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Subduction beneath the Queen Charlotte Islands? : the results of a seismic refraction survey Mackie, David 1985

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SUBDUCTION BENEATH THE QUEEN CHARLOTTE ISLANDS? A SEISMIC REFRACTION SURVEY  THE RESULTS OF  by DAVID MACKIE B.  Sc. (Honours), U n i v e r s i t y  of T o r o n t o ,  1982  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES Department of Geophysics  and A s t r o n o m y  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 to the required  standard  THE UNIVERSITY OF B R I T I S H COLUMBIA O c t o b e r 1985  ©  David Mackie,  1985  In presenting  this  thesis i n partial  f u l f i l m e n t of the  r e q u i r e m e n t s f o r an a d v a n c e d d e g r e e a t t h e of B r i t i s h Columbia, I agree that it  freely  t h e L i b r a r y s h a l l make  a v a i l a b l e f o r r e f e r e n c e and s t u d y .  agree t h a t p e r m i s s i o n f o r extensive for  University  s c h o l a r l y p u r p o s e s may  for  financial  of  The U n i v e r s i t y o f B r i t i s h 1956 Main M a l l V a n c o u v e r , Canada V6T 1Y3  DE-6  (3/81)  Columbia  my  It is thesis  s h a l l n o t be a l l o w e d w i t h o u t my  permission.  Department  thesis  be g r a n t e d by t h e h e a d o f  copying or p u b l i c a t i o n of t h i s  gain  further  copying of t h i s  d e p a r t m e n t o r by h i s o r h e r r e p r e s e n t a t i v e s . understood that  I  written  Abstract The  Queen  immediately  Charlotte  east  of  boundary between the American  plates.  underthrusting presently this on  crustal  seismic  the  of  the  b o u n d a r y and  across  Six  this the  along  a  the  Queen C h a r l o t t e  and  t w e n t y 60  long The  kg  east-west  200  km  the  region,  an  the  North oblique  11  investigate subduction  The  survey  Queen C h a r l o t t e  land  l i n e extending  British  based  km  Columbia.  c h a r g e s were d e t o n a t e d a l o n g  m u l t i p l e s h o t s r e c o r d e d on  were  w e s t o f .the  Thirteen  ocean t o the west of  was  Islands  stations  f r o m 20  t o the mainland.  be  onshore-offshore  c o n d u c t e d i n 1983.  i n the  same l i n e , e f f e c t i v e l y  To  oblique  t o the m a i n l a n d of  Islands  explosive  continental  fault.  c r u s t beneath the  Strait  line  the  transform  o c e a n b o t t o m s e i s m o g r a p h s and  deployed  and  lies  I s l a n d s , marks  i m p l i c a t i o n s of  s u r v e y was  Hecate  Charlotte  which  p l a t e b e n e a t h N o r t h A m e r i c a may  s t r u c t u r e beneath  refraction  zone,  p l a t e motions suggest that  Pacific along  fault  Pacific  Relative  d e s i g n e d t o sample the and  Queen  oceanic  occurring  plate  transform  the  540 a  kg  110  receivers.  m u l t i p l e r e c e i v e r s , a l l along  reverses  the  profile  over  km  some  the  of  its  length. The deep  crustal  Charlotte the  the  of  and  d a t a s e t was deep  of  the  study  beneath  i s to provide the  fault  Hecate S t r a i t . selected  ocean the  t h e Queen C h a r l o t t e  west  this  structure  Islands,  extensive  Beneath At  objective  to  An  a model of  zone,  a c t i v e Queen C h a r l o t t e  km  meet  wide  fault,  Queen  exemplary subset this  Moho d i p s a t a b o u t 2°  t e r r a c e , a 25  the  the  zone the  of  objective. to the  east.  immediately  d i p of  the  Moho  iii  increases terrace  t o a b o u t 5 ° . The c r u s t i s a b o u t and  18  km  thick  at  the  12  km  eastern  thick  at  the  edge o f t h e Queen  C h a r l o t t e I s l a n d s , a n d i n e x c e s s o f 30 km t h i c k a t t h e m a i n l a n d . The t e r r a c e u n i t i t s e l f unit  with  low  velocity  km/s/km) a n d a l o w e r low  gradient  supports the  p l a t e beneath  terrace  unit  could  is the  compatible Strait.  taken form  the  with  crustal the  -  high  an  upper  g r a d i e n t (0.3  ( 6 . 5 km/s) a n d  a  This model, while not d e f i n i t i v e ,  Queen  represent  shallow  u n d e r t h r u s t i n g of  Charlotte  Islands.  a sedimentary  slab.  o f t h e Queen C h a r l o t t e  shortening  and  The  accretionary  t h e Queen C h a r l o t t e t r a n s f o r m  up by d e f o r m a t i o n of  and  terrace unit - the subducting  in which compression across  in  km/s)  units  unit with a high v e l o c i t y  ( 0 . 0 5 km/s/km).  wedge a n d t h e l o w e r  zone  (4.1  t h e i n t e r p r e t a t i o n of o b l i q u e  Pacific  upper  i s d i v i d e d i n t o two  thickening  A model fault Islands i s not  t h i n c r u s t b e n e a t h t h e i s l a n d s and Hecate  iv  Table of Contents Abstract L i s t of Tables L i s t of F i g u r e s Acknowledgement Chapter I INTRODUCTION 1.1 The T e c t o n i c  ii v viii  ..1 Islands 1 1.1.1 A c c r e t i o n a r y H i s t o r y 1 1.1.2 P o s t A c c r e t i o n a r y H i s t o r y o f t h e Queen C h a r l o t t e Islands 11 1.1.3 P r e s e n t T e c t o n i c S i t u a t i o n a t t h e Queen C h a r l o t t e Islands 15 H i s t o r y o f t h e Queen C h a r l o t t e  Chapter I I DATA ACQUISITION AND PROCESSING 2.1 E x p e r i m e n t 2.2 I n s t r u m e n t a t i o n 2.3 I n i t i a l D a t a P r o c e s s i n g 2.3.1 D i g i t i z a t i o n 2.3.2 Time a n d D i s t a n c e C o r r e c t i o n s 2.3.3 D a t a Q u a l i t y a n d F i l t e r i n g . 2.4 Summary  33 33 35 38 38 40 43 48  Chapter I I I INTERPRETATION 3.1 M o d e l l i n g a n d U n i q u e n e s s 3.2 D e s c r i p t i o n o f t h e M o d e l l i n g A l g o r i t h m 3.3 I n t e r p r e t a t i o n o f I n d i v i d u a l P r o f i l e s 3.3.1 The F i n a l M o d e l - A P r e v i e w 3.3.2 The C h o i c e o f D a t a S e t s f o r M o d e l l i n g 3.3.3 Common R e c e i v e r P r o f i l e 1 3.3.4 Common R e c e i v e r P r o f i l e 3 3.3.5 Common R e c e i v e r P r o f i l e 15 3.3.6 Common R e c e i v e r P r o f i l e s 16 a n d 17 3.3.7 Common S h o t P r o f i l e 4 3.3.8 Common S h o t P r o f i l e 16 3.4 The F i n a l M o d e l - A R e c a p  49 49 51 52 52 54 55 61 64 69 75 79 83  Chapter IV DISCUSSION AND CONCLUSIONS Bibliography  87 100  A p p e n d i x A - ESTIMATION OF EXPLOSION DETONATION TIMES ...109 A p p e n d i x B - COMMON RECEIVER RECORD SECTION PLOTS  113  V  List  I.  Cenozoic  of  Tables  P l a t e I n t e r a c t i o n a t t h e Queen  I slands  Charlotte 16  II.  Instrument  Type  III.  Errors  i n time of f i r s t  IV.  Travel  time e r r o r s  38 sample  41 42  vi  List  of F i g u r e s  1. P r e s e n t d a y p l a t e t e c t o n i c c o n f i g u r a t i o n a l o n g coast of North America 2. S u s p e c t t e r r a n e s  of the Canadian C o r d i l l e r a  4. The a c c r e t i o n a r y h i s t o r y o f W r a n g e l l i a , C o a s t P l u t o n i c Complex  terranes  7. P h y s i o g r a p h i c Region  features  s l a b beneath North  o f t h e Queen C h a r l o t t e  8. B a t h y m e t r y o f t h e Queen C h a r l o t t e 9. T o t a l . u p l i f t ,  with  respect  Islands  t o sea l e v e l , s i n c e  C o n t e m p o r a r y v e r t i c a l l a n d movements r e l a t i v e l e v e l i n mm/yr f r o m t i d a l s t a t i o n s  11.  R e f l e c t i o n seismic terrace  12.  Crustal structure across fault  13.  Location  14.  D e s i g n o f t h e 1983 r e f r a c t i o n e x p e r i m e n t  across  t h e Queen ..  t h e Queen C h a r l o t t e  of composite P-nodal f a u l t  15. V e l o c i t y s e n s i t i v i t y  12  America. Islands  20  10 Ma.  23  t o sea  23  Charlotte  25  transform  27  p l a n e s o l u t i o n s . .28 34  o f t h e OBSs  16.  P e r i o d o g r a m power f o r a l a n d s t a t i o n  17.  Power s p e c t r a o f n o i s e  18.  Power s p e c t r a  19.  The f i n a l  20.  Comparison of the data  37 .43  a n d s i g n a l f o r an OBS  s i g n a l and n o i s e  17 18  region  10.  profile  7  S t i k i n i a and the 9  p l a t e t e c t o n i c r e c o n s t r u c t i o n f r o m 80 Ma  6. P o s s i b l e s u b d u c t e d P a c i f i c  2 4  3. The a c c r e t i o n o f t h e A l e x a n d e r a n d W r a n g e l l i a with North America  5. P a c i f i c  t h e west  f o r Hecate S t r a i t  v e l o c i t y model and s y n t h e t i c s f o r P r o f i l e  45 OBSs.  47 53  1. .56  vi i  21. F i n a l  ray tracing  d i a g r a m f o r Common R e c e i v e r P r o f i l e 1. 59  22. C o m p a r i s o n o f d a t a a n d s y n t h e t i c s f o r Common R e c e i v e r P r o f i l e 3 23. F i n a l  ray tracing  62  d i a g r a m f o r Common R e c e i v e r P r o f i l e 3. 63  24. C o m p a r i s o n o f d a t a a n d s y n t h e t i c f o r Common R e c e i v e r P r o f i l e 15.  65  25. F i n a l r a y t r a c i n g d i a g r a m f o r Common R e c e i v e r P r o f i l e 15  67  26. C o m p a r i s o n o f d a t a a n d s y n t h e t i c f o r Common R e c e i v e r P r o f i l e 16  70  27. F i n a l r a y t r a c i n g d i a g r a m f o r Common R e c e i v e r P r o f i l e 16  71  28. C o m p a r i s o n o f d a t a a n d s y n t h e t i c f o r Common R e c e i v e r P r o f i l e 17  72  29. F i n a l r a y t r a c i n g d i a g r a m f o r Common R e c e i v e r P r o f i l e 17  73  30. C o m p a r i s o n o f d a t a a n d s y n t h e t i c f o r Common S h o t P r o f i l e 31. F i n a l  ray tracing  4.  76  d i a g r a m f o r Common S h o t P r o f i l e  4. .78  32. C o m p a r i s o n o f d a t a a n d s y n t h e t i c f o r Common S h o t P r o f i l e 16 80 33. F i n a l r a y t r a c i n g d i a g r a m f o r Common S h o t P r o f i l e 16. 82 34. Two p o s s i b l e m o d e l s f o r t h e t e c t o n i c Charlotte Islands  r e g i m e o f t h e Queen 88  35. P r e l i m i n a r y g r a v i t y m o d e l a c r o s s t h e s o u t h e r n Moresby I s l a n d  t i p of  36. G e o m e t r i c a l method o f d e t e r m i n i n g  t i m e . .110  shot o r i g i n  92  vi i i  Acknowledgement  This support  thesis and  would  guidance  not of  and  me  throughout  my  Dr.  R. M. E l l i s  forcritically  the  success  helpful criticism research.  I  Bob  digitizing lessons helped many  and  the  Sonya  as  also this  of  bugs  in  my  Dehler  i n b u i l d i n g the  with  introductory  t o p i c s . Brad  the  would  and  interpretation i n the last  also  like  to  my s t a y h e r e t r u l y  p a r t i c u l a r l y C h r i s P i k e , K a t h y P e n n e y , Todd  and  lastly,  Joane Berube f o r h e r s c i e n t i f i c i n preparing  but not l e a s t ,  Data  acquisition  was  possible  thank  enjoyable  Parizeau.  -  Bostwick  t o thank  this  thesis  "ma  and  support. only  through t h e h e l p of  many. The D e p a r t m e n t o f F i s h e r i e s a n d O c e a n s g r a c i o u s l y t h e u s e o f t h e s h i p C.S.S.  the  advice, her unflagging  the f i n a l diagrams of f o r her moral  process  stages of  Kimi  t h e c o f f e e room c r e w . Most o f a l l I w o u l d l i k e  assistance  Prager  with  aided  computers  John  computer programs. D i s c u s s i o n s  made  chouette"  and  fix  people  and  t h e r e s u l t of  Meldrum  me  was i n d i s p e n s a b l e  who  thank  helped  I  Vancouver  to  work.  a n d many o t h e r  the workings of  enthusiasm,  like  Bob  providing  p r e p a r a t i o n of t h i s t h e s i s . in  His  were a l s o i n s t r u m e n t a l  G e o r g e S p e n c e a n d Don W h i t e greatly.  reading  without the  was i n v a l u a b l e i n g u i d i n g  would  work  instrumentation  to demystify of  John  system as w e l l  in  possible  of the f i e l d program i s l a r g e l y  k n o w l e d g e a b l e and c a r e f u l  Bennest.  been  D r . R. M. C l o w e s .  encouragement,  The  have  The  cooperation  provided of  her  ix  captain program  and  crew  helped  t o make t h e m a r i n e p a r t o f t h e f i e l d  a s u c c e s s . The a s s i s t a n c e  of  Chief  A l Wood  G e o r g e C o x , who e x p e r t l y d e t o n a t e d t h e e x p l o s i v e s , appreciated.  The  M o r e l a n d crew for  their  I would a l s o l i k e  from t h e E a r t h P h y s i c s B r a n c h  work  in carrying  M/S  i s especially  h e l p o f p e r s o n n e l from t h e P a c i f i c  Centre i s always a p p r e c i a t e d .  and  Geoscience  t o thank  Patrick  and Dr. J . Wright  out the land-based aspect of t h i s  exper iment. Financial  support f o r data  provided  by  Strategic  ( O c e a n s ) G r a n t G0738; D.S.S c o n t r a c t  from  NSERC  the Earth  06SB.23227-4-0904  Operating  acquisition  Physics from  Grant  Branch,  A7707  E.M.R.;  and  analysis  and  A2617;  was NSERC  05SU.23235-3-1089 D.S.S.  Contract  t h e P a c i f i c G e o s c i e n c e C e n t r e , E.M.R.;  and E.M.R. R e s e a r c h A g r e e m e n t 2 8 7 , 1983/84.. A d d i t i o n a l s u p p o r t was p r o v i d e d by C h e v r o n  Canada R e s o u r c e s L t d .  financial-'  1  I. 1.1  The  1.1.1  The  Tectonic  INTRODUCTION  H i s t o r y o f t h e Queen C h a r l o t t e  Accretionary  History  Queen C h a r l o t t e I s l a n d s a r e  s i t u a t e d immediately  a c t i v e p l a t e boundary between the plates  (figure  1).  To  North  understand the  Queen C h a r l o t t e  Islands  framework  which the North  upon  Islands  it  is  American  of  the North crustal  necessary  to  (Monger e t a l . , identification on  of  1972;  past  combined that  helping  'suspect'  terranes  i s termed  uncertain  with  paleogeographical craton, The  to  Queen  1980).  the  was  early  the  formed. Cenozoic  1980;  of  the  craton  Monger, 1984).  The  i s based p r i m a r i l y a tectonic origin  However,  interdisciplinary  biostratigraphic,  paleomagnetic,  and  seismic  techniques,  cross-cutting r e l a t i o n s h i p s provide  unravel (Saleeby,  'suspect' respect setting  i t is called  edge  n e c e s s a r i l y imply  paleobiogeographic,  with geologic  is  terrane  does not  using  paleophysiographic,  leading  fragments or t e r r a n e s  (Coney e t a l . ,  studies  the  Pacific  grew by p e r i p h e r a l a c c r e t i o n  .Coney et a l . ,  these  s t r a t i g r a p h y and  or  to  the  understand  American C o r d i l l e r a  American C o r d i l l e r a  fragments  and  of  t e c t o n i c h i s t o r y of  R e c e n t c o n c e n s u s i s t h a t t h r o u g h o u t M e s o z o i c and time  east  the  tectonic  1983;  history  Coney e t a l . ,  i f i t s paleogeographical  to the C o r d i l l e r a .  of  data these  1980).  A  setting  is  I f a t e r r a n e has  t h a t i s known t o be d i f f e r e n t  a  from  the  of  the  'allochthonous'.  Charlotte  Islands  are  members  2  Figure  1 - P r e s e n t day p l a t e t e c t o n i c c o n f i g u r a t i o n a l o n g t h e west c o a s t of N o r t h A m e r i c a .  The Queen C h a r l o t t e transform fault marks the boundary between the P a c i f i c and N o r t h American p l a t e s a d j a c e n t t o t h e Queen C h a r l o t t e I s l a n d s . PA - P a c i f i c p l a t e , NA - N o r t h A m e r i c a n p l a t e , J F - J u a n de F u c a p l a t e s y s t e m .  3  allochthonous  t e r r a n e W r a n g e l l i a , which a l s o i n c l u d e s Vancouver  I s l a n d , p a r t s of  southeast  eastern  (Yorath  Figure  Oregon  A l a s k a , and and  C h a s e , 1981;  2 shows t h e g e o g r a p h i c a l  major  terranes  important  in  in  the  the  Wrangellia,  extent  Canadian  following  Alexander terrane, S t i k i n i a , first  and  (1981)  also  (Maude and  Cordillera.  by  The  other terranes  Wrangellia,  Jones et a l .  (1977),  t o Upper T r i a s s i c  sedimentary  include  are  and  rocks.  is a  tholeiitic  Yorath  and  Chase  volcanic  and  sedimentary J u r a s s i c  Y a k o u n F o r m a t i o n s on  the  Queen  Charlotte  rocks  Islands;  B o n a n z a G r o u p on V a n c o u v e r I s l a n d ) as p a r t of W r a n g e l l i a . two  J u r a s s i c F o r m a t i o n s l i e c o n f o r m a b l y on  Lower J u r a s s i c  Karmutsen  Charlotte  Islands  rocks are  known  however,  on  Sicker  Group.  Kunga  (Sutherland to  occur  Vancouver  u n c o n f o r m a b l y on  and  the The  the Late  Formtions  Brown,  1968).  on  the  Queen  Island  the  Karmutsen  Paleozoic exotic  nature  of  the  Wrangell  Wrangellia  Mountains  l a y w i t h i n 15°  of  the p a l e o e q u a t o r  This requires that Wrangellia or  Formation  km  Triassic  Yole  and  the first  Greenstone that  Triassic.  moved n o r t h w a r d e i t h e r 3000  hemisphere s o l u t i o n i s favoured) s i n c e the position.  upon  of  was  the  northern  present  (depending  rests  indicating  a  its  6300  has  in  Queen  Islands;  sediments  1977)  whether  Irving  to  No p r e - T r i a s s i c  Wrangellia  (Hillhouse,  the  Charlotte  calcareous  These  Triassic  on  d e m o n s t r a t e d by p a l e o m a g n e t i c s t u d i e s o f t h e N i c o l a i in  the  Quesnellia.  defined  calcareous  in  Jones et a l . , 1977).  of W r a n g e l l i a  discussion  M e s o z o i c t e r r a n e composed of M i d d l e b a s a l t s and  t h e H e l l s Canyon a r e a  (1980) r e p o r t  or  km  southern  to  occupy  paleopoles  4  AX  Alexander Bridge  River  Cassiar Cache  Creek  Chugach Cascade Kootenay Monashee Olympic Pacific  Rim  Quesnellia Slide  Mountain  Stikinia Wrangellia Yukon-Tanana Coast P l u t o n i c Complex * * may r e p r e s e n t s u t u r e between W r a n g e l l i a and S t i k i n i a .  Figure  2 - Suspect t e r r a n e s of t h e Canadian  (from Monger,  1984).  Cordillera  5  from  the Vancouver I s l a n d  with an  Pacific  that  hemisphere o r i g i n  ( P a c k e r and S t o n e ,  the  present  Formation  that,  plate reconstructions (Hilde et a l . ,  18° s o u t h e r n  results  Karmutsen  Jurassic  location  1977),  for Wrangellia.  1974) f r o m  southern  together favour  Paleomagnetic  Alaska  indicate  o f W r a n g e l l i a was p a r a l l e l  day l o c a t i o n o f s o u t h e r n O r e g o n .  to the  Clearly, Wrangellia i s  an a l l o c h t h o n o u s t e r r a n e . The A l e x a n d e r (~650  t e r r a n e i s an a s s e m b l a g e o f L a t e  Precambrian  Ma) t o L a t e P a l e o z o i c (~250 Ma) s e d i m e n t a r y ,  igneous, and  metamorphic r o c k s metasedimentary of t h e C o a s t  from  Tipper et a l . ,  and m e t a v o l c a n i c  of  the  limestones  about  terrane,  Late  (Berg e t a l . ,  1800  c r a t o n i c North America Triassic Hound  time  Island  (Van  1978).  indicate  no  that  Alexander  the  position  which  significant  by  In the  rocks  overlie  Paleomagnetic  results  Alexander  terrane  was  the Late T r i a s s i c  1980).  overlie  latitudinal in  terrane  displacement  between L a t e C a r b o n i f e r o u s d e r Voo e t a l . ,  the  The Upper  and  Triassic  Paleozoic  i t s present  has  relative to  (~280 Ma)  displacement,  (~230 Ma)  margin  terrane.  Triassic  km o f n o r t h w a r d  volcanics  include  rocks along the western  southeast Alaska i n d i c a t e that the  undergone  (1981)  M o u n t a i n s as p a r t of t h e A l e x a n d e r  southern part Permian  (figure 2).  rocks  suggesting latitudinal  ( H i l l h o u s e and Gromme,  1980). Although terranes is  paleomagnetic  originated  known a b o u t  hypotheses  have  their been  s t u d i e s i n d i c a t e t h a t each of  these  f a r south of t h e i r p r e s e n t p o s i t i o n  little  northward proposed  journeys.  Several  i n the l i t e r a t u r e .  different Y o r a t h and  6  Chase and  (1981) r e c o n s t r u c t  Wrangellia (figure  drifting  northward  the h i s t o r y  3).  