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Electrical conductivity structure of the lower crust and upper mantle in western Canada Caner, Bernard 1969

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ELECTRICAL CONDUCTIVITY STRUCTURE OF THE LOWER CRUST AND UPPER MANTLE IN WESTERN CANADA . by BERNARD CANER " B . S c , U n i v e r s i t y of A l b e r t a , I960 M.Sc, U n i v e r s i t y of B r i t i s h Columbia, 1964 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n the Department of GEOPHYSICS We accept t h i s t h e s i s as conforming to the re q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA J u l y , 1969 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced degree a t the U n i v e r s i t y o f B r i t i s h C olumbia, I agr e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and Study. I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y purposes may be g r a n t e d by the Head o f my Department or by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g or p u b l i c a t i o n o f t h i s thes.is f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Bernard Caner G e o p h y s i c s Department o f The U n i v e r s i t y o f B r i t i s h Columbia Vancouver 8, Canada Date July 25, 1969 ( i ) ABSTRACT Geomagnetic induction techniques have been used to study the structure of the crust and upper mantle in western Canada. Geomagnetic depth-sounding (GDS) has been used primarily for mapping, and magneto- te l l u r i c s (MT) for quantitative interpretation. Self-consistent models of e l e c t r i c a l conductivity structure have been derived from the combined MT/GDS data. The conductivity structure models have been considered in con junction with other relevant geophysical information: heat-flow, seis mology and aeromagnetic surveys. No definite petrological models can be derived because of the order-of-magnitude uncertainties in the relations between el e c t r i c a l conductivity, temperature and composition. However, i f we exclude geocheinically improbable solutions, the following two distinct results can be extracted: a) In southwestern Canada (boundaries not clearly defined, but at least as far east as Lethbridge), the uppermost mantle i s moderately conducting (r e s i s t i v i t y 30-50 ohm-meters). This indicates a temperature of at least 750°C at depth 35 km., and provides independent confirmation (without assumptions of crustal structure) of the heat-flow derived estimates of Roy et a l (1968b). b) In a sharply delineated region starting from about 0-30 km west of the Rocky Mountain Trench, the lower crust (from a depth of about 10-15 km) i s conductive. The most l i k e l y interpretation i s a hydrated lower crust, as proposed by Hyndman and Hyndman, 1968. Hydration alone* i s sufficient to explain the observed conductivities, i.e. higher temperatures are not necessarily required for this model. However, given the information from (a) above, some partial melting of hydrated granitic materials should occur in this zone; this i s i n good agreement with the geological evidence of granitic intrusives i n this region. ( i i ) TABLE OF CONTENTS PAGE ABSTRACT ( i ) TABLE OF CONTENTS ( i i ) LIST OF FIGURES (iv) I. INTRODUCTION A) Preface and acknowledgements 1 B) Historical background 2 C) Thesis outline 8 D) Theory 11 I I . MAGNETO-TELLURICS A) Introduction 18 B) Instrumentation 23 C) Data ' ' 25 D) Interpretation - eastern stations 33 E) Interpretation - western stations 42 F) Summary - conductivity structure 47 I I I . GEOMAGNETIC DEPTH-SOUNDING A) Introduction 50 B) Instrumentation . 5 0 C) Mapping 52 D) Quantitative interpretation 61 IV. OTHER GEOPHYSICAL INFORMATION A) Introduction 7& B) Heat flow 7& C) Seismology 81 D) Aeromagnetic surveys i-> 84 ( i i i ) PAGE V. FETR0L0GICA1 INTERPRETATION A) Summary of data 98 B) E l e c t r i c a l conductivity of the lower crust and upper mantle 98 C) Petrological models 105 D) Conclusions 109 REFERENCES 113 (iv) LIST OF FIGURES PAGE 1-1. *Location of GDS stations i n North America up to 1966 5 I - 2. Location of GDS and MT stations i n western Canada 10 ,11-1. Location of MT stations used i n t h i s report 21 I I - 2. Schematic lay-out and frequency response of MT instrumentation used at Pincher and Penticton 22 II-3a. Section of MT recording i n the long-period band, eastern stations 26 II-3b. Section of MT recording i n the long-period band, western stations \ 2 7 II-4. Sections of MT. recording, short-period band 2 8 II-5. Apparent r e s i s t i v i t y plotted against period f o r a l l s i x stations 2 9 I I - 6 . Polarization plot at Fernie 32 II-7. Conductivity structure models, eastern stations 35 II-8. Conductivity structure models, western stations 43 I I - 9. Combined conductivity structure for both regions 48 I I I - l . Magnetogram copies from pairs of stations at latitudes 51°N, 49.5°N, and 33°-35°N. 53 III-2. ' Location of GDS stations i n western Canada 54 III-3. GDS recordings from Cache Creek - Prince George p r o f i l e and V i c t o r i a 57 III-4. M-ratios as a function of geomagnetic la t i t u d e f o r Cache Creek - Prince George p r o f i l e 58 III-5« Sections of GDS recordings, Pincher, Salmo, and Penticton 62 I I I - 6 . M-ratios as a function of period, Penticton norma].ized with respect to Pincher 63 III-7. 14-day recordings of Z, Pincher and Penticton 64 III-8. Fourier amplitudes for the diurnal variations at Pincher and Penticton 65 (v) PAGE III-9. GDS c o n d u c t i v i t y s t r u c t u r e models, P e n t i c t o n / P i n c h e r , s p a t i a l wavelengths ^ 3000 km 68 111-10. GDS c o n d u c t i v i t y s t r u c t u r e models, P e n t i c t o n / P i n c h e r , s p a t i a l wavelengths 1000 km 69 I I I - l l . GDS c o n d u c t i v i t y s t r u c t u r e models, Pincher v s . hypo t h e t i c a l "eastern" s t r u c t u r e 73 I I I - 12. GDS c o n d u c t i v i t y s t r u c t u r e models, P e n t i c t o n / P i n c h e r , w i t h surface l a y e r 74 IV- 1. L o c a t i o n of aeromagnetic p r o f i l e s 87 IV-2. Magnetic data and topography, p r o f i l e A 88 IV-3. Magnetic data and topography, p r o f i l e B 89 IV -4 . Superimposed f i l t e r e d magnetic data from p r o f i l e s A and B 91 IV-5. F i l t e r e d magnetic data, p r o f i l e C 92 IV- -6. F i l t e r e d magnetic data, p r o f i l e D 93 V- l . O u t l i n e of d i s c o n t i n u i t y d e f i n e d by GDS and aero magnetic data 99 V-2. F i n a l c o n d u c t i v i t y s t r u c t u r e models 100 V-3. E l e c t r i c a l c o n d u c t i v i t y of some rocks and minerals 101 V -4 . E l e c t r i c a l c o n d u c t i v i t y as a f u n c t i o n of temperature 104 V-5. E f f e c t s of hyd r a t i o n and melti n g on e l e c t r i c a l c o n d u c t i v i t y of rocks 108 V-6. Geotherms and melti n g zones 110 V-7. Combined p e t r o l o g i c a l model, l a t . 49.3°N 111 1 I-A) Preface and Acknowledgements The major pa r t of t h i s t h e s i s has been w r i t t e n a t the "publishable paper" l e v e l , i . e . r e l a t i v e l y l e a n and concentrated. In the course of p r e p a r a t i o n , some of the chapters have al r e a d y been published or submitted f o r p u b l i c a t i o n , as o u t l i n e d below. In each case the primary work (p l a n n i n g , o r g a n i z a t i o n , almost a l l data p r o c e s s i n g , and a l l i n t e r p r e t a t i o n ) was c a r r i e d out. by the candidate; the c o n t r i b u t i o n of the co-authors was l i m i t e d t o o p eration of f i e l d - s t a t i o n s and some r o u t i n e data processing. However, s i g n i f i c a n t c o n t r i b u t i o n s were r e c e i v e d from these co-authors i n reviewing the completed r e s u l t s , as w e l l as from i n t e r n a l reviewers, j o u r n a l e d i t o r s , and r e f e r e e s . T h i s feed-back has been in c o r p o r a t e d i n the t h e s i s . In p a r t i c u l a r , Dr. R.M. E l l i s and Dr. W.F. Slawson a t the U n i v e r s i t y of B r i t i s h Columbia, and Dr. E.R. N i b l e t t , Dr. P.H. Serson, and Dr. K. Whitham at the Dominion Observatory have g r e a t l y c o n t r i b u t e d towards the r e l i a b i l i t y of t h i s work. I should a l s o l i k e to acknowledge the o r g a n i z a t i o n a l support and p e r s o n a l encouragement provided by Dr. J.A. Jacobs, former head of the Department of Geophysics; by Dr. R.D. R u s s e l l , h i s successor, and by Dr. P.H. Serson, C h i e f of the D i v i s i o n of Geomagnetism at the Dominion Observatory. F i n a n c i a l support from the Department of Energy, Mines and Resources i s a l s o g r a t e f u l l y acknowledged. Reference t o published s e c t i o n s of t h i s t h e s i s : Chapter I I : Caner, B., P.A. Camfield, F. Andersen, and E.R. N i b l e t t . "A l a r g e - s c a l e m a g n e t o - t e l l u r i c survey i n western Canada", sub m i t t e d t o Can. J . E a r t h Sciences, 1969. 2 Chapter I I I : Caner, B., D.R. Auld, and P.A. Camfield. "Geomagnetic depth- sounding i n western Canada". In p r e p a r a t i o n , t o be submitted t o the J o u r n a l of Geophysical Research. Chapter IV ( S e c t i o n D): Caner, B. "Long aeromagnetic p r o f i l e s and c r u s t a l /: s t r u c t u r e i n western Canada", t o be p ublished i n E a r t h and P l a n e t a r y Science L e t t e r s , 1969. Chapter V: Caner, B., " E l e c t r i c a l c o n d u c t i v i t y s t r u c t u r e i n western Canada and p e t r o l o g i c a l i n t e r p r e t a t i o n " . To be published i n the J o u r n a l of Geomagnetism and G e o e l e c t r i c i t y . (Symposium on m u l t i d i s c i p l i n a r y s t u d i e s of unusual regions of the upper mantle, Madrid, Sept. 1969). I-B) H i s t o r i c a l Background The e l e c t r i c a l c o n d u c t i v i t y s t r u c t u r e of the earth's c r u s t and upper mantle can be determined by observations of f l u c t u a t i o n s i n the n a t u r a l geomagnetic and g e o e l e c t r i c ( t e l l u r i c ) f i e l d s . The p e r i o d i c i t i e s 5 of i n t e r e s t f o r t h i s type of work range from about 1 second t o about 10 seconds. Two b a s i c methods a r e ' i n use: a) m a g n e t o t e l l u r i c s (MT), where the two h o r i z o n t a l magnetic (Hx, Hy) and two h o r i z o n t a l t e l l u r i c (Ex, Ey) components are recorded; b) geomagnetic depth-sounding (GDS) where the t h r ee components (two h o r i z o n t a l , one v e r t i c a l ) of the magnetic f i e l d are recorded (Hx, Hy, Hz, or more u s u a l l y H, D, Z i n magnetic c o o r d i n a t e s ) . T h e o r e t i c a l l y , the two methods are e q u i v a l e n t , but i n p r a c t i c e t h e i r execution (and sometimes the r e s u l t s obtained) are d i f f e r e n t . R i k i t a k e (1966) has o u t l i n e d the r e l a t i v e advantages of the two methods and t h e i r l i m i t a t i o n s f o r p a r t i c u l a r s i t u a t i o n s . A v e r y s u p e r f i c i a l o u t l i n e of the comparative m e r i t s and ranges of u s e f u l n e s s i s shown i n the f o l l o w i n g t a b l e . A summary and b i b l i o g r a p h y of the l i t e r a t u r e has been published by Fournier (1966). MAGNETO-TELLURICS (MT)  Advantages 1. S i n g l e - s t a t i o n i n t e r p r e t a t i o n p o s s i b l e . 2. Source dependent only f o r periods > 1000 sec. 3. I n t e r p r e t a t i o n subject t o only moderate b i a s i n s e l e c t i o n of c o n d u c t i v i t y s t r u c t u r e models. Disadvantages 1. Re s u l t s ( e n t i r e spectrum) s t r o n g l y dependent on surface c o n d i t i o n s . 2. Experimental d i f f i c u l t i e s a t long p e r i o d s . 3. L o g i s t i c s complicated (long l i n e s , ground c o n t a c t s ) . Primary u s e f u l n e s s (open t o argument!) GEOMAGNETIC DEPTH-SOUNDING | i Disadvantages Network or p r o f i l e r e q u i r e d . I n t e r p r e t a t i o n s t r o n g l y source j dependent. I n t e r p r e t a t i o n s t r o n g l y depen dent on perso n a l b i a s , since two s t a t i o n models are i n v o l v e d f o r each data s e t . Advantages Re s u l t s r e l a t i v e l y independent of surface c o n d i t i o n s . E x p e r i m e n t a l l y simple, e n t i r e spectrum. L o g i s t i c s simple, cheap. 1. Periods <. 1000 sec. (surface l a y e r s and c r u s t ) . 2. Q u a n t i t a t i v e c o n f i r m a t i o n (at a few s e l e c t e d s i t e s ) of r e s u l t s p r e v i o u s l y obtained by GDS. Periods > 1000 sec. (lower c r u s t and mantle). Mapping and p r e l i m i n a r y surveys of l a r g e areas f o r d e l i n e a t i o n of anomalous r e g i o n s . I n North America, MT data obtained a t a few sc a t t e r e d l o c a t i o n s i n d i c a t e d the exis t e n c e of a h i g h l y conducting zone at depths v a r i o u s l y reported as between 35 km and 140 km ( f o r example, Cantwell and Madden, | I960; N i b l e t t and Sayn-Wittgenstein, I960; S r i v a s t a v a and Jacobs, 1964; V o z o f f and E l l i s , 1966; P l o u f f , 1966). The s t a t i o n s were too s c a t t e r e d t o provide any l a r g e - s c a l e models f o r the s t r u c t u r e under the co n t i n e n t , • i and the main impetus f o r systematic work i n t h i s f i e l d came from GDS. Although low Z/H r a t i o s had been noted e a r l i e r a t Tucson ( B a r t e l s et a l , 1939), the d e n s i t y of permanent o b s e r v a t o r i e s i n North America was too low t o permit the type of pione e r i n g GDS work which was p o s s i b l e i n Japan ( R i k i t a k e , 1959) and Europe (Wiese, 1956"). The i n t r o d u c t i o n of p o r t a b l e variographs d u r i n g the IGY opened up t h i s f i e l d i n North America. Matsushita (i960) noted t h a t the Z amplitudes of sudden commencements recorded a t s e v e r a l s t a t i o n s a t l a t i t u d e 39.5°N were d i f f e r e n t , and suggested d i f f e r e n c e s i n subsurface c o n d u c t i v i t y s t r u c t u r e as a p o s s i b l e e x p l a n a t i o n ; however, the s t a t i o n s were too wi d e l y spaced (about 400 km) f o r any c o n s i s t e n t p a t t e r n t o be d e r i v e d . The main break came w i t h Schmucker's work d u r i n g 1959-1962 i n the southwest U.S.A. (Schmucker, 1964). He d e l i n e a t e d a sharp d i s c o n t i n u i t y i n the charac t e r of recorded geo magnetic data ( s p e c i f i c a l l y i n the amplitude of f l u c t u a t i o n s i n Z w i t h p e r i o d s of 15-60 minutes) along an Er-W p r o f i l e ' c r o s s i n g the C o r d i l l e r a a t l a t i t u d e 32°N ( p r o f i l e A on F i g . I - l ) . The d i s c o n t i n u i t y occurred between Las Cruces (LAC) and Cornudas (COR) i n New Mexico; t o the east of t h i s t r a n s i t i o n , the r a t i o of v e r t i c a l to h o r i z o n t a l amplitudes f o r bay-type f e a t u r e s was roughly three times higher than a t the western s t a t i o n s . Schmucker (1964) i n t e r p r e t e d t h i s a t t e n u a t i o n t o be caused by a step i n a h i g h l y conducting sub-stratum from a depth of 320 km under the e a stern region t o a depth of 160 km under the western r e g i o n . Two of the permanent Canadian o b s e r v a t o r i e s ( A l e r t and Mould Bay) were a l s o found t o be "anomalous" i n t h e i r Z/H r a t i o s (Whitham, 1965); i t became 5 F i g . 1 - 1 . L o c a t i o n of GDS s t a t i o n s i n North America, up t o 1 9 6 6 ( a f t e r Caner et a l , 1 9 6 7 ) . 6 c l e a r t h a t these GDS "anomalies" are not n e a r l y as r a r e as p r e v i o u s l y supposed. In 1963, a long-term GDS p r o j e c t was i n i t i a t e d by the I n s t i t u t e of E a r t h Sciences a t the U n i v e r s i t y of B r i t i s h Columbia, i n cooperation w i t h the Dominion Observatory's D i v i s i o n of Geomagnetism a t V i c t o r i a , Vand co-ordinated by the author. Hyndman (1963) operated an east-west GDS p r o f i l e at l a t i t u d e 49.5°N, from Westham I s l a n d (near Vancouver) to Lethbridge i n A l b e r t a - p r o f i l e C on Figure 1-1. His r e s u l t s provided the b a s i c ground work f o r a l l subsequent surveys i n t h i s area. Hyndman observed the same p a t t e r n as Schmucker i n the southwest U.S.A.: s t a t i o n s t o the east of a d i s c o n t i n u i t y ( l o c a t e d i n the Kootenay Lake region) showed Z/H r a t i o s about 2-3 times higher than s t a t i o n s t o the west. Whitham (1965) estimated that the observed a t t e n u a t i o n could be explained by a r i s e of conducting m a t e r i a l t o w i t h i n about 200 km of the surface under the western r e g i o n , a s t r u c t u r e compatible w i t h Schmucker's i n t e r  p r e t a t i o n i n the southwest U.S.A. In 1964, Hyndman's p r o f i l e v/as extended towards the west i n order t o study the "coast e f f e c t " on geomagnetic r e c o r d i n g s . The r e s u l t s (Lambert and Caner, 1965) are not d i r e c t l y r e l e v a n t t o the present study of the main i n l a n d d i s c o n t i n u i t y . During 1965 and 1966, a t t e n t i o n was again focussed on the i n l a n d d i s c o n t i n u i t y and two f u r t h e r p r o f i l e s were operated by W.H, Cannon and C.E. L i v i n g s t o n e : one at l a t i t u d e 35°N i n the south west U.S.A. ( p r o f i l e B on Figure I - l ) and one at l a t i t u d e 51°N ( p r o f i l e D) between Cache Creek, B.C., and Calgary. The aim of these two p r o f i l e s , which were about 200 km t o the no r t h of the two e a r l i e r p r o f i l e s of Schmucker and Hyndman, was t o v e r i f y t h a t these d i s c o n t i n u i t i e s were i n f a c t the borders of l a r g e - s c a l e c o n t i n e n t a l f e a t u r e s r a t h e r than j u s t i s o l a t e d anomalies. The r e s u l t s (Caner et a l , 1967) confirmed t h i s f a c t , although on p r o f i l e B the d i s c o n t i n u i t y was found about 200 km t o the east of i t s expected p o s i t i o n . Caner et a l (1967) attempted q u a n t i t a t i v e i n t e r  p r e t a t i o n of the combined data from a l l p r o f i l e s ; u s i n g d i f f e r e n t s t r u c  t u r a l models, they concluded t h a t a r i s e of conducting m a t e r i a l t o w i t h i n 25-35 km of the surface under the western region could e x p l a i n the ^observed a t t e n u a t i o n i n Z - c o n s i d e r a b l y shallower than the previous e s t i m a t e s . In October 1966, Gough and R e i t z e l operated a GDS p r o f i l e a t l a t i t u d e 38.5°N, and confirmed the p r e l i m i n a r y r e s u l t s p r e v i o u s l y i n f e r r e d from the more w i d e l y spaced IGY s t a t i o n s of Matsushita (i960); the d i s  c o n t i n u i t y was c l e a r l y l o c a t e d at l o n g i t u d e 106°. Subsequent work (Gough and Anderson, 1968) at intermediate l a t i t u d e s has s i n c e confirmed the e x i s t e n c e of t h i s f e a t u r e across the e n t i r e U.S.A. The d i s c o n t i n u i t y f o l l o w s roughly the l i n e of the Rocky Mountains, but d e t a i l e d p r o f i l i n g shows s i g n i f i c a n t departures (100-200 km) t o e i t h e r s i d e of t h i s l i n e . I t i s c l e a r t h a t the term "anomaly" can h a r d l y be a p p l i e d t o a f e a t u r e covering perhaps as much as a quarter of the c o n t i n e n t a l area. In the f o l l o w i n g work, the term "anomalous" has been a p p l i e d only t o s t a t i o n s where the r a t i o Z/H i s a f u n c t i o n of azimuth, i . e . i n d i c a t i v e of deep- seated l a t e r a l inhomogeneities - f o r example, A l e r t (Whitham and Andersen, 1962) and the Kootenay Lake S t a t i o n (Hyndman, 1963; Caner et a l , 1967). A l l other GDS d a t a , whatever the a c t u a l value of the Z/H r a t i o , i s considered "normal", r e p r e s e n t a t i v e of a p a r t i c u l a r c o n d u c t i v i t y s t r u c t u r e 0 Although s e v e r a l second-order d i f f e r e n c e s have been observed, the feature of main i n t e r e s t remains the l a r g e region of low Z/H r a t i o defined by the above r e s u l t s f o r western North America. The i n t e r e s t i n t h i s f e a t u r e i s heightened by the f a c t t h a t the same re g i o n (although not so s h a r p l y d e l i n e a t e d ) i s a l s o c h a r a c t e r i s e d by high heat-flow (Lee and Uyeda, 1965; 8 Roy et a l , 1968), low seismic Pn v e l o c i t i e s , and absence of long-wavelength " s t a t i c " magnetic features (Pakiser and. Z i e t z , 1965). I f successfully t i e d i n with t h i s other geophysical evidence, the geomagnetic induction results could therefore be of considerable interest to: a) delineate the exact boundaries of t h i s "western-type" or "Cordilleran" geophysical region, and b) help interpret i t s causes and o r i g i n . In p a r t i c u l a r , i t has been suggested (Caner and Cannon, 1965) that the observed effects could be the surface expressions of an inland continuation of the East P a c i f i c Rise. With the major conductivity structure regions being delineated by GDS, i t becomes possible to interpret the data from the widely scattered MT stations i n a more systematic manner as representative of certain regions. Swift (1967) operated several MT stations i n the southwest U.S.A., follow ing Schmucker's GDS p r o f i l e ; his results confirmed the existence of a zone of high conductivity at shallow depth under the western region. I-C) Thesis outline The main objectives of t h i s work are: 1) Determination of the e l e c t r i c a l conductivity structure of the lower crust and upper mantle i n western Canada, using geomagnetic induction methods. Integrated GDS/MT methodology has been used; such an approach has not previously been applied to large-scale investigations of t h i s nature. • 2) Derivation of a petrological model for these depths (10-60 km) i n t h i s region. This model i s based primarily on the e l e c t r i c a l conductivity structure, but compatibility with other relevant geophysical information i s maintained. .9 • To achieve the above o b j e c t i v e s , the e a r l i e r work described i n the preceding s e c t i o n has been extended, and s e v e r a l new l i n e s o f approach were s t a r t e d (Figure 1 - 2 ) . In GDS, mapping has been continued w i t h a north-south p r o f i l e from Pri n c e George t o Cache Creek, and w i t h s e v e r a l f i l l - i n s t a t i o n s i n the v i c i n i t y o f d i s c o n t i n u i t i e s ; a l s o a hig h e r - q u a l i t y data set was obtained f o r one p a i r of s t a t i o n s t o permit d e t a i l e d q u a n t i t a t i v e a n a l y s i s . The main emphasis of the work has s h i f t e d t o MT, t o provide more r e l i a b l e i n f o r m a t i o n on the subsurface c o n d u c t i v i t y s t r u c t u r e s r e s p o n s i b l e f o r the observed GDS e f f e c t s . The MT work i n v o l v e d a l a r g e - s c a l e f i e l d survey i n v o l v i n g simultaneous operation at 5 s t a t i o n s . Although c h r o n o l o g i c a l l y the GDS work preceded the MT work, they have been presented i n the reverse order f o r a more l o g i c a l p a t t e r n ( s i n c e the MT s t r u c t u r e models are re q u i r e d f o r t e s t i n g of the GDS r e s u l t s ) . The experimental requirements f o r the two methods vary w i d e l y and most work i n these two f i e l d s i s c a r r i e d out i n separate surveys. The GDS and MT p r o j e c t s described i n t h i s r e p o r t were a l s o c a r r i e d out i n separate operations, but o p e r a t i o n a l c o n d i t i o n s were set up i n such a way as t o permit combined i n t e r p r e t a t i o n , or at l e a s t mutual c o n t r o l . S p e c i f i c a l l y , GDS has been used mainly f o r l a r g e - s c a l e mapping and MT f o r d e t a i l e d q u a n t i t a t i v e i n t e r  p r e t a t i o n at s e l e c t e d " r e p r e s e n t a t i v e " l o c a t i o n s . The r e l a t i v e f u n c t i o n s performed by the MT and GDS data i n t h i s work can perhaps be l o o s e l y d e s c r i b e d by analogy w i t h d r i l l - h o l e l o g g i n g and seismic e x p l o r a t i o n : d r i l l i n g (MT) provides d e t a i l e d i n f o r m a t i o n a t high cost a t a few s e l e c t e d l o c a t i o n s ; the s t r u c t u r e s ( h o r i z o n s ) can then be mapped economically over much wider areas by seismic surveys (GDS). 