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Geomagnetic variation anomaly at Kootenay Lake, B.C. Lajoie, Jules Joseph 1970

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THE GEOMAGNETIC VARIATION  ANOMALY  AT KOOTENAY LAKE, B.C. by JULES JOSEPH L A J O I E B.Sc,  University  o f Ottawa,  1968  A THESIS SUBMITTED I N P A R T I A L F U L F I L L M E N T OF THE REQUIREMENTS FOR THE DEGREE OF M.Sc. i n t h e Department of . GEOPHYSICS  We a c c e p t t h i s t h e s i s a s c o n f o r m i n g t o t h e required  standard  THE UNIVERSITY OF B R I T I S H COLUMBIA April,  1970  In p r e s e n t i n g t h i s t h e s i s  in p a r t i a l  f u l f i l m e n t o f the requirements  an advanced degree at the U n i v e r s i t y of B r i t i s h C o l u m b i a , I agree the L i b r a r y  s h a l l make i t f r e e l y  I f u r t h e r agree tha  available for  permission for extensive  It  copying o f t h i s  thesis  i s understood that c o p y i n g o r  permission.  Department  of  The U n i v e r s i t y o f B r i t i s h Columbia Vancouver 8, Canada  or  publication  o f t h i s t h e s i s 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 written  that  r e f e r e n c e and s t u d y .  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 of my Department by h i s r e p r e s e n t a t i v e s .  for  ABSTRACT  The  p u r p o s e o f t h i s t h e s i s was t o s t u d y t h e  g e o m a g n e t i c v a r i a t i o n a n o m a l y a t K o o t e n a y L a k e , B.C., r e p o r t e d b y Hyndman i n 1 9 6 3 . by  first  The a n o m a l y i s c h a r a c t e r i z e d  a v e r y h i g h c o r r e l a t i o n b e t w e e n t h e v e r t i c a l and  horizontal north-south  geomagnetic components;  this  i n d i c a t e s anomalous c u r r e n t s s t r i k i n g magnetic e a s t - w e s t , the  to  south of Kootenay Lake. During  t h e summer o f 1 9 6 8 ,  twenty recording  were s e t up i n t h e K o o t e n a y L a k e a r e a , u s i n g f o u r  stations  Askania  v a r i o g r a p h s and t h r e e f l u x g a t e m a g n e t o m e t e r s . S p e c t r a l analysis  shows t h a t t h e m a i n  was t r a v e r s e d o v e r north-south Polarization  a relatively  profile,  s h o r t d i s t a n c e on a  t o the south of Kootenay Lake.  studies indicate  magnetic east-west  'low I - h i g h I ' d i s c o n t i n u i t y  anomalous c u r r e n t s  striking  i n t h e same g e n e r a l a r e a o f t h e a b o v e  discontinuity. The main  model proposed i s a l o c a l i z e d  'low I - h i g h 1  1  discontinuity,  d i s t o r t i o n of the  resulting  i n a well  defined c o n d u c t i v i t y step, s t r i k i n g magnetic east-west, the  south of Kootenay Lake.  to  ii TABLE  OF CONTENTS Page  I  1  INTRODUCTION A  1  The A n o m a l y o  II  Hyndman's I n t e r p r e t a t i o n  C  L o w e r C r u s t and U p p e r M a n t l e i n We s t e r n C a n a d a  7  Induction  I n a Step D i s c o n t i n u i t y  B  Parkinson  Diagrams,  Induction  Mercator P r o j e c t i o n  Arrows,  .  Power S p e c t r a l A n a l y s i s  EXPERIMENTAL PROCEDURE  7 and 8 10 13  A  Instrumentation  13  B  Field  14  Experiment  A N A L Y S I S AND A  Sections  INTERPRETATION o f G.D.S. M a g n e t o g r a m s  17 ' 17  Power S p e c t r a l A n a l y s i s  20  C  Magnetic  25  D  P o l a r Diagrams  29  E  Interpretation  34  • B  V  4  A  C  IV  Structure  THEORY  the  III  2  B  CONCLUSION  Survey  40  Iii L I S T OP FIGURES  Magnetic Record at Boswell,  J u l y k,  1968  L o c a t i o n Map o f G e o m a g n e t i c D e p t h S o u n d i n g and M a g n e t o - T e l l u r i c  Stations  i n Western  Canada P e t r o l o g i c a l Model f o r Western Parkinson  Canada  Diagram  L o c a t i o n Map S h o w i n g A s k a n i a and F l u x g a t e Stations Two S i m u l t a n e o u s P r o f i l e s o f . G . D . S . Power S p e c t r a l  Estimates:  a  M i r r o r L a k e t o R y k e r t s and  b  B r o u s e t o Salmo  Power S p e c t r a l  Records  Newport  and N e w p o r t  Ratios:  a  M i r r o r L a k e t o R y k e r t s and  b  B r o u s e t o Salmo  and  Newport  Newport  L o c a t i o n Map S h o w i n g M a g n e t i c S u r v e y Profile Filtered Cutoffs  Magnetic Survey w i t h  Wavelength  Shown  P a r k i n s o n D i a g r a m s and C o r r e s p o n d i n g Mercator  Projections:  a  M i r r o r Lake t o R y k e r t s  b  B r o u s e t o Salmo  P o l a r D i a g r a m s and C o r r e s p o n d i n g Mercator  Projections:  a  M i r r o r Lake t o R y k e r t s  b  B r o u s e t o Salmo  iv Page 13  L o c a t i o n Map  14  Proposed  Showing Anomalous  Model Expressed  Highly•Conducting Layer  35  Stations  i n Depth Contours  to 37  V  ACKNOWLEDGEMENTS  I wish and  t o a c k n o w l e d g e the. a d v i c e ,  encouragement,  p a t i e n t s u p e r v i s i o n o f D r . G.K.C. C l a r k e  course  of t h i s  research.  S p e c i a l thanks are given  t o D r . B. C a n e r .of t h e  Dominion A s t r o p h y s i c a l Observatory directing  during the  and s u p e r v i s i n g t h e f i e l d  i nV i c t o r i a , f o r operations,  and f o r  many h o u r s o f d i s c u s s i o n s w h i c h l e d t o t h e i n t e r p r e t a t i o n of t h e anomaly. I would a l s o l i k e  t o thank the Dominion  i n Ottawa, f o r t h e use o f t h e t h r e e the  survey;  fluxgate units  and t h e U.S. C o a s t a n d G e o d e t i c  Newport Magnetic  Observatory  Observatory  magnetograms.  