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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? THE RESULTS OF A SEISMIC REFRACTION SURVEY by DAVID MACKIE B. Sc. (Honours), U n i v e r s i t y of Toro n t o , 1982 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES Department of Geophysics and Astronomy We ac c e p t t h i s t h e s i s as conforming t o the r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA October 1985 © D a v i d Mackie, 1985 I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an advanced degree a t the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I agree t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y purposes may be g r a n t e d by t h e head o f my department 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 . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f The U n i v e r s i t y o f B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 DE-6 (3/81) A b s t r a c t The Queen C h a r l o t t e t r a n s f o r m f a u l t zone, which l i e s i m m e d i a t e l y e a s t of the Queen C h a r l o t t e I s l a n d s , marks the boundary between the o c e a n i c P a c i f i c and the c o n t i n e n t a l N o r t h American p l a t e s . R e l a t i v e p l a t e motions suggest t h a t o b l i q u e u n d e r t h r u s t i n g of the P a c i f i c p l a t e beneath N o r t h America may be p r e s e n t l y o c c u r r i n g a l o n g t h i s t r a n s f o r m f a u l t . To i n v e s t i g a t e t h i s p l a t e boundary and t h e i m p l i c a t i o n s of o b l i q u e s u b d u c t i o n on c r u s t a l s t r u c t u r e beneath the r e g i o n , an o n s h o r e - o f f s h o r e s e i s m i c r e f r a c t i o n s u r v e y was c o n d u c t e d i n 1983. The s u r v e y was d e s i g n e d t o sample the c r u s t beneath the Queen C h a r l o t t e I s l a n d s and a c r o s s Hecate S t r a i t t o the m a i n l a n d of B r i t i s h C o lumbia. S i x ocean bottom seismographs and 11 l a n d based s t a t i o n s were d e p l o y e d a l o n g a 200 km l i n e e x t e n d i n g from 20 km west of .the the Queen C h a r l o t t e I s l a n d s t o the m a i n l a n d . T h i r t e e n 540 kg and twenty 60 kg 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 a l o n g a 110 km l o n g e a s t - w e s t l i n e i n the ocean t o the west of the r e c e i v e r s . The m u l t i p l e s h o t s r e c o r d e d on m u l t i p l e r e c e i v e r s , a l l a l o n g the same l i n e , e f f e c t i v e l y r e v e r s e s the p r o f i l e o v e r some of i t s l e n g t h . The o b j e c t i v e of t h i s s t u d y i s t o p r o v i d e a model of the deep c r u s t a l s t r u c t u r e b e n eath the f a u l t zone, the Queen C h a r l o t t e I s l a n d s , and H e c a t e S t r a i t . An exemplary subset of the e x t e n s i v e d a t a s e t was s e l e c t e d t o meet t h i s o b j e c t i v e . Beneath the deep ocean t h e Moho d i p s a t about 2° t o the e a s t . At the Queen C h a r l o t t e t e r r a c e , a 25 km wide zone i m m e d i a t e l y west of the a c t i v e Queen C h a r l o t t e f a u l t , t h e d i p of the Moho i i i i n c r e a s e s t o about 5°. The c r u s t i s about 12 km t h i c k a t the t e r r a c e and 18 km t h i c k a t the e a s t e r n edge of the Queen C h a r l o t t e I s l a n d s , and i n excess of 30 km t h i c k a t the 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 i s d i v i d e d i n t o two u n i t s - an upper u n i t w i t h low v e l o c i t y (4.1 km/s) and h i g h g r a d i e n t (0.3 km/s/km) and a lower u n i t w i t h a h i g h v e l o c i t y (6.5 km/s) and a low g r a d i e n t (0.05 km/s/km). T h i s model, w h i l e not d e f i n i t i v e , s u p p o r t s the i n t e r p r e t a t i o n of o b l i q u e s h a l l o w u n d e r t h r u s t i n g of the P a c i f i c p l a t e beneath the Queen C h a r l o t t e I s l a n d s . The upper t e r r a c e u n i t c o u l d r e p r e s e n t a s e d i m e n t a r y a c c r e t i o n a r y wedge and the lower t e r r a c e u n i t - the s u b d u c t i n g s l a b . A model i n which c o m p r e s s i o n a c r o s s the Queen C h a r l o t t e t r a n s f o r m f a u l t zone i s t a k e n up by d e f o r m a t i o n of the Queen C h a r l o t t e I s l a n d s i n the form of c r u s t a l s h o r t e n i n g and t h i c k e n i n g i s not c o m p a t i b l e w i t h the t h i n c r u s t beneath the i s l a n d s and Hecate S t r a i t . i v T a b l e of Contents A b s t r a c t i i L i s t of T a b l e s v L i s t of F i g u r e s Acknowledgement v i i i C h apter I INTRODUCTION ..1 1.1 The T e c t o n i c H i s t o r y of the Queen C h a r l o t t e I s l a n d s 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 Post A c c r e t i o n a r y H i s t o r y of the Queen C h a r l o t t e I s l a n d s 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 the Queen C h a r l o t t e I s l a n d s 15 Chapter I I DATA ACQUISITION AND PROCESSING 33 2.1 Experiment 33 2.2 I n s t r u m e n t a t i o n 35 2.3 I n i t i a l Data P r o c e s s i n g 38 2.3.1 D i g i t i z a t i o n 38 2.3.2 Time and D i s t a n c e C o r r e c t i o n s 40 2.3.3 Data Q u a l i t y and F i l t e r i n g 43 . 2.4 Summary 48 Chapter I I I INTERPRETATION 4 9 3.1 M o d e l l i n g and Uniqueness 49 3.2 D e s c r i p t i o n of the M o d e l l i n g A l g o r i t h m 51 3.3 I n t e r p r e t a t i o n of I n d i v i d u a l P r o f i l e s 52 3.3.1 The F i n a l Model - A Preview 52 3.3.2 The Choice of Data S e t s f o r M o d e l l i n g 54 3.3.3 Common R e c e i v e r P r o f i l e 1 55 3.3.4 Common R e c e i v e r P r o f i l e 3 61 3.3.5 Common R e c e i v e r P r o f i l e 15 64 3.3.6 Common R e c e i v e r P r o f i l e s 16 and 17 69 3.3.7 Common Shot P r o f i l e 4 75 3.3.8 Common Shot P r o f i l e 16 79 3.4 The F i n a l Model - A Recap 83 Chapter IV DISCUSSION AND CONCLUSIONS 87 B i b l i o g r a p h y 100 Appendix A - ESTIMATION OF EXPLOSION DETONATION TIMES ...109 Appendix B - COMMON RECEIVER RECORD SECTION PLOTS 113 V L i s t of T a b l e s I . C e nozoic P l a t e I n t e r a c t i o n a t the Queen C h a r l o t t e I s l a n d s 16 I I . I nstrument Type 38 I I I . E r r o r s i n time of f i r s t sample 41 IV. T r a v e l time e r r o r s 42 v i L i s t of F i g u r e s 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 2 2. Suspect t e r r a n e s of the Canadian C o r d i l l e r a 4 3. The a c c r e t i o n of t h e A l e x a n d e r and W r a n g e l l i a t e r r a n e s w i t h N o r t h America 7 4. The a c c r e t i o n a r y h i s t o r y of W r a n g e l l i a , S t i k i n i a and the Coast P l u t o n i c Complex 9 5. P a c i f i c 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 from 80 Ma 12 6. P o s s i b l e subducted P a c i f i c s l a b beneath N o r t h A m e r i c a . 17 7. P h y s i o g r a p h i c f e a t u r e s of the Queen C h a r l o t t e I s l a n d s Region 18 8. Bathymetry of the Queen C h a r l o t t e I s l a n d s r e g i o n 20 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 l e v e l , s i n c e 10 Ma. 23 10. Contemporary v e r t i c a l l a n d movements r e l a t i v e t o sea l e v e l i n mm/yr from t i d a l s t a t i o n s 23 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 the Queen C h a r l o t t e t e r r a c e . . 25 12. C r u s t a l s t r u c t u r e a c r o s s the Queen C h a r l o t t e t r a n s f o r m f a u l t 27 13. L o c a t i o n of c o m p o s i t e P-nodal f a u l t p l a n e s o l u t i o n s . .28 14. D e s i g n of the 1983 r e f r a c t i o n experiment 34 15. V e l o c i t y s e n s i t i v i t y of the OBSs 37 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 .43 17. Power s p e c t r a of n o i s e and s i g n a l f o r an OBS 45 18. Power s p e c t r a s i g n a l and n o i s e f o r Hecate S t r a i t OBSs. 47 19. The f i n a l v e l o c i t y model 53 20. Comparison of the d a t a and s y n t h e t i c s f o r P r o f i l e 1. .56 v i i 21. F i n a l r ay t r a c i n g diagram f o r Common R e c e i v e r P r o f i l e 1. 59 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 62 23. F i n a l r ay t r a c i n g diagram f o r Common R e c e i v e r P r o f i l e 3. 63 24. Comparison of 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 15. 65 25. F i n a l r ay t r a c i n g diagram f o r Common R e c e i v e r P r o f i l e 15 67 26. Comparison of 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 70 27. F i n a l ray t r a c i n g diagram f o r Common R e c e i v e r P r o f i l e 16 71 28. Comparison of 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 17 72 29. F i n a l ray t r a c i n g diagram f o r Common R e c e i v e r P r o f i l e 17 73 30. Comparison of d a t a and s y n t h e t i c f o r Common Shot P r o f i l e 4. 76 31. F i n a l r ay t r a c i n g diagram f o r Common Shot P r o f i l e 4. .78 32. Comparison of d a t a and s y n t h e t i c f o r Common Shot P r o f i l e 16 80 33. F i n a l ray t r a c i n g diagram f o r Common Shot P r o f i l e 16. 82 34. Two p o s s i b l e models f o r t h e t e c t o n i c regime of the Queen C h a r l o t t e I s l a n d s 88 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 the so u t h e r n t i p of Moresby I s l a n d 92 36. G e o m e t r i c a l method of d e t e r m i n i n g shot o r i g i n t i m e . .110 v i i i Acknowledgement T h i s t h e s i s would not have been p o s s i b l e w i t h o u t the support and guidance of Dr. R. M. Clowes. H i s enth u s i a s m , encouragement, and h e l p f u l c r i t i c i s m was i n v a l u a b l e i n g u i d i n g me throughout my r e s e a r c h . I would a l s o l i k e t o thank Dr. R. M. E l l i s f o r c r i t i c a l l y r e a d i n g t h i s work. The s u c c e s s of the f i e l d program i s l a r g e l y the r e s u l t of the knowledgeable and c a r e f u l work of Bob Meldrum and John Bennest. Bob and John were a l s o i n s t r u m e n t a l i n b u i l d i n g the d i g i t i z i n g system as w e l l as p r o v i d i n g me w i t h i n t r o d u c t o r y l e s s o n s i n i n s t r u m e n t a t i o n and many o t h e r t o p i c s . Brad P r a g e r h e l p e d t o d e m y s t i f y the workings of computers and h e l p e d f i x many of the bugs i n my computer programs. D i s c u s s i o n s w i t h George Spence and Don White a i d e d the i n t e r p r e t a t i o n p r o c e s s g r e a t l y . Sonya D e h l e r was i n d i s p e n s a b l e i n the l a s t s t a g e s of p r e p a r a t i o n of t h i s t h e s i s . I would a l s o l i k e t o thank the people i n Vancouver who made my s t a y here t r u l y e n j o y a b l e -p a r t i c u l a r l y C h r i s P i k e , Kathy Penney, Todd and K i m i B o s t w i c k and the c o f f e e room crew. Most of a l l I would l i k e t o thank "ma c h o u e t t e " Joane Berube f o r her s c i e n t i f i c a d v i c e , her u n f l a g g i n g a s s i s t a n c e i n p r e p a r i n g the f i n a l diagrams of t h i s t h e s i s and l a s t l y , but not l e a s t , f o r her moral s u p p o r t . Data a c q u i s i t i o n was p o s s i b l e o n l y t h r o u g h the h e l p of many. The Department of F i s h e r i e s and Oceans g r a c i o u s l y p r o v i d e d the use of the s h i p C.S.S. P a r i z e a u . The c o o p e r a t i o n of her i x c a p t a i n and crew h e l p e d t o make the marine p a r t of the f i e l d program a s u c c e s s . The a s s i s t a n c e of C h i e f A l Wood and M/S George Cox, who e x p e r t l y d e t o n a t e d the e x p l o s i v e s , i s e s p e c i a l l y a p p r e c i a t e d . The h e l p of p e r s o n n e l from the P a c i f i c G e o s c i e n c e C e n t r e i s always a p p r e c i a t e d . I would a l s o l i k e t o thank P a t r i c k M o r e l and crew from t h e E a r t h P h y s i c s Branch and Dr. J . Wright f o r t h e i r work i n c a r r y i n g out the la n d - b a s e d a s p e c t of t h i s exper iment. F i n a n c i a l support f o r da t a a c q u i s i t i o n and a n a l y s i s was p r o v i d e d by NSERC O p e r a t i n g Grant A7707 and A2617; NSERC S t r a t e g i c (Oceans) G r a n t G0738; D.S.S c o n t r a c t 05SU.23235-3-1089 from the E a r t h P h y s i c s Branch, E.M.R.; D.S.S. C o n t r a c t 06SB.23227-4-0904 from the 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. Research Agreement 287, 1983/84.. A d d i t i o n a l f i n a n c i a l - ' s u p p ort was p r o v i d e d by Chevron Canada Resources L t d . 1 I . INTRODUCTION 1.1 The T e c t o n i c H i s t o r y of the Queen C h a r l o t t e I s l a n d s 1.1.1 A c c r e t i o n a r y H i s t o r y The 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 i m m e d i a t e l y e a s t of the a c t i v e p l a t e boundary between the N o r t h American and P a c i f i c p l a t e s ( f i g u r e 1 ) . To u n d e r s t a n d the t e c t o n i c h i s t o r y of the Queen C h a r l o t t e I s l a n d s i t i s n e c e s s a r y t o u n d e r s t a n d the framework upon which the N o r t h American C o r d i l l e r a was formed. Recent concensus i s t h a t throughout Mesozoic and e a r l y Cenozoic time the N o r t h American C o r d i l l e r a grew by p e r i p h e r a l a c c r e t i o n of c r u s t a l f r a g m e n t s t o the l e a d i n g edge of the c r a t o n (Monger et a l . , 1972; .Coney et a l . , 1980; Monger, 1984). The i d e n t i f i c a t i o n of t h e s e fragments or t e r r a n e s i s based p r i m a r i l y on s t r a t i g r a p h y and does not n e c e s s a r i l y imply a t e c t o n i c o r i g i n or p a s t (Coney e t a l . , 1980). However, i n t e r d i s c i p l i n a r y s t u d i e s u s i n g b i o s t r a t i g r a p h i c , p a l e o m a g n e t i c , p a l e o p h y s i o g r a p h i c , p a l e o b i o g e o g r a p h i c , and s e i s m i c t e c h n i q u e s , combined w i t h g e o l o g i c c r o s s - c u t t i n g r e l a t i o n s h i p s p r o v i d e data t h a t i s h e l p i n g t o u n r a v e l the t e c t o n i c h i s t o r y of these ' s u s p e c t ' t e r r a n e s ( S a l e e b y , 1983; Coney et a l . , 1980). A t e r r a n e i s termed ' s u s p e c t ' i f i t s p a l e o g e o g r a p h i c a l s e t t i n g i s u n c e r t a i n w i t h r e s p e c t t o the C o r d i l l e r a . I f a t e r r a n e has a p a l e o g e o g r a p h i c a l s e t t i n g t h a t i s known t o be d i f f e r e n t from the c r a t o n , i t i s c a l l e d ' a l l o c h t h o n o u s ' . The Queen C h a r l o t t e I s l a n d s a r e members of the 2 F i g u r e 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 the 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 t r a n s f o r m f a u l t 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 the 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 American p l a t e , JF - Juan de Fuca p l a t e system. 3 a l l o c h t h o n o u s 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 s o u t h e a s t A l a s k a , and the H e l l s Canyon a r e a i n e a s t e r n Oregon ( Y o r a t h and Chase, 1981; Jones e t a l . , 1977). F i g u r e 2 shows the g e o g r a p h i c a l e x t e n t of W r a n g e l l i a and o t h e r major t e r r a n e s i n the Canadian C o r d i l l e r a . The t e r r a n e s i m p o r t a n t i n the f o l l o w i n g d i s c u s s i o n a r e W r a n g e l l i a , the A l e x a n d e r t e r r a n e , S t i k i n i a , and Q u e s n e l l i a . W r a n g e l l i a , f i r s t d e f i n e d by Jones et a l . (1977), i s a Mesozoic t e r r a n e composed of M i d d l e t o Upper T r i a s s i c t h o l e i i t i c b a s a l t s and c a l c a r e o u s s e d i m e n t a r y r o c k s . Y o r a t h and Chase (1981) a l s o i n c l u d e v o l c a n i c and s e d i m e n t a r y J u r a s s i c r o c k s (Maude and Yakoun Formations on the Queen C h a r l o t t e I s l a n d s ; Bonanza Group on Vancouver I s l a n d ) as p a r t of W r a n g e l l i a . These 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 the L a t e T r i a s s i c t o Lower J u r a s s i c Karmutsen and Kunga F o r m t i o n s on the Queen C h a r l o t t e I s l a n d s ( S u t h e r l a n d Brown, 1968). No p r e - T r i a s s i c r o c k s a r e known to occur on the Queen C h a r l o t t e I s l a n d s ; however, on Vancouver I s l a n d the Karmutsen F o r m a t i o n r e s t s unconformably on the P a l e o z o i c c a l c a r e o u s sediments of the S i c k e r Group. The e x o t i c n a t u r e of W r a n g e l l i a was f i r s t d e m o nstrated by p a l eomagnetic s t u d i e s of the N i c o l a i Greenstone i n the W r a n g e l l Mountains ( H i l l h o u s e , 1977) i n d i c a t i n g t h a t W r a n g e l l i a l a y w i t h i n 15° of the p a l e o e q u a t o r i n the T r i a s s i c . T h i s r e q u i r e s t h a t W r a n g e l l i a has moved northward e i t h e r 3000 km or 6300 km (depending upon whether a n o r t h e r n or s o u t h e r n hemisphere s o l u t i o n i s f a v o u r e d ) s i n c e the T r i a s s i c t o occupy i t s p r e s e n t p o s i t i o n . Y o l e and I r v i n g (1980) r e p o r t p a l e o p o l e s 4 AX A l e x a n d e r B r i d g e R i v e r C a s s i a r Cache Creek Chugach Cascade Kootenay Monashee Olympic P a c i f i c Rim Q u e s n e l l i a S l i d e Mountain S t i k i n i a W r a n g e l l i a 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 . F i g u r e 2 - Suspect t e r r a n e s of the Canadian C o r d i l l e r a (from Monger, 1984). 5 from the Vancouver I s l a n d Karmutsen F o r m a t i o n t h a t , t o g e t h e r w i t h P a c i f i c p l a t e r e c o n s t r u c t i o n s ( H i l d e et a l . , 1977), f a v o u r an 18° s o u t h e r n hemisphere o r i g i n f o r W r a n g e l l i a . Paleomagnetic r e s u l t s (Packer and Stone, 1974) from s o u t h e r n A l a s k a i n d i c a t e t h a t the J u r a s s i c l o c a t i o n of W r a n g e l l i a was p a r a l l e l t o the p r e s e n t day l o c a t i o n of s o u t h e r n Oregon. C l e a r l y , W r a n g e l l i a 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 t e r r a n e i s an assemblage of L a t e Precambrian (~650 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 ( f i g u r e 2 ) . T i p p e r e t a l . , (1981) i n c l u d e metasedimentary and m e t a v o l c a n i c r o c k s a l o n g the western margin of the Coast Mountains as p a r t of the A l e x a n d e r t e r r a n e . In the s o u t h e r n p a r t of the t e r r a n e , L a t e T r i a s s i c r o c k s o v e r l i e Permian l i m e s t o n e s (Berg e t a l . , 1978). Paleomagnetic r e s u l t s from s o u t h e a s t A l a s k a i n d i c a t e t h a t the A l e x a n d e r t e r r a n e has undergone about 1800 km of northward d i s p l a c e m e n t r e l a t i v e t o c r a t o n i c N o r t h America between L a t e C a r b o n i f e r o u s (~280 Ma) and T r i a s s i c time (Van der Voo e t a l . , 1980). The Upper T r i a s s i c Hound I s l a n d v o l c a n i c s which o v e r l i e the P a l e o z o i c r o c k s i n d i c a t e no s i g n i f i c a n t l a t i t u d i n a l d i s p l a c e m e n t , s u g g e s t i n g t h a t the A l e x a n d e r t e r r a n e was i n i t s p r e s e n t l a t i t u d i n a l p o s i t i o n by the L a t e T r i a s s i c (~230 Ma) ( H i l l h o u s e and Gromme, 1980). A l t h o u g h p a l e o m a g n e t i c s t u d i e s i n d i c a t e t h a t each of the s e t e r r a n e s o r i g i n a t e d f a r s o u t h of t h e i r p r e s e n t p o s i t i o n l i t t l e i s known about t h e i r n o r t h w a r d j o u r n e y s . S e v e r a l d i f f e r e n t h ypotheses have been proposed i n the l i t e r a t u r e . Y o r a t h and 6 Chase (1981) r e c o n s t r u c t the h i s t o r y of the Alexander t e r r a n e and W r a n g e l l i a ( f i g u r e 3 ) . They suggest that both t e r r a n e s were d r i f t i n g northward s e p a r a t e l y u n t i l they c o l l i d e d with each other i n the Late J u r a s s i c to e a r l i e s t Cretaceous (— 140 Ma), c l o s e to t h e i r present l a t i t u d i n a l p o s i t i o n , amalgamating i n t o a l a r g e r t e r r a n e . Both the Alexander terrane and W r a n g e l l i a are i n t r u d e d by a s e r i e s of s i m i l a r synorogenic p l u t o n s that date the c o l l i s i o n (Yorath and Chase, 1981). Young (1981) gives the average age of these p l u t o n s as 144 Ma (Late J u r a s s i c ) . The Lower Cretaceous Longarm Formation c o n t a i n s c l a s t s of these p l u t o n s i n i t s coarse sandstones and conglomerates. T h i s l i t h o l o g y r e p r e s e n t s e r o s i o n due to r a p i d u p l i f t - expected when two landmasses c o l l i d e . The two t e r r a n e s were most c e r t a i n l y amalgamated by at l e a s t the Late J u r a s s i c as .both are overlapped by the Lower Cretaceous to Upper J u r a s s i c G r a v i n a - N u t z o t i n assemblage (Berg et a l . , 1972). T h i s superterrane then a c c r e t e d to the North American c r a t o n sometime in the Late Cretaceous to m i d - T e r t i a r y (90 Ma to 40 Ma). As i t approached North America, u p l i f t which has been dated by Roddick and Hutchison (1974) at 50 to 80 Ma began in the Coast P l u t o n i c Complex. Monger et a l . (1972) and Monger (1984) a l s o favour t h i s h y p othesis. Monger (1984) g i v e s a very complete review of C o r d i l l e r a n t e c t o n i c s . Between 90 Ma and 40 Ma the west coast of W r a n g e l l i a , and thus the Queen C h a r l o t t e I s l a n d s , became the a c t i v e edge of c r a t o n i c North America. I r v i n g et a l . (1985) propose a somewhat d i f f e r e n t s c e n a r i o . They report new paleomagnetic data from the Coast > 140 M I L L I O N Y E A R S A G O W R A N G E L L I A M O V E S N O R T H W A R D P R O M T H E S O U T H E R N H E M I S P H E R E SOUTHEAST ALASKA- LV^'.o ' . ' f , ; . ' QUEEN CHARLOTTE ISLANDS VANCOUVER ISLAND A B O U T 140 M I L L I O N Y E A R S A G O W R A N G E L L I A C O L L I D E S W I T H T H E A L E X A N D E R T E R R A N E I ' N O RTH ;o 'cV-^'^vAM ERICA i - ^ ' y - - ' - ' " - ' - - M» ^ 7 -x ' 90 T O 40 M I L L I O N Y E A R S A G O C O M B I N E D W R A N G E L L I A A N D A L E X A N D E R T E R R A N E D O C K W I T H N O R T H A M E R I C A 3. F i g u r e 3 - The a c c r e t i o n of the A l e x a n d e r and W r a n g e l l i a t e r r a n e s w i t h N o r t h A m e r i c a . 