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A geological reconnaissance of the dellwood seamount area, northeast pacific ocean, and its relationship… Bertrand, Wayne Gerrard 1972

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A GEOLOGICAL RECONNAISSANCE OF THE DELLWOOD SEAMOUNT AREA, NORTHEAST PACIFIC OCEAN, AND ITS RELATIONSHIP TO PLATE TECTONICS by WAYNE GERRARD BERTRAND B . S c , U n i v e r s i t y o f the West I n d i e s , 1969 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department o f Geology I n s t i t u t e o f Oceanography We a c c e p t t h i s t h e s i s as c o n f o r m i n g to the r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA J a n u a r y , 1972 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t freely available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the Head of my Department or by h i s representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of Geology The University of B r i t i s h Columbia Vancouver 8, Canada January 1972. i ABSTRACT The Dellwood Seamount A r e a , an area o f a p p r o x i m a t e l y 10,000 square km., about 185 km. west o f the n o r t h e r n t i p of Vancouver I s l a n d i s l o c a t e d a t o r n e a r the n o r t h e r n m o s t end o f the Juan de Fuca p l a t e . The main purpose o f the s t u d y was to t e s t the h y p o t h e s i s t h a t the Dellwood K n o l l s mark the s i t e o f a s h o r t s p r e a d i n g segment connected to the E x p l o r e r S p r e a d i n g Seg-ment at one end by a t r a n s f o r m f a u l t t r e n d i n g a l o n g the s o u t h -w e s t e r n s l o p e o f P a u l Revere R i d g e ; and meeting the Queen C h a r l o t t e r i g h t l a t e r a l t r a n s f o r m f a u l t a t the o t h e r end. An a n a l y s i s of more than 950 km. o f 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 s , i n c o n j u n c t i o n w i t h o t h e r g e o p h y s i c a l d a t a , shows t h a t ( i ) the Revere - Dellwood f a u l t zone i s a d e x t r a l t r a n s f o r m f a u l t zone c o n n e c t i n g the E x p l o r e r and Dellwood s p r e a d -i n g segments. ( i i ) 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 d i e s out at the s o u t h e a s t e r n end o f the S c o t t Channel near the n o r t h -e a s t e r n end o f the Dellwood S p r e a d i n g Segment. ( i i i ) In the channel between the Dellwood K n o l l s (one o f two p o s s i b l e l o c a t i o n s o f the Dellwood S p r e a d i n g Segment), the sediments and v o l c a n i c basement are c u t by normal f a u l t s , a f e a t u r e which i s c h a r a c -t e r i s t i c o f s p r e a d i n g c e n t r e s w i t h median v a l l e y s . The heat f l o w i n t h i s c h a n n e l and i n the Revere - Dellwood f a u l t zone i s h i g h , ( i v ) The lower c o n t i n e n t a l s l o p e sediments west o f Queen C h a r l o t t e Sound i s f a u l t e d and crumpled and may be the n o r t h e r l y e x t e n s i o n o f the S c o t t I s l a n d s f a u l t , zone and a s i t e o f slow contemporaneous o r r e c e n t l y ceased s u b d u c t i o n . The d e f o r m a t i o n i i of the t h i c k t u r b i d i t e sequence i n the Winona Basin may also be due to subduction. B a s a l t from the area i s chemically intermediate between t h o l e i i t i c and a l k a l i c types. That from the Northwest Dellwood K n o l l s , however, i s l e a s t d i f f e r e n t i a t e d and l e s s than 1 myr. o l d i n c o n t r a s t to b a s a l t from the sediment-draped Southeast Dellwood K n o l l s , the l a t t e r b a s a l t having Mn-coating up to 50 mm. t h i c k and i s thus r e l a t i v e l y o l d . This suggests that spreading may be o c c u r r i n g at the Northwest Dellwood K n o l l s and not i n the channel between the k n o l l s . The texture of b a s a l t s from the Dellwood Seamount Range vary depending on s i z e of p i l l o w and depth below p i l l o w s u r f a c e , but the mineralogy i s e s s e n t i a l l y s i m i l a r . An unusual rock probably best described as a p l a g i o -c l a s e - o l i v i n e b a s a l t porphyry was also recovered from the D e l l -wood Seamount Range. Non-volcanic rocks recovered in c l u d e g l a c i a l e r r a t i c s , an authigenic sandstone comprising g l a c i a l fragments i n an i r o n - r i c h cement, a laminated l i m o n i t i c sediment and manganese nodules. The Dellwood Spreading Segment may have o r i g i n a t e d by l e f t l a t e r a l t r a n s c u r r e n t o f f s e t from the E x p l o r e r Spreading Segment, the o f f s e t caused by a change i n the d i r e c t i o n of motion: of the Juan de Fuca p l a t e . i i i TABLE OF CONTENTS ABSTRACT i TABLE OF CONTENTS i i i LIST OF TABLES v i LIST OF ILLUSTRATIONS v i i ACKNOWLEDGMENTS x i I INTRODUCTION 1 L o c a t i o n 1 Purpose and Scope o f the I n v e s t i g a t i o n 1 I I A SUMMARY OF THE LITERATURE ON THE NORTHEAST PACIFIC PLATES AND ON THE PETROLOGY OF OCEANIC BASALTS 5 R e g i o n a l T e c t o n i c s 5 G r a v i t y and Heat Flow Data 12 G r a v i t y Data 12 Heat Flow Data 15 R e g i o n a l T e c t o n i c H i s t o r y D u r i n g the Ce n o z o i c 19 P e t r o l o g y o f Oceanic B a s a l t s 23 Types o f Oceanic B a s a l t s 23 O r i g i n o f Oceanic B a s a l t s 27 I I I SHIPBORNE AND LABORATORY INVESTIGATIONS 32 Shipborne O p e r a t i o n s 32 I n t r o d u c t i o n 32 Continuous S e i s m i c R e f l e c t i o n P r o f i l i n g 32 Dredging 34 Other O p e r a t i o n s 34 L a b o r a t o r y Methods 34 CSP A n a l y s i s 34 P e t r o g r a p h y 38 Chemical A n a l y s i s 38 i v IV SEISMIC DATA. 41 I n t r o d u c t i o n 41 Major T e c t o n i c and P h y s i o g r a p h i c F e a t u r e s o f the Dellwood Seamount Ar e a 42 E x p l o r e r S p r e a d i n g Zone 42 Dellwood Seamount Range 42 Sou t h e a s t Dellwood K n o l l s 42 Northwest Dellwood K n o l l s 43 Dellwood S p r e a d i n g Zone 43 S c o t t Channel 44 Queen C h a r l o t t e F a u l t Zone 45 C o n t i n e n t a l Slope 45 P a u l Revere Ridge 46 Winona B a s i n 46 Revere - Dellwood F a u l t Zone 47 Other F e a t u r e s 49 V ANALYSIS OF DREDGE HAULS AND CORE 51 P e t r o g r a p h y o f B a s a l t s 51 EN 70-025-2D 51 EN 70-025-3D 55 Dellwood Seamount Range 5 8 Che m i s t r y o f B a s a l t s . 