n t i l the late Neogene 118 (Monger et a l 1972). Middle Cenozoic u p l i f t The a x i a l region of the Coast Mountains from latitude 52°-55°N experienced u p l i f t at rates of 0.1-0.2 km/Ma during the period from 30 to 10 Ma (Chapter 1). In contrast, f i s s i o n track data and modeling indicate that rates in the south were no more than 0.1 km/Ma, and perhaps near zero. There is evidence that paleogeothermal gradients were low in the region near Ocean F a l l s (l7°/km), which may have been representative of the northern (52° to 55°N) region as a whole. Thus, the more rapid u p l i f t in the north was probably not related to the expansion of crust or mantle. The igneous a c t i v i t y of the Anahim Volcanic Belt largely postdates t h i s middle Cenozoic period of u p l i f t . The coincidence of most rapid (0.1-0.2 km/Ma) Miocene u p l i f t rates and maximum middle Cenozoic t o t a l u p l i f t (Chapter 1) 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 thickened Eocene crust in the northern area would have induced rapid u p l i f t during orogenesis and diminishing u p l i f t afterwards. There would l i k e l y have been a si g n i f i c a n t crustal root supporting a mountain system for some time after culmination of tectonic a c t i v i t y . Since the u p l i f t rates in the region were probably quite steady (as in the Ocean Falls-King Island area), i t i s suggested that the middle Cenozoic u p l i f t was simply related to the gradual 1 19 erosion of the mountains bouyed by such a c r u s t a l root. Erosion in this case induces additional u p l i f t at a slowly decreasing rate in the absence of any thermal anomaly. Over a time of several tens of mi l l i o n s of years, such a root would e s s e n t i a l l y disappear. A mature mountain physiography has been documented in the Bella Coola region prior to the eruption of 10 Ma basalts (Chapter 1, Baer 1973), and this was probably the same kind of topography that persisted throughout the middle Cenozoic in the central and northern Coast Mountains (52° to 55°N). In contrast, u p l i f t rates in the southern Coast Mountains (50° to 52°N) during the middle Cenozoic were low and l i t t l e r e l i e f is preserved on the erosion surface beneath basalts of the same Late Miocene age. F i s s i o n track dates place tight constraints on the t o t a l amount of middle Cenozoic u p l i f t and erosion. It seems l i k e l y that the southern area was very stable, lacked a cr u s t a l root, and had l i t t l e r e l i e f during most of the Cenozoic, except in areas near active volcanoes (Chapter 1). Late Neogene u p l i f t Following the prolonged middle Cenozoic period of r e l a t i v e tectonic quiescence, late Neogene u p l i f t , e specially in the south, elevated the crust up to 3 km. U p l i f t rates in the south exceeded 0.5 km/Ma for at least the last 5 Ma (Chapter 1), and geologic evidence alone suggests t o t a l u p l i f t of 2-3 km. The southern segment has 120 been above an active subduction zone for the entire Neogene and probably since 100 Ma ago. The northern segment of the Coast Mountains from 52-55°N had an average a l t i t u d e and l o c a l r e l i e f of about 1.0 km and 1.5 km, respectively, prior to the eruption of 10 Ma basalts (Chapter 1). Fi s s i o n track data and heat flow models for the axi a l zone suggest additional late Neogene u p l i f t of about 2-3 km. Since the average a l t i t u d e i s lower in the north than in the south, i t is suggested that t h i s additional u p l i f t occured mainly in the Late Miocene rather than the Pliocene, as i s the case in the south. The late Neogene u p l i f t in the north, outside the ax i a l region, is thus somewhat less than in the south. Since the northern Coast Mountains (north of 52°N) are adjacent to a transform fault plate boundary whereas the southern Coast Mountains are opposite a subduction zone, i t appears that plate setting may have played an important role in the mechanism of u p l i f t in the two regions. U p l i f t in the north (52° to 55°N) In part, the late Neogene u p l i f t in the north i s a continuation of the middle Cenozoic u p l i f t pattern and as such requires no special explanation. An acceleration of the rate from 0.2 km/Ma to 0.4 km/Ma in the Bella Coola region (Chapter 1) may be related in part to the Anahim volcanic belt, or hot spot, which passed beneath the Coast Mountains from 14-8 Ma ago (Bevier et a l 1979). This thermal anomaly, 121 which may have coincided with the Early Miocene t r i p l e junction between Vancouver Island and Queen Charlotte Islands (Yorath and Chase 1981), was l i k e l y a broad zone of asthenospheric upwelling that interacted with the overlying lithosphere. Yorath and Chase (1981) suggested that thermally-induced, crustal-pervasive faults i n i t i a t e d basin development and volcanism in both Queen Charlotte Islands and southern Queen Charlotte Basin. As the hotspot passed beneath the c r y s t a l l i n e rocks of the Coast Mountains, the thickness of the g r a n i t i c crust probably inhibited s i m i l i a r f a u l t i n g and the thinning of the lithosphere was probably accomplished e n t i r e l y within the lithospheric mantle beneath the crust. If the Early Miocene (20 Ma) lithospheric thickness was 100 km corresponding to an approximate surface heat flow of about 40 kW/km2, then thinning of the lithosphere to 40 km by the Anahim hot spot would induce about 1 km of u p l i f t by thermal expansion. This c a l c u l a t i o n assumes that the c o e f f i c i e n t of thermal expansion i s 30 x 10" 6/°C (an average for f o r s t e r i t e in Skinner 1966), and the average r i s e in temperature over the 60 km of heated lithosphere was 500°C. If this were accomplished over a period of a few m i l l i o n years, the u p l i f t rates would be approximately those observed. Moreover, an increase in sub-crustal heat flow about 15 Ma ago i s required by f i s s i o n track dates, past and present heat flow data, and thermal models (Table V and Figure 15b). Whether the transverse path of the Anahim hot 1 22 spot could induce a longitudinal NW-trending u p l i f t in the northern Coast Mountains is an open question; i f there was any remanent Eocene thermal structure the hot spot may have enhanced i t . At any rate there i s clear heat flow evidence that a thermal perturbation occurred in the Bella Coola area of the northern Coast Mountains at about the same time as somewhat increased regional u p l i f t , and the two are probably related. U p l i f t in the south (49° to 52°N) The size of the mountains in western B r i t i s h Columbia in i t s e l f suggests analogy to the world's great mountain systems such as the Himalaya, European Alps, Alaska Range, and the Southern Alps of New Zealand. For several reasons, however, the orig i n of the u p l i f t in the Coast Mountains cannot be the same as in these other systems. Most great mountain systems owe their high elevation to cru s t a l thickening (Walcott 1978, LeFort 1975). In the examples c i t e d , the u p l i f t i s a direct response to large scale horizontal motions of c r u s t a l plates, and present plate boundaries l i e within the mountain belts as deduced by seismicity and d i s t r i b u t i o n of active f a u l t s . In the Coast Mountains, the lack of seismicity (Milne et a l 1978), absence of thickened crust (Berry and Forsyth 1975) and present structural i n t e g r i t y strongly suggest that the cause of the u p l i f t must l i e within the upper mantle and that the u p l i f t i s not the result of ongoing cr u s t a l 1 23 shortening. The pattern of u p l i f t i s related to various geological and geophysical features. The 2-3 km late Neogene u p l i f t in the southern (50° to 52°N) Coast Mountains (Chapter 1, Mathews 1968) is d i r e c t l y opposite the plate boundary offshore where subduction is occurring. However, the pattern of late Neogene u p l i f t i s less intense in southern Washington and Oregon adjacent to the main part of the Juan de Fuca plate. One interesting and probably important correlation i s that the u p l i f t began during a pronounced hiatus in arc volcanism in the Late Miocene and Pliocene. The youngest volcanic event within the Pemberton Volcanic Belt on the east side of the Coast Mountains is about 8 Ma (Berman and Armstrong 1980), and the oldest within the Garibaldi Volcanic Belt to the west i s about 2 Ma (Bevier et a l 1979). The westward migration of the magmatic front to produce the Garibaldi belt represents a marked change in the volcanic pattern. Thermal and geophysical data indicate that the upper mantle opposite the subduct ion-related plate boundary is d i f f e r e n t from that farther north adjacent to the Queen Charlotte transform f a u l t . Upper mantle Pn wave v e l o c i t i e s are 7.8 km/sec in both the southern Coast Mountains and the southern Interior in contrast to more normal continental v e l o c i t i e s of 8.1 km/sec farther north (Berry and Forsyth 1975, Cumming et a_l 1979). Heat flow data in Jervis and Bute Inlets and northeast of Vancouver indicate a sharp 1 24 tr a n s i t i o n from low fore-arc (<40 kW/km2) to high arc and back-arc values (>60 kW/km2) near the present volcanic front (Hyndman 1976, Jessop and Judge 1971). Heat flow data are unfortunately sparse in central and northern B r i t i s h Columbia. The heat flow data (>60 kW/km2) and seismic v e l o c i t i e s (7.8 km/sec) in southern B r i t i s h Columbia indicate an upper mantle with moderately low density and f a i r l y high temperature, probably corresponding to hot, s o l i d p e r i d o t i t e . In the southern Interior a d i s t i n c t mantle low ve l o c i t y layer (MLVL) i s c l e a r l y present and l i e s beneath a thin mantle l i d (Wickens 1977). A well-developed MLVL is not observed beneath the Coast Mountains (wickens 1977). The lithospheric thickness in the southern Interior can be inferred to be about 50-80km, but i t may be considerably more (>100 km) beneath both the northern Interior and Coast Mountains (Wickens 1977). Crustal thickness (Figure 16), derived from seismic refraction and ray tracing, increases eastwards from 25 to 40 km beneath the southern Coast Mountains (Berry and Forsyth 1975, Wickens 1977) and across the southern Interior from about 33 km just east of the Coast Mountains to over 40 km near the Rocky Mountain Trench (Cumming et a_l 1979). The cr u s t a l thickness gradient in the Coast Mountains i s cl e a r l y r e f l e c t e d in the gradient of Bouguer gravity, shown in Figure 16. The close c o r r e l a t i o n between cru s t a l thickness, regional a l t i t u d e , and Bouguer gravity and the lack of appreciable seismicity in the Coast Mountains 125 Figure 16. Gravity anomalies and c r u s t a l thickness i n the Coast Mountains area. Gravity anomalies are bouguer on land and free a i r at sea, and are from the Earth Physics Branch (1980). Crustal thickness i s from Berry and Forsyth (1975). 1 26 strongly suggests that the topography is i s o s t a t i c a l l y compensated (Stacey 1973). If the c r u s t a l thickness values of Berry and Forsyth (1975) are correct, the mantle density must decrease southwards to explain the lower gravity and higher average a l t i t u d e of the southern (50° to 52°N) Coast Mountains which have equally thick crust as the northern part (52° to 55°N). Changes in the geometry and motion of oceanic plates as deduced from marine geophysical data can also be compared with the u p l i f t history. Riddihough (1977) has suggested that important changes occurred about 4-5 Ma ago in the interaction between the American, Juan de Fuca, and Explorer plates. Prior to about 5 Ma, the Explorer plate was being subducted beneath the f u l l extent of the southern Coast Mountains of B r i t i s h Columbia (Figure 17a). Subsequently, the Explorer-Juan de Fuca boundary, the Nootka fa u l t zone, became more active and moved northwards to i t s position today off central Vancouver Island. The Pacific-Explorer-America t r i p l e junction has remained near the north end of Vancouver Island for at least 10 Ma and probably most of the Miocene. In addition to these geographic changes, the rates of convergence, as measured orthogonal to the continental margin, have not remained constant (Figure 17b). Riddihough (1977) notes a decrease in Explorer-America convergence from 4 cm/yr to 1.5 cm/yr from 3 to 5 Ma ago while Juan de Fuca-America rates declined from 4 to 3 cm/yr. 