i n the Late J u r a s s i c  close  to t h e i r  the  average Lower  age  its  coarse  landmasses  the  Lower  assemblage to  uplift 50  (90 Ma  which  t o 80 Ma  (1972)  and  The  to  has  began  90  t h e Queen C h a r l o t t e North  40 Ma  clasts  The  of  these  conglomerates. uplift  This  - e x p e c t e d when certainly  as .both a r e o v e r l a p p e d Gravina-Nutzotin accreted  i n the L a t e C r e t a c e o u s  i t approached  Plutonic  and  America,  and H u t c h i s o n  (1974) a t  Complex.  favour t h i s of  to  North  Monger e t a l .  hypothesis.  Cordilleran  Monger  tectonics.  t h e west c o a s t of W r a n g e l l i a , and  Islands,  a  date  Jurassic).  s u p e r t e r r a n e then  i n the Coast  review  into  (1981) g i v e s t h e  Jurassic  by R o d d i c k  (1984) a l s o  each Ma),  that  t e r r a n e s were most  been d a t e d  Monger  Ma  and  This  As  (— 140  plutons  contains  Upper  t o 40 Ma).  with  amalgamating  (Late  to rapid two  collided  Young  Ma  c r a t o n sometime  (1984) g i v e s a v e r y c o m p l e t e Between  due  ( B e r g e t a l . , 1972).  mid-Tertiary  1981).  144  t e r r a n e s were  Cretaceous  the L a t e J u r a s s i c  Cretaceous  the N o r t h American  both  terrane  t e r r a n e and W r a n g e l l i a a r e  sandstones  collide.  that  position,  Formation  represents erosion  amalgamated by a t l e a s t by  Chase,  Longarm  Alexander  synorogenic  o f t h e s e p l u t o n s as  in  lithology  of s i m i l a r  the  they  earliest  the Alexander  ( Y o r a t h and  Cretaceous  plutons  two  Both  by a s e r i e s  collision  to  until  present l a t i t u d i n a l  terrane.  intruded  suggest  separately  other  larger  They  of  became t h e a c t i v e  edge of  thus  cratonic  America. Irving  scenario.  et a l . They  report  (1985) new  propose  paleomagnetic  a  somewhat data  from  different the  Coast  >  140 M I L L I O N Y E A R S A G O  WRANGELLIA MOVES PROM THESOUTHERN  SOUTHEAST ALASKA-  NORTHWARD HEMISPHERE  A B O U T 140 M I L L I O N Y E A R S A G O  90 T O 40 M I L L I O N Y E A R S A G O  WRANGELLIA COLLIDES ALEXANDER TERRANE  COMBINED WRANGELLIA AND A L E X A N D E R T E R R A N E DOCK WITH NORTH AMERICA  LV^'.o'.'f,;.'  WITH T H E  I ' N O RTH ;o 'cV-^'^vAM ERICA --  i-^'y--'-'"-' M» ^ 7 - ' x  QUEEN CHARLOTTE ISLANDS VANCOUVER ISLAND  3.  Figure 3 - The a c c r e t i o n o f t h e A l e x a n d e r a n d Wrangellia terranes with North America. See t e x t f o r e x p l a n a t i o n .  (from Y o r a t h  a n d Cameron,  1982).  8  Plutonic  Complex i n d i c a t i n g  that  mid-Cretaceous.  This  s i n c e the consistent  with  a  30°  tilt  i t h a s moved 2400 km paleomagnetic  t o the southwest  C o a s t P l u t o n i c Complex and t h e C a s c a d e s ; translation tilt,  as  of the r o c k s , r a t h e r a  more  paleopoles  from  Wrangellia  on  Schwarz e t a l . , with  probable  in  Vancouver  Island  the  and Q u e s n e l l i a  of  Triassic (Yole  they  rock  and  regional that  units  of 1980,  1976)  agree  Complex r e s u l t s and  Wrangellia,  This the  thus  places  southern  and p e r h a p s  the Coast  Quesnellia  ( I r v i n g et a l . ,  1985).  It  i s suggested, therefore, that these terranes accreted  south  of  their  l a t i t u d e of M e x i c o  regard  Irving,  (Symons, 1973,  P l u t o n i c Complex, the Cascades, S t i k i n i a , a t t h e p r e s e n t day  also  They a l s o n o t e  a r g u e t h a t t h e s e p a l e o p o l e s a r e of t h a t age. position  is  of the s o u t h e r n  however,  explanation.  the mid-Cretaceous Coast P l u t o n i c  mid-Cretaceous  data  than such a c o n s i s t e n t  overprints  1980)  northward  p r e s e n t l o c a t i o n and t h e n c o n t i n u e d n o r t h w a r d a s one  (figure 4). joining  The  Stikinia  C o a s t P l u t o n i c Complex with Wrangellia.  paleomagnetic r e s u l t s America  from  the  represents  Concordance of  Cascades  and  the  unit  suture  mid-Tertiary  cratonic  North  i n d i c a t e t h a t r e l a t i v e m o t i o n b e t w e e n t h e two had  ceased  by t h e n . I r v i n g e t a l . ' s model  indicates  t h a t t h e Queen  I s l a n d s became t h e l e a d i n g edge o f t h e N o r t h A m e r i c a n by  at least  the m i d - T e r t i a r y  S t i k i n i a must h a v e s l i p p e d had been l y i n g o f f s h o r e day  latitude  (—40  Ma).  If this  Charlotte continent  i s correct  i n behind the Alexander terrane  then which  from N o r t h America c l o s e t o i t s p r e s e n t  s i n c e the Late T r i a s s i c ,  as h a s been s u g g e s t e d by  9  Figure 4 - The a c c r e t i o n a r y h i s t o r y o f W r a n g e l l i a , S t i k i n i a and t h e C o a s t P l u t o n i c C o m p l e x . Compare t h i s w i t h t h e s c e n a r i o i n F i g u r e 3. W - W r a n g e l l i a , S - S t i k i n i a and C - C o a s t P l u t o n i c C o m p l e x . See the text for explanation. The two s t i p p l e d bands m a r k e d T N and T S indicate t h e two p o s s i b l e T r i a s s i c origins (northern hemisphere and southern hemisphere respectively) for W r a n g e l l i a and S t i k i n i a before accreting together (from I r v i n g e t a l . , 1985). R  R  10  Monger  and  Subsequent  Irving  (1980)  and  Yorath  and  Chase  (1981).  t o t h i s , W r a n g e l l i a must h a v e been s m e a r e d a l o n g t h e  w e s t e r n edge o f t h e A l e x a n d e r t e r r a n e by a t r a n s c u r r e n t movement to a t t a i n  the  present  situation  sandwiched between W r a n g e l l i a There  these  North  exotic  the  and S t i k i n i a  have been s e v e r a l p r o p o s a l s  boundary separated carried  with  America  and  terranes  Alexander  (figure 2).  a s t o what k i n d o f p l a t e the  oceanic  northward.  a  major  California.  transcurrent  Yorath  and Chase  infer  that both Wrangellia  North  America along  previous  the  a subduction  North  American c r a t o n .  present times  zone.  day  latitudinal  from  Wrangellia  out  the C o r d i l l e r a  along  from (1979)  o f t h e two  ( 1 9 8 4 ) who s u g g e s t s t h a t a the  Late  Cretaceous  to  d i s p l a c e d northwards  p l a t e s , c o l l i d e d w i t h a westward moving Irving et a l . spread  that of i t s p a l e o l a t i t u d i n a l  Perhaps,  displaced  northward  A combination  The m o b i l e t e r r a n e s , b e i n g  various Pacific  that  and t h e A l e x a n d e r t e r r a n e a c c r e t e d t o  i d e a s was p r o p o s e d by Monger  early Tertiary.  running  was  ( 1 9 8 1 ) a n d Monger a n d P r i c e  zone o f ' t r a n s p r e s s i o n ' e x i s t e d  by  fault  plate  Van d e r Voo e t a l .  (1980) h a v e s u g g e s t e d t h a t t h e A l e x a n d e r t e r r a n e along  terrane  of  (1985) Wrangellia  spread  a n d some o f t h e o t h e r  note  that  i s about  i n the Late  the three  Triassic.  t e r r a n e s were smeared  d u r i n g t h i s phase of ' t r a n s p r e s s i o n ' .  11  1.1.2  Post  A c c r e t i o n a r y H i s t o r y of t h e Queen C h a r l o t t e  After the  t h e m i d - C r e t a c e o u s t h e Queen C h a r l o t t e I s l a n d s became  l e a d i n g edge of t h e C a n a d i a n C o r d i l l e r a .  tectonic  regime along  unclear. Pacific  The  plate  chain, provide along  the  motions  (eg.  1976;  1977;  wholly  anomaly 1970).  the  Atwater,  about  the  Ma  is  stripes  on  the  motions  1982a).  Kula  that  west  ( f i g u r e 5a, and  Hayes,  1968;  ridges.  ridge trend  i s constrained only after  ridge  the  and  convergence a l l  questions respect  anomalies  (Atwater, of  the  it  1970).  magnetic  North  America  60 Ma  produced (Is  it  is  top).  is  Magnetic  Bight  Grow and of  paleomagnetic  data  and  the  other  two on  Atwater,  the  Kula-  b e c a u s e most of  the  been consumed  somehow  by  significant  would answer  p o s i t i o n of t h i s  (1976) c o n s t r u c t e d  now  1970)  our  ridge  with  i n d e f i n i n g the  plate  r e g i m e a t t h e Queen C h a r l o t t e I s l a n d s f r o m a b o u t 90 Ma Cooper et a l .  The  (KF)  which  important  North  Kula-Farallon  have  not  anomalies  h a v e been s u b d u c t e d ? ) The to  The  models  of  Atwater,  T h i s anomaly p a t t e r n w e l l d e f i n e s t r e n d s Pacific-Farallon  occurred  A l l of t h e s e  t o e x p l a i n t h e e x i s t e n c e of t h e A l a s k a n  and  with  Cooper et a l . ,  plates  (Grow  and  along  Hawaiian-Emperor  Coney, 1976;  P a c i f i c , F a r a l l o n , and  (Pitman  the  30  system,  from  oceanic  pattern  Pacific  that  d e t a i l s of  S e v e r a l d i f f e r e n t m o d e l s have  1970;  plate  to  plate  plate  Riddihough,  Kula  The  magnetic  e x i s t e n c e of t h r e e  subducted  necessary  Fuca  American c o a s t .  been p u b l i s h e d  America:  of  determined  evidence f o r  Stone,  f r o m 90 Ma  patterns  t h e J u a n de  North  r e q u i r e the  the c o a s t  complex  p l a t e and  absolute  Islands  m o d e l s : one absolute  t o 30  Ma.  based  on  p l a t e motions  r-o  A North American plate, K - Kula plate, F - Farallon plate, P - Pacific plate. 0 r e p r e s e n t s the l a t i t u d e of the Queen Charlotte I s l a n d s , and SF r e p r e s e n t s t h e l a t i t u d e o f San F r a n c i s c o . (a) the Kula-Faral1 on-North America triple junction migrates from the G u l f of A l a s k a southwards a l o n g the c o a s t ( a f t e r R i d d i h o u g h , 1982a). (b) the Ku1a-Fara11 on-North America triple junction m i g r a t e s northwards from s o u t h e r n C a l i f o r n i a ( a f t e r Atwater, 1970).  13  determined models  from  show  the  the  Hawaiian-Emperor  Chain.  Both  K u l a - F a r a l l o n (KF) r i d g e p e r p e n d i c u l a r  t o the  A l e u t i a n t r e n c h a t 80 Ma  (figure  5a).  Kula-Farallon-North  Seamount  Assuming  America  constant  (KFN) t r i p l e  plate  motion,  the  migrated  s o u t h w a r d t o t h e Queen C h a r l o t t e I s l a n d s by 50 Ma t o 30  Ma w i t h o b l i q u e c o n v e r g e n c e o r t r a n s c u r r e n t m o t i o n o f and  North  American  plates  replacing  the boundary.  is  c o n s t r a i n e d but i t appears t o have had a  component o f s t r i k e - s l i p (1981)  infer  a  motion  region.  vector  M o r g a n , 1969; A t w a t e r ,  was  (1970),  The  reconstructed (figure 5b). Farallon  plate  northwards America  motions  (McKenzie and a t 100  ( 1 9 7 0 ) shows  KFN  .mm/y  (1973)  r e l a t i v e p l a t e motions  t o the c o a s t l i n e . the  American  and A t w a t e r and Molnar  junction o f f the coast  t h e KF r i d g e p e r p e n d i c u l a r  Harper e t a l .  e t a l . , 1981).  m o d e l s b a c k t o 80 Ma u s i n g  (KPF) t r i p l e  relative  convergent  1970) a n d h a s been e s t i m a t e d  A t 80 Ma, A t w a t e r  significant  Farallon-North  largely  Coney ( 1 9 7 0 ) ,  vector  o f 120 mm/yr a t N8°E f o r t h e  a t N57°E n e a r -Vancouver I s l a n d ( H a r p e r Atwater  1970).  Kula  Farallon  America motion  (Atwater  relative velocity  Queen C h a r l o t t e I s l a n d s relative  The K u l a - N o r t h  motion  the  the subducting  p l a t e along poorly  junction  triple  the K u l a - P a c i f i c of C a l i f o r n i a Using  with  extrapolated  junction  migrates  t o V a n c o u v e r I s l a n d by 40 Ma w i t h t h e F a r a l l o n - N o r t h  trench  Cooper e t a l .  developing (1976),  behind  Atwater  i t .  (1970),  The  Riddihough  models  of  (1982a),  and  Stone  (1977) i n d i c a t e t h a t s o m e t i m e b e t w e e n 50 Ma a n d 30 Ma  KFN  triple  junction  ( f i g u r e 5a a n d b ) .  lay  North  of  just this  north  of  triple  Vancouver junction,  the  Island oblique  14  convergence American being  or  transcurrent  plates occurred.  subducted  m o t i o n between t h e K u l a and N o r t h  South of i t t h e  beneath  North  America.  p r e s e n t t h e p l a t e movements a r e l e s s  ridge  transcurrent  began motion  (figure  5b).  America  i t remained  absence  As  of  time  be  began  the  subducted along  Farallon  two  Riddihough, All  Plate  of  i n d e p e n d e n t l y a n d t h e J u a n de 1970;  the  20 Ma ( D i c k i n s o n portions  of  10  beneath  North  subducted,  region. triple  Ma  and  20  Ma  by t h e  reached m e l t i n g  Farallon was  1979).  At  began t o move born  (Atwater,  America.  t h e s u b d u c t i o n o f t h e KF  t h e KF r i d g e was t o t a l l y  subducted  When motion  Queen  the  KPF  between the  triple  junction  the P a c i f i c  Queen  was  and N o r t h  Charlotte  Islands  t h e K u l a c o n t i n u e d t o s u b d u c t t o t h e n o r t h t h e KPN  j u n c t i o n migrated northwards Pacific-North  Aleutians (1979)  created  North  Islands.  along  (Riddihough,  the  America boundary  i n c o n t a c t with North America  Byrne  beneath  Snyder,  Ma t h e K u l a p l a t e was t o t a l l y s u b d u c t e d a n d  the  Fault  Charlotte  p l a t e s was i n i t i a t e d i n  transcurrent  was  gross  1982a)..  transcurrent  As  the  Andreas  Fault  plate  r i d g e beneath North America near t h e  American  San  and  these models a l s o p r e d i c t  Between  Ma t o t h e  and  descended  the  Fuca  30  was  beneath C a l i f o r n i a and  s u b d u c t i o n a t t h e San A n d r e a s  the  plate  A t 29-30 Ma t h e F a r a l l o n -  i n one p i e c e u n t i l t h e g a p  temperature a t about this  to  From  uncertain  c h a r a c t e r i s t i c s o f most m o d e l s a g r e e . Pacific  Farallon  and t h e  lengthened. the  from Vancouver  1982a;  coast  Atwater,  Pacific  By 20 plate  Island north to 1970); a l t h o u g h ,  s u g g e s t s t h a t t h e K u l a p l a t e may h a v e c e a s e d t o a c t  15  a s an i n d e p e n d e n t p l a t e a t a b o u t 55 Ma. San  Andreas  plate  transform  continued  to  fault  At the  subduct.  The  (JPN) t r i p l e  southern  America  to i t s present  p o s i t i o n a t Cape M e n d o c i n o  northern  JPN t r i p l e  of Vancouver I s l a n d f o r Table the  I  provides  foregoing  55  the  last  10  Tectonic  the P a c i f i c  (Minster  and  (Riddihough,  Ma  (Minster et a l . ,  indicate  that  i smainly  1978).  Global  1974; M i n s t e r  and  Figure slab  to  be  vector  given  fault  at a rate  consistent  6 i l l u s t r a t e s the probable  at  1978)  o f t h e t r a c e o f t h e Queen C h a r l o t t e  i s necessary  The  p l a t e motion  Jordan,  20  mm/yr  day  strike-slip  10  discrepancy.  Islands  American p l a t e s .  1 ) . A component o f o b l i q u e u n d e r t h r u s t i n g  with  extent  of this  of the  a c o n v e r g e n c e r a t e o f 20 mm/yr f o r t h e  6 Ma. T h e r e i s an i n c r e a s i n g amount o f  and  1977).  c h a r a c t e r i s t i c s of  (figure  past  north  (Riddihough,  the P a c i f i c - N o r t h America r e l a t i v e motion  10° t o 20° e a s t  underthrust  1982a).  f a u l t marks t h e p r e s e n t  and North  Jordan,  analyses  to  Fuca-  northwards  S i t u a t i o n a t t h e Queen C h a r l o t t e  A m e r i c a - P a c i f i c r e l a t i v e motion  trends  de  junction migrated  a comparison of the gross  between  mm/yr  Juan  j u n c t i o n has remained s t a b l e j u s t  The Queen C h a r l o t t e t r a n s f o r m  North  the  models.  1.1.3 P r e s e n t  boundary  time  grew i n l e n g t h a s t h e J u a n de F u c a  Pacific-North  The  same  geophysical  Pacific transform  plate fault.  evidence that supports beneath Figure  North  physiographic, oblique  America along  7 shows a s c h e m a t i c  geologic,  subduction  of t h e  t h e Queen C h a r l o t t e representation  of  Q> l~h  rt fl> i-l  •-3 0)  DATA SOURCE  Atwater(1970) Atwater & Mo1nar(1973) Coney(197G)  Cooper  MODEL  Constant mot i on  Hot s p o t , seamount pa 1 e o m a g n e t 1 c s , r e l a t i v e motion  cr  K O C  relative  e t a 1 .( 197G )  S t o n e ( 1977 )  R1ddihough(1982a)  Constant motion P a c i f i c Hotspot  Re 1 a t i ve mot i on  T r a n s f o r m (N. A m e r i c a -Pac i f i c )  Pac1fic-N Transform  re  3 lQ  AGE  i  (MA) 0  o  10 00  (0 3  to  o  O  N O  o> o >-••  10  1  o I— rr 0) 1  n- ft U) M 0» 3 0>  3 rr fD  .  rr O 3  20  Transform Pacific)  (N. A m e r i c a -  t  Interaction Farallon -Pacific ridgeN. A m e r i c a  n  America  Farallon subduction  A  Q>  w o  30  1  0J  fins' ri)  t  Transform or oblique convergence(N. America -Kula)  40  O C fl> (D 3  50  Transform or oblique c o n v e r g e n c e (N. A m e r i c a -Kula)  1 1  Farallon  Interaction KulaF a r a 1 1 on r 1 d g e N. A m e r i c a  Interaction KulaF a r a l l o n r1dge-N. Amer i c a  Subduction  Subduction plate .  (Farallon)  Transform or oblique convergence Kula-N. Amer i c a  I n t e r a c t i o n Kula F a r a l l o n r i d g e - N. Amer i c a Subduction (Farallon)  17  Figure  6 - P o s s i b l e subducted P a c i f i c North America.  s l a b beneath  The shaded region represents the p o s s i b l e extent of a subducted P a c i f i c p l a t e beneath North American given that oblique subduction h a s o c c u r r e d f o r t h e p a s t 6 Ma a t 20 mm/yr. The l o n g a r r o w r e p r e s e n t s t h e r e l a t i v e m o t i o n o f t h e N o r t h A m e r i c a n a n d P a c i f i c p l a t e s f o r t h e l a s t 6 Ma. Medium l e n g t h a r r o w s p e r p e n d i c u l a r t o t h e Queen C h a r l o t t e fault indicate t h e d i r e c t i o n o f t h e component o f s u b d u c t i o n . The s m a l l e s t a r r o w s i n d i c a t e t r a n s c u r r e n t m o t i o n t a k e n up along the transform faults i n the region (after Yorath and Hyndman, 1 9 8 3 ) .  18  OFFSHORE  Figure  TERRACE ACCRETIONARY SEDIMENTARY WEDGE  '7 - P h y s i o g r a p h i c f e a t u r e s o f t h e Queen Charlotte Islands Region.  This three dimensional drawing shows the observed physiographic features that a r e consistent with subduction b e n e a t h t h e Queen C h a r l o t t e Islands (from Yorath and Hyndman, 1 9 8 3 ) .  19  the  major  Charlotte other  physiographic Islands  features  region.  Such  broad  gentle  Oshawa R i s e , about  associated gravity  t h e Queen  consistent  with  i s present  about  100  km w i d e .  with  the r i s e .  anomaly  Talwani,  with  1974).  A positive free-air The  this  that  The  i s considered  compressive  stress  convergence  of  rises  in  the  the  oceanic  overriding  along  studies.  such  is a  1979;  feature  Watts  feature  a  and  indicate  bathymetric  high,  (Chase e t a l . , 1975). response t o h o r i z o n t a l  plate  caused  continental  and  plate  by  the  (Watts and  ( f i g u r e 7)  i n Hecate S t r a i t  the  Charlotte  studied  Charlotte  uplift  rates  Islands  Uplift  along  h a s been o c c u r r i n g  in  using  on  sea  tidal  levels,  Coast  fission-track  the coast f o r the l a s t  geodetic  several  the  (1982b) a l s o s u g g e s t s a s i m i l a r c o n t e m p o r a r y  based  also  Islands  h a s been d o c u m e n t e d by  Queen C h a r l o t t e  Islands  has  zones of convergence around t h e w o r l d .  d a t i n g o f a p a t i t e s and z i r c o n s .  Riddihough  continent  t h e w e s t e r n m a r g i n on t h e Queen  P a r r i s h (1982)  Mountains  pattern  of  rise  this  the  t o be a f l e x u r a l  i n the o v e r r i d i n g  subsidence  Queen  with  i s a tectonic feature  observed i n various  Uplift  correlation  p r o f i l e s across  and  1974).  Flexure been  close  (Uyeda a n d K a n a m o r i ,  basement  suggesting  Talwani,  C a l l e d the  g r a v i t y anomaly i s  the seaward t o p o g r a p h i c  Seismic  the acoustic  bulge  ( f i g u r e 7 and f i g u r e 8 ) .  i t e x t e n d s t o t h e s o u t h e r n edge o f t h e i s l a n d s  of many s u b d u c t i o n z o n e s  and  are  bulge i n the ocean f l o o r  100 km s e a w a r d o f t h e s h o r e  that  features  with  known s u b d u c t i o n z o n e s a r o u n d t h e w o r l d . A  is  associated  of  the  10 Ma. flexure  r e l e v e l l i n g , and  20  Figure  8 - B a t h y m e t r y o f t h e Queen C h a r l o t t e region.  