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 and Cameron, 1982). 8 P l u t o n i c Complex i n d i c a t i n g t h a t i t has moved 2400 km northward s i n c e the m i d - C r e t a c e o u s . T h i s p a l e o m a g n e t i c d a t a i s a l s o c o n s i s t e n t w i t h a 30° t i l t t o the southwest of the s o u t h e r n Coast P l u t o n i c Complex and the Cascades; however, they r e g a r d t r a n s l a t i o n of the r o c k s , r a t h e r than such a c o n s i s t e n t r e g i o n a l t i l t , as a more p r o b a b l e e x p l a n a t i o n . They a l s o note t h a t p a l e o p o l e s from o v e r p r i n t s i n the T r i a s s i c rock u n i t s of W r a n g e l l i a on Vancouver I s l a n d ( Y o l e and I r v i n g , 1980, Schwarz et a l . , 1980) and Q u e s n e l l i a (Symons, 1973, 1976) agree w i t h the mid-Cretaceous Coast P l u t o n i c Complex r e s u l t s and thus argue 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. T h i s p l a c e s the mid-Cretaceous p o s i t i o n of W r a n g e l l i a , the s o u t h e r n Coast P l u t o n i c Complex, the Cascades, S t i k i n i a , and perhaps Q u e s n e l l i a a t the p r e s e n t day l a t i t u d e of Mexico ( I r v i n g e t a l . , 1985). I t i s s u g g e s t e d , t h e r e f o r e , t h a t t h e s e t e r r a n e s a c c r e t e d s o u t h of t h e i r p r e s e n t l o c a t i o n and then c o n t i n u e d northward as one u n i t ( f i g u r e 4 ) . The Coast P l u t o n i c Complex r e p r e s e n t s the s u t u r e j o i n i n g S t i k i n i a w i t h W r a n g e l l i a . Concordance of m i d - T e r t i a r y p a l e o m a g n e t i c r e s u l t s from the Cascades and c r a t o n i c N o r t h America i n d i c a t e t h a t r e l a t i v e motion between the two had ceased by t h e n . I r v i n g e t a l . ' s model i n d i c a t e s t h a t the Queen C h a r l o t t e I s l a n d s became the l e a d i n g edge of the N o r t h American c o n t i n e n t by a t l e a s t the m i d - T e r t i a r y (—40 Ma). I f t h i s i s c o r r e c t then S t i k i n i a must have s l i p p e d i n b e h i n d t h e A l e x a n d e r t e r r a n e which had been l y i n g o f f s h o r e from N o r t h America c l o s e t o i t s p r e s e n t day l a t i t u d e s i n c e the L a t e T r i a s s i c , as has been suggested by 9 F i g u r e 4 - The a c c r e t i o n a r y h i s t o r y of W r a n g e l l i a , S t i k i n i a and the Coast P l u t o n i c Complex. Compare t h i s w i t h the 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 - Coast P l u t o n i c Complex. See the t e x t f o r e x p l a n a t i o n . The two s t i p p l e d bands marked T RN and T RS i n d i c a t e the two p o s s i b l e T r i a s s i c o r i g i n s ( n o r t h e r n hemisphere and s o u t h e r n hemisphere r e s p e c t i v e l y ) f o r W r a n g e l l i a and S t i k i n i a b e f o r e a c c r e t i n g t o g e t h e r (from I r v i n g e t a l . , 1985). 10 Monger and I r v i n g (1980) and Y o r a t h and Chase (1981). Subsequent t o t h i s , W r a n g e l l i a must have been smeared a l o n g the western edge of the 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 t o a t t a i n the p r e s e n t s i t u a t i o n w i t h the A l e x a n d e r t e r r a n e sandwiched between W r a n g e l l i a and S t i k i n i a ( f i g u r e 2 ) . There have been s e v e r a l p r o p o s a l s as t o what k i n d of p l a t e boundary s e p a r a t e d N o r t h America and the o c e a n i c p l a t e t h a t c a r r i e d t h e s e e x o t i c t e r r a n e s n o r t h w a r d . Van der Voo et a l . (1980) have suggested t h a t the A l e x a n d e r t e r r a n e was d i s p l a c e d a l o n g a major t r a n s c u r r e n t f a u l t r u n n i n g northward from C a l i f o r n i a . Y o r a t h and Chase (1981) and Monger and P r i c e (1979) i n f e r t h a t b oth W r a n g e l l i a and the 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 N o r t h America a l o n g a s u b d u c t i o n zone. A c o m b i n a t i o n of the two p r e v i o u s i d e a s was proposed by Monger (1984) who su g g e s t s t h a t a zone of ' t r a n s p r e s s i o n ' e x i s t e d from the L a t e C r e t a c e o u s t o e a r l y T e r t i a r y . The m o b i l e t e r r a n e s , b e i n g d i s p l a c e d northwards by the v a r i o u s P a c i f i c p l a t e s , c o l l i d e d w i t h a westward moving N o r t h American c r a t o n . I r v i n g e t a l . (1985) note t h a t the p r e s e n t day l a t i t u d i n a l s p r e a d of W r a n g e l l i a i s about t h r e e t i m e s t h a t of i t s p a l e o l a t i t u d i n a l s p r e a d i n the L a t e T r i a s s i c . P e r h a p s , W r a n g e l l i a and some of the o t h e r t e r r a n e s were smeared out a l o n g the C o r d i l l e r a 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 P o s t A c c r e t i o n a r y H i s t o r y of the Queen C h a r l o t t e I s l a n d s A f t e r the mid-Cretaceous the Queen C h a r l o t t e I s l a n d s became the l e a d i n g edge of the Canadian C o r d i l l e r a . The d e t a i l s of the t e c t o n i c regime a l o n g the c o a s t from 90 Ma t o about 30 Ma i s u n c l e a r . The complex p a t t e r n s of magnetic s t r i p e s on the P a c i f i c p l a t e and the Juan de Fuca p l a t e system, a l o n g w i t h a b s o l u t e p l a t e motions d e t e r m i n e d from the Hawaiian-Emperor c h a i n , p r o v i d e evidence f o r the p l a t e motions t h a t o c c u r r e d a l o n g the N o r t h American c o a s t . S e v e r a l d i f f e r e n t models have been p u b l i s h e d (eg. A t w a t e r , 1970; Coney, 1976; Cooper et a l . , 1976; Stone, 1977; R i d d i h o u g h , 1982a). A l l of t h e s e models r e q u i r e the e x i s t e n c e of t h r e e o c e a n i c p l a t e s west of N o r t h A m e r i c a : P a c i f i c , F a r a l l o n , and K u l a ( f i g u r e 5a, t o p ) . The now w h o l l y subducted K u l a p l a t e (Grow and A t w a t e r , 1970) i s n e c e s s a r y t o e x p l a i n the e x i s t e n c e of the A l a s k a n Magnetic B i g h t anomaly p a t t e r n (Pitman and Hayes, 1968; Grow and A t w a t e r , 1970). 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 of the K u l a -P a c i f i c and P a c i f i c - F a r a l l o n r i d g e s . The K u l a - F a r a l l o n (KF) r i d g e t r e n d i s c o n s t r a i n e d o n l y a f t e r 60 Ma because most of the r i d g e and the anomalies i t produced have been consumed by convergence ( A t w a t e r , 1970). ( I s i t not somehow s i g n i f i c a n t t h a t a l l of the magnetic anomalies which would answer our q u e s t i o n s have been subducted?) The p o s i t i o n of t h i s r i d g e w i t h r e s p e c t t o N o r t h America i s i m p o r t a n t i n d e f i n i n g the p l a t e regime a t the Queen C h a r l o t t e I s l a n d s from about 90 Ma t o 30 Ma. Cooper et a l . (1976) c o n s t r u c t e d two models: one based on p a l e o m a g n e t i c data and t h e o t h e r on a b s o l u t e p l a t e motions r-o A - North American p l a t e , K - K u l a p l a t e , F - F a r a l l o n p l a t e , P - P a c i f i c p l a t e . 0 r e p r e s e n t s the l a t i t u d e of the Queen C h a r l o t t e I s l a n d s , and SF r e p r e s e n t s the l a t i t u d e of San F r a n c i s c o . (a) - the K u l a - F a r a l 1 on-North America t r i p l e j u n c t i o n m i g r a t e s 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 Riddihough, 1982a). (b) - the Ku1a-Fara11 on-North America t r i p l e j u n c t i o n m i g r a t e s northwards from southern C a l i f o r n i a ( a f t e r Atwater, 1970). 13 d e termined from the Hawaiian-Emperor Seamount C h a i n . Both models show the 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 ( f i g u r e 5 a ) . Assuming c o n s t a n t p l a t e m o t i o n , the K u l a - F a r a l l o n - N o r t h America (KFN) t r i p l e j u n c t i o n m i g r a t e d southward t o the 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 convergence or t r a n s c u r r e n t motion of the K u l a and N o r t h American p l a t e s r e p l a c i n g t h e s u b d u c t i n g F a r a l l o n p l a t e a l o n g the boundary. The K u l a - N o r t h A m e r i c a motion v e c t o r i s p o o r l y c o n s t r a i n e d but i t appears t o have had a s i g n i f i c a n t component of s t r i k e - s l i p motion (Atwater 1970). Harper e t a l . (1981) i n f e r a r e l a t i v e v e l o c i t y of 120 mm/yr at N8°E f o r the Queen C h a r l o t t e I s l a n d s r e g i o n . The F a r a l l o n - N o r t h American r e l a t i v e motion v e c t o r was l a r g e l y c o n v e r g e n t (McKenzie and Morgan, 1969; A t w a t e r , 1970) and has been e s t i m a t e d a t 100 .mm/y at N57°E near -Vancouver I s l a n d (Harper e t a l . , 1981). Atwater (1970), Coney (1970), and A t w a t e r and Molnar (1973) r e c o n s t r u c t e d models back t o 80 Ma u s i n g r e l a t i v e p l a t e motions ( f i g u r e 5 b ). At 80 Ma, Atwater (1970) shows the K u l a - P a c i f i c -F a r a l l o n (KPF) t r i p l e j u n c t i o n o f f the c o a s t of C a l i f o r n i a w i t h the KF r i d g e p e r p e n d i c u l a r t o the c o a s t l i n e . U s i n g e x t r a p o l a t e d r e l a t i v e p l a t e motions the KFN t r i p l e j u n c t i o n m i g r a t e s northwards t o Vancouver I s l a n d by 40 Ma w i t h the F a r a l l o n - N o r t h America t r e n c h d e v e l o p i n g b e h i n d i t . The models of Cooper e t a l . (1976), A t w a t e r (1970), R i d d i h o u g h (1982a), and Stone (1977) i n d i c a t e t h a t sometime between 50 Ma and 30 Ma the KFN t r i p l e j u n c t i o n l a y j u s t n o r t h of Vancouver I s l a n d ( f i g u r e 5a and b ) . N o r t h of t h i s t r i p l e j u n c t i o n , o b l i q u e 1 4 convergence or t r a n s c u r r e n t motion between the K u l a and N o r t h American p l a t e s o c c u r r e d . South of i t the F a r a l l o n p l a t e was b e i n g subducted beneath N o r t h A m e r i c a . From 30 Ma t o the p r e s e n t the p l a t e movements a r e l e s s u n c e r t a i n and the g r o s s c h a r a c t e r i s t i c s of most models ag r e e . At 29-30 Ma the F a r a l l o n -P a c i f i c r i d g e began t o be subducted beneath C a l i f o r n i a and t r a n s c u r r e n t motion began a l o n g the San Andreas F a u l t ( f i g u r e 5 b ) . As the F a r a l l o n P l a t e descended beneath N o r t h America i t remained i n one p i e c e u n t i l t he gap c r e a t e d by the absence of s u b d u c t i o n a t the San Andreas F a u l t reached m e l t i n g t e m p e r a t u r e a t about 20 Ma ( D i c k i n s o n and Snyder, 1979). At t h i s time the two p o r t i o n s of the F a r a l l o n began t o move i n d e p e n d e n t l y and the Juan de Fuca p l a t e was born ( A t w a t e r , 1970; R i d d i h o u g h , 1982a).. A l l of the s e models a l s o p r e d i c t the s u b d u c t i o n of the KF r i d g e beneath N o r t h America near the Queen C h a r l o t t e I s l a n d s . Between 10 Ma and 20 Ma the KF r i d g e was t o t a l l y subducted beneath N o r t h America. When the KPF t r i p l e j u n c t i o n was sub d u c t e d , t r a n s c u r r e n t motion between the P a c i f i c and N o r t h American p l a t e s was i n i t i a t e d i n the Queen C h a r l o t t e I s l a n d s r e g i o n . As the K u l a c o n t i n u e d t o subduct t o the n o r t h the KPN t r i p l e j u n c t i o n m i g r a t e d northwards a l o n g the c o a s t and the t r a n s c u r r e n t P a c i f i c - N o r t h America boundary l e n g t h e n e d . By 20 Ma the K u l a p l a t e was t o t a l l y subducted and the P a c i f i c p l a t e was i n c o n t a c t w i t h N o r t h America from Vancouver I s l a n d n o r t h t o the A l e u t i a n s ( R i d d i h o u g h , 1982a; A t w a t e r , 1970); a l t h o u g h , Byrne (1979) s u g g e s t s t h a t the K u l a p l a t e may have ceased t o a c t 1 5 as an independent p l a t e a t about 55 Ma. At the same time the San Andreas t r a n s f o r m f a u l t grew i n l e n g t h as the Juan de Fuca p l a t e c o n t i n u e d t o subduct. The sou t h e r n Juan de Fuca-P a c i f i c - N o r t h America (JPN) t r i p l e j u n c t i o n m i g r a t e d northwards t o i t s p r e s e n t p o s i t i o n a t Cape Mendocino ( R i d d i h o u g h , 1982a). The n o r t h e r n JPN t r i p l e j u n c t i o n has remained s t a b l e j u s t n o r t h of Vancouver I s l a n d f o r the l a s t 10 Ma ( R i d d i h o u g h , 1977). T a b l e I p r o v i d e s a comparison of the gr o s s c h a r a c t e r i s t i c s of the f o r e g o i n g models. 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 the Queen C h a r l o t t e I s l a n d s The Queen C h a r l o t t e t r a n s f o r m f a u l t marks the p r e s e n t day boundary between the P a c i f i c and No r t h American p l a t e s . The N o r t h A m e r i c a - P a c i f i c r e l a t i v e motion i s m a i n l y s t r i k e - s l i p a t 55 mm/yr ( M i n s t e r and J o r d a n , 1978). G l o b a l p l a t e motion a n a l y s e s ( M i n s t e r e t a l . , 1974; M i n s t e r and J o r d a n , 1978) i n d i c a t e t h a t the P a c i f i c - N o r t h America r e l a t i v e motion v e c t o r t r e n d s 10° t o 20° ea s t of the t r a c e of the Queen C h a r l o t t e f a u l t ( f i g u r e 1 ) . A component of o b l i q u e u n d e r t h r u s t i n g a t a r a t e of 10 t o 20 mm/yr i s n e c e s s a r y t o be c o n s i s t e n t w i t h t h i s d i s c r e p a n c y . F i g u r e 6 i l l u s t r a t e s t he p r o b a b l e e x t e n t of the u n d e r t h r u s t s l a b g i v e n a convergence r a t e of 20 mm/yr f o r the pa s t 6 Ma. There i s an i n c r e a s i n g amount of p h y s i o g r a p h i c , g e o l o g i c , and g e o p h y s i c a l e v i d e n c e t h a t s u p p o r t s o b l i q u e s u b d u c t i o n of the P a c i f i c p l a t e beneath N o r t h America a l o n g the Queen C h a r l o t t e t r a n s f o r m f a u l t . F i g u r e 7 shows a schematic r e p r e s e n t a t i o n of Q> l~h r t fl> i-l K O C 3 l Q 10 00 to •-3 0) cr re i o (0 3 o N O O >-•• o> o o I—1 rr 0) n- ft U) 3 M rr 0» fD 3 n 0> Q> w o . rr O 3 0J fi-n-s ' ri) O C fl> (D 3 DATA SOURCE MODEL AGE (MA) 0 10 20 30 40 50 Atwater(1970) Atwater & Mo1nar(1973) Coney(197G) Constant r e l a t i v e mot i on Transform (N. America-P a c i f i c ) t Transform or o b l i q u e convergence(N. America -Kula) 1 Cooper et a 1 .( 197G ) Hot spot, seamount pa 1eomagnet1cs, r e l a t i v e motion Transform or o b l i q u e convergence (N. America -Kula) 1 I n t e r a c t i o n K u l a -F a r a l l o n r1dge-N. Amer i ca 1 Subduction ( F a r a l l o n ) Stone( 1977 ) Constant motion P a c i f i c Hotspot Transform (N. America -Pac i f i c ) 1 I n t e r a c t i o n F a r a l l o n - P a c i f i c r i d g e -N. America S u b d u c t i o n F a r a l l o n p l a t e . I n t e r a c t i o n K u l a -Fara11 on r1dge-N. America Transform or o b l i q u e convergence Kula-N. Amer i ca R1ddihough(1982a) Re 1 at i ve mot i on P a c 1 f i c - N America Transform t F a r a l l o n s u b d u c t i o n 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 ca Su b d u c t i o n ( F a r a l l o n ) 17 F i g u r e 6 - P o s s i b l e subducted P a c i f i c s l a b beneath N o r t h A m e r i c a . The shaded r e g i o n r e p r e s e n t s the p o s s i b l e e x t e n t of a subduc t e d P a c i f i c p l a t e beneath N o r t h American g i v e n t h a t o b l i q u e s u b d u c t i o n has o c c u r r e d f o r the p a s t 6 Ma a t 20 mm/yr. The l o n g arrow r e p r e s e n t s the r e l a t i v e motion of the N o r t h American and P a c i f i c p l a t e s f o r the l a s t 6 Ma. Medium l e n g t h a rrows p e r p e n d i c u l a r t o the Queen C h a r l o t t e f a u l t i n d i c a t e the d i r e c t i o n of the component of s u b d u c t i o n . The s m a l l e s t arrows i n d i c a t e t r a n s c u r r e n t motion t a k e n up a l o n g the t r a n s f o r m f a u l t s i n the r e g i o n ( a f t e r Y o r a t h and Hyndman, 1983). 18 TERRACE ACCRETIONARY OFFSHORE SEDIMENTARY WEDGE F i g u r e '7 - P h y s i o g r a p h i c f e a t u r e s of the Queen C h a r l o t t e I s l a n d s R e g i o n . T h i s t h r e e d i m e n s i o n a l d r awing shows the ob s e r v e d p h y s i o g r a p h i c f e a t u r e s t h a t a r e c o n s i s t e n t w i t h s u b d u c t i o n beneath the Queen C h a r l o t t e I s l a n d s (from Y o r a t h and Hyndman, 1983). 