10 - s r E l MAGNETO -TELLURIC , STATIONS GEOMAGNETIC STATIONS: d LOW -1 © HIGH - I TRANSITION A ANOMALOUS 0* O * 120' I i i i "v. 100 KM h l Vv O o o* o 6 ^ o' \ *0* O A ® L R ^ ^ W A S H I N G T O N 120' L_„ ^! j I IDAHO [ 6 \ NEWPORT % \ i J F i g . 1-2. Location o f GDS and MT s t a t i o n s i n western Canada 11 The r e s u l t s from a l l the GDS and MT data have been combined t o \ provide a r e l i a b l e and s e l f - c o n s i s t e n t c o n d u c t i v i t y s t r u c t u r e model f o r southwest Canada. Other r e l e v a n t g e o p h y s i c a l evidence has a l s o been examined. D e t a i l e d a n a l y s i s has been c a r r i e d out on s e v e r a l long aero magnetic p r o f i l e s ; the observed smoothing of long-wavelength f e a t u r e s i n the western r e g i o n has been t i e d i n w i t h the geomagnetic i n d u c t i o n r e s u l t s . A s e l f - c o n s i s t e n t p e t r o l o g i c a l model f o r the lower c r u s t a l and upper mantle s t r u c t u r e has been d e r i v e d t o s a t i s f y a l l the above data. I t i s shown t h a t t h i s i s not a unique s o l u t i o n , since the o r d e r - o f - magnitude u n c e r t a i n t i e s i n the r e l a t i o n s between c o n d u c t i v i t y , temperature, and composition preclude any such confidence i n the p e t r o l o g i c a l models. I-D) Theory A b r i e f o u t l i n e of the theory a p p l i c a b l e i n geomagnetic i n d u c t i o n work i s given below. The o u t l i n e i s based on the t h e o r e t i c a l work of P r i c e (1962) and Wait (1962), and i s e s s e n t i a l l y an adaptation of subsequent developments by others ( p a r t i c u l a r l y S r i v a s t a v a , 1965 and Whitham, 1963). From the f i e l d equations, f o r a g e n e r a l l i n e a r i s o t r o p i c medium we can o b t a i n expressions f o r the e l e c t r i c and magnetic f i e l d s w i t h i n a conductor, f o r e x t e r n a l inducing f i e l d s . Assuming Ez = 0, and n e g l e c t i n g displacement c u r r e n t s , these ar e : e (1) H — £ . . U + . — T — 1 (2) V i s a constant which d e f i n e s the h o r i z o n t a l s c a l e of the source f i e l d (X = 2 l i / V ) , and the f u n c t i o n s P and Z are defined by the f o l l o w i n g 12 r e l a t i o n s : ^K*} + + V*P = 0 (3) Consider a h o r i z o n t a l l y l a y e r e d e a r t h ; w i t h i n each l a y e r of thi c k n e s s h, the c o n d u c t i v i t y S i s constant; the s o l u t i o n f o r (4) i s : = Re + Be. (5) where U * « V * + «+M i lO S S u b s t i t u t i n g (5) i n t o ( l ) and (2) and t a k i n g the r a t i o , we o b t a i n an exp r e s s i o n f o r the "impedance" w i t h i n the l a y e r : (6) or - - ±Ji c t k [U z - JU. C ft/e>V/;l] (V) Taking i n t o account the c o n t i n u i t y at the i n t e r f a c e s , the "impedance" in each l a y e r can be expressed as a f u n c t i o n of the impedance i n the adjacent l a y e r above i t , and the "surface impedance" by a complex f u n c t i o n of a l l "n" l a y e r s : (8) w i t h an i d e n t i c a l e xpression (opposite s i g n ) f o r the orthogonal set Ey/Hx. 13 The above equation (8) i s the b a s i c g e n e r a l i z e d formula f o r MT work, d e f i n i n g the impedance of an n- l a y e r c o n d u c t i v i t y s t r u c t u r e as a f u n c t i o n of frequency. The curves of the o b s e r v a t i o n a l parameter observed at the surface (Ex/Hy or Ey/Hx as a f u n c t i o n of frequency) are compared t o model curves f o r v a r i o u s assumed c o n d u c t i v i t y s t r u c t u r e s . More u s u a l l y ft 2 the "apparent r e s i s t i v i t y " ,V a = 0.2(E/H) T, i s p l o t t e d a g a i n s t p e r i o d T. The preceding t h e o r e t i c a l development assumes d i f f u s i o n o f the electromagnetic f i e l d through the conductors. I f plane p o l a r i z e d e l e c t r o  magnetic waves normally i n c i d e n t on the surface are assumed, the r e l a t i o n s can be de r i v e d i n a d i f f e r e n t way by using the t r a n s m i s s i o n and r e f l e c t i o n c o e f f i c i e n t s at the boundaries. The "Cagniard-type" curves thus d e r i v e d are n u m e r i c a l l y e q u i v a l e n t t o the s p e c i a l case of V = 0, i . e . i n f i n i t e s p a t i a l wavelength, i n the g e n e r a l i z e d equation. I n p r a c t i c e , i t i s found t h a t the e f f e c t of f i n i t e sources (V / 0) becomes s i g n i f i c a n t o n l y f o r f a i r l y extreme values (V> 0.005 km - 1, i . e . \<1000 km), and even then only f o r the l o n g e r - p e r i o d range of most" prac t i c a l MT data s e t s . Below these p e r i o d s , the two formulae ( " d i f f u s i o n " and "Cagniard") are n u m e r i c a l l y i d e n t i c a l f o r a l l r e a l i s t i c values of V. I t i s t h e r e f o r e p o s s i b l e to use the much simpler Cagniard formulae f o r p r e l i m i n a r y model f i t t i n g and f o r many-layer cases where the g e n e r a l i z e d equation becomes p r o h i b i t i v e l y complicated f o r computation. Nomographs f o r g r a p h i c a l s o l u t i o n s t o "Cagniard" models are a v a i l a b l e ( F o u r n i e r , 1965) and can be used f o r rough " f i r s t - g u e s s " approximations, t o be r e f i n e d by comparison w i t h published master curves ( S r i v a s t a v a , 1967), and. f i n a l l y by comparison w i t h computer-generated model curves based on the g e n e r a l i z e d formula. 14 For i n t e r p r e t a t i o n of GDS data , the r a t i o Hz/Hy or Hz/Hx i s the o b s e r v a t i o n a l parameter. S u b s t i t u t i n g (5) i n t o ("2) and t a k i n g the r a t i o , we o b t a i n : F o l l o w i n g the same procedure as before, the r a t i o Hx/Hx at the surface can be obtained f o r an n- l a y e r c o n d u c t i v i t y s t r u c t u r e : / H - A P v* , (.. i s a m e a s 1 f - 1 ^ ) * — r - C0tK > U.H, • • ' eq'n No. 8 \ . (10) This i s the b a s i c equation used f o r i n t e r p r e t i n g the GDS data i n t h i s r e p o r t . The main d i f f e r e n c e between t h i s e xpression and the one used f o r MT i s the presence of the f a c t o r V^^/o^/^a^. . The MT expression (equation 8) i s a f u l l y e x p l i c i t f u n c t i o n of frequency (or pe r i o d T ) , s p a t i a l wavelength, and c o n d u c t i v i t y s t r u c t u r e parameters (h,S) at the s i t e : E/H = f(T,V,h,S). The GDS expression (equation 10) i s of the form Z/H = V 2 . F ( P ) . f 1 ( T , V , h , S ) , where F(P) i s not an e x p l i c i t f u n c t i o n of the s t a t i o n parameters. This d i f f e r e n c e emphasizes the main weakness of the GDS method when used f o r q u a n t i t a t i v e i n t e r p r e t a t i o n . The V term i n d i c a t e s t h a t the Z/H r a t i o i s h e a v i l y a f f e c t e d along the e n t i r e  frequency band by the value of the ind u c i n g f i e l d wavelength (whereas i t a f f e c t e d only the long periods i n MT). More s e r i o u s i s the f a c t t h a t the term F(P) = r/^o^/^x i s e n t i r e l y indeterminate. The p o t e n t i a l f u n c t i o n P and i t s h o r i z o n t a l g r a d i e n t s can be determined only by data from a lar g e - s c a l e network of s t a t i o n s covering an area much l a r g e r than the conductiv i t y s t r u c t u r e being s t u d i e d . Such a n a l y t i c determinations from surveys u s i n g l a r g e numbers (20-40) of simultaneous s t a t i o n s are being planned (Gough and R e i t z e l , 1967). However, f o r the p r o f i l e - t y p e surveys 15 d e s c r i b e d i n t h i s work (4-8 s t a t i o n s ) , the f a c t o r F(P) remains i n d e t e r  minate. I n p r a c t i c e , the method of a n a l y s i s i s t h e r e f o r e based on s i m u l  t a n e o u s l y recorded data from at l e a s t two s t a t i o n s , one re p r e s e n t i n g the unknown c o n d u c t i v i t y s t r u c t u r e , and one a " s t a n d a r d i z i n g " s t a t i o n w i t h (hopefully.') known c o n d u c t i v i t y s t r u c t u r e (Whitham, 1963; Caner et a l , 1967). For such t w o - s t a t i o n work, the unknown term V .F(P) drops out i n the r a t i o : (Z/H) S t a t i o n I _ ^(T.V.h.S) S t a t i o n 1 _ Q, (Z/H) S t a t i o n 2 f (T,V,h,S) S t a t i o n 2 Q 2 y t h i s r a t i o can be evaluated as a f u n c t i o n of T f o r v a r i o u s combinations of c o n d u c t i v i t y s t r u c t u r e parameters at the two s t a t i o n s . In p r a c t i c e , i n t e r p r e t a t i o n i s u s u a l l y c a r r i e d out on the squares of the above Q - f a c t o r s , 2 2 i . e . on the r a t i o M = ^i/^2y P a r t l J " by analogy w i t h the t r a d i t i o n a l MT parameter Vo, = 0,2(E/H) T, p a r t l y f o r computational convenience when us i n g power s p e c t r a l components r a t h e r than F o u r i e r amplitudes. I t i s c l e a r t h a t such " t w o - s t a t i o n " model f i t t i n g leaves f a r too much l a t i t u d e f o r p e r s o n a l b i a s and other u n c e r t a i n t i e s , unless one of the s t r u c t u r e s 2 i s r e l i a b l y known. I t w i l l a l s o be shown t h a t although the main V term has been c a n c e l l e d out, f o r many p r a c t i c a l models the M-r a t i o i n GDS i s s t i l l more dependent on V than the apparent r e s i s t i v i t y i n MT. The two formulae d e r i v e d f o r the MT and GDS cases (equations 8 and 10) are very s i m i l a r i n s t r u c t u r e , and t h e i r r e l a t i o n i s i n f a c t d e c e p t i v e l y simple: E _ ti> { W ~L (ii) 16 The use of combined MT/GDS methods ( i . e . recording o f a l l 5 components simultaneously) would t h e r e f o r e seem t o be the obvious way to overcome some o f the a m b i g u i t i e s ; s i n c e each set can provide independent i n t e r  p r e t a t i o n , the redundancy of i n f o r m a t i o n i n the combined data should provide a more r e l i a b l e s o l u t i o n . For example, the above r e l a t i o n between E/H and Z/H could be used t o evaluate the source f i e l d parameters. In p a r t i c u l a r , i f the f a c t o r V^/^^/^x, i s independent of frequency (as has been assumed i n most c a s e s ) , the value of V can be d e r i v e d d i r e c t l y . Watanabe (1964) and S r i v a s t a v a (1965) have suggested methods f o r d e r i v i n g s e l f - c o n s i s t e n t models from such combined data. In p r a c t i c e , such "5-component" work runs i n t o d i f f i c u l t i e s . For example, broadband MT i n f o r m a t i o n i s u s u a l l y obtained from d i f f e r e n t sets of data ( s h o r t - p e r i o d band, long-period band), not n e c e s s a r i l y simultaneous, and t h e r e  f o r e not n e c e s s a r i l y possessing the same s o u r c e - f i e l d parameters. Even i f obtained during the same d i s t u r b a n c e , there i s no reason t o presuppose t h a t the s h o r t e r - p e r i o d (say 10-100 sec) f l u c t u a t i o n s are n e c e s s a r i l y caused by the same generating mechanism as the l o n g e r - p e r i o d (100-10000 sec) ones; there i s i n f a c t good reason t o b e l i e v e t h a t t h i s i s not the case, as the s p a t i a l coherence of s h o r t e r - p e r i o d f l u c t u a t i o n s i s f a r lower than t h a t of long-period ones, i n d i c a t i n g generating currents of sm a l l e r s c a l e . For the work d e s c r i b e d i n t h i s r e p o r t , such s e l f - c o n s i s t e n t GDS/MT c o n d u c t i v i t y s t r u c t u r e models were de r i v e d i n a more p e d e s t r i a n manner d i c t a t e d by p r a c t i c a l c o n s i d e r a t i o n s (mainly the non- s i m u l t a n e i t y of most o f the GDS and MT surveys). The MT models f o r c e r t a i n areas were de r i v e d s e p a r a t e l y , w i t h the maximum amount of i n t e r n a l c o n t r o l ( c l u s t e r i n g of s t a t i o n s , e t c . ) ; the GDS model curves f o r these s t r u c t u r e s were then computed and compared t o the observed GDS data f o r these areas. I f 17 necessary, the MT models were then readjusted u n t i l s a t i s f a c t o r y agree ment cou l d be reached. A l l the above t h e o r e t i c a l developments assume homogeneous h o r i  z o n t a l l y s t r a t i f i e d s t r u c t u r e s , and of s m a l l enough s c a l e so t h a t the s p h e r i c i t y of the e a r t h can be ignored. The assumption of homogeneity can be checked e x p e r i m e n t a l l y f o r each set of d a t a : i n MT, a p p a r e n t ' r e s i s t i v  i t i e s must be equal i n the two orthogonal d i r e c t i o n s , i . e . Ex/Hy = Ey/Hx f o r a l l f r e q u e n c i e s ; i n GDS, the r a t i o Z/(H 2x + H 2y)^" must be independent of the azimuth of the i n d u c i n g f i e l d v e c t o r , i . e . of the r a t i o Hx/Hy. R e l i a b l e a n a l y t i c methods f o r handling n o n - i s o t r o p i c data are as yet not a v a i l a b l e . I n MT some s p e c i a l cases of f a u l t s and d i k e s have been worked out w i t h some s i m p l i f y i n g assumptions; i n GDS, such attempts have been d i s a p p o i n t i n g l y u n s u c c e s s f u l i n p r o v i d i n g reasonable i n t e r p r e t a t i o n (see f o r example R i k i t a k e and Whitham, 1964). Although the d i r e c t i o n o f the a n i s o t r o p y axes can u s u a l l y be d e r i v e d from both MT ( B o s t i c k and Smith, 1962) and GDS (Parkinson, 1962) a n i s o t r o p i c data, the determination of c o n d u c t i v i t y s t r u c t u r e s remains u n r e l i a b l e , i n s p i t e of the i n c r e a s i n g l y complex and elegant computational methods which are being used t o process the a n i s o t r o p i c data. Even more embarrassing i s the f a c t t h a t MT a n i s o  t r o p i c s at v a r i o u s "apparent depths" are found i n areas where no.such a n i s o t r o p i e s are shown by GDS, I n view of the above l i m i t a t i o n s , the q u a n t i t a t i v e work described i n t h i s r e p o r t has been r e s t r i c t e d as f a r as p o s s i b l e t o i s o t r o p i c data. Since the primary purpose of the work i s p r a c t i c a l determination of c o n d u c t i v i t y s t r u c t u r e , such an approach i s j u s t i f i e d . I t simply means choosing ones experimental c o n d i t i o n s i n t e l l i g e n t l y i n order t o minimize a n a l y t i c a l d i f f i c u l t i e s and the r e s u l t i n g u n c e r t a i n t i e s i n i n t e r p r e t a t i o n . 18 I n p r a c t i c e , t h i s means a v o i d i n g a) a n a l y s i s on data from s t a t i o n s w i t h known a n i s o t r o p i c s (such as Kootenay Lake), and b) s i t i n g of new MT s t a t i o n s i n the v i c i n i t y of known boundaries between c o n d u c t i v i t y s t r u c t u r e r e g i o n s . Since the p a r t i c u l a r s t r u c t u r e being s t u d i e d covers an area o f c o n t i n e n t a l p r o p o r t i o n s , there i s no d i f f i c u l t y i n s a t i s f y  i n g these simple r e s t r i c t i o n s on s t a t i o n choice. The term " a n i s o t r o p i c " as used i n t h i s t h e s i s r e f e r s only t o d a t a , i . e . t o an azimuth-dependence o f the o b s e r v a t i o n a l parameter. The term "inhomogeneity" i s used f o r a c t u a l c o n d u c t i v i t y s t r u c t u r e s i n the e a r t h . These are somewhat a r b i t r a r y semantic d e f i n i t i o n s , s i n c e "inhomogeneity" as used here would r e s u l t from both a n i s o t r o p i c c o n d u c t i v i t y media and s t r u c t u r a l inhomogeneities such as f a u l t s o r dykes. I I . MAGNETO-TELLURICS II - A ) I n t r o d u c t i o n GDS mapping has d e f i n e d a c l e a r d i s t i n c t i o n between two regions i n Canada, w i t h the d i s c o n t i n u i t y f o l l o w i n g roughly' the l i n e of the Rocky Mountain Trench. The GDS data have been shown t o be compatible w i t h the exi s t e n c e o f a conducting l a y e r about 15 km. t h i c k a t depth about 25 km., i . e . i n the lower c r u s t or upper mantle (Caner et a l , 1967). However, p r o f i l e - t y p e GDS surveys do not lend themselves r e a d i l y to q u a n t i t a t i v e i n t e r p r e t a t i o n - although they are i d e a l l y s u i t e d f o r l a r g e - s c a l e mapping. The o b s e r v a t i o n a l parameter (z/H, the r a t i o of v e r t i c a l t o h o r i z o n t a l amplitude) i s not an e x p l i c i t f u n c t i o n of the subsurface c o n d u c t i v i t y s t r u c t u r e parameters. Unless a wide s t a t i o n network i s a v a i l a b l e t o permit s e p a r a t i o n of i n t e r n a l and e x t e r n a l f i e l d s , i n t e r p r e t a t i o n has t o be c a r r i e d out on the data obtained simultaneously a t two s t a t i o n s , i n order t o e l i m i n a t e the unknown terms. The use of two assumed 1 . = • I • c o n d u c t i v i t y s t r u c t u r e s f o r model f i t t i n g leaves an unacceptably l a r g e ! amount of l a t i t u d e f o r personal b i a s ; f o r a l l p r a c t i c a l purposes the s o l u t i o n i s indeterminate unless the c o n d u c t i v i t y s t r u c t u r e i s known f o r a t l e a s t one of the s t a t i o n s . i By c o n t r a s t the MT method provides r e s u l t s which can be i n t e r  preted f o r a s i n g l e s t a t i o n w i t h only the u s u a l amount o f perso n a l b i a s , i . e . i n the choice of the p a r t i c u l a r models to be checked a g a i n s t the o b s e r v a t i o n a l data.. Although the method has many disadvantages compared t o GDS, such MT observations a t a number of s e l e c t e d s i t e s can t h e r e f o r e be used t o " c a l i b r a t e " the GDS r e s u l t s f o r a p a r t i c u l a r r e g i o n . The MT survey d e s c r i b e d i n t h i s paper was d e l i b e r a t e l y designed t o provide such c a l i b r a t i o n f o r the GDS r e s u l t s i n western Canada. A s i m i l a r MT survey was c a r r i e d out i n the southwest U.S.A. by S w i f t (1967). Most of the MT data p r e v i o u s l y obtained i n western Canada covered only the s h o r t e r periods ( < 1000 sees.) and are t h e r e f o r e inadequate t o provide i n f o r m a t i o n at the lower c r u s t a l and upper mantle depths r e q u i r e d f o r c a l i b r a t i o n of the GDS dat a . Longer-period data sets were obtained a t Meanook i n northern A l b e r t a ( N i b l e t t and Sayn-Wittgenstein, I960; S r i v a s t a v a and Jacobs, 1964) and a t V i c t o r i a (Caner and Auld , 1968). However, both these s i t e s are too f a r from the areas of d i r e