Islands  T h e Oshawa R i s e r e p r e s e n t s a b r o a d g e n t l e b u l g e i n t h e o c e a n floor. The Queen Charlotte t e r r a c e 1s a 25 km w i d e z o n e i m m e d i a t e l y west o f the islands. (after Chase et a l 1975)  21  Q u a t e r n a r y beach l e v e l s . coast  is  observed  not  the  Charlotte  consistent  with  other  the world  Zealand).  Islands oblique  (eg.  transform  transform  (Riddihough, The  Figure  or  along  9 a n d f i g u r e 10 The p a t t e r n  and  landward  of u p l i f t  the  of w e l l s  (+1  along is  mm/yr)  Fault,  along  the  New  Queen  i s compatible with a p o s i t i o n close to  highly  oblique  convergent  plate  boundary  1982b).  s u b s i d e n c e of Hecate S t r a i t  Pacific  plate.  and Queen  Charlotte  a n d Queen C h a r l o t t e  a t e c t o n i c s u b s i d e n c e h i s t o r y of t h e b a s i n . s u b s i d e n c e began a t 6 Ma a f t e r a p r e v i o u s that  Sound  subduction  Y o r a t h a n d Hyndman ( 1 9 8 3 ) u s e d a s e r i e s  i n Hecate S t r a i t  therefore,  the  subsidence  may a l s o be a r e s u l t o f f l e x u r e c a u s e d by t h e o b l i q u e of  patterns show  Sangami T r o u g h , J a p a n ; A l p i n e  fault  the  zones of compression or s u b d u c t i o n  The v e r t i c a l movement  Charlotte a  1982b).  recovery  the contemporary u p l i f t  b e t w e e n t h e two s t u d i e s .  Queen  around  compatible with  (Riddihough,  agreement  Glacio-isostatic  oblique  underthrusting  corroborated  of  uplift  began a t t h a t t i m e .  This  p l a t e c h a n g e d m o t i o n t o a more c o n v e r g e n t d i r e c t i o n w i t h  respect  t o N o r t h A m e r i c a a t 5 Ma  chain  with  San  geologic  Andreas  small  (Cox  and  i n d i c a t i n g that the  Engebretson,  Fault.  This  along  c h a n g e i n p l a t e m o t i o n a t 5 Ma  plate  b e n e a t h t h e Queen C h a r l o t t e Queen C h a r l o t t e  1985).  bend i n t h e H a w a i i a n - E m p e r o r seamount  e v i d e n c e f o r c o n v e r g e n c e a t 5 Ma  have c a u s e d t h e P a c i f i c  The  study  and  Pacific  a  new  period  that  is  correlated  a  They c o n c l u d e d  timing  They  by  Sound t o c o n s t r u c t  to  begin  subducting  the could  obliquely  Islands.  trough resembles trenchs  i n a r e a s were  22  Figure  9 - T o t a l u p l i f t , w i t h r e s p e c t t o sea s i n c e 10 Ma.  The u p l i f t f i g u r e s i n t h e n o r t h e r n c o a s t m o u n t a i n s maximum v a l u e s ( f r o m P a r r i s h , 1 9 8 2 ) .  level, represent  23  Figure relative  10 - C o n t e m p o r a r y v e r t i c a l l a n d movements t o s e a l e v e l i n mm/yr f r o m t i d a l s t a t i o n s .  (from Riddihough,  1982b).  24  shallow It  subduction  has  been  profiles up  i s known t o o c c u r  documented  55  km  (Chase e t a l . , indicate  wide  and  1975).  seismic  however,  profiles  flat  lying  type  trench.  is  end  also  The t r o u g h  between  Seismic  that the oceanic  reflections,  Chile-Peru  2.8  profiles  basement d i p s abruptly  i s 300 to  3.0  across  the  i n d i c a t e that the trough  sediments which i s  characteristic  trough,  the  gradient  constant  (about 7°).  terrace  only  of  Even  slightly  the  narrow f e a t u r e and  scarps  one a d j a c e n t  shore that drops profiles the  a  The  is filled  with  a  Chile-Peru  is  slope  Queen  Charlotte  characterized  by  two  steep  t o t h e s h o r e a n d a s e c o n d 25 km f r o m t h e  the  sedimentary  abyssal  an  sequence  1971;  C u r r i e e t a l . , 1980). be  i s very  The t e r r a c e i s a  deeps  (figure 8).  ( f i g u r e 11). A g r a v i t y low over the t e r r a c e  thick  Islands to  i s composed o f d e f o r m e d s t r a t a w i t h h i g h  Srivastava et a l . ,  may  wide  trough  terrace.  continental km  deep  Seismic  ( C h a s e e t a l . , 1975; D a v i s a n d Seemann, 1981) show  terrace  folds  to  25  km  The basement  of  interrupts this trend.  relatively -  the  long,  the  landward.  under  seismic  km  From t h e m o u n t a i n s u m m i t s on t h e Queen C h a r l o t t e the  trench).  b o t h on b a t h y m e t r y c h a r t s a n d  ( f i g u r e 8 and f i g u r e 1 1 ) .  to  (eg.  underlies  Srivastava,  1973;  The Queen C h a r l o t t e  accretionary  i t  s e d i m e n t a r y wedge  that  amplitude  suggests  (Couch,  that 1969;  R i d d i h o u g h , 1981;  terrace (Yorath  (figure 7), a n d Hyndman  1983). A previous crossed  the  r e f r a c t i o n survey  Queen  Charlotte  ( H o r n e t a l . , 1984; H o r n , 1 9 8 2 )  fault  zone  at  52.25°N.  The  25  10  20  30  40  KILOMETERS  Figure  11 - R e f l e c t i o n s e i s m i c p r o f i l e a c r o s s Charlotte terrace.  the  Queen  This l i n e d r a w i n g of a r e f l e c t i o n s e i s m i c p r o f i l e shows t h e f l a t l y i n g s e d i m e n t s d e p o s i t e d i n t h e Queen C h a r l o t t e trough created by downwarping of the basement j u s t west of t h e t e r r a c e e d g e . The t e r r a c e a p p e a r s t o be d e f o r m e d s e d i m e n t s . The basement i s n o t v i s i b l e b e n e a t h t h e t e r r a c e ( f r o m Chase et a l . , 1975).  26  interpretation indicates that high  at  shown  layer  10  accretionary  km  wedge  f i g u r e 12.  layer  1984).  This  highly  Islands  region  is  consistent  compressed  adjacent  studies  (Hyndman a n d E l l i s ,  most  of  the  terrace,  to  Moresby  seismic  Berube  indicate the  several  c l u s t e r along shows  Queen  Island.  activity  (1985)  with  (unit  a  3  that  ( u n i t 6) 7)  (Horn  month  a  is  the  Queen  compressive transform  microseismicity  1985)  occurs along scarp  in  indicate  this the  northern  an  earthquake  of the  active  plate  130 e a r t h q u a k e s f r o m a period  in  1983.  f a u l t plane s o l u t i o n s f o r  ( f i g u r e 13).  The c o m p o s i t e s o l u t i o n f o r  p o r t i o n of the f a u l t .  fault adjacent  to  Graham  One c o m p o s i t e s o l u t i o n  on t h e f a u l t a d j a c e n t t o M o r e s b y  shows a t h r u s t m e c h a n i s m .  A n o t h e r s o l u t i o n , f r o m an  swarm i n t h e same l o c a t i o n , i n d i c a t e s v e r t i c a l o c e a n s i d e down.  She  P-nodal  t h e Queen C h a r l o t t e  cluster  that  scarp  that mostly s t r i k e - s l i p motion i s o c c u r r i n g  the  an  below about  Charlotte  Recent  l o c a t e d about  composite  c l u s t e r s of earthquakes  Island  crust  higher extends  sediments  conducted  1981; B e r u b e ,  o f 310 r e c o r d e d o v e r  compiled  the  also  i n d i c a t i n g that the inner  boundary. total  studies  regime i s a c t i v e along  fault  velocity  of t h e experiment.  S e i s m i c i t y and heat f l o w  tectonic  low  The m o d e l i s n o t w e l l c o n s t r a i n e d  10 km b e c a u s e o f t h e d e s i g n  Charlotte  thin  ( u n i t 7 i n f i g u r e 12) t h a t  o v e r l y i n g a sheared and f r a c t u r e d o c e a n i c et a l . ,  interpretation  6 i n f i g u r e 12) o v e r l y i n g a  depth. of  This  i s composed o f a  (unit  lower gradient least  in  the terrace  gradient  velocity to  is  along from Island  earthquake  f a u l t i n g with the  A c l u s t e r o f e v e n t s f r o m i n l a n d Graham  Island,  27  0  Distance (km) 40 60 80  20 N o r t h America Plate  Q . C. Terrace  Pacific  w  Plate  o C\2  O  V.E.:  3.5:1  CO Figure (from Horn  12 - C r u s t a l s t r u c t u r e a c r o s s t h e Queen C h a r l o t t e Transform f a u l t . et a l . ,  1984).  100  28  Figure  13 - L o c a t i o n o f c o m p o s i t e P - n o d a l solutions.  f a u l t plane  The X's enclosed in c i r c l e s represent clusters of e a r t h q u a k e s used t o produce composite f a u l t p l a n e solutions f r o m a m i c r o e a r t h q u a k e s t u d y . The s o l u t i o n s a r e : (1) s t r i k e s l i p m o t i o n p a r a l l e l t o t h e f a u l t z o n e . (2) 1 t h r u s t mechanism s o l u t i o n and 1 v e r t i c a l m o t i o n s o l u t i o n ( e a s t s i d e i s u p l i f t e d ) f o r two c l u s t e r s of e a r t h q u a k e s i n t h e same l o c a t i o n . (3) 2 w e l l c o n s t r a i n e d t h r u s t m e c h a n i s m s o l u t i o n s . (4) t h r u s t m e c h a n i s m s o l u t i o n . ( a f t e r Berube, 1985).  29  t h a t c a n n o t be c o r r e l a t e d w i t h any constrained  thrust  mechanism.  known f a u l t s , A l l  of  a l s o has a  the t h r u s t  well  mechanisms  i n d i c a t e a n o r t h - s o u t h d i r e c t i o n of c o m p r e s s i o n , c o n s i s t e n t oblique  convergence  indicating  studies  i s convergence fault  zone.  first  motion  Pacific  Two  z o n e were  No  earthquakes  identified.  of l a r g e e a r t h q u a k e s i n d i c a t e t h a t t h e r e  earthquakes  fault  to  (1949,  plane solutions.  earthquake  t h r u s t component  is  (Rogers, the  fault  c o m p r e s s i v e s t r e s s was Bostwick  plane  The  suggests  have w e l l motion  Charlotte constrained the  1949  s t r i k e - s l i p with a  small  The  for  horizontal  (about  motion).  not r e l e a s e d  (1984)  1970)  mainly  1983).  P a c i f i c - N o r t h America r e l a t i v e  1983).  plate.  o r c o m p r e s s i v e s t r e s s a c r o s s t h e Queen  8.1  parallel  the  the p r e s e n c e of a B e n i o f f  Previous  magnitude  of  with  15°  motion  is  different  from  This indicates that  in this  earthquake  that the f i r s t  the  (Rogers,  motion  fault  p l a n e mechanism s o l u t i o n s a r e not i n d i c a t i v e  of t h e c h a r a c t e r  motion along the rupture.  difference  the  aftershock  displacement motion  z o n e and  offset  fault  plane show  east.  surface  identified E.E. fault fault.  on SEA  Davis,  a  found a l a r g e  between  the rupture length s u g g e s t i n g that  along solutions  earthquake The  He  thrust  t h e f a u l t was from  the  mechanism  expression  of  uneven. 1970,  p e r s o n a l communication,  magnitude  earthquake  MARK i m a g i n g o f t h e s e a f l o o r 1985).  (G.C. The  indicates a significant  7.0, t o the  has Rogers  newly  s c a r p t r e n d s 10° more n o r t h - s o u t h t h a n t h e Queen This l a r g e earthquake  the  i First  w i t h a d i p o f 50°  this  of  been and  located  Charlotte component  30  of c o n v e r g e n c e Recent  (Rogers,  studies  1983).  indicate  Ma o f s u b d u c t i o n . the  trench  continuous trough,  Low h e a t f l o w e x t e n d i n g  a x i s h a s been o b s e r v e d transition  and  paleo-heat  heat  flow  values  by Hyndman e t a l .  decrease  by a  thermal The  contrast  factor  t h e o r e t i c a l value near  of three  (Hyndman e t a l . , 1 9 8 2 ) .  but i s similar  from west t o e a s t .  Cape  to other  are closer  flow  transform  fault  Hyndman e t a l .  (Sass  2  (1982)  oceanic  The Queen C h a r l o t t e measurement  to the  on  Insular coastal  (Sass  et a l . ,  1985).  would  be e x p e c t e d a b o v e a  than  the characteristic  1985).  indicates subduction  that  Numerical  modelling  by  the observations a r e  and that  boundary cannot s a t i s f y  that the subducting  i s closet o  w i t h i n 50-100 km o f t h e San A n d r e a s  et a l . ,  consistent with oblique ocean/continent  t o what 2  mWirr  terrace.  and Vancouver I s l a n d which has a  z o n e ( 2 5 - 3 5 mWm" )  o f 70-80  The m a i n  Pacific  I t i s also similar  Mendocino  values  areas i n the coast  2  subduction  Ma  only  h e a t f l o w w i t h a mean o f 42 mWm" values  shallow  flow  2  Belt  These  The heat  (47 mWm" ) i s t h e  island  uniform  I s l a n d s was  Charlotte Islands.  the  between  t o low  Charlotte  f o r t h e age o f t h e 7  I s l a n d s heat flow value  zone  terrace,  i n t h e Queen C h a r l o t t e t r o u g h  t h e Queen  A  i n t h e Queen C h a r l o t t e  on t h e  (1982).  zones.  i s l o c a t e d a t t h e s e a w a r d edge o f t h e  average heat flow  crust  flow  o n t h e Queen  observed  50-200 km i n l a n d f r o m  i n many s u b d u c t i o n  from h i g h heat  t o intermediate  continental  heat  the present  i n t h e Queen C h a r l o t t e I s l a n d s r e g i o n a r e c o n s i s t e n t w i t h 6  flow  the  that  a  the data.  s l a b i s a c t i n g as a heat  sink  "steady-state" They  suggest  beneath t h e  s  31  Queen  Charlotte  Islands  southeastern Alaska is  parallel  to  the  the fault  Insular  strike,  The  paleo-heat  has  wells  drilled  also  the  the  Vitrinite past  Pacific  flow of Hecate S t r a i t  been in  studied.  Hecate  past.  Hecate  Hyndman  as f a r  Yorath  Strait  and  Queen  flow i s lower a n d Hyndman  flow  Queen  history.  the  from and  Charlotte  (1983),  the  day  heat  reflectance  Charlotte  reduced  flow  flow.  present  areas  (1983) have l o o k e d a t  vitrinite  Queen  Sound  i n both these  These s t u d i e s i n d i c a t e ,  present  Charlotte  H e a t f l o w m e a s u r e m e n t s made i n  Strait heat  and  i s u s e f u l because i t s r e f l e c t a n c e i s  thermal  of  value  s l a b does not e x t e n d  Queen C h a r l o t t e S o u n d , t h e p a l e o - h e a t that  the average  heat  T h i s may i n d i c a t e t h a t t h e  2  e s t i m a t e s of p a l e o - h e a t from  motion  i n d i c a t e s t h a t t h e mean  (47mWm" ).  indicate that the present in  where t h e p l a t e  2  Belt  A new s t u d y i n  Alaska.  Sound  than  Strait.  ( a t a b o u t 59 mWrrr ) t h a n  n o r t h e r n edge o f a s u b d u c t e d as s o u t h e a s t  Hecate  (Sass e t a l . , 1985),  flow there i s higher for  and  was  Sound  related  data  wells. to i t s  that at least i n probably  twice  A c c o r d i n g t o Y o r a t h and day  heat  flow  is  a  c o n s e q u e n c e o f u n d e r t h r u s t i n g b e g i n n i n g a t 6 Ma. The evidence the  physiographic, presented  large  geological,  i n t h i s chapter  onshore-offshore  conducted  across the f a u l t  Columbia  i n 1983.  zone  Much e v i d e n c e  geophysical  provides  refraction to  the  suggests  c o n v e r g e n c e a l o n g t h e Queen C h a r l o t t e f a u l t  and t e c t o n i c  justification seismic  mainland  for  experiment of  British  that there i s oblique zone, a t l e a s t  M o r e s b y I s l a n d , where t h e d i s c r e p a n c y b e t w e e n t h e  along  Pacific-North  32  A m e r i c a r e l a t i v e motion v e c t o r and t h e t r e n d of t h e f a u l t greatest.  The Queen C h a r l o t t e f a u l t zone i s a c o m p l i c a t e d  supporting compression designed  i s the  b o t h t r a n s c u r r e n t m o t i o n and p r o b a b l y or  underthrusting.  The r e f r a c t i o n  t o a d d r e s s some o f t h e q u e s t i o n s  about t h i s e n i g m a t i c  area.  some  area  form  of  e x p e r i m e n t was  t h a t h a v e been  raised  33  II.  2.1  DATA ACQUISITION AND PROCESSING  Experiment In  the  August  1983, t h e C a n a d i a n  U n i v e r s i t y of  Centre  and  the  British  COCRUST g r o u p  Columbia,  the  Earth Physics Branch  ocean  across  mainland this of  of B r i t i s h Columbia  were t o d e t e r m i n e  t h e Queen C h a r l o t t e and  seismographs  (3)  fault  below  on  14).  The  the lithospheric  zone,  Strait,  Strait.  the  i n Hecate  Queen  Strait  Charlotte  mainland  Charlotte  Eight  Strait.  bottom  line,  a n d two w e s t o f  and i s l a n d s e a s t o f Hecate  each  e x t e n d i n g westward from  land-based more  shot  spacing  along  NITROPEL® was u s e d time  the l i n e  3 km.  f o r a l l of t h e  of d e t o n a t i o n ( o r i g i n  time)  on  T w e l v e 540 km  25 km o f f s h o r e .  540 km c h a r g e , two 60 kg c h a r g e s  four  Moresby  e x p l o s i v e c h a r g e s were d e t o n a t e d , u s i n g t i m e d f u s e s , e v e r y along a l i n e  of  s t r u c t u r e (1)  s t a t i o n s were d e p l o y e d a c r o s s M o r e s b y I s l a n d a n d t h r e e the  t o the  S i x ocean  terrace.  deep  objectives  ( 2 ) b e l o w t h e Queen  Hecate  carried  survey from t h e  (OBS) were d e p l o y e d a l o n g t h e r e f r a c t i o n  e a s t of Moresby I s l a n d Island  (figure  Geoscience  project)  n o r t h e r n Moresby I s l a n d and Hecate  experiment  Islands,  Pacific  forthis  o u t an o f f s h o r e - o n s h o r e s e i s m i c r e f r a c t i o n  ( r e p r e s e n t e d by  10 km  I n between  were d e t o n a t e d m a k i n g  the  The p e l l e t e d TNT e x p l o s i v e  charges. f o r each  To  calculate  the  shot, the output  from  a h y d r o p h o n e t o w e d b e h i n d t h e s h i p a n d a geophone p l a c e d on  the  s h i p d e c k w e r e r e c o r d e d a l o n g w i t h t h e WWVB r a d i o t i m e s i g n a l on an  FM  tape  recorder  and  monitored  on  a chart recorder.  A  vQ C fD  .54°  I  Hecate  a ro w  Strait  AN  3  Island B'anks^\ Island OO  oo  L53°  pi  ro  t-ti  o  7 >»' 10 13.  o  16  3 fD  *  A  Land stations  B  Ocean bottom seismographs  *  Big shots (540 kg)  19  25  TJ  fD  >•  28 31  3  Moresby ^0 Island  3  fD 3  152°  —  /  V *  Shot line  .... Air gun line 0  134°  10 20 30 40 50  133°  132°  Queen Charlotte Sound  131°  130°  co it*  35  d e t a i l e d e x p l a n a t i o n of t h e o r i g i n Appendix A  time c a l c u l a t i o n  i s given  in  A. 32  1 (2000  refraction profiles: Charlottes,  (2)  perpendicular  i n ) a i r g u n was u s e d t o p r o v i d e 3 a d d i t i o n a l 3  ( 1 ) o v e r t h e two OBSs  over  the  OBSs  t o t h e OBSs i n a  minute  at  in  Hecate  west  Hecate  of  the  Strait,  Strait.  The  fired  once  psi).  When t h e a i r g u n was d e p l o y e d t h e s h i p steamed  Queen  and  (3)  airgun  was  a n o m i n a l p r e s s u r e o f 13.8 MPa  g i v i n g a shot s p a c i n g of about  200 m.  (2000  a t 11 km/hr  The e x p e r i m e n t a l  pattern  was d e s i g n e d t o p r o v i d e g o o d c o v e r a g e a c r o s s t h e Queen C h a r l o t t e transform  fault  and  Moresby  Island  while  satisfying  environmental c o n s t r a i n t s which d i d not allow the d e t o n a t i o n charges Moresby  in  Hecate  Strait  or  in  the  of  s h a l l o w w a t e r s west o f  Island.  2.2 I n s t r u m e n t a t i o n The the  s i x OBSs u s e d  Department  of  i n t h i s experiment  were  G e o p h y s i c s and Astronomy  the A t l a n t i c  Geoscience  Substantial  technical  Centre  (Heffler  revisions  p u b l i c a t i o n of the paper;  most  have  notably  constructed  in  a f t e r a d e s i g n from and  been  Barret, made  being  a  1979).  since new  the  release  mechanism  and  t h e u s e o f Benthos® g l a s s s p h e r e s f o r f l o t a t i o n .  The  are  equipped  OBSs  horizontal  with  two  Hz  seismometers  a n d one v e r t i c a l ) a n d a h y d r o p h o n e .  t h e s e c o m p o n e n t s p l u s an i n t e r n a l l y recorded  4.5  in  direct  enable s u f f i c i e n t l y  mode  onto  (one  The o u t p u t  generated clock  signal  a 4 channel cassette tape.  long deployment  t h e tape speed  was  from are To  set at  36  0.2  mm/s.  The i n t e r n a l  Hz c a r r i e r of  frequency.  t h e hydrophone  help identify response  t i m e c o d e i s an a m p l i t u d e m o d u l a t e d  The e n v e l o p e o f t h e h i g h  c h a n n e l was s u p e r i m p o s e d  t h e w a t e r wave a r r i v a l  of t h e whole  system  frequency  part  on t h e t i m e t r a c k t o  accurately.  (including  10  The  frequency  the p l a y b a c k system) i s  band l i m i t e d between 4.5 and 30 Hz ( f i g u r e 1 5 ) . F o u r o f t h e 11 l a n d - b a s e d s t a t i o n s MCR-600  Microcorders  samples  p e r second  corner  at  vertical and  (sps).  