70 H y a l o c l a s t i t e s B r e c c i a s 79 Chemical Weathering § A l t e r a t i o n o f B a s a l t i c Fragments 83 V e s i c l e L i n i n g s and F i l l i n g s 86 Pos t V o l c a n i c D e p o s i t s 89 Ferromanganese C r u s t s and Nodules 89 I r o n D e p o s i t 93 Sediment 96 L o o s e l y C o n s o l i d a t e d Sediments 96 V VI ANALYSIS OF DREDGE HAULS AND CORE (Continued) Mud G l a c i a l E r r a t i c s JOIDES D r i l l Hole No. 177 ( P a u l Revere Ridge) DISCUSSION: THE RELATIONSHIP OF THE DELLWOOD SEAMOUNT AREA TO THE JUAN DE FUCA PLATE V I I SUMMARY AND CONCLUSIONS V I I I BIBLIOGRAPHY APPENDICES APPENDIX A Summary o f major t e c t o n i c and p h y s i o g r a p h i c f e a t u r e s and t h e i r c h a r a c t e r i s t i c s as o b s e r v e d on CSP APPENDIX B - B r i e f d e s c r i p t i o n o f dredge h a u l s 98 98 100 103 118 125 131 135 v i LIST OF TABLES I Average c o m p o s i t i o n and mean d e v i a t i o n s o f o c e a n i c t h o l e i i t e s and a l k a l i b a s a l t s . 26 I I Chemical and n o r m a t i v e c o m p o s i t i o n s o r some b a s a l t s from the Dellwood Seamount A r e a . 57 I I I Chemical c o m p o s i t i o n s o f b a s a l t s from the Dellwood Seamount A r e a . 71 IV Chemical c o m p o s i t i o n s of b a s a l t s from known s p r e a d i n g c e n t r e s , seamounts and o c e a n i c i s l a n d s . 72 V Chemical c o m p o s i t i o n s o f h y a l o c l a s t i t e s from the Dellwood Seamount A r e a 81 VI T h i c k n e s s o f ferromanganese c o a t i n g s and e s t i m a t e d age range f o r r o c k s o f each dredge h a u l . 90 V I I E l e m e n t a l c h e m i c a l c o m p o s i t i o n p o s t v o l c a n i c d e p o s i t s . 92 V I I I P a r t i a l c h e m i c a l c o m p o s i t i o n o f some deep sea i r o n d e p o s i t s . 95 IX Types o f g l a c i a l e r r a t i c s from the Dellwood Seamount A r e a . 100 X D e s c r i p t i o n o f JOIDES P a u l Revere Ridge c o r e . 101 V l l LIST OF ILLUSTRATIONS FIGURES 1. L o c a t i o n o f the Dellwood Seamount A r e a 2. Map o f earthquake e p i c e n t r e s o c c u r r i n g between 1954 and 1963 3. Index anomaly map o f the t o t a l magnetic f i e l d i n t h e ; N o r t h e a s t P a c i f i c Ocean 4. Bathymetry o f the deep sea f l o o r o f f Washington and B r i t i s h Columbia 5. B o u n d a r i e s o f the Juan de F u c a . P l a t e 6. R e l a t i o n s h i p between s p r e a d i n g r i d g e s , s u b d u c t i o n zones and Q u a t e r n a r y a n d e s i t i c v o l c a n o e s i n the N o r t h e a s t P a c i f i c 7. G e n e r a l i z e d map o f the Cape Mendocino Ar e a s c h e m a t i c a l l y showing major t e c t o n i c f e a t u r e s 8. F r e e - a i r g r a v i t y anomaly map, west c o a s t o f Washington and B r i t i s h Columbia 9. F r e e - a i r g r a v i t y anomaly map, west c o a s t o f Oregon 10. F r e e - a i r g r a v i t y anomaly map, west c o a s t o f N o r t h e r n C a l i f o r n i a 11. Summary of heat f l o w v a l u e s i n the N o r t h e a s t P a c i f i c 12. Heat f l o w v a l u e s i n the Dellwood Seamount -E x p l o r e r Trench Area 13. Some major t e c t o n i c f e a t u r e s o f Western N o r t h Ame r i c a 14. M a g n e t i c anomalies i n the N o r t h e a s t P a c i f i c 15. Schematic model o f p l a t e i n t e r a c t i o n s and e v o l u t i o n w i t h time o f boundary regimes a l o n g the west c o a s t o f N o r t h America assuming a model of changing motions, 16. R e c o n s t r u c t i o n o f p l a t e e v o l u t i o n and d e f o r m a t i o n r e l a t e d t o l a t e C e n o z o i c i n t e r a c t i o n o f the N o r t h American and P a c i f i c P l a t e s v m LIST OF ILLUSTRATIONS (Continued) FIGURES 17. L o c a t i o n o f p l a t e boundary regimes w i t h r e s p e c t to p o i n t s i n w e s t e r n N o r t h A m e r i c a assuming motions and d e f o r m a t i o n s d e s c r i b e d i n F i g u r e 16 18. Major t y p e s o f b a s a l t 19. R a r e - e a r t h p a t t e r n r e l a t i o n between t h o l e i i t i c b a s a l t s o f s h i e l v o l c a n o e s and ocean r i d g e t h o l e i i t e s . 20. Diagrammatic summary o f the e f f e c t s and d i r e c t i o n s o f f r a c t i o n a t i o n o f b a s a l t i c l i q u i d s at moderate to h i g h p r e s s u r e 21. Model of magmatic e v o l u t i o n o f the M i d - A t l a n t i c Ridge 22. Shipborne O p e r a t i o n s - Dredging 23. - L o c a t i o n o f CSP l i n e s 24. C o n s t r u c t e d p r o f i l e a l o n g the Dellwood b a s i n between the Dellwood Seamount Range and the Dellwood K n o l l s 25. L o c a t i o n o f Dredge S t a t i o n s 26. B a s a l t s r e c o v e r e d from the Northwest Dellwood K n o l l s 27. Photomicrographs o f o l i v i n e - p l a g i o c l a s e g l o m e r o p o r p h y r i t i c b a s a l t from the Northwest Dellwood K n o l l s 28. B a s a l t s from the S o u t h e a s t Dellwood K n o l l s 29. C h a r a c t e r i s t i c f e a t u r e s o f b a s a l t i c p i l l o w fragments from the Dellwood Seamount Range 30. Photomicrographs o f b a s a l t s from the D e l l w o o d Seamount Range. Dredge Haul No. EN 70-025-7D 31. Photomicrographs o f b a s a l t s from the Dellwood Seamount Range. Dredge Haul Nos. EN 70 :025-8D and EN 70-025-9D 32. Photomicrographs showing changes i n t e x t u r e w i t h depth i n t o p i l l o w fragment 33. R e l a t i o n s h i p between o b s e r v e d groundmass t e x t u r e s , p i l l o w s i z e and c o o l i n g r a t e i x LIST OF ILLUSTRATIONS (Continued) t FIGURES 34. H o l o c r y s t a l l i n e b l o c k l a v a s from the Dellwood Seamount Range 35. P o r p h y r i t i c p l a g i o c l a s e b a s a l t from the Dellwood Seamount Range 36. S i l i c a v a r i a t i o n diagrams f o r b a s a l t s o f the Dellwood Seamount Ar e a 37. T r i a n g u l a r diagram f o r b a s a l t s o f the Dellwood Seamount Area 38. S.I. v a r i a t i o n diagrams f o r b a s a l t s o f the Dellwood Seamount Ar e a 39. H y a l o c l a s t i t e B r e c c i a fragments from the Dellwood Seamount A r e a 40. Weathered zones i n b a s a l t s from the Dellwood Seamount Ar e a 41. V e s i c l e l i n i n g s and amygdules i n b a s a l t s from the Dellwood Seamount Ar e a 42. Ferromanganese n o d u l e s and c r u s t s from the Dellwood Seamount Range 43. I r o n d e p o s i t from the Dellwood Seamount Range 44. A u t h i g e n i c l o o s e l y c o n s o l i d a t e d sediments from the Dellwood Seamount Ar e a 45. G l a c i a l e r r a t i c s r e c o v e r e d from the Dellwood Seamount Ar e a 46. V i n e ' s t o t a l f i e l d m agnetic anomaly map o f the Dellwood Seamount and a d j a c e n t areas 47. I n t e r p r e t a t i o n o f t o t a l f i e l d magnetic anomalies assuming the Revere - Dellwood F a u l t zone o r i g i n a t e d as a l e f t l a t e r a l t r a n s c u r r e n t f a u l t 48. I n t e r p r e t a t i o n o f t o t a l f i e l d m agnetic anomalies assuming t h a t the Dellwood S p r e a d i n g Centre o r i g i n a t e d i n S i t u l e s s . t h a n 2 myr. ago 49. Summary o f major t e c t o n i