during  Survey f o r the  1  I  INTRODUCTION  I n t h e summer o f 1 9 6 8 ,  t h e Department  at the U n i v e r s i t y of B r i t i s h Columbia,  of Geophysics  i n cooperation with  t h e . D i v i s i o n o f Geomagnetism o f t h e O b s e r v a t o r i e s B r a n c h , at V i c t o r i a ,  sponsored a geomagnetic  survey t o i n v e s t i g a t e  t h e geomagnetic  K o o t e n a y L a k e , B.C. B e c a u s e in  1968,  d e p t h s o u n d i n g (G.D.S.) v a r i a t i o n anomaly a t  of the high s o l a r f l a r e  activity  t h i s p r o v e d t o be a most f a v o u r a b l e t i m e t o c o n d u c t  t h e e x p e r i m e n t . F o u r A s k a n i a GV-3 v a r i o g r a p h s and t h r e e f l u x g a t e - t y p e c o n t i n u o u s r e c o r d i n g m a g n e t o m e t e r s were u s e d . A total  o f t w e n t y s t a t i o n s was s e t up t o map t h e a n o m a l o u s  zone. A  The A n o m a l y The  anomaly a t B o s w e l l ,  K o o t e n a y L a k e , was f i r s t  r e p o r t e d b y Hyndman, o n t h e b a s i s  o f a n e a s t - w e s t G.D.S. p r o f i l e 1963).  on t h e e a s t e r n s h o r e o f  at latitude  (Hyndman,  I t i s c h a r a c t e r i z e d by a s t r o n g c o r r e l a t i o n  the v e r t i c a l  (z) and m a g n e t i c  station  ( F i g . 1).  reported  related  structure  magnetogram f o r t h i s  A s i m i l a r b u t w e a k e r c o r r e l a t i o n was a l s o  at Crescent Valley, The  between  n o r t h - s o u t h (H) i n t e n s i t i e s .  T h i s i s q u i t e e v i d e n t i n a sample  be  49.5°N  B.C.  anomaly i s o f p a r t i c u l a r  i n t e r e s t because  t o a change i n upper mantle  and/or  lower  i t may  crustal  a s s o c i a t e d w i t h t h e e a s t e r n edge o f t h e w e s t e r n  2 cordillera anomaly as the  (Caner  i s at the center  the Kootenay  1965).  a major  Geologically, tectonic  sweeps i n a broad  (Crosby,  of the induced  conductivity south  of  A r c , which  Nelson batholith  dependence  the  and Cannon,  1968). The  Z field  inhomogeneity  the  feature curve  known  around  directional  suggests  at once  s t r i k i n g magnetic  a  lateral  east-west,  to  of Boswell, B . C .  12:00  15:00 U.T.  U.T.  D  Fig.  B  1  Hyndman's  Magnetic  Boswell of the  (1963)  to the data.  a cylinder,  attempted  July  4,  1968.  s t r i k i n g magnetic  conductivity of  t o f i t an I n f i n i t e  His calculations  and C r e s c e n t  radius  at B o s w e l l ,  Interpretation  Hyndman model  Record  Valley,  and r a d i u s ,  an i n f i n i t e l y  disclosed,  east-west, required  to  satisfy  conducting  cylinder  however,  to the south  unusually the data. cylinder  that of  large  values  For  example,  required  i t  Pig.  2  L o c a t i o n Map Telluric  o f G e o m a g n e t i c D e p t h S o u n d i n g and  S t a t i o n s i n Western Canada  Magneto-  ( a f t e r C a n e r , 1970).  4 to  extend  contrast radius.  beyond for  the  could  shape  the  of  detailed anomaly  Lower  a very  survey  resolve  the  surface  completely however, depends their  of  the  depths,  The  parameter  the  I  ratio  of  shows highly  the  and t h e  used  1964).  c o r d i l l e r a was  The  I  ratios  and C o r n u d a s , of  stations  to  prompted  this  the  thesis,  Structure  of  to  ~ [(AD)  +  the  more  to  map  the  the  of  Canada  Z  at  induced  field.  the  i n Western field  'flat' For  earth,  the  i n the  earth,  Z  component  subsurface  the  layers,  incident  measure  E . M . wave.  this'attenuation  is  (AH)]"*  G.D.S.  carried  out  to  1962,  of  stations  New M e x i c o , the  Infinite .  lAZl  extensive  f r o m 1959  large  as  1  Schmucker  the  conducting  frequency  generally  first  that  attenuation  T _  western  in  that  conductivities  defined  The  an u n r e a s o n a b l y  This  inducing Z  the  conductivity  problem.  an i d e a l  extent  on t h e  zone.  and Upper M a n t l e  cancels  a finite  rough, a p p r o x i m a t i o n to  reported  R i k i t a k e . (1966) the  demanded  conductivity  Crust  Hence,  Hyndman c o n c l u d e d  o n l y be  G.D.S. and  surface.  cylinder  Therefore,  cylinder  C  the  west.  at  were This  profile i n the  latitude to  the  three  U.S.A. 32°N  east  times  prompted  across  of  by  (Schmucker, Las  higher  further  the  Cruces than  profiles  those to  be c a r r i e d o u t a c r o s s t h e c o r d i l l e r a , t o d e t e r m i n e i f t h i s d i s c o n t i n u i t y was e i t h e r a l o c a l o r c o n t i n e n t a l feature.  SELKIRK KOOTENAY MOUNTAINS LAKE  PURCELL MOUNTAINS  ROCKY MTN. TRENCH (KOOTENAY VALLEY)  ROCKY MOUNTAINS  PLAINS  t  \ \ t I0-I5KM\ CONDUCTIVE,HYDRATED LOWER C R U S T (PROBABLY PARTIAL MELTING)  20  \ \ \ \ R E S I S T I V E , NO RESOLUTION A \ \ \ \  * 40  60  \ \ \t\ \ \ 30-35K >750°C X  MODERATELY CONDUCTIVE UPPER M A N T L E , T> 8 0 0 ° C , C O M P O S I T I O N U N D E F I N E D  a 80 -  Fig.  3  Petrological (horizontal  Model scale  f o r W e s t e r n Canada, = vertical  Latitude  scale).(after  49.3  Caner,  N.  1970)  The  n e x t two p r o f i l e s i n t h e U n i t e d S t a t e s ,  at latitudes  34°N ( C a n e r , C a n n o n , and L i v i n g s t o n ,  1967)  Gough and R e i t z e l i n 1 9 6 6 ,  that the d i s c o n t i n u i t y  was r o u g h l y c o r r e l a t e d  indicated  and 3<3.5°N b y  t o t h e e a s t e r n edge o f t h e c o r d i l l e r a .  I n s o u t h w e s t e r n C a n a d a , s u r v e y s by Hyndman Cannon (1967),  D r a g e r t i n 19&9,  are i n t e g r a t e d  i n t o F i g u r e 2,  s t a t i o n a t Kootenay approximate  Lake  which  Observatory  i n c l u d e s t h e anomalous  (K00). T h i s f i g u r e d e f i n e s t h e  l o c a t i o n of the low I-high I d i s c o n t i n u i t y i n  southwestern  Canada.  Caner  (1970)  i n t e g r a t e d these r e s u l t s w i t h  seismological,  aeromagnetic,  a petrological  and e l e c t r i c a l  lower crust  and t h e D o m i n i o n  (1963),  and u p p e r  In the western region,  mantle  and h e a t f l o w d a t a t o c o n s t r u c t c o n d u c t i v i t y model o f t h e f o r w e s t e r n Canada  (Fig. 3).  