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 Reconstruction of t h i s geometry indicates that prior to 5 Ma, rapid subduction of a single Explorer plate beneath southern B r i t i s h Columbia was occurring. The motion of this subducted plate slowed at 4 Ma and as the Nootka fault zone migrated northwards, the slow moving (<2cm/yr) Explorer plate was progressively replaced by a more rapidly moving (3-4cm/yr) Juan de Fuca plate. Thus, under the Coast Mountains north of Vancouver, the subducted slab was f i r s t a rapidly moving Explorer plate prior to 5 Ma, then a slowed Explorer plate, and f i n a l l y a more rapidly moving Juan de Fuca plate. Figure 17c shows the trace of the northern edge of the plate since the Late Miocene. It seems reasonable that the observed discontinuity in volcanism may be an expression of these changes. The most recent period of volcanism, l o c a l i z e d in the Garibaldi Volcanic Belt, is probably related to the renewed more rapid subduction of the Juan de Fuca plate, probably at a steeper angle as suggested by the westward jump in the locus of volcanism. What plate tectonic and mantle processes could be responsible for the abrupt u p l i f t documented by t h i s study? Since the cause of u p l i f t most l i k e l y resides in the mantle, a gross reduction in mantle density must be suspected. This could be thermal expansion caused by heating of the lithosphere, either i n t e r n a l l y by magma transfer or at i t s base by asthenospheric upwelling, or due to phase changes induced by heating. This change in mantle density must occur mainly beneath the Coast Mountains as opposed to the 1 29 Interior in order to explain the documented d i f f e r e n t i a l u p l i f t . The following sequence of events i s suggested as an explanation. Since Pemberton volcanic belt volcanism was concentrated in the southeastern corner of the Coast Mountains region (Figure 1 ) , and temporally overlapping basaltic volcanism occurred in the Interior Plateau (Bevier et a l 1979), the Late Miocene subducted slab may have had a f a i r l y shallow dip, perhaps less than 30°. The upper mantle and c r u s t a l region west of the Late Miocene Pemberton arc would have been in the low heat flow regime with surface heat flow less than 40 kW/km2. In this western region, lithospheric thickness (including the America plate and the subducted slab) would have l i k e l y been 100-150 km (Figure 18) . During the reorganization of plate motion during the Late Miocene-Pliocene, as the Juan de Fuca plate replaced the Explorer plate, the subduction zone steepened to produce a westward s h i f t in the locus of volcanism. The shallow-dipping, Late Miocene, subducted slab probably broke off and remained in place beneath the arc in a manner suggested by Thompson and Zoback (1979) for the Colorado Plateau region. Subduction then proceeded at a steeper angle as shown in Figure 18. The hiatus in volcanic a c t i v i t y in the Late Miocene and Pliocene probably resulted from both slowed Explorer convergence rates as well as i n i t i a t i o n of the new zone of steeper subduction. 1 30 The material between the Pemberton and Garibaldi Volcanic Belts that was experiencing Late Miocene low heat flow (Figure 18) would have experienced r e l a t i v e l y sudden heating resulting from steepened subduction and associated asthenospheric upwelling and magmatism. The warming of thi s lithospheric material above the slab would result in thermal expansion and u p l i f t . A >50% increase in surface heat flow from fore-arc low (<40 kW/km2) to arc high (>60 kW/km2) corresponds to an average temperature increase of about 500° for the fore-arc lithospheric thickness of about 100-150km. If the co e f f i c i e n t of thermal expansion for peridotite is assumed to be 30 x 10- 6/ oC, then th i s temperature increase results in a density decrease of 1.5%. Mass balance implies 1.8 km of u p l i f t by this process alone. Additional u p l i f t may have occurred by the conversion of eclogite to gabbro in the heated mantle region (Figure 18). E c l o g i t i c material could have composed part of the lowermost crust or more l i k e l y the cut-off shallow slab. Assuming 5 km of eclogite converted to gabbro in thi s stagnant slab when the heat flux suddenly increased, an additional u p l i f t of 0.5 km would have resulted (10% density decrease x 5 km thickness). Any p a r t i a l melting would increase u p l i f t even more. Convective upwelling of asthenospheric material above the region of subduction-related melting i s essential to these models so that heat can be transported rapidly. The u p l i f t of the Coast Mountains Figure 18. Schematic s t r u c t u r a l section across the southern Coast Mountains. Postulated lithosphere-asthenosphere boundaries are shown with the approximate p o s i t i o n of the subducted plate i n Late Miocene and Recent time; continental and oceanic crust, and the predominant volcanic belts are also o u t l i n e d . The area of Miocene low heat flow that was heated as a consequence of steeper subduction is shown by horizontal l i n e s . The adjacent area to the west where l i t h o s p h e r i c thickening resulted 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 related to heating of a formerly cool fore-arc upper mantle region by the upwelling of magma or hot asthenospheric material above a steepened subduction zone. The region just west of th i s heated mantle region, shown in Figure 18, would have experienced lithospheric cooling and thickening as a dir e c t result of steepened subduction in the Pliocene. This cooling and thickening would, by contrast, induce thermal contraction and subsidence. The lithospheric thickening, resulting from a density increase of 1-1.5% of former asthenospheric material, might be 20 to 30 km, and could induce 0.2 to 0.4 km of surface subsidence, approximately that observed in the S t r a i t of Georgia. The southern Interior was the locus of high back-arc heat flow and asthenospheric upwelling in both Miocene and Pliocene to Recent times; late Neogene u p l i f t (about 0.5 to 1.0 km) in the southern Interior (Douglas et a l 1970, Holland 1964) cannot be explained simply by the region's high heat flow. The generally elevated southern Interior (average a l t i t u d e about 1.0-1.5 km) could be the result of the gradual warming, eclogite to gabbro conversion, and induced expansion of the f l a t t e r , cut-off, Miocene subducted slab that may have been present beneath much of the southern Int e r i o r . This explanation i s similar to that of Thompson and Zoback (1979) for the u p l i f t of the Colorado Plateau. 