c f e a t u r e s o f the Dellwood Seamount Area 50. Symbols used i n i n t e r p r e t a t i o n o f S e i s m i c P r o f i l e s shown i n P l a t e s I to X I I I X LIST OF ILLUSTRATIONS (Continued) PLATES I CSP and Magnetometer l i n e s I I CSP and Magnetometer l i n e s and I I I CSP and Magnetometer l i n e s and IV CSP and Magnetometer l i n e s V CSP l i n e EN 70-025-3 VI CSP and Magnetometer l i n e s and V I I CSP and Magnetometer l i n e s and V I I I CSP l i n e EN 70 -025 -1 IX CSP l i n e EN 70 -025 -2 X CSP l i n e EN 70 -025 -9 XI CSP l i n e s EN 70 -025 -10 and XI I CSP l i n e BI 69 -050 -HUDSON X I I I CSP L i n e BI 69 -050 HUDSON IOUBC.70-16-15 139 IOUBC 70-16-16 IOUBC 70-16-17 140 IOUBC 70-16-18 IOUBC 70-16-19 141 IOUBC 70-16-20 142 .14 3 IOUBC 70-16-21A IOUBC 70-16-21B 144 IOUBC 70-16-23 IOUBC 70-16-26 145 146 14 7 148 EN 70-025-11 149 3 ISO 6 151. x i ACKNOWLEDGMENTS T h i s t h e s i s was. u n d e r t a k e n under the s u p e r v i s i o n o f Dr. R.L. Chase to whom the au t h o r i s g r e a t l y i n d e b t e d . H i s a d v i c e and a s s i s t a n c e throughout the p r o j e c t i s a p p r e c i a t e d . Thanks are due to Dr. W.K. F l e t c h e r and t o M i s s Sandra B a r r f o r t h e i r a s s i s t a n c e i n the c h e m i c a l a n a l y s e s o f the r o c k s . The use o f the atomic a b s o r p t i o n u n i t i n Dr. G r i l l ' s c h e m i c a l oceanography l a b o r a t o r y was most welcome. The e n t h u s i a s t i c c o - o p e r a t i o n o f t h e . o f f i c e r s and crew o f CNAV ENDEAVOUR and. CSS HUDSON, o f the s c i e n t i s t s o f the A t l a n t i c Oceanographic L a b o r a t o r y ( B e d f o r d I n s t i t u t e ) , and o f the t e c h n i c i a n s and s t a f f o f the UBC Geology De p a r t m e n t , i s a l s o a p p r e c i a t e d . Dr. R . L i Chase and Dr. P.B. Read c r i t i c a l l y , r e v i e w e d the m a n u s c r i p t and made many h e l p f u l s u g g e s t i o n s . F i n a n c i a l s u p p o r t f o r t h i s p r o j e c t was p r o v i d e d by NRC Grant No. A-7032, DRB Grant No. 9511-95 (both t o Dr. R.L: Chase), g r a n t s from s e v e r a l o i l companies and a UBC Graduate F e l l o w s h i p . Funds from A t l a n t i c Oceanographic L a b o r a t o r y s u p p o r t e d c a t a l o g u i n g o f rock samples c o l l e c t e d d u r i n g Hudson 70 phase V I I . CHAPTER ONE INTRODUCTION LOCATION The D e l l w o o d Seamount A r e a , s i t u a t e d b e t w e e n 50°15" and 51°15' N o r t h l a t i t u d e and b e t w e e n 129°40' and 131°00' West l o n g i t u d e , i s an a r e a o f some 10,000 s q u a r e k i l o m e t e r s b o u n d e d by t h e c o n t i n e n t a l s h e l f and s l o p e o f f Queen C h a r l o t t e Sound on t h e n o r t h a nd n o r t h e a s t , by t h e W i n o n a R i d g e and S c o t t I s l a n d s F a u l t Zone on t h e e a s t , b y t h e E x p l o r e r S p r e a d i n g Zone on t h e s o u t h and t h e S c o t t Seamount R a n g e 1 on t h e w e s t ( F i g u r e 1 ) . T e c t o n i c a l l y , t h e D e l l w o o d Seamount A r e a i s s i t u a t e d a t t h e e a s t e r n m a r g i n o f t h e P a c i f i c p l a t e , n e a r t h e s o u t h w e s t e r n m a r g i n o f t h e A m e r i c a n p l a t e a n d n e a r t h e n o r t h e r n e n d o f t h e J u a n de F u c a P l a t e . PURPOSE AND SCOPE OF THE INVESTIGATION E w i n g e t a l (1968) s u g g e s t e d t h a t a r i d g e r u n n i n g f r o m t h e n o r t h w e s t e n d o f t h e D e l l w o o d Seamount Range t o w a r d t h e c o n -t i n e n t a l s l o p e may be a s h o r t segment o f t h e E a s t P a c i f i c R i s e o f f s e t f r o m t h e E x p l o r e r S p r e a d i n g Segment by a f r a c t u r e z o n e a l o n g o r p a r a l l e l t o t h e D e l l w o o d Seamount Range. A c l u s t e r o f e a r t h q u a k e e p i c e n t r e s n e a r t h e n o r t h e n d o f t h e E x p l o r e r Segment ( T a l w a n i e t a l , 1 9 6 5 ; W e t m i l l e r , 1969) s u p p o r t s t h i s i n t e r p r e t a -t i o n . The p u r p o s e o f t h e s t u d y was t o t e s t t h e h y p o t h e s i s t h a t t h e D e l l w o o d K n o l l s mark s u c h a s e g m e n t , o f f s e t f r o m t h e The S c o t t Seamount Range i s a new name c h o s e n by t h e a u t h o r b e c a u s e i t i s t h e l a r g e s t unnamed seamount r a n g e s i t u a t e d on t h e s o u t h bank o f t h e S c o t t C h a n n e l . 2 3 Explorer Spreading Segment by the Revere - Dellwood Fault Zone, and meeting the Queen Charlotte right-lateral transform fault, thus forming the northwesternmost boundary of the Juan de Fuca plate. If this is true, then the Dellwood Seamount Area may be a triple junction between the Pacific, American and Juan de Fuca plates, i.e. between the Dellwood Spreading Zone, the Queen Charlotte transform fault and the northern end of the zone of subduction which marks the eastern margin of the Juan de Fuca plate. In this thesis, geophysical data (seismic reflection and magnetic profiles; heat flow, gravity and earthquake epicentral distribution data) were used to produce a tectonic map of the area. Basalts from the Dellwood Seamount Range and Dellwood Knolls were compared petrographically and chemically with similar rocks from known ocean ridge spreading centres, transform fault zones and seamounts,. especially, those of the northeast Pacific, to determine whether or not they represent ocean ridge basalts. Due to insufficient shiptime, extensive sampling was impossible and although about one ton of rocks was recovered, they came only from six dredge sites which, however, represent a l l the main seamounts of the area. Due to lack of unequivocal data resulting from insuffic-ient shiptime, equipment malfunction, seismic reflection records with low signal to noise ratio and inaccurate locations of some seismic and magnetic profiles and earthquake epicentres, the study became a geological and geophysical reconnaissance of the area with special reference to the tectonics, and only tentative conclusions could be reached. 4 FIGURE 2. I/.ap of earthquake epicenters, occuring between 1954 and 19o3. (f roin Tbbin and Sykes, 1968.) 