Hz,  record The  in  with  a  format  filter,  (15/160  seismometer  w i t h t h e WWVB t i m e c o d e on a f i f t h seismometers  were  remainder of the seismographs instruments  were r e c o r d e d  Branch.  These r e c o r d e r s sampled  between  2  and  25  component seismometers lists  track.  were  Hz.  -  1.0  0.238  (each  at  tracks along  B o t h t h e v e r t i c a l and  Willmore  used  a  The o u t p u t s f r o m one  MK EMR  (Backpacks) d e s i g n e d and b u i l t  Hz  with  ips  two g a i n s e t t i n g s s e p a r a t e d by 18 db) on 4 p a r a l l e l  horizontal  a t 60  b a n d l i m i t s t h e system between  7 - t r a c k G e o t e c h FM a n a l o g r e c o r d e r . -  Geotech  Mark P r o d u c t s L-4C 1 Hz  R e c e i v e r 9 was a s l o w s p e e d  v e r t i c a l a n d one h o r i z o n t a l  Teledyne  digital  anti-aliasing  combined  component s e i s m o m e t e r  9.5 H z .  mm/s)  9.5  which  were  II  models.  Mark  The  II  digital  by t h e E a r t h  Physics  a t 60 s p s a n d were  bandlimited  Two Hz Mark P r o d u c t s L4A v e r t i c a l  were u s e d w i t h t h e B a c k p a c k s .  t h e instrument type f o r each  receiver.  Table  II  37  Figure  15  - Velocity  sensitivity  of  the  OBSs.  38  R e c e i v e r Number  Instrument  1 2 11 12 13 14  OBS  3 4 6 15 16 17  Backpack  9  FM  5 7 8 10  Microcorder  Table  2.3 I n i t i a l 2.3.1  Data  (analog) (digital)  (analog)  I I - Instrument  (digital)  Type  Processing  Digitization A PDP 11/34 was u s e d  and  Type  to  provide  plots  to d i g i t i z e  t h e OBS a n d FM a n a l o g  for quality control.  h a n d l i n g s o f t w a r e was w r i t t e n by t h e a u t h o r . h a n d l i n g p a c k a g e was u s e d and  plot  the  data.  to  The  digitize,  data  Much o f t h e d a t a The PDP 11/34 d a t a  demultiplex,  OBSs, i n p a r t i c u l a r ,  organize,  presented  some  accommodate  long  problems f o r d i g i t i z a t i o n . The  OBSs h a v e a v e r y s l o w t a p e  deployment logistics required  times of  the  and  recording  1983  Queen  on  a  conducted  to  single  Charlotte  t h a t t h e OBSs i n H e c a t e S t r a i t  data over a p e r i o d of 6 days. was  speed  Since the  cassette.  Islands  The  experiment  be c a p a b l e o f r e c o r d i n g refraction  c o n c u r r e n t l y w i t h a UBC s e i s m i c i t y  study  experiment (Berube,  39  1985) the  on t h e Queen C h a r l o t t e I s l a n d s , OBSs  deployed  as  long  i t was d e s i r a b l e  as p o s s i b l e .  speed  o f t h e OBS t a p e r e c o r d e r p l a y i n g  mm/s  (1  in  7/8 i p s ) on a c o n v e n t i o n a l  t h e l a b would  rate  seriously  limit  due t o t h e maximum t h r o u g h p u t  system.  Therefore, the f i e l d  the  factor  between  the  per  14,400 s p s .  second  would  introduce  data.  As w e l l ,  capability  significant  overcome t h e s e problems slaved  to  parallel in  the  in principle, FM  the time code,  corrected  digitization  recorder giving  t o 1/4  a  simultaneously  a  total  and  final  code  at  throughput  tape  120  rate of  recorder  flutter  OBS  speed  i n the recorded  recorder  was  instrument to instrument.  the d i g i t i z i n g time  47.6  rate  f o r t h e OBS d a t a  carrier  frequency,  set To was  recorded  w i t h t h e d a t a t r a c k s , u s i n g a d e v i c e d e s i g n e d and b u i l t  multi-channel  are  Hz  from  t h e Department of Geophysics  similar,  to  10  wow  of each  i n d e p e n d e n t l y and thus v a r i e d  at  of the d i g i t i z i n g  t h a t t h e s l o w OBS t a p e  t h e speed  back  f i e l d a n d l a b t a p e s o f 30 t i m e s .  (sps) giving  I t was f e l t  data  realizable  E a c h o f t h e f o u r c h a n n e l s was d i g i t i z e d samples  Because of t h e slow  r e c o r d i n g s were t r a n s f e r r e d  the  have  h i g h q u a l i t y c a s s e t t e deck  i n c h t a p e u s i n g a h i g h q u a l i t y FM t a p e speed-up  to  to a flutter  tape r e c o r d e r s .  fluctuations  a l i a s i n g , a 2-pole  By s l a v i n g to  device  circuit  one  Hz  the  digitized  data  are  analog f i l t e r  rate well  variations  verify  s a m p l i n g r a t e a r e l e s s t h a n 0.1 s p s . lowpass  is  used i n  the sampling  of tape s t r e t c h and speed  Spot checks of in  The  compensation  s p e e d v a r i a t i o n s o f up  and t h e problems  avoided.  and Astronomy.  that  To a v o i d  ( S a l l e n a n d Key c i r c u i t  c o n f i g u r a t i o n ) w i t h a c o r n e r a t 43 Hz was u s e d  to f i l t e r  t h e OBS  40  data before  digitizing.  The FM a n a l o g d a t a a l s o were d i g i t i z e d a t 120 s p s u s i n g t h e PDP 11/34 s y s t e m .  The d i g i t i z i n g  by an a c c u r a t e f r e q u e n c y  generator.  due t o t a p e s t r e t c h a n d t a p e sps.  The  aliasing is  FM  speed  analog system  filter  r a t e was c o n t r o l l e d Sampling  provided a s u f f i c i e n t  r a t e chosen  3 db down a t 20 Hz w i t h a r o l l o f f  2.3.2  fluctuations  v a r i a t i o n s were l e s s t h a n 0.1  itself  f o r the sampling  rate  externally  anti-  (the response  curve  o f 30 d b / o c t a v e ) .  Time a n d D i s t a n c e C o r r e c t i o n s All  LORAN  s h o t l o c a t i o n s a n d OBS p o s i t i o n s were C  navigation  available.  supplemented  by  satellite  R e l a t i v e accuracy of these p o s i t i o n s  w i t h an a b s o l u t e a c c u r a c y o f 300 m (Hyndman land  determined  receivers  fixes  i s a b o u t 200  et a l . ,  150 m.  T r a v e l time e r r o r s  by l o c a t i o n e r r o r s a r e n e g l i g i b l e c o m p a r e d t o o r i g i n  An  1979).  m  The  introduced time  errors  errors.  important  problem  in  marine  timed  fuse-detonated explosives i s the  shot  depth.  refraction accurate  studies using estimation  of  The t i m e d f u s e s h a v e a b u r n i n g r a t e t h a t i n c r e a s e s  non-linearly accurately  when  were l o c a t e d on 1:50,000 s c a l e t o p o g r a p h i c maps  w i t h an a c c u r a c y o f a b o u t  and p i c k i n g  from  with  depth  estimate  methods were u s e d  the  i n Appendix  A.  times  caused  shot  (Appendix A ) .  depth  therefore  cannot  of d e t o n a t i o n .  t o e s t i m a t e shot depths  detail  by  and  errors  Two  and a r e  The maximum p r o b a b l e e r r o r depth  be  used  to  independent  described i n shot  in  origin  i s a b o u t ± 0.03 s e c o n d s  41  The  internal  corrected  fordrift.  deployment internal  clocks  assumed t o be  of  f o r both  linear  linearity  just  after  in  analog  each  t h e OBSs a n d t h e M i c r o c o r d e r s was ratings.  (1985)  our experiment internal  This  assumption  i s  they a r e h e l d  seawater.  on  A  t h e Queen  concluded.  test  Charlotte  She p e r i o d i c a l l y  c l o c k w i t h WWVB o v e r a p e r i o d o f  t h a t the assumption  of l i n e a r  drift  was v a l i d .  r e c o r d e d WWVB on a t r a c k p a r a l l e l t o t h e s e i s m i c c o r r e c t i o n s were  of t h e time  of  necessary.  Receiver  Time o f F i r s t Sample E r r o r (seconds)  OBS  ± 0.013  Microcorder  ± 0.006  FM  ± 0.006  Analog  Errors  f i r s t samples of the d i g i t i z e d  t r a c e s a r e summarized i n Table I I I .  Table  day of  f o r one o f t h e M i c r o c o r d e r s u s e d i n t h e  a n d t h u s no d r i f t  estimates  were  The M i c r o c o r d e r  and a f t e r  by t h e s u r r o u n d i n g  was done by B e r u b e  12 d a y s and f o u n d  channels  before  between  of d r i f t  rated the Microcorder  FM  recovery.  v a l i d f o r t h e OBSs s i n c e , o n c e d e p l o y e d ,  experiment  The  after  rated  a constant temperature  Islands  and Microcorders  The OBS c l o c k s were r a t e d j u s t p r e v i o u s t o  were  Drift  probably  i n t h e OBSs  and immediately  shooting.  at  clocks  I I I - E r r o r s i n time o f f i r s t  sample  42  The d a t a  from the  Branch  without  ratings. travel first  were  documentation  supplied  of  drift  errors  arrivals.  of  ± 0.05  T a b l e IV  shows  a  Physics  list  or c l o c k  give  total  s f o r good q u a l i t y  of  probable  maximum  f o r d i f f e r e n t p o r t i o n s of t h e d a t a s e t .  1 2 (shots  T r a v e l Time E r r o r (seconds)  1 t o 16)  ±  0.01  3 4 5 6  ±  0.05  7 8 9 10  ± 0. 100  15 16 17 ( s h o t s  1-8)  ' ± 0.300  15 16 17 ( s h o t s  9-33)  ± 0. 100  T a b l e IV - T r a v e l  D a t a Q u a l i t y and land  stations  recorded  very  good  (figure Hz  time e r r o r s  Filtering  The  1.5  Earth  errors,  s t o ± 0.10  Receiver  2.3.3  by  corrections  These e r r o r s , as w e l l as p i c k i n g time  errors  Backpacks  l o c a t e d on t h e Queen C h a r l o t t e  data.  An  exemplary  16) shows t h a t t h e s e i s m i c and  6  Hz  with  r e c e i v e r s on t h e m a i n l a n d  power  recorded  periodogram  signal i s bandlimited  a peak a t a b o u t 3 Hz. good  data  Islands  between  The t h r e e with  the  land same  43  Figure  16 - P e r i o d o g r a m power f o r a l a n d  station.  T h i s power s p e c t r u m was c a l c u l a t e d o v e r a 2 s e c o n d window o f t h e s e i s m i c s i g n a l r e c o r d e d on R e c e i v e r 3.  44  frequency  characteristics  frequency land  of the  receiver  well  n o i s e was  All  first of  good  two  section  in  the  kg c h a r g e s a t f a r o f f s e t s were  not  7,8,9, and  Hz  10 were c l o s e e n o u g h t o  microseisms.  and  was  The  comparison  of  a  distance  B).  o f a b o u t 85  Prominent  the  these  two  i s below 5  land r e c e i v e r s .  level  Filtering  spectra Hz.  There i s  frequencies  o f t h e OBSs.  A  due  a  Past (see  This  km.  background  is  seismic  data  did  Figure  17a  of  seismic A  t h a t most of  the  with  energy recorded amount  of  zero-phase Butterworth  bandpass  were from  applied 0.1-15  to  the  on  the  power  at  Hz  filters  the  provided  enhancement f o r t h i s d a t a  set.  Attempts to f i l t e r  peak a t 7 Hz  first  breaks past  acceptable.  are  noise.  consistent  significant  limtis  degraded the  record  arrivals  of these  indicates  this  to background noise which i s v i s i b l e  Eight pole  frequency 1.  the  recorded  km.  secondary  same f o r a p o r t i o n o f  c h a r a c t e r i s t i c s of the  different  with  Islands  s e c t i o n b e y o n d a d i s t a n c e , of 85  seismic energy frequency  of  filtering.  u n f i l t e r e d data  power p e r i o d o g r a m f o r a 2 s e c o n d i n t e r v a l 17b  the  frequency  e a s y remove w i t h  receiver 1 for a l l shots.  the  receiver  to  Appendix  s i g n a l and  all  of a l l  a characteristic  a r r i v a l s were b e l o w t h e n o i s e  improve the  higher  low  p i c k s were made on  data  first  shows  The  induced  receivers.  OBSs w e s t o f t h e Queen C h a r l o t t e  distance  on  island  sections.  quality  visible  60  b e l o w 1.5  filtered  The  not  The  Receivers  arrival  the  s i g n a l was  to r e c o r d ocean  the  aid  data.  recorded.  water  seismic  as  85  km  out  data the the  on  with from best noise  t o o much t o  be  45  Figure  17 - Power s p e c t r a o f n o i s e and s i g n a l OBS.  f o r an  These periodograms were c o m p u t e d from d a t a r e c o r d e d on R e c e i v e r 7 o v e r a two s e c o n d window o f s e i s m i c signal (a), and background noise (b). The seismic signal i s band l i m i t e d b e t w e e n a b o u t 1 t o 5 Hz. There i s s i g n i f i c a n t n o i s e i n t h e 5 t o 10 Hz r a n g e .  46  The OBSs d e p l o y e d i n H e c a t e S t r a i t d i d n o t r e c o r d any except  for  seismometer data  some  strong  events  c h a n n e l of r e c e i v e r s  quality  in  Hecate  12, 13, and  Strait  environment of the i n s t r u m e n t s . body o f w a t e r w i t h a s o f t that  tidal  seismic  n o i s e and signal and  11  Figure and  12  signal.  The  i s recoverable.  Hecate S t r a i t  sediment bottom  stations  5  and  it  and  Hz  was  Hz  illustrated 'noise'  identical  is  is significant  e n e r g y between  1.5  spectrum that  i s a b s e n t on t h e  observed  was  seismic  OBSs.  successful  4.5  Island.  Hz  (figure  and c o n t a i n l i t t l e  8-pole  of  1  12, 13,  and  14  recovered.  The  OBS  15), m a i n l y because  Thus e n e r g y  i s attenuated relative  'signal'  An  on  i n r e c o v e r i n g the strong events,  used.  significant  signals  A v e r y narrow bandpass  s e i s m i c e n e r g y was  below  'noise'  used t o attempt t o r e c o v e r the  i n the frequency  OBSs.  This  on a l l t h e o t h e r OBSs and  spectra  11  (figure  18 c  for receiver  energy below  of  to the higher frequency  Strait  by t h e s p e c t r a o f r e c e i v e r and  background  There  n o i s e r e c o r d e d on a l l o f t h e H e c a t e that  from  the  seismometers  range of i n t e r e s t  The  believed  12 i n d i c a t e t h a t  t h e OBSs west o f M o r e s b y  r e s p o n s e i s n o t good  problem  is  for receiver  b u t no o t h e r r e c o g n i z a b l e  4.5  poor  i s a very shallow  c o r r e s p o n d i n g t o s h o t s 1 t h r o u g h 4, on r e c e i v e r  the  very  spectra  on t h e H e c a t e S t r a i t  to  The  o v e r 2 s e c o n d windows o f b o t h  zero-phase B u t t e r w o r t h f i l t e r  Hz  14.  vertical  i s t h o u g h t t o be c a u s e d by t h e  s p e c t r u m , a band c o n s i s t e n t w i t h  signal  the  18 shows s a m p l e power p e r i o d o g r a m s  5 Hz on t h e ' s i g n a l '  land  on  c u r r e n t s and p e r h a p s s h i p i n d u c e d n o i s e o b s c u r e d t h e  signal.  receivers  observed  data  5 Hz.  is  a  i s well and  d).  11 a r e a l m o s t  Both  have  the  47 1.0  noise  0.5  10  15 20 FREQUENCY (HZ)  25  30  25  30  0.0  1 10  15 20 FREQUENCY (HZ)  10  15 FREQUENCY (HZ)  1.0  c signal  0.5  0.0  10  {  15  20  FREQUENCY (HZ)  Figure  18 - Power s p e c t r a s i g n a l and S t r a i t OBSs.  noise  f o r Hecate  window of T h e s e p e r l o d o g r a m s were computed o v e r a 2 s e c o n d data . ( a ) R e c e i v e r 12 - s e i s m i c s i g n a l ( b ) R e c e i v e r 12 - b a c k g r o u n d n o i s e ( c ) R e c e i v e r 11 - s e i s m i c s i g n a l ( d ) R e c e i v e r 11 - b a c k g r o u n d n o i s e r e c o r d e d o n r e c e i v e r 12 i s b a n d l i m i t e d selmlc signal The There i s s i g n i f i c a n t n o i s e above 5 b e t w e e n 1 a n d 5 Hz ( a ) filter of 1-5 Hz was n e c e s s a r y t o A bandpass Hz (b) r e c o v e r any r e c o g n i z a b l e s e i s m i c s i g n a l . F i l t e r i n g t h e data from r e c e i v e r 11 d i d not improve the quality. The similarity of c and d i n d i c a t e s t h a t any s e i m i c s i g n a l p r e s e n t i s c o m p l e t e l y o b s c u r r e d by background noise.  25  30  48  same and  two  peaks,  10 H z .  frequency  one b e t w e e n 15 a n d 20 Hz a n d a n o t h e r  The same f i l t e r  a s a b o v e was  l i m i t s b u t no o b s e r v a b l e  applied  s e i s m i c energy  between 5  with  varying  was r e c o v e r e d .  2.4 Summary Except  f o r t h e 4 OBSs d e p l o y e d  i n Hecate S t r a i t  a l l of t h e  r e c e i v e r s y i e l d e d good q u a l i t y d a t a .  The l a n d s t a t i o n s  deployed  on M o r e s b y I s l a n d  data  mainland  recorded excellent  were  sources.  The two OBSs w e s t o f M o r e s b y I s l a n d a l s o r e c o r d e d  of  the  Some s e i s m i c s i g n a l was r e c o v e r e d  OBSs full  because  the  stations  data.  noisier  while  by  filtering  far offset  from  the Hecate  from t h e  Strait  t h e data u s i n g a v e r y narrow bandpass.  data s e t i s described the  i n an  explosion  open  1984).  A l l of  data  receiver  r e c o r d s e c t i o n s i n A p p e n d i x B.  file are  report  good  The  (Clowes,  p r e s e n t e d a s common  49  III.  3.1  M o d e l l i n g and  INTERPRETATION  Uniqueness  A forward modelling process dimensional Although using of  synthetic  was  seismograms  used  to  that  matched  other  models  technique  that w i l l  r e f r a c t i o n problem i s  t h e r e e x i s t s an  also satisfy  very  non-unique  c o n s t r a i n t s o f t e n outnumber the p a r a m e t e r s . stems  from  model.  For  exist  a l l  foremost,  of  concrete  data  the n o n - u n i q u e n e s s of the  The data  geologically  seismic  from  s h o t s and  process.  receivers i t i s  therefore, First  used  constrained  seismic  refraction profile  refracting refracting  design  to  to  in  is  a  technique.  i n t e r f a c e and interface  choosing  parameters  the common  and  Obviously, to  further  of  determined  i n the  t h e optimum a r r a y of  reduce  the  model.  the  number  The  example  I t a l l o w s both  the v e l o c i t y be  to  problem.  possible  poorly  experimental  By  the  gradient for that  p r o b l e m of n o n - u n i q u e n e s s must a l s o be a d d r e s s e d  acquisition  data  p a r a m e t e r s i n the  reasonable. be  number  the  can m u s t e r .  must  data  non-uniqueness  I t i s necessary,  t h e c o n s t r a i n t s t h a t one  other a v a i l a b l e  restrict  This  i n a l a y e r the v e l o c i t y  t h e model must be  The  t u r n i n g rays or r e f l e c t i o n s  i s not w e l l c o n s t r a i n e d .  employ  the  though  l a c k o f c o n s t r a i n t s on a few  e x a m p l e , i f no  boundary  layer  any  the  data.  infinite  the d a t a . even  two-  the  i t i s p o s s i b l e t o c o n s t r u c t a model t h a t f i t s  such a m o d e l l i n g  lower  calculate  of  of  reversed such  an  t h e d i p of  the  layer  uniquely.  below  the  B e c a u s e of  50  environmental 1983 An  r e g u l a t i o n s i t was n o t  refraction  attempt  at  receivers.  reverse  increasing  offset  from  the  (figure 14).  linear  reverses the l i n e  'pseudo-reversal'  i sexploited  over  by  i n b o t h common s h o t a n d common r e c e i v e r g a t h e r s  the  same  u s u a l sense; shot  of  the earth.  however, i t i s necessary  that  both  the  sample  i s not reversed  to satisfy  model f i t s The  the data  sufficiently  forward  complicated  data  to  alter  fit  the data.  modelling  i n the  t h e common  common  well. technique,  T h i s lack of c o n s i s t e n c y  applied  to  It is  may  affect  The method i s i m p r a c t i c a l  such  a  impossible  the  final  f o r large data  P e r h a p s a more s a t i s f y i n g a p p r o a c h w o u l d be t o c o n s t r u c t  ensure t h a t t h e f i n a l model f i t s consistent  manner.  seismic refraction  inversion technique. the  dimensional  travel  set similar  experiment  total  data  This  set  would  i n some  The t w o - d i m e n s i o n a l  inverse problem a p p l i e d  i s , f o r the reason  s t a t e d p r e v i o u s l y , very  non-unique and t h e r e f o r e u n s t a b l e .  found  a  Each  e a c h m o d e l i n a c o n s i s t e n t manner w h i l e a t t e m p t i n g t o  a model u s i n g a t w o - d i m e n s i o n a l  data  separately u n t i l  s e t , h a s two m a i n d r a w b a c k s .  m o d e l i n unknown w a y s . sets.  i s modelled  of  some o f i t s  a n d common r e c e i v e r p r o f i l e s w i t h an i d e n t i c a l m o d e l .  record section or p r o f i l e  to  The l i n e  array  modelling  data  area  the  t h i s p r o b l e m was made by d e t o n a t i n g many  This, i n effect,  This  to  by s h o o t i n g i n H e c a t e S t r a i t  to alleviate  explosions  length.  