t h e l o w e r c r u s t i s assumed t o be  h i g h l y c o n d u c t i v e due t o h y d r a t i o n . W a t e r s a t u r a t i o n was first  proposed  b y Hyndman and Hyndman ( 1 9 6 8 )  i n order to  e x p l a i n the h i g h e l e c t r i c a l c o n d u c t i v i t y found c r u s t o f young, t e c t o n i c a l l y a c t i v e , earth.  i n the lower  c o n t i n e n t a l areas of the  7 II  A  THEORY  Induction  i n a Step D i s c o n t i n u i t y (1969)  Madden and S w i f t effect  i n a t w o - d i m e n s i o n a l model o f an anomalous u p p e r  mantle feature- resembling in the  a poorly defined  conductivity. I t included s u r f a c e . They r e p o r t e d  field  studied the magneto-telluric  was p e r p e n d i c u l a r  electric  field  transition  a 10/i-m. s e d i m e n t a r y l a y e r o n  t h a t f o r t h e c a s e when t h e e l e c t r i c  t o the s t r i k e  tended t o i n c r e a s e  of the f e a t u r e , the  when a p p r o a c h i n g t h e  z o n e f r o m t h e more r e s i s t i v e  s i d e , and t o d e c r e a s e  when a p p r o a c h i n g i t f r o m t h e c o n d u c t i v e was no n o t i c e a b l e c h a n g e a c r o s s field  was p a r a l l e l  c u r r e n t s induced  parallel  to the strike  s i d e . However,  This  i n d i c a t e d that the  by the e x t e r n a l f i e l d  were  flowing  o f t h e a n o m a l o u s f e a t u r e . The  shallow  s e d i m e n t a r y l a y e r t e n d e d t o mask t h e d e e p e r features  at the higher Schmucker  in the of  (1964)  i n v e s t i g a t e d the step d i s c o n t i n u i t y  of magnetic f i e l d  an e x t e r n a l t i m e - v a r y i n g  defined  Weaver  induced  are then  t o i t . The more  t h e more c u r r e n t w i l l  (1963)  and showed  one b e i n g  corner  Intensified  sharply  flow.  studied a two-dimensional  c o n s i s t i n g of'two regions,  that  i n such a c o n d u c t o r by  magnetic f i e l d ,  and f l o w p a r a l l e l  the step,  state case,  converged onto the upper  t h e s t e p . Hence, c u r r e n t s  at the corner  anomalous  frequencies.  c o n d u c t i v i t y f o r the steady lines  there  t h e z o n e when t h e e l e c t r i c  to the s t r i k e .  electric  step d i s c o n t i n u i t y  model  more r e s i s t i v e  than the  8 other,  representing  ratio  |Z| / IYI w h e r e  perpendicular as  the  to  boundary  away f r o m t h e rapidly over  the  model of  over  for  is  occurred  and  by  the  than  on t h e  of  lower  crust  the  currents  currents  step  primary  field  at  induced  field  w o u l d be  variations  resulting the  upper  induced  surface  Parkinson Diagrams,  result  theoretical  vertical  in a  mantle  component  semi-infinite  field  of  the  to  the  of  strike  added  to  the  secondary  primary the  crust  step.  The  of  Induction Arrows,  upper  the  earth. the  lower  the  w o u l d be  to  conductivity  depths,  through  corner  correlated  perpendicular  The  discontinuity in  and u p p e r  i n the  the  side.  more  1966).  E . M . waves would p e n e t r a t e  induce  However,  d i m i n i s h e d much  source  the  increased  side.  landward  that  field  discontinuity,  with Rikitake's  induced  defined  He f o u n d  from either  ratio  (Rikitake,  a well  the  approached  Locally,  B  of  i n agreement  field  at  frequency  sea  interface.  h o r i z o n t a l magnetic  strike  cancellation  c o n d u c t i n g body If  was  the  boundary,  the  magnetic  Y is  the  the  sea  a land-sea  or  field  discontinuity.  and t h e  Mercator  Projection. Let geomagnetic  T define field  the  in a  total  vector  specified  change  interval  T = \/(AD) +(AH) +(AZ) 2  and  its  direction is  b e t w e e n T\ a n d clockwise  the  specified  vertical,  2  by  two  a n d (J), t h e  f r o m H . © a n d (J) a r e  of  of  time.  Then,  2  angles: azimuth  conveniently  the  6,  the  angle  measured  plotted  as  points  9 N  N  Fig.  4  Parkinson Diagram.  on a d i a g r a m a s shown i n F i g . 4 ( P a r k i n s o n , two  circles  vertical  represents  component.  declination, North.  of  and downward  The h o r i z o n t a l v e c t o r N i n F i g . 4,  The r a d i a l d i s t a n c e  6, w h e r e center.  so t h a t  upward  1959), changes  6=90° i s a t t h e c i r c u m f e r e n c e ,  the  i n the  i s corrected  represents  from the center  where  for  geographic  of the c i r c l e  is  a n d 6=0° i s a t t h e  Hence,  the closer  a point .is plotted  to the  center  the c i r c l e ,  the larger  i s the Z v a r i a t i o n  of the  event.  If, surface  of  f o r many e v e n t s  a unit  sphere,  T,  points  corresponding  are plotted  on the  to the d i r e c t i o n of  10 T from the non-polar  sphere's stations,  'preferred  plane',  diagram  then  the  is  horizontal  1962;  For in  'UP  dipping This  the  the  at  cluster  sphere of  found,  about  in half.  the  of  the  circle Fig.  of  plane  1964;  1962),  great  at  a  The  preferred  Parkinson plane  striking  perpendicular  on  arrow  defined  as  .  and  preferred  i n the  a vector  by. t h e  is  (f) - l 8 o ° 1  Z and  the  1964;  tilted.  dashed  a preferred  line  plane  direction.  horizontal Inhomogeneity  (J)' d i r e c t i o n . whose  1959,  Sposito,  plane  a conductivity  the  defining  (Parkinson,  represents  90° - 6'  to  horizontal,  represented  4,  and hence  1  is  Simeo  a c o r r e l a t i o n between  (}) - l 8 0 ° v e c t o r ,  is  generally  i n many c a s e s  Rikitake,  an angle  implies  the  However,  circle  1  is  points  preferred  Anderson,  example, the  cutting  1962;  Whitham and  the  it  plane.  