c t i n t e r e s t and not r e p r e s e n t a t i v e of the main regions i n other ways ( p r o x i m i t y t o source c u r r e n t s a t Meanook, and p o s s i b l y c o a s t a l e f f e c t s at V i c t o r i a ) , The s t a t i o n s used i n t h i s survey (Figure I I - l ) were not l a i d out i n p r o f i l e s to cover maximum a r e a , since the region had already been thoroughly mapped by GDS. Instead, they were c l u s t e r e d i n two groups: one c l u s t e r o f three s t a t i o n s i n the western r e g i o n ( P e n t i c t o n , Grand Forks, Osoyoos) and one c l u s t e r (Pinches, Fernie ) i n the e a s t e r n r e g i o n . In a d d i t i o n , data from the Vulcan s t a t i o n of Vozoff and E l l i s (1966) has i been used i n support of the eastern c l u s t e r . The s t a t i o n s were l o c a t e d i n areas known (from GDS mapping) t o be "normal", i . e . not i n the immediate v i c i n i t y o f deep-seated a n i s o t r o p i c s o r of the main d i s c o n - i t i n u i t y between the two zones. The purpose of t h i s c l u s t e r i n g i s t o improve the r e l i a b i l i t y of the d e r i v e d c o n d u c t i v i t y s t r u c t u r e models. The i n t e r p r e t a t i o n of MT data u s u a l l y s u f f e r s from a number of a m b i g u i t i e s : surface l a y e r e f f e c t s , a n i s o t r o p i c s , and t o a l e s s e r extent source f i e l d wavelengths. By r e c o r d  i n g s imultaneously i n a c l o s e l y spaced c l u s t e r , some of these ambigu i t i e s can be e l i m i n a t e d . C l e a r l y the models f o r the s t a t i o n s w i t h i n each c l u s t e r must agree w i t h i n reason f o r depths g r e a t e r than t h e i r s e p a r a t i n g d i s t a n c e , and f o r simultaneously recorded disturbances must use the same source f i e l d parameters. By combining the r e s u l t s from s e v e r a l s t a t i o n s , r e s t r a i n t s are imposed on the s o l u t i o n which minimize the i n f l u e n c e of p e r s o n a l b i a s . In a d d i t i o n i t can be expected t h a t some s t a t i o n s would show a n i s o t r o p i c c h a r a c t e r i s t i c s , i . e . the observed apparent r e s i s t i v i t i e s are a f u n c t i o n of azimuth. The treatment of a n i s o t r o p i c data presents one of the main d i f f i c u l t i e s i n the i n t e r p r e t a t i o n of MT data. T h e o r e t i c a l treatments f o r such data have been worked out ( f o r example B o s t i c k and Smith, 1962; Wiese, 1965; O'Brien and Morrison, 1967), but t h e i r p r a c t i c a l v alue has not been v e r i f i e d e x p e r i m e n t a l l y . By r e c o r d i n g i n c l u s t e r s w i t h i n a r e g i o n known t o be homogeneous below upper c r u s t a l depths, the treatment of any a n i s o t r o p i c data from one s t a t i o n can be v e r i f i e d e x p e r i m e n t a l l y by comparison with data from the other s t a t i o n s ( h o p e f u l l y i s o t r o p i c ) w i t h i n the c l u s t e r . F i g . I I - l . Location of MT sta t i o n s covered i n Chapter I I , as w e l l as the Vulcan s t a t i o n of Vozoff and E l l i s (1966) and the V i c t o r i a s t a t i o n o f Caner and Auld (1968). F L U X G A T E DETECTOR F L U X G A T E MAGNETOMETER i A.C. C A L O.OOI TO PO CPS 60 C.P.S _ D.C _ REJECT - BUCKING MATCHED ACTIVE BAND P A S S F I L T E R S G • 1.0 12 V. 20 TO 100 SEC. G • 0.5 AT SO AND 900 SEC. SYSTEM A ( SHORT - PERIOD ) S T R I P - C H A R T RECORDER IZ IN/„ R SYSTEM B ( LONG-PERIOD ) S T R I P - C H A R T 10' 10* PERIOD (SEC.) TO r o Figure II-2. Schematic lay-out and frequency response.of MT instrumentation used at Pincher and Penticton. . 2 3 II-B) Instrumentation Within each cluster one station was considered as "primary" (Pincher i n the east, Penticton i n the west), and took priority i n the allocation of equipment, servicing, and installation quality (length of t e l l u r i c lines in particular). The equipment used at these two stations i s shown schematically i n Figure II-2. Two separate systems were used for the magnetic components: portable three component Askania GV-3 variographs for the long-period band (DC to 400 sec. period), and three-component fluxgate magnetometers for the short-period band (800 to 20 sec. "period). The latter i s a portable transistorized version of Serson's (1957) station magnetometer. The same t e l l u r i c system was used for both frequency bands. The electrodes were formed by lead plates (2' x 2') buried at depths of about 5'j contact resistances were negligible i n comparison to the input impedance of the amplifiers. The t e l l u r i c systems were aligned in magnetic coordinates, E^g or E^ and or E^, i.e. rotated 21-22° clockwise of geographic coordinates. This provides orthogonality with the recorded magnetic components D (magnetic east west) and H (magnetic north south). Line lengths varied between 400 and 700 meters at the secondary stations and between 860 and 1000 meters at Pincher and Penticton. The t e l l u r i c signals were amplified by Mediator type A-6LRB DC microvoltmeter amplifiers, modified for MT work by inclusion of calibration circuits and 60 Hz rejection f i l t e r s (see Figure II-2). Fi l t e r s were used to shape the amplifier output for the two frequency bands. The same basic equipment was used at the secondary stations (Fernie and Grand Forks), with the following exceptions: a) Medistor amplifiers were not available for the t e l l u r i c s , and comparable circuits constructed in the laboratory were used; b) no Askania variographs were operated, and 24 apparent resistivities in the long-period band were computed using the magnetic data from the nearest primary station. "As w i l l be shown later, the horizontal magnetic components at these long periods are homogeneous over long distances and can be considered constant in amplitude over the short distances between the primary and secondary stations ( < 100 km) within each cluster. At the fifth station (Osoyoos) no magnetic detectors were available, and a l l apparent resistivity data (long and short period bands) were computed using the magnetic data from Penticton or Grand Forks. Since the short-period fluctuations cannot be reliably assumed constant even over these short distances, the accuracy of the short-period data at Osoyoos is therefore not very high. In addition, some difficulty was encountered in aligning the telluric lines in the magnetic coordinate system. The station was located on a plateau on top of Mount Kobau, at altitude 6000 f t . , and because of topographical limitations, the telluric lines had to be misaligned by 15° with respect to magnetic co ordinates. A l l telluric data were therefore derived by computational axis rotation, adding to the inaccuracies at this station. The long-period equipment was operated continuously at a l l stations for the entire seven-week duration of the survey (May 17 to July 7, 1967). Only two sets of short-period equipment were available, and these were shifted halfway through the survey to provide about 3 weeks operation at each station. .An attempt was made to extend the data to the diurnal fluctuations (24, 12 and 8 hour periods), hence the relatively long telluric lines and deeply buried electrodes at the two primary stations. The attempt was unsuccessful, as no uninterrupted recording of sufficient duration (say > 10 days) could be obtained. Apart from accidental interruptions (line breakages or equipment failures), the dynamic range . . . . . 25 of the t e l l u r i c r e c o r d i n g systems was i n s u f f i c i e n t to accommodate both the high s e n s i t i v i t i e s r e q u i r e d f o r the measurement of the d i u r n a l f l u c t u a t i o n s (of the order o f 0.5 mv/km), and the requirements f o r continuous "on-scale" range during major disturbances w i t h excursions exceeding hundreds of mv/km. II-C) Data ~ k An example of a s e c t i o n of long-period r e c o r d i n g i s shown i n Fig u r e s II-3a and II-3b, f o r a disturbance recorded simultaneously a t a l l s t a t i o n s . Scale bars are 50 gamma or 25 mv/km, and a l l components are. p l o t t e d t o the same s c a l e except Fernie E^. The coherency between the e l e c t r i c and orthogonal magnetic components i s g e n e r a l l y very good, although i n v i s u a l comparisons the r a p i d f l u c t u a t i o n s are over-accentuated i n the t e l l u r i c t r a c e s and tend t o obscure the lo n g e r - p e r i o d f e a t u r e s which are more prominent i n the magnetic t r a c e s . A comparison of the h o r i z o n t a l magnetic components (D and H) between P e n t i c t o n and Pincher (400 krn apart) i n d i c a t e s the s p a t i a l u n i f o r m i t y of the h o r i z o n t a l f i e l d f o r l o n g -period v a r i a t i o n s , and j u s t i f i e s the use of the amplitudes from primary s t a t i o n s f o r computations of apparent r e s i s t i v i t y a t secondary s t a t i o n s l e s s than 100 km d i s t a n t . A t h i r d Askania variograph was a l s o operated simultaneously at Salmo, about half-way between the two c l u s t e r s , and confirmed the u n i f o r m i t y of the h o r i z o n t a l f i e l d over the r e g i o n . An example of s e c t i o n s o f s h o r t - p e r i o d r e c o r d i n g i s shown i n Figure 11-45 these are not simultaneous a t any of the s t a t i o n s . At the two primary s t a t i o n s , PIN (Pincher) and PEN ( P e n t i c t o n ) , v i s u a l coherence between the t e l l u r i c and magnetic t r a c e s i s s t i l l good, although not as c l e a r as f o r the l o n g e r - p e r i o d data. At the secondary s t a t i o n s , the recordings are of poorer q u a l i t y ; some of t h e i r l i m i t a t i o n s w i l l be 2? 1 hr. H v (D) PEN H, •J.I V W E T PEN oso '-^/'A.^^r/V* E Y CRA F i g . II-3b. S e c t i o n of simultaneous MT re c o r d i n g i n the long-period band, western s t a t i o n s . Scale bars are' 50 gamma or 2'5 mv/km, and a l l t r a c e s are p l o t t e d t o same s c a l e . 28 F i g . S e c t i o n s of MT recordings i n the. s h o r t - p e r i o d band. 29 r_ 200 > ^ p </>100 • »E t (/) |_ 50 U J U J K £ H I »o z s U I Z 10 cc o < — OSOYOOS ENS/HEW P E R I O D ( S E C . ) 102 103 10* V U L C A N (VOZOFF AND ELLIS, F i g . II-5. Apparent r e s i s t i v i t y p l o t t e d as a f u n c t i o n of period f o r a l l s t a t i o n s . Note d i f f e r e n t r e s i s t i v i t y s c a l e f o r F e r n i e , and d i f f e r e n t p e r i o d s c a l e f o r Vulcan. 30 discussed f u r t h e r on. S e l e c t e d s e c t i o n s of r e c o r d from a l l s t a t i o n s were d i g i t i z e d f o r p r o c e s s i n g , u s i n g e i t h e r commercial s e r v i c e s or a Dobbie-Mclnnes P e n c i l Follower. The d i g i t i z i n g i n t e r v a l f o r the l o n g - p e r i o d band i ( o r i g i n a l l y recorded a t 1 or 2 i n / h r ) i s 72 seconds. For the s h o r t - p e r i o d band (recorded at 12 i n / h r ) , the d i g i t i z i n g i n t e r v a l i s 6 or 15 \ seconds, although some s e c t i o n s were d i g i t i z e d at c l o s e r i n t e r v a l s i n an u n s u c c e s s f u l attempt to e x t r a c t i n f o r m a t i o n f o r periods below 10 seconds. The r e s u l t i n g time s e r i e s v a r i e d i n l e n g t h between N = 500 and N - '1500 data p o i n t s . At l e a s t three s e r i e s f o r each band were used a t each s t a t i o n f o r each of the f o u r components. A t o t a l of 122 time s e r i e s were used i n the analyses. The time s e r i e s were processed u s i n g r o u t i n e power s p e c t r a l techniques (Blackman & Tuckey, 1958), and apparent r e s i s t i v i t i e s computed as a f u n c t i o n of periodica = 0.2 (E/H) T . The p l o t s of apparent r e s i s t i v i t y as a f u n c t i o n of p e r i o d are shown i n Figures I I - 5 , a t o f . Only data p o i n t s f o r which the coherency between orthogonal E and H exceeded 0.75 have been i n c l u d e d ; p o i n t s f o r which i t exceeded 0.95 are i d e n t i f i e d by s o l i d symbols. For the Vulcan data of Vozoff and E l l i s (1966) a l l p o i n t s shown are f o r coherency R>0.90. On each p l o t are a l s o drawn smoothed bands of mean + standard d e v i a t i o n ; these have been computed u s i n g t r i p l e weighting f o r the high-coherence (R>0.95) p o i n t s . These p l o t s are discussed s e p a r a t e l y f o r each s t a t i o n ; Pincher. An adequate amount of data were c o l l e c t e d between 20 and 7000 sec. p e r i o d s . The data are i s o t r o p i c , i . e . apparent r e s i s t i v i t i e s com puted from k^jg/Hg^ f a l l w i t h i n the same range as those computed from E j ^ / H N S ; t h i s permits i n t e r p r e t a t i o n u s i n g simple, h o r i z o n t a l l y l a y e r e d , c o n d u c t i v i t y s t r u c t u r e models. 31 Vulcan. The data points shown have been replotted from Vozoff and E l l i s (1966); the data are also isotropic . The sol id curve represents their interpretation of these data: a sedimentary surface layer of thickness 3.6 km, a resist ive upper crust (1000 ohm-meters), and a conducting zone (30 ohm-meters) starting at depth 35 km. Fernie. The data are highly anisotropic, with apparent r e s i s t i v i t y ratios of about 1:50 between the two axes ( i . e . t e l l u r i c amplitudes i n the EW direction about 6-8 times higher than those i n the NS direct ion) . Fortuitously the observational axes coincide with the pr incipal axes of the anisotropy; polarization plots i n both frequency bands show a highly eccentric t e l l u r i c e l l i p s e , the major axis coinciding with the observational EW axis (Fig. II-6) . Interpretation can therefore be carried out direct ly i n the pr incipal directions of the anisotropy, without computational axis rotation. Penticton. A large amount of coherent data were collected i n a l l components, including some good data at very long periods (to 7500 sees). The data are isotropic and can be interpreted with hor izontal ly-s t ra t i f ied conduct i v i t y structures. Grand Forks. The location was found to be very noisy, mainly atmospheric e lectr ic discharges, but possibly including industr ia l interference and amplifier noise as wel l . The E^g component i n particular was almost never free of noise. A reasonable amount of data were collected for the E^yA^g set, but only a few widely scattered points were obtained from E ^ g / H ^ Superf ic ia l consideration of the data points in Figure 5 might indicate some anisotropy. However, the gradual convergence of the E^g/Kg^ points towards the ^ . j / H ^ g band at long periods i s more consistent with the assumption that the E^g data are dominated by short-period noise. If 32 5 MV/KM F i g . II-6. P o l a r i z a t i o n p l o t at Fernie. 33 a p p r o p r i a t e f i l t e r i n g i s a p p l i e d t o prevent s p e c t r a l leakage from s h o r t e r - p e r i o d n o i s e , the long-period (> 1000 sec) data are i n f a c t s h i f t e d s l i g h t l y downwards i n t o the E ^ band. No f i l t e r i n g was found p r a c t i c a b l e f o r the s h o r t e r - p e r i o d data. I n t e r p r e t a t i o n was c a r r i e d out on the /^NS ^ a ^ a a l ° n e > o n the assumption t h a t i t i s r e p r e s e n t a t i v e of both components of an i s o t r o p i c s e t . Osoyoos. The Osoyoos data i s apparently a n i s o t r o p i c , w i t h apparent r e s i s t i v i t i e s i n the EW d i r e c t i o n higher by a f a c t o r of about two than those i n the NS d i r e c t i o n (a f a c t o r of about 1.2 - 1.4 d i f f e r e n c e i n the amplitudes of t e l l u r i c components). Since no magnetic components were recorded at t h i s s i t e , computational t e n s o r a n a l y s i s could not be attempted. In any case i t i s d o u b t f u l i f the d i f f e r e n c e i s s i g n i f i c a n t i n view of the low q u a l i t y of the s i t e , w i t h the somewhat g r e a t e r u n c e r t a i n t i e s i n l i n e - l e n g t h measurements, s i g n i f i c a n t e l e ctrode a l t i t u d e d i f f e r e n c e s ( i . e . p o s s i b l e c o n t r i b u t i o n s from v e r t i c a l t e l l u r i c components), and edge e f f e c t s caused by the p r o x i m i t y of the e l e c t r o d e s t o the sharp a l t i t u d e d r o p - o f f s from the mountain-top p l a t e a u . The observed d i f f e r e n c e ( f a c t o r of about 1.2 - 1.4) i s frequency independent, and i s w e l l w i t h i n the p o s s i b l e range of combined i n a c c u r a c i e s i n the two components caused by the above e f f e c t s . II-D) I n t e r p r e t a t i o n - Eastern S t a t i o n s Model curves f o r d i f f e r e n t c o n d u c t i v i t y s t r u c t u r e s have been f i t t e d t o the experimental d a t a , u s i n g the formulas of S r i v a s t a v a (1965, 1967). These are based on Wait's (1962) and P r i c e ' s (1962) th e o r y , i n v o l v i n g f i n i t e h o r i z o n t a l wavelengths f o r the inducing magnetic f i e l d s (see S e c t i o n I-D). In p r a c t i c e , f o r almost a l l the cases reported i n t h i s paper, the r e s u l t s were i n d i s t i n g u i s h a b l e from those which could have been obtained from the s i m p l e r Cagniard (1953) plane-wave theory, i . e . u s i n g i n f i n i t e s p a t i a l wavelengths; t h i s confirms Madden and Nelson's (1964) analysis of the difference between the two methods. The Pincher data can best be considered in conjunction with the Vulcan data of Voz:off and E l l i s (1966) - see Figure II-7a. The frequency ranges of the two data sets are complementary, and since the two stations are only 110 km apart, the major deep structural features should be the same at the two s i tes . The heavy l ine (Model 72) on the Vulcan data of Figure II-7a represents the or ig inal interpretation of Vozoff and E l l i s (1966): a sedimentary surface layer of thickness 3»6 km, a resist ive upper crust of thickness 35 km, and a "base" r e s i s t i v i t y of 30 ohm-meters. The surface layer was composed of three dis t inc t layers, using information from o i l - w e l l conductivity logs; for purposes of f i t t i n g long-period models i t can be replaced by a single layer of same overal l thickness (3.6 km) with an equivalent integrated r e s i s t i v i t y of 16 ohm-meters. O i l - w e l l logs i n the Pincher area were examined, and the two deepest wells (one 4.8 km deep 15 km to the SW, and one 4.1 km deep 15 km to the SE) did not penetrate the basement rocks. We have assumed a thickness of 4.8 km for the sedimentary surface layer; for this thick ness an integrated r e s i s t i v i t y of 11 ohm-meters i s required to f i t the data. From the point of view of f i t t i n g models to long-period data this assumption i s arbitrary, since completely equivalent models can be con structed using different combinations of thickness and r e s i s t i v i t y for . the surface layer. Model 82 on Figure II-7a represents a good f i t to the Pincher data, i n reasonable agreement with the Vulcan model. The slight discrepancy i n the depth to the conducting layer (35 km at Vulcan, 30 km at Pincher) can readily be resolved: a) i t could be r e a l ; a dip of 5 km over a distance of 110 km i s quite reasonable, part icularly since MT data from stations Figure II-7. Conductivity structure models, eastern s t a t i o n s . 