line  possible  time  i n design  Spence  inversion  (1984)  techniques  t o t h e 1983 Queen  ( m u l t i p l e shots recorded  applied  to a refraction  Charlotte  Islands,  on m u l t i p l e r e c e i v e r s ) .  t h a t because t h e problem i s unstable  two-  i t c a n be  used  He only  51  to  'fine  forward  tune'  modelling  dimensional this  a  model  t h a t h a s l a r g e l y been c o n s t r u c t e d  methods.  travel  Because of  time  constraints,  by  two-  t i m e i n v e r s i o n was n o t a t t e m p t e d a s p a r t of  thesis. It  i s a l s o important  objectively This  determine  is particularly  uniformly  such  to  how  note  in  model  this  in  unknown  ways.  inversion procedure i t i s which  features  of  the  is  thesis.  c h a r a c t e r i s t i c s of t h e f i n a l model a r e model  it  In  this by  the  difficult  objectively  model  are  non-  case  modelling  to  to  f i t the data.  I n any f o r w a r d  or  the  initial iterative determine  r e q u i r e d by t h e d a t a . Noting  The  these  concerns  u s i n g a r a y method  synthetic  inhomogeneous m e d i a  (Spence,  p r o c e e d i n a c a r e f u l manner.  3.2 D e s c r i p t i o n of t h e M o d e l l i n g All  models  were  seismogram a l g o r i t h m 1984;  difficult  parameterized  biased  g o o d n e s s o f f i t t h u s becomes s u b j e c t i v e . we w i l l  is  w e l l t h e model s h o u l d  true i f the  as  that  constructed  for laterally  Spence e t a l . ,  1984).  calculated using a modified (1979)  ray  Algorithm  tracing  Ray  paths  and t r a v e l  v e r s i o n of the W h i t t a l l  program;  amplitudes  are  and  ray theory.  of  b l o c k s w i t h t h e v e l o c i t y and l i n e a r v e l o c i t y  d e f i n e d a t t h e t o p of each b l o c k .  as  are  Clowes  computed  asymptotic large  The model i s p a r a m e t e r i z e d  times  a  using series  gradient  The r a y p a t h s a r e t h e n  traced  a s a r c s o f c i r c l e s w i t h a r a d i u s d e f i n e d by  the  value  gradient  (see  Law  determines the  behaviour  of rays c r o s s i n g boundaries.  Gebrande,  1976).  Snell's  Because t h i s  is  of  a  the  ray  52  method, a l l t h e l i m i t a t i o n s of r a y t h e o r y algorithm,  except  that  apply  t o the modelling  " p s e u d o " h e a d waves c a n be g e n e r a t e d a s  described  by W h i t t a l l  and  Clowes  (1979)  and  Spence  (1984).  Thus t u r n i n g r a y s , p r e - a n d p o s t - c r i t i c a l  reflections,  m u l t i p l e r e f l e c t i o n s a n d h e a d waves c a n be i n v o k e d The  algorithm  i s f a s t and e f f i c i e n t  and  economically  et a l .  i f  a l l o w i n g the user  desired. to easily  t e s t many v a r i a t i o n s o f a m o d e l .  3.3 I n t e r p r e t a t i o n o f I n d i v i d u a l P r o f i l e s 3.3.1  The F i n a l M o d e l - A P r e v i e w The  final  model ( f i g u r e 19) was c o n s t r u c t e d  common r e c e i v e r a n d 2 common s h o t p r o f i l e s . of  the  final  velocity  interpretations feature  of  oceanic  of  the  model  crust to  Islands  and  each  profile i s the  thicker  Hecate  crust  will in  Strait.  The  structure  of t h e lower oceanic  crust  perspective.  modelling  depth.  this  Beneath  t o the east  may r e p r e s e n t  procedure.  Queen  the main  the  model,  i s very  The o c e a n i c  Charlotte  Charlotte  terrace  the  similar of  velocity  t o t h a t of the  fault  l i m i t a t i o n s of t h e data s e t Moho i s a t  the t e r r a c e and f u r t h e r e a s t ,  reaching  The  Queen  lower t e r r a c e region and t h e lower c r u s t east although  placing  the  In  5°  aid in  beneath  regions.  and  A short d e s c r i p t i o n  change f r o m a t y p i c a l  these  zone,  5  lateral  separates  the  two  model  by m o d e l l i n g  a d e p t h o f 35 km  about  10  km  t h e Moho d i p s a t  beneath  the  mainland  coast. The constrained  shallow  oceanic  sediment  layer  i n d e p t h by a c o n t i n u o u s s e i s m i c  (1.8  reflection  km/s) i s profile  53  o  0  Figure  S  01  SI  19 - The f i n a l  02  S2  velocity  0C  SG  model.  S o l i d l i n e s i n d i c a t e b o u n d a r i e s t h a t a r e w e l l c o n s t r a i n e d by the data. Dashed lines i n d i c a t e boundaries that a r e not well constrained. The f i r s t number i n e a c h block i s the velocity ( i n km/s) a t t h e t o p b o u n d a r y o f t h e b l o c k . The s e c o n d number i s t h e v e l o c i t y g r a d i e n t ( i n km/s/km). The model distance i s p l o t t e d across the t o p of the f i g u r e . P o s i t i o n s of shots and r e c e i v e r s used in this study are superimposed on t h e m o d e l . QCI = Queen C h a r l o t t e I s l a n d s . HS = H e c a t e S t r a i t .  54  collinear (Davis  with  is  not  structure  well  structure  survey  in  The  our  of  layering by  our r e f r a c t i o n l i n e below  our data  refraction profile  1982) p a r a l l e l  of our  section  constrained  from a s e i s m i c  Horn,  south  for  offshore  a n d Seemann, 1 9 8 1 ) .  zone  and  the  was  this  s e t . The  model.  basis  fault  .for choosing  results  from  the  the  The l a c k o f any n e a r s u r f a c e  design  1984  zone and  t h e Queen C h a r l o t t e I s l a n d s e a s t w a r d s i n t h e f i n a l  model  oceanic  (Horn e t a l . ,  t o t h e Queen C h a r l o t t e the  sediment  of t h e e x p e r i m e n t .  ocean details  velocity More  shots  n e a r t h e i s l a n d and m a i n l a n d s t a t i o n s w o u l d be n e e d e d t o p r o v i d e the  constraints to construct  region.  The  Forsyth  et a l .  approximate  refraction (1974)  east  models  provided  velocity  mainland region  an u p p e r  structure  in this  the  e n t i r e data research  some and  purposes  to  Queen C h a r l o t t e chosen  study  to  as  to  mantle  the  i n the  Neither  a detailed velocity  of  model  of  for Modelling this  t h e s i s , only  interpreted.  The  t h e deep c r u s t a l  The c r i t e r i a  a p o r t i o n of the  objective  of  this  s t r u c t u r e beneath the  used i n choosing  set  was  the p r o f i l e s  were:  (1) t h e d a t a must s a m p l e t h e p a r t be  depth  (1972) a n d  I s l a n d s and Hecate S t r a i t and t h e d a t a  accordingly.  t o be m o d e l l e d  constraints  this  area.  s e t was f u l l y  was  for  o f t h e Queen C h a r l o t t e I s l a n d s .  3.3.2 The C h o i c e o f D a t a S e t s For  model  of Johnson e t a l .  t h e s e s u r v e y s was a d e q u a t e t o p r o v i d e of t h e c r u s t  crustal  studied,  of the e a r t h t h a t  i s to  55  (2) of  the  d a t a must t a k e a d v a n t a g e o f t h e ' p s e u d o - r e v e r s a l '  p r o f i l e s , and  (3) t h e d a t a must be o f s u f f i c i e n t After  to  interpret.  p r e l i m i n a r y m o d e l l i n g o f e v e r y common r e c e i v e r p r o f i l e i t  became a p p a r e n t receiver and  quality  which  profiles  Common  1, 3, 15, 16, 17, a n d common s h o t p r o f i l e s  16 were c h o s e n  distances  o n e s b e s t met t h e a b o v e c r i t e r i a .  for modelling.  referred  to  in  Except  this  chapter  where  4,  noted, a l l  are shot-receiver  d i s t a n c e s a n d , where a p p l i c a b l e , a r e f o l l o w e d by m o d e l d i s t a n c e s in  parentheses.  profile 3.3.3  of  the  interpretation  (figure  1, an OBS, i s t h e most w e s t w a r d r e c e i v e r a l o n g t h e 14).  It  was  i n 900 m o f w a t e r .  receiver  distance  deployed  on  quality  to  a  distance  of  (by  shot-  (figure  The  data  20).  The f i r s t  a r e of the f i r s t  arrivals  to  49  e r r o r s l e s s t h a n ±0.05 s; a r r i v a l s p a s t 49 km a r e more  ±0.1 s.  the  d i s t a n c e ) and f o r shot amplitudes  time e r r o r s a r e i n the  The d a t a h a v e been c o r r e c t e d f o r s p h e r i c a l  multiplying  the shot  correlate  80 km b e y o n d w h i c h  u n c e r t a i n and t h e e s t i m a t e d t r a v e l of  Queen C h a r l o t t e  t o m o d e l d i s t a n c e by v i e w i n g t h e a p p r o p r i a t e  a r r i v a l s a r e not v i s i b l e have  the  I t i spossible to  f i g u r e d i s p l a y i n g both data and s y n t h e t i c s .  km  each  Common R e c e i v e r P r o f i l e 1  terrace,  good  of  follows.  Receiver line  A discussion  are  data size  proportional  by  r by  2  where r i s t h e s h o t  assuming  that  the  order  spreading receiver recorded  to W/,  where W i s t h e w e i g h t o f  i n k i l o g r a m s ( O ' B r i e n , 1960).  This d i d not c o r r e c t the  2  3  56  Figure  20 - C o m p a r i s o n o f t h e d a t a P r o f i l e 1.  and s y n t h e t i c s f o r  The d a t a f o r common r e c e i v e r p r o f i l e 1 ( a ) i s c o m p a r e d with t h e s y n t h e t i c s e i s m o g r a m (b) c o m p u t e d u s i n g t h e f i n a l m o d e l . Arrowheads on (a) denote first arrival p i c k s of m a n t l e refracted rays. The s h o t - r e c e i v e r d i s t a n c e i s p l o t t e d a l o n g t h e t o p o f ( a ) . The m o d e l d i s t a n c e i s p l o t t e d along the b o t t o m o f ( b ) and b e t w e e n ( a ) and ( b ) . The d a t a a r e p l o t t e d w i t h a r e d u c i n g v e l o c i t y s u c h t h a t a r r i v a l s w i t h an a p p a r e n t v e l o c i t y o f 8 km/s a p p e a r h o r i z o n t a l .  135  125  115  S h o t - R e c e i v e r D i s t a n c e (km) 105 95 85 75 65 55 45 35  25  15  -5 0  5  10  15 20 25 30 35 40 45 50 55 80 65 70 75 80 85 90 95 100 105 110 1]  -5 0  5  10  15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90  Model  Distance  (km)  5  95 100 105 110 115 120  58  close shots f o r receiver  1 w e l l a s i s i n d i c a t e d by t h e  v a r i a t i o n over the f i r s t  six traces  kilogram  break  from  o c c u r s a t 25 km, mantle.  The  result  of  profile  crustal  p r o v i d i n g an  low a p p a r e n t  out  f r o m 40 km These  velocity by  arrivals  The then  85 km.  t o 125  corresponds  and  estimate  of  the  The  km have an  The  to  m o d e l l i n g procedure modelling  the  of  topography  refractions  have  b u t t h e y a r e v e r y weak  velocity  of  arrivals  6.8  km/s. which  point. entailed  fitting  the  travel  times  the a m p l i t u d e c h a r a c t e r i s t i c s of the d a t a . to  degraded  the  travel  time f i t .  were t h e n m o d e l l e d  f i t amplitude  in  characteristics  Both t r a v e l  conjunction  t i m e s and  until  a  often  amplitude  satisfactory  resulted. The  data  from  receiver  1 s a m p l e s t h e o c e a n i c and  r e g i o n s w e s t o f t h e Queen C h a r l o t t e tracing  synthetic in  the e f f e c t  Pn m a n t l e  apparent  model  ray  depth  merge w i t h t h e Pn a r r i v a l s a t a b o u t 40 km  to the c r i t i c a l  The  refractions  h i g h amplitude secondary  the  model  540  of the c r u s t a l a r r i v a l s a r e a  o v e r 8 km/s  Altering  fits  is a  important.  mantle  velocity  slightly  very to  s l o w e r c r u s t a l m a t e r i a l and  an a p p a r e n t die  are  arrivals  o f t h e Queen C h a r l o t t e t e r r a c e .  and  (every fourth trace  shot).  Some f e a t u r e s o f t h i s sharp  amplitude  through  this  Islands.  portion  seismogram f o r t h i s p r o f i l e  f i g u r e 20.  The  are traced through terrace region.  rays a r r i v i n g the  low  A low v e l o c i t y  F i g u r e 21  and  shows  of  the  f i n a l model.  and  the data are presented  b e t w e e n 8 ( 1 1 7 ) km a n d  velocity  terrace  -  high  The  25(99)  gradient  km  upper  h i g h g r a d i e n t were n e c e s s a r y  MODEL  DISTANCE  (km)  F i g u r e 21 - F i n a l r a y t r a c i n g d i a g r a m Common R e c e i v e r P r o f i l e 1. (see  text for  explanation).  for  60  to  f i t both  the travel  these a r r i v a l s . km  to  25(99)  The s y n t h e t i c a m p l i t u d e s i n c r e a s e km  where  a r r i v a l s a t 3 0 ( 9 5 ) km. velocity upper  time and a m p l i t u d e c h a r a c t e r i s t i c s f o r  controlled  they  d i p of  constrained.  the  may  arrivals.  The s t r o n g  125(0)  In other words,  Moho  in  I t may i n d e e d be h o r i z o n t a l  structure  account  f o r the secondary  and  apparent  (figure 20).  These  arrivals  from  Variations  more  the  i n t h e upper  example,  km/s w o u l d  average  depth  arrivals also constrain  good  velocity  t o the  Moho which  t o t h e Moho.  2  km  shallower.  the v e l o c i t y  gradient i n  l o w e r o c e a n i c c r u s t t o be v e r y l o w (0.015 km/s/km) i n o r d e r energy  from t h e f a r t h e s t  t h i c k n e s s o f t h e 1.8 sediments,  final  from t h e  oceanic layers,  f o r c e t h e o c e a n i c Moho t o be a b o u t  to propagate  area  to  a change i n v e l o c i t y o f t h e 3.7 km/s l a y e r t o 4.2  These secondary the  km  f i t is  are not w e l l c o n s t r a i n e d , c o u l d i n f l u e n c e the depth For  these  30(90)  amplitude  constrain  2°  shallow of  reflections  a b o v e t h e Moho and. t h u s a l l o w t h e a p p r o x i m a t e t o be d e t e r m i n e d .  a  velocity  arrivals  time and  the  t h e model i s not w e l l  km a r e i n t e r p r e t e d a s p o s t - c r i t i c a l  Moho ( f i g u r e 2 1 ) . The t r a v e l  apparent  v e l o c i t y , a n d t h e d i p on an  the shots.  oceanic  8(117)  secondary  r e f r a c t i o n s h a v e an  the mantle  o c e a n i c l a y e r beneath  eastward  merge w i t h t h e s t r o n g  The m a n t l e by  from  km/s  layer,  representing  model i s v e r y s i m i l a r terrace  lower b l o c k .  region  The o c e a n i c  structure  t o t h a t o f Horn e t a l . was d i v i d e d  The  unconsolidated  i s c o n s t r a i n e d by c o n t i n u o u s s e i s m i c p r o f i l e s  ( D a v i s a n d Seemann, 1 9 8 1 ) .  The  shot t o t h e r e c e i v e r .  i n t o an u p p e r  i n the of t h e  (1984). b l o c k and a  The p o s i t i o n o f t h e b o u n d a r y s e p a r a t i n g t h e  upper  61  and  lower  terrace  regions  i s not  block  must h a v e a much s m a l l e r  order  f o r the  velocity  s t r u c t u r e of  zone  is  Charlotte it  the  Islands  exhibited  prominent  s.  receiver  1  is  terrace  most  are  (see  The  km  travelled 33(115) block  ( a t 33  to  to with  and  The  strong data  any  first 1.  the  significant the  of  crustal  t h a n 8 km/s.  chosen  is  the  at  far  B).  correspond to energy that  velocity  of the  increase  with  data i l l u s t r a t e  at  velocity  and  First  the  over  large amplitude a r r i v a l s  gradient.  the  similar in  break  its  km)  of  suggests  an " a p p a r e n t  region.  on  crust across  arrivals  with  less  offset  This  r e c e i v e r 3 are  arrivals upper  from has from  terrace  Consequently  offset.  Traces  this effect.  so  excellent  The  terrace  l a r g e and  Queen  because  r e c e i v e r s east  have t r a v e l l e d t h r o u g h the  high  the  secondary a r r i v a l s  change i n the  refractions  fault  time u n c e r t a i n t i e s are  a r r i v a l s on The  was  on  quality  Appendix  lateral  Charlotte  4 5 ( 1 0 7 ) km  are 42  Pn  are  through  amplitudes  in  receiver.  station  profile  travel  zone  significant  km  The  fault  about  t o 68  the  on  to receiver  km  Queen  land  This  evident  greater  the  secondary a r r i v a l s v i s i b l e  character  45  of  not  region.  slightly  1.  Estimated  The  a  45  block  i s s u s p i c i o u s l y s i m i l a r to  westward  receiver  Queen C h a r l o t t e there  j u s t east  ( f i g u r e 14).  a).  t h a n 0.05  top  lower  3  a marked l a c k of  on  ( f i g u r e 22  t h i s block  Profile  3 i s located  and  than the  The  layer.  Common R e c e i v e r  Receiver  gradient  wide-angle r e f l e c t i o n s to reach the  lower oceanic 3.3.4  well constrained.  1 and  Traces  the 4 2  155  145  135  S hot- Rec e i v er Distance (km) 125 115 105 95 85 75 65 55  45  35  -5 0  5  10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120  -5 0  5  10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100105 110 115 120125  MODEL  DISTANCE  (km)  F i g u r e 22 - Comparison of d a t a and s y n t h e t i c s f o r Common R e c e i v e r P r o f i l e 3 . (a) d a t a . (b) s y n t h e t i c s p r o d u c e d f r o m f i n a l r a y t r a c i n g d i a g r a m . The t r a v e l t i m e c u r v e s f r o m t h e s y n t h e t i c a r r i v a l s a r e drawn over the data section. See F i g u r e 2 0 f o r an e x p l a n a t i o n o f d i s t a n c e and time axes.  63  and  3  were  f r o m 60 kg c h a r g e s , a n d a s n o t e d p r e v i o u s l y ,  i t is  b e l i e v e d t h a t t h e s t a n d a r d c h a r g e - s i z e c o r r e c t i o n has not worked w e l l a t near o f f s e t d i s t a n c e s . arrivals  45(105)  68(78)  km.  i s about  T h i s phase  change  (figure  in  23).  The  58(90)  km  the upper m a n t l e . amplitudes  6.8 km/s.  They c o n t i n u e t o  I t ends a b r u p t l y because  synthetic  seismograms  (figure 22).  t o 148(0)  mimic  rays turning not  substantial distance 3.3.5  energy  f o r t h i s phase.  travel  the  in  data  the  coda  and  A secondary a r r i v a l  from  Common R e c e i v e r P r o f i l e  (figure  although  phase  there  is  observed t r a c e s over  this  15  is  the  15  most  westward  chosen  because  they  were  ( t h e r e f o r e r a y p a t h s t o them s a m p l e d  Appendix  i s due t o  mainland  station  14). T h i s r e c o r d s e c t i o n , and those of r e c e i v e r s  were  because  section  Such a p r o m i n e n t  section of  through  range.  Receiver  17,  on  these  times  i n the lower oceanic c r u s t .  observed  scarp  well  km a r e due t o r a y s r e f r a c t e d  The o b s e r v e d a n d s y n t h e t i c  agree w e l l  of  F i r s t a r r i v a l s extending  80 t o 112 km (model d i s t a n c e ) on t h e s y n t h e t i c  is  these  d i p o f t h e Moho b e n e a t h t h e o u t e r t e r r a c e  amplitude c h a r a c t e r i s t i c s from  distance  h a s been i n t e r p r e t e d a s r e f l e c t i o n s  f r o m t h e Moho b e n e a t h t h e t e r r a c e . the  km  merge w i t h a n o t h e r s e t o f l a r g e a m p l i t u d e a r r i v a l s f o r  which the apparent v e l o c i t y about  At  they e x h i b i t e d B).  distant  deepest  arrivals  175 t o 195 km a r e v e r y weak  and  the shots  i n t o t h e e a r t h ) and  strong secondary a r r i v a l s  The f i r s t  from  16 a n d  (figure  24  and  from s h o t - r e c e i v e r d i s t a n c e of emergent.  Because  of  their  Model 0  10  20  30  40  50  60  Distance 70  80  90  (km) 100  110  F i g u r e 23 - F i n a l r a y t r a c i n g d i a g r a m Common R e c e i v e r P r o f i l e 3. (see  text for  explanation).  for  120  130  140  150  290  280  Shot-Receiver Distance (km) 270 260 250 240 230 220 210 200 190  180  170  160  oH 63 00 CO  9 r~  00  \  —r  5  10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100105110 115 120 125  5  10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125  CO •—N  o H  co  co-  oo  Q  CO  tO-  B  -5 0  M o d eC olm p a rD i s t a n c e (km) i s o n o f d a t a and s y n t h e t i c f o r  Figure  The  24  Common R e c e i v e r P r o f i l e 15. (a) data. over the d a t a s e c t i o n . T h e a r r o w h e a d s mark f i r s t p i c k s for (b) s y n t h e t i c s produced from f i n a l r a y t r a c i n g diagram. mantle r e f r a c t e d a r r i v a l s . See f i g u r e 20 f o r a n e x p l a n a t i o n t r a v e l t i m e c u r v e s f r o m t h e s y n t h e t i c a r r i v a l s a r e drawn of d i s t a n c e and time axes.  66  emergent  nature  and  small  difficult  to accurately  amplitudes  determine  and  picks  do n o t r e p r e s e n t t h e a c t u a l  285  the  km  first  estimated  arrivals  travel  mainland data Islands  first  uncertainty  (figure  22)  that  From 200  clearer  about  o f t h e Pn a r r i v a l s p a s t 210  very  strong  190  km.  aid  of  secondary These  the  filtered  The  data  this  profile.  