5 CHAPTER TWO A SLMMARY OF THE LITERATURE ON THE NORTHEAST PACIFIC PLATES AND ON THE PETROLOGY OF OCEANIC BASALTS REGIONAL TECTONICS The h i g h degree o f s e i s m i c a c t i v i t y below the c o n t i n -e n t a l s l o p e and a d j a c e n t s e a f l o o r west of B r i t i s h Columbia shows the a r e a to be t e c t o n i c a l l y a c t i v e ( M i l n e , 1967; T o b i n and Sykes, 1968; W e t m i l l e r , 1969). The w o r l d w i d e d i s t r i b u t i o n o f earthquake e p i c e n t r e s shows the s e to be l o c a t e d m a i n l y i n areas o f subduc-t i o n and a l o n g t r a n s f o r m f a u l t s c o n n e c t i n g ocean r i d g e s p r e a d i n g segments. F i g u r e 2, t a k e n from T o b i n and Sykes ( 1 9 6 8 ) , shows the l o c a t i o n o f e p i c e n t r e s o f major ea r t h q u a k e s o c c u r r i n g between 1954 and 1963 i n the n o r t h e a s t P a c i f i c . W i l s o n ( 1 9 6 5 ) , V i n e and W i l s o n ( 1 9 6 5 ) , V i n e (1968) and o t h e r s have i n t e r p r e t e d the magnetic d a t a o f R a f f and Mason (1961) ( F i g u r e 3) i n terms o f s e a f l o o r s p r e a d i n g . Work by s e v e r a l o t h e r a u t h o r s , i n c l u d i n g Pitman and Hayes (1968), S r i v a s t a v a e t a l (1971) and E m i l i a e t a l ( 1 9 6 8 ) , have c o n f i r m e d and extended the d a t a o f R a f f and Mason. Thus from magnetic and s e i s m i c d a t a , the ocean f l o o r o f f B r i t i s h C o lumbia, Washington, Oregon and N o r t h e r n C a l i f o r n i a has been p o s t u l a t e d to c o n t a i n l a r g e t r a n s f o r m f a u l t s and s p r e a d i n g ocean r i d g e s . B a t h y m e t r i c c h a r t s by Menard (1961) , McManus (1967), Mammerickx and T a y l o r ( 1 9 7 1 ) , and Chase e t a l (1970) show major f e a t u r e s i d e n t i f i e d from s e i s m i c and magnetic d a t a w i t h v a r y i n g degree o f c l a r i t y ( F i g u r e 4 ) , as w e l l as minor f e a t u r e s such as seamounts, t r e n c h e s , canyons, fans and p l a i n s . 6 FIGURE 3 .Index anomaly map of the t o t a l magnetic f i e l d ir>„ the N o r t h e a s t P a c i f i c Ocean. The p o s i t i v e anomalies are shovm i n b l a c k . (from Raff and Kason, 1961.) 7 FIGURE 4 BATHYMETRY of the PIONEER SURVEY AREA NORTH of 45 N LAT. OY J. UM!:>rR:C\X ana . L. JAYLCff Gfrt.CP>l'*L M M CENTIA SCfcPPS INSTITUTION of C \ f ANCGPAPMY St I C:AL C«»»T VJ I 8 Morgan (1968) r e f e r r e d to the s m a l l a r e a between the w e s t e r n margin o f N o r t h America and the Juan de Fuca and Gorda r i d g e s as the Juan de Fuca b l o c k . The w e s t e r n margin o f t h i s Juan de Fuca b l o c k o r p l a t e c o n s i s t s o f a s e r i e s o f s p r e a d i n g c e n t r e s and t r a n s f o r m f a u l t s ( F i g u r e 5 ) . From n o r t h to s o u t h t h e s e a r e : the Dellwood S p r e a d i n g Segment (Ewing e t a l , 1968) c o n n e c t e d by the Revere - Dellwood f a u l t zone t o the E x p l o r e r Segment (McManus, 1967; Ewing e t a l , 1968) i n t u r n c o n n e c t e d by the Sovanco F r a c t u r e Zone ( P a v o n i , 1966) to the Juan de Fuca segment ( W i l s o n , 1965). T h i s i s c o n n e c t e d v i a the B l a n c o F r a c - . t u r e Zone to the Gorda R i s e which i s c o n n e c t e d v i a the Mendocino F r a c t u r e Zone t o the San Andreas t r a n s f o r m f a u l t and thus t o the E a s t P a c i f i c R i s e i n the G u l f o f C a l i f o r n i a . The n o r t h e r n end o f t h i s s e r i e s o f t r a n s f o r m f a u l t s and s p r e a d i n g segments i s c o n n e c t e d to the A l e u t i a n I s l a n d A r c v i a r i g h t l a t e r a l t r a n s -form movement a l o n g the Queen C h a r l o t t e and r e l a t e d f a u l t s ( T obin and Sykes , 196 8 ) . The l o c a t i o n o f the e a s t e r n margin o f t h i s p l a t e i s u n c e r t a i n , though i t i s assumed t o be at the base o f the c o n t i n -e n t a l s l o p e o f f w e s t e r n N o r t h A m e r i c a between Cape Mendocino i n the s o u t h and the S c o t t I s l a n d s i n the n o r t h ; t h i s based on s e v e r a l l i n e s o f e v i d e n c e which i n c l u d e : ( i ) A zone o f moderate s e i s m i c i t y n e a r the c o a s t o f Oregon, Washington, s o u t h e r n B r i t i s h Columbia and Vancouver I s l a n d w i t h some s e i s m i c e v e nts h a v i n g h y p o c e n t r e s to depths o f 60 km. ( T o b i n and Sykes, 1968). ( i i ) D e f o r m a t i o n i n P l i o c e n e and P l e i s t o c e n e sediments on the c o n t i n e n t a l s h e l f and s l o p e and i n Winona B a s i n ( B a r r , 1971, SCAL€ a so'H 10 p e r s o n a l communication; S i l v e r , 1969, and p r e s e n t s t u d y ) . ( i i i ) The e x i s t e n c e o f P l i o c e n e and P l e i s t o c e n e a n d e s i t i c v o l c a n o e s n e a r the c o a s t o f Oregon and Washington from Mt. G a r i b a l d i i n B r i t i s h Columbia t o Mt. Lassen i n N o r t h e r n C a l i f -o r n i a (Morgan, 1968; Hays and N i n k o v i c h , 1970; F i g u r e 6 ) . McKenzie and Morgan (1969) i n t e r p r e t e d the Cape Mendocino a r e a as a t r i p l e j u n c t i o n between the zone o f subduc-t i o n a l o n g the c o n t i n e n t a l margin n o r t h o f Cape Mendocino, the Mendocino F r a c t u r e Zone and the San Andreas t r a n s f o r m f a u l t . They c o n s i d e r e d the Juan de Fuca p l a t e as b e i n g consumed a l o n g the c o a s t n o r t h o f Cape Mendocino. F i r s t - m o t i o n s t u d i e s and f a u l t - p l a n e s o l u t i o n s by B o l t e t a l (1968), Seeber e t a l (1970), as w e l l as an a n a l y s i s o f the. magnetic anomaly p a t t e r n a l o n g the Mendocino F a u l t by S i l v e r (1969) i n d i c a t e t h a t n o r t h - s o u t h r e g i o n a l c o m p r e s s i o n as w e l l as s t r i k e - s l i p m o tion may be o c c u r -r i n g at Cape Mendocino and perhaps a l o n g the r e s t o f the Mendocino F a u l t to the Gorda Ridge ( F i g u r e 7) and thus i t i s p o s s i b l e t h a t consumption o f the Gorda B a s i n i s o c c u r r i n g a l o n g the Mendocino F a u l t as w e l l as a l o n g the w e s t e r n c o n t i n e n t a l m a r g i n. The e v i d e n c e from f i r s t - m o t i o n s t u d i e s and f a u l t - p l a n e s o l u t i o n s s u g g e s t s t h a t a new t e c t o n i c regime i s d e v e l o p i n g w i t h i n the Juan de Fuce p l a t e i n v o l v i n g i n t e r n a l d e f o r m a t i o n o f the Gorda B a s i n and n o r t h - s o u t h c o m p r e s s i o n a l o n g the Mendocino F a u l t e a s t o f the Gorda Ridge (Seeber e t a l , 1970). The Juan de Fuca p l a t e may thus be b r e a k i n g up i n t o a s e r i e s o f s m a l l e r p l a t e s perhaps due to i t s becoming welded to the American c o n t i n e n t as p a r t o f the American p l a t e o r due 11 ? A C I O C E A N — M FIGURE 6* R e l a t i o n s h i p between spreading; r i d g e s , sv.bd*aotlch zones and Quaternary A n d e s i t i c volcanoes i n the Northeast P a c i f i c * (from Hays and ITinkovich, 1970.) 39" OREGON T CALIFORNIA j ^ //GORDA BASIN\X\ } ": NORTH AMERICANI PACIFIC, PLATE , . Mfc ND O C • f*0 v .,^r PLATE A S F A U L T iGURE 7. Generalized map of Cape Mendocino Area schenatical": y showing major t e c t o n i c f e a t u r e s . . (from Seeber et al., 1970.) 