19 the major p h y s i o g r a p h i c f e a t u r e s a s s o c i a t e d w i t h the Queen C h a r l o t t e I s l a n d s r e g i o n . Such f e a t u r e s a re c o n s i s t e n t w i t h o t h e r known s u b d u c t i o n zones around the w o r l d . A broad g e n t l e bulge i n the ocean f l o o r i s p r e s e n t about 100 km seaward of the shore ( f i g u r e 7 and f i g u r e 8 ) . C a l l e d the Oshawa R i s e , i t extends t o the s o u t h e r n edge of the i s l a n d s and i s about 100 km wide. A p o s i t i v e f r e e - a i r g r a v i t y anomaly i s a s s o c i a t e d w i t h the r i s e . The c l o s e c o r r e l a t i o n of such a g r a v i t y anomaly w i t h the seaward t o p o g r a p h i c r i s e i s a f e a t u r e of many s u b d u c t i o n zones (Uyeda and Kanamori, 1979; Watts and T a l w a n i , 1974). S e i s m i c p r o f i l e s a c r o s s t h i s f e a t u r e i n d i c a t e t h a t the a c o u s t i c basement r i s e s w i t h the b a t h y m e t r i c h i g h , s u g g e s t i n g t h a t t h i s i s a t e c t o n i c f e a t u r e (Chase e t a l . , 1975). The bulge i s c o n s i d e r e d t o be a f l e x u r a l response t o h o r i z o n t a l c o m p r e s s i v e s t r e s s i n the o c e a n i c p l a t e caused by the convergence of the o v e r r i d i n g c o n t i n e n t a l p l a t e (Watts and T a l w a n i , 1974). F l e x u r e i n the o v e r r i d i n g c o n t i n e n t ( f i g u r e 7) has a l s o been obse r v e d i n v a r i o u s zones of convergence around the w o r l d . U p l i f t a l o n g the western margin on the Queen C h a r l o t t e I s l a n d s and s u b s i d e n c e i n Hecate S t r a i t has been documented by s e v e r a l s t u d i e s . P a r r i s h (1982) s t u d i e d u p l i f t r a t e s i n the Coast Mountains and the Queen C h a r l o t t e I s l a n d s u s i n g f i s s i o n - t r a c k d a t i n g of a p a t i t e s and z i r c o n s . U p l i f t a l o n g the c o a s t of the Queen C h a r l o t t e I s l a n d s has been o c c u r r i n g f o r the l a s t 10 Ma. R i d d i h o u g h (1982b) a l s o s u g g e s t s a s i m i l a r contemporary f l e x u r e p a t t e r n based on sea t i d a l l e v e l s , g e o d e t i c r e l e v e l l i n g , and 2 0 F i g u r e 8 - Bathymetry of the Queen C h a r l o t t e I s l a n d s r e g i o n . The Oshawa R i s e r e p r e s e n t s a br o a d g e n t l e b u l g e i n the ocean f l o o r . The Queen C h a r l o t t e t e r r a c e 1s a 25 km wide zone immediately west of the i s l a n d s . ( a f t e r Chase et a l 1975) 21 Q u a t e r n a r y beach l e v e l s . G l a c i o - i s o s t a t i c r e c o v e r y a l o n g the c o a s t i s not c o m p a t i b l e w i t h the contemporary u p l i f t p a t t e r n s o b s e r v e d ( R i d d i h o u g h , 1982b). F i g u r e 9 and f i g u r e 10 show the agreement between the two s t u d i e s . The p a t t e r n of u p l i f t a l o n g the Queen C h a r l o t t e I s l a n d s and landward s u b s i d e n c e i s c o n s i s t e n t w i t h o t h e r o b l i q u e zones of compression or s u b d u c t i o n around the w o r l d (eg. Sangami Trough, Japan; A l p i n e F a u l t , New Z e a l a n d ) . The v e r t i c a l movement (+1 mm/yr) a l o n g the Queen C h a r l o t t e t r a n s f o r m f a u l t i s c o m p a t i b l e w i t h a p o s i t i o n c l o s e t o a t r a n s f o r m or h i g h l y o b l i q u e c onvergent p l a t e boundary ( R i d d i h o u g h , 1982b). The s u b s i d e n c e of Hecate S t r a i t and Queen C h a r l o t t e Sound may a l s o be a r e s u l t of f l e x u r e caused by the o b l i q u e s u b d u c t i o n of t h e P a c i f i c p l a t e . Y o r a t h and Hyndman (1983) used a s e r i e s of w e l l s i n Hecate S t r a i t and Queen C h a r l o t t e Sound t o c o n s t r u c t 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 the b a s i n . They c o n c l u d e d t h a t 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 p e r i o d of u p l i f t and t h e r e f o r e , t h a t o b l i q u e u n d e r t h r u s t i n g began a t t h a t t i m e . T h i s t i m i n g i s c o r r o b o r a t e d by a new st u d y i n d i c a t i n g t h a t the P a c i f i c p l a t e changed motion t o a more conv e r g e n t d i r e c t i o n w i t h r e s p e c t t o N o r t h America a t 5 Ma (Cox and E n g e b r e t s o n , 1985). They c o r r e l a t e d a s m a l l bend i n the Hawaiian-Emperor seamount c h a i n w i t h g e o l o g i c e v i d e n c e f o r convergence a t 5 Ma a l o n g the San A ndreas F a u l t . T h i s change i n p l a t e motion at 5 Ma c o u l d have c a u s e d the P a c i f i c p l a t e t o b e g i n s u b d u c t i n g o b l i q u e l y b eneath t h e Queen C h a r l o t t e I s l a n d s . The Queen C h a r l o t t e t r o u g h resembles t r e n c h s i n a r e a s were 22 F i g u r e 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 l e v e l , s i n c e 10 Ma. The u p l i f t f i g u r e s i n the n o r t h e r n c o a s t mountains r e p r e s e n t maximum v a l u e s (from P a r r i s h , 1982). 23 F i g u r e 10 - Contemporary v e r t i c a l l a n d movements r e l a t i v e t o sea l e v e l i n mm/yr from t i d a l s t a t i o n s . (from R i d d i h o u g h , 1982b). 24 s h a l l o w s u b d u c t i o n i s known t o o c c u r (eg. C h i l e - P e r u t r e n c h ) . I t has been documented b o t h on bathymetry c h a r t s and s e i s m i c p r o f i l e s ( f i g u r e 8 and f i g u r e 11). The t r o u g h i s 300 km l o n g , up t o 55 km wide and i s between 2.8 t o 3.0 km deep (Chase e t a l . , 1975). S e i s m i c p r o f i l e s a c r o s s the t r o u g h i n d i c a t e t h a t the o c e a n i c basement d i p s l a n dward. The basement r e f l e c t i o n s , however, end a b r u p t l y under the t e r r a c e . The s e i s m i c p r o f i l e s a l s o i n d i c a t e t h a t the t r o u g h i s f i l l e d w i t h f l a t l y i n g sediments which i s c h a r a c t e r i s t i c of a C h i l e - P e r u t y p e t r e n c h . From the mountain summits on the Queen C h a r l o t t e I s l a n d s t o th e t r o u g h , the g r a d i e n t of t h e c o n t i n e n t a l s l o p e i s v e r y c o n s t a n t (about 7°). Even the 25 km wide Queen C h a r l o t t e t e r r a c e o n l y s l i g h t l y i n t e r r u p t s t h i s t r e n d . The t e r r a c e i s a r e l a t i v e l y narrow f e a t u r e and i s c h a r a c t e r i z e d by two s t e e p s c a r p s - one a d j a c e n t t o the s h o r e and a second 25 km from the shore t h a t drops t o the a b y s s a l deeps ( f i g u r e 8 ) . S e i s m i c p r o f i l e s (Chase et a l . , 1975; D a v i s and Seemann, 1981) show t h a t t h e t e r r a c e i s composed of deformed s t r a t a w i t h h i g h a m p l i t u d e f o l d s ( 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 s u g g e s t s t h a t a t h i c k s e d i m e n t a r y sequence u n d e r l i e s i t (Couch, 1969; S r i v a s t a v a e t a l . , 1971; S r i v a s t a v a , 1973; R i d d i h o u g h , 1981; C u r r i e e t a l . , 1980). The Queen C h a r l o t t e t e r r a c e ( f i g u r e 7 ) , may be an a c c r e t i o n a r y s e d i m e n t a r y wedge ( Y o r a t h and Hyndman 1983). A p r e v i o u s r e f r a c t i o n s u r v e y (Horn e t a l . , 1984; Horn,1982) c r o s s e d the Queen C h a r l o t t e f a u l t zone a t 52.25°N. The 25 10 20 30 40 KILOMETERS F i g u r e 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 the Queen C h a r l o t t e t e r r a c e . T h i s l i n e drawing 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 the f l a t l y i n g sediments d e p o s i t e d i n the Queen C h a r l o t t e t r o u g h c r e a t e d by downwarping of the basement j u s t west of the t e r r a c e edge. The t e r r a c e appears t o be deformed s e d i m e n t s . The basement i s not v i s i b l e beneath the t e r r a c e (from Chase et a l . , 1975). 26 i n t e r p r e t a t i o n i s shown i n f i g u r e 12. T h i s i n t e r p r e t a t i o n i n d i c a t e s t h a t the t e r r a c e i s composed of a t h i n low v e l o c i t y h i g h g r a d i e n t l a y e r ( u n i t 6 i n f i g u r e 12) o v e r l y i n g a h i g h e r v e l o c i t y lower g r a d i e n t l a y e r ( u n i t 7 i n f i g u r e 12) t h a t extends t o a t l e a s t 10 km depth. T h i s i s c o n s i s t e n t w i t h an a c c r e t i o n a r y wedge of h i g h l y compressed sediments ( u n i t 6) 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 c r u s t ( u n i t 7) (Horn et a l . , 1984). The model i s not w e l l c o n s t r a i n e d below about 10 km because of the d e s i g n of the experiment. S e i s m i c i t y and heat f l o w s t u d i e s conducted i n the Queen C h a r l o t t e I s l a n d s r e g i o n a l s o i n d i c a t e t h a t a c o m p r e s s i v e t e c t o n i c regime i s a c t i v e a l o n g the Queen C h a r l o t t e t r a n s f o r m f a u l t a d j a c e n t t o Moresby I s l a n d . Recent m i c r o s e i s m i c i t y s t u d i e s (Hyndman and E l l i s , 1981; Berube, 1985) i n d i c a t e t h a t most of the s e i s m i c a c t i v i t y o c c u r s a l o n g t h i s s c a r p of the t e r r a c e , i n d i c a t i n g t h a t the i n n e r s c a r p i s the a c t i v e p l a t e boundary. Berube (1985) l o c a t e d about 130 e a r t h q u a k e s from a t o t a l of 310 r e c o r d e d over a 3 month p e r i o d i n 1983. She c o m p i l e d s e v e r a l composite P-nodal f a u l t p l a n e s o l u t i o n s f o r c l u s t e r s of e a r t h q u a k e s ( f i g u r e 13). The composite s o l u t i o n f o r the c l u s t e r a l o n g the Queen C h a r l o t t e f a u l t a d j a c e n t t o Graham I s l a n d shows t h a t m o s t l y s t r i k e - s l i p motion i s o c c u r r i n g a l o n g the n o r t h e r n p o r t i o n of the f a u l t . One composite s o l u t i o n from an earthquake c l u s t e r on the f a u l t a d j a c e n t t o Moresby I s l a n d shows a t h r u s t mechanism. Another s o l u t i o n , from an earthquake swarm i n the 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 f a u l t i n g w i t h the ocean s i d e down. A c l u s t e r of e v e n t s from i n l a n d Graham I s l a n d , 27 0 20 Distance (km) 40 60 80 w 100 North America Plate Q . C. Terrace Pacific Plate o C\2 O CO V.E.: 3.5:1 F i g u r e 12 - C r u s t a l s t r u c t u r e a c r o s s the Queen C h a r l o t t e T r a n s f o r m f a u l t . (from Horn e t a l . , 1984). 28 F i g u r e 13 - L o c a t i o n of composite P-nodal f a u l t p l a n e s o l u t i o n s . The X's e n c l o s e d i n c i r c l e s r e p r e s e n t c l u s t e r s 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 s o l u t i o n s from 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 motion p a r a l l e l t o the f a u l t zone. (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 motion 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 the 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 mechanism s o l u t i o n s . (4) t h r u s t mechanism s o l u t i o n . ( a f t e r Berube, 1985). 29 t h a t cannot be c o r r e l a t e d w i t h any known f a u l t s , a l s o has a w e l l c o n s t r a i n e d t h r u s t mechanism. A l l of the t h r u s t 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 w i t h o b l i q u e convergence of the P a c i f i c p l a t e . No e a r t h q u a k e s i n d i c a t i n g the p r e s e n c e of a B e n i o f f zone were i d e n t i f i e d . P r e v i o u s s t u d i e s 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 i s convergence or c o m p r e s s i v e s t r e s s a c r o s s the Queen C h a r l o t t e f a u l t zone. Two e a r t h q u a k e s (1949, 1970) have w e l l c o n s t r a i n e d f i r s t motion f a u l t p l a n e s o l u t i o n s . The motion f o r the 1949 magnitude 8.1 earthquake i s m a i n l y s t r i k e - s l i p w i t h a s m a l l t h r u s t component (Rogers, 1983). The h o r i z o n t a l motion i s p a r a l l e l t o the f a u l t p l a n e (about 15° d i f f e r e n t from P a c i f i c - N o r t h America r e l a t i v e m o t i o n ) . T h i s i n d i c a t e s t h a t the c o m p r e s s i v e s t r e s s was not r e l e a s e d i n t h i s e arthquake (Rogers, 1983). B o s t w i c k (1984) s u g g e s t s t h a t the f i r s t m otion f a u l t 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 the c h a r a c t e r of motion a l o n g t h e r u p t u r e . He found a l a r g e d i f f e r e n c e between the a f t e r s h o c k zone and the r u p t u r e l e n g t h s u g g e s t i n g t h a t the d i s p l a c e m e n t o f f s e t a l o n g the f a u l t was u n e v e n . i F i r s t m otion f a u l t p l a n e s o l u t i o n s from the 1970, magnitude 7.0, e a r t h q u a k e show a t h r u s t mechanism w i t h a d i p of 50° t o the e a s t . The s u r f a c e e x p r e s s i o n of t h i s e a r t h q uake has been i d e n t i f i e d on SEA MARK imaging of the s e a f l o o r (G.C. Rogers and E.E. D a v i s , p e r s o n a l communication, 1985). The newly l o c a t e d f a u l t s c a r p t r e n d s 10° more n o r t h - s o u t h than the Queen C h a r l o t t e f a u l t . T h i s l a r g e e a r t h q u a k e i n d i c a t e s a s i g n i f i c a n t component 30 of convergence (Rogers, 1983). Recent s t u d i e s i n d i c a t e t h a t t h e p r e s e n t and p a l e o - h e a t f l o w i n the 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 Ma of s u b d u c t i o n . Low heat f l o w e x t e n d i n g 50-200 km i n l a n d from the t r e n c h a x i s has been o b s e r v e d i n many s u b d u c t i o n zones. A c o n t i n u o u s t r a n s i t i o n from h i g h heat f l o w i n the Queen C h a r l o t t e t r o u g h , t o i n t e r m e d i a t e v a l u e s on t h e t e r r a c e , t o low c o n t i n e n t a l heat f l o w on the Queen C h a r l o t t e I s l a n d s was obse r v e d by Hyndman e t a l . ( 1 9 8 2 ) . The heat f l o w v a l u e s d e c r e a s e by a f a c t o r of t h r e e from west t o e a s t . The main t h e r m a l c o n t r a s t i s l o c a t e d a t t h e seaward edge of the t e r r a c e . The average heat f l o w i n t h e Queen C h a r l o t t e t r o u g h i s c l o s e t o the t h e o r e t i c a l v a l u e f o r the age of the 7 Ma P a c i f i c o c e a n i c c r u s t near the Queen C h a r l o t t e I s l a n d s . The Queen C h a r l o t t e I s l a n d s heat f l o w v a l u e (47 mWm"2) i s the o n l y measurement on the i s l a n d but i s s i m i l a r t o o t h e r a r e a s i n the c o a s t I n s u l a r B e l t (Hyndman e t a l . , 1982). I t i s a l s o s i m i l a r t o the c o a s t a l zone between Cape Mendocino and Vancouver I s l a n d which has a u n i f o r m heat f l o w w i t h a mean of 42 mWm"2 (Sass e t a l . , 1985). These v a l u e s a r e c l o s e r t o what would be e x p e c t e d above a sh a l l o w s u b d u c t i o n zone (25-35 mWm"2) than the c h a r a c t e r i s t i c heat f l o w of 70-80 m W i r r 2 w i t h i n 50-100 km of the San Andreas t r a n s f o r m f a u l t (Sass e t a l . , 1985). N u m e r i c a l m o d e l l i n g by Hyndman e t a l . (1982) i n d i c a t e s t h a t the o b s e r v a t i o n s a r e c o n s i s t e n t w i t h o b l i q u e s u b d u c t i o n and t h a t a " s t e a d y - s t a t e " o c e a n / c o n t i n e n t boundary cannot s a t i s f y the d a t a . They suggest t h a t t h e s u b d u c t i n g s l a b i s a c t i n g as a heat s i n k beneath t h e 31 s Queen C h a r l o t t e I s l a n d s and Hecate S t r a i t . A new study i n s o u t h e a s t e r n A l a s k a (Sass e t a l . , 1985), where the p l a t e motion i s p a r a l l e l t o the f a u l t s t r i k e , i n d i c a t e s t h a t the mean heat f l o w t h e r e i s h i g h e r ( a t about 59 mWrrr2) than the average v a l u e f o r the I n s u l a r B e l t (47mWm"2). T h i s may i n d i c a t e t h a t the n o r t h e r n edge of a subducted P a c i f i c s l a b does not extend as f a r as s o u t h e a s t A l a s k a . The p a l e o - h e a t f l o w of Hecate S t r a i t and Queen C h a r l o t t e Sound has a l s o been s t u d i e d . Heat f l o w measurements made i n w e l l s d r i l l e d i n Hecate S t r a i t and Queen C h a r l o t t e Sound i n d i c a t e t h a t the p r e s e n t heat f l o w i s lower i n both these a r e a s than i n the p a s t . Y o r a t h and Hyndman (1983) have l o o k e d a t e s t i m a t e s of p a l e o - h e a t f l o w from v i t r i n i t e r e f l e c t a n c e d a t a from the H e c a t e S t r a i t and Queen C h a r l o t t e Sound w e l l s . V i t r i n i t e 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 r e l a t e d t o i t s pa s t t h e r m a l h i s t o r y . These s t u d i e s i n d i c a t e , t h a t a t l e a s t i n Queen C h a r l o t t e Sound, the p a l e o - h e a t f l o w was p r o b a b l y t w i c e t h a t of the p r e s e n t day heat f l o w . A c c o r d i n g t o Y o r a t h and Hyndman (1983), the reduced p r e s e n t day heat f l o w i s a consequence of 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 p h y s i o g r a p h i c , g e o l o g i c a l , g e o p h y s i c a l and t e c t o n i c e v i d e n c e p r e s e n t e d i n t h i s c h a p t e r p r o v i d e s j u s t i f i c a t i o n f o r the l a r g e o n s h o r e - o f f s h o r e r e f r a c t i o n s e i s m i c experiment conducted a c r o s s the f a u l t zone t o t h e main l a n d of B r i t i s h Columbia i n 1983. Much e v i d e n c e s u g g e s t s t h a t t h e r e i s o b l i q u e convergence a l o n g the Queen C h a r l o t t e f a u l t zone, a t l e a s t a l o n g Moresby I s l a n d , where the d i s c r e p a n c y between the P a c i f i c - N o r t h 32 America r e l a t i v e motion v e c t o r and the t r e n d of the f a u l t i s the g r e a t e s t . 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 area s u p p o r t i n g b oth t r a n s c u r r e n t motion and p r o b a b l y some form of compression or u n d e r t h r u s t i n g . The r e f r a c t i o n experiment was de s i g n e d t o a d d r e s s some of the q u e s t i o n s t h a t have been r a i s e d about t h i s e n i g m a t i c a r e a . 33 I I . DATA ACQUISITION AND PROCESSING 2.1 Experiment In August 1983, the Canadian COCRUST group ( r e p r e s e n t e d by the U n i v e r s i t y of B r i t i s h C o l u m b i a , the P a c i f i c G e o s c i e n c e Centre and the E a r t h P h y s i c s B r a n c h f o r t h i s p r o j e c t ) c a r r i e d out 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 survey from the deep ocean a c r o s s n o r t h e r n Moresby I s l a n d and Hecate S t r a i t , t o the mainland of B r i t i s h Columbia ( f i g u r e 14). The o b j e c t i v e s of t h i s experiment were t o dete r m i n e t h e l i t h o s p h e r i c s t r u c t u r e (1) of the Queen C h a r l o t t e f a u l t zone, (2) below the Queen C h a r l o t t e I s l a n d s , and (3) below Hecate S t r a i t . S i x ocean bottom seismographs (OBS) were d e p l o y e d a l o n g the r e f r a c t i o n l i n e , f o u r e a s t of Moresby I s l a n d i n Hecate S t r a i t and two west of Moresby I s l a n d on the Queen C h a r l o t t e t e r r a c e . E i g h t l a n d - b a s e d s t a t i o n s were de p l o y e d a c r o s s Moresby I s l a n d and t h r e e more on the m a i n l a n d and i s l a n d s e a s t of Hecate S t r a i t . Twelve 540 km 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 timed f u s e s , e v e r y 10 km a l o n g a l i n e e x t e n d i n g westward from 25 km o f f s h o r e . In between each 540 km c h a r g e , two 60 kg c h a r g e s were d e t o n a t e d making the shot s p a c i n g a l o n g the l i n e 3 km. The p e l l e t e d TNT e x p l o s i v e NITROPEL® was used f o r a l l of the c h a r g e s . To c a l c u l a t e the time of d e t o n a t i o n ( o r i g i n t i m e ) f o r each s h o t , the output from a hydrophone towed be h i n d the s h i p and a geophone p l a c e d on the s h i p deck were r e c o r d e d a l o n g w i t h the WWVB r a d i o time s i g n a l on an FM tape r e c o r d e r and m o n i t o r e d on a c h a r t r e c o r d e r . A vQ C fD I a ro w 3 OO oo ro t-ti o o 3 fD TJ fD 3 fD 3 .54° AN L53° Hecate Strait 13. 7 > » ' 10 pi Moresby Island ^0 16 19* 25 >• 28 31 / 3 V * 152° 0 10 20 30 40 50 Island B'anks^- \ Island 134° 133° 132° A Land stations B Ocean bottom seismographs * Big shots (540 kg) — Shot line .... Air gun line 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 time c a l c u l a t i o n i s g i v e n i n Appendix A. A 32 1 (2000 i n 3 ) a i r g u n was used t o p r o v i d e 3 a d d i t i o n a l r e f r a c t i o n p r o f i l e s : (1) over the two OBSs west of the Queen C h a r l o t t e s , (2) over the OBSs i n Hecate S t r a i t , and (3) p e r p e n d i c u l a r t o the OBSs i n Hecate S t r a i t . The a i r g u n was f i r e d once a minute a t a nominal p r e s s u r e of 13.8 MPa (2000 p s i ) . When the a i r g u n was d e p l o y e d the s h i p steamed at 11 km/hr g i v i n g a shot s p a c i n g of about 200 m. The e x p e r i m e n t a l p a t t e r n was d e s i g n e d t o p r o v i d e good coverage a c r o s s the Queen C h a r l o t t e t r a n s f o r m f a u l t and Moresby I s l a n d w h i l e s a t i s f y i n g e n v i r o n m e n t a l c o n s t r a i n t s which d i d not a l l o w the d e t o n a t i o n of charges i n Hecate S t r a i t or i n the s h a l l o w waters west of Moresby I s l a n d . 