strong  recorded  km  on  secondary  8.0  s  the  The  slightly  small.  receivers  traced  arrivals  16 and  rays  travel  times  is  shown  f i t well  the  abruptly  at  terrace.  in  figure  Queen  km  (model  25.  Charlotte  of  arrivals  on  distance)  and  are do the  these r e f l e c t i o n s i s  m o d e l l e d c o r r e c t l y by a c o m b i n a t i o n o f t h e c h a n g e i n d i p o f 100  were  r e f l e c t i o n events  end  abrupt  The  but the a m p l i t u d e s  synthetic  The  km/s.  have been i n t e r p r e t e d a s p o s t -  Nonetheless, the secondary  section.  end  8.5  apparent  17 a r e v e r y s i m i l a r t o  a l l other  distances  The  shot-receiver  Charlotte  have a much l a r g e r a m p l i t u d e t h a n  The  an  for shots that  Queen  r e f l e c t i o n s f r o m t h e Moho b e n e a t h  Islands.  terrace  to  Charlotte  The  i s about  a r r i v a l s occur only  f i n a l model w i t h  strong  at  km  s.  Queen  sections.  a r r i v a l s a t about  detonated over or very c l o s e t o  Moho  the  with  ±0.10  of t h e l a r g e  were  A l l of t h e t r a v e l t i m e p i c k s were made on u n f i l t e r e d  velocity  critical  are  of  because  the  The  possible  breaks.  phase)  data with  The  i t is  ( f i g u r e 24) a r e n o i s i e r t h a n t h e  data  distances.  time  (Pn  t r a v e l time p i c k s  the high gradient  the upper  region. very from  weak  mantle  175(115)  refractions  for  shot-receiver  km t o 1 9 0 ( 9 5 ) km a r e m o d e l l e d w e l l i n  Model  Figure  w  (see  text for  Distance  (km)  25 - F i n a l r a y t r a c i n g d i a g r a m f o r Common R e c e i v e r P r o f i l e 15. explanation).  68  amplitude. the  The e a s t w a r d d i p o f t h e Moho b e n e a t h t h e t e r r a c e a n d  low g r a d i e n t  arrivals. arrivals  i n the mantle c o n t r i b u t e  The m o d e l t r a v e l t i m e s a r e e a r l i e r by  weak  16 a n d 17 e x h i b i t t h i s  arrivals  are  r e l i a b l e as they are at Other the  arrivals  terrace  misfit  region  The  reasonably  late with  level  of  the  structure  of  the  by r e c e i v e r s  shadow  to the data.  terrace  The  the  this  is  1, 3, a n d common s h o t on  receiver,  15  Forcing the  the  terrace  region  w o u l d be with  20  km  required. the  data.  segment o f Moho j o i n i n g t h e  This  i n turn creates  a  large  w e s t o f 2 0 0 ( 9 0 ) km where no m a n t l e r e f r a c t i o n s c a n  t o the surface.  mantle  sample  region  To c o r r e c t t h i s an 18 t o  t o the terrace crust.  zone  penetrate  sections  a s t h e weak a r r i v a l s .  require a steeply dipping  ocean c r u s t  noise.  s t r u c t u r e beneath the shots over the  constrained  beneath  would  travel  background  T h i s model h a s been t e s t e d a n d i s i n c o n s i s t e n t It  Synthetic  t o f i t t h e d a t a t r a v e l t i m e s makes t h e r e f l e c t i o n s  respect  Moho  picked  and f i t t h e m o d e l w e l l , i n d i c a t i n g t h a t  t h e same r e g i o n  weak a r r i v a l s  the  emergent and t h e p i c k s a r e not  The w i d e - a n g l e r e f l e c t i o n s r e c o r d e d  sample  deep  the  amplitude  same c h a r a c t e r i s t i c .  t h i s d a t a s e c t i o n and o t h e r  velocity  well  g a t h e r 4. also  on  very  i s n o t due t o l o c a l  terrace.  than  a s much a s 0.3 s on some t r a c e s .  times f o r r e c e i v e r s These  to the small  refractions  285(0)km c o n s t r a i n of t h e o c e a n i c that  the  first  arrivals  This on  receiver  observed  on  the  data.  15  205(90)  km t o  t h e Moho t o be d i p p i n g  c r u s t eastward.  misfit  i s not  from  gently  from  the  edge  F o r t h e above r e a s o n s i t i s f e l t  may be a r e s u l t o f n o t b e i n g  able  t o pick the  because they a r e below t h e background n o i s e  level  69  and  n o t due t o s t r u c t u r e i n t h e t e r r a c e r e g i o n . The  increase i n amplitude  225(85)  km i s m o d e l l e d  the outer  edge o f  amplitudes  than  the  mantle  terrace. weak  These  arrivals  energy recorded  Profiles Receiver coast  Profiles  and  17  are  most e a s t w a r d displayed  14). Receiver station.  very  long  receiver and  Profile  i n figure  km  arrivals and  have  an  even  data.  larger  estimated  17 d a t a , a l s o  gain  required  at  The  data  recorded  on  Arrivals  between  245  u n c e r t a i n t y o f ±0.3 s.  uncertainties.  a  28 a n d A p p e n d i x B.  weak  and  Receiver  The weak e m e r g e n t a r r i v a l s b e t w e e n  km h a v e an e s t i m a t e d  i s the  high  b e t w e e n 215 km a n d 235 km a r e v e r y  have  quality  distances.  17 a r e o f m a r g i n a l q u a l i t y .  325  another.  16, a common r e c e i v e r g a t h e r , i s  i s found  shot-receiver  one  I s l a n d j u s t west o f t h e m a i n l a n d  The d a t a a r e v e r y n o i s y b e c a u s e o f t h e such  to  17, l o c a t e d on t h e m a i n l a n d ,  Profile  gather,  larger  horizontal.  similar  i n f i g u r e 26 and A p p e n d i x B.  receiver  have  16 a n d 17  16 i s l o c a t e d on P i t t  (figure  common  16  at  f r o m 1 7 5 ( 1 1 5 ) km t o  1 9 5 ( 9 0 ) km b e c a u s e t h e Moho b o u n d a r y i s more 3.3.6 Common R e c e i v e r  refractions  w i t h a c h a n g e i n d i p o f t h e Moho b e n e a t h  the the  of  e r r o r o f a b o u t 0.3 s.  km  First  emergent  16 h a s b e t t e r 195 km a n d 222  The a r r i v a l s  between  225 km a n d 302 km have u n c e r t a i n t i e s o f a b o u t 0.15 s.  Since the  c h a r a c t e r i s t i c s of t h e s e  they  be d i s c u s s e d  two p r o f i l e s a r e v e r y s i m i l a r  will  together.  T h e s e d a t a were m o d e l l e d  t o help c o n s t r a i n the p o s i t i o n  t h e Moho f u r t h e r d o w n d i p b e n e a t h H e c a t e S t r a i t .  The f i n a l  of  model  Shot-Receiver 285 275 265 255  315  Distance (km) 245 235 225 215  195  205 185  10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125  O  w as  9  oo H  00  coin  i i i i i i i i i f f f f B  -5 0  5  10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125  Mo d e l D i s t a n c e Figure  The  26  (km)  - Comparison o f d a t a and s y n t h e t i c f o r Common R e c e i v e r P r o f i l e 16. (a) data. over the data s e c t i o n . T h e a r r o w h e a d s mark f i r s t p i c k s f o r (b) s y n t h e t i c s produced from f i n a l ray t r a c i n g diagram, mantle r e f r a c t e d a r r i v a l s . See f i g u r e 20 f o r an e x p l a n a t i o n t r a v e l t i m e c u r v e s from t h e s y n t h e t i c a r r i v a l s a r e drawn o f d i s t a n c e a n d t i m e a x e s .  Model  Distance  (km)  F i g u r e 27 - F i n a l r a y t r a c i n g d i a g r a m Common R e c e i v e r P r o f i l e 16. (see  text for  explanation).  for  72  with  traced  figure than  rays  25 e x c e p t receiver  ( f i g u r e 27 a n d f i g u r e 29) i s v e r y s i m i l a r t o  that the crust  15.  times  for receiver travel  time  slope and  modelled  travel  is in  the  and  travel  times  layers  of  the  mantle  distance).  refractions  (west o f t h e t e r r a c e  the  reflections These  rays  shadow  zone  The a m p l i t u d e s up  through  profile  the data w e l l .  f o r these r e f r a c t e d a r r i v a l s  g e n t l y eastward beneath  late  the  (figures  by a s h a l l o w e r Moho  corner  i n t r o d u c e d by b e n d i n g  mantle  a r r i v a l s at f a r offset.  data.  constrained shot-receiver  The by  low the  16  and  small  6.4 km/s b l o c k  19 a n d 2 7 ) . beneath  dips  16 a n d 17  immediately  The same e f f e c t c o u l d  the  receivers  but the  t h e Moho c r e a t e s a shadow z o n e f o r  amplitudes on  just  region.  This effect  i s not  g r a d i e n t i n the mantle  distances  The l a c k  i n d i c a t e s t h a t t h e Moho  the terrace  without  the stations  be c a u s e d  well.  surfacing  I t was f o u n d t h a t a l l m o d e l a r r i v a l s on r e c e i v e r s too  the  17 c o r r e s p o n d s t o t h e c h a n g e i n d i p o f t h e  west o f t h e t e r r a c e r e g i o n f u r t h e r  the  of  region) are modelled  Moho a t t h e o u t e r t e r r a c e edge a n d f i t s  beneath  arrivals but the  order  time.  i n c r e a s e i n a m p l i t u d e b e y o n d 2 2 5 ( 8 9 ) km on  were  east  26 a n d 2 8 ) , a s was t h e c a s e  t o a d i s t a n c e o f 200 km (model  a  the  well  The s t r o n g , w i d e - a n g l e  i n amplitude  2 4 0 ( 9 0 ) km on p r o f i l e  of  to  t h e Moho f u r t h e r d o w n d i p a n d h e l p t o d e f i n e i t s e a s t w a r d  oceanic The  were  However, t h e m i s f i t  uncertainties.  are modelled w e l l reflect  distances  were t o o e a r l y ( f i g u r e 15.  farther  The a m p l i t u d e s o f t h e weak emergent  at the s h o r t e r o f f s e t travel  i s sampled  of these  Pn  in  (0.005 km/s/km) i s  arrivals three  observed  at  long  receivers.  The  335  325  S h o t - R e c e i v e r D i s t a n c e (km) 315 305 295 285 275 265 255 245  235  225 215 205  10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95" 100 105110 115 120125 O.  CO  ce-  o coco \\ Q Z>I E1  i  60-  B  -5 0  5  10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125  Model Figure  The  Distance  (km)  28 - C o m p a r i s o n o f d a t a a n d s y n t h e t i c f o r Common R e c e i v e r P r o f i l e 17. (a) data. over the data s e c t i o n . The a r r o w h e a d s mark f i r s t p i c k s f o r (b) s y n t h e t i c s p r o d u c e d from f i n a l r a y t r a c i n g diagram. mantle r e f r a c t e d a r r i v a l s . See f i g u r e 20 f o r a n e x p l a n a t i o n t r a v e l t i m e c u r v e s f r o m t h e s y n t h e t i c a r r i v a l s a r e drawn of d i s t a n c e and time axes.  Model 0  20  40  60  80  100  120  140  Distance 160  180  200  (km) 220  240  F i g u r e 29 - F i n a l r a y t r a c i n g d i a g r a m Common R e c e i v e r P r o f i l e 17. (see  text for  explanation).  260  for  280  300  320  340  75  amplitudes the  of these a r r i v a l s a r e very s e n s i t i v e  gradient  higher than the  because  their  0.005 km/s/km r e s u l t s  farthest  amplitudes  of  distances.  are a l i t t l e  Even  larger  to  changes  long mantle path. i n much l a r g e r with  in  A gradient  amplitudes  at  this  small gradient the  than d e s i r e d .  However, a s m a l l e r  g r a d i e n t does n o t a l l o w s h o t s a t f a r o f f s e t  t o propagate  energy  to the mainland.  3.3.7 Common S h o t P r o f i l e This The  profile  r e p r e s e n t s shot  d a t a a r e shown  corrected were  in  f o r the  deployed  4  f i g u r e 30.  response  along  the  instruments with d i f f e r e n t trace  to  Localized amplitude data  from  amplitudes variation  trace  line.  amplitudes  Because  response  receiver first  much  i s thought  smaller  types  that of  is may  sometimes also  large.  account  for  two t r a c e s , c o r r e s p o n d i n g t o  than  t o be a r e s u l t  been  instruments  several  t h e two OBSs w e s t o f M o r e s b y I s l a n d very  have  f u n c t i o n s were d e p l o y e d , t h e  variation  s t r u c t u r e beneath a The  The  of the d i f f e r e n t  amplitude  variations.  4 r e c o r d e d on a l l r e c e i v e r s .  (figure  14),  the land receivers.  of the c o u p l i n g of  the  have This OBS  S ho t - R e c e ive r Distance 60  80  180  100  200  Model Figure  120  220  140  240  Distance  160  (km) 180  240  260  (km)  30 - C o m p a r i s o n o f d a t a a n d s y n t h e t i c f o r Common S h o t P r o f i l e 4.  . . (a) d a t a . ( b ) s y n t h e t i c s p r o d u c e d f r o m f i n a l r a y t r a c i n g diagram.T h e t r a v e l t i m e c u r v e s f r o m t h e s y n t h e t i c a r r i v a l s a r e drawn over the data section. Arowheads denote mantle r e f r a c t e d arrival f i r s t picks. The d a t a were b a n d p a s s f i l t e r e d f r o m 1  - 5 Hz. E x c e p t f o r t h e d a t a b e t w e e n 120 km a n d 160 km shot receiver distance the f i r s t p i c k s w e r e made on u n f l l t e r e d data. S e e f i g u r e 20 f o r a n e x p l a n a t i o n o f d i s t a n c e a n d t i m e axes .  77  with  the  likely land  o c e a n b o t t o m and  that  the  instrument response.  t h e OBSs were s i t t i n g  stations  equipped with  were 4.5  placed  Hz  on  on  soft  b e l o w 4.5  Hz  t h e OBSs i s g r e a t l y a t t e n u a t e d  Hz  and  thus  (figures  C h a p t e r I I , most of it  16  seismometers.  is  and  partially  17a).  travel  ocean  first  arrivals  of  0.05  s or  less.  weak and  e m e r g e n t and  from the  near-offset  15,  and  16,  17  arrivals  b e t w e e n 120  final  profile  ( f i g u r e 21 that  the  As  i s below OBS  for  the  place  the  5  itself first  two  OBSs  on  km  on  of  a r r i v a l s b e y o n d 82  each of the 26,  about km  and  (shot-receiver  distance)  and 8  230  28).  km/s. km  4  (the  km  in  the  are  very  fourth  common r e c e i v e r  trace gathers  T h e s e a r r i v a l s have The  h a v e an  strong  an  secondary  a p p a r e n t v e l o c i t y of  km/s. The  This  recorded  t r a v e l time u n c e r t a i n t i e s  First  ( f i g u r e 24,  velocity  82  correspond to shot end)  apparent  6.6  energy  by  to e f f e c t i v e l y  before  of e x c e l l e n t q u a l i t y w i t h  order  are  surface.  The are  stations  ( f i g u r e 15).  seismic  times  the is  Thus e n e r g y  attenuated  The  r e c e i v e r s h a v e been c o r r e c t e d the  the  while  w e l l t h e OBS  land  1  in  Hz  the  with  indicated  2  As  equipped  on  or  sediments  bedrock.  seismometers while  I t i s most  have  6 5 ( 1 7 0 ) km  and  model, w i t h is  the  are  terrace block.  km  modelled  are  the  m o d e l l e d as  gradient  as  rays,  'pseudo-reversal'  f i g u r e 23).  traversed  traced  i s shown  turning  8 0 ( 1 8 5 ) km  region. rays  The  from  through the  the  1 and are  arrivals  A r r i v a l s b e t w e e n 6 5 ( 1 7 0 ) km reflections  figure  for receivers  A r r i v a l s before terrace  in  Moho  3. rays  out  upper and  31.  to high  80(185)  beneath  the  Model  Distance  (km)  F i g u r e 31 F i n a l r a y t r a c i n g diagram f o r Common Shot P r o f i l e 4. (see  text for explanation).  79  terrace. well.  The m o d e l t r a v e l Two d i f f e r e n t  times and  types of rays  amplitudes  f i t the  ( r e f r a c t e d and r e f l e c t e d ) t o  m o d e l t h e s e a r r i v a l s were c h o s e n b e c a u s e o f t h e l o c a t i o n boundary  separating  weak f i r s t the  arrivals  mantle  t h e upper from  of  of t h e  and lower t e r r a c e b l o c k s .  1 8 0 ( 2 8 5 ) km t o  refractions  receivers  230(335)  km  15,  and  16,  be d a t a r e l a t e d with  the  secondary  17. The  arrivals as  I t i s c o n s i d e r e d t h a t t h i s m i s f i t may  from  of  receiver  1 2 0 ( 2 2 5 ) km t o  post-critical  15.  230(335)  reflections  from  Charlotte  Islands  amplitudes  and t r a v e l  times a r e w e l l modelled  shot p r o f i l e  response.  Strait.  column.  Both  the  Moho  distance).  16 ( f i g u r e 32) g a t h e r s d a t a Amplitudes  Receivers  1  from a f a r  h a v e been  and  2 were  corrected effectively  through  The a m p l i t u d e s o f t h e s e t w o a r r i v a l s  r e c o r d e d on t h e l a n d s t a t i o n s . t h e OBS-ocean  beneath  the d i p of the  f i g u r e 32 w i t h a r r o w h e a d s ) a r e v e r y much s m a l l e r  to  been  by t h e s y n t h e t i c s  p l a c e d on t h e s u r f a c e o f t h e o c e a n by r a y t r a c i n g water  have  16  s h o t r e c o r d e d on a l l r e c e i v e r s . receiver  km  fully  amplitude  t h e Moho  b e t w e e n 140 km a n d 185 km ( m o d e l  3.3.8 Common S h o t P r o f i l e  for  Hecate  These a r r i v a l s h e l p c o n s t r a i n  in the i n t e r v a l  Common  and  The l a r g e  Queen  seismograms.  travel  r a t h e r than model r e l a t e d and i s d i s c u s s e d  interpretation  interpreted the  early.  The  'reverse'  a m p l i t u d e s of t h e s y n t h e t i c s match t h e data w e l l but t h e times a r e s l i g h t l y  data  (marked on arrivals  T h i s i s c o n s i d e r e d t o be r e l a t e d  bottom c o u p l i n g and t h e poor  OBSs a t f r e q u e n c i e s b e l o w 5  than  the  Hz.  This  response  i s discussed  of t h e  i n more  Shot-Receiver 110  130  150  Di s t m c e (km) 170  190  210  230  250  270  120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320 330 340  L0 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320 330 340  Model Figure  32  Distance  (km)  - Comparison of d a t a and s y n t h e t i c f o r Common S h o t P r o f i l e 1G.  (a) d a t a . (b) s y n t h e t i c s p r o d u c e d from f i n a l r a y t r a c i n g diagram. The t r a v e l t i m e c u r v e s f r o m t h e s y n t h e t i c a r r i v a l s a r e d r a w n over the data s e c t i o n . Arrowheads denote f i r s t p i c k s . See f i g u r e 20 f o r a n e x p l a n a t i o n o f d i s t a n c e a n d t i m e a x e s .  CD  o  81  detail The  w i t h t h e i n t e r p r e t a t i o n o f s h o t 4 common r e c e i v e r  travel  t i m e u n c e r t a i n t i e s f o r t h e s e two a r r i v a l s  o r d e r o f 0.1 s.  The f i r s t  arrivals  have u n c e r t a i n t i e s o f a b o u t stations in  deployed  11,  T h i s energy  12,  13,  and  noise,  t h e y d i d n o t r e c o r d any v i s i b l e  The t h r e e a r r i v a l s  three  points. All  first  ( f i g u r e 33).  unknown  The gap  four  high  amplitude  s e i s m i c energy  from  arrivals  velocity  of s l i g h t l y l e s s  have" been m o d e l l e d a s m a n t l e  The o n l y s e c o n d a r y  receiver  because  from  profile  from shot  the  than  the 1  oceanic  arrivals  It  is  only  model t r a v e l to  This  on  Wide  on  angle  receiver  1  the  reason  high  gradient  eastward that  through  a l l first  refractions.  times and a m p l i t u d e s f i t t h e data w e l l f o r  1 3 0 ( 1 9 0 ) km.  t h e Moho d i p i n t h i s  and  t o propagate  for this  were m o d e l l e d a s m a n t l e  out  1.  i s observed  pg. 5 8 ) .  16 a r e i d e n t i f i a b l e  common  c h a n g e i n Moho d i p a t t h e o u t e r t e r r a c e edge ( a t a  the t e r r a c e r e g i o n .  The  Moho  ( f i g u r e 20  t e r r a c e do n o t a l l o w e n e r g y  arrivals  8  refractions  a r r i v a l o b s e r v e d on t h i s  m o d e l d i s t a n c e o f 100 km i n f i g u r e 33) upper  shot  i t i s hard t o estimate t h i s a c c u r a t e l y with only  wide a n g l e r e f l e c t i o n  reflections  OBSs  i n Hecate  s h o t p r o f i l e a t 60 km a n d 6.3 s c o r r e s p o n d s t o r e c e i v e r  common  km  b e y o n d 220 km c o r r e s p o n d t o t h e m a i n l a n d  s t a t i o n s a n d h a v e an a p p a r e n t although  125  was r e c o r d e d on  14) t h a t were d e p l o y e d  Due t o a s o u r c e o r s o u r c e s o f  km/s  and  130 km a n d 220 km r e p r e s e n t s , t h e  Strait.  16.  km  i s i n the  a c r o s s t h e Queen C h a r l o t t e I s l a n d s .  t h e d a t a between  (receivers  0.05 s.  b e t w e e n 80  gather.  This p r o f i l e helps to constrain  r e g i o n and t h e average  velocity  structure  83  above  the  Moho.  