12 t o a tendency f o r s t r i k e - s l i p m o t i o n to become c o n t i n u o u s a l o n g the c o n t i n e n t a l margin j o i n i n g the San Andreas f a u l t system to the Queen C h a r l o t t e f a u l t zone. GRAVITY AND HEAT FLOW DATA G r a v i t y Data D e h l i n g e r e t a l (1970) , r e p o r t i n g on g r a v i t y work done by Gemperle and L o v e l l (1967), Couch ( 1 9 6 9 ) , and D e h l i n g e r e t a l ( 1 9 6 7 ) , o b s e r v e d t h a t the g r a v i t y i n the a r e a has e s s e n t i a l l y z e r o -average o v e r b o t h the e n t i r e r e g i o n and many major s t r u c t u r a l f e a t u r e s i m p l y i n g t h a t the a r e a i s e s s e n t i a l l y i n i s o s t a t i c e q u i l -i b r i u m and t h a t i s o s t a t i c r e a d j u s t m e n t o c c u r s f a r more r a p i d l y f o r t h e s e f e a t u r e s than does t e c t o n i c u p h e a v a l . Regions o f s t r o n g n e g a t i v e f r e e - a i r anomalies w h i c h r e s u l t from s t r o n g t e c -t o n i c a c t i v i t y i n c l u d e the Puget Sound a r e a , the B l a n c o f a u l t , Winona B a s i n and a d j a c e n t c o n t i n e n t a l s l o p e 1 and the Queen C h a r l o t t e f a u l t zone. Many of the l o c a l f r e e - a i r anomalies are due t o the e f f e c t s o f topography as f o r example th o s e a s s o c i a t e d w i t h the Mendocino and Gorda escarpments and w i t h the numerous seamounts. A f r e e - a i r anomaly o f a p p r o x i m a t e l y -100 m i l l i g a l s i s a s s o c i a t e d w i t h the p a r t i a l l y sediment - f i l l e d Queen C h a r l o t t e t r o u g h . O f f the n o r t h end o f Vancouver I s l a n d , a f r e e - a i r anomaly g r e a t e r t h a n -150 m i l l i g a l s i s n o t a s s o c i a t e d w i t h a t o p o g r a p h i c t r o u g h and i n d i c a t e s t h a t t h i s a r e a (the Winona ^'he S c o t t I s l a n d s f r a c t u r e zone o f Couch, D e h l i n g e r , and o t h e r s i n c l u d e s the Winona B a s i n and a d j a c e n t c o n t i n e n t a l s l o p e . FIGURE 8. Free-air gravity anomaly map, west coast of Washington and B r i t i s h Columbia. Contour Interval 50 milligals. (after Couch, 1969.) FIGURE 9. F r e e - a i r g r a v i t y anomaly map, west coast of Oregon. Contour I n t e r v a l 50 m i l l i g a l s . ( a f t e r Dehlinger et a l . , 1970.) 1-32 130 . 128 126 124 FIGURE 10, F r e e - a i r g r a v i t y anomaly map, west coasx of Northern C a l i f o r n i a . Contour I n t e r v a l 50 m i l l i g a l s . ( a f t e r Dehlinger et a l . , 1970.) 15 B a s i n and adjacent c o n t i n e n t a l slope) may r e p r e s e n t a sediment-f i l l e d downfaulted s t r u c t u r e . Figures 8, 9, and 10 are f r e e - a i r g r a v i t y anomaly maps over the e n t i r e r e g i o n of the Juan de Fuca p l a t e and adjacent areas, a l l taken from Dehlinger et a l (1970). Heat Flow Lata Dehlinger et a l (1970) have a l s o summarised the heat flow measurements made i n the area between 1962 and 1970 (Figure 11) as f o l l o w s : "The heat flow i n tiie area i s g e n e r a l l y high f o r oceanic regions averaging 2.01 uca l s / c m 2 . sec which i s about 60% h i g h e r than the average f o r ocean basins (1.28 u c a l s / c m 2 . sec) High heat flow values are seen to be r e l a t e d to the Mendocino escarpment east of the Gorda Ridge, to the Gorda B a s i n , Gorda Ridge, Juan de Fuca Ridge, Cascadia B a s i n and Dixon Entrance. The average value f o r 31 s t a t i o n s along these f e a t u r e s i s 2.95 uca l s / c m 2 . sec approximately 601 h i g h e r than the average f o r ocean r i d g e s A s t r i k i n g observa-t i o n on the heat flow map (Figure 11) i s t h a t heat flow values are below normal at r e l a t i v e l y short d i s t a n c e s west of the r i d g e s , whereas the hig h heat flow extends from the r i d g e s across the p l a i n s to the e a s t . The low heat flow i s a s s o c i a t e d with normal to above normal mantle v e l o c i t i e s and the high heat flow with below normal mantle v e l o c i t i e s . 1 ' T h i s r e l a t i o n s h i p between heat flow and mantle v e l o c -i t i e s i s supported by s e i s m i c r e f r a c t i o n work by Shor et a l (1968). Heat flow values i n the Dellwood Seamount Area and Paul Revere Ridge - E x p l o r e r Trench Area were measured by L. L i s t e r and E. Davis aboard the CSS Hudson d u r i n g Phase VII o f the C i r c u m n a v i g a t i o n of the Americas i n 1970. Figure 12 shows the r e l a t i o n of these values to topography. T h e i r s i g n i f i c a n c e to the t e c t o n i c s of the area i s d i s c u s s e d i n a l a t e r s e c t i o n . (See D i s c u s s i o n ) 16 | 1 2 " |.10* 133* 136' 134° 132' 130* 128* 126* 124* 122* 120' FIGURE II. Sunrarvry of heat flow values ( i n pcal/cr.2.sec) i n the northeast Pacific. (fras D'ehiinger et al. ,1970.) CONTOUR INTERVAL 200 METRES 18 'IGUR5 1 3 . Some major t e c t o n i c f e a t u r e s o f Western N o r t h America,, Host t h r u s t s a r e o m i t t e d . Q u a t e r n a r y v o l c a n l c s a r e b l a c k , g r a n i t i c p l u t o n i c s a r e g r a y . (from Atwater, 1970.) Fi -ure li}..Magnetic anomalies in the northeast Pacific from Atwater and Menard (1970). Numbering of anomalies and their ages shown i n the scale follow lUir tz ler and others (196S); ages of geologic epochs follow Bcrggren (1969). 19 REGIONAL TECTONIC HISTORY DURING THE CENOZOIC Using the major t e c t o n i c f e a t u r e s of western North America (Figure 13) and the magnetic anomaly p a t t e r n i n the n o r t h e a s t P a c i f i c (Figure 14), Atwater (1970) a p p l i e d two models o f . p l a t e i n t e r a c t i o n s to e x p l a i n the Cenozoic h i s t o r y , of western North America. The f i r s t model i s i l l u s t r a t e d by, Figure .15 which shows s c h e m a t i c a l l y p l a t e i n t e r a c t i o n s ( Figure 15A) and t e c t o n i c h i s t o r y ( F i g u r e 15B) along the.coast of western North America, assuming t h a t the North American and P a c i f i c P l a t e s were f i x e d r e l a t i v e to one another between 30 myr. and 5 myr, ago; i . e . that the San Andreas F a u l t was i n i t i a t e d as a s t r i k e - s l i p f a u l t only 5 myr. ago when the North American and P a c i f i c P l a t e s broke apart and began to move past one another at the r a t e of 6 cm/yr. P r i o r to 30 myr. ago, t h i s model shows the coast of C a l i f o r n i a as a subduction zone. In Atwater's second model, the P a c i f i c - A m e r i c a n motion i s assumed