1 33 Summary Heat flow models have been presented that simulate time-dependent u p l i f t in a column of the earth's crust which is subjected to variable sub-crustal geothermal flux, changing surface temperature, erosion lagging behind u p l i f t , and an exponentially downward declining d i s t r i b u t i o n of heat production. An u p l i f t history of the Coast Mountains, derived from f i s s i o n track and geological data (Chapter 1) has been used with estimates of present and past heat flow (Hyndman 1976, Chapter 1) to derive models consistent with f i s s i o n track and other isotopic data, heat flow measurements, and geological and physiographic data. These self-consistent models reinforce conclusions on u p l i f t history drawn from Chapter 1 . The rapid u p l i f t in the central and northern Coast Mountains (53° to 56°N) during the Eocene was f e l t around the periphery of the Eocene high-grade plutonic and metamorphic core zone with rates up to 1 km/Ma. This orogenic a c t i v i t y resulted in a substantially thickened crust which, during the middle Cenozoic, was gradually thinned by denudation-induced u p l i f t . A Late Miocene episode of somewhat accelerated u p l i f t was probably related to the passage of the Anahim hot spot beneath the area. The southern Coast Mountains (50° to 52°N) experienced orogenic plutonic and metamorphic a c t i v i t y mainly during the Cretaceous and was t e c t o n i c a l l y stable throughout most of the Cenozoic, except for scattered volcanism. Late Neogene, 1 34 probably Pliocene to Recent u p l i f t in the south, which resulted in 2 to 3 km of u p l i f t of a broad area, was caused by Pliocene steepening and reorganization of subducted plate geometry and westward migration of the magmatic front. This migration caused abrupt heating of the formerly cool Miocene fore-arc mantle and consequent thermal expansion, leading to u p l i f t . Steepened subduction also led to lithospheric thickening west of the magmatic front of the Garibaldi Volcanic Belt and subsidence of the S t r a i t of Georgia. Warming of a proposed cut-off Miocene shallow-dipping slab under the southern Interior of B r i t i s h Columbia induced moderate u p l i f t by thermal and possibly phase transiton expansion. The v e r t i c a l movements documented in the Late Neogene were a l l thermally induced and were caused by changes in the plate tectonic regime. 1 3 5 Acknowledgements During' the course of thi s study, the author was supported by a Pre-doctoral fellowship at the University of B r i t i s h Columbia. Financial support was provided by a Natural Sciences and Engineering Research Council of Canada grant awarded to R.L. Armstrong, and through a Geological Society of America Grant-in-Aid. R.L. Armstrong was a constant source of support, advice, and ideas during the course of t h i s study. The computing aspects were greatly improved through the advice of G.K.C. Clarke and E.H. Perkins, and W.H. Mathews, G.T. Nixon, T. Lewis, R.D. Hyndman, and G.K.C. Clarke provided ideas which improved both the manuscript and my understanding of geophysical and thermal processes. L. Gilmore assisted with drafting and cheerful f i e l d assistance, and K. 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Riddihough, R.P. 1977. A model for the recent plate interactions off Canada's west coast. Canadian Journal of Earth Sciences, _1_4, pp. 384-396. Roddick, J.A., and Hutchison, W.W. 1974. Setting of the Coast Plutonic Complex, B r i t i s h Columbia. P a c i f i c Geology, 8, pp.91-108. Roddick, J.A., and Woodsworth, G.J. 1977. Bute Inlet (92K) 1 39 map-area. Geological Survey of Canada, Open f i l e map 480. 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., Blackwell, D.D., and Birch, F. 1968. Heat generation of plutonic rocks and continental heat-flow provinces. Earth and Planetary Science Letters, 5, pp.1 - 12. Schaer, J.P., Reimer, G.M., and Wagner, G.A. 1975. Actual and ancient u p l i f t rate in the Gotthard region, Swiss Alps: A comparison between precise leveling and f i s s i o n track apatite age. Tectonophysics, 29^, pp. 293-300. Skinner, B.J. 1966. Thermal expansion. I_n Handbook of physical constants. Clark, S.P., ed. Geological Society of America Memoir 97, pp.75-96. Souther, J.G. 1977. Volcanism and tectonic environments in the Canadian C o r d i l l e r a - A second look. I_n Volcanic regimes in Canada. Baragar, W.R.A., and others, eds., Geological Association of Canada Special Paper 16, pp. 3-24. Stacey, R.A. 1973. Gravity anomalies, cr u s t a l structure, and plate tectonics in the Canadian C o r d i l l e r a . Canadian Journal of Earth Sciences, j_0, pp.615-628. Strahler, A.N. 1973. Introduction to physical geography, 3rd ed. John Wiley and Sons, Inc. New York. Thompson, G.A., and Zoback, M.L. 1979 Regional geophysics of the Colorado Plateau. Tectonophysics, 6J_, pp. 149-181. Walcott, R.I. 1978. Present tectonics and Late Cenozoic evolution of New Zealand. Geophysical Journal of the Royal Astronomical Society, 5_2, pp.137-164. Wanless, R.K., Stevens, R.D., Lachance, G.R., and Delabio, R.N. 1979. Age determinations and geologic studies. Geological Survey of Canada, Paper 79-2. Wellman, H.W. 1979. An u p l i f t map for the South Island of New Zealand, and a model for u p l i f t of the Southern Alps. Royal Society of New Zealand, B u l l e t i n 18, pp.13-20. Wickens, A.J. 1977. The upper mantle of southern B r i t i s h Columbia. Canadian Journal of Earth Sciences, 14, pp.1100-1115. 1 40 Woodhouse, J.H., and Birch, F. 1980. Comment on 'Erosion, u p l i f t , exponential heat source d i s t r i b u t i o n , and transient heat flux' by T.-C.Lee. Journal of Geophysical Research, 85, pp.2691-2693. Woodsworth, G.J. 1979. Metamorphism, deformation, and plutonism in the Mount Raleigh pendant, Coast Mountains, B r i t i s h Columbia. Geological 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 Plutonic Complex, western B r i t i s h Columbia. Geological Association of Canada Cordilleran Section, Programme and Abstracts, pp.32-34. Yorath, C.J., and Chase, R.L. 1981. Tectonic history of allochthonous terranes: Northern Canadian P a c i f i c continental margin. Geological Association of Canada Cordilleran Section, Programme and Abstracts, pp. 42-43. CHAPTER 3. REFINEMENT OF APPARENT UPLIFT RATES DETERMINED BY FISSION TRACK DATING Abstract Fi s s i o n track dating of apatite and zircon is widely used to infer cooling and u p l i f t h i s t o r i e s of rocks. Dating minerals from rocks c o l l e c t e d at various a l t i t u d e s usually reveals an increase in date with a l t i t u d e which translates into an apparent u p l i f t rate. This apparent rate w i l l equal true u p l i f t rate only i f erosion equals u p l i f t , isotherms remain fixed with respect to the surface, and when l a t e r a l heat flow i s negligible compared to v e r t i c a l heat flow. These conditions are rarely met; a heat flow model is used to derive corrections to f i s s i o n track-derived apparent u p l i f t rates so that they can be used with greater confidence in teconic interpretations. Apatite-derived apparent rates w i l l c l o s e l y approximate true u p l i f t rates 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 5 Ma after the i n i t i a t i o n or cessation of u p l i f t . Apparent u p l i f t rates derived from zircon dates must be used with caution since excessive or d e f i c i e n t rates can result from gradual isotherm relaxation or slow response to u p l i f t i n i t i a t i o n . S l i g h t l y excessive rates can also result from prolonged u p l i f t accompanied by erosion of heat producing elements concentrated near the surface of the crust. 