A  Charlotte Islands variations  small  was  between  bend i n t h e Moho b e n e a t h t h e Queen  used  to  model  the  apparent  80(140)  km  a n d 1 3 0 ( 1 9 0 ) km.  bend i n t h e Moho o r s t r u c t u r e a t a s h a l l o w e r of  this  arrivals  variation 'reverse'  rays" that This  i s not c o n s t r a i n e d  cross  travel  helps  are  late  difficult  and  the  site-dependent  depths.  be  as w e l l as t h e d a t a l o w e r e d by a s m a l l e r  inconsistent gradient region receiver  quite  amplitudes points  as  17 f o r Islands.  good.  much  The  too large.  i n that  s t r u c t u r e or  region  features  The u n c e r t a i n t i e s o f t h e t r a v e l  l a r g e because of t h e n o i s e . fit  These  i t  t o determine whether t h e a m p l i t u d e v a r i a t i o n s a r e  c a u s e d by l o c a l crustal  cause  set. and  this  c o n s t r a i n t h e d i p o f t h e Moho.  Because t h e a r r i v a l s a r e t h e o n l y data is  15, 16,  model f i t a t f a r o f f s e t s i s n o t times  depth i s the  t h e Moho b e n e a t h t h e Queen C h a r l o t t e  'pseudo-reversal' The  Whether  by t h i s d a t a  common r e c e i v e r g a t h e r s  velocity  with  It i s felt  quality requires. gradient  receiver  a  refracted  these a r r i v a l s The a m p l i t u d e s  15,  shadow rays  lower  time p i c k s a r e  i n the mantle although  gathers  below 0.005 km/s/km, f o r mantle  that  at  16,  zone  i s created  would  this  a n d 17. in  are  the  is  With a terrace  f o r these  common  gathers.  3.4 The F i n a l M o d e l - A R e c a p The  final  constrained of  model  has  by t h e d a t a .  crust  beneath  features  which  are  well  The t e r r a c e r e g i o n , t h e c h a n g e i n d i p  t h e Moho b e n e a t h t h e o u t e r  shallow  several  the  edge  of  the  terrace,  and  the  Queen C h a r l o t t e I s l a n d s a n d H e c a t e  84  Strait  will  be d i s c u s s e d .  unconsolidated profile  Other  1.8  km/s  layer,  and  Seemann,  s t r u c t u r e i s very  refraction  1981) a l o n g similar  studies  to  Horn e t a l .  et a l . ,  e t a l . , 1974) i n d i c a t e t h a t t h e c r u s t  i s about  t h e west c o a s t The  terrace  region  km/s/km) a n d a l o w e r a  takes  low place  gradient  has  ( 0 . 0 5 km/s/km).  i s constrained only t h e upper b l o c k  reflections  1972; 30  (4.1 km/s) a n d h i g h  Forsyth  km  thick  gradient  velocity  b e t w e e n 6 a n d 10 km.  i s necessary  from t h e o c e a n i c  (0.3  ( 6 . 5 km/s)  E x a c t l y where t h i s  o f c l o s e a r r i v a l s on r e c e i v e r s 1 a n d 3. angle  (1984).  b e e n d i v i d e d i n t o two b l o c k s : an  u n i t w i t h a much h i g h e r  gradient  in  line.  of t h e m a i n l a n d .  upper u n i t w i t h low v e l o c i t y  and  seismic  the refraction  (Johnson  along  representing  s e d i m e n t s , i s c o n s t r a i n e d by a c o n t i n u o u s  (Davis  The o c e a n i c  The  change  The  to explain  high  amplitudes  The l a c k o f ' a n y  wide-  Moho t h a t a r e s o p r o m i n e n t on  receiver  1 ( b u t n o t o b s e r v e d on a n y r e c e i v e r s e a s t  o f t h e Queen  Charlotte  fault  block  focuses  the  zone) i s a l s o  energy  to  the  explained  by  this  which  surface.  The h i g h g r a d i e n t  also  c o n t r i b u t e s t o t h e a b r u p t e n d o f t h e Moho r e f l e c t i o n s v i s i b l e on receivers  15, 16, a n d  gradient  of  0.05  17.  km/s/km,  r e f l e c t i o n s on r e c e i v e r 1. energy t o reach  The is  The  r e f l e c t i o n s a t f a r o f f s e t on s h o t low  the  terrace region  terrace  gradient  (figure  i n the lower  terrace  required  A higher  the s t a t i o n .  of  lower  by  unit, the  with  wide  a  angle  g r a d i e n t does not a l l o w the refractions  and  wide-angle  gather  4 also require a region  terrace.  The a v e r a g e v e l o c i t y o f  i s constrained  by f i r s t  arrivals  3 3 , f i g u r e 21 a n d f i g u r e 2 3 ) .  through  the  85  The  5° e a s t w a r d d i p o f t h e Moho shown i n t h e f i n a l model i s  reasonably (figure 16,  well  constrained  1 9 ) . Wide a n g l e  and  17  help  f i x the  horizontal  to  reflections  p r o g r e s s i v e l y eastward. almost  out  abrupt  figure a  depth  of  of  these  km  to  shallow  A  factor  t o 9° i n o r d e r  shallow  gathers oceanic figure  Moho  Moho t o a (page  shadow z o n e e a s t  deep  reflections vertically  4 provide  first  recorded  1 9 , page 53) were  time  have fit.  i s r e q u i r e d by  arrivals  on  such  as  receiver  well  oceanic  shown  in  t h e mantle rays and c r e a t e a None o f t h e e n e r g y  from  r e c e i v e r s 15, 16, o r 17.  of t h e ocean c r u s t and constrained  on r e c e i v e r 1.  through the  would  the travel  Moho,  of the t e r r a c e r e g i o n .  are  from  A s t e e p l y d i p p i n g boundary j o i n i n g t h e  2 7 ) , would block  Moho  17.  t h e model above t h e  (figure  to satisfy  continental  average v e l o c i t y  oceanic  of  t h e d i p o f t h e Moho  i n t h e shadow zone w o u l d r e a c h The  gather  beneath the t e r r a c e region  15, 16, a n d 17.  12  an  controlling  I f t h e 5.5 km/s b l o c k  km, m o d e l d i s t a n c e s ,  the c o n t i n u i t y of mantle r e f r a c t i o n  the  major  structure  r e p l a c e d w i t h a 6.65 km/s b l o c k t o be i n c r e a s e d  15,  from  o f r e f l e c t i o n s on r e c e i v e r s 15, 16, a n d  the  340  receivers  t h i s boundary a t p o i n t s  2 4 ) . The w i d e - a n g l e r e f l e c t i o n s on s h o t  in  distance  wide-angle r e f l e c t i o n s (eg.  d i p p i n g Moho c a n i n f l u e n c e i t s d i p .  shots  on  The c h a n g e i n d i p o f t h e Moho  termination  Variations  model  d i s c o n t i n u i t y beneath t h e ocean t o a d i p p i n g  'pseudo-reversal'  220  km  recorded  i n t e r f a c e beneath the t e r r a c e i s the the  200  by  the the  Since a l l other  region  the  upper  p o o r l y c o n s t r a i n e d a n d c o u l d e a s i l y be r e p r e s e n t e d  depth  of  wide-angle rays  travel  layers  are  by one l a y e r .  86  The  oceanic  s t r u c t u r e u s e d was t a k e n f r o m H o r n  et a l .  (1984),  parallel  to  who  the  also  Queen  modelled Charlotte  r e f r a c t i o n s through the terrace and  3 help The  constrain solid  constrained that  are  boundaries  well  region  Charlotte  in  The d a s h e d l i n e s  and  dipping  Islands  slow  diagnostic be  velocity  significant  and s h a l l o w  and  high  53)  represent  the  underthrusted  s e d i m e n t a r y wedge?  structure  Charlotte  Islands  oceanic  material?  In the  a t t e m p t t o answer t h e s e q u e s t i o n s  are well  modelling that  has  The t e r r a c e the  a r e such  velocity  Queen  features. structure?  of t h e upper t e r r a c e i s  I s the very  Queen  1  boundaries  Moho b e n e a t h  gradient  mantle  region.  features.  this  shot  receivers  of h i g h l y sheared and deformed s e d i m e n t s .  an a c c r e t i o n a r y  beneath  (page  and western Hecate S t r a i t  velocity  The  by t h e d a t a s e t . The  What i s t h e t e c t o n i c s i g n i f i c a n c e o f The  line  zone.  r e c o r d e d on  f i g u r e 19  us w i t h a  constrained  and a g e n t l y  fault  region  well constrained  p r o c e d u r e has p r o v i d e d some  refraction  t h e d e p t h t o t h e Moho i n t h a t  by t h e d a t a . not  a  (1982) and Horn  Could  shallow  continental following  i s made.  this Moho  crust  or  chapter,  an  87  IV. This  seismic  information  DISCUSSION AND CONCLUSIONS refraction  study  on t h e Queen C h a r l o t t e  o b j e c t i v e of t h e s t u d y , velocity  model  tectonic  sense.  significant  from  the  Only the f e a t u r e s well  are:  (1) t h e t h i n c r u s t  Islands  and  Hecate S t r a i t ;  Moho s t a r t i n g  terrace either  region side.  without,  will  be  this  the  final  take  that  are  beneath  the  Queen  both These  Charlotte  e a s t w a r d d i p (5°) o f  edge o f t h e t e r r a c e ; a n d ( 3 ) t h e  which  discussed  To meet  be c o n s i d e r e d .  separates  d i f f e r e n t m a t e r i a l on  Two d i f f e r e n t m o d e l s , one w i t h  will  more  and i n t e r p r e t i t i n a  (2) t h e g e n t l e  at the outer itself  data  to  o f t h e model  constrained  features  the  Islands region.  i t i s necessary  derived  and  was p l a n n e d t o o b t a i n  in  this  subduction  chapter.  and  one  Both of these  m o d e l s a r e shown i n f i g u r e 34. Evidence Pacific Islands  plate  indicating  the  possible  beneath  North  America  was d i s c u s s e d  refraction  i n Chapter  experiment.  structure that could  The  I  underthrusting at  as  data  Charlotte  justification  f o r the  c o m p r i s e d of normal c o n t i n e n t a l m a t e r i a l . oceanic  Islands ocean in  crust  to  the  t o standard  a  defined  Islands  and  i s not  transition Charlotte  from  standard  c o n t i n e n t a l c r u s t where a l a r g e c h a n g e  by t h e r e f r a c t i o n d a t a Hecate  subducting  crust  The g e n t l e  transition  t h i c k n e s s c a n be e x p e c t e d o v e r a s m a l l  Moho  the  c r u s t b e n e a t h t h e Queen  i s a l s o not i n d i c a t i v e of crust  do n o t d e l i n e a t e a  i d e n t i f i e d as a  s l a b ; however, t h e model does i n d i c a t e t h a t  from  the  t h e Queen  analyzed  be u n a m b i g u o u s l y  of  Strait  may  be  distance.  Thus  b e n e a t h t h e Queen the  bottom  the  Charlotte  edge  of  an  88  These two drawings a r e s c h e m a t i c r e p r e s e n t a t i o n s of (a) a s u b d u c t i o n t e c t o n i c r e g i m e and (b) a non-subduction regime at the Queen C h a r l o t t e I s l a n d s . The t o p of t h e s u b d u c t i n g s l a b drawn i n (a) i s n o t s e e n by t h e r e f r a c t i o n d a t a and is shown h e r e j u s t a s a p o s s i b l e m o d e l . V e r t i c a l e x a g g e r a t i o n i s 6:1.  89  underthrust  Pacific  Further  plate.  credence to t h i s  i n t e r p r e t a t i o n i s provided  lower t e r r a c e u n i t which i s very to the  oceanic  Charlotte  p l a t e t o the  Islands  to  underlies  terrace  region.  The  and  gradient  (4.1  high  west and  the  c r u s t most l i k e l y  the  km/s;  velocity  1.7-2.0 km/s  at the  5.6  km/s  for  c e n t r a l Peru.  The  km/s/km)  Huene  et a l .  f r o n t of  the  1972  and  C h a s e e t a l . , 1975)  work  ( H o r n e t a l . , 1984  and  this  the  terrace  are  accreting  in  this  closely  sedimentary m a t e r i a l  study  indicates  resembles  prominent  in  Kanamori,  1979  between  constrained  and  others).  underthrust  the  lower t e r r a c e  plate.  Moho b e n d s downward f r o m b e i n g ocean  to  an  by  the abrupt  zones  1977).  the  almost  end  of  1979;  i s not the  trough region horizontal The  that  The  model  material that  are  Uyeda  and  sloping  i t represents  In  composed  step-like profile  (Seely,  regions  e a s t w a r d d i p o f a b o u t 5°.  well constrained  be  t h i c k sequence of  eastward  (Chase  refraction  sedimentary prisms  The  data to c l a i m  Pacific  seismic  Karig,  subduction zones  u p p e r and by  this  accretionary  shallow  the  that  sediments i n  subduction  (see  a  sediments  reflection  study) appears to  i s s i m i l a r t o many known  report  accreted  The  the  represents  (1985)  and  h i g h l y deformed compressed sediments.  of  i t s low v e l o c i t y  probably  t e r r a c e , from b o t h s e i s m i c  Tiffin,  of  with  Queen oceanic  sediments  o l d e r more c o m p r e s s e d a c c r e t e d  and  of  region  the  structure  indicates that  compressed  0.3  Von  velocity  c r u s t beneath the  This  upper t e r r a c e  sediments.  and  the  east.  compressed of  similar in  by  boundary  well  enough  top  of  the  oceanic  beneath  an  the  b e n d i n t h e Moho i s  reflections  from  this  90  horizon 17).  on  any  receivers  The g e n t l e n e s s  Strait  e a s t of t h e t e r r a c e ( r e c e i v e r s 3 t o  of t h e d i p and i t s e x t e n s i o n  i s also well constrained.  w i t h an u n d e r t h r u s t  shallow  expected.  The  the  Charlotte  Queen  below  Hecate  This feature c o r r e l a t e s w e l l  s l a b where a  gentle  d i p would  be  l a c k of a b i l i t y t o d i s t i n g u i s h t h e s l a b beneath Islands  and  Hecate  Strait  from  the  c o n t i n e n t a l m a t e r i a l above i t may due t o t h e d a t a s e t o r i n - s i t u physical it.  properties  A l l raypaths  turning  points  of t h e s l a b or the c o n t i n e n t a l c r u s t above  pass t h r o u g h t h e proposed s l a b (no i n the  slab)  and  thus  rays  have  i t i s not r e a d i l y  d i s t i n g u i s h a b l e from m a t e r i a l above i t . The  extremely  not s u r p r i s i n g . Islands and  The  shallow Pacific  i s about 7 Ma o l d .  Wortel  (1976)  subduction  note  plate  of t h e P a c i f i c p l a t e i s  at  the  Queen  Ruff and Kanamori (1980), that  there  is a  Charlotte and V l a a r  strong  inverse  c o r r e l a t i o n between l i t h o s p h e r i c age and depth of p e n e t r a t i o n of the  slab.  subduction  The  young,  h o t , and buoyant s l a b tends t o r e s i s t  i n t o the mantle.  Uyeda and Kanamori (1979) note t h a t  i n p l a c e s where t h e o v e r t h r u s t i n g p l a t e has velocity  toward t h e t r e n c h  mantle) than t h e u n d e r t h r u s t This  s l a b , the  and W o r t e l ,  AM1 ( M i n s t e r e t a l . , reference at  about  1974)  absolute  1980).  subduction  is  shallow.  (10 -30° d i p ) , P e r u (10°), and 0  N o r t h Honshu Japan (35°-40°) s u b d u c t i o n 1979  higher  (assuming the t r e n c h i s f i x e d i n t h e  i s observed i n t h e C h i l e a n  Kanamori  a  zones  (see Uyeda  and  The a b s o l u t e motion of model  indicates  that  from  the  hotspot  frame N o r t h America i s moving i n a southwest d i r e c t i o n 2.7 cm/yr.  The P a c i f i c p l a t e i s moving almost n o r t h -  91  northwest  at  overthrusting  5.6  to  the  trench  mostly p a r a l l e l the  larger  6.1  cm/yr. while  to the trench.  component  subduction should  of  Thus the  final  shallowly  convergence  shown and  personal  slab  about  22 km.  This  region  shallow  Crustal thickness  gravity  well  model  subduction of the P a c i f i c  indicates that  with  the  lying  reaching  model  the  plate  a  beneath  region i s velocity  with very  the  In  maximum  refraction  i s consistent  is  between 4°  below t h e t e r r a c e  with  a  data (S.  The g r a v i t y t h e Moho d i p s  of  of g r a v i t y  and western Hecate S t r a i t .  t h e Moho i s f l a t  12 t o 15 km a g r e e i n g  model.  1985).  I t indicates that  Hecate S t r a i t of  that  a gravity interpretation  i s consistent  communication,  9° b e n e a t h t h e t e r r a c e  depth  having  indicates  t h e s o u t h e r n t i p of Moresby I s l a n d  i n f i g u r e 35.  eastern  i s moving  plate  model but p r e l i m i n a r y m o d e l l i n g  subducting  Carbotte,  plate  is  be o c c u r r i n g .  velocity  data across  Pacific  America  The o v e r t h r u s t i n g  Time c o n s t r a i n t s d i d n o t a l l o w the  North  Queen  shallow Charlotte  Islands. There  are  many  places  in  s u b d u c t i o n h a s been d o c u m e n t e d . indicate between that  that  beneath  Gorda  plate  world  where v e r y  F o c a l mechanisms of  c e n t r a l and n o r t h e r n  10° a n d 15° ( S t a u d e r ,  the  the  1975).  shallow  seismicity  Peru the slab  Cockerham  d i p s a t 10° f o r a l e n g t h  (1984)  dips  reports  o f 120 km t o a  d e p t h o f 30-35 km where i t s t e e p e n s t o a d i p o f 25°.  Closer  home,  subduction.  the  Preliminary profiles  J u a n de F u c a p l a t e a l s o e x h i b i t s s h a l l o w analysis across  of  the  southern  Lithoprobe  Vancouver  seismic Island  to  reflection  indicates  that  92  Figure (S.  35 - P r e l i m i n a r y g r a v i t y model a c r o s s s o u t h e r n t i p of Moresby I s l a n d  C a r b o t t e , personal communication,  1985).  the  93  W r a n g e l l i a i s between 15 and zone  of  slab,  possibly  beneath  20 km t h i c k and  underplated  which  lies  i s underlain  by  a  a l l o c t h o n o u s m a t e r i a l or an o l d the  actual  subducting  slab  (Yorath et a l . , 1985). The  model  of  shallow subduction  phenomena  observed  in  discussed  previously  the in  Queen  Chapter  i s c o n s i s t e n t with other  Charlotte I.  Islands  region,  The broad g e n t l e o f f s h o r e  topographic bulge and a s s o c i a t e d g r a v i t y high i s observed  here.  The  km of  Queen C h a r l o t t e trough i s shallow and  flat-lying shallow  sediments  angle  resembling  trenches  with 2-3  associated  with  of subduction such as c e n t r a l and northern  the A l e u t i a n s , and o t h e r s . Queen C h a r l o t t e trough has typical  filled  of North P a c i f i c  trench which i s f i l l e d  Scholl  (1974)  observes  a  Peru,  that  the  s t r a t i g r a p h i c and s t r u c t u r a l f e a t u r e s trenches such as the Washington-Oregon  with 1-2  km of sedimentary  deposits.  U p l i f t has been o c c u r r i n g along the west coast of the Queen C h a r l o t t e I s l a n d s f o r the l a s t probably  continuing  10  Ma  p r e s e n t l y (Riddihough,  has been o c c u r r i n g i n Hecate S t r a i t , the l a s t uplift  6 Ma. and  (Parrish,  South Kanto d i s t r i c t  the  trench;  is  Subsidence  j u s t 80 km to the e a s t , f o r this  pattern  of  Scholz and Kato (1978) s t u d i e d the  i n Japan where h i g h l y o b l i q u e subduction of  the P a c i f i c p l a t e occurs at 3 cm/yr along Elevation profiles  and  but s e v e r a l i n p a r t i c u l a r are s i m i l a r to  the Queen C h a r l o t t e I s l a n d s .  near  1982b).  Many subduction zones d i s p l a y  subsidence  1982)  i n d i c a t e that u p l i f t the  "uplift"  landward of the trench a x i s .  Seno  the  Sangami  trough.  of about 20 mm/yr occurs  f a l l s to negative v a l u e s 30 (1977)  has  documented  km the  94  same  pattern  New Z e a l a n d the  i n the Nankai trough  and I n d i a n p l a t e s ( W a l c o t t ,  oblique  occurring  convergence along  the  edge  (Lensen, of  1975).  t h e Queen  at  10°-15°  fault.  r e a c h +10 mm/yr b u t f a l l trench  compatible  boundaries. highly  Charlotte  than  in  areas  subduction. occur  with highly  oblique  (Berube,  compressional boundary. inland  the  on  western  subduction  indicates  and  that  regime  1982)  which  transform  subsidence  at  t o t h e margin  is  dominantly  i n both  t h e Andes and A l a s k a a p p e a r s t o  trench  than  i n t h e Queen  Charlotte  1972). recorded 1985  in  and  s t r e s s i s being  Berube  subduction  the  1982b, a n d P a r r i s h ,  tectonic  the  Subsidence  there  I s l a n d s o f +2 mm/yr d r o p s t o -1  oblique  (1982b)  along  where  seismicity  region  uplift  margins occurs c l o s e r  (see P l a f k e r ,  The  i s no  convergent  f u r t h e r from  Islands  there  Although  t o n e g a t i v e v a l u e s w i t h i n 120 km o f t h e The p r e s e n t  Riddihough  1978).  between  V e r t i c a l movements n e a r t h e f a u l t  mm/yr w i t h i n 80 km ( R i d d i h o u g h , is  The A l p i n e F a u l t i n  i s a z o n e o f o b l i q u e c o n v e r g e n c e a t 4 cm/yx  Pacific  is  region.  Rogers,  1983)  indicates  released i n earthquakes  (1985) i d e n t i f i e d  Graham I s l a n d .  