2.2 I n s t r u m e n t a t i o n The s i x OBSs used i n t h i s experiment were c o n s t r u c t e d i n the Department of G e o p h y s i c s and Astronomy a f t e r a d e s i g n from the A t l a n t i c G e o s c i e n c e C e n t r e ( H e f f l e r and B a r r e t , 1979). S u b s t a n t i a l t e c h n i c a l r e v i s i o n s have been made s i n c e the p u b l i c a t i o n of the paper; most n o t a b l y b e i n g a new r e l e a s e mechanism and the use of Benthos® g l a s s spheres f o r f l o t a t i o n . The OBSs a r e equipped w i t h two 4.5 Hz seismometers (one h o r i z o n t a l and one v e r t i c a l ) and a hydrophone. The output from these components p l u s an i n t e r n a l l y g e n e r a t e d c l o c k s i g n a l a re r e c o r d e d i n d i r e c t mode onto a 4 c h a n n e l c a s s e t t e t a p e . To enable s u f f i c i e n t l y l o n g deployment the tape speed was s e t at 3 6 0.2 mm/s. The i n t e r n a l time code i s an a m p l i t u d e modulated 10 Hz c a r r i e r f r e q u e n c y . The envelope of the h i g h f r e q u e n c y p a r t of t he hydrophone c h a n n e l was superimposed on the time t r a c k t o h e l p i d e n t i f y the water wave a r r i v a l a c c u r a t e l y . The frequency response of the whole system ( i n c l u d i n g 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 15). Four of the 11 la n d - b a s e d s t a t i o n s were Teledyne Geotech MCR-600 M i c r o c o r d e r s which r e c o r d i n d i g i t a l format a t 60 samples per second ( s p s ) . The a n t i - a l i a s i n g f i l t e r , w i t h a c o r n e r a t 9.5 Hz, combined w i t h a Mark P r o d u c t s L-4C 1 Hz v e r t i c a l component seismometer b a n d l i m i t s the system between 1.0 and 9.5 Hz. R e c e i v e r 9 was a slow speed (15/160 i p s - 0.238 mm/s) 7 - t r a c k Geotech FM a n a l o g r e c o r d e r . The o u t p u t s from one v e r t i c a l -and one h o r i z o n t a l seismometer were r e c o r d e d (each a t 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 t r a c k s a l o n g w i t h the WWVB time code on a f i f t h t r a c k . Both the v e r t i c a l and h o r i z o n t a l seismometers were W i l l m o r e MK I I models. The remainder of the seismographs used were EMR Mark I I d i g i t a l i n s t r u m e n t s (Backpacks) d e s i g n e d and b u i l t by the E a r t h P h y s i c s B r a n c h . These r e c o r d e r s sampled a t 60 sps and were b a n d l i m i t e d between 2 Hz and 25 Hz. Two Hz Mark P r o d u c t s L4A v e r t i c a l component seismometers were used w i t h the Backpacks. Table I I l i s t s t he i n s t r u m e n t type f o r each r e c e i v e r . 37 F i g u r e 15 - V e l o c i t y s e n s i t i v i t y of the OBSs. 38 R e c e i v e r Number Instrum e n t Type 1 2 11 12 13 14 3 4 6 15 16 17 9 5 7 8 10 OBS (a n a l o g ) Backpack ( d i g i t a l ) FM (a n a l o g ) M i c r o c o r d e r ( d i g i t a l ) T a b le I I - Instrument Type 2.3 I n i t i a l Data 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 A PDP 11/34 was used t o d i g i t i z e the OBS and FM a n a l o g d a t a and t o p r o v i d e p l o t s f o r q u a l i t y c o n t r o l . Much of the data 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 the a u t h o r . The PDP 11/34 data h a n d l i n g package was used t o d i g i t i z e , d e m u l t i p l e x , o r g a n i z e , and p l o t the d a t a . The OBSs, i n p a r t i c u l a r , p r e s e n t e d some problems f o r d i g i t i z a t i o n . The OBSs have a very slow tape speed t o accommodate l o n g deployment t i m e s and r e c o r d i n g on a s i n g l e c a s s e t t e . The l o g i s t i c s of the 1983 Queen C h a r l o t t e I s l a n d s experiment r e q u i r e d t h a t the OBSs i n Hecate S t r a i t be c a p a b l e of r e c o r d i n g d a t a over a p e r i o d of 6 days. S i n c e the r e f r a c t i o n experiment was conducted 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 s tudy (Berube, 39 1985) on the Queen C h a r l o t t e I s l a n d s , i t was d e s i r a b l e t o have the OBSs d e p l o y e d as l o n g as p o s s i b l e . Because of the slow speed of the OBS tape r e c o r d e r p l a y i n g the d a t a back a t 47.6 mm/s (1 7/8 i p s ) on a c o n v e n t i o n a l h i g h q u a l i t y c a s s e t t e deck i n t he l a b would s e r i o u s l y l i m i t the r e a l i z a b l e d i g i t i z a t i o n r a t e due t o the maximum thr o u g h p u t c a p a b i l i t y of t h e d i g i t i z i n g system. T h e r e f o r e , the f i e l d r e c o r d i n g s were t r a n s f e r r e d t o 1/4 i n c h tape u s i n g a h i g h q u a l i t y FM tape r e c o r d e r g i v i n g a f i n a l speed-up f a c t o r between the f i e l d and l a b t a p e s of 30 t i m e s . Each of the f o u r c h a n n e l s was d i g i t i z e d s i m u l t a n e o u s l y a t 120 samples per second (sps) g i v i n g a t o t a l t h r o u g h p u t r a t e of 14,400 sps. I t was f e l t t h a t the slow OBS tape r e c o r d e r speed would i n t r o d u c e s i g n i f i c a n t wow and f l u t t e r i n the r e c o r d e d d a t a . As w e l l , the speed of each OBS tape r e c o r d e r was s e t i n d e p e n d e n t l y and thus v a r i e d from i n s t r u m e n t t o i n s t r u m e n t . To overcome th e s e problems the d i g i t i z i n g r a t e f o r the OBS d a t a was s l a v e d t o the 10 Hz time code c a r r i e r f r e q u e n c y , r e c o r d e d p a r a l l e l w i t h the 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 i n t h e Department of G e o p h y s i c s and Astronomy. The d e v i c e i s s i m i l a r , i n p r i n c i p l e , t o a f l u t t e r compensation c i r c u i t used i n m u l t i - c h a n n e l FM tape r e c o r d e r s . By s l a v i n g the s a m p l i n g r a t e t o t he time code, speed v a r i a t i o n s of up t o one Hz a r e w e l l c o r r e c t e d and t h e problems of tape s t r e t c h and speed v a r i a t i o n s a r e a v o i d e d . Spot checks of the d i g i t i z e d d a t a v e r i f y t h a t f l u c t u a t i o n s i n s a m p l i n g r a t e a r e l e s s than 0.1 s p s . To a v o i d a l i a s i n g , a 2-pole lowpass a n a l o g f i l t e r ( S a l l e n and 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 used t o f i l t e r t he OBS 40 data b e f o r e d i g i t i z i n g . 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 sps u s i n g the PDP 11/34 system. The d i g i t i z i n g r a t e was c o n t r o l l e d e x t e r n a l l y by an a c c u r a t e frequency g e n e r a t o r . Sampling r a t e f l u c t u a t i o n s due t o tape s t r e t c h and tape speed v a r i a t i o n s were l e s s than 0.1 sps. The FM a n a l o g system i t s e l f p r o v i d e d a s u f f i c i e n t a n t i -a l i a s i n g f i l t e r f o r the sa m p l i n g r a t e chosen (the response c u r v e i s 3 db down a t 20 Hz w i t h a r o l l o f f of 30 d b / o c t a v e ) . 2.3.2 Time and D i s t a n c e C o r r e c t i o n s A l l shot l o c a t i o n s and OBS p o s i t i o n s were d e t e r m i n e d from LORAN C n a v i g a t i o n supplemented by s a t e l l i t e f i x e s when a v a i l a b l e . R e l a t i v e a c c u r a c y of t h e s e p o s i t i o n s i s about 200 m w i t h an a b s o l u t e a c c u r a c y of 300 m (Hyndman e t a l . , 1979). The l a n d r e c e i v e r s 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 of about 150 m. T r a v e l time e r r o r s i n t r o d u c e d by l o c a t i o n e r r o r s a re n e g l i g i b l e compared t o o r i g i n t ime e r r o r s and p i c k i n g e r r o r s . An i m p o r t a n t problem i n marine r e f r a c t i o n s t u d i e s u s i n g timed f u s e - d e t o n a t e d e x p l o s i v e s i s the a c c u r a t e e s t i m a t i o n of shot d e p t h . The timed f u s e s have 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 n o n - l i n e a r l y w i t h depth and t h e r e f o r e cannot be used t o a c c u r a t e l y e s t i m a t e the depth of d e t o n a t i o n . Two independent methods were used t o e s t i m a t e shot depths and a r e d e s c r i b e d i n d e t a i l i n Appendix A. The maximum p r o b a b l e e r r o r i n shot o r i g i n t i m e s caused by shot depth e r r o r s i s about ± 0.03 seconds (Appendix A ) . 41 The i n t e r n a l c l o c k s i n the OBSs and M i c r o c o r d e r s were c o r r e c t e d f o r d r i f t . 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 deployment and imm e d i a t e l y a f t e r r e c o v e r y . The M i c r o c o r d e r i n t e r n a l c l o c k s were r a t e d b e f o r e and a f t e r each day of s h o o t i n g . D r i f t f o r both the OBSs and the M i c r o c o r d e r s was assumed t o be l i n e a r between r a t i n g s . T h i s assumption i s p r o b a b l y v a l i d f o r the OBSs s i n c e , once d e p l o y e d , they a re h e l d a t a c o n s t a n t temperature by the s u r r o u n d i n g seawater. A t e s t of l i n e a r i t y of d r i f t f o r one of the M i c r o c o r d e r s used i n the experiment was done by Berube (1985) on the Queen C h a r l o t t e I s l a n d s j u s t a f t e r our experiment c o n c l u d e d . She p e r i o d i c a l l y r a t e d the M i c r o c o r d e r i n t e r n a l c l o c k w i t h WWVB over a p e r i o d of 12 days and found t h a t the assumption of l i n e a r d r i f t was v a l i d . The FM a n a l o g 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 the s e i s m i c c h a n n e l s and thus no d r i f t c o r r e c t i o n s were n e c e s s a r y . E r r o r s i n e s t i m a t e s of the time of f i r s t samples of the d i g i t i z e d t r a c e s a re summarized i n Table I I I . R e c e i v e r Time of F i r s t Sample E r r o r (seconds) OBS M i c r o c o r d e r FM Ana l o g ± 0.013 ± 0.006 ± 0.006 Tab l e I I I - E r r o r s i n time of f i r s t sample 42 The d a t a from the Backpacks were s u p p l i e d by E a r t h P h y s i c s Branch w i t h o u t documentation of d r i f t c o r r e c t i o n s or c l o c k r a t i n g s . These e r r o r s , as w e l l as p i c k i n g e r r o r s , g i v e t o t a l t r a v e l time e r r o r s of ± 0.05 s t o ± 0.10 s f o r good q u a l i t y f i r s t a r r i v a l s . T a b l e IV shows a l i s t of p r o b a b l e maximum e r r o r s f o r d i f f e r e n t p o r t i o n s of the d a t a s e t . R e c e i v e r T r a v e l Time E r r o r (seconds) 1 2 ( s h o t s 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 Table IV - T r a v e l time e r r o r s 2.3.3 Data Q u a l i t y and F i l t e r i n g The l a n d s t a t i o n s l o c a t e d on the Queen C h a r l o t t e I s l a n d s r e c o r d e d v e r y good d a t a . An exemplary power p e r i o d o g r a m ( f i g u r e 16) shows t h a t the s e i s m i c s i g n a l i s b a n d l i m i t e d between 1.5 Hz and 6 Hz w i t h a peak a t about 3 Hz. The t h r e e l a n d r e c e i v e r s on the m a i n l a n d r e c o r d e d good d a t a w i t h the same 43 F i g u r e 16 - Periodogram power f o r a l a n d s t a t i o n . T h i s power spectrum was c a l c u l a t e d over a 2 second window of the 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. 4 4 f r e q u e n c y c h a r a c t e r i s t i c s as the i s l a n d r e c e i v e r s . The low f r e q u e n c y of t h e s e i s m i c s i g n a l was a c h a r a c t e r i s t i c of a l l the l a n d r e c e i v e r d a t a . The 60 kg c h a r g e s at f a r o f f s e t s were not w e l l r e c o r d e d . R e c e i v e r s 7,8,9, and 10 were c l o s e enough t o the w a t e r t o r e c o r d ocean i n d u c e d m i c r o s e i s m s . The f r e q u e n c y o f the n o i s e was below 1.5 Hz and was easy remove w i t h f i l t e r i n g . A l l f i r s t a r r i v a l p i c k s were made on u n f i l t e r e d d a t a w i t h the a i d of f i l t e r e d s e c t i o n s . The two OBSs west of t h e Queen C h a r l o t t e I s l a n d s r e c o r d e d good q u a l i t y d a t a t o a d i s t a n c e of about 85 km. P a s t t h i s d i s t a n c e f i r s t a r r i v a l s were below the n o i s e l e v e l (see r e c o r d s e c t i o n i n Appendix B ) . Prominent secondary a r r i v a l s are v i s i b l e on r e c e i v e r 1 f o r a l l s h o t s . F i l t e r i n g o f t h e s e d a t a d i d not improve th e s e c t i o n beyond a d i s t a n c e , of 85 km. F i g u r e 17a shows the power p e r i o d o g r a m f o r a 2 second i n t e r v a l of s e i s m i c s i g n a l and 17b the same f o r a p o r t i o n of background n o i s e . A comparison of t h e s e two s p e c t r a i n d i c a t e s t h a t most of the s e i s m i c energy i s below 5 Hz. T h i s i s c o n s i s t e n t w i t h the f r e q u e n c y c h a r a c t e r i s t i c s of the s e i s m i c energy r e c o r d e d on the l a n d r e c e i v e r s . There i s a s i g n i f i c a n t amount of power a t h i g h e r f r e q u e n c i e s due t o background n o i s e which i s v i s i b l e on a l l of the OBSs. E i g h t p o l e z e ro-phase B u t t e r w o r t h f i l t e r s w i t h d i f f e r e n t f r e q u e n c y l i m t i s were a p p l i e d t o the d a t a from r e c e i v e r 1. A bandpass from 0.1-15 Hz p r o v i d e d the b e s t enhancement f o r t h i s d a t a s e t . A t t e m p t s t o f i l t e r out t h e n o i s e peak at 7 Hz degraded the f i r s t b r e a k s p a s t 85 km t o o much t o be a c c e p t a b l e . 45 F i g u r e 17 - Power s p e c t r a of n o i s e and s i g n a l f o r an OBS. These periodograms were computed from d a t a r e c o r d e d on R e c e i v e r 7 over a two second window of s e i s m i c s i g n a l ( a ) , and background n o i s e ( b ) . The s e i s m i c s i g n a l i s band l i m i t e d between about 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 the 5 t o 10 Hz range. 46 The OBSs d e p l o y e d i n Hecate S t r a i t d i d not r e c o r d any d a t a except f o r some s t r o n g e v e n t s observed on the v e r t i c a l seismometer c h a n n e l of r e c e i v e r s 12, 13, and 14. The v e r y poor d a t a q u a l i t y i n Hecate S t r a i t i s thought t o be caused by the environment of the i n s t r u m e n t s . Hecate S t r a i t i s a v e r y s h a l l o w body of water w i t h a s o f t sediment bottom and i t i s b e l i e v e d t h a t t i d a l c u r r e n t s and perhaps s h i p induced n o i s e obscured the s e i s m i c s i g n a l . F i g u r e 18 shows sample power periodograms from r e c e i v e r s 11 and 12 over 2 second windows of both background n o i s e and s i g n a l . The s p e c t r a f o r r e c e i v e r 12 i n d i c a t e t h a t the s i g n a l i s r e c o v e r a b l e . There i s s i g n i f i c a n t energy between 1.5 and 5 Hz on the ' s i g n a l ' spectrum t h a t i s absent on the ' n o i s e ' spectrum, a band c o n s i s t e n t w i t h o b s e r v e d s e i s m i c s i g n a l s on l a n d s t a t i o n s and the OBSs west of Moresby I s l a n d . An 8-pole zero-phase B u t t e r w o r t h f i l t e r was used t o attempt t o r e c o v e r the s i g n a l on the Hecate S t r a i t OBSs. A v e r y narrow bandpass of 1 Hz t o 5 Hz was s u c c e s s f u l i n r e c o v e r i n g the s t r o n g e v e n t s , 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 12, 13, and 14 but no o t h e r r e c o g n i z a b l e s e i s m i c energy was r e c o v e r e d . The OBS response i s not good below 4.5 Hz ( f i g u r e 15), m a i n l y because of the 4.5 Hz seismometers used. Thus energy i n the frequency range of i n t e r e s t i s a t t e n u a t e d r e l a t i v e t o the h i g h e r frequency n o i s e r e c o r d e d on a l l of the Hecate S t r a i t OBSs. T h i s i s a problem t h a t i s s i g n i f i c a n t on a l l the o t h e r OBSs and i s w e l l i l l u s t r a t e d by the s p e c t r a of r e c e i v e r 11 ( f i g u r e 18 c and d ) . The ' n o i s e ' and ' s i g n a l ' s p e c t r a f o r r e c e i v e r 11 a r e almost i d e n t i c a l and c o n t a i n l i t t l e energy below 5 Hz. Both have the 47 1.0 0.5 n o i s e 10 15 20 FREQUENCY (HZ) 0.0 1 25 30 10 15 20 FREQUENCY (HZ) 25 30 1.0 0.5 0.0 c s i g n a l { 10 15 FREQUENCY 20 25 30 (HZ) 10 15 FREQUENCY (HZ) F i g u r e 18 - Power s p e c t r a s i g n a l S t r a i t OBSs. and n o i s e f o r Hecate These d a t a . perlodograms were computed over a 2 second window of (a) R e c e i v e r 12 (b) R e c e i v e r 12 ( c ) R e c e i v e r 11 (d) R e c e i v e r 11 s e l m l c s i g n a l 1 and 5 Hz (a) A bandpass - s e i s m i c s i g n a l - background n o i s e - s e i s m i c s i g n a l - background n o i s e r e c o r d e d on r e c e i v e r 12 i s band l i m i t e d There i s s i g n i f i c a n t n o i s e above 5 f i l t e r of 1-5 Hz was n e c e s s a r y to The between 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 the d a t a from r e c e i v e r 11 d i d not improve the q u a l i t y . The s i m i l a r i t y 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 n o i s e . 48 same two peaks, one between 15 and 20 Hz and another between 5 and 10 Hz. The same f i l t e r as above was a p p l i e d w i t h v a r y i n g f r e q u e n c y l i m i t s but no o b s e r v a b l e s e i s m i c energy was r e c o v e r e d . 2.4 Summary Except f o r the 4 OBSs de p l o y e d i n Hecate S t r a i t a l l of the 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 d e p l o y e d on Moresby I s l a n d r e c o r d e d e x c e l l e n t d a t a w h i l e the mainland s t a t i o n s were n o i s i e r because of the f a r o f f s e t from the s o u r c e s . The two OBSs west of Moresby I s l a n d a l s o r e c o r d e d good d a t a . Some s e i s m i c s i g n a l was r e c o v e r e d from the Hecate S t r a i t OBSs by f i l t e r i n g the d a t a u s i n g a v e r y narrow bandpass. The f u l l d a t a s e t i s d e s c r i b e d i n an open f i l e r e p o r t (Clowes, 1984). A l l of the e x p l o s i o n d a t a a r e p r e s e n t e d as common r e c e i v e r r e c o r d s e c t i o n s i n Appendix B. 49 I I I . INTERPRETATION 3.1 M o d e l l i n g and Uniqueness A f o r w a r d m o d e l l i n g p r o c e s s was used t o c a l c u l a t e two-d i m e n s i o n a l s y n t h e t i c seismograms t h a t matched the d a t a . A l t h o u g h 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 the data u s i n g such a m o d e l l i n g t e c h n i q u e t h e r e e x i s t s an i n f i n i t e number of o t h e r models t h a t w i l l a l s o s a t i s f y the d a t a . The s e i s m i c r e f r a c t i o n problem i s v e r y non-unique even though the data 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 . T h i s non-uniqueness stems from the l a c k of c o n s t r a i n t s on a few parameters i n the model. For example, i f no t u r n i n g r a y s or r e f l e c t i o n s from the lower boundary e x i s t i n a l a y e r the v e l o c i t y g r a d i e n t f o r t h a t l a y e r i s not w e l l c o n s t r a i n e d . I t i s n e c e s s a r y , t h e r e f o r e , to employ a l l of the c o n s t r a i n t s t h a t one can muster. F i r s t and f o r e m o s t , the model must be g e o l o g i c a l l y r e a s o n a b l e . O b v i o u s l y , any o t h e r a v a i l a b l e c o n c r e t e d a t a must be used t o f u r t h e r r e s t r i c t the non-uniqueness of the problem. The problem of non-uniqueness must a l s o be a d d r e s s e d i n the d a t a a c q u i s i t i o n p r o c e s s . By c h o o s i n g the optimum a r r a y of s h o t s and r e c e i v e r s i t i s p o s s i b l e t o reduce the number of p o o r l y c o n s t r a i n e d p a r a m e t e r s i n the model. The r e v e r s e d s e i s m i c r e f r a c t i o n p r o f i l e i s a common example of such an e x p e r i m e n t a l d e s i g n t e c h n i q u e . I t a l l o w s b o t h the d i p of the r e f r a c t i n g i n t e r f a c e and t h e v e l o c i t y of the l a y e r below the r e f r a c t i n g i n t e r f a c e t o be d e t e r m i n e d u n i q u e l y . Because of 50 e n v i r o n m e n t a l r e g u l a t i o n s i t was not p o s s i b l e t o r e v e r s e t h e 1983 r e f r a c t i o n l i n e by s h o o t i n g i n Hecate S t r a i t ( f i g u r e 14). An attempt t o a l l e v i a t e t h i s problem was made by d e t o n a t i n g many e x p l o s i o n s a t i n c r e a s i n g o f f s e t from the l i n e a r a r r a y of r e c e i v e r s . T h i s , i n e f f e c t , r e v e r s e s the l i n e over some of i t s l e n g t h . T h i s ' p s e u d o - r e v e r s a l ' i s e x p l o i t e d by m o d e l l i n g the d a t a i n bo t h common shot and common r e c e i v e r g a t h e r s t h a t sample the same a r e a of the e a r t h . The l i n e i s not r e v e r s e d i n the u s u a l sense; however, i t i s n e c e s s a r y t o s a t i s f y b o th the common shot and 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 model. Each r e c o r d s e c t i o n o r p r o f i l e i s m o d e l l e d s e p a r a t e l y u n t i l a common model f i t s t he d a t a s u f f i c i e n t l y w e l l . The f o r w a r d m o d e l l i n g t e c h n i q u e , a p p l i e d t o such a c o m p l i c a t e d d a t a s e t , has two main drawbacks. I