constant f o r s e v e r a l tens of m i l l i o n s of years and' t h i s motion i s assumed to be taken up by transform motion along the c o a s t . Figure 16 i s a r e c o n s t r u c t i o n of p l a t e e v o l u t i o n and deformation r e l a t e d to middle - l a t e Cenozoic i n t e r a c t i o n of the North American and P a c i f i c p l a t e s under such assumptions and Fi g u r e 17 shows the t e c t o n i c h i s t o r y , along t h e , c o a s t of western North America dur i n g the same p e r i o d . In Figure 16, the North American p l a t e i s a r b i t r a r i l y h e l d f i x e d and going back i n time, the P a c i f i c p l a t e i s h o r i z o n t a l l y moved to the r i g h t by the amount i n d i c a t e d . A l a t e Mesozoic - e a r l y Cenozoic e x i s t e n c e -of a Kula 20 • ^ <-PRESENT 5M 300 im 10 Ml S.F LA fjORTH OS ' 71 / AMERICAN-\ / % PACIFIC PLATE J SF LA GS • WZ "" 1 v '" 20 KT N^. */ NORTH AMERICAN PLATE-1 30 NT. QUEEN CHARLOTTE SAN LOS ISLANDS SEATTLE FRANCISCO ANGELES GUMMAS MAZAT LAN STR.KE SLIP 1 STRIKE SLIP 1 NO RELATIVE MOTION BEING TAKEN UP — AT MARGIN / i • / -SF LA FIGURE 15. A - Schematic model of plate interactions assuming that the North American and Pacific plates were fixed to one another unt i l 5 m.y. ago, at which time they broke apart and began, to move at a rate of 6 cm/vr. B - Evolution with time of boundary regimes along the coast <3f North .America assuming the model of. changing motions described i n Av (from Atwater, 1970.) NOTE: In figures 15 and 16, single lines are transform faults, double lines are spreading centers and hatched lines are subduction zones. Large arrows show motion of the plates with respect to the Pacific plate i n figure 15 and the North American plate in figure 16. Small arrows show relative motions at points along plate boundaries. I n i t i a l s are places listed i n figure 17. 21 20MX I 2 0 0 K M . / FIiVP.3 16 . R e c o n s t r u c t i o n o f p l a t e e v o l u t i o n and d e f o r m a t i o n r e l a t e d to l a t e C e n o t o i c i n t e r a c t i o n of the ! io - th A n c r i c a n and P a c i f i c p l a t e s . D iar . rans i n G show the d e r i v a t i o n o f v a r i o u s v e c t o r s . C a p t i o n s e i v e r - i l l l o n s o f y e a r s b e f o r e p r e s e n t and ar.ount o f o f f s e t which must s u b s e q u e n t l y o c c u r to br in i? P a c i f i c and Inner Nor th Ar- .erican p l a t e s back to t h e i r p r e s e n t r e l a t i v e p o s i t i o n s . P a c i f i c North A m e r i c a n mot ion (PA ) i s assumed c o n s t a n t a t 6cr- , /yr . i n a h o r i z o n t a l d i r e c t i o n . F o r the l a s t 2 0 r y r . , 4 c n / y r . i s a s s u r e d taken up on c o a s t a l f a u l t s w h i l e 2 c n / y r . I s accommodated by i n l a n d f a u l t s . Thus C a l i f o r n i a was r .ov ing n o r t h w e s t a t 2 c n / y r . P r i o r to 2 0 n y r . a l l m o t i o n is presumed taken up a lonr ; the c o a s t . B l a c k r e g i o n s a r e u n a c c e p t a b l e o v e r l a p s of o c e a n i c and c o n t i n e n t a l c r u s t , ( a f t e r A t w a t e r , 1 9 7 0 ) . Jt » J u a n de F u e a - F . n c i f i c r o t l o n . J A • 'uan de ? u c a - A n o r i c a n n o t i o n . .!< " . 'uan de F u c a - C a l i f o m i a m o t i o n . P A - r a c i f i c - A n o r i c a n n o t i o n . C„ • C a l l f o m i a - A n e r i c a n n o t i o n . F , • F a r a l l o n - F a c i f l c m o t i o n . F< • F a r a l l o n - A m e r l c a n n o t i o n . K» - X u l a - P a c i f i c n o t i o n . K» • K u l a - A n e r i c a n m o t i o n . 22 C 5 SP H B SF SL CC LV D LA PH EP CI GS CH HZ BC GU Figure 17. Location of plate boundary regimes with respect to points in western North America, assuming motions and deformations described for Figure 16. Inland cities are raised above the line. They have been projected to the coast vertically in the projection of Figure 16. roughly parallel to the direction of Farallon-North American underthrusting. Fine lines trace the shifting locations of the cities as the continent deforms. Gray areas show times and places where tectonic and igneous activity related to subduction are predicted. White areas show times and places where North America was in contact with the Pacific plate or with the Kula plate » (from Atwater, 1970.) 23 P l a t e has been p o s t u l a t e d (Pitman and Hayes, 1968; A t w a t e r , 1970 and o t h e r s ) t o e x p l a i n the East-West t r e n d i n g magnetic anomalies and the Great Magnetic B i g h t l y i n g s o u t h o f the A l e u t i a n T r e n c h , w i t h a t h i r d r i d g e l y i n g between the K u l a and F a r a l l o n p l a t e s t o generate these East-West a n o m a l i e s . P r i o r to 40 myr. ago, however, the h i s t o r y i s d i f f i c u l t t o u n r a v e l , as s u b d u c t i o n o f the A l e u t i a n Trench has d e s t r o y e d the magnetic anomaly p a t t e r n s . A l t h o u g h A t w a t e r p r e f e r r e d t h i s second model o f c o n s t a n t motions as the most p r o b a b l e one, i t i s n o t as y e t d e f i n i t e l y e s t a b l i s h e d w h i c h i s c o r r e c t . In the f i r s t model ( c h a n g i n g m o t i o n s ) , the n o r t h e r n t r i p l e p o i n t between the A m e r i c a n , P a c i f i c and F a r a l l o n P l a t e (now the Juan de Fuca P l a t e ) moved r a p i d l y , down the c o a s t o f S o u t h e r n A l a s k a and B r i t i s h Columbia s i n c e b e f o r e 30 myr. ago u n t i l 5 myr. ago and then remained j u s t o f f n o r t h e r n Vancouver I s l a n d ( p r o b a b l y w i t h i n the Dellwood Seamount area) between 5 myr. ago and p r e s e n t , d u r i n g which time the San Andreas f a u l t was a c t i v e . In the second model ( c o n s t a n t m o t i o n s ) , t h i s t r i p l e p o i n t remained o f f Vancouver I s l a n d d u r i n g the whole o f the m i d d l e t o l a t e T e r t i a r y but m i g r a t e d n o r t h {not south as p r o p o s e d i n the f i r s t model) from C a l i f o r n i a d u r i n g the e a r l y T e r t i a r y . PETROLOGY OF OCEANIC BASALTS Types o f Oceanic B a s a l t s Oceanic b a s a l t s comprise two main t y p e s - o c e a n i c t h o l e i i t e s and a l k a l i - r i c h b a s a l t s . E n g e l and E n g e l (1964) and E n g e l e t . a l (1965) s t u d i e d b a s a l t s from i s l a n d s and seamounts 24 Di (cpx) C p x cpx C p x C p x O l 01 . A l k a l i B o s o l t Group O l i v ine T h o l e i i t e Group Thole i i te ' Group FIGURE 18. Major types o f b a s a l t . • (fro*: Yoder and T i l l e y , 1962.) A — Generalized normative b a s a l t tetrahedron. B — Exploded view o f g e n e r a l i z e d simple b a s a l t system i l l u s t r