36 f u r t h e r north and east i n d i c a t e even g r e a t e r depths (Vozoff and E l l i s , 1966j N i b l e t t and Sayn-Wittgenstein, I960); b) the s h o r t - p e r i o d data at Vulcan does not provide high r e s o l u t i o n f o r depths and can e a s i l y be r e i n t e r p r e t e d ; f o r example, Model 73 provides as good f i t t o the Vulcan data as Model 72, and i s i n exact agreement w i t h Model 83 at Pincher (conducting l a y e r of r e s i s t i v i t y 40 ohm-meters s t a r t i n g at depth 30 km). A r e s i s t i v i t y o f 1000 ohm-meters has been assigned t o the upper c r u s t a l l a y e r f o r a l l the above models. This chosen value i s f l e x i b l e w i t h i n f a i r l y wide l i m i t s . For example, Model 82A ( r e s i s t i v i t y 6000 ohm-meters) i s i n d i s t i n g u i s h a b l e from Model 82 (1000 ohm-meters); Model 82B ( r e s i s t i v i t y 250 ohm-meters) can a l s o be accommodated w i t h only s l i g h t changes i n surface l a y e r parameters. I n f a c t the Pincher data by i t s e l f can be f i t t e d without any r e s i s t i v e upper c r u s t at a l l : Model 84 f o r example, w i t h m a t e r i a l of r e s i s t i v i t y 50 ohm-meters s t a r t i n g r i g h t below a t h i n surface l a y e r . I t i s only i n c l u s i o n o f the s h o r t e r - p e r i o d data from Vulcan which c l e a r l y i n d i c a t e s the e x i s t e n c e of a r e s i s t i v e upper c r u s t , but even these s h o r t - p e r i o d data cannot r e s o l v e the a c t u a l r e s i s t i v i t y value i n t h i s l a y e r . For example, Model 72A (6000 ohm-meters) i s b a r e l y d i s t i n g u i s h a b l e from Model 72 (1000 ohm-meters); even Model 72B (250 ohm-meters) can be accommodated, although b a r e l y so, by some adjustments i n surface l a y e r parameters. Consequently, the combined Vulcan/Pincher data can be s a t i s f i e d by any upper c r u s t a l r e s i s t i v i t y value above about 250 ohm-meters, without any upper l i m i t . Although most MT models have been constructed w i t h v a l u e s of 1,000 or 10,000 ohm-meters f o r the upper c r u s t a l l a y e r , i t should be c l e a r l y understood t h a t t h i s i s simply a convenient choice r e p r e s e n t i n g a v/ide range of p o s s i b l e values. T h i s l a c k of r e s o l v i n g power f o r h i g h - r e s i s t i v i t y l a y e r s i s inherent i n almost a l l MT da t a , even though not always e x p l i c t l y 37 - | recognized. Even s h o r t - p e r i o d data cannot provide adequate r e s o l u t i o n i f obtained over conductive surface l a y e r s , as i s the case f o r Vulcan and f o r the vast m a j o r i t y of published MT data. Only s h o r t - p e r i o d data (1-2 second pe r i o d s ) obtained d i r e c t l y over the r e s i s t i v e medium ( f o r example, C a n t w e l l and Madden, I960; Caner and Auld, 1968) can define i t s r e s i s t i v i t y v a l u e . I f the s h o r t e s t periods are attenuated by a conductive surface l a y e r , t h i s r e s o l u t i o n i s l o s t ; f o r periods over 10 sec these r e l a t i v e l y t h i n r e s i s t i v e l a y e r s are v i r t u a l l y transparent ( f o r example, f o r periods of 10 and 100 seconds, the electromagnetic s k i n depth i n m a t e r i a l of 1000 ohm-meters i s 50 and 160 km r e s p e c t i v e l y ) . Moving t o the Fernie data (Figure II-7b) we are faced w i t h the d i f f i c u l t y o f i n t e r p r e t i n g the s t r o n g l y a n i s o t r o p i c data. As p r e v i o u s l y e x p l a i n e d , the two sets of curves represent the apparent r e s i s t i v i t i e s i n the two " p r i n c i p a l " d i r e c t i o n s , ' i . e . p a r a l l e l and perpendicular t o the axes of a presumed inhomogeneity i n subsurface c o n d u c t i v i t y s t r u c t u r e . The mathematical methods developed f o r handling a n i s o t r o p i c data have been a p p l i e d mostly t o r e l a t i v e l y s h o r t - p e r i o d data (<10 - 100 sec p e r i o d s ) , and could be e x p l a i n e d by a n i s o t r o p i e s or s t r u c t u r a l inhomogeneities i n the surface or upper c r u s t a l l a y e r s , presumably on the assumption t h a t the two apparent r e s i s t i v i t y curves converge at much longer p e r i o d s . Although such an approach may be v a l i d f o r converging curves, i t i s d i f f i c u l t t o j u s t i f y f o r curves which remain s eparated at very l o n g p e r i o d s . Inhomogeneities can reasonably be expected i n the sedimentary surface l a y e r s and upper c r u s t a l r o c k s , but become p r o g r e s s i v e l y more d i f f i c u l t t o conceive as the depth i n c r e a s e s . S t r u c t u r e s r e q u i r e d to i n t e r p r e t h i g h l y a n i s o t r o p i c long-period data such as t h a t of Figure II-7b 38 •-• I • - ' ! strain the c r e d i b i l i t y , particularly since no deep-seated anisotropics  are indicated by GDS, It i s therefore the author's view that such drastic broad-band anisotropics with parallel curves and no GDS confirmation have a more t r i v i a l origin: an anomalous surface distortion of the t e l l u r i c currents, in effect a "measurement error" caused by poor sampling (electrodes too shallow, lines too short, unsuitable surface medium, etc.). This view i s strengthened by the fact that in these cases (see for example the Victoria data, Caner and Auld, 1968) the two curves are practically parallel along a very wide frequency range; a fixed multiplying factor applied to the amplitudes of either one of the measured E components brings the two curves into agreement along the entire frequency range. The Fernie data provides an excellent opportunity to test this particular type of anisotropic data. The area has previously been mapped by GDS, and we know that there are no major conductivity inhomogeneities at lower crustal depths. The Fernie location was bracketed by two normal (i.e . isotropic) GDS stations: Kimberley 55 km to the west and Crowsnest 40 km to the east. Simultaneous recordings of the same type (eastern, high Z) were obtained at Pincher Creek and Lethbridge (Hyndman, 1963). Both Pincher and Fernie therefore l i e within the horizontally-stratified eastern region; constraints should therefore be applied to the. choice of models so that reasonable structural similarity i s maintained between the two sites. The data for the major principal direction of the anisotropy (EW, upper curves on Figure II-7b) can readily be interpreted by a uni form medium of r e s i s t i v i t y 1200-1500 ohm-meters, with or without a thin conducting surface layer, and extending to at least 400 km in depth. 39 C l e a r l y such an EW s t r u c t u r e i s unacceptable i n view of the models de r i v e d f o r P i ncher, 70 km t o the east and w i t h i n the same c o n d u c t i v i t y s t r u c t u r e zone. I f we look f o r a l a y e r of c o n d u c t i v i t y 30.ohm-meters at depth 30 km as i n d i c a t e d at Pincher and Vulcan, we f i n d t h a t f o r Fernie EV/ i t cannot exceed 1.5 km i n t h i c k n e s s (Model 52); Model 53, w i t h t h i c k n e s s 2 km, i s a l r e a d y outside the acceptable range. I f we assume a d i f f e r e n c e i n depth, \ i . e . t h a t the conducting l a y e r has dipped from 30 km at Pincher t o 50 km at F e r n i e , a l a y e r as t h i c k as 2 km of 30 ohm-meters can be f i t t e d t o the data (Model 54). I f we accept a moderate change i n r e s i s t i v i t y f o r the conducting l a y e r , say 100 ohm-meters i n s t e a d of 30, a l a y e r t h i c k n e s s of 5 km at depth 30 km becomes acceptable. (Model 55, i n d i s t i n g u i s h a b l e from Model 52). I f we accept both changes i n depth and r e s i s t i v i t y , a t h i c k n e s s of up to 7 km could be acceptable f o r a l a y e r of r e s i s t i v i t y 100 ohm-meters at depth 50 km. I t i s c l e a r t h a t not"even remotely acceptable agreement can be found between the Pincher/Vulcan Models and the Fernie EW data. Models 85 and 86 on Figure II-7a show t h a t the t h i c k n e s s of the conducting l a y e r (30 - 40 ohm-meters) at Pincher must be at l e a s t 75 km i f i t i s u n d e r l a i n by r e s i s t i v e m a t e r i a l . Although moderate changes i n the parameters of a conducting l a y e r can be accepted over the 70 km d i s t a n c e between Pincher and F e r n i e , i t i s i n c o n c e i v a b l e t h a t i t s t h i c k n e s s could have pinched out from over 75 km t o l e s s than 5 km over such a short d i s t a n c e - p a r t i c u l a r l y s i n c e the GDS mapping shows no d i s c o n t i n u i t y i n t h i s r e g i o n . No such d i f f i c u l t y i s encountered i n f i n d i n g agreement between the Pincher/Vulcan models and the Fernie NS data (lower set of curves on Figure II-7b). Models 62 and 62B both f i t the data, i n exact agreement w i t h 40 Models 73 at Vulcan and 83 a t Pincher. The p o s t u l a t e d surface l a y e r (0,6 km of 1 ohm-meters) i s of course e n t i r e l y a r b i t r a r y , since no nearby w e l l - l o g s or s t a t i o n s w i t h very s h o r t - p e r i o d MT data are a v a i l a b l e ; the r e s u l t s would be unchanged by the assumption of e n t i r e l y d i f f e r e n t s urface c o n d i t i o n s w i t h roughly the same r a t i o H /(? . The r e s i s t i v i t y of the s s upper c r u s t i s a l s o i n d e f i n i t e , as f o r the Pincher models, and any value above about 250 ohm-meters could f i t the data. An i n o r d i n a t e amount of e f f o r t appears t o have been spent on d e t a i l e d d i s c u s s i o n of the Fernie d a t a . From the primary p o i n t of view of t h i s survey, i . e . determination of c o n d u c t i v i t y s t r u c t u r e i n a p a r t i c u l a r r e g i o n , i t adds only moderate support to the Pincher/Vulcan models. However i t i s of more g e n e r a l i n t e r e s t as an experimental check on the treatment of h i g h l y a n i s o t r o p i c data of a p a r t i c u l a r type (broadband p a r a l l e l c u r v e s ) . I t confirms t h a t only one of the curves has r e a l p h y s i c a l meaning (apparently the minor p r i n c i p a l d i r e c t i o n ) . The other curve (major p r i n c i p a l d i r e c t i o n ) represents an a r t i f i c i a l enhancement of the measured t e l l u r i c s i g n a l , probably by anomalous concentrations of c u r r e n t s by surface f e a t u r e s . I t i s very d i f f i c u l t t o express t h i s a n a l y t i c a l l y , but i t i n d i c a t e s t h a t unless a n i s o t r o p i e s are a l s o demonstrated by GDS d a t a , l o n g - p e r i o d a n i s o t r o p i c MT data do not n e c e s s a r i l y prove the e x i s t e n c e of a r e a l inhomogeneity at depth. To summarize, the data at the e a s t e r n c l u s t e r (Pincher, Vulcan, F e m i e NS) can be f i t t e d by the f o l l o w i n g c o n d u c t i v i t y s t r u c t u r e model: an a r b i t r a r y conducting surface l a y e r , a r e s i s t i v e l a y e r (> 250 ohm- meters) of t h i c k n e s s 30-35 km, u n d e r l a i n by m a t e r i a l of r e s i s t i v i t y 30-40 ohm-meters ( p o s s i b l y as h i g h as 50 ohm-meters). The t h i c k n e s s of t h i s conducting zone i s u n c e r t a i n , but lower l i m i t s can be derived 41 from the Pincher data (Figures I I - 7 a and 7 c ) . No drastic changes in r e s i s t i v i t y (increase to 1000 ohm-meters, Models 85 and 86 on Figure I I - 7 a , or decrease.to 1 ohm-meter, Model 82F on Figure I I - 7 c ) can be accepted for thicknesses of less than about 90 - 100 km (i.e. overall depth of 120-130 km). Moderate changes (to 10 ohm-meters) are acceptable for thicknesses as low as 75 km, i.e. overall depth about 100 km (Model 82E). One further relevant point i s the question of spatial source f i e l d parameters. A l l the above models were derived using very large horizontal wavelengths (">10000 km), and are indistinguishable from models derived using the simpler method of Cagniard (1953) which i s equivalent to the i n f i n i t e wavelength case. Madden and Nelson (1964), Srivastava (1965), and others have shown that the effect of f i n i t e wave lengths i s not serious for most MT data; the effect becomes significant only at the long-period end of the data. In GDS however, the f i n i t e wavelengths affect the data over the entire frequency range. Quantitative work i n GDS (Whitham, 1963; Caner et a l , 1967) has provided acceptable models only for values of V, the wave-number parameter in Price's (1962) expressions, of the order of 0,01 km \ This corresponds to wavelengths of the order of 600-800'km (\ = 2Tf/v). Figure I I - 7 c (lower set of curves) shows the effect of vaiying the value of V. Model 82H (V = 0.002 km-1) i s indistinguishable from V = 0, the i n f i n i t e wavelength case. Model 82J (V = 0.0050 km"1) i s readily acceptable, and even V = 0.0085 km"1 (Model 82K) i s possible without drastically altering the conductivity structure interpretation.' In fact, the observational data point scatter at the long periods can readily be explained by variable V values in the range 0 to 42 0.0080 km i.e. horizontal wavelengths between i n f i n i t y and 800 km. If we increase the base-layer conductivity to 50 ohm-meters, V as low as 0.01 km ^ i s acceptable - though barely so (Model 87). This confirms that for this type of MT data, the Cagniard (1953) method i s sufficiently accurate, yet compatibility with the lower wavelengths derived in GDS i s not excluded. . II-E) Interpretation - Western Stations ' The data for Penticton (PEN) and Grand Forks (GRA) are of the same type and can be considered together (Figure II-8a). The conductivity structure models require a thin conducting surface layer at both stations i n order to f i t these data. We have used 0.45 km at Penticton and 0.60 km at Grand Forks, with r e s i s t i v i t y 2 ohm-meters, but this i s a flexible choice; identical results at the longer periods can be obtained with various other combinations of surface layer thickness and r e s i s t i v i t y . The resistive upper crust i s much thinner than at the eastern stations - about 15 km. Here again the same uncertainty applies for the exact value of r e s i s t i v i t y i n this layer. For example models 31 and 31A (1000 and 6000 ohm-meters respectively) at Grand Forks are indistinguishable, and even Model 3IB (250 ohm-meters) can barely be resolved (Figure II-8a). In fact, since no short-period station i s available in the western cluster, the resistive layer could be omitted entirely from the models; for example i n Model 24 at Penticton, a slightly thicker (19 km) upper crustal layer of r e s i s t i v i t y 35 ohm-meters can replace the previous combination of a highly conducting surface layer and a resistive upper crust. The existence of a resistive upper crustal layer can be inferred from other evidence: the eastern region models, the Victoria data (Caner and Auld, 1968) and DC 44 r e s i s t i v i t y surveys i n western North America (Cantwell et a l , 1965; Cantwell and Orange, 1965; K e l l e r et a l , 1966; Jackson, 1966). Such a layer has therefore been included i n the subsequent models, with an a r b i t r a r i l y assigned r e s i s t i v i t y of 1000 ohm-meters. Fortunately the conductivity structure models are not c r i t i c a l l y affected by any assumptions about t h i s layer, as can be seen from comparison of the models shown i n Figure II-Ba. Below the r e s i s t i v e upper crust, the data indicate very c l e a r l y a conducting layer of r e s i s t i v i t y 8-10 ohm-meters; t h i s i s i n turn underlain by higher r e s i s t i v i t y material. The "best f i t " f o r the para meters of t h i s layer i s s l i g h t l y d i f f e r e n t f o r the two stations. At Penticton i t indicates a r e s i s t i v i t y of 10-12 ohm-meters starting at depth 13-15 km (Models 21-23 on Figure II - 8 a ) , whereas at Grand Forks, i t indicates a r e s i s t i v i t y of 6-8 ohm-meters sta r t i n g at depth 15-16 km (Models 31-34)• However, the data bands are wide enough to provide l a t i t u d e f o r f u l l agreement on a value of 8-10 ohm-meters at depth 15+2 km. We have avoided showing a "perfect agreement" pair of models f o r the two stations with r e s i s t i v i t y 9 ohm-meters; quoting an odd value such as "9 ohm-meters" would imply a degree of accuracy which i s c e r t a i n l y not j u s t i f i e d by the data used to derive these models. Below t h i s conducting layer the resolution becomes poor. Although the r e s i s t i v i t y does c l e a r l y increase again below t h i s layer, i t i s hard to define at what depth and towards what value t h i s occurs. A layer thickness of 30 km underlain by 30 ohm-meters (Model 22 on Figure II - 8a ) or a thickness of 40 km underlain by 50 ohm-meters (Model 23 give equally good f i t to the Penticton data. Similar combinations of these parameters f i t at Grand Forks (Models 31-33); even a layer t h i c k  ness of 50 km underlain by 100 ohm-meter material (Model 34) can be 45 acceptable. I t i s clear that conditions at depths below about 30-40 km can be defined only within a f a i r l y broad range of values at the western stations. . x'i? The above models were derived on the assumption that there i s a d i s t i n c t boundary between the conducting layer (8-10 ohm-meters) and the base material (30-50 ohm-meters). This need not be the case; additional " t r a n s i t i o n " layers can be postulated' with equally good f i t to the data, such as for example the models shown i n Figure II-8b. The top of the conducting layer i s s t i l l at the same value (about 10 ohm-meters) and at the.same depth (15+2 km); however, i t s thickness can be reduced from 30-40 km to 20 km, i f we introduce a t r a n s i t i o n layer (20 ohm-meters) between i t and the base medium. In e f f e c t , below the conducting layer we cannot distinguish between the case of sharply layered structures and the case of a gradual t r a n s i t i o n from 10 ohm-meters at depth 35 km to 30-50 ohm-meters at some undefined depth of the order of 75 km. Moving to the Osoyoos data, i t proved d i f f i c u l t to f i n d good agreement with the Penticton/Grand Forks models. As previously explained, the Osoyoos data i s of very poor qual i t y ; nonetheless an attempt has been made to derive some models which would agree with the other stations. Models were f i t t e d to the NS data; t h i s a r b i t r a r y choice i s based on: a) the evidence of Fernie, where the lower set of curves was proven to be the "correct" one, and b) i n t e r n a l consistency - i . e . somewhat better agreement with the data from Penticton. Figure II-8c shows several model curves f i t t e d to these data. The depth to the conducting layer has been taken as 22 km.for the upper set on t h i s Figure; t h i s i s somewhat extreme i n comparison to the depth of 15-16 km derived f o r Penticton, but not 46 impossible. With this depth we find that the minimum r e s i s t i v i t y of a conducting layer of comparable thickness (^-25 km) i s 25 ohm-meters (Model 43), i . e . 2-3 times as high as the values derived for Penticton and Grand Forks. A second group of model curves i s shown i n the lower set of Figure II-8c. These indicate that lower r e s i s t i v i t i e s (15-20 ohm-meters) can be obtained for the conducting layer i f we thin i t down to about 8-10 km, (Models 45 and 46), or i f we accept i t s depth at 25 km (Model 47). Such "structural" disagreement between Osoyoos and the Penticton/Grand Forks models i s even more unacceptable than the previous set of models, which at least agreed i n structure i f not i n the exact value of r e s i s t  i v i t i e s . We conclude that no satisfactory agreement can be reached between the Osoyoos data and the combined Penticton/Grand Forks models, although the factor of 2-3 discrepancy i n r e s i s t i v i t i e s would perhaps not be considered prohibitive by the standards commonly applied to the consistency of MT data obtained at different locations. The Osoyoos data has therefore not been included i n the f i n a l combined conductivity structure models. The j u s t i f i c a t i o n for this omission i s simply the lack of internal consistency, i . e . lack of agreement with a l l other stations. The omission can however also be rationalized on the more objective grounds that the site was far from i d e a l . In particular , i t i s l i k e l y that v e r t i c a l earth-currents contributed towards the observed t e l l u r i c amplitudes. Although theo r e t i c a l l y irrelevant for horizontally s t ra t i f ied structures, such coherent v e r t i c a l currents have been observed i n practice (for example Jones and Geldart, 1967),.with amplitudes comparable to those of the horizontal components. They could cause a significant increase i n the observed 47 t e l l u r i c amplitudes a t a s i t e such as Osoyoos where a) the el e c t r o d e s were not at the same l e v e l o r over l e v e l i n t e r v e n i n g ground, and b) there are s i g n i f i c a n t changes i n topography (few thousand f e e t ) a t short d i s t a n c e s from the e l e c t r o d e s . I t should be pointed out t h a t the shape of the apparent r e s i s t i v i t y curves a t Osoyoos i s the same as t h a t obtained a t the other two s t a t i o n s ; a v e r t i c a l frequency-independent s h i f t by a f a c t o r of 4 i n apparent r e s i s t i v i t y ( f a c t o r o f 2 change i n t e l l u r i c amplitude) would b r i n g the curve i n t o almost p e r f e c t overlap w i t h the P e n t i c t o n data. Such a frequency-independent f a c t o r may be caused by- e i t h e r s u r f a c e - a n i s o t r o p i c e f f e c t s or v e c t o r a d d i t i o n of a v e r t i c a l component. I t i s very tempting t o omit the Osoyoos data e n t i r e l y from t h i s paper, as without i t the r e s u l t s could have been presented much more e l e g a n t l y i n a p e r f e c t l y c o n s i s t e n t p a t t e r n . They have nevertheless been i n c l u d e d , even though not considered f o r the f i n a l models, i n order t o emphasize one of the weaknesses of MT models: the d i f f i c u l t y i n o b t a i n i n g c o n s i s t e n t r e s u l t s from s e v e r a l s t a t i o n s . II-F)Summary - C o n d u c t i v i t y S t r u c t u r e The combined data can be summarized i n Figure II-9, w i t h the a d d i t i o n of the f o l l o w i n g remarks: 1) A r e s i s t i v e upper c r u s t i s i n f e r r e d at the eastern c l u s t e r . The exact value of the r e s i s t i v i t y i s i n d e f i n i t e ; any value from about 250 ohm-meters up could f i t the d a t a , without upper l i m i t . 2) At the eas t e r n c l u s t e r the t h i c k n e s s of t h i s r e s i s t i v e upper c r u s t i s 20-35 km; i t i s u n d e r l a i n by moderately conducting m a t e r i a l , r e s i s t i v i t y 30-50 ohm-meters. 48 DC t— CL UJ O 20 40 60 80 100 \ \ \ \ \ \ \ \ \ \ V 15 i 5 KM 2 0 - 4 0 KM \ \ \"\ \ 30 - 35 KM > 250 - ^ - M \ \ \ W / 10 -20 - ^ - M ? / / / / / { Figure II - 9 . Summary, combined conductivity structure models for the two regions. 