t i s i m p o s s i b l e t o a l t e r each model 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 f i t t he d a t a . T h i s l a c k of c o n s i s t e n c y may a f f e c t the f i n a l model i n unknown ways. The method i s i m p r a c t i c a l f o r l a r g e d a t a s e t s . Perhaps a more s a t i s f y i n g approach would be t o c o n s t r u c t a model u s i n g a t w o - d i m e n s i o n a l i n v e r s i o n t e c h n i q u e . T h i s would ensure t h a t t h e f i n a l model f i t s the t o t a l d a t a s e t i n some c o n s i s t e n t manner. The t w o - d i m e n s i o n a l i n v e r s e problem a p p l i e d t o s e i s m i c r e f r a c t i o n i s , f o r the reason s t a t e d p r e v i o u s l y , v e r y non-unique and t h e r e f o r e u n s t a b l e . Spence (1984) a p p l i e d two-d i m e n s i o n a l t r a v e l time i n v e r s i o n t e c h n i q u e s t o a r e f r a c t i o n d a t a s e t s i m i l a r i n d e s i g n t o the 1983 Queen C h a r l o t t e I s l a n d s , experiment ( m u l t i p l e s h o t s r e c o r d e d on m u l t i p l e r e c e i v e r s ) . He found t h a t because the problem i s u n s t a b l e i t can be used o n l y 51 t o ' f i n e tune' a model t h a t has l a r g e l y been c o n s t r u c t e d by f o r w a r d m o d e l l i n g methods. Because of time c o n s t r a i n t s , two-d i m e n s i o n a l t r a v e l time i n v e r s i o n was not atte m p t e d as p a r t of t h i s t h e s i s . I t i s a l s o i m p o r t a n t t o note t h a t i t i s d i f f i c u l t t o o b j e c t i v e l y determine how w e l l the model s h o u l d f i t the d a t a . T h i s i s p a r t i c u l a r l y t r u e i f the model i s p a r a m e t e r i z e d non-u n i f o r m l y such as i n t h i s t h e s i s . In t h i s case the c h a r a c t e r i s t i c s of the f i n a l model a r e b i a s e d by the i n i t i a l model i n unknown ways. In any f o r w a r d m o d e l l i n g or i t e r a t i v e i n v e r s i o n p r o c e d u r e i t i s d i f f i c u l t t o o b j e c t i v e l y determine which f e a t u r e s of the model a r e r e q u i r e d by the d a t a . The goodness of f i t thus becomes s u b j e c t i v e . N o t i n g t h e s e concerns we w i l l 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 the M o d e l l i n g A l g o r i t h m A l l models were c o n s t r u c t e d u s i n g a ray method s y n t h e t i c seismogram a l g o r i t h m f o r l a t e r a l l y inhomogeneous media (Spence, 1984; Spence et a l . , 1984). Ray paths and t r a v e l times a r e c a l c u l a t e d u s i n g a m o d i f i e d v e r s i o n of the W h i t t a l l and Clowes (1979) r a y t r a c i n g program; a m p l i t u d e s a r e computed u s i n g a s y m p t o t i c r a y t h e o r y . The model i s p a r a m e t e r i z e d as a s e r i e s of l a r g e b l o c k s w i t h the v e l o c i t y and l i n e a r v e l o c i t y g r a d i e n t d e f i n e d a t t h e top of each b l o c k . The ray paths a r e then t r a c e d as a r c s of 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 v a l u e of the g r a d i e n t (see Gebrande, 1976). S n e l l ' s Law d e t e r m i n e s the be h a v i o u r of r a y s c r o s s i n g b o u n d a r i e s . Because t h i s i s a ray 52 method, a l l the l i m i t a t i o n s of ray t h e o r y a p p l y t o the m o d e l l i n g a l g o r i t h m , e xcept t h a t "pseudo" head waves can be generated as d e s c r i b e d by W h i t t a l l and Clowes (1979) and Spence et a l . (1984). Thus t u r n i n g r a y s , p r e - and p o s t - c r i t i c a l r e f l e c t i o n s , m u l t i p l e r e f l e c t i o n s and head waves can be invoked i f d e s i r e d . The a l g o r i t h m i s f a s t and e f f i c i e n t a l l o w i n g the user t o e a s i l y and e c o n o m i c a l l y t e s t many v a r i a t i o n s of a model. 3.3 I n t e r p r e t a t i o n of 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 Model - A Pr e v i e w The f i n a l model ( f i g u r e 19) was c o n s t r u c t e d by m o d e l l i n g 5 common r e c e i v e r and 2 common shot p r o f i l e s . A s h o r t d e s c r i p t i o n of the f i n a l v e l o c i t y model w i l l a i d i n p l a c i n g the i n t e r p r e t a t i o n s of each p r o f i l e i n p e r s p e c t i v e . The main f e a t u r e of the model i s the l a t e r a l change from a t y p i c a l o c e a n i c c r u s t t o t h i c k e r c r u s t beneath the Queen C h a r l o t t e I s l a n d s and Hecate S t r a i t . The Queen C h a r l o t t e t e r r a c e s e p a r a t e s t h e s e two r e g i o n s . In the model, the v e l o c i t y s t r u c t u r e of the lower o c e a n i c c r u s t i s v e r y s i m i l a r t o t h a t of the lower t e r r a c e r e g i o n and the lower c r u s t e a s t of the f a u l t zone, a l t h o u g h t h i s may r e p r e s e n t l i m i t a t i o n s of the da t a s et and m o d e l l i n g p r o c e d u r e . The o c e a n i c Moho i s a t about 10 km de p t h . Beneath the t e r r a c e and f u r t h e r e a s t , the Moho d i p s a t 5° t o the e a s t r e a c h i n g a d e p t h of 35 km beneath the mainland c o a s t . The s h a l l o w o c e a n i c sediment l a y e r (1.8 km/s) i s c o n s t r a i n e d i n depth by a c o n t i n u o u s s e i s m i c r e f l e c t i o n p r o f i l e 53 o 0 S 01 SI 02 S2 0C SG F i g u r e 19 - The f i n a l v e l o c i t y 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 d a t a . Dashed 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 not w e l l c o n s t r a i n e d . The f i r s t number i n each b l o c k i s the v e l o c i t y ( i n km/s) a t the t o p boundary of the b l o c k . The second number i s the v e l o c i t y g r a d i e n t ( i n km/s/km). The model d i s t a n c e i s p l o t t e d a c r o s s the t o p of the f i g u r e . P o s i t i o n s of s h o t s and r e c e i v e r s used i n t h i s study a r e superimposed on the model. QCI = Queen C h a r l o t t e I s l a n d s . HS = Hecate S t r a i t . 54 c o l l i n e a r w i t h the o f f s h o r e s e c t i o n of our r e f r a c t i o n l i n e ( D a v i s and Seemann, 1981). The l a y e r i n g below t h i s sediment zone i s not w e l l c o n s t r a i n e d by our d a t a s e t . The o c e a n i c s t r u c t u r e from a s e i s m i c r e f r a c t i o n p r o f i l e (Horn e t a l . , 1984 and Horn, 1982) p a r a l l e l t o the Queen C h a r l o t t e f a u l t zone and so u t h of our survey was the b a s i s . f o r c h o o s i n g the ocean s t r u c t u r e i n our model. The l a c k of any near s u r f a c e d e t a i l s f o r t h e Queen C h a r l o t t e I s l a n d s e astwards i n the f i n a l v e l o c i t y model r e s u l t s from the d e s i g n of the e x p e r i m e n t . More s h o t s near the i s l a n d and mai n l a n d s t a t i o n s would be needed t o p r o v i d e the c o n s t r a i n t s t o c o n s t r u c t an upper c r u s t a l model f o r t h i s r e g i o n . The r e f r a c t i o n models of Johnson e t a l . (1972) and F o r s y t h et a l . (1974) p r o v i d e d some c o n s t r a i n t s as t o the app r o x i m a t e v e l o c i t y s t r u c t u r e and depth t o mantle i n the m a i n l a n d r e g i o n e a s t of the Queen C h a r l o t t e I s l a n d s . N e i t h e r of th e s e s u r v e y s was adequate t o p r o v i d e a d e t a i l e d v e l o c i t y model of the c r u s t i n t h i s a r e a . 3.3.2 The C h o i c e of Data S e t s f o r M o d e l l i n g For the purposes of t h i s t h e s i s , o n l y a p o r t i o n of the e n t i r e d a t a s e t was f u l l y i n t e r p r e t e d . The o b j e c t i v e of t h i s r e s e a r c h was t o st u d y the deep c r u s t a l s t r u c t u r e beneath the Queen C h a r l o t t e I s l a n d s and Hecate S t r a i t and the d a t a s e t was chosen a c c o r d i n g l y . The c r i t e r i a used i n c h o o s i n g the p r o f i l e s t o be m o d e l l e d were: (1) the d a t a must sample the p a r t of the e a r t h t h a t i s t o be s t u d i e d , 55 (2) the da t a must t a k e advantage of the ' p s e u d o - r e v e r s a l ' of p r o f i l e s , and (3) the d a t a must be of s u f f i c i e n t q u a l i t y t o i n t e r p r e t . A f t e r p r e l i m i n a r y m o d e l l i n g of e v e r y common r e c e i v e r p r o f i l e i t became apparent which ones b e s t met the above c r i t e r i a . Common r e c e i v e r p r o f i l e s 1, 3, 15, 16, 17, and common shot p r o f i l e s 4, and 16 were chosen f o r m o d e l l i n g . Except where n o t e d , a l l d i s t a n c e s r e f e r r e d t o i n t h i s c h a p t e r a r e s h o t - r e c e i v e r d i s t a n c e s and, where a p p l i c a b l e , a r e f o l l o w e d by model d i s t a n c e s i n p a r e n t h e s e s . A d i s c u s s i o n of the i n t e r p r e t a t i o n of each p r o f i l e f o l l o w s . 3.3.3 Common R e c e i v e r P r o f i l e 1 R e c e i v e r 1, an OBS, i s t h e most westward r e c e i v e r a l o n g the l i n e ( f i g u r e 14). I t was d e p l o y e d on the Queen C h a r l o t t e t e r r a c e , i n 900 m of water. I t i s p o s s i b l e t o c o r r e l a t e s h o t -r e c e i v e r d i s t a n c e t o model d i s t a n c e by v i e w i n g the a p p r o p r i a t e f i g u r e d i s p l a y i n g both d a t a and s y n t h e t i c s . The d a t a a r e of good q u a l i t y t o a d i s t a n c e of 80 km beyond which the f i r s t a r r i v a l s a r e not v i s i b l e ( f i g u r e 2 0 ) . The f i r s t a r r i v a l s t o 49 km have e r r o r s l e s s than ±0.05 s; a r r i v a l s p a s t 49 km a r e more u n c e r t a i n and the e s t i m a t e d t r a v e l time e r r o r s a r e i n the or d e r of ±0.1 s. The da t a have been c o r r e c t e d f o r s p h e r i c a l s p r e a d i n g (by m u l t i p l y i n g the d a t a by r 2 where r i s the shot r e c e i v e r d i s t a n c e ) and f o r shot s i z e by assuming t h a t the r e c o r d e d a m p l i t u d e s a r e p r o p o r t i o n a l t o W2/3, where W i s the weight of the shot i n k i l o g r a m s ( O ' B r i e n , 1960). T h i s d i d not c o r r e c t the 56 F i g u r e 20 - Comparison of the d a t a and s y n t h e t i c s f o r P r o f i l e 1. 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 compared w i t h the s y n t h e t i c seismogram (b) computed u s i n g the f i n a l model. Arrowheads on (a) denote f i r s t a r r i v a l p i c k s of mantle r e f r a c t e d r a y s . 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 the t o p of ( a ) . The model d i s t a n c e i s p l o t t e d a l o n g the bottom of (b) and between (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 such t h a t a r r i v a l s w i t h an apparent v e l o c i t y of 8 km/s appear h o r i z o n t a l . 135 S h o t - R e c e i v e r D i s t a n c e (km) 125 115 105 95 85 75 65 55 45 35 25 15 5 - 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 95 100 105 110 115 120 Model D i s t a n c e (km) 58 c l o s e s h o t s f o r r e c e i v e r 1 w e l l as i s i n d i c a t e d by the a m p l i t u d e v a r i a t i o n over the f i r s t s i x t r a c e s ( e v e r y f o u r t h t r a c e i s a 540 k i l o g r a m s h o t ) . Some f e a t u r e s of t h i s p r o f i l e a r e v e r y i m p o r t a n t . The s h a r p break from c r u s t a l a r r i v a l s t o mantle r e f r a c t i o n s o c c u r s a t 25 km, p r o v i d i n g an e s t i m a t e of the depth t o the m a n t l e . The low apparent v e l o c i t y of the c r u s t a l a r r i v a l s a r e a r e s u l t of s l o w e r c r u s t a l m a t e r i a l and the e f f e c t of topography of the Queen C h a r l o t t e t e r r a c e . The Pn mantle r e f r a c t i o n s have an apparent v e l o c i t y s l i g h t l y over 8 km/s but they a r e v e r y weak and d i e out by 85 km. The h i g h a m p l i t u d e secondary a r r i v a l s from 40 km t o 125 km have an a p p a r e n t v e l o c i t y of 6.8 km/s. These a r r i v a l s merge w i t h the Pn a r r i v a l s a t about 40 km which c o r r e s p o n d s t o the c r i t i c a l p o i n t . The m o d e l l i n g procedure e n t a i l e d f i t t i n g the t r a v e l t i m e s and then m o d e l l i n g 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 . A l t e r i n g the model t o f i t a m p l i t u d e c h a r a c t e r i s t i c s o f t e n degraded the t r a v e l time f i t . Both t r a v e l t i m e s and a m p l i t u d e f i t s were then m o d e l l e d i n c o n j u n c t i o n u n t i l a s a t i s f a c t o r y model r e s u l t e d . The d a t a from r e c e i v e r 1 samples the o c e a n i c and t e r r a c e r e g i o n s west of the Queen C h a r l o t t e I s l a n d s . F i g u r e 21 shows ra y t r a c i n g t h r o u g h t h i s p o r t i o n of the f i n a l model. The s y n t h e t i c seismogram f o r t h i s p r o f i l e and the d a t a a r e p r e s e n t e d i n f i g u r e 20. The r a y s a r r i v i n g between 8(117) km and 25(99) km a r e t r a c e d t h r o u g h the low v e l o c i t y - h i g h g r a d i e n t upper t e r r a c e r e g i o n . A low v e l o c i t y and h i g h g r a d i e n t were n e c e s s a r y MODEL D I S T A N C E (km) F i g u r e 21 - F i n a l ray t r a c i n g diagram f o r Common R e c e i v e r P r o f i l e 1. (see t e x t f o r e x p l a n a t i o n ) . 60 t o f i t both the t r a v e l 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 thes e a r r i v a l s . 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 from 8(117) km t o 25(99) km where they merge w i t h the s t r o n g secondary a r r i v a l s a t 30(95) km. The mantle r e f r a c t i o n s have an apparent v e l o c i t y c o n t r o l l e d by the mantle v e l o c i t y , and the d i p on an upper o c e a n i c l a y e r beneath the s h o t s . In o t h e r words, the 2° east w a r d d i p of the o c e a n i c Moho i n the model i s not w e l l c o n s t r a i n e d . I t may indeed be h o r i z o n t a l and a more s h a l l o w s t r u c t u r e may account f o r the apparent v e l o c i t y of these a r r i v a l s . The s t r o n g secondary a r r i v a l s from 30(90) km t o 125(0) km are i n t e r p r e t e d as p o s t - c r i t i c a l r e f l e c t i o n s from the Moho ( f i g u r e 2 1). The t r a v e l time and a m p l i t u d e f i t i s good ( f i g u r e 2 0 ) . These a r r i v a l s c o n s t r a i n the aver a g e v e l o c i t y above the Moho and. thus a l l o w the approximate d e p t h t o the Moho t o be d e t e r m i n e d . V a r i a t i o n s i n the upper o c e a n i c l a y e r s , which a r e 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 d e p t h t o the Moho. For example, a change i n v e l o c i t y of the 3.7 km/s l a y e r t o 4.2 km/s would f o r c e the o c e a n i c Moho t o be about 2 km s h a l l o w e r . These secondary a r r i v a l s a l s o c o n s t r a i n the v e l o c i t y g r a d i e n t i n the lower o c e a n i c c r u s t t o be v e r y low (0.015 km/s/km) i n orde r t o propagate energy from the f a r t h e s t shot t o t h e r e c e i v e r . The t h i c k n e s s of the 1.8 km/s l a y e r , r e p r e s e n t i n g u n c o n s o l i d a t e d s e d i m e n t s , 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 i n the a r e a ( D a v i s and Seemann, 1981). The o c e a n i c s t r u c t u r e of the f i n a l model i s v e r y s i m i l a r t o t h a t of Horn e t a l . (1984). The t e r r a c e r e g i o n was d i v i d e d i n t o an upper b l o c k and a lower b l o c k . The p o s i t i o n of t h e boundary s e p a r a t i n g the upper 61 and lower t e r r a c e r e g i o n s i s not w e l l c o n s t r a i n e d . The lower b l o c k must have a much s m a l l e r g r a d i e n t than th e t o p b l o c k i n o r d e r f o r the w i d e - a n g l e r e f l e c t i o n s t o r e a c h the r e c e i v e r . The v e l o c i t y s t r u c t u r e of t h i s b l o c k i s s u s p i c i o u s l y s i m i l a r t o the lower o c e a n i c l a y e r . 3.3.4 Common R e c e i v e r P r o f i l e 3 R e c e i v e r 3 i s l o c a t e d j u s t e a s t of t h e Queen C h a r l o t t e f a u l t zone and i s the most westward l a n d s t a t i o n on the Queen C h a r l o t t e I s l a n d s ( f i g u r e 14). T h i s p r o f i l e was chosen because i t e x h i b i t e d a marked l a c k of the s t r o n g s econdary a r r i v a l s so prominent on r e c e i v e r 1. The d a t a q u a l i t y i s e x c e l l e n t ( f i g u r e 22 a ) . E s t i m a t e d t r a v e l time u n c e r t a i n t i e s a r e l e s s than 0.05 s. The secondary a r r i v a l s v i s i b l e a t f a r o f f s e t on r e c e i v e r 1 a r e not e v i d e n t on any of the r e c e i v e r s e a s t of the Queen C h a r l o t t e f a u l t zone (see Appendix B ) . T h i s s u g g e s t s t h e r e i s a s i g n i f i c a n t l a t e r a l change i n the c r u s t a c r o s s the t e r r a c e r e g i o n . The f i r s t a r r i v a l s on r e c e i v e r 3 a r e s i m i l a r i n c h a r a c t e r t o r e c e i v e r 1. The c r u s t a l a r r i v a l s break over a t about 45 km t o Pn r e f r a c t i o n s w i t h an " apparent v e l o c i t y s l i g h t l y g r e a t e r than 8 km/s. The l a r g e a m p l i t u d e a r r i v a l s from 45 km t o 68 km a r e s i g n i f i c a n t and c o r r e s p o n d t o energy t h a t has t r a v e l l e d t h r o u g h the t e r r a c e r e g i o n . F i r s t a r r i v a l s from 33(115) t o 45(107) km have t r a v e l l e d t h r o u g h the upper t e r r a c e b l o c k w i t h i t s h i g h v e l o c i t y g r a d i e n t . C o n s e q u e n t l y the a m p l i t u d e s a r e l a r g e and i n c r e a s e w i t h o f f s e t . T r a c e s 1 and 4 ( a t 33 and 42 km) of the d a t a i l l u s t r a t e t h i s e f f e c t . T r a c e s 2 S h o t - R e c e i v e r D i s t a n c e (km) 155 145 135 125 115 105 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 D I S T A N C E (km) F i g u r e 22 - Comparison of da 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 produced from f i n a l ray t r a c i n g diagram. The t r a v e l time c u r v e s from the s y n t h e t i c a r r i v a l s a r e drawn over the da t a s e c t i o n . See F i g u r e 20 f o r an e x p l a n a t i o n of d i s t a n c e and time axes. 63 and 3 were from 60 kg c h a r g e s , and as noted p r e v i o u s l y , i t i s b e l i e v e d t h a t the 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 . At 45(105) km d i s t a n c e t h e s e a r r i v a l s merge w i t h a n o t h e r s e t of 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 ap p a r e n t v e l o c i t y i s about 6.8 km/s. They c o n t i n u e t o about 68(78) km. T h i s phase has been i n t e r p r e t e d as r e f l e c t i o n s from the Moho beneath the t e r r a c e . I t ends a b r u p t l y because of the change i n d i p of the Moho beneath the o u t e r t e r r a c e s c a r p ( f i g u r e 2 3 ) . The s y n t h e t i c seismograms mimic w e l l t h e s e 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 i g u r e 2 2 ) . F i r s t a r r i v a l s e x t e n d i n g from 58(90) km t o 148(0) km a r e due t o r a y s r e f r a c t e d t h r o u g h the upper m a n t l e . The o b s e r v e d and s y n t h e t i c t r a v e l t i m e s and a m p l i t u d e s agree w e l l f o r t h i s phase. A secondary a r r i v a l from 80 t o 112 km (model d i s t a n c e ) on the s y n t h e t i c s e c t i o n i s due t o r a y s t u r n i n g i n the lower o c e a n i c c r u s t . Such a prominent phase i s not o b s e r v e d on the da t a s e c t i o n a l t h o u g h t h e r e i s s u b s t a n t i a l e n e rgy i n the coda of obser v e d t r a c e s over t h i s d i s t a n c e range. 3.3.5 Common R e c e i v e r P r o f i l e 15 R e c e i v e r 15 i s the most westward m a i n l a n d s t a t i o n ( f i g u r e 14). T h i s r e c o r d s e c t i o n , and tho s e of r e c e i v e r s 16 and 17, were chosen because they were d i s t a n t from the s h o t s ( t h e r e f o r e r a y p a t h s t o them sampled deepest i n t o the e a r t h ) and because they e x h i b i t e d s t r o n g secondary a r r i v a l s ( f i g u r e 24 and Appendix B ) . The f i r s t a r r i v a l s from s h o t - r e c e i v e r d i s t a n c e of 175 t o 195 km a r e v e r y weak and emergent. Because of t h e i r Mode l D i s t a n c e (km) 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 F i g u r e 23 - F i n a l ray t r a c i n g diagram f o r Common R e c e i v e r P r o f i l e 3. (see t e x t f o r e x p l a n a t i o n ) . 