t h e Queen C h a r l o t t e I s l a n d s  some  that  along  shallow thrust  the  events  The m i c r o s e i s m i c i t y i s s h a l l o w ( 5 - 1 5  km) a n d i t c o u l d n o t be a s s o c i a t e d w i t h a B e n i o f f z o n e . T h i s may be  a r e s u l t o f t h e s h o r t s t u d y l e n g t h (3 m o n t h s ) .  seismicity  associated with a subducting (1) a s e i s m i c  slip,  the lack of subduction.  of  s l a b may be c a u s e d b y :  (2) a p r e s e n t l y l o c k e d s u b d u c t i o n (3)  This lack  process, or  95  Since the  we  are  first  two  considering  lower  slabs  of  cause  Scholz  subducting  and  Kato  below  Sangami  subduction  s u b d u c t i o n of Island north  is  15 km  the  and  the  (1984) s u g g e s t boundary  (3-4  the  s l a b and  area  of  zone.  J u a n de  along  that  that  aseismic  (1983)  is little  slow  notes  not  H e a t o n and  result in  Island  may  The  McCann ( 1 9 7 6 ) i n d i c a t e t h a t interact  with  frequently. aseismic  the  subducting areas  Vancouver  Vancouver Kanamori  across  this between coastal  resistance  where  1984).  subduction  plate.  is  suggests occurring  that  it  may  Kelleher  bathymetric  is  possible  b e n e a t h t h e Queen  to zone  o v e r l y i n g p l a t e large earthquakes occur  A l l of t h i s  subduction  in  the  gaps ( P e n n i n g t o n ,  on  the  that  subduction  be  topography  the  of the  low  shallow  seismic  is  of  coupling  Back t i l t i n g indicate  1984).  several  rate  strong  of  i n d i c a t i n g that  slab.  t h e s e a r e a s of a s e i s m i c  to  subduction  between  convergence  N.E.  seismicity.  coupling  dipping  and  (eg.  periods  Pennington suggests that due  upper  examples  low  inter-seismic  overriding plate.  has  exhibit  Islands shallowly  Fuca p l a t e beneath southern  the  (Riddihough,  Columbia  There are  a s e i s m i c a l l y at present,  c m / y r ) may the  do  Rogers  shallowly  Vancouver  subduction  the  with  type earthquakes  report  o f P u g e t Sound t h e r e  Island  that  during  proceeding  zones  South America).  (1978)  Charlotte  f o r c e s between the  thrust  plates  occurring  Queen  subduction  large  J a p a n , t h e A l e u t i a n s , and shallowly  the  where l a r g e c o u p l i n g  plates  only  relevant.  seismicity,  characteristics  dipping  s u b d u c t i o n model a t p r e s e n t  p o s s i b i l i t i e s are  Except f o r the exhibit  the  and  highs less that  Charlotte  96  Islands. and  the  large  The  oceanic  crust  coupling  continent of  convergence r a t e  between  would not  Vancouver  i s very  be  the  J u a n de  (see  aseismic  at  figure  transform  fault  observed. boundary  Since at  s l a b must be this  present,  Relative west c o a s t and  of  North  underthrusting  of  similar  Islands  the be  Pacific Finally  that  the  along  the that  possible  that  the  Pacific  the  occurring  up  plate it  be  to  is  Queen  Queen  locked  plate by  Pacific  t o the p l a t e boundary between  the the  west  i s not  plate. the  that  no  i n some d e t a i l . along  subdsidence across  the  occurring of  Pacific If  then the  and  this Queen  T h i s model i s Pacific  the  inland,  presently.  deformation  for  transform  u p l i f t patterns  exists  the  underthrust  Charlotte  compression  is  along  possible  considered  Islands,  boundaries  taken or  at  s u b d u c t i o n zone i s  is  motions, current  plate  topography.  p r e s e n c e of  be  would i n d i c a t e that  along  indicate  c o m p r e s s i o n must Charlotte  this  possibility will  America  subduction  junction may  north  r e l a t e d to underthrusting  motion  t h e Queen C h a r l o t t e  sesimicity  the  activity  exist.  plate  It  of  large  a  overriding  plate  initiation  occurring  that  s e p a r a t e from the  This  the  Pacific  triple  2).  therefore  and  very  maximum),  zone.  seismic  to  the  the  presently  i s occurring  zone.  page  transcurrent  condition  subduction fault  no  slab  Also  America  i t is possible  p r e s e n t and  ductile;  I s l a n d s , has  1,  is  course  (2 cm/yr a t  p r o b a b l y e x i s t e d s i n c e the  subduction  Charlotte  and  s i t e of  Fuca-Pacific-North  position  Of  has  low  Pacific  expected.  Island,  topography  young  the  b e n e a t h t h e Queen C h a r l o t t e This  i s very  very  Indian  97  plates  at  the  Alpine  Fault  i n New Z e a l a n d .  The A l p i n e  c o n n e c t s t h e Tonga-Kermadec s u b d u c t i o n zone i n the  Macquarie  Fault  i s t e c h n i c a l l y a transform  indicates that The  fault  oblique  Indian  plate.  cm/yr  (Walcott,  convergence  1978). and  region.  discussed  the  are  Queen C h a r l o t t e  Uplift  Islands.  (1978) s u g g e s t s t h a t  per this that  the  i s similar  The P a c i f i c Zealand along  1978).  Alpine  thickening  been  i s possible  that  type of transform  not,  however,  Fault.  Walcott  is  the Alpine  most  of  shortening  Queen  some s e i s m i c i t y was f o u n d  terrace  scarp  1985).  will  There  t a k e n up by Fault.  e x p l a i n the  on t h e o r d e r  of  20  a n d one f i f t h o f  Walcott also  suggests  northwards  to the  Island.  The s e i s m i c i t y  to  i nthe  Alpine  Charlotte  i t occurs along  terrace,  (Berube,  the  squeezed  boundary.  been  plate  the North  the  has  to that  i s m a n i f e s t e d as u p l i f t .  s u b d u c t i o n zone a l o n g  although  Fault  i s thought t o occur,  some m a t e r i a l may h a v e  similar  crustal  Charlotte  i n pattern  Crustal thickening  cent of the shortening  It  and  of  2  i s s h a l l o w (4-15  the compressive s t r e s s i s being  kilometres  (Walcott,  Hikurangi  by  p l a t e toward the  a s i n t h e Queen  deformation of a broad c o r r i d o r p a r a l l e l i n g  uplift  1978).  characterized  Pacific  1975)  across  and  b e n e a t h New  Twenty-five  of s e i s m i c i t y  (Walcott,  S e i s m i c i t y i n t h i s area  Frohlich,  previously  subducting  of  with  The p r e s e n t component o f c o n v e r g e n c e i s a b o u t  (Caldwell  Islands  the pattern  area  north  Although the Alpine  i t i s a wide zone of d e f o r m a t i o n  r e l a t i v e p l a t e motions i n t h i s  highly  km)  Ridge complex i n t h e south.  the  Fault  the inner  occur  along  Islands  isa  is  shallow  scarp  of the  the  outer  i s a zone o f a c t i v i t y  98  b e t w e e n t h e two p l a t e s . seismicity  along  cannot e x p l a i n  The d e f o r m a t i o n  the  the  Queen  thin  Charlotte  crust  gravity  model  both i n d i c a t e that  of  Carbotte  the c r u s t  Given  that  (personal  indicated Cox  by  and E n g e b r e t s o n ,  taken This  up  crustal is  (Walcott  shortening  incompatible  Charlotte  at  the  of  6  Ma  Pacific  movement  (as  is  crustal  with also  p l a t e m o t i o n by must  have  i n t h e Queen C h a r l o t t e I s l a n d s  12 km o f  east  p l a t e h a s been c o n v e r g e n t  1985) 60 km  i n f e r s t h a t about  occurred.  1 8 km t h i c k  b a s e d on a b s o l u t e  by d e f o r m a t i o n  Queen  c o m m u n i c a t i o n , 1985)  1 cm/yr f o r t h e p a s t  a study  f a u l t but i t  i s 12 km t h i c k a t t h e west c o a s t o f  the P a c i f i c  North America a t over  the  the  The f i n a l v e l o c i t y model and  t h e Queen C h a r l o t t e I s l a n d s a n d a b o u t coast.  explain  transform  beneath  I s l a n d s and w e s t e r n Hecate S t r a i t . the  model c a n  thickening  been  region.  must  have  ( 1 9 7 8 ) . u s e s a f i g u r e o f 20 p e r c e n t - o f  f o r t h e amount o f c r u s t a l  thickening).  w i t h t h e t h i n c r u s t b e n e a t h t h e Queen  the  This  Charlotte  I s l a n d s and w e s t e r n Hecate S t r a i t .  On t h e b a s i s o f e v i d e n c e f o r  a  along  thin  crust  transform  of  the  f a u l t an o b l i q u e  non-subduction  collected  Pacific  subduction  Queen  Charlotte  model i s p r e f e r r e d o v e r a  thus  geologic, far  physiographic,  indicate  and  geophysical  that underthrusting  p l a t e beneath North America along  transform  the  model.  I n summary t h e data  continent  the  Queen  f a u l t may have been o c c u r r i n g s i n c e 6 Ma.  Charlotte These  models  The  cannot d e l i n e a t e c l e a r l y the  existence  analyzed  in  this  o f an u n d e r t h r u s t  study Pacific  plate  outlined  data  do n o t c o n c l u s i v e l y r u l e o u t o t h e r data  as  of the  beneath  the  above.  Queen  99  Charlotte this  Islands.  question  Further  work  i s necessary.  which would a i d i n r e s o l v i n g  A  seismic  r u n n i n g n o r t h - s o u t h on t h e Queen C h a r l o t t e many  additional  region. survey possibly  constraints  Ultimately, a  on  refraction Islands  the c r u s t a l  multi-channel  deep  answer  some  of  the  concerning t h i s enigmatic plate  questions boundary.  would  provide  structure  in this  seismic  i n H e c a t e S t r a i t w o u l d a i d i n mapping  profile  reflection  t h e deep c r u s t and  which  still  remain  100  Bibliography  A t w a t e r T., ( 1 9 7 0 ) . I m p l i c a t i o n s of p l a t e t e c t o n i c s f o r the Cenozoic t e c t o n i c e v o l u t i o n of Western North America. G e o l o g i c a l S o c i e t y of A m e r i c a B u l l e t i n , 81, 3513-3536. A t w a t e r T., P. M o l n a r , ( 1 9 7 3 ) . 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A c q u i s i t i o n of a c r u s t a l r e f r a c t i o n p r o f i l e a c r o s s t h e Queen C h a r l o t t e I s l a n d s and H e c a t e Strait. E a r t h P h y s i c s B r a n c h Open F i l e R e p o r t , No. 84-22, O t t a w a , 55 pp. C o c k e r h a m R.S, ( 1 9 8 4 ) . E v i d e n c e f o r a 180-km l o n g s u b d u c t e d s l a b beneath n o r t h e r n C a l i f o r n i a . B u l l e t i n of t h e S e i s m o l o g i c a l S o c i e t y o f A m e r i c a , 74, 5 6 9 - 5 7 6 . Coney P . J . , ( 1 9 7 6 ) . P l a t e t e c t o n i c s and t h e L a r a m i d e O r o g e n y , I n : T e c t o n i c s and M i n e r a l R e s o u r c e s o f S o u t h w e s t e r n N o r t h America. New M e x i c o G e o l o g i c a l S o c i e t y S p e c i a l P u b l i c a t i o n , 6/ 5-10. Coney P.J.,-D.L. J o n e s , J.W.H. M o n g e r , ( 1 9 8 0 ) . suspect terranes. N a t u r e , 288, 3 2 9 - 3 3 3 .  Cordilleran  Coney P . J . , ( 1 9 7 0 ) . 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OBS d e v e l o p m e n t a t B e d f o r d I n s t i t u t e o f O c e a n o g r a p h y . M a r i n e G e o p h y s i c a l R e s e a r c h e s , 4j_ 227-245. ' • * H i l d e T.W.C., S. U y e d a , L. K r o e n k e , ( 1 9 7 7 ) . E v o l u t i o n of the Western P a c i f i c and i t s margin. T e c t o n o p h y s i c s , 3 8 , 145^ 165. H i l l h o u s e J.W. , ( 1 9 7 7 ) . Paleomagnetism of the T r i a s s i c N i k o l a i greenstone, McCarthy quadrangle, A l a s k a . Canadian J o u r n a l o f E a r t h S c i e n c e s , 14, 2 5 7 8 - 2 5 9 2 . H i l l h o u s e J.W., C.S. Gromme, ( 1 9 8 0 ) . Paleomagnetism of the T r i a s s i c Hound I s l a n d V o l c a n i c s , A l e x a n d e r T e r r a n e , s o u t h e a s t e r n A l a s k a . J o u r n a l of G e o p h y s i c a l R e s e a r c h , 85, 2594-2602. H o r n J.R., ( 1 9 8 2 ) . A s n a p s h o t o f t h e Queen C h a r l o t t e f a u l t z o n e o b t a i n e d f r o m P wave r e f r a c t i o n d a t a . M.Sc. 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T h e s i s , 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 , V a n c o u v e r , 380 pp.  109  APPENDIX A - ESTIMATION OF Two  m e t h o d s were u s e d  t o e s t i m a t e the d e t o n a t i o n time  the time f u s e d e x p l o s i o n s . geometry of t h e s h i p , measuring the  EXPLOSION DETONATION TIMES  The  s h o t , and  first  method r e l i e s on  ocean bottom;  the  the second  t h e p e r i o d o f t h e o s c i l l a t i n g gas b u b b l e  of  on  produced  by  explosion. The  be u s e d  g e o m e t r i c a l method, f i r s t  when t h e o c e a n f l o o r b e n e a t h  horizontal  (figure  36).  The  explosion  i s dropped  the d i t c h  t i m e , tjj ); and  over  the d i t c h  and  u s i n g echo sounding V = 1 . 4 9 km/s.  and  water  wave a r r i v a l  the s h i p ' s average  ground  (called  velocity  The  depth  o f t h e w a t e r , D,  equipment assuming  a water  was  (v ) $  the  estimated  velocity  of  a hydrophone immediately behind  the  r e c o r d i n g t h e o u t p u t a l o n g w i t h t h e WWVB t i m e c o d e , t i m e s of the d i r e c t  reflected  water  d  wave ( t j ), and  t h e n be e x p r e s s e d  2  a  4D  b  8xD  c =  8x D t  t o second  - 4ac  2  1 6 D X + 8 x D y At - 2 x D w - t and 3  2  a  2  3  the  bottom  (1982) shows, order as:  (1 )  2  2  the  As H o r n  r  -b ± • b 2A  At = t  water  wave ( t ) , were o b t a i n e d .  t h e s h o t d e p t h , d, c a n  where  is flat  can  t i m e l a g between t h e t i m e when t h e  the d i r e c t  By t r a i l i n g  w  s h i p and  the shot  (1982),  t i m e were m e a s u r e d a l l o w i n g an e s t i m a t e of  s h i p - s h o t d i s t a n c e x.  arrival  d e s c r i b e d i n Horn  gure  36 - G e o m e t r i c a l method o f d e t e r m i n i n g o r i g i n time.  shot  111  v  = 1.49  w  km/s.  K n o w i n g d, t h e o r i g i n  t = c  t  d  time, t  D  , i s t h e n g i v e n by:  - • d* - x*  (2)  The g e o m e t r i c a l method was u s e d  for final  when t h e o c e a n f l o o r  beneath  topography  flat  (shots 6 to 33).  flat  s e a f l o o r assumption  origin  time e s t i m a t e s  the shot p r o f i l e  was  Above t h e Queen C h a r l o t t e t e r r a c e t h e i s i n v a l i d and a n o t h e r a p p r o a c h  was  attempted. The b u b b l e p u l s e m e t h o d , u s e d o v e r t h e Queen C h a r l o t t e terrace,  r e l i e s on m e a s u r i n g  oscillation  the p e r i o d ,  of t h e gas b u b b l e produced  r , of the  first  by t h e e x p l o s i o n .  a f u n c t i o n o f t h e s h o t d e p t h , d, s i n c e t h e h y d r o s t a t i c  T is  pressure  o p p o s i n g t h e e x p a n s i o n o f t h e gas b u b b l e d e p e n d s on d e p t h . by S t r u t t  ( L o r d R a y l e i g h ) i n 1917, a n d W i l l i s  the R a y l e i g h - W i l l i s bubble  T  = 2.13 W ^ (d+10) / 1  where  d =  (3) 6  shot depth  W = energy  (1941) r e s u l t e d i n  formula:  3  5  Work  and  of e x p l o s i o n e x p r e s s e d as e x p l o s i v e weight  ( k i l o g r a m s ) i n TNT  equivalents.  E r r o r s i n t h i s method a r i s e b e c a u s e t h e R a y l e i g h - W i l l i s f o r m u l a assumes a s p h e r i c a l l y e x p a n d i n g depth.  In r e a l i t y ,  bubble a t a constant  b e c a u s e o f t h e t h e n o n - s p h e r i c a l symmetry o f  11 2  the e x p l o s i o n and t h e r i s e of t h e bubble method  i s not as accurate as equation  as i t o s c i l l a t e s ,  ( 1 ) . The b u b b l e  method d o e s n o t r e l y , h o w e v e r , on t h e o c e a n b o t t o m The s h o t d e p t h s methods.  f o r the experiment  this  pulse  topography.  were e s t i m a t e d by b o t h  A g r e e m e n t b e t w e e n t h e m e t h o d s was good e x c e p t  f o r the  s h o t s a b o v e t h e Queen C h a r l o t t e t e r r a c e where t h e b o t t o m topography 30 m e x c e p t  i s variable.  f o r shots 3 t o 5 which  Charlotte terrace. shot  The d e p t h s  a r e i n agreement t o w i t h i n were l o c a t e d o v e r  t h e Queen  A g e o m e t r i c a l s o l u t i o n was n o t p o s s i b l e f o r  1 and 2 a s t h e r o o t s of e q u a t i o n  ( 1 ) were c o m p l e x .  o n l y o t h e r d i s c r e p a n c i e s were f o r s h o t s  10 a n d 24 a n d t o be  c o n s i s t e n t w i t h t h e o t h e r s h o t s t h e g e o m e t r i c a l method were u s e d .  The maximum p r o b a b l e  This error,  in addition  maximum e s t i m a t e d e r r o r  error  to errors in t i n the o r i g i n  The  f o r shot depth r  ,v , v  time  t  0  s  and t  results i s ± 30 m.  d  give a  o f a b o u t ± 0.03 s.  113  APPENDIX  B - COMMON RECEIVER RECORD SECTION  A p l o t of the t o t a l f o r m o f common r e c e i v e r true  explosion sections.  r e l a t i v e amplitudes.  data set i s included  i n the  The d a t a a r e p l o t t e d  with  A correction  for varying  was a t t e m p t e d  by m u l t i p l y i n g t h e a m p l i t u d e s by W / ,  the e x p l o s i v e  weight  spherical also  distance.  2  i n TNT e q u i v a l e n t s .  s p r e a d i n g and o t h e r energy  scaled  by t h e f a c t o r r No c o r r e c t i o n s  2  filtered,  zero-phase  as noted  shot  3  size  where W i s  To c o r r e c t f o r  l o s s t h e d a t a a m p l i t u d e was  , where r i s t h e s h o t - r e c e i v e r  forreceiver  h a v e been made t o t h e d a t a p r e s e n t e d were  PLOTS  on  B u t t e r w o r t h bandpass  elevation  o r shot  i n t h i s Appendix.  specific filter.  p l o t s , using  depth The d a t a  an 8-pole  S h ot - R ec eiver Distance RECEIVER 1  (km)  ro ro o  c\j —• ro fo o o  Ol  o M  00 o  I  s  o  (M (\J  CO ID CM (\J C\J O O o CM CM  CM  CO f\J  CM  CM  f\J  o  co o  — o cn co r - t o i n r\j CM — — — —• o o o o o o O  CM CM  (M  (\J  N  IM  CM  ro —  r\j —  CM CM  CM  —•  O O  O  —  —  Q Q  o m o o r - t D u i ^ m N o o o o o o o o o  O O O O O O O O O  ( M ( M ( M W N ( M W r g ( \ | ( \ J ( V J  cn  75  65  h ot- Rec eiver Distance RECEIVER 2  55  (km )  ro  145  CO  r\i  135  co  r\j  r-  CM  S3  125  co in rM  CM  0  cn  OsjrM  CM — o rM  rMrvi  cn co r— i ^- _  co  in _  •tf  ro  CM — _  o c n c o r - c o i n ' ^ r n r M — • '  o  o  o  o  o  o  o  o  o  ? § ? § S S § S S S § 3 3 § 3 S S § § § 3 S § § ?  85 95 75 65 55 115 105 S h o t - R e c e i v e r Di s t a n c e (km) RECEIVER  45  35  25  175  165  155  H5  135  125  115  SHOT-RECEIVER  105  95  DISTANCE  RECEIVER 6  85  75  (km)  65  55  45  Shot-Receiver  Distance  RECEIVER 7  filter: 1.5-20Hz  (km)  CO CO o CO  185  <\l • CO r o o o CO CO  cn co CM o CO  CM CM o o  CO  CO  CD in CM CM o o CO oo  cn CM CM o o 00 00  CM CM o 00  — « o c n o o r ^ c o CM CM — — — — o o o o o o CO CO GO 00 CO CO  m ^ m c M ^ o c n c o r - c o m ^ r c —- —< — « — — — O O O O O o o o o o o o o o o o 00 0 0 0 0 00 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0  175  S h o t - R e c e i v e r D i s t a n c e (km) RECEIVER 8  filter: 1.5-20Hz  o c M — O O O O o o o o 0000000  195  185  175  165  155  145  135  125  H5  S hot- Re c e iv e r Distance RECEIVER 10  filter:2-20Hz  105  (km)  95  85  75  65  cn on o  co cvj o  cn o  CVJ CVJ  IT) CVJ  cn o  o  o  co o  cn o  o o  o o  o o  o o  K)  CO  210  200  190  180  170  160  150  Shot-Receiver  140  130  120  D i s t a n c e (km)  RECEIVER 11  filter: 1-5Hz  110  100  90  80  fO  S h ot - R ec eiver Distance RECEIVER 12  filter: 1-5Hz  (km)  180  SHOT-RECEIVER  DISTANCE  RECEIVER 13  filter: 1-5Hz  (km)  cn o  CO CVJ o in  280  260 250 240 230 2^6 210 200 270 S H O T - R E C E I V E R D I S T A N C E (km)  ID  290  in CVJ o in  cvj  o t n o o ^ c o  CVJ  pvl  o m  i n ^  ,  c o N - < o o ) o o t  N  i o i r )  ,  f c o ( v j - '  _• « ^ - 4 ^ J r t ^ - i ^ o O O O O O O O O  o o o o o o o o o o o o o o o o o o o o i n i n i n i n i n i n i n i n i n i n i n i n i n i n i n i n i n i n i n i n  RECEIVER 15  filter : 2-10Hz  190 170 180  160  305  295  265 255 Shot-Receiver  245 235 Distance  RECEIVER 16  filter: 2-10Hz  225 215 (km)  195  205  185  CO CM  CO o  o  r—  335  325  315  305  CM CM o r-  O CM o  CO — o  295 285 275 Shot-Receiver  co  CO o r-  o  o  o o r-  co r o o o o r— r -  to LO ^ co CM —• o o o o o o o o o o o o r - r - r-- r - r-~ r -  265 255 245 235 Distance (km)  RECEIVER 17  filter: 2-8Hz  225  215  205  

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