1 43 Introduct ion F i s s i o n track dating of apatite has been used by numerous workers to infer p a l e o - u p l i f t rates in mountains bel t s . The technique has been useful to infer u p l i f t and cooling h i s t o r i e s where l i t t l e or no stratigraphic or geologic evidence has been available. In the Alpine-Himalaya system, Wagner and Reimer (1972), Wagner et a l (1977), Schaer et a l (1975), and Z e i t l e r (in preparation) documented an increase in f i s s i o n track date with increasing a l t i t u d e and inferred that the altitude-date slope represented the u p l i f t rate of the rocks. This slope on the altitude-date graph w i l l be referred to as the apparent u p l i f t rate (Chapter 1). In an interesting application to the Bergell Massif, Wagner e_t aJL (1979) dated jjn s i t u plutonic rocks from d i f f e r e n t a l t i t u d e s and c l a s t s of Bergell intrusives in sediments of the adjacent Po Plain and documented not only inferred u p l i f t rates, but also the o r i g i n a l pre-erosion position of the transported c l a s t s . Other workers including Gleadow and Brooks (1979) and Sharma et a_l (1980) have determined the cooling rates of rocks by dating d i f f e r e n t minerals.(zircon, apatite) and inferred u p l i f t rates by making an assumption of the paleo-geothermal gradient. Chapter 1 summarizes f i s s i o n track dating of apatite and zircon c o l l e c t e d at various a l t i t u d e s from the Coast Mountains of B r i t i s h Columbia, Canada, and in conjunction with heat flow models, documents a Cenozoic u p l i f t history. 144 Fis s i o n track dating of zircon and apatite has widespread application and importance to the study of r e l a t i v e l y recent u p l i f t and related tectonic processes, but the assumption of equating apparent u p l i f t rates with actual rates is s i m p l i s t i c and v a l i d only when several conditions are s a t i s f i e d . These are that 1) erosion must equal u p l i f t , 2) isotherms must have remained horizontal, uninfluenced by topography or heterogeneities in conductivity or geothermal flux, and 3) isotherms must remain fixed with respect to the surface regardless of the rate of u p l i f t . C l early, these assumptions w i l l rarely be met. It 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 derived apparent u p l i f t rates in order to confidently use them in tectonic interpretation. Ways to Generate Apparent U p l i f t Rates If a rock mass i s suddenly u p l i f t e d and simultaneously denuded such that the surface remains at a constant a l t i t u d e , then isotherms w i l l be carried upwards with respect to the surface. This effect produces an i n i t i a l increase in surface heat flow. Since the apparent u p l i f t rate determined from f i s s i o n track studies i s in essence the rate that the c r i t i c a l track retention isotherm moves downward through the rock column, the apparent u p l i f t rate w i l l i n i t i a l l y be less than the true u p l i f t rate. Eventually, with continued u p l i f t , and with the condition that u p l i f t equal erosion, isotherms can either 1 45 s t a b i l i z e , continue slowly r i s i n g , or slowly f a l l , depending on the v e r t i c a l d i s t r i b u t i o n of heat producing elements (see Woodhouse and Birch 1980). In t h i s situation, apparent u p l i f t rates w i l l equal, be s l i g h t l y below, or s l i g h t l y exceed the true u p l i f t rate. Either way, they w i l l be very close to the actual rate, once the i n i t i a l transient lag has decayed away. Upon the cessation of u p l i f t or after the influx of heat has ceased in an area subject to either rapid u p l i f t or higher than normal heat flow, isotherms w i l l relax, sometimes rapidly. This thermal relaxation w i l l produce positive apparent u p l i f t rates, even when no u p l i f t has occurred and w i l l lead to misleading inferences of u p l i f t . C learly one must distinguish between these p o s s i b i l i t i e s when interpreting f i s s i o n track data. The best way to evaluate t h i s i s through quantitative heat flow models. A Heat Flow Model and i t s Application A one-dimensional f i n i t e difference thermal model was developed to study the effects of variable u p l i f t and erosion rates, changing sub-crustal geothermal flux and fluctuating surface temperature on a column of crust with an exponential downwards decreasing d i s t r i b u t i o n of heat production. This model, described in Chapter 2, allows for a r e a l i s t i c approximation of heat flow and isotherm migration in areas of active u p l i f t where l a t e r a l heat flow and 1 46 igneous intrusion are not important. To test the effect of u p l i f t on the movement of isotherms and the consequent setting of f i s s i o n track clocks, eight models representing a wide variety of u p l i f t rates have been run. The models incorporate an i n i t i a l temperature d i s t r i b u t i o n , V(Z) = (Q*Z/K) + (D 2AO/K)(1-exp(-Z/D)) + a where V ( Z ) i s the temperature at depth Z below the surface, Q* i s the reduced heat flow, K i s the conductivity, D is scale height, Ao i s the surface heat production, and a is the mean surface temperature. In the models that follow, K=2.5 kW/km°C (5.98x10-3 cal/sec-cm-°C), D=10 km, a=0°C, and Q*=25 kW/km2, and u p l i f t i s assumed to be matched by erosion. Four of the models have Ao=1.0 kW/km3 and u p l i f t rates of 1.0, 0.6, 0.3, and 0.1 km/Ma, and are shown as the dotted curves A, B, C, and D of Figure 19a, respectively. U p l i f t i s allowed to continue for 20 Ma, after which i t ceases and isotherms are allowed to relax. Also shown in Figure 19a are the curves of "steady state" heat flow for each case. This is the value of heat flow, Q, that would eventually result i f u p l i f t was stopped at any time and isotherms allowed to f u l l y relax. Increasing rates of u p l i f t result in lower "steady state" equilibrium heat flow values since more of the heat producing elements are removed by erosion. Figure 147 Surface Heat Flow, Q (kW/km2) a) e s eo 65 60 45 40 35 30 25 A Q = 1 . 