49 3) No d e t a i l can be d i s t i n g u i s h e d w i t h i n t h i s moderately conducting m a t e r i a l , but there are no d r a s t i c changes i n r e s i s t i v i t y ( t o 1 ohm-meter or.1000 ohm-meters) down to a depth of at l e a s t 125 km, and not even moderate changes (to about 10 ohm-meters) t o a t l e a s t 100 km. 4) At the western c l u s t e r the upper c r u s t a l l a y e r i s only 15 + 5 km t h i c k , and i t s r e s i s t i v i t y cannot be r e s o l v e d . I t i s u n d e r l a i n by a conductive l a y e r of r e s i s t i v i t y 10+5 ohm-meters and t h i c k n e s s at l e a s t 20 km. 5) Below t h i s l a y e r the r e s i s t i v i t y i n c r e a s e s again, apparently t o the same "base" value as at the e a s t e r n c l u s t e r (30-50 ohm-meters). 6) No d e t a i l can be d i s t i n g u i s h e d below the conductive l a y e r . The l a y e r could be uniform, extending t o a depth of 35-55 km (t h i c k n e s s 20-40 km) and o v e r l y i n g d i r e c t l y the base m a t e r i a l . A l t e r n a t i v e l y , the r e s i s t i v i t y could shade g r a d u a l l y from 10 ohm-meters at depth 35 km towards the base-value (30-50 ohm-meters) at some undefined depth of the order of 75 km. 50 I I I . GEOMAGNETIC DEPTH-SOUNDING I I I - A ) I n t r o d u c t i o n The GDS r e s u l t s are presented i n two separate p a r t s - "mapping" ( s e c t i o n I I I - C ) and " q u a n t i t a t i v e i n t e r p r e t a t i o n " ( s e c t i o n I I I - D ) . The former i s the main purpose of the GDS work; s i n c e GDS models are ambiguous, q u a n t i t a t i v e i n f o r m a t i o n i s best obtained from the r e p r e s e n t a t i v e MT c l u s t e r s . Nevertheless, the MT models should a t l e a s t f i t as one of the p o s s i b l e s o l u t i o n s t o the GDS data , t o confirm the v a l i d i t y of usin g the MT models as " c a l i b r a t i o n " f o r the GDS surveys. I t should however be c l e a r l y understood t h a t the GDS models d e r i v e d i n s e c t i o n I I I - D are not independently d e r i v e d s o l u t i o n s which can be used t o r e i n f o r c e the MT i n t e r p r e t a t i o n . I t i s simply a demonstration that the MT de r i v e d con d u c t i v i t y s t r u c t u r e model i s one of the p o s s i b l e s o l u t i o n s to the GDS data. I I I - B ) Instrumentation The b a s i c instruments used f o r almost a l l the GDS work d e s c r i b e d i n t h i s r e p o r t are Askania type GV-3 three-component p o r t a b l e variographs. On occasion these have been supplemented by Fluxgate magnetometers - Serson's (1957) IGY s t a t i o n magnetometer, or a l a t e r t r a n s i s t o r i z e d v e r s i o n developed a t the Dominion Observatory i n Ottawa. The Askania GV-3 v a r i o - graph i s a s e l f - c o n t a i n e d u n i t comprising the three variometers (suspended- f i b r e t y p e ) , l i g h t - s o u r c e and o p t i c a l t r a n s m i s s i o n system, c a l i b r a t i o n c o i l s , r e c o r d i n g magazine (motor d r i v e n f i l m s p o o l ) , and i n t e r n a l thermo s t a t c o n t r o l l e d heaters. T h e i r main advantage i s t h e i r s t a b i l i t y . A f t e r a 24-hour " s e t t l i n g - i n " p e r i o d , the recorded t r a c e s are u s u a l l y s t a b l e t o w i t h i n 1-2 gamma; temperature e f f e c t s are minimized by temperature-51 compensated suspension systems ( d u a l f i b r e s ) and i n t e r n a l thermostats. Q u a n t i t a t i v e work can t h e r e f o r e be c a r r i e d out t o very long p e r i o d s , such as the d i u r n a l 24-hour v a r i a t i o n . ' ' i The frequency response of the variographs i s l i n e a r from DC t o about 2 sec p e r i o d , but i n p r a c t i c e i s l i m i t e d t o about 200 sec p e r i o d by the ti m e - s c a l e r e s o l u t i o n and s e n s i t i v i t y of the r e c o r d i n g s . T h i s i s adequate f o r GDS work, but f o r MT work the magnetic data a t s h o r t e r periods must be obtained from another set of instruments. Recording i s on continuous r o l l s (10 meters long) of photographic paper (12 cm. wide), p e r m i t t i n g up. t o 3 weeks of unattended o p e r a t i o n when r e c o r d i n g at 2 cm/hour. The o r i g i n a l d r i v e system f o r the r e c o r d i n g r e e l s was found t o be h i g h l y u n s a t i s f a c t o r y ; designed by the manufacturer f o r 50 cps o p e r a t i o n , the motors were found t o be e r r a t i c and u n r e l i a b l e when operated on 60 cps l i n e power. In e a r l i e r years up t o 25$ l o s s i n record was encountered, r e q u i r i n g i n e f f e c t continuous s u p e r v i s i o n of the s t a t i o n s . A new set of d r i v e u n i t s was t h e r e f o r e designed and b u i l t at V i c t o r i a , u s i n g 60 cps motors w i t h appropriate gear t r a i n s . Record l o s s w i t h the new u n i t s i s n e g l i g i b l e ; they were a l s o designed to provide v a r i a b l e chart speeds by changes i n an e x t e r n a l l y a c c e s s i b l e gear s e t . In p a r t i c u l a r , non-metric cha r t speeds ( l or 2 inches/hour) are now a v a i l a b l e i n a d d i t i o n t o metric speeds (20 or 40 mm/hour). This permits b e t t e r t i m e - s c a l e r e s o l u t i o n and a l s o f a c i l i t a t e s d i r e c t v i s u a l comparison w i t h t e l l u r i c s t r i p - c h a r t r e c o r d i n g s obtained on non-metric chart d r i v e s . The accuracy o f the instruments i s of the order of 2-3$, and even i n c l u d i n g reading e r r o r s an o v e r a l l accuracy of b e t t e r than 5% i n amplitude 52 can r e a d i l y be maintained f o r moderate and l a r g e disturbances (say 25-50 gamma). No attempt has u s u a l l y been made to use m a r g i n a l - q u a l i t y data, i . e . s m a l l amplitude f l u c t u a t i o n s where reading e r r o r s become s i g n i f i c a n t . Operation at each s t a t i o n was u s u a l l y continued f o r as long as necessary t o o b t a i n the r e q u i r e d amount of "good" dat a , i . e . 2 or 3 reasonably l a r g e d i s t u r b a n c e s . Even d u r i n g the "quiet-sun" years t h i s d i d not u s u a l l y exceed 3-6 weeks. The main l i m i t a t i o n on the more extensive use of Askania v a r i o - graphs i s t h e i r high cost - over $12,000 each. With the advent of r e a  sonably s t a b l e t r a n s i s t o r i z e d f l u x g a t e instruments and of s i m p l i f i e d , and c o n s i d e r a b l y cheaper variographs (Gough and R e i t z e l , 1 967), i t would appear that the use of Askania variographs f o r l a r g e - s c a l e surveys w i l l be l i m i t e d i n the f u t u r e . They are more l i k e l y t o be used f o r e i t h e r s m a l l e r - s c a l e surveys of d i s c o n t i n u i t i e s and l o c a l anomalies, or as "anchor" s t a t i o n s i n l a r g e - s c a l e surveys i n combination w i t h l a r g e r numbers of l o w e r - q u a l i t y instruments. I I I - C ) Mapping The r e s u l t s of e a r l i e r work i n North America have already been o u t l i n e d i n s e c t i o n I-B. Figure I I I - l shows samples of record obtained on the 3 p r o f i l e s operated on t h i s p r o j e c t before 1966: the same type of a t t e n u a t i o n i n the v e r t i c a l component (Z) i s observed at the western s t a t i o n of each p a i r . S p e c t r a l a n a l y s i s of these data i n d i c a t e d t h a t the frequency dependence of the a t t e n u a t i o n was w i t h i n the same range f o r a l l t h r ee p r o f i l e s (Caner et a l , 1967). The d e r i v e d a t t e n u a t i o n curve has been used i n subsequent mapping p r o f i l e s as the " c h a r a c t e r i s t i c s i g n a t u r e " of the c o n d u c t i v i t y s t r u c t u r e c o n t r a s t . 53 REVELSTOKE JULY 11.-1945 CALGARY GRAND FORKS LETKBRIDGE ] <»> . [ ., NEWKIRK / MAT tt -nu ] MAT 3t.-196* 1 I DALLAS [ F i g . I I I - l . Magnetogram copies from p a i r s of s t a t i o n s a t : (a) l a t i t u d e 51°N ( p r o f i l e D of F i g . I - l ) (b) l a t i t u d e 49.5°N ( p r o f i l e C) (c) l a t i t u d e 33°-35°N ( p r o f i l e B) S c a l e bars are 50 gammas, time marks are 1 hour. 1 2 6 ° GEOMAGNETIC STATIONS : o LOW - I ® HIGH -1 TRANSITION A ANOMALOUS . 12 0 I i I - 114 o 100 KM ' o ^ o 1 \ PACIFIC h- OCEAN Pv~q 33 A 31 -1 ..._o-. O A $ 5 ? - ^ 6 ^ <? IDAHO WASHINGTON 1 2 0 ° o NEWPORT MONTANA \ J L F i g . III-2. . L o c a t i o n o f GDS s t a t i o n s i n western Canada, and of the permanent magnetic o b s e r v a t o r i e s a t V i c t o r i a and Newport. New (post -1966) s t a t i o n s are u n d e r l i n e d . 55 Figure I I I - 2 shows i n more d e t a i l the l o c a t i o n of the GDS s t a t i o n s i n southwestern Canada, both f o r the e a r l i e r surveys reported p r e v i o u s l y and f o r the subsequent work o u t l i n e d i n the f o l l o w i n g r e p o r t . The two p r o f i l e s at l a t i t u d e s 49.5°N and 51°N have been improved by the a d d i t i o n o f a few s t a t i o n s i n the v i c i n i t y of the d i s c o n t i n u i t y . On the northern p r o f i l e , s t a t i o n s at Banff and F i e l d were found t o be of the same "ea s t e r n " type as Johnston Canyon (JOH on F i g . I I I - 2 ) and Calgary. The s t a t i o n at Golden i s t r a n s i t i o n a l i n Z/H r a t i o between t h i s " e a s t e r n " type and the "western" type recorded at Revelstoke, Salmon Arm (SAL), and f u r t h e r west. The d i s c o n t i n u i t y i s t h e r e f o r e placed at or s l i g h t l y ( l 0 - 2 0 k m ) t o the west of the Rocky Mountain Trench. For the southern p r o f i l e , no t r a n s i t i o n s t a t i o n could be i d e n t i f i e d . The s t a t i o n a t Kimberley a i r p o r t (KIM), i n the Kootenay R i v e r V a l l e y , i s of e a s t e r n type, same as Crowsnest t o L e t h b r i d g e ; the s t a t i o n s , at Crescent V a l l e y (CRE) and Salmo are western- t y p e . D e t a i l e d mapping of the d i s c o n t i n u i t y i n t h i s r e g i o n i s complicated by the e x i s t e n c e of a l o c a l anomaly ( i . e . a n i s o t r o p y i n Z/H r a t i o ) near Kootenay Lake (K00), as reported by Hyndman, 1963, and Caner et a l , 1967. T his anomaly i s p r e s e n t l y being s t u d i e d i n d e t a i l by J . L a j o i e of t h i s Department; i t i s not y e t c l e a r i f i t i s caused by an independent shallow c o n d u c t i v i t y s t r u c t u r e , or by a c o n v o l u t i o n i n the main d i s c o n t i n u i t y . However, below Kootenay Lake the d i s c o n t i n u i t y can be placed w i t h reasonable confidence w i t h i n 2 0 - 3 0 km t o the west of the Kootenay R i v e r V a l l e y , the presumed southern c o n t i n u a t i o n of the Rocky Mountain Trench (Robinson, 1968). The s t a t i o n s i n the v i c i n i t y of the d i s c o n t i n u i t y are shown i n more d e t a i l i n Figure V - l (page 99 ) o f the summary. I t i s tempting t o i n t e r p o l a t e between the two p o i n t s defined by the above p r o f i l e s , u s i n g g e o l o g i c a l or t e c t o n i c boundary trends as a guide; f o r example, we could s p e c i f y the d i s c o n t i n u i t y as "running 10-30 km to the west of the Rocky Mountain Trench from 49° t o 51° l a t i t u d e " . However, experience i n other i areas ( p a r t i c u l a r l y i n Texas-Oklahoma, p r o f i l e B on Figure I - l ) has shown t h a t surface f e a t u r e s are not a r e l i a b l e i n d i c a t o r f o r the course of the d i s c o n t i n u i t y . U n t i l very c l o s e l y spaced p r o f i l e s are a v a i l a b l e , the above i n t e r p o l a t i o n over about 200 km must be considered as t e n t a t i v e , p a r t i c u l a r l y i n view of the e x i s t e n c e of a n i s o t r o p i c f e a t u r e s which may i n d i c a t e convolutions i n the d i s c o n t i n u i t y . During 1967 a new p r o f i l e was operated running north from Cache Creek towards P r i n c e George, i n an attempt t o d e f i n e a northern boundary f o r the "western-type" c o n d u c t i v i t y s t r u c t u r e r e g i o n . The s i x s t a t i o n s (Figure III-2) were occupied simultaneously, using f o u r Askania v a r i o  graphs and two t r a n s i s t o r i z e d Fluxgate magnetometers. The p r o f i l e t i e s i n at Cache Creek w i t h the e a r l i e r east-west p r o f i l e (Cache Creek t o C a l g a r y ) , and with a short north-south p r o f i l e between Cache Creek and Hope. A l l these e a r l i e r data, although not simultaneous, can t h e r e f o r e be reduced t o a common reference s t a t i o n used as the "western-type" standard - e i t h e r Cache Creek, or V i c t o r i a which was operating continuously throughout a l l these surveys. Figure III-3 shows s e c t i o n s of record obtained simultaneously at the s i x s t a t i o n s , as w e l l as at the V i c t o r i a Observatory. U n l i k e the e a r l i e r p r o f i l e s which were operated i n east-west d i r e c t i o n s , the inducing f i e l d i s not even roughly constant over the l e n g t h of the p r o f i l e , and l o c a t i o n of d i s c o n t i n u i t i e s can no longer be c a r r i e d out by simple v i s u a l 57 Z H D I I I - 3 . Recordings from Cache Creek - P r i n c e George p r o f i l e , and the magnetic observatory at V i c t o r i a . 58 3.0 2.0 UJ UJ oc o UJ X o < 1.0 o X C L N C L 0.5 < X 0.3 M D_ 0.2 0.1 50-60 MIN.- 12-30 MIN. > t LU O > ._ < LU Lt < O CO O LU o 54 c f 56' 58 GEOMAGNETIC L A T I T U D E F i g . I I I - 4 . M-ratios as a f u n c t i o n of geomagnetic l a t i t u d e f o r Cache Creek - Pr i n c e George p r o f i l e . 59 | i n s p e c t i o n . In p a r t i c u l a r , as the p r o f i l e reaches higher geomagnetic j l a t i t u d e s ( i . e . as the source currents are approached), the u n i f o r m i t y i s l o s t even over r e l a t i v e l y short d i s t a n c e s . P r i n c e George l i e s a t geomagnetic l a t i t u d e 59.6°N; i t i s d o u b t f u l i f much u s e f u l p r o f i l e work can be c a r r i e d out f u r t h e r north than t h i s l a t i t u d e , unless the p r o f i l e s can be l a i d out i n arcs along geomagnetic p a r a l l e l s o f l a t i t u d e , i . e . p a r a l l e l to the source c u r r e n t s . An attempt was made t o i n c l u d e data from Meanook Magnetic Observatory (geomagnetic l a t i t u d e 6l.8°N) i n t h i s survey, but the f l u c t u a t i o n s were b a r e l y recognizable as coherent w i t h those recorded simultaneously a t P r i n c e George. The v e r t i c a l component (Z) amplitudes recorded along the p r o f i l e ( F i g . III-3) are seen t o increase towards the n o r t h ; a t the s t a r t , from Cache Creek t o Seventy-Mile House (SEV) or Wright (WRI), the increase i s gr a d u a l , but i t becomes q u i t e steep towards the higher l a t i t u d e s - p a r t i c u l a r l y i f the r e l a t i v e l y c l o s e spacing (<75-100.km) between the s t a t i o n s i s considered. However, i t can be seen that the h o r i z o n t a l component amplitude (H) incre a s e s at roughly the same r a t e , i n d i c a t i n g t h a t the i n c r e a s e i s a source e f f e c t r a t h e r than a change i n subsurface c o n d u c t i v i t y s t r u c t u r e towards the "eastern" high-Z type. - Figure III-4 shows a p l o t of some Z/H power s p e c t r a l r a t i o s , normalized w i t h respect t o Cache Creek and p l o t t e d a g a i n s t the geomagnetic l a t i t u d e s of the s t a t i o n s . The r a t i o s from the permanent o b s e r v a t o r i e s at V i c t o r i a and Newport have a l s o been i n c l u d e d . The data i s g e n e r a l l y of poor q u a l i t y . The primary o b j e c t i v e of t h i s p r o f i l e was mapping, since no q u a n t i t a t i v e i n t e r p r e t a t i o n was t o be attempted on a north-south p r o f i l e a t high l a t i t u d e ; consequently the s t a t i o n s were checked and s e r v i c e d i n t e r m i t t e n t l y only d u r i n g the 8-week per i o d o f op e r a t i o n . As a r e s u l t only two short (5 hour) s e c t i o n s of record w i t h an adequate .1 d i s t u r b a n c e l e v e l were obtained f o r a l l s t a t i o n s simultaneously. These were d i g i t i z e d and s p e c t r a l amplitudes obtained. Figure III-5 shows the r e s u l t s f o r three s p e c t r a l bands which contained measurable energy: periods 12, 30 and 50 minutes. The s c a t t e r i s r a t h e r h i g h , but i t should be kept i n mind t h a t these p o i n t s represent the r a t i o s of f o u r powers: r e l a t i v e l y minor i n a c c u r a c i e s i n each component can r e s u l t i n l a r g e e r r o r s i n these r a t i o s . For example, 10$ e r r o r s i n each of two a m p l i  tudes can expand a z/H power r a t i o i n t o the range 0.67 t o 1.48. A normalized r a t i o of two such power r a t i o s can r e a d i l y account f o r f a c t o r - of-two s c a t t e r , p a r t i c u l a r l y i n view of the poor q u a l i t y of the samples used (short l e n g t h of time s e r i e s a v a i l a b l e f o r s p e c t r a l a n a l y s i s ) . I t i s c l e a r from Figure III-4 t h a t t h e r e i s no d r a s t i c d i s c o n t i n u  i t y i n Z/H r a t i o between Cache Creek and Prince George. Previous work on the Cache Creek-Hope s e c t i o n i n d i c a t e d t h a t there i s no d i s c o n t i n u i t y between Cache Creek and V i c t o r i a (Cannon,•1967; Caner et a l , 1967), apart from the s l i g h t l a t i t u d e dependence of the Z/H r a t i o . To i n d i c a t e the magnitude of the d i s c o n t i n u i t y between the "eastern" and "western" r e g i o n s , the change i n power r a t i o observed between F i e l d and Revelstoke ( d i s t a n c e 125 km) i s p l o t t e d on the same f i g u r e , t o the same h o r i z o n t a l and v e r t i c a l s c a l e . C l e a r l y no such d i s c o n t i n u i t y occurs along the Cache Creek/Prince George p r o f i l e . This means t h a t the northern extent of the d i s c o n t i n u i t y remains undefined, except t h a t i t l i e s east of Prince George at l a t i t u d e 54°N. I t w i l l be shown i n a subsequent chapter ( s e c t i o n IV-D) t h a t i t can be l o c a t e d more c l o s e l y u s i n g other g e o p h y s i c a l i n f o r m a t i o n , and t h a t i t s most l i k e l y course f o l l o w s roughly the boundary between the Rocky Mountains to the east and the Cariboo Mountains to the west, as d e l i n e a t e d by the 61 Rocky Mountain Trench. The above i s not proposed as evidence f o r any deep g e o p h y s i c a l s i g n i f i c a n c e of the l o c a t i o n of the Rocky Mountain Trench; the Trench i s here used simply as a convenient g e o g r a p h i c a l landmark f o r e x p r e s s i n g the course of the d i s c o n t i n u i t y . However, t h i s apparent c o i n c i  dence between the Trench and a d i s c o n t i n u i t y i n a t l e a s t two g e o p h y s i c a l parameters ( s e e " s e c t i o n IV-D) j u s t i f i e s a c l o s e r examination of i t s p o s s i b l e t e c t o n i c s i g n i f i c a n c e . I I I - D ) Q u a n t i t a t i v e I n t e r p r e t a t i o n E a r l i e r q u a n t i t a t i v e work was c a r r i e d out on the combined data from s e v e r a l p r o f i l e s , to o b t a i n some "average" or " t y p i c a l " s t r u c t u r e model, and was based on f a i r l y poor q u a l i t y data (Caner et a l , 1967). In order t o o b t a i n r e l i a b l e data f o r a s p e c i f i c l o c a t i o n , two GDS s t a t i o n s were operated f o r 7 weeks a t Pincher and P e n t i c t o n ; these are the two "primary" MT s t a t i o n s d e s c r i b e d i n S e c t i o n I I , f o r which MT-derived c o n d u c t i v i t y s t r u c t u r e models are a v a i l a b l e . Figure III-5 shows examples of r e c o r d obtained a t these two s t a t i o n s , as w e l l as at an intermediate western-type s t a t i o n a t Salmo. The a t t e n u a t i o n i n Z at the western s t a t i o n s i s c l e a r l y v i s i b l e . S p e c t r a l a n a l y s i s of three data samples ( d u r a t i o n 21+ hours, sampling i n t e r v a l 72 sees) was c a r r i e d out, and the computed normalized M-ratios are shown i n Figure III-6. These have been p l o t t e d as a f u n c t i o n .of l o g p e r i o d r a t h e r than on the f r e q u e n c y - l i n e a r s c a l e p r e v i o u s l y used f o r GDS work. Only data p o i n t s f o r which the coherence Z ( P e n t i c t o n ) - Z( P i n c h e r ) exceeded 0.75 have been i n c l u d e d , and those f o r which i t exceeded 0.95 are i d e n t i f i e d by s o l i d symbols. Using t h i s c r i t e r i o n , no " v a l i d " data were obtained f o r periods l e s s than 1200 seconds, i n d i c a t i n g t h a t f o r these short p e r i o d s : a) the i n h e r e n t l y lower amplitudes combined 62 Fig. I I I - 5 . S e c t i o n s of GDS recordings from Pincher (PIN), Salmo (SAL), and P e n t i c t o n (FEN). 63 2.0 1.0 0.5 h o . 2 0.1 300 Pig. III-6. - R - R z z z z 0 . 7 5 0 . 9 5 A - D I U R N A L VARIAT IONS ( 2 4 , 1 2 , 8 , HOUR P E R I O D ) o o o o © oo ©o 103 o o P E R I O D ( S E C . ) 104 '10' M - r a t i o s , ^z/?y) Penticton/(P„/Py) Pincher, p l o t t e d as a f u n c t i o n of p e r i o d ( P e n t i c t o n normalized w i t h respect t o P i n c h e r ) . i OQJTJ: ' ' 1 1 : 1 < 1 1 1 1 < ! — i : 1 1 JUNE II JUNE 14 JUNE 17 JUNE 20 JUNE 2S Figure III-7. 14 day recordings of the vertical component, Pincher and Penticton. 65 Z PENTICTON O '-5 K0.5< Z PINCHER PERIOD ( H O U R S ) Fig. III-8. Fourier amplitudes and r a t i o (Penticton/Pincher) f o r the d i u r n a l v a r i a t i o n s . 