station.  Wiese,  that  then  a representation  Usually, a normal  center,  The  geographic  induction  azimuth  is  9 0 ° - e'  (p  1  and m a g n i t u d e Another  Mercator change  i)  plotted  Spectral  Continuous The  is  useful  projection,  is  Power  C  ^QS—  defined  where  of  the  representation  dip  a function  of  of  the  is  the  geomagnetic  geographic  field  azimuth.  Analysis  Case  power as:  as  method  (Lathi,  spectral  1968)  density  of  a finite  function  f (t) T  11 = 11m Frp(w) T ^ T-»a> T  S (w)  r  f  w h e r e , F,p(w) i s t h e F o u r i e r f (t)  =  T  An density  (1)  ;  transform  o f f ( t ) , and,  (f(t)  lt| < T / 2  I  otherwise  0  a l t e r n a t e method o f o b t a i n i n g  i s given  autocorrelation  by t h e F o u r i e r function  (2)  the spectral  transform  of the time  of f ( t ) : CO  S (w)  =/R (T)e-  f  j W T  f  dT  (3)  -oo  where t h e t i m e a u t o c o r r e l a t i o n  function i s x  R (T)  = l i m - ^ - J f ( t ) f (t+T)dt  f  ii)  Discrete The  Case  f u n c t i o n f ( t ) i s now s a m p l e d N t i m e s a t e q u a l T  i n t e r v a l must be s m a l l c 2n~ =  i  s  a  t  l  e  a  s  t  aliasing  e n o u g h so t h a t  greater  of f ( t ) a r e denoted T  estimated  effects,  autocorrelation  frequency  frequency  t h e N sampled  {x-^X^X.^  set { x ^ i s f i r s t  the sampling  the cutoff  than the l a r g e s t  component i n f ( t ) . F o r s i m p l i c i t y ,  The  O  1967)  ( B e n d a t and P i e r s o l ,  i n t e r v a l s h (Nh=T). To a v o i d  f  G  values  .  transformed  such t h a t  X-0. The  function at the displacement r h i s  1  R  r  = R(  r h  ) = _  ^  V W  r  =  0 , 1 , 2 , ....m  (5)  w h e r e r i s t h e l a g number a n d m i s t h e maximum l a g number. An range  estimate  0 ^.f < f  i s  of the true  spectral density  i n the  m+1 frequencies  estimates f = k f /m,  a r e made o f S ( f ) a t t h e d i s c r e t e k = l , 2,  The F o u r i e r t r a n s f o r m f  ( t ) , i s equivalent  estimate  having  ,m.-  (6) becomes  of a f i n i t e  length of record,  t o the c o n v o l u t i o n of F(w), the s p e c t r a l  of the i n f i n i t e  function.  c J  c  r-1  random p r o c e s s f ( t ) w i t h a s i n e  A sine f u n c t i o n has the u n d e s i r a b l e  e f f e c t of  c o n s i d e r a b l y l a r g e s i d e l o b e s , so t h a t t h e power  s p e c t r a l d e n s i t y u s i n g t h i s m e t h o d c a n o n l y be a 'raw' estimate  of the true  may be o b t a i n e d  spectral density. A better  b y u s i n g a H a n n i n g window,  estimate  since i t s  s i d e l o b e s i n t h e f r e q u e n c y d o m a i n a r e much s m a l l e r . b e t t e r w i n d o w s now e x i s t ,  t h e H a n n i n g window was r e t a i n e d t o  obtain results consistent with previous The H a n n i n g window i s d e f i n e d  D(rh)  -•  -^-(l+cos(Trr/m)) 0  Although  r=0,1,2  workers.  by  > • • • •>  m  r>m  (8)  T h i s may be m u l t i p l i e d b y t h e a u t o c o r r e l a t i o n f u n c t i o n i n (7) t o g i v e S ( k f /m)  r m-1 = 2h'R + 2 ] > Z D ^ c o s ( 7 r r k / m ) r=l  s i n c e D = 1 , D =0 o ' m  (9)  13 III  EXPERIMENTAL PROCEDURE  A  Instrumentation i)  The A s k a n i a The  Variograph  A s k a n i a . v a r i o g r a p h r e c o r d s time  the e a r t h ' s magnetic f i e l d per hour, Hourly and  and s e n s i t i v i t y  a t t h e r a t e o f one i n c h  o f a b o u t t h r e e gammas p e r mm.  time marks ( u s i n g an e x t e r n a l chronometer),  instrument  three  on f i l m ,  v a r i a t i o n s of  temperature  independent  optical  are also recorded.  base-line,  I t houses  s y s t e m s (D,Z,H) w i t h m i r r o r s  mounted on s m a l l A l n i c o - a l l o y magnets, each system equipped  w i t h c a l i b r a t i o n Helmhotz These i n s t r u m e n t s proved  survey,  i n the  t h o u g h c o n s i d e r a b l e c a r e was n e c e s s a r y  i n site  The A s k a n i a v a r i o g r a p h must be h o u s e d i n a  weatherproof  building with solid  magnetic d i s t u r b a n c e s such  is  coils.  most r e l i a b l e  selection.  etc.  being  Once a s u i t a b l e  site  floor,  p o w e r , and f a r f r o m  as t r a n s f o r m e r s , moving i s found,  vehicles,  however, t h e i n s t r u m e n t  o p e r a t i o n a l w i t h i n t h e hour.  II)  The P l u x g a t e The  field  Magnetometer  Dominion Observatory  fluxgate unit consists of a  s e n s i t i v e head c o n t a i n i n g t h r e e o r t h o g o n a l  detectors  ( S e r s o n , 1957),  a transistorized  magnetic  electronic  a s s e m b l y , and a c h a r t r e c o r d e r . The m a g n e t i c h e a d may be operated  out of doors  artificial  disturbances. Although  g e n e r a l l y poorer it  a t t h e e n d o f a l o n g c a b l e , away the q u a l i t y  than t h a t of the photographic  from  of the data i s variometer,  has t h e advantage o f p r o v i d i n g t h e o b s e r v e r w i t h an  1^ immediate v i s u a l  r e c o r d , and  site  selection i s less  stringent. This instrument from the Askania type  was  and  t o supplement the  resulted  used e a r l y i n the  (Mosley)  recorders.  B  Experiment R.  Hyndman had  s t r i k i n g east-west B o s w e l l and  survey,  proved  Hewlett-Packard  postulated a cylindrical  to the  s o u t h o f two  Crescent V a l l e y .  s e t up  body  of h i s s t a t i o n s  I n an a t t e m p t  a n o m a l o u s z o n e , a p r o f i l e was  to cross  at  the  between S l o e a n  Park  and  B l u e b e r r y C r e e k w i t h i n t e r m e d i a t e s t a t i o n s a t Thrums  and  Shoreacres  ( F i g . 