290 280 270 S h o t - R e c e i v e r 260 250 240 230 D i s t a n c e (km) 220 210 200 190 180 170 160 o H 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 CO • — N o H co-co oo Q CO tO-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 Mode l D i s t a n c e (km) F i g u r e 24 Comparison of 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 15. (a) d a t a . (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. The t r a v e l time c u r v e s from the s y n t h e t i c a r r i v a l s are drawn over the d a t a s e c t i o n . The arrowheads mark f i r s t p i c k s f o r 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 of d i s t a n c e and time axes. 66 emergent n a t u r e and s m a l l a m p l i t u d e s t r a v e l time p i c k s were d i f f i c u l t t o a c c u r a t e l y d e t e r m i n e and i t i s p o s s i b l e t h a t the p i c k s do not r e p r e s e n t the a c t u a l f i r s t b r e a k s . From 200 km t o 285 km the f i r s t a r r i v a l s (Pn phase) are c l e a r e r w i t h an e s t i m a t e d t r a v e l time u n c e r t a i n t y of about ±0.10 s. The m a i n l a n d d a t a ( f i g u r e 24) a r e n o i s i e r than the Queen C h a r l o t t e I s l a n d s d a t a ( f i g u r e 22) because of the l a r g e s h o t - r e c e i v e r d i s t a n c e s . A l l of the t r a v e l time p i c k s were made on u n f i l t e r e d d a t a w i t h the a i d of the f i l t e r e d s e c t i o n s . The apparent v e l o c i t y of the Pn a r r i v a l s p a s t 210 km i s about 8.5 km/s. The v e r y s t r o n g secondary a r r i v a l s a t about 8.0 s end a b r u p t l y a t 190 km. These s t r o n g a r r i v a l s o c cur o n l y f o r s h o t s t h a t were d e t o n a t e d over or v e r y c l o s e t o the Queen C h a r l o t t e t e r r a c e . The d a t a r e c o r d e d on r e c e i v e r s 16 and 17 are v e r y s i m i l a r t o t h i s p r o f i l e . The f i n a l model w i t h t r a c e d r a y s i s shown i n f i g u r e 25. The s t r o n g secondary a r r i v a l s have been i n t e r p r e t e d as p o s t -c r i t i c a l r e f l e c t i o n s from the Moho beneath the Queen C h a r l o t t e I s l a n d s . The t r a v e l t i m e s f i t w e l l but the a m p l i t u d e s a r e s l i g h t l y s m a l l . N o n e t h e l e s s , the secondary r e f l e c t i o n e v e n t s do have a much l a r g e r a m p l i t u d e than a l l o t h e r a r r i v a l s on the s y n t h e t i c s e c t i o n . The a b r u p t end of t h e s e 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 of the change i n d i p of the Moho a t 100 km (model d i s t a n c e ) and the h i g h g r a d i e n t upper t e r r a c e r e g i o n . The v e r y weak mantle r e f r a c t i o n s f o r s h o t - r e c e i v e r d i s t a n c e s from 175(115) km t o 190(95) km a r e m o d e l l e d w e l l i n Mode l D i s t a n c e (km) w F i g u r e 25 - F i n a l ray t r a c i n g diagram f o r Common R e c e i v e r P r o f i l e 15. (see t e x t f o r e x p l a n a t i o n ) . 68 a m p l i t u d e . The eastward d i p of the Moho beneath the t e r r a c e and the low g r a d i e n t i n the mantle c o n t r i b u t e t o the s m a l l a m p l i t u d e a r r i v a l s . The model t r a v e l t i m e s a r e e a r l i e r than the p i c k e d a r r i v a l s by as much as 0.3 s on some t r a c e s . S y n t h e t i c t r a v e l times f o r r e c e i v e r s 16 and 17 e x h i b i t t h i s same c h a r a c t e r i s t i c . These weak a r r i v a l s a r e v e r y emergent and the p i c k s a r e not r e l i a b l e as t h e y a r e a t the l e v e l of the background n o i s e . Other a r r i v a l s on t h i s d ata s e c t i o n and o t h e r s e c t i o n s sample the t e r r a c e r e g i o n and f i t the model w e l l , i n d i c a t i n g t h a t t h i s m i s f i t i s not due t o l o c a l s t r u c t u r e beneath the s h o t s over the t e r r a c e . The v e l o c i t y s t r u c t u r e of the t e r r a c e r e g i o n i s r e a s o n a b l y w e l l c o n s t r a i n e d by r e c e i v e r s 1, 3, and common shot g a t h e r 4. The wid 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 on r e c e i v e r , 15 a l s o sample t h e same r e g i o n as t h e weak a r r i v a l s . F o r c i n g the weak a r r i v a l s t o f i t the data 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 l a t e w i t h r e s p e c t t o the d a t a . To c o r r e c t t h i s an 18 t o 20 km deep Moho beneath the the t e r r a c e r e g i o n would be r e q u i r e d . T h i s model has been t e s t e d and i s i n c o n s i s t e n t w i t h the d a t a . I t would r e q u i r e a s t e e p l y d i p p i n g segment of Moho j o i n i n g the ocean c r u s t t o the t e r r a c e c r u s t . T h i s i n t u r n c r e a t e s a l a r g e shadow zone west of 200(90) km where no mantle r e f r a c t i o n s can p e n e t r a t e t o t h e s u r f a c e . T h i s i s not observed on the d a t a . The mantle r e f r a c t i o n s on r e c e i v e r 15 from 205(90) km t o 285(0)km c o n s t r a i n the Moho t o be d i p p i n g g e n t l y from the edge of the o c e a n i c c r u s t e a s t w a r d . For the above rea s o n s i t i s f e l t t h a t the m i s f i t may be a r e s u l t of not bei n g a b l e t o p i c k the f i r s t a r r i v a l s because they a r e below the background n o i s e l e v e l 69 and not due t o s t r u c t u r e i n the t e r r a c e r e g i o n . The i n c r e a s e i n a m p l i t u d e of the mantle r e f r a c t i o n s a t 225(85) km i s mo d e l l e d w i t h a change i n d i p of the Moho beneath the o u t e r edge of the t e r r a c e . These a r r i v a l s have l a r g e r a m p l i t u d e s than the weak energy r e c o r d e d from 175(115) km t o 195(90) km because the Moho boundary i s more h o r i z o n t a l . 3.3.6 Common R e c e i v e r P r o f i l e s 16 and 17 P r o f i l e s 16 and 17 are v e r y s i m i l a r t o one a n o t h e r . R e c e i v e r 16 i s l o c a t e d on P i t t I s l a n d j u s t west of the ma i n l a n d c o a s t ( f i g u r e 14). R e c e i v e r 17, l o c a t e d on the m a i n l a n d , i s the most e a s t w a r d s t a t i o n . P r o f i l e 16, a common r e c e i v e r g a t h e r , i s d i s p l a y e d i n f i g u r e 26 and Appendix B. P r o f i l e 17 d a t a , a l s o a common r e c e i v e r g a t h e r , i s found i n f i g u r e 28 and Appendix B. The d a t a a r e v e r y n o i s y because of the h i g h g a i n r e q u i r e d a t such l o n g s h o t - r e c e i v e r d i s t a n c e s . The data r e c o r d e d on r e c e i v e r 17 a r e of m a r g i n a l q u a l i t y . A r r i v a l s between 245 km and 325 km have an e s t i m a t e d u n c e r t a i n t y of ±0.3 s. F i r s t a r r i v a l s between 215 km and 235 km a r e v e r y weak and emergent and have even l a r g e r u n c e r t a i n t i e s . R e c e i v e r 16 has b e t t e r q u a l i t y d a t a . The weak emergent a r r i v a l s between 195 km and 222 km have an e s t i m a t e d e r r o r of about 0.3 s. The a r r i v a l s between 225 km and 302 km have u n c e r t a i n t i e s of about 0.15 s. S i n c e the c h a r a c t e r i s t i c s of the s e two p r o f i l e s a r e v e r y s i m i l a r t hey w i l l be d i s c u s s e d t o g e t h e r . These d a t a were m o d e l l e d t o h e l p c o n s t r a i n the p o s i t i o n of the Moho f u r t h e r downdip beneath Hecate S t r a i t . The f i n a l model 315 S h o t - R e c e i v e r 285 275 265 255 D i s t a n c e (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 co-in B i i i i i i i i i f f f f -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 del D i s t a n c e (km) F i g u r e 26 - Comparison of data 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) d a t a . over the d a t a s e c t i o n . The arrowheads 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 The t r a v e l time c u r v e s from the s y n t h e t i c a r r i v a l s a re drawn of d i s t a n c e and time axes. Mode l D i s t a n c e (km) F i g u r e 27 - F i n a l ray t r a c i n g diagram f o r Common R e c e i v e r P r o f i l e 16. (see t e x t f o r e x p l a n a t i o n ) . 72 w i t h t r a c e d r a y s ( f i g u r e 27 and f i g u r e 29) i s v e r y s i m i l a r t o f i g u r e 25 except t h a t the c r u s t i s sampled f a r t h e r t o the e a s t than r e c e i v e r 15. The a m p l i t u d e s of the weak emergent a r r i v a l s a t the s h o r t e r o f f s e t d i s t a n c e s were mo d e l l e d w e l l but the t r a v e l t i m e s were too e a r l y ( f i g u r e 26 and 28), as was the case f o r r e c e i v e r 15. However, the m i s f i t i s i n the o r d e r of the t r a v e l time u n c e r t a i n t i e s . The s t r o n g , wide-angle r e f l e c t i o n s a re m o d e l l e d w e l l i n a m p l i t u d e and t r a v e l t i m e . These r a y s r e f l e c t the Moho f u r t h e r downdip and h e l p t o d e f i n e i t s e a s t w a r d s l o p e t o a d i s t a n c e of 200 km (model d i s t a n c e ) . The a m p l i t u d e s and t r a v e l t i m e s of the mantle r e f r a c t i o n s up th r o u g h the o c e a n i c l a y e r s (west of the t e r r a c e r e g i o n ) a r e m o d e l l e d w e l l . The i n c r e a s e i n a m p l i t u d e beyond 225(89) km on p r o f i l e 16 and 240(90) km on p r o f i l e 17 c o r r e s p o n d s t o the change i n d i p of the Moho a t the o u t e r t e r r a c e edge and f i t s the d a t a w e l l . The l a c k of a shadow zone f o r these r e f r a c t e d a r r i v a l s s u r f a c i n g j u s t west of the t e r r a c e r e g i o n f u r t h e r i n d i c a t e s t h a t the Moho d i p s g e n t l y e a s t w a r d beneath the t e r r a c e r e g i o n . I t was found t h a t a l l model a r r i v a l s on r e c e i v e r s 16 and 17 were t o o l a t e w i t h o u t the s m a l l 6.4 km/s b l o c k i m m e d i a t e l y beneath the s t a t i o n s ( f i g u r e s 19 and 2 7 ) . The same e f f e c t c o u l d be caused by a s h a l l o w e r Moho beneath the r e c e i v e r s but the c o r n e r i n t r o d u c e d by bending the Moho c r e a t e s a shadow zone f o r mantle a r r i v a l s a t f a r o f f s e t . T h i s e f f e c t i s not obser v e d i n the d a t a . The low g r a d i e n t i n the mantle (0.005 km/s/km) i s c o n s t r a i n e d by the a m p l i t u d e s of Pn a r r i v a l s a t l o n g s h o t - r e c e i v e r d i s t a n c e s on the s e t h r e e r e c e i v e r s . 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. ce-CO o co-co \ \ Q I Z>-E-1 60-B i -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 D i s t a n c e (km) F i g u r e 28 - Comparison of data 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 17. (a) d a t a . over the d a t a s e c t i o n . The arrowheads 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 The t r a v e l time c u r v e s from the s y n t h e t i c a r r i v a l s are drawn of d i s t a n c e and time axes. Mode l D i s t a n c e (km) 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 F i g u r e 29 - F i n a l ray t r a c i n g diagram f o r Common R e c e i v e r P r o f i l e 17. (see t e x t f o r e x p l a n a t i o n ) . 75 a m p l i t u d e s of t h e s e a r r i v a l s a r e v e r y s e n s i t i v e t o changes i n the g r a d i e n t because of t h e i r l o n g mantle p a t h . A g r a d i e n t h i g h e r than 0.005 km/s/km r e s u l t s i n much l a r g e r a m p l i t u d e s a t the f a r t h e s t d i s t a n c e s . Even w i t h t h i s s m a l l g r a d i e n t the a m p l i t u d e s a r e a l i t t l e l a r g e r 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 not a l l o w s h o t s a t f a r o f f s e t t o prop a g a t e energy t o the m a i n l a n d . 3.3.7 Common Shot P r o f i l e 4 T h i s p r o f i l e r e p r e s e n t s shot 4 r e c o r d e d on a l l r e c e i v e r s . The d a t a a r e shown i n f i g u r e 30. The a m p l i t u d e s have been c o r r e c t e d f o r the response of the d i f f e r e n t i n s t r u m e n t s t h a t were d e p l o y e d a l o n g the l i n e . Because s e v e r a l t y p e s of i n s t r u m e n t s w i t h d i f f e r e n t response f u n c t i o n s were d e p l o y e d , the t r a c e t o t r a c e a m p l i t u d e v a r i a t i o n i s sometimes l a r g e . L o c a l i z e d s t r u c t u r e beneath a r e c e i v e r may a l s o account f o r a m p l i t u d e v a r i a t i o n s . The f i r s t two t r a c e s , c o r r e s p o n d i n g t o dat a from t h e two OBSs west of Moresby I s l a n d ( f i g u r e 14), have a m p l i t u d e s v e r y much s m a l l e r than the l a n d r e c e i v e r s . T h i s v a r i a t i o n i s thought t o be a r e s u l t of the c o u p l i n g of the OBS S h o t - R e c e i v e r D i s t a n c e (km) 60 80 100 120 140 160 180 240 180 200 220 240 260 Model D i s t a n c e (km) F i g u r e 30 - Comparison of data and s y n t h e t i c f o r . . Common Shot P r o f i l e 4. (a) d a t a . (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.-The t r a v e l time c u r v e s from the s y n t h e t i c a r r i v a l s a re drawn over the d a t a s e c t i o n . Arowheads denote mantle r e f r a c t e d a r r i v a l f i r s t p i c k s . The data were bandpass f i l t e r e d from 1 - 5 Hz. Except f o r the data between 120 km and 160 km shot r e c e i v e r d i s t a n c e the f i r s t p i c k s were made on u n f l l t e r e d d a t a . See f i g u r e 20 f o r an e x p l a n a t i o n of d i s t a n c e and time axes . 77 w i t h the ocean bottom and the i n s t r u m e n t response. I t i s most l i k e l y t h a t the OBSs were s i t t i n g on s o f t sediments w h i l e the l a n d s t a t i o n s were p l a c e d on bedrock. As w e l l the OBS i s equipped w i t h 4.5 Hz seismometers w h i l e the l a n d s t a t i o n s a r e equipped w i t h 1 or 2 Hz seismometers. Thus energy r e c o r d e d below 4.5 Hz on the OBSs i s g r e a t l y a t t e n u a t e d ( f i g u r e 15). As i n d i c a t e d i n Chapter I I , most of the s e i s m i c energy i s below 5 Hz and t h u s i t i s p a r t i a l l y a t t e n u a t e d by the OBS i t s e l f ( f i g u r e s 16 and 17a). The t r a v e l t i m e s f o r the f i r s t two r e c e i v e r s have been c o r r e c t e d t o e f f e c t i v e l y p l a c e the OBSs on the ocean s u r f a c e . The f i r s t a r r i v a l s b e f o r e 82 km ( s h o t - r e c e i v e r d i s t a n c e ) a r e of e x c e l l e n t q u a l i t y w i t h t r a v e l time u n c e r t a i n t i e s i n the o r d e r of 0.05 s or l e s s . F i r s t a r r i v a l s beyond 82 km a r e v e r y weak and emergent and c o r r e s p o n d t o shot 4 (the f o u r t h t r a c e from the n e a r - o f f s e t end) on each of the common r e c e i v e r g a t h e r s 15, 16, and 17 ( f i g u r e 24, 26, and 2 8 ) . These a r r i v a l s have an apparent v e l o c i t y of about 8 km/s. The s t r o n g secondary a r r i v a l s between 120 km and 230 km have an apparent v e l o c i t y of 6.6 km/s. The f i n a l model, w i t h t r a c e d r a y s , i s shown i n f i g u r e 31. T h i s p r o f i l e i s the ' p s e u d o - r e v e r s a l ' f o r r e c e i v e r s 1 and 3. ( f i g u r e 21 and f i g u r e 2 3 ) . A r r i v a l s b e f o r e 80(185) km a r e r a y s t h a t have t r a v e r s e d the t e r r a c e r e g i o n . The a r r i v a l s out t o 65(170) km a r e m o d e l l e d as t u r n i n g r a y s t h r o u g h the upper h i g h g r a d i e n t t e r r a c e b l o c k . A r r i v a l s between 65(170) km and 80(185) km a r e m o d e l l e d as r e f l e c t i o n s from the Moho beneath the Mode l D i s t a n c e (km) F i g u r e 31 F i n a l r ay t r a c i n g diagram f o r Common Shot P r o f i l e 4. (see t e x t f o r e x p l a n a t i o n ) . 79 t e r r a c e . The model t r a v e l t i m e s and a m p l i t u d e s f i t the da t a w e l l . Two d i f f e r e n t t y p e s of r a y s ( r e f r a c t e d and r e f l e c t e d ) t o model th e s e a r r i v a l s were chosen because of the l o c a t i o n of the boundary s e p a r a t i n g the upper and lower t e r r a c e b l o c k s . The weak f i r s t a r r i v a l s from 180(285) km t o 230(335) km ' r e v e r s e ' the mantle r e f r a c t i o n s of r e c e i v e r s 15, 16, and 17. The a m p l i t u d e s of the s y n t h e t i c s match the da t a w e l l but the t r a v e l t i m e s a r e s l i g h t l y e a r l y . 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 be d a t a r e l a t e d 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 f u l l y w i t h the i n t e r p r e t a t i o n of r e c e i v e r 15. The l a r g e a m p l i t u d e secondary a r r i v a l s from 120(225) km t o 230(335) km have been i n t e r p r e t e d as p o s t - c r i t i c a l r e f l e c t i o n s from the Moho beneath the Queen C h a r l o t t e I s l a n d s and Hecate S t r a i t . Both the a m p l i t u d e s and t r a v e l t i m e s a r e w e l l m o d e l l e d by t h e s y n t h e t i c s seismograms. These a r r i v a l s h e l p c o n s t r a i n the d i p of the Moho i n the i n t e r v a l between 140 km and 185 km (model d i s t a n c e ) . 3.3.8 Common Shot P r o f i l e 16 Common shot p r o f i l e 16 ( f i g u r e 32) g a t h e r s d a t a from a f a r shot r e c o r d e d on a l l r e c e i v e r s . A m p l i t u d e s have been c o r r e c t e d f o r r e c e i v e r r e s p o n s e . R e c e i v e r s 1 and 2 were e f f e c t i v e l y p l a c e d on the s u r f a c e of the ocean by ray t r a c i n g t h rough the water column. The a m p l i t u d e s of t h e s e two a r r i v a l s (marked on f i g u r e 32 w i t h arrowheads) a r e v e r y much s m a l l e r than 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 i s i s c o n s i d e r e d t o be r e l a t e d t o the OBS-ocean bottom c o u p l i n g and the poor response of the OBSs a t f r e q u e n c i e s below 5 Hz. T h i s i s d i s c u s s e d i n more S h o t - R e c e i v e r 110 130 150 Di s t m c e 170 190 (km) 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 CD o L0 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320 330 340 Mode l D i s t a n c e (km) F i g u r e 32 - Comparison of data and s y n t h e t i c f o r Common Shot P r o f i l e 1G. (a) d a t a . (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. The t r a v e l time c u r v e s from the s y n t h e t i c a r r i v a l s a re drawn 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 an e x p l a n a t i o n of d i s t a n c e and time axes. 81 d e t a i l w i t h the i n t e r p r e t a t i o n of shot 4 common r e c e i v e r g a t h e r . The t r a v e l time u n c e r t a i n t i e s f o r the s e two a r r i v a l s i s i n the or d e r of 0.1 s. The f i r s t a r r i v a l s between 80 km and 125 km have u n c e r t a i n t i e s of about 0.05 s. T h i s energy was r e c o r d e d on s t a t i o n s d e p l o y e d a c r o s s the Queen C h a r l o t t e I s l a n d s . The gap i n the data between 130 km and 220 km r e p r e s e n t s , the f o u r OBSs ( r e c e i v e r s 11, 12, 13, and 14) t h a t were d e p l o y e d i n Hecate S t r a i t . Due t o a source or sources of unknown h i g h a m p l i t u d e n o i s e , they d i d not r e c o r d any v i s i b l e s e i s m i c energy from shot 16. The t h r e e a r r i v a l s beyond 220 km c o r r e s p o n d t o the main l a n d s t a t i o n s and have an apparent v e l o c i t y of s l i g h t l y l e s s than 8 km/s a l t h o u g h i t i s hard t o e s t i m a t e t h i s a c c u r a t e l y w i t h o n l y t h r e e p o i n t s . A l l f i r s t a r r i v a l s have" been m o d e l l e d as mantle r e f r a c t i o n s ( f i g u r e 3 3 ) . The o n l y secondary a r r i v a l observed on t h i s common shot p r o f i l e a t 60 km and 6.3 s c o r r e s p o n d s t o r e c e i v e r 1. T h i s wide a n g l e r e f l e c t i o n from the o c e a n i c Moho i s ob s e r v e d on common r e c e i v e r p r o f i l e 1 ( f i g u r e 20 pg. 58). Wide a n g l e r e f l e c t i o n s from shot 16 a r e i d e n t i f i a b l e o n l y on r e c e i v e r 1 because the change i n Moho d i p a t the o u t e r t e r r a c e edge ( a t a model d i s t a n c e of 100 km i n f i g u r e 33) and the h i g h g r a d i e n t upper t e r r a c e do not a l l o w energy t o propagate eastward through the t e r r a c e r e g i o n . I t i s f o r t h i s reason t h a t a l l f i r s t a r r i v a l s were m o d e l l e d as mantle r e f r a c t i o n s . The model t r a v e l t i m e s and a m p l i t u d e s f i t the da t a w e l l f o r a r r i v a l s out t o 130(190) km. T h i s p r o f i l e h e l p s t o c o n s t r a i n the Moho d i p i n t h i s r e g i o n and the average v e l o c i t y s t r u c t u r e 83 above the Moho. A s m a l l bend i n t h e Moho beneath the Queen C h a r l o t t e I s l a n d s was used t o model the apparent v e l o c i t y v a r i a t i o n s between 80(140) km and 130(190) km. Whether t h i s bend i n the Moho or s t r u c t u r e a t a s h a l l o w e r depth i s the cause of t h i s v a r i a t i o n i s not c o n s t r a i n e d by t h i s d ata s e t . These a r r i v a l s ' r e v e r s e ' common r e c e i v e r g a t h e r s 15, 16, and 17 f o r r a y s " t h a t c r o s s the Moho beneath t h e Queen C h a r l o t t e I s l a n d s . T h i s ' p s e u d o - r e v e r s a l ' h e l p s c o n s t r a i n t h e d i p of the Moho. The model f i t a t f a r o f f s e t s i s not q u i t e as good. The t r a v e l t imes a r e l a t e and the a m p l i t u d e s much too l a r g e . Because the a r r i v a l s a r e the o n l y d a t a p o i n t s i n t h a t r e g i o n i t i s d i f f i c u l 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 caused by l o c a l s i t e - d e p e n d e n t s t r u c t u r e or f e a t u r e s a t lower c r u s t a l d e p t h s . The u n c e r t a i n t i e s of the t r a v e l time p i c k s a r e l a r g e because of the n o i s e . I t i s f e l t t h a t these a r r i v a l s a r e f i t as w e l l as the d a t a q u a l i t y r e q u i r e s . The a m p l i t u d e s would be lowered by a s m a l l e r g r a d i e n t i n the mantle a l t h o u g h t h i s i s i n c o n s i s t e n t w i t h r e c e i v e r g a t h e r s 15, 16, and 17. W i t h a g r a d i e n t below 0.005 km/s/km, a shadow zone i n the t e r r a c e r e g i o n f o r mantle r e f r a c t e d r a y s i s c r e a t e d f o r the s e common r e c e i v e r g a t h e r s . 