a t e d i n A. 25 o f the E a s t P a c i f i c R i s e and the ocean b a s i n s and concluded, t h a t t h o l e i i t i c b a s a l t s form most of the d e e p l y submerged v o l c a n i c f e a t u r e s i n the oceans a l o n g s c a r p l i k e r i d g e s o f o c e a n i c r i f t zones and on deeper f l a n k s o f the v a s t number of submarine v o l c a n o e s . In c o n t r a s t , they found t h a t a l k a l i - r i c h b a s a l t s cap submarine and i s l a n d v o l c a n o e s . N i c h o l l s e t a l (1964) n o t e d t h a t most o f the b a s a l t s from the r i d g e s o f the m i d - A t l a n t i c and I n d i a n oceans are t h o l e i i t i c . Kay e t a l (1970) c o n c l u d e d t h a t mid-ocean r i d g e v o l c a n i c r o c k s are c h a r a c t e r i s e d by o l i v i n e t h o l e i i t i c n o r m a t i v e c o m p o s i t i o n w i t h v a r i a b l e A 1 2 0 3 and t o t a l - i r o n c o n t e n t s . Aumento (1967) remarked t h a t a v a r i e t y o f v o l c a n i c e x t r u s i v e s i n c l u d i n g b o t h t h o l e i i t e s and a l k a l i b a s a l t s o c c u r on the M i d - A t l a n t i c R i d g e , y e t even h e r e , o l i v i n e t h o l e i i t e s are t h e . o n l y t y p e . r e p o r t e d to have e x t r u d e d i n the depths o f the median r i f t v a l l e y , w h i l e on the h i g h e r reaches o f the r i d g e and on o c e a n i c i s l a n d s , t h e r e o c c u r a l s o , t r a n s i t i o n -a l o l i y i n e t h o l e i i t e s and a l k a l i b a s a l t s . S e v e r a l c h e m i c a l and p e t r o l o g i c a l c h a r a c t e r i s t i c s d i s t i n g u i s h the two types o f o c e a n i c b a s a l t . Yoder and T i l l e y (1962) d i s t i n g u i s h e d them on the b a s i s o f n o r m a t i v e c o m p o s i t i o n . A l k a l i b a s a l t s and o l i v i n e t h o l e i i t e s are d i v i d e d i n the g e n e r a l -i z e d n o r m a t i v e b a s a l t t e t r a h e d r o n by the c r i t i c a l p l a n e o f s i l i c a u n d e r s a t u r a t i o n , ( F i g u r e 1 8 ) , o l i v i n e t h o l e i i t e s c o n t a i n -i n g n o r m a t i v e o l i v i n e and h y p e r s t h e n e , w h i l e a l k a l i b a s a l t s c o n t a i n n o r m a t i v e o l i v i n e and n e p h e l i n e . Chemical d i s t i n c t i o n between o l i v i n e t h o l e i i t e s and a l k a l i b a s a l t s have been r e p o r t e d by E n g e l and E n g e l (1964), E n g e l e t a l (1965) and o t h e r s . Such 26 .. 0 I, , M U X. \ \ i . u \ . . i C >MI ' i " . | 1 ) 1 1 N N V M I NI 1 S \ 1) VI \ T I ' i V s T i l ' H i II > n : , L . * v l l l l M I ' V U I 11 U | I'll Al .k M l l i l M I h 1 ! t u \ l S l . \M H A I ' 1 ! A S H Ki sit*. . lliolriitii: Mi-aii AU..M Men 1 Cumjxml ion l K l v . l l ! deviation basalt ( ICVKI I '.on (pans per million) Ha II 7 49H 136 Co *2 3 25 5 Cr 297 73 67 57 Cu 77 6 36 13 C,:i 17 •> 22 2 I.a < SO 90? Li 9 6 11 5 Nh < 30 72 9 Ni 97 19 51 .33 Kb < 10 33 Sc CI 19 26 4 Sr 130 21 H15 375 V 292 57 252 32 Y •13 10 54 7 Yb 5 1.5 4 1 Zf 95 15 333 48 (wciglit per cent) SiOj 49.34 .54 47^ 41 3.08 TiOj 1.49 .39 2.87 .24 M2O3 17.04 1.78 IS.02 1.71 1.99 .65 4.17 1.16 1-cO 6.82 1.50 5.80 1.17 M11O .17 .03 .16 .03 Mi;0 7.19 .67 4.79 1.35 o o 11.72 .69 8.65 .91 \:.;0 2.73 .20 3.99 .41 K a O .16 .06 1.66 .38 H ; 0 + .(>9 .79 H 2 O - .5S .61 '" i\-Oi .16 .05 .92 .22 . 29 ' .72 K/i<!i • 1300" 418 Sr/Rb 130* 25 Na/K 16 2 K / Z r 14 .4 K/Ba 121 ' 28 i K / C r 4 206 • . . . ( from En-el et a l . , 1965.) 1 i r y 30 10 ._L_J L _ J L ( Y ) (ft) ~ _ l L _ J L L a Co P r N d P m S m E u G d T b D y H o E r T m Yl> L u 60 70 a t o m i c n u m b e r F IGURE 1 9 . R a r e - e a r t h pa t t e rn r e l a t ion be tween (o) tho le i i t i c basalts o f sh ie ld volcanoes a n d (o) spa t ia l ly re la ted m i d - o c e a n r idge tholei i tcs . N o t e the l igh t r ;c . en r i chmen t for sh ie ld volcanoes i n contrast to the heavy rare earths w h i c h are s i m i l a r for b o t h the sh ie ld v o l c a n o a n d the m i d - o c e a n r idge , (a) J c b c l T c i r tholei i te and average of five R e d S e a a x i a l t rough basa l i s ; (b) C u l p e p p e r thole i i te a n d average o f six Eas t Pac i f i c rise tholei i tcs . . .(frop S c h i l l i n g , 1971.) 27 d i s t i n g u i s h i n g f e a t u r e s are summarised i n Table I t a k e n from E n g e l e t a l (1965). S c h i l l i n g (1971) a l s o shows t h a t on the b a s i s o f r a r e e a r t h d i s t r i b u t i o n p a t t e r n , mid-ocean r i d g e t h o l e i i t e s are d i s t i n g u i s h a b l e from s p a t i a l l y r e l a t e d o c e a n i c s h i e l d v o l c a n o e s o f t h o l e i i t i c c o m p o s i t i o n ( F i g u r e 1 9 ) , f o r t h e r e i s "a n o t i c e a b l e e n r i c h m e n t o f the l i g h t r . e . c o n t e n t f o r the s u b a e r i a l s h i e l d v o l c a n o e s r e l a t i v e to the s p a t i a l l y r e l a t e d mid-ocean r i d g e bas-a l t s , w h i l e the heavy r . e . c o n t e n t s s t a y more c o n s t a n t . " Gast (1971) demonstrated t h a t o c e a n i c t h o l e i i t e b a s a l t s had about the same p r o p o r t i o n s o f r a r e e a r t h s as c h o n d r i t i c m e t e o r i t e s e x c e p t f o r a s l i g h t d e f i c i e n c y i n the l i g h t e r e l e m e n t s , w h i l e the a l k a l i b a s a l t s o c c a s i o n a l l y produced at the r i d g e were g r e a t l y e n r i c h e d i n the l i g h t r a r e e a r t h s . O r i g i n o f Oceanic B a s a l t s Yoder and T i l l e y ( 1 9 6 2 ) , from an e x p e r i m e n t a l s t u d y o f b a s a l t s , c o n c l u d e d t h a t "The two p r i n c i p a l magma types r e c o g n i s e d by f i e l d i n v e s t i g a t o r s - t h o l e i i t i c and a l k a l i b a s a l t types - appear t o be s e p a r a t e d by e q u i l i b r i u m t h e r m a l d i v i d e s a t 1 atm. The p r i n c i p a l d i v i d e s were found by expe r i m e n t at e l e v a t e d p r e s s u r e s to g i v e way t o a new s e t o f e q u i l i b r i u m t h e r m a l d i v i d e s r e s u l t i n g from a new m i n e r a l o g y . The change of the e q u i l i b r i u m t h e r m a l d i v i d e s w i t h p r e s s u r e l e a d s t o the d e r i v a t i o n o f the two p r i n c i p a l magma t r e n d s from the same b u l k c o m p o s i t i o n I n g e n e r a l , a l k a l i b a s a l t - t y p e magmas are to be e x p e c t e d to be g e n e r a t e d a t g r e a t e r depths than t h o l e i i t i c t ype magmas from the same p r i m a r y s o u r c e r o c k , " Kuno (1959) would d e r i v e t h e s e two magma t y p e s from d i f f e r e n t p a r e n t m a t e r i a l s m e l t i n g under