66 with the heavy attenuation i n the western region to produce low signal/ noise ratios, and/or b) the spatial coherence over the 400 km distance between the two stations i s too low for short periods. The scatter of the data points i s very high, but as mentioned in section III-C this i s not unreasonable for a ratio of four powers, each with i t s own errors i n calibration, measurement, and spectral analysis. In addition i t w i l l be shown that in the range 10^ - K A sees, the M- ratios are very sensitive to variations in spatial source wavelength, justifying some additional scatter for combined data from several dis turbances. The analysis has been extended to very long periods with a 14-day section digitized at half hour intervals, i n order to derive the ampli- tudes of the diurnal fluctuation and i t s harmonics. These data are shown in Figure III-7. Spectral analysis was carried out, and the diurnal peaks * are clearly resolved (Fig. III-8). The computed energy ratios for periods 24, 12, 8 hours have been included on Figure III-6. The amplitude ratios are within 15$ of unity, which i s about the accuracy which can be expected from such a short sample containing some irregular activity as well (see Fig. III-7). No significance has therefore been ascribed to the departure from unity of the ratio at 24 hours period. Theoretical models of conductivity structure have been f i t t e d to the data of Figure III-6, using the MT-derived structures as a guide to choice of models. The following table specifies the parameters of the various models shown i n Figures III-9, 10, 12. Unless otherwise indicated on the drawings, the models are normalized with respect to the structure designated as " I " at Pincher. 67 PENTICTON ' PINCHER MODEL 101 102 103 104 105 106 107 I II I I I Hi 15 15 10 10 10 10 10 H & . - - 4.8 1000 1000 1000 100 1000 1000 1000 - - 11 Ha 20 40 20 20 10 40 10 H, 35 35 35 i o 10 5 5 5 5 2 ft 1000 9900 1000 40 40 40 50 40 40 40 ^ 6 40 50 40 SUFFIX NONE A B C D VCKM-1) 0.000157 0.001250 0.002100 0.006280 0.010000 X(KM) 40,000 5,000 3,000 1,000 628 Model 101 on Figure III-9 shows the M-ratios for a structure with the median values derived from the MT data, i.e. a conducting layer of thickness 20 km and r e s i s t i v i t y 10 ohm-meters starting at depth 15 km. It i s clear that not enough attenuation at short periods can be obtained with this model, even i f we increase the thickness of the conducting layer to 40 km (Model 102). I f we change the western parameters to the jjimit of the acceptable MT-derived range, we obtain Model 103 (H, = 10 km, ^*2_- 5 ohm-m); this provides enough attenuation at the short periods, but too much attenuation at the long periods. If we push a l l the parameters (east and west) to the l i m i t , including base r e s i s t i v i t i e s and maximum contrast in upper layer r e s i s t i v i t i e s (100 vs. 10,000 ohm-m), a slight improvement i s obtained (Model 104/H); however, the change i s not significant enough to warrant such complications. Nor do more complex models (such as the 4-layers structure suggested i n Section II-E, Fig. II-8b) provide any better f i t . The only really effective way of modifying the curves are either a) changes i n the spatial source f i e l d Fig. III - 9 . GDS c o n d u c t i v i t y s t r u c t u r e models f o r long s p a t i a l wavelengths (^3000 km), compared t o M-ratios P e n t i c t o n / P i n c h e r . See page 67 f o r parameters. REN f. PINCHER H,: H2 P. P2 Pi PB pp. -2.0 o o PERIOD (SEC.) 300 IO3 10h .10 Fig. 111-10. GDS c o n d u c t i v i t y s t r u c t u r e models f o r s h o r t s p a t i a l wavelengths (^1000 km), compared t o M - r a t i o s P e n t i c t o n / P i n c h e r . See page 67 f o r parameters. 70 parameter V (discussed in the next paragraph), or b) changes in conduct ing layer parameters (depth and/or r e s i s t i v i t y ) which are outside the limits imposed by the MT models. Models 101-103 were derived with an assumed spatial wave-number V = 0.000157 km \ corresponding to a wavelength of 40,000 km, the limiting value suggested by Price (1962). The models for longer wave lengths (V = 0, k = <*>), and those for shorter ones down to about 15,000 km, are indistinguishable from the above within the frequency range under consideration, and for this particular assumed conductivity structure model (this needs to be emphasized - dependence of the models on V i s strongly affected by the assumed structures). Even for k = 10,000 km, the change i n M i s less than 3% at period 10^ sec. Models 103A and 103B on Figure III-9 show the effects of decrea sing the wavelength even further: 5,000 and 3,000 km respectively, compared to Model 103 with k = 15,000 km. This does provide some improve ment in the long-period response, but not enough, and we have to move to even shorter wave-lengths to obtain the characteristic__/~shape of the curves - i.e. no attenuation at long periods and heavy attenuation O l 0 db) at short periods. , Several model curves for shorter spatial wavelengths are shown i n Figure 111-10. For V = 0.00628 (k = 1,000 km), Models 101C and 103C show significant improvement over the equivalent long-wavelength models (101 and 103). The effect of altering the thickness of the conducting layer from 20 km to either 10 km (Model 1050) or 40 km (Model 106C) i s apparently significant, and the models would indicate that the thinner layers (i.e. 10-20 km) provide better f i t to the data. In practice 71 however, the GDS data i s not u s e f u l t o help d e f i n e t h i s parameter, i s i n c e at t h i s stage the models become very s e n s i t i v e t o even s m a l l changes i n s p a t i a l wavelength; i t i s v i r t u a l l y i mpossible t o d i s t i n g u i s h reasonable changes i n c o n d u c t i v i t y s t r u c t u r e parameters from s l i g h t • | v a r i a t i o n s i n s p a t i a l wavelength. For example, Model 103D (A. = 628 km) w i t h H 2 =20 km f a l l s r i g h t between the H 2 = 10 km and H2 = 20 km curves f o r X = 1,000 km. S i m i l a r l y , the curve f o r H 2 = 40 km w i t h X = 628 km (not shown) i s v i r t u a l l y i n d i s t i n g u i s h a b l e from Model 103C ( H 2 = 20 km w i t h X = 1,000 km). Since there i s no GDS mapping coverage east of Lethb r i d g e , i t i s not c l e a r i f the Pincher s t r u c t u r e o u t l i n e d above i s r e p r e s e n t a t i v e of the e n t i r e e a s t e r n r e g i o n , or j u s t of some l o c a l s t r u c t u r e i n the south west corner of A l b e r t a . The l a t t e r appears t o be i n d i c a t e d , s i n c e a l l published MT data f u r t h e r east or north i n d i c a t e g r e a t e r depths (70 - 150 km) to the conducting l a y e r ( N i b l e t t and Sayn-Wittgenstein, I960; S r i v a s t a v a and Jacobs, 1964). Even a t Brooks (only about 100 km t o the east of V u l c a n ) , the conducting l a y e r was estimated t o s t a r t near depth 100 km (Vozoff and E l l i s , 1966). This would mean th a t the Pincher/Vulcan r e g i o n a l r e a d y l i e s above the "western" u p l i f t e d geotherm zone, or above a t r a n s i t i o n a l s t r u c t u r e ; the d i s t i n c t i o n between t h i s r e g i o n and the western "low-I" r e g i o n which i s so sharply d e l i n e a t e d west of the Rocky Mountain Trench, i s mainly the absence of the conducting lower c r u s t a l . l a y e r , not a s i g n i f i c a n t d i f f e r e n c e i n upper mantle c o n d u c t i v i t i e s . The q u e s t i o n w i l l probably be re s o l v e d once GDS mapping coverage i s extended towards the east. I t should however be pointed out t h a t t h i s may not be as s t r a i g h t f o r w a r d as d e t e c t i o n of the main western d i s c o n t i n u i t y : GDS data i s not p a r t i c u l a r l y s e n s i t i v e t o changes i n the depth of a i moderately conducting (\* = 30-50 ohm-m) zone. Figure I I I - l l shows M-ra t i o s computed f o r a comparison between the Pincher/Vulcan s t r u c t u r e and a h y p o t h e t i c a l "normal e a s t e r n " s t r u c t u r e (depth of 70 km t o the moderately conducting zone). In the p e r i o d range 2000 to 5000 sees i n which most of the GDS data i s concentrated, the amplitude a t t e n u a t i o n f o r t h i s s t r u c t u r a l d i f f e r e n c e ranges between 0.81 and 0.87 f o r " i n f i n i t e " ( i . e . > 20,000 km) s p a t i a l wavelength; i t i s b a r e l y s i g n i f i c a n t (>0.90) f o r the s h o r t e r s p a t i a l wavelengths {r^i 1,000 km) i n d i c a t e d by the P e n t i c t o n / P i n c h e r models. I f the "normal e a s t e r n " depth i s g r e a t e r than 70 km, then the d i f f e r e n c e may be r e s o l v a b l e ; f o r example i f H^ e a g t^ = 100 km, the r a t i o could be as low as 0.75 i n amplitude f o r t h i s p e r i o d range ( F i g . I I I - l l ) . I t i s c l e a r t h a t the GDS data cannot r e s o l v e changes i n depth i n the range 35-80 km t o a moderately conducting zone. Consequently, the mapping of t h i s s t r u c t u r e towards the east would not be expected t o show an e a s i l y r e c o g n i z a b l e f i r s t - o r d e r d i s c o n t i n u i t y such as t h a t observed west of the Trench, p a r t i c u l a r l y i f the change i s g r a d u a l and/or p a r t l y masked by d i f f e r e n c e s i n conducting surface l a y e r s . The preceding models were a l l based on a simple 2-layer s t r u c t u r e at Pincher. I f we i n c l u d e a t h i c k conducting surface l a y e r at Pincher, as estimated from the MT data and w e l l - l o g s , the P e n t i c t o n / P i n c h e r GDS model curves are d r a s t i c a l l y a l t e r e d - p a r t i c u l a r l y at the s h o r t e r periods. Figure III-12 shows two such models ( 1 0 3 / i l l and 103C/III), w i t h the e q u i v a l e n t n o - s u r f a c e - l a y e r models shown i n dashed l i n e s . I f such a massive surface l a y e r i s t o he used at Pincher but not at P e n t i c t o n , the 73 -o.2 _ H = 7 Q K M : H = 1 0 0 KM - 0.1 P E R I O D ( S E C . ) .'300 _ : i o 3 •IO" 10' F i g . I I I - l l . GDS c o n d u c t i v i t y s t r u c t u r e models: K - r a t i o as a f u n c t i o n of p e r i o d , Pincher normalized w i t h respect t o h y p o t h e t i c a l "eastern" s t r u c t u r e . PENT. PINCHER H,; Pi • ' '////////,+- HSj ps PI [ P2 PB PB -2.0 PERIOD (SEC.) L i — t 300 i o 4 . 10' Figure 111-12. GDS c o n d u c t i v i t y s t r u c t u r e models"with surface l a y e r at Pinche compared t o M-ratio P e n t i c t o n / P i n c h e r . See page 67 f o r para- meters. 75 parameters of the western structure would have t o be altered beyond the l i m i t s imposed by the MT data. For example, reducing the r e s i s t i v i t y of the lower c r u s t a l layer from 5 ohm-m to 2 ohm-m (Model 107C/III) compensates f o r the addition of the surface layer. However, there i s no r e a l need f o r - i such changes, as a f a i r l y s i g n i f i c a n t upper c r u s t a l conductivity i s i n d i  cated by the MT data at Penticton as w e l l . Since no short-period MT data or well-logs were available near Penticton, the parameters of the upper crust could not be resolved. However, the integrated upper c r u s t a l conductivity appears to be of the same order as at Pincher - see f o r example the MT models on Figure II-8 (0.45 km of 2 ohm-m + 15 km of 1000 ohrn-m, or 19 km of 3 5 ohm-m, or other equivalent combinations). This would compensate f o r the surface layer effects at Pincher, leaving GDS models which are compatible with those derived from MT. This s e n s i t i v i t y of the M-ratio to surface layers (for t h i s p a r t i c u l a r pair of st r u c t u r a l models) raises some questions about the v a l i d i t y of the GDS models previously derived f o r the southwest USA - p r o f i l e B on Fig. 1-1. The region of high attenuation i n Z was found to continue w e l l east of the Rocky Mountains, into an area of very thick sedimentary layers (Caner et a l , 196?; Livingstone, 1967). Quantitative work was based on dif f e r e n t assumed conductivity structures, since no MT " c a l i b r a t i o n " was available; for these models the effects of surface layers were found to be 'negligible, and i t was concluded that the main discontinuity does indeed swing that far east - even though t h i s caused some d i f f i c u l t i e s i n interpretation: a) poorer agreement with the area of high heat flow; b) very sharp swings i n the discontinuity would be required, since i t was recovered much nearer to the Rockies both to the south and to the north (Schmuckej-,, 1964; Gough and Anderson, 1968). 76 More d e t a i l e d work has since been c a r r i e d out i n t h i s area (Gough and Anderson, 1968; Porath, 1969), and has confirmed the e x t e n s i o n of the low-Z type of rec o r d i n g towards the east. However, t h e i r i n t e r p r e t a t i o n i n d i c a t e d t h a t t h i s could be caused by the e f f e c t s of massive conducting surface l a y e r s i n t h i s area (P o r a t h , 1969). In view of the r e s u l t s obtained on the present models (using MT " c a l i b r a t i o n " r a t h e r than assumed s t r u c t u r e s ) , t h i s i n t e r p r e t a t i o n i s now considered p o s s i b l e . This would remove one of the o b s t a c l e s t o b e t t e r agreement w i t h heat f l o w and other g e o p h y s i c a l data. . I t i s c l e a r t h a t the GDS data by i t s e l f i s incapable of indepen dent s o l u t i o n , even i f i t were l e s s s c a t t e r e d . By v a r y i n g one or s e v e r a l parameters at e i t h e r one or both of two s t a t i o n s , as w e l l as the s p a t i a l wavelength, any number of p o s s i b l e s o l u t i o n s can be "confirmed", p a r t i c u l a r l y f o r data s e t s w i t h h i g h s c a t t e r . Some u s e f u l conclusions can however be drawn: a) The MT-derived c o n d u c t i v i t y s t r u c t u r e s can f i t the GDS da t a , provided we use the more conductive end of the acceptable MT-derived l i m i t s : 10 km depth r a t h e r than 15+5 km, and r e s i s t i v i t y 5 ohm-m r a t h e r than 10+5 ohm-m. With these values the t h i c k n e s s of the l a y e r would a l s o be at the lower end of the MT-derived range - about 20 km r a t h e r than 20-40 km. b) The t h r e e - l a y e r nature of the western s t r u c t u r e i s confirmed, although the base (upper mantle) r e s i s t i v i t y remains u n c e r t a i n . The f a c t t h a t the M - r a t i o a t 24-hour per i o d trends above u n i t y might i n d i c a t e s l i g h t l y higher base r e s i s t i v i t i e s i n the west than i n the.east - f o r example 75 vs 40 ohm- m, which i s w i t h i n the MT-derived range. 77 c) The s p a t i a l wavelengths f o r which GDS models can be f i t t e d vary between about 600 km and 1,500 km, which i s compatible w i t h the MT r e s u l t s . W i t h i n t h i s range, the models are very s e n s i t i v e t o even s l i g h t v a r i a t i o n s i n t h i s parameter; t h i s probably accounts f o r the l a r g e s c a t t e r observed when combining data from s e v e r a l events ( q u i t e apart from the^ a l r e a d y high s c a t t e r which can be expected f o r the r a t i o of f o u r powers). The s e n s i t i v i t y t o changes i n s p a t i a l wavelength i s p a r t i c u l a r l y evident i n the c r u c i a l p e r i o d range - 10^ sees i n which most of the GDS data i s concentrated. d) No coherent s h o r t - p e r i o d ( < 1,000 sec) data could be obtained over the 400 km d i s t a n c e between the two s t a t i o n s . T his i s p a r t l y due t o the s e n s i t i v i t y l i m i t a t i o n s of the instruments ( p a r t i c u l a r l y i n , Z a t the western s t a t i o n s ) . However, i t probably a l s o i n d i c a t e s t h a t the s p a t i a l wavelengths f o r these periods are s m a l l e r than those d e r i v e d above f o r the l o n g - p e r i o d range. T h i s would not a f f e c t the v a l i d i t y of the MT models, s i n c e i n MT only the longest periods are a f f e c t e d by short s p a t i a l wave lengths . 78 - • i IV. OTHER GEOPHYSICAL INFORMATION I IV-A) Introduction For purposes of interpretation of the lower crustal and upper mantle structure delineated by the geomagnetic induction work, data from three disciplines i s of particular relevance and i s discussed in the following sections: heat-flow, seismology, and aeromagnetic surveys. It should be stated that the relations between the results derived from these disciplines and those derived from geomagnetic induction are not at a l l clear-cut. The "western region" delineated by GDS i s also a distinct geological region, and i t i s hardly surprising to find various geophysical parameters differing between this region and the rest of North America. Although i t i s tempting to find a common interpretation, some of these differences may be entirely unconnected with the electrical conductivity structure, having neither cause/effect connection nor even a common cause. Although the following sections are a l l compatible with a common causative agent (higher temperatures i n the upper mantle under the western region), i t w i l l be shown that alternative explanations are possible. IV-B) Heat-flow - Heat-flow data i n western Canada i s as yet too sparse for regional analysis. The few available observations indicate higher heat-flow values in southern Alberta than in the Canadian Shield - of the order of 1.5 HFU as compared to 0.8 - 1.0 HFU over the Shield (Garland and Lennox, 1962; Anglin and Beck, 1965). The HFU (Heat-flow unit) i s defined as 1 microcal /cm sec. In Canada, there i s no evidence for (or against) any further 79 i n c r e a s e i n heat - f l o w i n the western region d e l i n e a t e d by GDS; the value l observed a t P e n t i c t o n i s 1.5 HFU (A.M. Jessop, p e r s o n a l communication). ! However, i n the U.S.A. the average heat-flow west of the Rockies i s i c o n s i d e r a b l y h igher than the average from other North American areas. Regions of high heat-flow have been d e l i n e a t e d i n the Basin and Range Province and west o f the southern Rocky Mountains i n Colorado (Roy et a l , 1968a; Decker, 1969), as w e l l as i n the northwest U.S.A. ( B l a c k w e l l , 1969). Two high heat-flow observations (2.0 and 2.3 HFU) have been reported on the border j u s t south of the GDS s t a t i o n at Salmo (see F i g . I I I - 2 ) . B l a c k w e l l (1969) concluded t h a t the Northern C o r d i l l e r a , Columbia Plateaus, and the Basin and Range Province form a continuous physiographic r e g i o n , the " C o r d i l l e r a n thermal anomaly zone". Roy et a l (1968b) combined heat- f l o w measurements at a number o f s e l e c t e d s i t e s , and d e r i v e d " r e p r e s e n t a t i v e " c r u s t a l geotherms f o r s e v e r a l r e g i o n s . They concluded t h a t the c o n t r i  b u t i o n from the mantle towards the observed heat-flow i s c o n s i d e r a b l y higher i n the Basin and Range Province than i n the eas t e r n U.S.A. The temperatures a t depth 35 km. d e r i v e d from these models are 460°C (eastern U.S.A.) and 860°C (Ba s i n and Range P r o v i n c e ) . S i m i l a r v a l u e s of temperature a t depth 37 km have been d e r i v e d i n A u s t r a l i a (Howard and Sass, 1964): 460°C f o r the s h i e l d a r e a , 650°C - 780°C f o r the o f f - s h i e l d areas. The geotherms d e r i v e d by Roy et a l (1968b) have been used i n a l l subsequent work i n t h i s t h e s i s . This choice i s based p r i m a r i l y on s u b j e c t i v e judgement: i t i s f e l t t h a t geotherms based on s e l e c t e d h i g h - q u a l i t y observations grouped i n dense c l u s t e r s are more r e l i a b l e than those d e r i v e d from r e g i o n a l "average" v a l u e s . However, i t i s c l e a r t h a t any assumed 80 geotherms f o r the lower c r u s t and upper mantle must be considered as s p e c u l a t i v e . I t i s simply impossible t o e x t r a p o l a t e r e l i a b l y to great depths from data obtained over the topmost few km. Th i s thread o f u n c e r t a i n t y runs throughout a l l the f o l l o w i n g d i s c u s s i o n s , and a q u a l i f y i n g phrase must be i m p l i c i t l y understood ahead of any mention o f a s p e c i f i c temperature: " i f the geotherms derived by Roy et a l (1968b) are valid..."