3.4 The F i n a l Model - A Recap The f i n a l model has s e v e r a l f e a t u r e s which a r e w e l l c o n s t r a i n e d by the d a t a . The t e r r a c e r e g i o n , the change i n d i p of the Moho beneath the o u t e r edge of the t e r r a c e , and the s h a l l o w c r u s t beneath the Queen C h a r l o t t e I s l a n d s and Hecate 84 S t r a i t w i l l be d i s c u s s e d . The 1.8 km/s l a y e r , r e p r e s e n t i n g u n c o n s o l i d a t e d 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 s e i s m i c p r o f i l e ( D a v i s and Seemann, 1981) a l o n g the r e f r a c t i o n l i n e . The o c e a n i c s t r u c t u r e i s v e r y s i m i l a r t o Horn e t a l . (1984). Other r e f r a c t i o n s t u d i e s (Johnson e t a l . , 1972; F o r s y t h et a l . , 1974) i n d i c a t e t h a t the c r u s t i s about 30 km t h i c k a l o n g the west c o a s t of the m a i n l a n d . The t e r r a c e r e g i o n has been d i v i d e d i n t o two b l o c k s : an upper u n i t w i t h low v e l o c i t y (4.1 km/s) and h i g h g r a d i e n t (0.3 km/s/km) and a lower u n i t w i t h a much h i g h e r v e l o c i t y (6.5 km/s) and a low g r a d i e n t (0.05 km/s/km). E x a c t l y where t h i s change t a k e s p l a c e i s c o n s t r a i n e d o n l y between 6 and 10 km. The h i g h g r a d i e n t i n the upper b l o c k i s n e c e s s a r y t o e x p l a i n a m p l i t u d e s of c l o s e a r r i v a l s on r e c e i v e r s 1 and 3. The l a c k o f ' any wide-a n g l e r e f l e c t i o n s from t h e o c e a n i c Moho t h a t a r e so prominent on r e c e i v e r 1 ( b u t not o b s e r v e d on any r e c e i v e r s e a s t of the Queen C h a r l o t t e f a u l t zone) i s a l s o e x p l a i n e d by t h i s b l o c k which f o c u s e s t h e energy t o the s u r f a c e . The h i g h g r a d i e n t a l s o c o n t r i b u t e s t o the abr u p t end of t h e Moho r e f l e c t i o n s v i s i b l e on r e c e i v e r s 15, 16, and 17. The low e r t e r r a c e u n i t , w i t h a g r a d i e n t of 0.05 km/s/km, i s r e q u i r e d by t h e wide a n g l e r e f l e c t i o n s on r e c e i v e r 1. A h i g h e r g r a d i e n t does not a l l o w the energy t o r e a c h the s t a t i o n . The r e f r a c t i o n s and w i d e - a n g l e r e f l e c t i o n s a t f a r o f f s e t on shot g a t h e r 4 a l s o r e q u i r e a r e g i o n of low g r a d i e n t i n the lower t e r r a c e . The average v e l o c i t y of the t e r r a c e r e g i o n i s c o n s t r a i n e d by f i r s t a r r i v a l s t h r o u g h the t e r r a c e ( f i g u r e 33, f i g u r e 21 and f i g u r e 2 3 ) . 85 The 5° eastward d i p of the Moho shown i n the f i n a l model i s r e a s o n a b l y w e l l c o n s t r a i n e d out t o 200 km model d i s t a n c e ( f i g u r e 19). Wide a n g l e r e f l e c t i o n s r e c o r d e d on r e c e i v e r s 15, 16, and 17 h e l p f i x the depth of t h i s boundary a t p o i n t s p r o g r e s s i v e l y e a s t w a r d . The change i n d i p of the Moho from an almost h o r i z o n t a l d i s c o n t i n u i t y beneath the ocean t o a d i p p i n g i n t e r f a c e beneath the t e r r a c e i s the major f a c t o r c o n t r o l l i n g the a b r u p t t e r m i n a t i o n of thes e w ide-angle r e f l e c t i o n s (eg. f i g u r e 2 4 ) . The wide-angle r e f l e c t i o n s on shot g a t h e r 4 p r o v i d e a ' p s e u d o - r e v e r s a l ' of r e f l e c t i o n s on r e c e i v e r s 15, 16, and 17. V a r i a t i o n s i n the s h a l l o w s t r u c t u r e of the model above the d i p p i n g Moho can i n f l u e n c e i t s d i p . I f the 5.5 km/s b l o c k from 220 km t o 340 km, model d i s t a n c e s , ( f i g u r e 1 9 , page 53) were r e p l a c e d w i t h a 6.65 km/s b l o c k the d i p of the Moho would have t o be i n c r e a s e d t o 9° i n o r d e r t o s a t i s f y t he t r a v e l time f i t . A s h a l l o w Moho beneath the t e r r a c e r e g i o n i s r e q u i r e d by the c o n t i n u i t y of mantle r e f r a c t i o n f i r s t a r r i v a l s on r e c e i v e r g a t h e r s 15, 16, and 17. A s t e e p l y d i p p i n g boundary j o i n i n g the o c e a n i c Moho t o a deep c o n t i n e n t a l Moho, such as shown i n f i g u r e 12 (page 27), would b l o c k the mantle r a y s and c r e a t e a shadow zone e a s t of the t e r r a c e r e g i o n . None of the energy from s h o t s i n t h e shadow zone would re a c h r e c e i v e r s 15, 16, or 17. The average v e l o c i t y of the ocean c r u s t and the depth of the o c e a n i c Moho are w e l l c o n s t r a i n e d by the wide - a n g l e r e f l e c t i o n s r e c o r d e d on r e c e i v e r 1. S i n c e a l l o t h e r r a y s t r a v e l v e r t i c a l l y t h r o u g h the o c e a n i c r e g i o n the upper l a y e r s a r e p o o r l y c o n s t r a i n e d and c o u l d e a s i l y be r e p r e s e n t e d by one l a y e r . 86 The o c e a n i c s t r u c t u r e used was taken from Horn (1982) and Horn et a l . (1984), who a l s o m o d e l l e d a r e f r a c t i o n l i n e shot p a r a l l e l t o the Queen C h a r l o t t e f a u l t zone. The mantle r e f r a c t i o n s t h r ough the t e r r a c e r e g i o n r e c o r d e d on r e c e i v e r s 1 and 3 h e l p c o n s t r a i n the depth t o the Moho i n t h a t r e g i o n . The s o l i d b o u n d a r i e s i n f i g u r e 19 (page 53) are w e l l c o n s t r a i n e d by the d a t a . The dashed l i n e s r e p r e s e n t b o u n d a r i e s t h a t a r e not w e l l c o n s t r a i n e d by the d a t a s e t . The m o d e l l i n g p r o c e d u r e has p r o v i d e d us w i t h a v e l o c i t y s t r u c t u r e t h a t has some w e l l c o n s t r a i n e d and s i g n i f i c a n t f e a t u r e s . The t e r r a c e r e g i o n and a g e n t l y d i p p i n g and s h a l l o w Moho beneath the Queen C h a r l o t t e I s l a n d s and western Hecate S t r a i t a r e such f e a t u r e s . What i s the t e c t o n i c s i g n i f i c a n c e of t h i s v e l o c i t y s t r u c t u r e ? The slow v e l o c i t y and h i g h g r a d i e n t of the upper t e r r a c e i s d i a g n o s t i c of h i g h l y sheared and deformed s e d i m e n t s . Could t h i s be an a c c r e t i o n a r y sedimentary wedge? I s the v e r y s h a l l o w Moho beneath the Queen C h a r l o t t e I s l a n d s c o n t i n e n t a l c r u s t or u n d e r t h r u s t e d o c e a n i c m a t e r i a l ? In the f o l l o w i n g c h a p t e r , an attempt t o answer th e s e q u e s t i o n s i s made. 87 IV. DISCUSSION AND CONCLUSIONS T h i s s e i s m i c r e f r a c t i o n study was plann e d t o o b t a i n more i n f o r m a t i o n on the Queen C h a r l o t t e I s l a n d s r e g i o n . To meet t h i s o b j e c t i v e of the s t u d y , i t i s n e c e s s a r y t o t a k e the f i n a l v e l o c i t y model d e r i v e d from the d a t a and i n t e r p r e t i t i n a t e c t o n i c sense. Only the f e a t u r e s of the model t h a t a re both s i g n i f i c a n t and w e l l c o n s t r a i n e d w i l l be c o n s i d e r e d . These f e a t u r e s a r e : (1) the t h i n c r u s t beneath the Queen C h a r l o t t e I s l a n d s and Hecate S t r a i t ; (2) the g e n t l e e a s t w a r d d i p (5°) of th e Moho s t a r t i n g at the o u t e r edge of the t e r r a c e ; and (3) the t e r r a c e r e g i o n i t s e l f which s e p a r a t e s d i f f e r e n t m a t e r i a l on e i t h e r s i d e . Two d i f f e r e n t models, one w i t h s u b d u c t i o n and one w i t h o u t , w i l l be d i s c u s s e d i n t h i s c h a p t e r . Both of the s e models a r e shown i n f i g u r e 34. Ev i d e n c e i n d i c a t i n g the p o s s i b l e u n d e r t h r u s t i n g of the P a c i f i c p l a t e beneath N o r t h America a t the Queen C h a r l o t t e I s l a n d s was d i s c u s s e d i n Chapter I as j u s t i f i c a t i o n f o r the r e f r a c t i o n e x p e r i m e n t . The d a t a a n a l y z e d do not d e l i n e a t e a s t r u c t u r e t h a t c o u l d be unambiguously i d e n t i f i e d as a s u b d u c t i n g s l a b ; however, the model does i n d i c a t e t h a t the c r u s t i s not 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 . The g e n t l e t r a n s i t i o n from o c e a n i c c r u s t t o the c r u s t beneath the Queen C h a r l o t t e I s l a n d s i s a l s o not i n d i c a t i v e of a t r a n s i t i o n from s t a n d a r d ocean c r u s t t o s t a n d a r d c o n t i n e n t a l c r u s t where a l a r g e change i n t h i c k n e s s can be e x p e c t e d over a s m a l l d i s t a n c e . Thus the Moho d e f i n e d by the r e f r a c t i o n d a t a beneath the Queen C h a r l o t t e I s l a n d s and Hecate S t r a i t may be the bottom edge of an 88 These two drawings a r e schematic 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 regime and (b) a n o n - s u b d u c t i o n 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 the s u b d u c t i n g s l a b drawn i n (a) i s not seen by the r e f r a c t i o n d a t a and i s shown here j u s t as a p o s s i b l e model. 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 u n d e r t h r u s t P a c i f i c p l a t e . F u r t h e r credence t o t h i s i n t e r p r e t a t i o n i s p r o v i d e d by the lower t e r r a c e u n i t which i s v e r y s i m i l a r i n v e l o c i t y s t r u c t u r e t o the o c e a n i c p l a t e t o the west and the c r u s t beneath the Queen C h a r l o t t e I s l a n d s t o the e a s t . T h i s i n d i c a t e s t h a t o c e a n i c c r u s t most l i k e l y u n d e r l i e s the compressed sediments of the t e r r a c e r e g i o n . The upper t e r r a c e r e g i o n w i t h i t s low v e l o c i t y and h i g h g r a d i e n t (4.1 km/s; 0.3 km/s/km) p r o b a b l y r e p r e s e n t s compressed s e d i m e n t s . Von Huene et a l . (1985) r e p o r t a v e l o c i t y of 1.7-2.0 km/s a t the f r o n t of the a c c r e t e d sediments and 5.6 km/s f o r o l d e r more compressed a c c r e t e d sediments i n c e n t r a l P e r u . The t e r r a c e , from b o t h s e i s m i c r e f l e c t i o n (Chase and T i f f i n , 1972 and Chase et a l . , 1975) and s e i s m i c r e f r a c t i o n work (Horn e t a l . , 1984 and t h i s s t u d y ) appears t o be composed of h i g h l y deformed compressed s e d i m e n t s . The s t e p - l i k e p r o f i l e of the t e r r a c e i s s i m i l a r t o many known s u b d u c t i o n zones t h a t a r e a c c r e t i n g s e d i m e n t a r y m a t e r i a l (see K a r i g , 1977). The model i n t h i s s t u d y i n d i c a t e s t h a t t h i s t h i c k sequence of m a t e r i a l c l o s e l y resembles a c c r e t i o n a r y s e d i m e n t a r y p r i s m s t h a t a r e prominent i n s h a l l o w s u b d u c t i o n zones ( S e e l y , 1979; Uyeda and Kanamori, 1979 and o t h e r s ) . The e a s t w a r d s l o p i n g boundary between the upper and lower t e r r a c e r e g i o n s i s not w e l l enough c o n s t r a i n e d by the d a t a t o c l a i m i t r e p r e s e n t s t h e t o p of an u n d e r t h r u s t P a c i f i c p l a t e . In t h e t r o u g h r e g i o n t h e o c e a n i c Moho bends downward from b e i n g a l m o s t h o r i z o n t a l beneath the ocean t o an e a stward d i p of about 5°. The bend i n the Moho i s w e l l c o n s t r a i n e d by the a b r u p t end of r e f l e c t i o n s from t h i s 90 horizon on any r e c e i v e r s east of the t e r r a c e ( r e c e i v e r s 3 to 17). The gentleness of the d i p and i t s extension below Hecate S t r a i t i s a l s o w e l l c o n s t r a i n e d . This feature c o r r e l a t e s w e l l with an underthrust shallow s l a b where a gentle d i p would be expected. The lack of a b i l i t y to d i s t i n g u i s h the s l a b beneath the Queen C h a r l o t t e I s l a n d s and Hecate S t r a i t 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 to the data set or i n - s i t u p h y s i c a l p r o p e r t i e s of the s l a b or the c o n t i n e n t a l c r u s t above i t . A l l raypaths pass through the proposed s l a b (no rays have t u r n i n g p o i n t s i n the slab) and thus 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 shallow subduction of the P a c i f i c p l a t e i s not s u r p r i s i n g . The P a c i f i c p l a t e at the Queen C h a r l o t t e Islands i s about 7 Ma o l d . Ruff and Kanamori (1980), and V l a a r and Wortel (1976) note that there i s a 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 pe n e t r a t i o n of the s l a b . The young, hot, and buoyant s l a b tends to r e s i s t subduction i n t o the mantle. Uyeda and Kanamori (1979) note that i n places where the o v e r t h r u s t i n g p l a t e has a higher absolute v e l o c i t y toward the trench (assuming the trench i s f i x e d i n the mantle) than the underthrust s l a b , the subduction i s shallow. This i s observed i n the Chilean (100-30° d i p ) , Peru (10°), and North Honshu Japan (35°-40°) subduction zones (see Uyeda and Kanamori 1979 and Wortel, 1980). The absolute motion of model AM1 (Minster et a l . , 1974) i n d i c a t e s that from the hotspot reference frame North America i s moving i n a southwest d i r e c t i o n at about 2.7 cm/yr. The P a c i f i c p l a t e i s moving almost north-91 n orthwest a t 5.6 t o 6.1 cm/yr. Thus N o r t h America i s o v e r t h r u s t i n g the t r e n c h w h i l e the P a c i f i c p l a t e i s moving m o s t l y p a r a l l e l t o the t r e n c h . The o v e r t h r u s t i n g p l a t e h a v i n g the l a r g e r component of convergence i n d i c a t e s t h a t s h a l l o w s u b d u c t i o n s h o u l d be o c c u r r i n g . Time c o n s t r a i n t s d i d not a l l o w a g r a v i t y i n t e r p r e t a t i o n of the f i n a l v e l o c i t y model but p r e l i m i n a r y m o d e l l i n g of g r a v i t y d a t a a c r o s s the sou t h e r n t i p of Moresby I s l a n d i n d i c a t e s t h a t a s h a l l o w l y s u b d u c t i n g s l a b i s c o n s i s t e n t w i t h the da t a (S. C a r b o t t e , p e r s o n a l communication, 1985). The g r a v i t y model i s shown i n f i g u r e 35. I t i n d i c a t e s t h a t the Moho d i p s between 4° and 9° beneath the t e r r a c e r e g i o n and western Hecate S t r a i t . In e a s t e r n Hecate S t r a i t the Moho i s f l a t l y i n g r e a c h i n g a maximum depth of 22 km. C r u s t a l t h i c k n e s s below the t e r r a c e r e g i o n i s about 12 t o 15 km a g r e e i n g w e l l w i t h the r e f r a c t i o n v e l o c i t y model. T h i s g r a v i t y model i s c o n s i s t e n t w i t h v e r y s h a l l o w s u b d u c t i o n of the P a c i f i c p l a t e beneath the Queen C h a r l o t t e I s l a n d s . There a r e many p l a c e s i n the w o r l d where v e r y s h a l l o w s u b d u c t i o n has been documented. F o c a l mechanisms of s e i s m i c i t y i n d i c a t e t h a t beneath c e n t r a l and n o r t h e r n P e r u the s l a b d i p s between 10° and 15° ( S t a u d e r , 1975). Cockerham (1984) r e p o r t s t h a t the Gorda p l a t e d i p s a t 10° f o r a l e n g t h of 120 km t o a depth of 30-35 km where i t steepens t o a d i p of 25°. C l o s e r t o home, the Juan de Fuca p l a t e a l s o e x h i b i t s s h a l l o w s u b d u c t i o n . P r e l i m i n a r y a n a l y s i s of the L i t h o p r o b e s e i s m i c r e f l e c t i o n p r o f i l e s a c r o s s s o u t h e r n Vancouver I s l a n d i n d i c a t e s t h a t 92 F i g u r e 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 the s o u t h e r n t i p of Moresby I s l a n d (S. C a r b o t t e , p e r s o n a l communication, 1985). 93 Wrangellia i s between 15 and 20 km thick and i s underlain by a zone of possibly underplated allocthonous material or an old slab, beneath which l i e s the actual subducting slab (Yorath et a l . , 1985). The model of shallow subduction is consistent with other phenomena observed in the Queen Charlotte Islands region, discussed previously in Chapter I. The broad gentle offshore topographic bulge and associated gravity high is observed here. The Queen Charlotte trough i s shallow and f i l l e d with 2-3 km of f l a t - l y i n g sediments resembling trenches associated with a shallow angle of subduction such as central and northern Peru, the Aleutians, and others. Scholl (1974) observes that the Queen Charlotte trough has stratigraphic and s t r u c t u r a l features t y p i c a l of North P a c i f i c trenches such as the Washington-Oregon trench which i s f i l l e d with 1-2 km of sedimentary deposits. U p l i f t has been occurring along the west coast of the Queen Charlotte Islands for the l a s t 10 Ma (Parrish, 1982) and is probably continuing presently (Riddihough, 1982b). Subsidence has been occurring in Hecate S t r a i t , just 80 km to the east, for the l a s t 6 Ma. Many subduction zones display t h i s pattern of u p l i f t and subsidence but several in particular are similar to the Queen Charlotte Islands. Scholz and Kato (1978) studied the South Kanto d i s t r i c t in Japan where highly oblique subduction of the P a c i f i c plate occurs at 3 cm/yr along the Sangami trough. Elevation p r o f i l e s indicate that u p l i f t of about 20 mm/yr occurs near the trench; the " u p l i f t " f a l l s to negative values 30 km landward of the trench axis. Seno (1977) has documented the 94 same p a t t e r n i n the Nankai t r o u g h r e g i o n . The A l p i n e F a u l t i n New Zealand i s a zone of o b l i q u e convergence a t 4 cm/yx between the P a c i f i c and I n d i a n p l a t e s ( W a l c o t t , 1978). A l t h o u g h t h e r e i s o b l i q u e convergence a t 10°-15° t h e r e i s no s u b d u c t i o n o c c u r r i n g a l o n g the f a u l t . V e r t i c a l movements near the f a u l t r e a c h +10 mm/yr but f a l l t o n e g a t i v e v a l u e s w i t h i n 120 km of the t r e n c h (Lensen, 1975). The p r e s e n t u p l i f t a l o n g t h e western edge of the Queen C h a r l o t t e I s l a n d s of +2 mm/yr drops t o -1 mm/yr w i t h i n 80 km ( R i d d i h o u g h , 1982b, and P a r r i s h , 1982) which i s c o m p a t i b l e w i t h h i g h l y o b l i q u e s u b d u c t i o n and t r a n s f o r m b o u n d a r i e s . R i d d i h o u g h (1982b) i n d i c a t e s t h a t s u b s i d e n c e a t h i g h l y o b l i q u e c o n v e r g e n t margins o c c u r s c l o s e r t o the margin than i n a r e a s where the t e c t o n i c regime i s d o m i n a n t l y s u b d u c t i o n . Subsidence i n both t h e Andes and A l a s k a a p p e a r s t o occur f u r t h e r from the t r e n c h t h a n i n the Queen C h a r l o t t e I s l a n d s (see P l a f k e r , 1972). The s e i s m i c i t y r e c o r d e d i n the Queen C h a r l o t t e I s l a n d s r e g i o n (Berube, 1985 and Rogers, 1983) i n d i c a t e s t h a t c o m p r e s s i o n a l s t r e s s i s b e i n g r e l e a s e d i n eart h q u a k e s a l o n g the boundary. Berube (1985) i d e n t i f i e d some s h a l l o w t h r u s t e v e n t s i n l a n d on Graham I s l a n d . 