0 (kW/km3) Q R = 2 5 (kW/km2) Observed Heat Flow "Steady State" Heat Flow 10 20 30 Time (Ma) 40 Surface Heat Flow, Q (kW/km2) b) A= 3.0 (kW/km3) o Q= 25 (kW/km2) Observed Heat Flow "Steady State" Heat Flow 10 20 30 Time (Ma) Figure 19. Surface heat flow vs. time curve for thermal models described in the text. Uplift rates are 1.0, 0.6, 0.3, and 0.1 km/Ma for models A, B, C, and D, respectively. The up 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 result i f , at any time, u p l i f t stopped and the iso- therms were allowed to ful 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 condition that Ao=3.0 kW/km3. "Steady state" heat flow values in this case have decreased more than when Ao=1.0 kW/km2, and tend with large u p l i f t to approach the value of Q*. Figure 19 c l e a r l y shows the effect that u p l i f t and erosion have on the surface heat flux. Apparent vs. True Upli f t Rates Estimates of apparent u p l i f t rate have been made for apatite and zircon by noteing the time that a rock at a sp e c i f i c depth passes below i t s respective track retention temperature during u p l i f t - r e l a t e d cooling. The same is done for rocks at successively greater depths for each of the eight models. A depth-time curve for the two (105°C and 175°C) c r i t i c a l isotherms can thus be constructed. Subtraction of the slope of th i s curve from the given u p l i f t rate (with respect to the surface) gives the apparent u p l i f t rate. Curves of apparent u p l i f t rate have been constructed for the four constant u p l i f t rates and for the cases when Ao changes from 1.0 to 3.0 kW/km3. These curves are shown in Figure 20. Note in Figure 20 that there can be a considerable difference between apparent u p l i f t rate and true u p l i f t rate. Though tracks are increasingly retained over a range of temperature (Naeser and Faul 1969), the concept of an ef f e c t i v e closure temperature i s applicable here (Dodson 1973, Haack 1977) since a l l rocks w i l l pass this closure 1 49 range in a similar fashion. In thi s c a l c u l a t i o n , closure temperatures of 105°C and 175°C were chosen for apatite (Zimmerman and Gaines 1978, Naeser and Forbes 1976) and zircon (Harrison e_t a l 1979), respectively. The s p e c i f i c choice of closure temperature w i l l not affect the conclusions of thi s paper. Apparent Upli f t Rates during Upli f t Because sudden inception of u p l i f t causes upward movement of isotherms, the i n i t i a l apparent rates w i l l be far below the true rate. This is especially important in the f i r s t few m i l l i o n years of u p l i f t (Figure 20), and the lag of apparent rates behind actual rates is longer for zircon than apatite since the isotherms move a greater distance upwards. The dashed l i n e s show the time when the apparent rate i s a fixed proportion (75%, 90%, 25%) of the actual or former u p l i f t rate. It i s evident from Figure 20 that when geothermal gradients are i n i t i a l l y higher, as when Ao=3.0 kW/km3, the apparent rates approach true rates more rapidly since the 105°C and 175°C isotherms are i n i t i a l l y nearer the surface and are displaced upwards by a smaller amount. Rocks passing upwards in thi s case move by isotherms more rapidly. This same eff e c t would occur i f Q* was higher. When Ao=1.0 kW/km3 and Q*=25 kW/km2, the geothermal gradient i s a rather low 14°C/km. U p l i f t proceeding in such a regime w i l l result in apatite apparent u p l i f t rates being Apatite Fission Track "Apparent" Uplift Rate (km/Ma) Actual Uplift Rate (km/Ma) Zircon Fission Track "Apparent" Uplift Rate (km/Ma) 1.0 0.8 0.6 0.4 0.2 k 0.0 1.0 0.5 0.0 1.0 0.8 0.6 0.4 0.2 0.0 A Q = 1.0 k W / k m 3 Actual Uplift Rate ! 90% A 75%.. B ;. i :\'\25% r.'P.. A B C D ..V ',75% i I ...vB " I I ic. -•/" ',90%; . 25% 10 20 Time (Ma) 30 1.0L 0.8 0.6 0.4 0.2 0.0 1.0 0.5 0.0 10L 0.8 0.6 0.4 0.2 0.0 (75%... i : •' 90% A Q = 3 . 0 k W / k m 3 c D ' 90% •75%i C D 40 10 Percentages represent the ratio: Apparent Uplift Rate Actual Rate During Uplift •\ 25% A B C D 25% 20 Time 30 40 (Ma) Figure 20. Apatite and zirc o n apparent u p l i f t r a t e vs. time curves f o r models described i n the text and shown i n Figure 19. Rates of u p l i f t f o r curves A, B, C, and D are shown by the middle diagram (actual u p l i f t r a t e vs. time). The apparent u p l i f t rate represents the rate at which the c r i t i c a l isotherms move downward through the rock column. 151 within 25% of the true value after a period of 3 to 5 Ma (Figure 20). Since most mountain systems are characterized by higher geothermal gradients, t h i s 5 Ma lag period is a maximum; in most cases, i t w i l l be less and apparent rates w i l l be very close to true rates within only a few m i l l i o n years. Consequently, in most cases of u p l i f t , apparent apatite rates w i l l closely approach true rates and may be used as such. The corresponding lag after which apparent rates are 75% of true rates i s greater for zircon, and may be up to 10 Ma after u p l i f t commences (Figure 20). It i s thus less responsive than apatite for u p l i f t studies, where u p l i f t rates are subject to rapid fl u c t u a t i o n . The u p l i f t rate and the denudation of heat producing elements are competing processes which determine the movement of isotherms and variations in heat flow. With a l l models, the f i r s t stage in thermal evolution after i n i t i a t i n g u p l i f t i s a r i s e in isotherms and Q. If u p l i f t rate i s s u f f i c i e n t l y high (>0.2 km/Ma for Ao=1.0 kW/km3, >0.6 km/Ma for Ao=3.0 kW/km3, >1.2 km/Ma for Ao=6.0 kW/km3), heat flow w i l l continue to increase, despite the removal of upwards-concentrated heat producing elements. This results in continued upward migration of isotherms with the consequence that apparent u p l i f t rates w i l l always be less than the true rate of u p l i f t . If u p l i f t rate i s less than these estimates for a given Ao, isotherms and resultant heat flow w i l l begin to decline, r e f l e c t i n g reduced heat 1 52 production in the upper crust. When isotherms decline, apparent u p l i f t rates w i l l exceed true rates of u p l i f t . Even for long (and sometimes u n r e a l i s t i c ) periods of u p l i f t , the apparent rates for apatite and zircon w i l l not exceed the true rate by more than about 