„ We can summarize the heat-flow r e s u l t s as f o l l o w i n g : a) average heat-flow i n the western U.S.A. i s about twice as high as i n the e a s t e r n U.S.A.; no r e l i a b l e conclusions about lower c r u s t a l and upper mantle temperatures can be drawn from such average v a l u e s , because of the o v e r r i d i n g c o n t r o l of heat-flow by upper c r u s t a l composition. b) the re g i o n of high heat-flow i s i n f a i r l y good s p a t i a l agreement w i t h the h i g h - c o n d u c t i v i t y r e g i o n d e l i n e a t e d by GDS. Only i n one area (south-west U.S.A.) i s the d e n s i t y of heat-flow observations s u f f i c i e n t t o c o n f i r m exact (+ 25 km) coincidence between the t r a n s i t i o n zones. c) d e t a i l e d s t u d i e s i n the Basin and Range Province i n d i c a t e a s i g n i f i  cant c o n t r i b u t i o n o f upward heat-flow from the mantle. In view of the c o n t i n u i t y of the high heat f l o w r e g i o n t o the Northwest U.S.A., these r e s u l t s can be a p p l i e d w i t h some confidence throughout the e n t i r e western r e g i o n . d) i n t h i s r e g i o n , temperatures a t depth 3 5 km are estimated a t 860°C - about 400° higher than i n the e a s t e r n U.S.A. e) s i m i l a r l y high temperatures (650° - 780°) have been d e r i v e d i n easte r n A u s t r a l i a ; there i s some evidence t h a t higher e l e c t r i c a l con d u c t i v i t i e s are a s s o c i a t e d w i t h t h i s area as w e l l ( E v e r e t t and Hyndman, 1967). j. 81 IV-C) Seismology The western r e g i o n d e l i n e a t e d by GDS i s c h a r a c t e r i z e d by low Pn v e l o c i t i e s (compressional seismic wave r e f r a c t e d along the top of the mantle), both i n Canada (White and Savage, 196$; White et a l , 1968) and i n the United S t a t e s ( H e r r i n and Taggart, 1962; P a k i s e r and Z i e t z , 1965). East of the Rocky Mountains the Pn v e l o c i t i e s are everywhere g r e a t e r than 8.0 km/sec, t y p i c a l l y 8.1 - 8.2 km/sec. West of the Rockies (except f o r the C a l i f o r n i a c o a s t a l region) the Pn v e l o c i t i e s are g e n e r a l l y l e s s than 8.0 km/sec, t y p i c a l l y 7.8 - 7«9 km/sec. Exact coincidence between the regions of high e l e c t r i c a l c o n d u c t i v i t y and of low Pn v e l o c i t y has not been e s t a b l i s h e d , mainly because of the poor h o r i z o n t a l r e s o l u t i o n of seis m i c r e f r a c t i o n data. I t i s u n l i k e l y t h a t such exact agreement can be achieved economically to the degree of accuracy w i t h which the GDS boundary can be e s t a b l i s h e d (about +10-20 km), s i n c e observed Pn v e l o c i t i e s are average values obtained from long p r o f i l e s . However, i t would seem worthwhile t o attempt such s p a t i a l c o r r e l a t i o n by long r e f r a c t i o n p r o f i l e s shot p a r a l l e l t o the GDS d i s c o n t i n u i t y , one on each s i d e of i t : f o r example, i n Canada a p r o f i l e between P r i n c e George and Creston i n the west, and between the Hart Mountain Range and Calgary i n the east . Such c o n f i r m a t i o n of the r e l a t i o n between the two d i s c o n t i n u i t i e s could be of considerable p r a c t i c a l i n t e r e s t ; i t could provide s e i s m o l o g i s t s w i t h a p r a c t i c a l method t o : a) p l a n the optimum l o c a t i o n of r e f r a c t i o n p r o f i l e s , and b) i n t e r p r e t data from p r o f i l e s which were shot across the d i s c o n t i n u i t y . The low Pn v e l o c i t i e s have g e n e r a l l y been i n t e r p r e t e d on the basis of composition changes r a t h e r than temperature e f f e c t s (Thompson and Talwani, 1964; Dehlinger e t a l , 1964); i n t h i s case they would be unconnected w i t h 82 the observed changes i n e l e c t r i c a l c o n d u c t i v i t y , since geomagnetic i n d u c t i o n data are i n s e n s i t i v e t o composition c o n t r a s t s (see Chapter V"), However, there i s ample evidence f o r the a l t e r n a t i v e (temperature e f f e c t ) e x p l a n a t i o n . Laboratory data f o r the temperature-dependence o f compression- a l wave v e l o c i t i e s are very sparse, p a r t i c u l a r l y f o r temperatures above about 400°C. Even at the lower temperatures, the c o e f f i c i e n t s f o r rock samples vary over a f a i r l y wide range: from about -2.5 x 10 4 km/sec/°C f o r some b a s a l t samples t o as high as -38 x 10 ^ f o r one dunite sample (Hughes and Maurette, 1957). Even f o r two d i f f e r e n t d u n i t e s , the co e f f i c i e n t s v a r i e d between -10 x 10 4 f o r a 90$ o l i v e n e sample (Hughes and Cross, 1951) and -38 x,10~ 4 f o r a 99$ o l i v e n e sample (Hughes and Maurette, 1957); both were measured a t the same pressure (5 Kbar). However, the l a t t e r value was obtained over a very narrow temperature range (25°C - 225CC); Hughes and Maurette (1957) s t a t e t h a t the data "should be suspected of being i n e r r o r " . For pure minerals of relevance t o upper mantle compositions, the thermal c o e f f i c i e n t s are b e t t e r d e f i n e d , and average about -4 x 10 4 km/sec /°C f o r F o r s t e r i t e (the magnesium end-member of the o l i v e n e s u i t e ) and Garnet, and -5 x 1 0 - 4 f o r P e r i c l a s e (Anderson et a l , 1968). Soga e t a l (1966) have d e r i v e d r e l a t i o n s f o r e x t r a p o l a t i n g the lower-temperature (<800°C) l a b o r a t o r y data f o r these minerals t o temperatures as high as 2200°C. For example, f o r F o r s t e r i t e the r e l a t i o n i s : V p(km/sec) = 7.75 - 3-62 x 10~ 4T - 7.46 x 10~ 8T 2+ 3.66 x 1 0 ~ 1 4 T 3 , where T i s the absolute temperature (°K). For low temperatures (<T500°C) only the f i r s t term i s important, but as the temperature i n c r e a s e s towards 1000°C the second term has t o be i n c l u d e d . For example, at T = 1000°K (727°C), the e f f e c t i v e mean c o e f f i c i e n t i s 4.4 x 10~ 4 km/sec/°C. 83 Although the m i n e r a l data show well-behaved r e l a t i o n s towards higher temperature, j u s t i f y i n g some e x t r a p o l a t i o n , the whole-rock data show s i g n i f i c a n t s c a t t e r , and steepening of the g r a d i e n t s i n some cases (Hughes and Maurette, 1957). This r a i s e s doubts about the v a l i d i t y of l i n e a r e x t r a p o l a t i o n from the l a b o r a t o r y range (400°C) t o the range of i n t e r e s t (^ 800°C) f o r the western upper mantle - p a r t i c u l a r l y i f we consider the second-order c o e f f i c i e n t s d e r i v e d f o r the m i n e r a l s . Never t h e l e s s , the e x t r a p o l a t e d low-temperature data can probably be accepted as the lower l i m i t f o r the c o e f f i c i e n t s at these temperatures. Using the c o e f f i c i e n t f o r F o r s t e r i t e ( -4.0 x 10 4 km/sec/°C) as the extreme lower l i m i t , the i n f e r r e d temperature d i f f e r e n c e of 400 °C between the western and eastern regions r e s u l t s i n a Vp d i f f e r e n t i a l of only 0.16 km/sec, not enough t o account f o r a l l the observed d i f f e r e n c e i n Pn v e l o c i t y . However, t h i s c o e f f i c i e n t i s almost c e r t a i n l y too low f o r r e a l i s t i c upper mantle m a t e r i a l s . Toksoz et a l (1967) considered the v e l o c i t y . d i s t r i b u t i o n p a t t e r n s under oceanic and c o n t i n e n t a l r e g i o n s , and concluded t h a t ("kVp/^>T)p of -5 x 10 4km/sec/°C was much too low f o r upper mantle m a t e r i a l s . Using the lowest l a b o r a t o r y - d e r i v e d c o e f f i c i e n t f o r dunite (10 x 10 4 km/sec/°C), a v e l o c i t y d i f f e r e n c e of 0.40 km/sec i s d e r i v e d f o r a temperature d i f f e r e n c e of 400°C - more than enough t o account f o r the observed decrease i n Pn v e l o c i t y . I t i s c l e a r t h a t no d e f i n i t e c onclusions can be drawn u n t i l b e t t e r l a b o r a t o r y data become a v a i l a b l e f o r whole-rock samples, but the observed r e d u c t i o n i n Pn v e l o c i t y can apparently be accounted f o r by incre a s e s i n temperature of the order i n d i c a t e d by heat-flow data. The above argument i s of course i n c o n c l u s i v e , 84 s i n c e changes i n composition ( p a r t i c u l a r l y towards a l e s s mafic upper mantle) provide a much more d i r e c t way to change the v e l o c i t i e s . To summarizethe seismic data: i f the geotherms derived by Roy e t a l (1968b) are v a l i d , the observed decrease i n Pn v e l o c i t y i n the western r e g i o n can be accounted f o r by temperature e f f e c t s . However, l a b o r a t o r y data on u l t r a m a f i c rocks at high temperatures and pressures i s r e q u i r e d f o r any v a l i d q u a n t i t a t i v e work i n t h i s f i e l d . IV-D) Aeromagnetic Surveys S t a t i s t i c a l s t u d i e s have been c a r r i e d out on s e v e r a l long aeromagnetic p r o f i l e s (Serson and Hannaford, 1957;. A l l d r e d g e and Van V o o r h i s , 1961; A l l d r e d g e et a l , 1963). I f short-wavelength f l u c t u a t i o n s are f i l t e r e d out, the remaining f l u c t u a t i o n s i n f i e l d s t r e n g t h cover a wide range of wavelengths. S p e c t r a l a n a l y s i s of one very long p r o f i l e (37,000 km) i n d i c a t e d t h a t the "energy" (amplitude squared i n a f i x e d wavelength- wid t h f i l t e r ) i s r e l a t i v e l y u n i f o r m l y d i s t r i b u t e d between about 20 km and 250 km, with a s i g n i f i c a n t d r o p - o f f f o r longer wavelengths (Al l d r e d g e et a l , 1963); these were a t t r i b u t e d t o c r u s t a l sources. A second block of s p e c t r a l "energy" a t much longer wavelengths (> 3,700 km) was a t t r i b u t e d t o sources w i t h i n the core. - I t i s i n t u i t i v e l y tempting t o equate longer wavelengths w i t h sources at g r e a t e r depth, but i t should be kept i n mind t h a t no unique determin a t i o n s can ever be d e r i v e d from such surface measurements of the s t a t i c magnetic f i e l d s t r e n g t h ; long-wavelength anomalies can be explained e q u a l l y w e l l by shallower sources of l a r g e r a r e a l e x t e n t . I t can only be argued t h a t c e r t a i n source c o n f i g u r a t i o n s are more a t t r a c t i v e than others on the b a s i s of other g e o p h y s i c a l and g e o l o g i c a l c o n s i d e r a t i o n s . 85 There i s generally no correlation between " s t a t i c magnetic" anomalies and "geomagnetic induction" anomalies, since they are sensitive to e n t i r e l y d i f f e r e n t parameters. However, a study of the U.S. Trans continental P r o f i l e has shown that the f i l t e r e d aeromagnetic p r o f i l e s are generally much f l a t t e r and more featureless i n the west than i n the east (Pakiser and Zi e t z , 1 9 6 5 ) . The p r o f i l e runs roughly along a great c i r c l e from Norfolk, V i r g i n i a , to near San Francisco, crossing the Rocky Mountains near Denver, Colorado. The t r a n s i t i o n to the smooth "western- type" p r o f i l e occurs about 1 6 0 km west of Denver, i . e . i n good agreement with the po s i t i o n of the GDS discontinuity at t h i s l a t i t u d e (Gough and Anderson, 1968). Pakiser and Zietz ( 1 9 6 5 ) and Zietz et a l ( 1 9 6 6 ) suggested that t h i s "smoothing" of long wavelength features could be caused by an upwelling of the Curie isotherm to shallower depth i n the western region, i . e . absence of sources i n the lower crust. They did however emphasize the ambiguity of aeromagnetic data, and the p o s s i b i l i t y of alternative explanations such as a more s i l i c i c crust or more abrupt l a t e r a l v a r i a t i o n s . In t h i s section, some aeromagnetic data from western Canada have been examined for s i m i l a r e f f e c t s . Figure IV-1 shows the location of the four p r o f i l e s used i n t h i s analysis. P r o f i l e s A and B were derived from Map 749G of the Geological Survey of Canada (Morley, 1 9 5 9 ) , which covered a 1 2 mile wide s t r i p north of lat i t u d e 4 9 °N; these are t o t a l - f i e l d (F) surveys obtained at f l i g h t a l t i t u d e s ranging from 1 , 0 0 0 to 1 1 , 0 0 0 feet, depending on topography. P r o f i l e s C and D are based on data from the Dominion Observatory's three-component survey (Serson et a l , 1 9 5 7 5 Dawson and Dalgetty, 1 9 6 6 ) , flown at an a l t i t u d e of 1 1 , 0 0 0 feet. For 86 p r o f i l e s C and D the v e r t i c a l component Z v/as used, as more complete coverage v/as a v a i l a b l e f o r t h i s component. At these l a t i t u d e s the anoma l i e s i n F and Z are very s i m i l a r ; i n p a r t i c u l a r the same c o n s i d e r a t i o n s of source depth v s . anomaly wavelength can be made f o r e i t h e r one of thes components. On p r o f i l e s A and B, the magnetic data were d i g i t i z e d at one-mile i n t e r v a l s , and r e p l o t t r a c e s of these data are shown on Figures IV-2 and IV-3. A l s o shovm on these f i g u r e s are topography and average f l i g h t a l t i t u d e s (dashed l i n e ) . The u n f i l t e r e d p l o t shows l a r g e f l u c t u a t i o n s (up t o 1000 gamma amplitude) over a wide range of wavelengths. Low-pass f i l t e r i n g was then a p p l i e d t o these d a t a , u s i n g computational operations i n the wave-number domain ( i . e . F o u r i e r transform, l i n e a r f i l t e r i n g , and inv e r s e F o u r i e r transform t o r e c o n s t i t u t e the t r a c e s ) . The numbers shown to the r i g h t of the f i l t e r e d t r a c e s are the c u t - o f f wavelengths. As the c u t - o f f wavelength i s i n c r e a s e d , the large-amplitude f e a t u r e s i n the v/est e m s e c t i o n are p r o g r e s s i v e l y smoothed out, and f o r c u t - o f f v/avelengths betv/een about 100 and 150 km the western s e c t i o n becomes smooth and f e a t u r e l e s s . The long-wavelength feature east of the P u r c e l l Mountains remains unattenuated at n e a r l y 1000 gamma amplitude. The l o c a t i o n of the " d i s c o n t i n u i t y " i s i n good agreement w i t h the l o c a t i o n of the GDS d i s  c o n t i n u i t y , i . e . about 50 km west of the Rocky Mountain Trench and i t s southern c o n t i n u a t i o n through the Kootenay R i v e r V a l l e y . Some of the p r o p e r t i e s of the computational f i l t e r should be noted a t t h i s stage. I t i s r e c t a n g u l a r and provides sharp c u t - o f f i n terms of wave-number. However, there i s some s p e c t r a l leakage i n t o adjacent bands and at the longer wavelengths the c u t - o f f i s l e s s s h a r p l y d e f i n e d when expressed i n terms of wavelength r a t h e r than wave-number, 87 F i g . IV-1. L o c a t i o n of aeromagnetic p r o f i l e s ; A and B from G e o l o g i c a l S u r v e y o f Canada, C and D from Dominion Observatory. 68 90 p a r t i c u l a r l y f o r pass-band c h a r a c t e r i s t i c s . For example, i f L = 100 km, the r e j e c t i o n i s s t i l l t o t a l f o r L<82 km, but there i s now s i g n i f i c a n t a t t e n u a t i o n of wavelengths as high as 120 km. This e x p l a i n s the apparent pro g r e s s i v e a t t e n u a t i o n of the eas t e r n f e a t u r e ( a c t u a l wavelength about 150 km) on the f i l t e r e d t r a c e s of F i g . IV-2 and IV-3. I t does not mean t h a t t h i s anomaly cont a i n s shorter-wavelength components which are p r o g r e s s i v e l y removed w i t h stronger f i l t e r i n g ; i t simply emphasizes the f a c t t h a t f o r t h i s p a r t i c u l a r p r o f i l e l ength (rJ 800 km) the f i l t e r i s not sharp enough t o d i s c r i m i n a t e between wavelengths of about 120 t o 250 km. • The r e s u l t s are i n good agreement w i t h those obtained i n the U.S.A., where the long - i/avelength anomalies which c h a r a c t e r i z e d the eastern r e g i o n were absent i n the western zone. Some longer-wavelength f e a t u r e s remain i n the western s e c t i o n of the f i l t e r e d t r a c e s shown i n Figures IV-2 and IV-3, although of l e s s e r amplitude than the eastern f e a t u r e . I n order to estimate t h e i r s i g n i f i c a n c e , the f i l t e r e d t r a c e s from both p r o f i l e s are shown superimposed on each other i n Figure I t i s c l e a r t h a t f o r two p r o f i l e s spaced only 11 km a p a r t , the e f f e c t s o f e i t h e r deep-seated or of are a l l y - e x t e n d e d sources should be coherent between the two p r o f i l e s . Figure IV-A shows th a t t h i s i s the case only f o r the eastern f e a t u r e ; the others are not coherent and can presumably be accounted f o r by e i t h e r f l i g h t a l t i t u d e e f f e c t s and d i u r n a l contamin a t i o n , o r by genuine sources which are l i n e a r l y extended only i n the d i r e c  t i o n of f l i g h t . The longer wavelength f e a t u r e i n the Fra s e r D e l t a could be "genuine"; i t s amplitude i s higher than could be accounted f o r by the change i n f l i g h t a l t i t u d e . U n f o r t u n a t e l y the p r o f i l e s are discontinuous a t t h i s p o i n t (no data across the S t r a i t of Georgia), and a n a l y s i s of t h i s f e a t u r e could not be c a r r i e d out. 100 KM LONGITUDE — r — — 1 — — 1 I 1 1 1 R T—- 1 1 — , 122 '20° 118° 116° 114° 112° F i g . I V - 4 . Superimposed f i l t e r e d t r a c e s f o r p r o f i l e s A ( s o l i d curves) and B (dotted c u r v e s ) . ROCKY MTN. TRENCH [05; LAT. 50°55' 52'43' 26 LONG. I I7°52' I I8°27 ' . IV-5. F i l t e r e d magnetic d a t a , P r o f i l e C. 100 KM [00 KM 150 KM vO 225 KM 54° 46' 118° 55' 57°00 I I9°59" 59°08' I20°56' 60°00' I2T32' ROCKY MTN.TRENCH 4 9 ° 0 0 ' L A T . 5 I ° I 3 ' 53° 31' 5 5 ° 4 4 ' 5 7 ° 5 5 ' 5 9 ° 3 l ' . . I I9 0 39" LONG. I20°2 l ' 121° 13* I 2 2 ° 0 4 ' I 2 3 ° 0 6 ' I 2 3 ° 5 0 ' F i g . IV-6. F i l t e r e d magnetic data, P r o f i l e D. 94 i S i m i l a r a n a l y s i s was c a r r i e d out on the data from p r o f i l e s C and D ( F i g u r e s IV-5 and IV -6) . These data were i n the form of average values over 5 minutes of f l i g h t (about 30 km), i . e . already pre-smoothed t o remove the short wavelengths which appeared so prominently i n the u n f i l t e r e d i data from p r o f i l e s A and B. Since the number o f data p o i n t s was con s i d e r  a b l y lower than f o r the previous p r o f i l e s , some care had t o be taken i n the F o u r i e r transform operations. In p a r t i c u l a r , exact coincidence of the end po i n t s had t o be enforced by s u b t r a c t i o n o f an a r b i t r a r y l i n e a r g r a d i e n t , i n order t o avoid end-range d i s t o r t i o n s of the r e c o n s t i t u t e d t r a c e s due to Gibbs o s c i l l a t i o n s . The p a t t e r n evident on the f i l t e r e d t r a c e s o f Figure 17-5 and IV-6 i s of the same type as t h a t observed on p r o f i l e s A and B. East of the Rocky Mountain Trench, long wavelength f l u c t u a t i o n s of s i g n i f i c a n t amplitude (+ 500 gamma) are observed, as reported by Serson and Hannaford (1957), and by P a k i s e r and Z i e t z (1965)in the U.S.A. West of the Trench, the f i l t e r e d p r o f i l e s a r e smooth t o a remarkable degree: on p r o f i l e C w i t h i n + 15 gamma up t o the Trench, and w i t h i n + 5 gamma t o a po i n t 25 km west of the Trench. On p r o f i l e D a one-point d i s c o n t i n u i t y ( p o s s i b l y of in s t r u m e n t a l o r i g i n ) introduced some d i s t o r t i o n , but even there the p r o f i l e i s smooth t o w i t h i n + 65 gamma from the U.S. border up to the Trench. The f l i g h t l i n e s i n t e r s e c t the Trench a t an angle of about 50°; i f the s t r i k e of the d i s c o n t i n u i t y does f o l l o w the Trench d i r e c t i o n , we can t h e r e f o r e not expect sharp d e f i n i t i o n f o r the l o c a t i o n of the d i s c o n t i n u i t y . How ever, judging from the most h e a v i l y f i l t e r e d t r a c e , we can place i t w i t h i n + 40 km of the Trench, w i t h somewhat higher l i k e l i h o o d towards the western l i m i t of t h i s range. . 