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 (5-15 km) and i t c o u l d not be a s s o c i a t e d w i t h a B e n i o f f zone. T h i s may be a r e s u l t o f the s h o r t s t u d y l e n g t h (3 months). T h i s l a c k of s e i s m i c i t y a s s o c i a t e d w i t h a s u b d u c t i n g s l a b may be caused by: (1) a s e i s m i c s l i p , (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 p r o c e s s , or (3) the l a c k of s u b d u c t i o n . 95 S i n c e we a r e c o n s i d e r i n g the s u b d u c t i o n model a t p r e s e n t o n l y the f i r s t two p o s s i b i l i t i e s a r e r e l e v a n t . Except f o r the s e i s m i c i t y , the Queen C h a r l o t t e I s l a n d s e x h i b i t c h a r a c t e r i s t i c s of s u b d u c t i o n zones w i t h s h a l l o w l y d i p p i n g s l a b s where l a r g e c o u p l i n g f o r c e s between the upper and lower p l a t e s cause l a r g e t h r u s t t ype e a r t h q u a k e s (eg. N.E. Japan, the A l e u t i a n s , and South A m e r i c a ) . There a r e examples of s h a l l o w l y s u b d u c t i n g p l a t e s t h a t do e x h i b i t low s e i s m i c i t y . S c h o l z and K a t o (1978) r e p o r t t h a t a s e i s m i c s u b d u c t i o n i s o c c u r r i n g below 15 km d u r i n g the i n t e r - s e i s m i c p e r i o d s of the Sangami s u b d u c t i o n zone. Rogers (1983) n o t e s t h a t the s u b d u c t i o n of t h e Juan de Fuca p l a t e beneath s o u t h e r n Vancouver I s l a n d i s p r o c e e d i n g a s e i s m i c a l l y a t p r e s e n t , i n d i c a t i n g t h a t n o r t h of Puget Sound t h e r e i s l i t t l e c o u p l i n g between Vancouver I s l a n d and the s h a l l o w l y d i p p i n g s l a b . Heaton and Kanamori (1984) suggest t h a t the slow convergence r a t e a c r o s s t h i s boundary (3-4 cm/yr) may not r e s u l t i n s t r o n g c o u p l i n g between the s l a b and the o v e r r i d i n g p l a t e . Back t i l t i n g o f the c o a s t a l a r e a of Vancouver I s l a n d may i n d i c a t e low r e s i s t a n c e t o s u b d u c t i o n ( R i d d i h o u g h , 1984). The s h a l l o w s u b d u c t i o n zone a l o n g Columbia has s e v e r a l s e i s m i c gaps ( P e n n i n g t o n , 1984). Pennington s u g g e s t s t h a t t h e s e a r e a s of a s e i s m i c s u b d u c t i o n may be due t o topography on the s u b d u c t i n g p l a t e . K e l l e h e r and McCann (1976) i n d i c a t e t h a t i n a r e a s where b a t h y m e t r i c h i g h s i n t e r a c t w i t h the o v e r l y i n g p l a t e l a r g e e a r t h q u a k e s o c c u r l e s s f r e q u e n t l y . A l l of t h i s s u g g e s t s t h a t i t i s p o s s i b l e t h a t a s e i s m i c s u b d u c t i o n i s o c c u r r i n g beneath th e Queen C h a r l o t t e 96 I s l a n d s . The convergence r a t e i s v e r y low (2 cm/yr a t maximum), and the o c e a n i c c r u s t i s v e r y young and d u c t i l e ; t h e r e f o r e a l a r g e c o u p l i n g between t h e P a c i f i c s l a b and the o v e r r i d i n g c o n t i n e n t would not be e x p e c t e d . A l s o the P a c i f i c p l a t e n o r t h of Vancouver I s l a n d , t h e s i t e of the i n i t i a t i o n of s u b d u c t i o n beneath the Queen C h a r l o t t e I s l a n d s , has v e r y l a r g e topography. T h i s topography has p r o b a b l y e x i s t e d s i n c e the p r e s ence of the Juan de F u c a - P a c i f i c - N o r t h A m e r i c a t r i p l e j u n c t i o n a t t h a t p o s i t i o n (see f i g u r e 1, page 2 ) . I t may be p o s s i b l e t h a t a s e i s m i c s u b d u c t i o n i s p r e s e n t l y o c c u r r i n g a l o n g the Queen C h a r l o t t e t r a n s f o r m f a u l t z one. Of c o u r s e i t i s p o s s i b l e t h a t the s u b d u c t i o n zone i s l o c k e d a t p r e s e n t and no s e i s m i c a c t i v i t y r e l a t e d t o u n d e r t h r u s t i n g i s o b s e r v e d . S i n c e t r a n s c u r r e n t motion i s o c c u r r i n g a l o n g the boundary a t p r e s e n t , t h i s would i n d i c a t e t h a t the u n d e r t h r u s t s l a b must be s e p a r a t e from t h e P a c i f i c p l a t e t o the west f o r t h i s c o n d i t i o n t o e x i s t . F i n a l l y i t i s p o s s i b l e t h a t no s u b d u c t i o n i s o c c u r r i n g a l o n g t h e Queen C h a r l o t t e t r a n s f o r m f a u l t zone. T h i s p o s s i b i l i t y w i l l be c o n s i d e r e d i n some d e t a i l . R e l a t i v e p l a t e m o t i o n s , c u r r e n t u p l i f t p a t t e r n s a l o n g the west c o a s t of the Queen C h a r l o t t e I s l a n d s , s u b d s i d e n c e i n l a n d , and s e s i m i c i t y i n d i c a t e t h a t c o m p r e s s i o n a c r o s s the P a c i f i c N o r t h America p l a t e b o u n d a r i e s e x i s t s p r e s e n t l y . I f u n d e r t h r u s t i n g of the P a c i f i c p l a t e i s not o c c u r r i n g then t h i s c o m p r e s s i o n must be t a k e n up by d e f o r m a t i o n of the Queen C h a r l o t t e I s l a n d s or the P a c i f i c p l a t e . T h i s model i s v e r y s i m i l a r t o the p l a t e boundary between the P a c i f i c and I n d i a n 97 p l a t e s a t the A l p i n e F a u l t i n New Zeal a n d . The A l p i n e F a u l t c o n n e c t s the Tonga-Kermadec s u b d u c t i o n zone i n the n o r t h w i t h the Macquarie Ridge complex i n the s o u t h . A l t h o u g h the A l p i n e F a u l t i s t e c h n i c a l l y a t r a n s f o r m f a u l t the p a t t e r n of s e i s m i c i t y i n d i c a t e s t h a t i t i s a wide zone of d e f o r m a t i o n ( W a l c o t t , 1978). The r e l a t i v e p l a t e m o t ions i n t h i s a r e a a re c h a r a c t e r i z e d by h i g h l y o b l i q u e convergence of the P a c i f i c p l a t e toward the I n d i a n p l a t e . The p r e s e n t component of convergence i s about 2 cm/yr ( W a l c o t t , 1978). S e i s m i c i t y i n t h i s area i s s h a l l o w (4-15 km) ( C a l d w e l l and F r o h l i c h , 1975) as i n the Queen C h a r l o t t e I s l a n d s r e g i o n . U p l i f t a c r o s s the A l p i n e F a u l t has been d i s c u s s e d p r e v i o u s l y and i s s i m i l a r i n p a t t e r n t o t h a t i n the Queen C h a r l o t t e I s l a n d s . The P a c i f i c p l a t e i s n o t , however, s u b d u c t i n g beneath New Zealand a l o n g the A l p i n e F a u l t . W a l c o t t (1978) s u g g e s t s t h a t the c o m p r e s s i v e s t r e s s i s b e i n g taken up by d e f o r m a t i o n of a broad c o r r i d o r p a r a l l e l i n g the A l p i n e F a u l t . T w e n t y - f i v e k i l o m e t r e s of c r u s t a l s h o r t e n i n g w i l l e x p l a i n the u p l i f t ( W a l c o t t , 1978). C r u s t a l t h i c k e n i n g on the o r d e r of 20 per c e n t of the s h o r t e n i n g i s thought t o o c c u r , and one f i f t h of t h i s t h i c k e n i n g i s m a n i f e s t e d as u p l i f t . W a l c o t t a l s o s u g gests t h a t some m a t e r i a l may have been squeezed northwards t o t h e H i k u r a n g i s u b d u c t i o n zone a l o n g the N o r t h I s l a n d . I t i s p o s s i b l e t h a t the Queen C h a r l o t t e I s l a n d s i s a s i m i l a r type of t r a n s f o r m boundary. The s e i s m i c i t y i s s h a l l o w and a l t h o u g h most of i t o c c u r s a l o n g the i n n e r s c a r p of the t e r r a c e , some s e i s m i c i t y was found t o occur a l o n g the o u t e r t e r r a c e s c a r p (Berube, 1985). There i s a zone of a c t i v i t y 98 between the two p l a t e s . The d e f o r m a t i o n model can e x p l a i n the s e i s m i c i t y a l o n g the Queen C h a r l o t t e t r a n s f o r m f a u l t but i t cannot e x p l a i n the t h i n c r u s t beneath the Queen C h a r l o t t e I s l a n d s and western Hecate S t r a i t . The f i n a l v e l o c i t y model and the g r a v i t y model of C a r b o t t e ( p e r s o n a l communication, 1985) both i n d i c a t e t h a t the c r u s t i s 12 km t h i c k a t the west c o a s t of the Queen C h a r l o t t e I s l a n d s and about 1 8 km t h i c k a t the east c o a s t . Given t h a t the P a c i f i c p l a t e has been convergent w i t h N o r t h America a t over 1 cm/yr f o r the pa s t 6 Ma (as i s a l s o i n d i c a t e d by a study based on a b s o l u t e P a c i f i c p l a t e motion by Cox and E n g e b r e t s o n , 1985) 60 km of movement must have been taken up by d e f o r m a t i o n i n the Queen C h a r l o t t e I s l a n d s r e g i o n . T h i s i n f e r s t h a t about 12 km of c r u s t a l t h i c k e n i n g must have o c c u r r e d . ( W a l c o t t ( 1 978).uses a f i g u r e of 20 per c e n t - o f the c r u s t a l s h o r t e n i n g f o r the amount of c r u s t a l t h i c k e n i n g ) . T h i s i s i n c o m p a t i b l e w i t h the t h i n c r u s t beneath the Queen C h a r l o t t e I s l a n d s and w e s t e r n Hecate S t r a i t . On the b a s i s of e v i d e n c e f o r a t h i n c r u s t of the c o n t i n e n t a l o n g the Queen C h a r l o t t e t r a n s f o r m f a u l t an o b l i q u e s u b d u c t i o n model i s p r e f e r r e d over a no n - s u b d u c t i o n model. In summary the g e o l o g i c , p h y s i o g r a p h i c , and g e o p h y s i c a l d a t a c o l l e c t e d thus f a r i n d i c a t e t h a t u n d e r t h r u s t i n g of the P a c i f i c p l a t e beneath N o r t h America a l o n g the Queen C h a r l o t t e t r a n s f o r m f a u l t may have been o c c u r r i n g s i n c e 6 Ma. These data do not c o n c l u s i v e l y r u l e out o t h e r models as o u t l i n e d above. The d a t a a n a l y z e d i n t h i s study cannot d e l i n e a t e c l e a r l y the e x i s t e n c e of an u n d e r t h r u s t P a c i f i c p l a t e beneath the Queen 99 C h a r l o t t e I s l a n d s . F u r t h e r work which would a i d i n r e s o l v i n g t h i s q u e s t i o n i s n e c e s s a r y . A s e i s m i c r e f r a c t i o n p r o f i l e r u n n i n g n o r t h - s o u t h on the Queen C h a r l o t t e I s l a n d s would p r o v i d e many a d d i t i o n a l c o n s t r a i n t s on the c r u s t a l s t r u c t u r e i n t h i s r e g i o n . U l t i m a t e l y , a m u l t i - c h a n n e l deep s e i s m i c r e f l e c t i o n s u r v e y i n Hecate S t r a i t would a i d i n mapping the deep c r u s t and p o s s i b l y answer some of the q u e s t i o n s which s t i l l remain c o n c e r n i n g t h i s e n i g m a t i c p l a t e boundary. 100 B i b l i o g r a p h y Atwater T., (1970). 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 Cen o z o i c t e c t o n i c e v o l u t i o n of Western N o r t h A m e r i c a . G e o l o g i c a l S o c i e t y of America B u l l e t i n , 81, 3513-3536. Atwater T., P. Molnar, (1973). R e l a t i v e motion of the P a c i f i c and N o r t h America P l a t e s deduced from s e a - f l o o r s p r e a d i n g i n the A t l a n t i c , I n d i a and South P a c i f i c Oceans. I n : Kovach R.L., A. Nur Eds. P r o c e e d i n g s of Conference on T e c t o n i c Problems of the San Andreas F a u l t System: S t r a t f o r d U n i v e r s i t y P u b l i c a t i o n s G e o l o g i c a l S c i e n c e s , 13, 136-148. Berg H.C, D.L. Jones, D.H R i c h t e r , (1972). G r a v i n a - N u t z o t i n b e l t - t e c t o n i c s i g n i f i c a n c e of an upper Mesozoic s e d i m e n t a r y and v o l c a n i c sequence i n so u t h e r n and s o u t h e a s t e r n A l a s k a . 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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 lumbia, Vancouver, 380 pp. 109 APPENDIX A - ESTIMATION OF EXPLOSION DETONATION TIMES Two methods were used t o e s t i m a t e the d e t o n a t i o n time of the time f u s e d e x p l o s i o n s . The f i r s t method r e l i e s on the geometry of the s h i p , s h o t , and ocean bottom; the second on measuring the p e r i o d of the o s c i l l a t i n g gas bubble produced by the e x p l o s i o n . The g e o m e t r i c a l method, f i r s t d e s c r i b e d i n Horn (1982), can be used when the ocean f l o o r beneath the shot i s f l a t and h o r i z o n t a l ( f i g u r e 36). The time l a g between the time when the e x p l o s i o n i s dropped and the d i r e c t water wave a r r i v a l ( c a l l e d the d i t c h t i m e , tjj ); and the s h i p ' s average ground v e l o c i t y ( v $ ) over the d i t c h time were measured a l l o w i n g an e s t i m a t e of the s h i p - s h o t d i s t a n c e x. The depth of the water, D, was e s t i m a t e d u s i n g echo sounding equipment assuming a water v e l o c i t y of V w=1.49 km/s. By t r a i l i n g a hydrophone imm e d i a t e l y behind the s h i p and r e c o r d i n g the output a l o n g w i t h the WWVB time code, the a r r i v a l t i m e s of the d i r e c t water wave ( t j ), and the bottom r e f l e c t e d water wave ( t r ) , were o b t a i n e d . As Horn (1982) shows, the shot d e p t h , d, can then be e x p r e s s e d t o second o r d e r a s : d -b ± • b 2 -2A 4ac (1 ) where a 4D 2 b 8 x D 2 c = 8x 2D 2 16D 3X + 8xD 2y At - 2x 3D w At = t t - t a and gure 36 - G e o m e t r i c a l method of o r i g i n t i m e . d e t e r m i n i n g shot 111 v w = 1.49 km/s. Knowing d, the o r i g i n t i m e , t D , i s then g i v e n by: t c = t d - • d* - x* (2) The g e o m e t r i c a l method was used f o r f i n a l o r i g i n time e s t i m a t e s when the ocean f l o o r topography beneath the shot p r o f i l e was f l a t ( s h o t s 6 t o 33). Above the Queen C h a r l o t t e t e r r a c e the f l a t s e a f l o o r assumption i s i n v a l i d and another approach was atte m p t e d . The bubble p u l s e method, used over the Queen C h a r l o t t e t e r r a c e , r e l i e s on measuring the p e r i o d , r , of the f i r s t o s c i l l a t i o n of the gas bubble produced by the e x p l o s i o n . T i s a f u n c t i o n of the shot d e p t h , d, s i n c e the h y d r o s t a t i c p r e s s u r e o p p o s i n g the e x p a n s i o n of the gas bubble depends on depth. Work by S t r u t t ( L o r d R a y l e i g h ) i n 1917, and W i l l i s (1941) r e s u l t e d i n the R a y l e i g h - W i l l i s bubble f o r m u l a : T = 2.13 W1^3 (3) ( d + 1 0 ) 5 / 6 where d = shot depth and W = energy 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 e q u i v a l e n t s . E r r o r s i n t h i s method a r i s e because the 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 expanding bubble a t a c o n s t a n t d e p t h . In r e a l i t y , because of the the n o n - s p h e r i c a l symmetry of 1 1 2 the e x p l o s i o n and the r i s e of the bubble as i t o s c i l l a t e s , t h i s method i s not as a c c u r a t e as e q u a t i o n ( 1 ) . The bubble p u l s e method does not r e l y , however, on the ocean bottom topography. The shot depths f o r the experiment were e s t i m a t e d by both methods. Agreement between the methods was good except f o r the s h o t s above the Queen C h a r l o t t e t e r r a c e where the bottom topography i s v a r i a b l e . The depths a r e i n agreement t o w i t h i n 30 m except f o r s h o t s 3 t o 5 which were l o c a t e d over the Queen C h a r l o t t e t e r r a c e . A g e o m e t r i c a l s o l u t i o n was not p o s s i b l e f o r shot 1 and 2 as the r o o t s of e q u a t i o n (1) were complex. The 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 and 24 and t o be c o n s i s t e n t w i t h the o t h e r s h o t s the g e o m e t r i c a l method r e s u l t s were used. The maximum p r o b a b l e e r r o r f o r shot depth i s ± 30 m. T h i s e r r o r , i n a d d i t i o n t o e r r o r s i n t r , v , v s and t d g i v e a maximum e s t i m a t e d e r r o r i n the o r i g i n time t 0 of about ± 0.03 s. 113 APPENDIX B - COMMON RECEIVER RECORD SECTION PLOTS A p l o t of the t o t a l e x p l o s i o n d a t a s e t i s i n c l u d e d i n the form of common r e c e i v e r s e c t i o n s . The d a t a a r e p l o t t e d w i t h t r u e r e l a t i v e a m p l i t u d e s . A c o r r e c t i o n f o r v a r y i n g shot s i z e was a t t e m p t e d by m u l t i p l y i n g the a m p l i t u d e s by W2/3, where W i s the e x p l o s i v e weight i n TNT e q u i v a l e n t s . To c o r r e c t f o r s p h e r i c a l s p r e a d i n g and o t h e r energy l o s s the d a t a a m p l i t u d e was a l s o s c a l e d by the f a c t o r r 2 , where r i s the s h o t - r e c e i v e r d i s t a n c e . No c o r r e c t i o n s f o r r e c e i v e r e l e v a t i o n or shot depth have been made t o the d a t a p r e s e n t e d i n t h i s Appendix. The d a t a were f i l t e r e d , as noted on s p e c i f i c p l o t s , u s i n g an 8-pole z e r o - p h a s e B u t t e r w o r t h bandpass f i l t e r . S h o t - R e c e i v e r D i s t a n c e (km) RECEIVER 1 ro c\j —• ro ro fo o o o O l 00 I s o o o M (M (\J CO ID CM (\J O O CM CM CO C\J f\J o o CM CM CM — o cn co r - to co r\j CM — — — —• o o o o o o o f\J CM CM (M (\J N IM i n ro r\j - o m o o r - t D u i ^ m N -—• — — — — o o o o o o o o o O O O O Q Q O O O O O O O O O CM CM CM CM ( M ( M ( M W N ( M W r g ( \ | ( \ J ( V J cn 75 65 55 h o t - R e c e i v e r D i s t a n c e (km RECEIVER 2 ) r o CO r\i co r -r\j CM cn CM — o cn co r- co in •tf ro CM — o c n c o r - c o i n ' ^ r n r M — • OsjrM rM rMrvi —i ^- _ _ _ ' o o o o o o o o o S 3 0 ? § ? § S S § S S S § 3 3 § 3 S S § § § 3 S § § ? co in rM CM 145 135 125 95 85 115 105 S h o t - R e c e i v e r Di RECEIVER 75 65 55 s t a n c e (km) 45 35 25 175 165 155 H5 135 125 115 105 95 85 75 65 55 45 S H O T - R E C E I V E R D I S T A N C E (km) RECEIVER 6 S h o t - R e c e i v e r D i s t a n c e (km) RECEIVER 7 filter: 1.5-20Hz CO <\l • CO CO ro o o o CO CO CO cn co CM CM CM o o o CO CO CO CD in CM CM o o CO oo cn CM CM o o 00 00 CM — « o c n o o r ^ c o m ^ m c M ^ o c n c o r - c o m ^ r c o c M — CM CM CM — — — — —- —< —« — — — 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 o o 00 CO CO GO 00 CO CO 00 0000 00 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 185 175 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 195 185 175 165 155 145 135 125 H5 105 95 85 75 65 S h o t - R e c e i v e r D i s t a n c e (km) RECEIVER 10 filter:2-20Hz cn on o cn o co cvj o IT) CVJ o CVJ CVJ o cn o co o cn o o o o o o o o o C O 210 K) 200 190 180 170 160 150 140 130 S h o t - R e c e i v e r D i s t a n c e RECEIVER 11 filter: 1-5Hz 120 110 (km) 100 90 80 fO S h o t - R e c e i v e r D i s t a n c e (km) RECEIVER 12 filter: 1-5Hz 180 S H O T - R E C E I V E R D I S T A N C E (km) RECEIVER 13 filter: 1-5Hz cn o I D CO CVJ o in in CVJ o in cvj o t n o o ^ c o i n ^ , c o N - < o o ) o o t N i o i r ) , f c o ( v j - ' CVJ pvl _• « ^ - 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 o m 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 290 280 270 260 250 240 230 2^6 210 200 190 S H O T - R E C E I V E R D I S T A N C E (km) RECEIVER 15 filter : 2-10Hz 180 170 160 305 295 265 255 S h o t - R e c e i v e r RECEIVER 16 filter: 2-10Hz 245 235 D i s t a n c e 225 215 (km) 195 205 185 CO o CO CM o r— CM CM o r -O CO CM — o o CO o r -co o o o o r -co r - to LO ^ co CM —• o o o o o o o o o o o o o o o o r— r - r- r- r-- r - r-~ r -335 325 315 305 295 285 275 265 255 245 235 S h o t - R e c e i v e r D i s t a n c e (km) RECEIVER 17 filter: 2-8Hz 225 215 205 

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