20% so that, this e f f e c t is not very important. After declining for a considerable time as a result of removal of heat producing elements, isotherms w i l l begin to slowly r i s e again. This w i l l cause apparent u p l i f t rates to be s l i g h t l y less than true rates. Clearly, however, once the i n i t i a l 3 to 5 Ma period since inception of u p l i f t has passed, apparent u p l i f t rates for apatite w i l l remain very close (±20%) to the true rate of u p l i f t . This situation is more complex when u p l i f t and erosion are not in balance, but w i l l average out over long u p l i f t periods. Apparent Rates during Isotherm Relaxation Misleading apparent u p l i f t rates are produced when isotherms relax through either a cessation of u p l i f t (Figure 20) or through the cooling of an intrusive body. For apatite, the f i c t i t i o u s apparent rates produced by downward-moving isotherms drop to less than 25% of the former rate within a few m i l l i o n years, but take several times longer for zircon. Zircon date-altitude trends, therefore, can be strongly affected by t h i s thermal relaxation and the interpretation of u p l i f t from such zircon data can be misleading. Clearly a combination of diminishing u p l i f t and isotherm relaxation w i l l produce an effect 153 between the extremes described here. Inspection of Figure 20 reveals that i f a r e l a t i v e l y constant u p l i f t continues for a s i g n i f i c a n t (10-20 Ma) period of time, then the f i s s i o n track-derived estimates of apatite apparent u p l i f t rates preserved in the rocks w i l l closely approximate the true u p l i f t history. A similar observation for zircon i s not possible since the position of the 175°C isotherm i s shifted much more during u p l i f t and thermal relaxation. The approach, described in Chapter 2, where f i s s i o n track-derived u p l i f t h i s t o r i e s are evaluated and/or v e r i f i e d by thermal modeling provides the best method of a r r i v i n g at an accurate u p l i f t history. In several examples in Chapters 1 and 2, zircon apparent rates are s i g n i f i c a n t l y in excess of the most l i k e l y u p l i f t rates consistent with thermal models and geologic data. Apatite apparent rates, on the other hand, are usually very good indicators of the actual rates of u p l i f t . Discussion A combined approach of heat flow modeling and f i s s i o n track dating of apatite and zircon can provide excellent documentation of the u p l i f t history of rocks in mountain bel t s . Thermal modeling does show, however, that in areas of low geothermal gradient (produced by low Q*, low Ao, or both) the corrections that must be made to f i s s i o n track-derived apparent u p l i f t rates can be large, and modeling should be done to find the actual u p l i f t rates that 1 54 w i l l produce the observed f i s s i o n track date-altitude trends. The f i s s i o n track-derived rates can be either more or less than the true u p l i f t rate depending on whether the dates preserved were frozen in the rocks at the end or beginning of an u p l i f t episode. A knowledge of the timing of u p l i f t from geological data can help to choose between these alternat ives. If a knowledge of Ao, Q*, or average geothermal gradient is available for an area where f i s s i o n track dating studies on samples from d i f f e r e n t a l t i t u d e s is done, the curves of Figure 20 can be used as a general guideline in evaluating the possible error in calculated apparent rate. Apatite can continue to be widely used in a manner similar to that of Wagner et a l (1979), Schaer et a l (1975) in Chapter 1, but the extension of the method to zircon should proceed with caution unless thermal modeling accompanies the analysis of zircon data. Geothermal inferences gained from such work (such as constraints on Q*) can provide useful insight on the causes of u p l i f t . 155 Acknowledgements This paper has benefited from discussions on heat flow with G.R.C.Clarke. W.H.Mathews and T.J.Lewis encouraged me to quantify the relationship between u p l i f t , movement of isotherms, and f i s s i o n track derived apparent u p l i f t rates. Their comments, and those of R.L.Armstrong, are appreciated. This work was supported by the Natural Sciences and Engineering Research Council of Canada as a grant awarded to R.L.Armstrong. 1 56 References Dodson, M. 1973. Closure temperature in cooling geochronologic and petrologic systems. Contributions to Mineralogy and Petrology, 40, pp.259-274. Gleadow, A.J.W., and Brooks, C.K. 1979. Fission track dating, thermal h i s t o r i e s and tectonics of igneous intrusions in east Greenland. Contributions to Mineralogy and Petrology, 7J_, pp.45-60. Haack, U. 1977. The closing temperature for f i s s i o n track retention in minerals. American Journal of Science, 277, pp.459-464. Harrison, T.M. Armstrong, R.L., Naeser, C.W., and Harakal, J.E. 1979. Geochronology and thermal history of the coast plutonic complex, near Prince Rupert, B r i t i s h Columbia. Canadian Journal of Earth Sciences, 16, pp.400-410. Naeser, C.W., and Faul, H. 1969. F i s s i o n track annealing in apatite and sphene. Journal of Geophysical Research, 74, pp.705-710. Naeser, C.W., and Forbes, R.B. 1976. Variation of f i s s i o n track ages with depth in two deep d r i l l holes (abstract). EOS, 57, p.363. Schaer, J.P., Reimer, G.M., and Wagner, G.A. 1975. Actual and ancient u p l i f t rate in the Gotthard region, Swiss Alps: A comparison between precise leveling and f i s s i o n track apatite age. Tectonophysics, 2j3, pp.293-300. Sharma, K.K., Bal, K.D., Parshad, R., L a i , N., and Nagpaul, K.K. 1980. Paleo-uplift and cooling rates from various orogenic belts of India, as revealed by radiometric ages. Tectonophysics, 70, pp.135-158. Wagner, G.A., and Reimer, G.M. 1972. Fission track tectonics: The tectonic interpretation of f i s s i o n track apatite ages. Earth and Planetary Science Letters, 14, pp.263-268. Wagner, G.A., Reimer, G.M., and Jager, E. 1977. The cooling ages derived by apatite f i s s i o n track, mica Rb-Sr, and K-Ar dating: The u p l i f t and cooling history of the Central Alps. Memoir of the Institute of Geology and Mineralogy, University of Padova, Padova, Italy, XXX, 27p. Wagner, G.A., M i l l e r , D.A., and Jager, E. 1979. F i s s i o n track ages on apatite of Bergell rocks from Central Alps and Bergell boulders in Oligocene sediments. Earth and 1 57 Planetary Science Letters, 45, pp.355-360. Woodhouse, J.H., and Birch, F. 1980. Comment on 'Erosion, u p l i f t , exponential heat source d i s t r i b u t i o n , and transient heat flux' by T.