95 Q u a n t i t a t i v e i n t e r p r e t a t i o n of the smoothed p r o f i l e s i s v i r t u a l l y i m p o s s i b l e ; i t i s d i f f i c u l t enough t o i n t e r p r e t the long-wavelength anoma l i e s i n the eastern s e c t i o n i n view of the inherent ambiguity of magnetic d a t a . Serson and Hannaford (1957) estimated t h a t most of the sources l i e i n the upper c r u s t (down t o depth 11 km under the c o n t i n e n t s ) . Vacquier and A f f l e c k (1941) estimated t h a t the bottom of magnetic inhomogeneities l i e s a t depths between 17.7 km and 24.1 km. However, a l l these estimates are o b v i o u s l y dependent upon some assumption of s t r u c t u r e - no unique s o l u t i o n i s p o s s i b l e . I n t e r p r e t a t i o n of some s p e c i f i c long-wavelength anomalies i n eastern Canada was c a r r i e d out by Bhattacharya and Morley (1965); by imposing r e s t r a i n t s on the shape and o r i e n t a t i o n of the s t r u c t u r e s , they d e r i v e d depths t o the bottom between 17.7 km and 24 km, w i t h a mean of 20 km - same as the value d e r i v e d by Vacquier and A f f l e c k (1941). These depths were assumed t o d e f i n e the Cu r i e p o i n t isotherm f o r c r u s t a l m a t e r i a l s . Although a figure- of 475°C i s sometimes quoted i n the l i t e r a t u r e as the Curie p o i n t f o r b a s a l t i c m a t e r i a l s , the Curie po i n t f o r magnetite (578°C) i s a more r e l e v a n t l i m i t i n g temperature; magnetite i s the most common ferromagnetic component o f igneous and metamorphic rocks, and v a r i a t i o n s i n magnetic i n t e n s i t y are p r i m a r i l y produced by v a r i a t i o n s i n magnetite content (Magata, 1961; Z i e t z e t a l , 1966). Using the geotherm d e r i v e d by Roy et a l (1968b) f o r the eastern U.S.A., magnetization could t h e r e f o r e p e r s i s t w e l l i n t o the uppermost 10 km of the mantle; the depths •derived by Vacquier and A f f l e c k (1961) and Bhattacharya and Morley (1965) are t h e r e f o r e w e l l w i t h i n the acceptable range f o r the exi s t e n c e of magnetic sources. The f a c t t h a t magnetic s t r u c t u r e s can e x i s t dovm to upper mantle depths does of course not imply t h a t sources do a c t u a l l y 96 e x i s t down t o t h i s depth. In the western r e g i o n , t h i s C u r i e p o i n t temperature would be reached at a much shallower depth (20-25 km according to the geotherm d e r i v e d by Roy et a l , 1968b). I t i s c l e a r t h a t no i n t e r p r e t a t i o n can be attempted on the absence of f e a t u r e s which cannot themselves be i n t e r p r e t e d uniquely. I f the long- wavelength anomalies observed east of the Trench are caused by deep-seated (lower c r u s t a l ) inhomogeneities, then t h e i r absence i n the western r e g i o n can best be expla i n e d by an u p w e l l i n g of the Curie p o i n t isotherm. How ever, i f the e a s t e r n anomalies are caused by shallower s t r u c t u r e s of l a r g e a r e a l extent and uniform magnetisation, then the most l i k e l y i n t e r p r e t a t i o n f o r t h e i r absence would be more v i o l e n t "jumbling-up" of these s t r u c t u r e s i n the west. Such an i n t e r p r e t a t i o n i s i n f u l l agreement w i t h the geo l o g i c a l evidence; i n the east ge o l o g i c lineaments can be fol l o w e d over long d i s t a n c e s , but west of the Rocky Mountain Trench the s t r u c t u r e s are v e r y complex, having been broken up by s e v e r a l ages of deformation, numerous i n t r u s i o n s , and widespread r e g i o n a l and contact metaraorphism ('White, 1959; Armstrong, 1959; Henderson, 1959). This e x p l a n a t i o n f o r the absence of long-wavelength anomalies i n the west would be v a l i d f o r whatever i n t e r  p r e t a t i o n i s accepted f o r the eastern anomalies. A t h i r d , even more " t r i v i a l " e x p l a n a t i o n can a l s o not be r u l e d out: a more s i l i c i c c r u s t i n the west. T h i s i s a l s o i n agreement w i t h the g e o l o g i c a l evidence; there are numerous and v a s t a c i d i c i n t r u s i o n s west of the Trench, but none i n the Rocky .Mountains or t o the e a s t . The r e s u l t s d e r i v e d from the aeromagnetic data can be summarized as f o l l o w i n g : a) Long aeromagnetic p r o f i l e s can apparently be used t o d e l i n e a t e a reg i o n which appears to c o i n c i d e w i t h the r e g i o n of high e l e c t r i c a l c o n d u c t i v i t y d e l i n e a t e d by GDS. This would provide a method f o r expanding GDS mapping 9 7 coverage t o areas beyond the reach of surface t r a n s p o r t a t i o n , b) The observed smoothing of long-wavelength magnetic anomalies i n the western r e g i o n cannot be i n t e r p r e t e d i n any unique way. P o s s i b l e explana t i o n s a r e : u p w e l l i n g of the Curie p o i n t isotherm, break-up of l a r g e s t r u c t u r e s by t e c t o n i c and metamorphic mechanisms, and/or a more s i l i c i c c r u s t . • . . . 9 8 V. FETROLOGIGAL INTERPRETATION V-A) Summary of Data The f o l l o w i n g two f i g u r e s ( V - l and V-2) summarize v i r t u a l l y a l l the i n f o r m a t i o n t h a t can be e x t r a c t e d from the previous s e c t i o n s : (a) From GDS and aeromagnetic surveys we d e f i n e the ea s t e r n boundary o f a re g i o n w i t h some d i s t i n c t magnetic and e l e c t r i c a l ' c h a r a c t e r  i s t i c s . The p o s i t i o n o f the boundary i s defined i n f o u r places between l a t i t u d e s 49°N and 54 °N; s i n c e only i t s l o c a t i o n (not i t s s t r i k e ) i s d e f i n e d , no "border" l i n e s have been shown on F i g . V - l . (b) From MT and GDS, s e l f - c o n s i s t e n t c o n d u c t i v i t y s t r u c t u r e models are obtained. The western s t r u c t u r e can be a p p l i e d t o the e n t i r e western re g i o n d e f i n e d above; the eastern s t r u c t u r e i s probably a p p l i c a b l e only t o the southwest corner of A l b e r t a . V-B) E l e c t r i c a l C o n d u c t i v i t y of the Lower Crust and Upper Mantle Figure V -3a shows the r e s i s t i v i t i e s of some r e l e v a n t m a t e r i a l s as a f u n c t i o n of temperature. A l l dry c r y s t a l l i n e rocks are cla s s e d as " s o l i d e l e c t r o l y t e s " or i n s u l a t o r s " , i . e . the dominant conduction mechanisms are i o n i c r a t h e r than e l e c t r o n i c . The c o n d u c t i v i t y can be expressed as a -U /kT summation of s e v e r a l terms of form S = S e ' , one term f o r each o conduction mechanism.- For any p a r t i c u l a r temperature range one of these mechanisms u s u a l l y predominates, so t h a t p l o t s of l o g c o n d u c t i v i t y a g a i n s t i n v e r s e absolute temperatures are formed by a succession of l i n e a r segments. On Figure V -3a the right-hand segments represent i o n i c i m p u r i t y conduction, merging i n t o the steeper segments r e p r e s e n t i n g i n t r i n s i c i o n i c conduction f o r temperatures above about 500 - 700°C. Throughout t h i s temperature range, e l e c t r o n i c conduction i s n e g l i g i b l e . 99 GEOMAGNETIC STATIONS : • O LOW-1 -• lAZl !•/, © HIGH-1 J '~[IO5) 2-HAH) 2] ' S \gj 1 AT PERIODS FROM " . O TO 120 MINUTES (D " TRANSITION AEROMAGNETIC PROFILES ": EASTERN-TYPE SMOOTHED 200 km F i g . V - l . O u t l i n e of d i s c o n t i n u i t y d e f i n e d by map a f t e r Robinson (1968). GDS and aeromagnetic data; base r ioi 10" 10' 10' K) 10' 10. ^ If j i .0006 .0003 .0010 .0012 .OCI4 .OCI6 1000 Irrverse obsoljte temperature, l/T a) Resistivity as a function of l /T for several rocks. The right-hand segments of the curves represent extrinsic conduction by impurities and crystal defects, the left-hand, or high-temperature portions, represent intrinsic conduction. . ( a f t e r K e l l e r and Fris c h k n e e h t , i960) 10" t u n ] 730 560 450°C R E C I P R O C A L T E M P E R A T U R E , i n 1000 c) FREQUENCY in Cycles p:.- Second ( a f t e r K e l l e r , 19o3) ' : ( a f t e r K e l l e r , 1963)' F i g . V-3. E l e c t r i c a l r e s i s t i v i t y of some rocks and mi n e r a l s , and frequency- dependence of such d a t a . 102 D e r i v a t i o n of the c o e f f i c i e n t s f o r the e x p o n e n t i a l terms i s based e n t i r e l y on l a b o r a t o r y data, since a n a l y t i c techniques cannot be a p p l i e d t o the i o n i c conduction, p a r t i c u l a r l y the lower temperature i m p u r i t y mechanism, f o r complex rock assemblages. The " t y p i c a l " c o e f f i c i e n t s o f t e n quoted i n the l i t e r a t u r e should be used w i t h c a u t i o n , f o r two reasons: a) the great v a r i a b i l i t y i n r e s u l t s obtained f o r the same type of rock, and b) the f a c t t h a t much of the l a b o r a t o r y data i s obtained w i t h A.C, techniques ( u s u a l l y i n the audio range). Figures V-3b and 3c ( K e l l e r , 1963) demonstrate the e f f e c t s of frequency on c o n d u c t i v i t y measurements. For higher temperatures the e f f e c t i s not too s e r i o u s . However, a t temperatures beloxv 700°C, order-of-magnitude e r r o r s may be introduced when e s s e n t i a l l y "D.C." geomagnetic i n d u c t i o n data i s compared with A.C. la b o r a t o r y measurements. T h i s frequency dependence i s o f t e n overlooked; f o r example, the 1 Kcps curve f o r g r a n o d i o r i t e ( F i g . 3b) has migrated i n t o the textbook l i t e r a t u r e ( F i g . 3a) without mention of i t s A.C. d e r i  v a t i o n , i . e . order-of-magnitude u n c e r t a i n t y at lower temperatures. S e v e r a l conductivity/temperature p l o t s f o r u l t r a b a s i c m a t e r i a l s are shown i n Figure V-4. The o l i v i n e data have been r e s t r i c t e d t o those obtained a t the pressures r e l e v a n t t o the lower c r u s t and uppermost mantle; the p e r i d o t i t e and b a s a l t data were obtained at atmospheric pressure. G e n e r a l l y , pressure e f f e c t s up t o 10 - 15 Kb are n e g l i g i b l e (< 50$) compared to the e f f e c t s of temperature ( K e l l e r and F r i s c h k n e c h t , 1966). A number of other determinations are a v a i l a b l e f o r b a s a l t s ( f o r example, No r i t o m i , 196l); these span s e v e r a l orders o f magnitude, presumably depending on exact composition. A l l the b a s a l t s d e s c r i b e d by Nori t o m i (I96l) have r e s i s t i v i t i e s above 1000 ohm-meters f o r temperatures up to 700°C. The 103 curve on F i g . V-4 has t h e r e f o r e been considered as the lower l i m i t f o r b a s a l t i c m a t e r i a l s . I t i s c l e a r from Figure V-4 t h a t where "eastern-type" (< 10°C/km) geothermal g r a d i e n t s e x i s t , p e r i d o t i t e and ba s a l t are r e s i s t i v e (>500 ohm-m) to depths of 50 km. As seen i n the previous s e c t i o n s , geomagnetic i n d u c t i o n methods cannot" r e s o l v e r e s i s t i v i t y d i f f e r e n c e s i n regions where the r e s i s  t i v i t y exceeds 250 ohm-meters. Consequently, geomagnetic i n d u c t i o n methods cannot be expected t o "see" any d i s c o n t i n u i t i e s i n composition w i t h i n t h i s v s range, such as the Conrad or Mohorovicic d i s c o n t i n u i t i e s . At gr e a t e r depths t h i s i s not n e c e s s a r i l y t r u e ; f o r example the o l i v i n e / s p i n e l phase t r a n s f o r m a t i o n appears t o be a s s o c i a t e d w i t h an in c r e a s e i n c o n d u c t i v i t y (Akimoto and Fujisawa, 1965) which may w e l l be d e t e c t a b l e by geomagnetic i n d u c t i o n methods. Exceptions to the above statement are of course p o s s i b l e , s i n c e the a v a i l a b l e l a b o r a t o r y data does not cover a l l r e l e v a n t m a t e r i a l s . However, the only exception which i s w e l l defined by experimental data i s i r o n - r i c h o l i v i n e . The o l i v i n e s u i t e , ( F e ^ g ^ S i O ^ , ranges con t i n u o u s l y from F a y a l i t e (Fe^SiO^) t o F o r s t e r i t e (Mg^SiO^), a n c j composition i s i n d i c a t e d by the mole percent content of one of these end-members. For F a y a l i t e mole percentages of 50 and over, the r e s i s t i v i t i e s are w e l l below 100 ohm-meters, even f o r moderate temperatures ( F i g . V-4); f o r the higher-pressure s p i n e l phase of F a y a l i t e the r e s i s t i v i t y i s even lower (under 1 ohm-meter; Akimoto and Fujisawa, 1965). The assumption t h a t upper mantle o l i v i n e s are magnesium-rich i s based on the evidence of surface samples, and to a l e s s e r extent on the composition of c h o n d r i t i c m e t e o r i t e s . For example, the composition of nine o l i v i n e nodules a s s o c i a t e d w i t h b a s a l t l a v a s ranges only between Fo 89.7 and Fo 91.0, even though they come from widely separated l o c a t i o n s (Wager, 1958); s i m i l a r ranges T E M P E R A T U R E ( ° C ) 4 0 0 5 0 0 7 0 0 1000 1500 F i g . V-4. E l e c t r i c a l c o n d u c t i v i t y as a f u n c t i o n of temperature, u s i n g data from Coster (1949), Bradley et a l (1964), and Hamilton (1965). Dashed l i n e s are AC-derived data. are obtained for alpine-type dunites. Although the olivines of peridotites associated with gabbros are somewhat less Mg-rich than those from alpine- type dunites, they are s t i l l above Fo 85. The evidence of surface samples i s of course not a f u l l y re l iable indicator of upper mantle composition. However, the fractionation processes of ultramafic materials are such that magma-derived surface samples would normally be Fe-enriched with respect to the parent stock (Wager, 1958). Consequently the upper mantle olivines can be expected to be even more Mg-rich than the surface samples, and Wager (1958) estimated the upper mantle ol ivine composition at Fo 90. The p o s s i b i l i t y of i ron-r ich olivines as an explanation for low r e s i s t i v i t i e s has therefore been rejected; other, as yet undefined, geochemical factors can of course not be ruled out i n view of the limited amount of laboratory data. V-C) Petrological Models l ) Eastern Region_ The conductivity structure i n the Pincher/Vulcan area (and ^ probably as far west as the Rocky Mountain Trench) i s characterized by a moderately conducting zone ( res is t iv i ty 30-50 ohm-meters) starting at depth 30-35 km. Using the "eastern-type" geotherm of Roy et a l (1968b), the temperature at depth 35 km would be 460°C and i s shown as a v e r t i c a l l ine on F i g . V-4- None of the usually accepted mafic or ultramafic materials for which laboratory data are available reach such low resis  t i v i t i e s at this temperature; i t i s therefore concluded (as had already been proposed ear l ier on other evidence) that this southwestern corner of Alberta i s not representative of the normal eastern structure. For example, i f the temperature at depth 35 km i s 860°C (see v e r t i c a l l ine 106 on Fig. V-4),. the extrapolated o l i v i n e (Fa 10%) curve gives a r e s i s t i v i t y of about 50 ohm-meters. I f we assume a steepening of the^/T curves f o r the higher temperatures, as observed f o r other samples, the observed r e s i s t i v i t y (30-50 ohm-meters) can be interpreted for even lower temper atures (about 750°C). I t i s concluded that under the Pincher/Vulcan area the temperature at depth 35 km must be at least 750°C, provided that t h i s region i s composed of ordinary basaltic materials. No GDS mapping i s available towards the east to v e r i f y the r e l a t i o n of t h i s area to the rest of North America. I t i s however l i k e l y that t h i s i s an anomalously high temperature, i . e . already within (or t r a n s i t i o n a l towards) the high-temperature western region. The suggested structure i s outlined i n Figure V-7. 2) Western Region_ Similar considerations indicate that the mantle below 35 km i s also moderately conductive, and consistent with a temperature greater than 750°C. The lower crust i s however d i f f e r e n t , being r e l a t i v e l y highly conductive. The explanation can hardly involve temperature alone, f o r two reasons: a) improbably high temperatures would be required - w e l l over 900°C at depth 20 km; and b) a temperature inversion would be required. This leaves two alternative explanations: a) p a r t i a l melting i n the lower crust, as suggested by Caner et a l (1967), or b) hydrated lower c r u s t a l materials, as suggested by Hyndman and Hyndman (1968). a) P a r t i a l melting: Complete melting of basalt increases the e l e c t r i c a l conductivity by about a factor of 10. Melting data f o r a basalt (after Barus and Iddings, 1392) i s shown on Figure V-5; point "b" denotes the s t a r t of melting, point "C" the temperature at which a homo geneous melt i s reached. Similar results have been reported by Khitarov and Slutskiy (1965). For granites the increase i n conductivity i s much 107 more pronounced - up to about 3 orders of magnitude. Data from Lebedev and Khitarov (1964.) and from Noritomi (1961) are shown on Fig. V-5. The main objection to partial.melting of dry rocks i s the required high temperature; s t a r t of melting for these materials i s at 1000°C or higher, which i s u n l i k e l y to be reached at the required depths of 10-15 km. b) Hydration: Hyndman and Hyndman (1968) proposed a tectonic model which would account f o r a hydrated lower crust under t e c t o n i c a l l y young areas. This lower crust i s subsequently dehydrated by the upwards migration of granite-water melts, leaving the stable (dehydrated and granite-depleted) lower crust of shield areas. The e l e c t r i c a l r e s i s t i v i t y of granite i s markedly lowered by any water content; f o r example, the r e s i s t i v i t y of granite at 600°C i s decreased by at least three orders of magnitude for P u „ = P, , , = 1 Kbar (Lebedev and Khitarov, 1964) - ^2 t o t a l see Fig. V-5. This corresponds to a water concentration of about l+% by weight. Even lower r e s i s t i v i t i e s (under 1 ohm-meter for T = 600°C) are obtained for higher p a r t i a l water pressures (Lebedev and Khitarov, 1964). Unfortunately, hydrous data i s available only f o r Q = ^ o t a l ' ^"e* rocks f u l l y saturated at the par t i c u l a r temperature. Since the saturating water content varies markedly with temperature, such hydrous experiments are not a r e l i a b l e measure of r e a l conditions; the changing water content as w e l l as the changing temperature affect the conductivity. No experimental data i s available for fixed water concentrations less than saturation. No experimental data i s available f o r hydrated mafic and ultrama f i c rocks at high temperatures and pressures. However, i t i s l i k e l y that a sim i l a r increase i n conductivity e x i s t s ; the melting point of mafic materials i s s i g n i f i c a n t l y lowered by water content, so that we could also expect lower ac t i v a t i o n energies f o r i o n i c conduction (Hyndman and Hyndman, 1968). 108 l/T(°K). o . o o ! 5 2 5 0 o . o o i o T DRY GRANITE ( N0R1T0MI) (BARUS BIDDINGS) ( LEBEDEV a'KHITRROV) PHOO = 1Kb 3 0 0 T E M P E R A " 4 0 0 5 0 0 ™ 0 (°C) „™L^UJ..„JJLL..LI 7 0 0 1 0 0 0 1 5 0 0 o X CD O - 5 _ 4 3 - 2 Fig.. V-5. E f f e c t s of h y d r a t i o n and me l t i n g on e l e c t r i c a l c o n d u c t i v i t y of r o c k s . B a s a l t data i s DC-derived, g r a n i t e data i s AC- de r i v e d . S e c t i o n s b - c of the curves are the p a r t i a l m e l t i n g zones. 109 j i I t i s c l e a r from F i g . V-5 t h a t a reasonable water c o n c e n t r a t i o n can readily" account f o r the c o n d u c t i v i t i e s observed f o r the lower c r u s t i n the western r e g i o n , e i t h e r w i t h or without any assumptions of higher . temperatures. However, i t should be noted t h a t the m e l t i n g p o i n t of g r a n i t i c and mafic m a t e r i a l s i s a l s o markedly lowered by h y d r a t i o n ( T u t t l e and Bowen, 1958; Yoder and T i l l e y , 1962); a t depth 20 km the m e l t i n g temperature can be lowered by as much as 250°C f o r f u l l h y d r a t i o n . Consequently, i f the lower c r u s t i s hydrated, and i f we accept the higher temperatures i n d i c a t e d by the moderate c o n d u c t i v i t i e s i n the upper mantle, then p a r t i a l m e l t i n g should occur i n t h i s lower c r u s t a l l a y e r . Figure V-6 shows some of the r e l e v a n t data f o r m e l t i n g and h y d r a t i o n depths. The "western" geotherm ( a f t e r Roy et a l , 1968b) c l e a r l y does not i n t e r s e c t the m e l t i n g zone f o r dry b a s a l t w i t h i n the c r u s t ; i t does however i n t e r s e c t the s o l i d u s f o r "wet" g r a n i t e w e l l w i t h i n the c r u s t . The width of the m e l t i n g zone ( i . e . between s t a r t and completion of m e l t i n g ) depends on the water content. The curves shown i n F i g . V-6 are f o r 2% H^O; f o r higher c o n c e n t r a t i o n s , the m e l t i n g zone would be narrower, and a t the l i m i t of f u l l s a t u r a t i o n (about 9-10$ H^O) i t would c o i n c i d e w i t h the s t a r t of m e l t i n g curve. By p o s t u l a t i n g s p e c i f i c water content percentages, the width of t h i s m e l t i n g zone can t h e r e f o r e be adjusted t o any d e s i r e d v a l u e , but the s t a r t i n g point remains f i x e d at 24 km f o r t h i s p a r t i c u l a r assumed geotherm. V-D) Conclusions Figure V-7 provides a summary of the p e t r o l o g i c a l models; there are two d i s t i n c t r e s u l t s : l ) In southwestern Canada (boundaries not c l e a r l y d e f i n e d , but a t l e a s t 110 1400 1200 - 1000 h 800 h 600 400 h 200 - F i g . V -6 . 10 20 30 40 50 60 Geotherms ( a f t e r Roy et a l , 1968b) and m e l t i n g zones, us i n g data from T u t t l e and Bowen, 1958; Yoder and T i l l e y , 1962; Lambert arid WyHie, 1968. • P L A I N S R O C K Y M O U N T A I N S R O C K Y MTN. T R E N C H ( K O O T E N A Y V A L L E Y ) P U R C E L L M O U N T A I N S K O O T E N A Y L A K E S E L K I R K M O U N T A I N S . D E P T H (K M) 112 j as f a r east as Pin c h e r ; , the uppermost mantle i s moderately conducting down t o a depth of at l e a s t 100 km. Exc l u d i n g geochemically improbable explanations (such as a r e g i o n a l l y i r o n - r i c h upper mantle), a temperature of a t l e a s t 750°C at depth 35 km i s i n d i c a t e d . This provides independent | c o n f i r m a t i o n of heat-flow d e r i v e d estimates; u n l i k e i n t e r p r e t a t i o n o f heat-flow d a t a , the geomagnetic i n d u c t i o n r e s u l t s are not dependent on any assumptions of c r u s t a l s t r u c t u r e and composition. Z) In a sh a r p l y d e l i n e a t e d r e g i o n , s t a r t i n g from about 0-50 km west of the Rocky Mountain Trench, the lower c r u s t i s conductive. The most l i k e l y i n t e r p r e t a t i o n i s a hydrated lower c r u s t , as proposed by Hyndman and Hyndman, • 1968. Hydration alone i s s u f f i c i e n t t o e x p l a i n the observed d a t a , i . e . higher temperatures are not n e c e s s a r i l y r e q u i r e d f o r t h i s model. However, given the i n f o r m a t i o n from ( l ) above, some p a r t i a l m e l t i n g probably occurs i n t h i s zone s i n c e the m e l t i n g temperature i s lowered by h y d r a t i o n . 113 i I REFERENCES ADAM, A . , 1964. 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