5).  A f l u x g a t e s t a t i o n was  midway b e t w e e n Thrums and  a n t i c i p a t e d c r o s s o v e r was  not  r e c o v e r Hyndman's o r i g i n a l  profile  found,  ( B l u e b e r r y C r e e k ) was  On  July  2nd,  the  and  2-21),  anomaly at t h a t  instruments  Hyndman's a n o m a l y a t B o s w e l l  (fluxgate),  at La France and  at a l l of these  one  of  (K00  Wynndel ( F i g . 5 ) . On  J u l y 13,  V a l l e y to  were moved t o a  second  to i n t e r c e p t  i n F i g . 2).  The  the  site.  Suitable sites  Creek, B o s w e l l , Sanca,  stations.  up  the  moved t o C r e s c e n t  a t K o o t e n a y L a k e , i n an a t t e m p t  were f o u n d  a l s o set  Shoreacres.  A f t e r t h r e e weeks o f o b s e r v a t i o n (June  Askanias  bar  i n high record loss. Later, better  r e s u l t s were o b t a i n e d w i t h t w o - c h a n n e l  Field  data  v a r i o g r a p h s . U n f o r t u n a t e l y , the chopper  r e c o r d e r , w h i c h was  unreliable  intended  Kuskanook  a n o m a l y was S a n c a and  recovered  LaFrance  15  117.5 w  1 16.5°V  miles  Schroeder  Mirror  Point  Lake  t.OLaFrance  X|  Askania _Fluxgate Roads  Fig 5  o X  L o c a t i o n Map S h o w i n g  B.C. Idaho  Askania  and F l u x g a t e  Ck.  *'CRESTON  IQ R y k e r t s  Staions.  16 C r e e k s t a t i o n s were moved t o M i r r o r L a k e and R y k e r t s t o outline  t h e boundary o f t h e anomaly. On J u l y  27, t h e t h i r d  and l a s t  profile  was s e t up  b e t w e e n B r o u s e and S a l m o , w i t h i n t e r m e d i a t e s t a t i o n s a t Sllverton, fluxgate  Slocan  i n s t r u m e n t s were a c q u i r e d f r o m  Observatory  ( O t t a w a ) and i n s t a l l e d  Howser t o e x t e n d operated of  ( f l u x g a t e ) and Taghum. Two' a d d i t i o n a l  t h e second  profile.  Observatory,  Idaho border. Records from  The  P o i n t and  The i n s t r u m e n t s  were  this  on t h e W a s h i n g t o n -  o b s e r v a t o r y were  t h e c o u r t e s y o f t h e U.S. C o a s t  l o c a t i o n i s of normal  as a c o m p a r i s o n In  reference f o r the f i e l d  an attempt  to obtain further  o f t h e anomaly, a s t a t i c  conducted  between R e v e l s t o k e 1969,  obtained  and G e o d e t i c  Survey.  'low I ' t y p e , and c o u l d be u s e d  boudaries  May 1 s t ,  at Schroeder  u n t i l . A u g u s t 27. Shown i n F i g . 2, i s t h e l o c a t i o n  t h e Newport Magnetic  through  the Dominion  recordings. i n f o r m a t i o n on t h e  magnetometer survey  and B o n n e r ' s F e r r y , I d a h o ,  u s i n g a McPhar v e r t i c a l  magnetometer.  was on  17 IV  A  A N A L Y S I S AND I N T E R P R E T A T I O N  Sections  of  G.D.S.  In F i g . profiles  are  sensitivity, i  Mirror  simultaneous  6,  and  w i t h Newport  a common  Lake  almost  -  this  identical,  vertical  component,  (western Newport  (NEW).  exhibit  very  as  has  s t i l l  already  amplitude; component  at is  H-correlated at The  6;  to  (WYN),  they  stations  very  which the  that to  The  between  normal  low-Z  observed  Mirror  1.  slightly  anomalous  at  Lake  correlated  in Fig.  with  (MIR)  to  H,  Kuskanook  is  reduced  (H-correlated)  are  still  (BRO),  short  recordings  were  f r o m Howser  Z fluctuations Brouse  to  Boswell  the  effect,  stations.  shows  strongly  H, but  components  character  Rykerts  simultaneous  recordings  Fig.  these  side)  perceptible.  Lake,  the  over  barely  Mirror  date;  obtained  Wynndel  no  of  of  correlated  right  latitude  from Sanca  shown f o r  6,  the for  Z amplitudes,  been  (Fig.  horizontal  identical  stations  Although north  side  The  equal  the  changes  (RYK).  character,  high  highly  field  however,  and R y k e r t s  type)  for  at  north-south  reference,  and Newport  except  f r o m two  replotted  profile,  a uniform source  (SAN)  as  Rykerts  implying  Sanca  records  shown d i g i t i z e d  Throughout are  Magnetograms  occupied  (HOW) a r e  strongly least  twice  only  about  50  of  d i s c o n t i n u i t y occurs  available  at  a  shown.on  km.  as to  the  the  left  with  large  approximately in a  subsequent  anomalous,  at  distance  are  as  those  west.  25  north-south  miles  Fig. 6  Two  S i m u l t a n e o u s P r o f i l e s o f G.D.S. R e c o r d s .  19 direction  i s now  emphasized.  sounding p r o f i l e s the  a c r o s s t h e e a s t e r n edge o f t h e  same d i s c o n t i n u i t y  r o u g h l y 150 ii  km.  Brouse  On o t h e r g e o m a g n e t i c  i n an e a s t - w e s t  o b s e r v e d on t h i s p r o f i l e  also,  ( F i g . 6,  left  Brouse  side)  i n Z are  but w i t h c o n s i d e r a b l y  reduced  s i m u l t a n e o u s r e c o r d i n g f r o m Howser, f r o m i s shown f o r c o m p a r i s o n t o i n d i c a t e  r e d u c t i o n i n a m p l i t u d e . The show a l m o s t i d e n t i c a l  of  direction,  Anomalous ( H - c o r r e l a t e d ) f l u c t u a t i o n s  eastern p r o f i l e ,  cordillera,  i s observed over a l a r g e r distance  - Salmo and N e w p o r t  a m p l i t u d e s . The  depth  the  s t a t i o n s f r o m Taghum (TAG) records;  anomalous Z a m p l i t u d e s are s l i g h t l y  a t Salmo  the  to  (SAL), the  reduced, but  still  c l e a r l y l a r g e compared t o t h e n o r m a l low Z s t a t i o n a t Newport. The  t r a n s i t i o n from  'anomalous'  l o w I ' s t a r t s a t Salmo on t h i s p r o f i l e . s t a t i o n s were a v a i l a b l e the  transition will  to  'normal  western  However, s i n c e  no  s o u t h o f Salmo, i t i s n o t c l e a r i f  be as s h a r p a s t h a t o b s e r v e d  K u s k a n o o k and R y k e r t s ; a c l o s e l y  spaced network  between of  stations  b e t w e e n Salmo and N e w p o r t w o u l d be r e q u i r e d t o d e t e r m i n e this detail.  