-C.Lee. Journal 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 Geological 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 FINITE DIFFERENCE SOLUTION TO THE 1-DIMENSIONAL HEAT FLOW C PROBLEM WITH VARIABLE UPLIFT, SURFACE TEMPERATURE, REDUCED C FLOW, AND EXPONENTIALLY DECREASING DISTRIBUTION OF HEAT C PRODUCTION C C THE PROGRAM IS DESIGNED FOR THE CENOZOIC HISTORY OF THE C BRITISH COLUMBIA COAST MOUNTAINS. THE SURFACE TEMPERATURE C CAN BE CHANGED TO SIMULATE CONDITIONS WHEN UPLIFT AND C EROSION ARE NOT IN BALANCE. C C THE PROGRAM IS DESIGNED FOR A MAXIMUM GRID OF 200X200. TO C BE REALISTIC, THE HEAT PRODUCTION AT THE BASE OF THE GRID C SHOULD BE LESS THAN 2% THAT OF THE SURFACE. C 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 SET LOWER BOUNDARY CONDITIONS-VARIABLE FLUX; 2 CHANGES IN C REDUCED HEAT FLUX ARE ALLOWED AT TIMES TQR1 AND TQR2; THE C THREE HEAT FLUX VALUES-OLDEST TO YOUNGEST-ARE QR1,QR2,AND C QR4. C WRITE (6,840) READ (5,940) QR1,TQR1,QR2,TQR2,QR3 C C SET UPLIFT RATE CONDITIONS-3 CHANGES IN UPLIFT RATE ARE C PERMITTED AT TIMES TUPL1, TUPL2, TUPL3; THE 4 RATES ARE C UPL1,UPL2,UPL3, AND UPL4. C WRITE (6,850) READ (5,950) TUPL1,TUPL2,TUPL3,UPL1,UPL2,UPL3,UPL4 C C SET CONDITIONS WHEN UPLIFT .NE. EROSION C 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 CALCULATE UPL VECTOR, WITH RESPECT TO SURFACE C 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 20 CONTINUE DO 25 JV=J101 A,J102 UPL(JV)=UPL2 25 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 30 CONTINUE DO 35 JV=J103A,J104 UPL(JV)=UPL4*BBB 35 CONTINUE C C CALCULATE QR VECTOR C 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 C C CALCULATE TSURF AND SURFEL VECTORS C 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(JZ)=LAM-GAM C C SET ASTAR (HEAT PRODUCTION TERM) C ASTAR1=-(Z-TOTUP(TT-1))/D ASTAR=(2.0*DIF*DT*A0/K)*EXP(ASTAR1) C C SET RIGHT HAND SIDE VECTOR DD C 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 C 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 C 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 C C COMPUTE NEW TEMPERATURES U(TT,JZ) USING SUBROUTINE TRIDAG C CALL TRIDAG (1,LAST,A,B,C,DD,V) C DO 500 JZ=1,LAST U(TT,JZ)=V(JZ) 500 CONTINUE U(TT,LAST+1)=U(TT,LAST)+DT*QR(TT-1)/K C C SET HEAT FLOW MATRIX C 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 r5)=TOTUP(l) C 100 CONTINUE C C PRINT PARAMETERS C WRITE (7,1001) WRITE (7,1003) A0,K,DIF,D,Q0,TSURF0 1 63 WRITE (7,1005) WRITE (7,1007) WRITE (7,1009) WRITE (7,1300) WRITE (7,1310) WRITE (7,1320) WRITE (7,1330) WRITE (7,1340) WRITE (7,1350) WRITE (7,1050) WRITE (7,1015) C C PRINT TIME HEADINGS C 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) C C PRINT SURFACE TEMPERATURES C 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) C C PRINT TEMPERATURE MATRIX C 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 C WRITE (7,1015) WRITE (7,1015) C C PRINT HEAT FLOW-TOTAL UPLIFT MATRIX C 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 DTSURF,ZMAX,TMAX,DZ,DT QR1,TQR1,QR2,TQR2,QR3 UPL1,TUPL1,UPL2,TUPL2,UPL3,TUPL3,UPL4 LAPSE AAA BBB TNONEQ 164 WRITE WRITE C C c (7,1024) (7,1015) 680 C C C C C C DO 680 JB=1,20 J9=(IFIX(ZDT)/20)*JB WRITE (7,1030) (W(J9,J10), J10=1,5) CONTINUE WRITE (7,1015) WRITE (7,1040) X15 IZDT=IFIX(ZDT) IZDZ=IFIX(ZDZ) WRITE (7,1042) ZDT, IZDT (7,1043) ZDZ, IZDZ (7,1015) (7, 1015) (7,1070) WRITE WRITE WRITE WRITE WRITE WRITE WRITE (7,1050) (7,1011)(P(J6),J6=1 (7,1015) 21 ) 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) PRINT SURFACE ALTITUDE VALUES WRITE (7,1190) (AQSURF(J20),J20=1,21) FORMAT STATEMENTS 800 FORMAT 900 FORMAT 810 FORMAT 910 FORMAT 820 FORMAT 920 FORMAT 830 FORMAT 930 FORMAT 840 FORMAT 940 FORMAT 850 FORMAT 950 FORMAT 1001 FORMAT '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 1003 1 005 1 007 1009 1011 1013 1014 1015 1 020 1 021 1022 1023 1024 1 030 1 040 1 042 1 043 1 050 1070 1080 1 1 00 1110 1 1 20 1 1 30 1 1 40 1 1 50 1 1 60 1 1 70 1 180 .1 190 1 300 1310 1 320 1 330 1340 1 350 SCALE HEIGHT= ',F6.1,/, 15X,'INITIAL TSURFACE= .F5.0,/, FORMAT ('SURFACE HEAT PROD. A0= ',F5.2,12X, 'CONDUCTIVITY= *,F5.2, /, 'DIFFUSIVITY= ',F5.2,22X, 'INITIAL Q(REDUCED)= ',F5.1 F5.1,/) FORMAT ('DTSURF(DEG/MA)= ',F5.2,19X,'ZMAX= 'TMAX(MA)= ' ,F5. 1 ,25X,'DZ(KM) = ',F5.2,/, 'DT(MA)= ',F5.2,//) FORMAT ('REDUCED HEAT FLOW PARAMETERS: QR1 = 'TQR1(MA)= ',F5.1,25X,'QR2= ',F5.1,/, 'TQR2(MA)= ',F5.1,25X,'QR3= ',F5.1,//) FORMAT ('UPLIFT PARAMETERS: UPL1= ',F5.3,/, 'TUPL1(MA)= ',F5.1,24X, 'TUPL2(MA)= ',F5.1,24X, 'TUPL3(MA)= ',F5.1,24X, FORMAT (1X,50F5.1) (1X,50F5.1) (1X,50F5.0) •F5.1,/, 'UPL2= 'UPL3= 'UPL4= ,F5 ,F5 ,F5 3,/, 3,/, 3,///) ) COL. COL. COL. COL. COL, FORMAT FORMAT FORMAT ( FORMAT ( FORMAT FORMAT FORMAT FORMAT FORMAT FORMAT FORMAT FORMAT FORMAT FORMAT FORMAT FORMAT FORMAT FORMAT (O.O)IF.EQ. FORMAT (1F5 FORMAT ('ERO/UPL .0.0-1.0') FORMAT (1F5.2) FORMAT ('TIME OF . ERO' ) FORMAT FORMAT .UPL.NE FORMAT FORMAT ( FORMAT ( FORMAT ( ' AAA= FORMAT ( FORMAT ( FORMAT ( F5.2, ,F5, ,F5, 2, 2, AT 1=TIME(MA) ') 2=Q AT DEPTH= 3=Q AT DEPTH= 4=Q AT DEPTH= 5=TOTAL DENUDATION ( 1X,5F10.3) ('DEPTH PRINTING INTERVAL = ' ( 'ZDT= ' ,F10.2,5X,'IFIX(ZDT) = ('ZDZ= ',F10.2,5X,'IFIX(ZDZ)= (50X,'TIME(MA)') CHEAT PRODUCTION AT SURFACE AND BASE OF GRID',/) ( 1X,50F5.2) ('ATMOSPHERIC LAPSE RATE: -DEG/KM') ( 1F5. 1 ) ( 'ERO.LT.UPL.,TYPE(- 1 ) 0) OR UPL/ERO, 'KM' 'KM' 'KM' SURFACE(AVE.ALT),(KM)') ',F5.2,'KM') RI3) ,13) 0);UPL.LT.ERO.(1.0) WHICHEVER, APPLIES,-FROM ONSET OF NON-EQUILIBRIUM BETWEEN UPL AND (1F5.0) ('ALTITUDE(KM) ERO' ) (2X,50F5.2) OF SURFACE ABOVE SEA LEVEL WHEN AAA= AAA= -1 1 . .0' ) 0' ,1 OX, •F5.1,//) LAPSE RATE= ',F5.1) IF EROSION IS LESS THAN UPLIFT, IF UPLIFT IS LESS THAN EROSION, ,F5.1 ) IF EROSION IS EQUAL TO UPLIFT, RATIO OF EROSION/UPLIFT OR UPLIFT/EROSION=',F5.2) TIME OF ONSET WHEN EROSION .NEQ. UPLIFT= ', AAA= 0.0') 166 C STOP END C r> C SUBROUTINE FOR SOLVING A SYSTEM OF LINEAR SIMULTANEOUS C EQUATIONS HAVING A TRIDIAGONAL COEFFICIENT MATRIX. THE C EQUATIONS ARE NUMBERED FROM 1 TO LAST, AND THEIR SUBDIAGONAL, C DIAGONAL, AND SUPER-DIAGONAL COEFFICIENTS ARE STORED IN THE C ARRAYS A, B, AND C. THE COMPUTED SOLUTION VECTOR C U(TT,1)...U(TT,LAST) IS STORED IN THE ARRAY V. C C 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) C C COMPUTE BETA AND GAMMA ARRAYS C 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 C C COMPUTE FINAL SOLUTION VECTOR V C 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 C RETURN END

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