At t h i s  s t a g e , i t c a n o n l y be c o n c l u d e d  a) t h e a n o m a l y on t h e B r o u s e l e s s pronounced b)  start  t h a n f o r the M i r r o r Lake  i s considerably  - Rykerts p r o f i l e ;  o f t h e t r a n s i t i o n o c c u r s a t r o u g h l y t h e same  latitude,  c o n f i r m i n g the east-west s t r i k e  One Fig.  - Salmo p r o f i l e  that  of the  structure.  o f t h e most s t r i k i n g f e a t u r e s d e m o n s t r a t e d  6 i s t h e d i f f e r e n c e b e t w e e n t h e two p r o f i l e s ;  a m p l i t u d e of the anomaly changes  by  the  very s t r o n g l y over  e a s t - w e s t d i s t a n c e o f t h e o r d e r o f o n l y a b o u t 40 km.  an (for  20 example,  Mirror  structural anomalies  Lake  change cannot  B  Power  W.  section  per  power  and  (9)  1-3  Rykerts  45^.  purposes  the  section  which  for  of  II  a  i.e.  strong  the  simple  profiles  sampling  Geophysics  The  Brouse  -  Salmo  estimates  and  in  of  the  of  64  by  U.B.C.  The  I B M 360  lengths  -  in  the  used  error  for  were  Lake  as  resolution  used  with  used  Mirror was  standard  are  built  at  A 20°/c l a g  increase  rate  on the  record  a normalized  resulting the  a  two  computed  C.  high  as  is  comparison  only. profiles  shown  horizontal  spectral  estimates  except  a  slight  latitude  more  noticeable  for  progressively Z estimates  of  (Fig.  7a)  are  Sanca  are  very  is  at  respectively.  the  since  from the  were  the  gives  In both  Rykerts  of  d i g i t i z i n g machine  estimates  profiles,  preferable  well,  i n terms  digitized  Department  of  However,  records  with  hours,  analysis,  as  indicates  Analysis  were  the  spectral  equation 12  hour  Cannon of  direction  explained  Simultaneous  points  This  structures.  Spectral  previous  Sllverton).  in.this be  two-dimensional  to  has  the of  special  similar  the  a known low  Mirror  same I  station.  (D a n d effect  Lake  at -  high  is  Jb,  the  closely  related,  becomes  higher  here.  frequency Wynndel  H) a r e  Rykerts  exhibit  and  7a  which  the  interest  and  high  in Pig.  frequencies.  and Newport Mirror  Lake  Z power  cutoff  as  profile and  levels.  Newport,  intermediate  The  which  between  the  21  .LOG  (3'7C.RM.)  10  -  0.02  • F  Pig.  7a  R  0.06 E  Q  U  E  N  C  0.10 Y  ,  Power S p e c t r a l E s t i m a t e s , Rykerts  and  MIRROR L A K E SANCA WYNNDEL RYKERTS NEWPORT  Newport.  C  R  M  .  M i r r o r Lake to  22  0.02  0.06 F  Fig.  7b  R  E  Q  U  E  N  0.10 C  Y  ,  C  R  M  ,  Power S p e c t r a l E s t i m a t e s , B r o u s e t o Salmo and N e w p o r t .  23  STATION LOCATION  ' F i g . 8a  Power S p e c t r a l R a t i o s , and  Newport.  M i r r o r Lake t o R y k e r t  24  .Fig 8b  Power and  Spectral Ratios,  Newport.  B r o u s e t o Salmo  25 two  groups.  visual  This  comparisons The  Newport  profile,  profile  in  do n o t section  III  stations  Brouse  to  anomalous  i.e.  western  From the  i n the  a  to  Mirror  between  Salmo  show  levels,  Lake  to  at  periods  coherency. station  These  i n each  geographic  the  profile  (Mirror  it  shows  on the C  Brouse  Magnetic  the  striking and  were  P /P  distinct Lake  -  8a  Newport  G.D.S. Newport.  being  strongly  ratios showed  respect  as  to  were  at  This Figure  a l l  dependence  the  I  of again  eastern  three  for  high  a low  a function  d i s c o n t i n u i t y on t h e  a frequency -  of  spectral  and 8 b ) .  Newport),  and  7b).  closely  a n d 50 m i n . w h i c h  and p l o t t e d  (Fig.  the  (Fig.  xl  normalized with  profile  studying  United  frequencies; discontinuity  profile.  the  west  of  1966).  The  eastern  an e a s t - w e s t  States,  difference  to  anomalies  35>  Salmo  and  Survey  Upon across  20,  latitude  illustrates  also  of  Salmo  spaced  evidence  to  the  Wynndel.  Z  found  -  and more  than  results,  Brouse  closely  of  section.  discontinuity  a more  Newport  expression  previous  the  sharp  A,  Z power  above  of  desirable  type  stations  out  show  been  anomalous,  a quantitative  estimates  would have  However,  related  is  carried  Z spectral  As mentioned  less  result  was the  passing found  near  between  Rocky Mountains  profile  and b r o a d e r  aeromagnetic Denver, profiles  Colorado, to  (Pakiser  included both  regional  profile  anomalies  sharp  the  and  a  east  Zietz,  local  whereas  the  latter  26  Pig.  9  L o c a t i o n Map  Showing Magnetic  Survey  Profile.  UJ  o  cc cc UJ  UJ 1C  CO  cr  _ l LU i> UJ CC  UJ CO o a:  cc o cc cc  cc  CO o 1X1  UJ }C  cc UJ o CQ  WIUERED  1 0 . 0 3 MJ  2 5 . 0 0 MI  n.oom  • 7 5 . 0 0 MI  20  Pig.  10  F i l t e r e d Magnetic Survey w i t h  HUES  Wave! ength C u t o f f s  Shown.  ro  28 was  a b s e n t i n t h e west. T h i s change i n c h a r a c t e r o c c u r r e d  about  100 m i l e s w e s t  o f Denver,  which i s approximatelythe  l o c a t i o n o f t h e G.D.S. d i s c o n t i n u i t y  (Caner, Cannon, and  1967). T h i s agreement between t h e p a t t e r n o f  Livingston, aeromagnetic  profiles  and G.D.S. d i s c o n t i n u i t i e s was a l s o  o b s e r v e d i n w e s t e r n Canada (Caner, 1 9 7 0 ) . effect  The s m o o t h i n g  i n t h e w e s t may be e x p l a i n e d b y a r i s e  1965) o r p e r h a p s by a  p o i n t i s o t h e r m ( P a k i s e r and Z i e t z , more s i l i c i c  i n the Curie  lower c r u s t w i t h low magnetite content  (Caner,  1969). Using long aeromagnetic required the  the f i l t e r i n g  magnetic  therefore  profiles,  Caner's  o f w a v e l e n g t h s up t o 100 km. b e f o r e  d i s c o n t i n u i t y became a p p a r e n t . I t w o u l d  seem r a t h e r u n l i k e l y t h a t t h i s c o u l d be d e t e c t e d  in  a s h o r t ground  to  d e t e c t t h e d i s c o n t i n u i t y on t h e M i r r o r Lake  profile  work  because  s u r v e y . However, i t was t h o u g h t  of the r e l a t i v e l y  possible  - Rykerts  short distance over  which  i t occurs. t h e s p r i n g o f 1969, a ground  In  s u r v e y was c o n d u c t e d  b e t w e e n R e v e l s t o k e , B.C., a n d S a n d p o i n t , I d a h o , u s i n g a McPhar v e r t i c a l magnetometer  ( P i g . 9 ) . The d a t a was  c o r r e c t e d f o r time v a r i a t i o n s ,  using the V i c t o r i a  m a g n e t o g r a m s , and a l s o f o r l a t i t u d e filtering  d e p e n d e n c e . Low p a s s  i n t h e f r e q u e n c y d o m a i n was a p p l i e d  s u c c e s s i v e wavelength c u t o f f s o f 10, The  results  with  2 5 , 5 0 , and 75 m i l e s .  shown i n F i g . 10 a r e i n c o n c l u s i v e . A  smoothing of t h e l o n g e r wavelength anomalies i sn o t apparent  29 south  of  absence  Rykerts. of  magnetic  disprove  the  D  Diagrams  Polar  Two of  the  dependence  of  Brouse,  to  and R y k e r t s .  These  used  in this  analysis  these  sites, About  storm of  above  derived, are  were  shown  the  in Fig.  directly  from  and  consuming  hand. 10  the  largest  450  of  as  and  made  task  of  these,  to  following  previous Mirror  were  Lake,  equipped  were  not  orthogonal all  be  recorded  at  hand  obtain  data,  to  measuring to  in the  magnetic magnetograms  Parkinson section  II  I B M 3^0  polar  diagrams B,  were  computer,  diagrams  eliminate magnetic  compute  digitized  plot  peak  from the  events,  of  to  lib.  to  composed on the  the  Salmo;  peak  defined  output  digitized  intervals  by  From these  was  A p r o g r a m was  minute  measured  11a  plotting  limitations.  and f i f t y  plotter  An a t t e m p t  time  not  in  the  records  could  projections,  using  fluxgate  field  hundred  at  stations  three  recorder  not  azimuthal  of  and  the  stations.  and M e r c a t o r  and  one  variations  the  to  the  because  magnetic due  The  the  does  which d i f f e r  profiles  Taghum,  variographs.  region,  component  Silverton,  Askania  of  field two  I  i.e.  section.  investigate  the  with  components  last  result,  high  diagrams,  vertical to  negative  i n the  the  polar  used  the  Wynndel,  of  of  belonging  section: Sanca,  were  this  sources  results  types  dip,  stations,  However,  the  tedious  variations  vector  section,  on a p o l a r  the  and  diagram.  by  changes choose To  in  the  obtain  O  Fig.  11a  P a r k i n s o n D i a g r a m s and C o r r e s p o n d i n g M e r c a t o r P r o j e c t i o n s Dip  (-90° t o 90°)  M i r r o r Lake.to  vs Geographic  Rykerts.  Azimuth  -  Measured from N o r t h  (-180°  to l80°)  i  Fig. l i b  P a r k i n s o n Diagrams and Corresponding Mercator P r o j e c t i o n s Dip  (-90°  to 9 0 ° )  Brouse to Salmo.  vs Geographic Azimuth Measured  from North ( - l 8 0  to 180  )  LO  ro  Fig.  12a  P o l a r D i a g r a m s and C o r r e s p o n d i n g M e r c a t o r P r o j e c t i o n s Dip  (-90°  t o 90°)  vs Geographic  M i r r o r Lake t o R y k e r t s .  Azimuth  -  Measured from N o r t h  (-180°  t o l80°)  Fig.  12b  P o l a r Diagrams Dip  (-90°  Brouse  and C o r r e s p o n d i n g M e r c a t o r P r o j e c t i o n s  t o 90°)  t o Salmo.  vs Geographic Azimuth Measured  -  f r o m N o r t h (-180  t o 180  )  34 a  true  horizontal projection  radial be  distance  cos  where  Parkinson  neglecting  center  of  §  dip  the  is  phase  magnetic  events.  Mercator  projections  here,  are  Sanca, whether at  polar types  is  evident  Brouse, Wynndel  between  taken  compared  i s -TJ- -  to to  components  12a  the  This  disadvantage  w i t h the  in Fig.  the  of  during  corresponding  and  12b,  and,  diagrams. of  shows  the  was  as  distance has  plane,  circle  event,  along  shown  diagrams,  at  the  Silverton,  a c o r r e l a t i o n between  following  Taghum,  any  and  sites:  Salmo.  correlation,  Mirror  dip  Lake,  It  is  debatable  and none  is  evident  section  II  Rykerts. Induction  derived is  results  preferred the  radial  however,  are  both  azimuth  the  of  difference  These  called  In and  the  procedure,  the  the  from the  d i a g r a m where  computational  of  from these  described  are  given  their such  arrows,  results,  i n the  next  results  are  and  are  very  equal  large  and  to  the  inhomogeneity  E  Interpretation anomalous  to  causing  that  Creek  of  errors,  of  the  the  were  14,  which  and  Boswell  since  course,  approximately  However,  B,  Sanca,  -  15°  general  i n d e t e r m i n i n g the  stations  from previous  shown i n F i g .  The  are  helpful  very  in  LaFrance  identical.  is  arrows  stations  are  amplitude.  the  other  and  20% f o r  of  The  defined  section..  i n d u c t i o n arrows  magnetograms  direction  as  in for  trend  direction  anomaly.  are  shown i n F i g .  profiles  are  13-  included  to  A  few show  UI  Fig.  13  L o c a t i o n Map  Showing Anomalous  Stations.  36 the  approximate boundary of the  Several magneto-telluric  low  also  i n d u c t i o n , a r r o w s i n F i g . 14  to  the  an  isolated  crust,  south  of the  s t a t i o n s . I f the  conductor,  arrows to reverse This  discontinuity  anticipat  indicate  magnetic  a n o m a l y was  one  would expect the  so. I n s t e a d ,  the  low  east-wes  caused  s u c h as a c y l i n d e r i n t h e  upon c r o s s i n g i t w i t h a  i s not  the  previously  s t r i k i n g approximately  s t r i k i n g east-west,  profile.  show t h a t  a deeper inhomogeneity. A l s o , they  anomalous c u r r e n t s  to  included.  a n o m a l o u s z o n e i s much l a r g e r t h a n was suggesting  I discontinuit  s t a t i o n s w h i c h were u s e d  c o n s t r u c t the model of F i g . 3 are The  I - high  by  lower induction  north-south I - high  I  i s e n c o u n t e r e d as a r a t h e r sharp s t e p .  This  then s u g g e s t s a c o n v o l u t i o n i n the major c o n t i n e n t a l low  I - high I d i s c o n t i n u i t y . The  presented  m o d e l p r o p o s e d i s shown i n F i g . 14,  i n t e r m s of d e p t h c o n t o u r s  highly conducting The  (hydrated?)  d e p t h s shown a r e b a s e d on two  regions  contours  are  of course  d e p t h i s c o n c e r n e d , but  discontinuity  arbitrary their  model presented  surface  of  3)  (refer to F i g .  i n so f a r a s  concentration  data  absolute  south  of  'cliff-like' Induced  and/or v e r t i c a l f a c e observed  the  intermediate  s t r i k i n g magnetic east-west.  then r e s p o n s i b l e f o r the The  The  r e p r e s e n t a t i o n of the  f l o w i n g i n the upper corner are  crust  is  the m a g n e t o - t e l l u r i c  (Caner, 1969).  from the  Wynndel i s a g r a p h i c  lower  t o the  and  currents  of the  ste  anomaly.  here i s q u a l i t a t i v e  only.  The  LO  Fig.  14  Proposed  Model Expressed  i n Depth Contours  to Highly Conducting  Layer.  38 power s p e c t r a l a n a l y s i s s e r v e d  as a mapping t e c h n i q u e t o  l o c a t e the low I - h i g h I d i s c o n t i n u i t y , arrows i n d i c a t e d contours  while the i n d u c t i o n  t h e d i r e c t i o n t o t h e a n o m a l o u s c u r r e n t s . The  shown i n P i g - . 14 c o u l d be v e r y much i n e r r o r .  Caner's r e s u l t s  (Caner,  Prom  1970), t h e c o n d u c t i v i t y c o n t r a s t f o r  the model i s a t l e a s t 50:1. The visualized  lower  c r u s t I n t h e a n o m a l o u s r e g i o n may be  a s t h a t o f P i g . 3,  s t r i k i n g magnetic east-west. unknown b u t i t s h o u l d  but with a s i n i s t r a l  fault  The l a t e r a l d i s p l a c e m e n t i s  be o f t h e o r d e r o f 50 t o 100 m i l e s ,  s i n c e the low I - h i g h I d i s c o n t i n u i t y i s recovered 100  m i l e s west of Denver, C o l o r a d o .  P. F o m i n o f f ,  At t h e time  of the Department of Geophysics  again  of w r i t i n g ,  a t U.B.C., i s  c o n s t r u c t i n g a n e x p e r i m e n t a l m o d e l b a s e d o n F i g . 14, t o investigate An low  the i n d u c t i o n e f f e c t s of t h i s attempt  was made t o c o r r e l a t e  model. the offset  I - high I d i s c o n t i n u i t y w i t h surface geology  i n the  and t h e  t e c t o n i c s of the area,  but without  be  that the d i r e c t i o n of the o f f s e t ,  i n t e r e s t i n g t o note  extended eastward,  s u c c e s s . H o w e v e r , i t may  i n t e r s e c t s the southern  Rocky Mountain Trench.  limit  Perhaps the f e a t u r e o r i g i n a t e d w i t h  the Nevadian d i s t u r b a n c e i n l a t e J u r a s s i c , r e v o l u t i o n i n late Cretaceous,  or the Laramide  two o f t h e m a j o r  r e s p o n s i b l e f o r the. f o r m a t i o n o f t h e w e s t e r n The  model l e n d s  c o n d u c t i v i t y observed  of the  support  cordillera.  t o the t h e s i s that the high  i n the lower  caused by h y d r a t i o n r a t h e r than  disturbances  c r u s t i n t h e west, i s  a rise  i n temperature.  In  39 the  latter  case,  d i f f u s i o n would not  a w e l l d e f i n e d step, u n l e s s of course in geologically  recent  times.  permit  the e x i s t e n c e  the f e a t u r e  of  originated  40 V  CONCLUSION  Prom t h i s r e s e a r c h , i t i s c o n c l u d e d t h a t t h e geomagnetic originates  v a r i a t i o n anomaly a t Kootenay  Lake,  B.C.,  i n a l o c a l i z e d d i s t o r t i o n of t h e main low I -  high I discontinuity., r e s u l t i n g i n a w e l l defined step i n conductivity Kootenay  s t r i k i n g magnetic  Lake,  qualitative conductivity  B.C. The f i n a l  east-west, t o the south of m o d e l i n F i g . 14, i s  only. I t supports the t h e s i s that the h i g h of the lower c r u s t ,  i n t h e west,  some c o m p o s i t i o n a l r a t h e r t h a n t h e r m a l e f f e c t , h y d r a t i o n as s u g g e s t e d by Caner  (Caner,  1969).  i s due t o perhaps  41 REFERENCES  1.  Bendat, of  2.  J . S . , A.G. P i e r s o l ,  random d a t a , 2 9 0 - 2 9 3 ,  C a n e r , B., W.H.  M e a s u r e m e n t and a n a l y s i s  W i l e y , New Y o r k ,  Cannon, G e o m a g n e t i c  depth  1967. sounding  and c o r r e l a t i o n w i t h o t h e r g e o p h y s i c a l d a t a i n w e s t e r n N o r t h A m e r i c a , N a t u r e , 207, 3.  C a n e r , B., W.H.  Cannon, C.E. L i v i n g s t o n ,  d e p t h s o u n d i n g and u p p e r m a n t l e Cordillera  C a n e r , B., L o n g structure Letters,  5.  Caner,  1965.  Geomagnetic  structure  i n the  region of western North America, J .  Geophys. Res., 72, 4.  927-928,  6335-6351/1967.  a e r o m a g n e t i c p r o f i l e s and c r u s t a l  i n w e s t e r n C a n a d a , E a r t h and P l a n e t . S c . 7,  3-11,  1969.  B., E l e c t r i c a l  conductivity  structure i n  w e s t e r n C a n a d a 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 , J . Geomag. G e o e l e c , 6.  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