d i f f e r e n t c o n d i t i o n s at d i f f e r e n t l e v e l s beneath the c r u s t . Macdonald (1959), and 28 E n g e l and En g e l (1964) , from a n a l y t i c a l d a t a c o u p l e d w i t h f i e l d r e l a t i o n s h i p s o f the p r i n c i p a l b a s a l t t y p e s on the H a w a i i a n I s l a n d s and E a s t P a c i f i c R i s e r e s p e c t i v e l y , c o n c l u d e d t h a t a l k a l i - r i c h r o c k s are d e r i v a t i v e r o c k s f r a c t i o n a t e d from o c e a n i c t h o l e i i t e s by p r o c e s s e s o f magmatic d i f f e r e n t i a t i o n and o c e a n i c t h o l e i i t e s are the p r i n c i p a l o r o n l y magma g e n e r a t e d i n the upper mantle beneath the oceans. S c h i l l i n g (1971) r e p o r t e d t h a t " T h o l e i i t i c s h i e l d v o l c a n o e s which are p r e s e n t l y o r were once l o c a t e d over the c r e s t of mid-ocean r i d g e s are n o t common. Most o c e a n i c i s l a n d s are c h a r a c t e r -i s e d by l a v a s o f the a l k a l i s e r i e s a t l e a s t subaer-i a l l y . Such o b s e r v a t i o n s p r o b a b l y have g e n e t i c i m p l i c a t i o n s . Dredge h a u l s from f l a n k s o f v o l c a n i c i s l a n d and seamounts are too s c a r c e to determine c r i t i c a l l y whether a g r a d a t i o n e x i s t s between sea-f l o o r t h o l e i i t i c b a s a l t s o v e r the r i d g e and a l k a l i b a s a l t s towards the top o f such v o l c a n i c e d i f i c e s . " Green and Ringwood (1967) demonstrated t h a t changes i n p r e s s u r e may be the c o n t r o l l i n g f a c t o r i n the t r e n d o f f r a c t i o n -a l m e l t i n g ; magma s e g r e g a t i o n from r e s i d u a l c r y s t a l s , and c r y s -t a l l i z a t i o n , o f a p a r e n t p y r o l i t e mantle p r o d u c i n g h i g h a l u m i n a o l i v i n e t h o l e i i t e from a p a r t i a l m e l t a t a depth o f 30 km. and a l k a l i b a s a l t from f r a c t i o n a l m e l t i n g at g r e a t e r d e p t h s . Aumento (1967) remarked, however, t h a t " t h i s i n t e r r u p t e d sequence o f magma g e n e r a t i o n b e g i n n i n g w i t h the i n c o r r e c t magma types does not c o r r e s p o n d to the f i e l d o b s e r v a t i o n s . " N e v e r t h e l e s s , Aumento (1967), used e x p e r i m e n t a l d a t a by Green and Ringwood (1967) - " t h a t 30% p a r t i a l m e l t i n g o f p y r o l i t e a t p r e s s u r e s o f 12-18 kb. (35-60 km.) w i l l y i e l d o l i v i n e t h o l e i i t e magma .... a l e s s e r degree o f p a r t i a l m e l t i n g (201) w i l l produce an a l k a l i - r i c h b a s a l t l i q u i d " , and t h e i r demonstra-t i o n t h a t " o l i v i n e - r i c h t h o l e i i t i c magma may f r a c t i o n a t e t o 29 C L O S E D S Y S T E M F R A C T I O N A T I O N O F BASALTIC AT V A R I O U S P R E S S U R E S M A G M A ALKALI OlIVINE BASALT High Al,q& normative ALKALI OLIVINE BASALT High AljOj OLIVINE &ASANITE normative nephdine Aluminous G'nopyroxen© i + Aluminous Erotatire CfinopyroJtena AUCAU OLIVINE BASALT Low A l , 0 , ; 25% normative olivine; 1*2% normative r>eph»Cn« AIum-ncHJi Eniratite AJuminoui & lub-caick cGno pyroxene OLIVINE THOLEIITE K$h AljOj 5-15% nor motive olivine & 5-15% hypenrhenc Enslatit* + Ctino pyroxene OLIVINE BASALT Low A i p 3 ; 25% normative otvinc; ^5% orthopyroxene Ahjrninouf En italic 9 Kb low AljO, EtutaHte OLIVINE THOLEIITE Low A l p , 20-25% nor ma live olivine ;5-I5% norma' five hypenthene 13-18 Kb l + Mgh Alft • Eratatite ? PICRITE FIGURE 20. DiagraiiJiiatic summary of. the effects and directions of fractionation of basaltic liquids at moderate to high pressures, (from Green and Ringwood, 1967,) W CREST MTNS. RECENT MEDIAN VALLEY 5 km. ALKALI BASALT •1 TRANS THOLEIITES OLIVINE 7 rIOLEIITES ppt. of olivine ppt of olivine a low Co pyroxene ppt of Mg - Al pyroxene % fractional melting sea level - 1 km. - 2km. 3 km. — 15 km-• 35 km. • ALKALI OLIVINE BASALT 20% 25% 30% 25% • 60km.-0 = PYROLITE • = PYROLITE = =PYROLITE==C> convection convection FIGURE 21., Model of magmatic evolution, of the Mid-Atlantic Ridge, (from Aumento, 1967.) 30 y i e l d d i s t i n c t i v e b a s a l t i c magma types a t d i f f e r e n t p r e s s u r e s " ( F i g u r e 20) to p r o v i d e a model o f magmatic e v o l u t i o n f o r the M i d - A t l a n t i c Ridge. O b s e r v a t i o n s by s e v e r a l a u t h o r s have shown t h a t "a v a r i e t y o f v o l c a n i c e x t r u s i v e s i n c l u d i n g t h o l e i -i t e s , h i g h a l u m i n a and a l k a l i b a s a l t s and t h e i r d i f f e r e n t i a t e s o c c u r on the M i d - A t l a n t i c Ridge .... i n c o n t i n u o u s r e p e a t e d v o l c a n i c c y c l e s commencing w i t h t h o l e i i t e s and en d i n g w i t h a l k a l i b a s a l t s .... - t h o l e i i t e s e x t r u d i n g on the f l o o r o f the median r i f t v a l l e y ; subsequent e x t r u s i o n s c a p p i n g the t h o l e i i t e s are p r o g r e s s i v e l y e n r i c h e d i n a l k a l i s . " Aumento's model ( F i g u r e 21) i s as f o l l o w s : "Sudden s t r e s s r e l e a s e due t o f a u l t i n g on the median r i d g e v a l l e y r e s u l t i n c o n s i d e r a b l e p a r t i a l m e l t i n g o f a p y r o l i t e mantle which i n i t i a t e s a v o l -c a n i c c y c l e w i t h subsequent e x t r u s i o n o f t h o l e i i t i c l a v a . As the o r i g i n a l energy i s consumed, the e x t e n t of p a r t i a l m e l t i n g p o s s i b l e i s g r a d u a l l y r e d u c e d so t h a t s m a l l e r q u a n t i t i e s o f magma p r o g r e s s i v e l y e n r i c h e d i n a l k a l i s are g e n e r a t e d . The l a s t e x t r u -s i o n s o f a v o l c a n i c c y c l e c o n s i s t of s m a l l q u a n t i t i e s o f a l k a l i o l i v i n e b a s a l t . S i n c e the magmas e x t r u d e onto an a c t i v e l y s p r e a d i n g f l o o r .... by the time a v o l c a n o r e a c h e s the end o f i t s c y c l e e x t r u d i n g the l a s t o f the a l k a l i l a v a s , i t w i l l have moved l a t e r a l l y a few km. from i t s o r i g i n a l a x i a l p o s i t i o n ..... H i g h a l u m i n a e q u i v a l e n t s o f these magmas w i l l be produce d by d i f f e r e n t i a t i o n o f the r i s i n g magmas at lower p r e s s u r e and/or by p a r t i a l m e l t i n g o f p y r o l i t e a t s h a l l o w e r d e p t h s . " Thus due t o d i f f e r e n t amounts o f p a r t i a l m e l t i n g o f a p y r o l i t e m a n t l e , c o u p l e d w i t h magmatic d i f f e r e n t i a t i o n and a s p r e a d i n g s e a f l o o r "a c o n t i n u o u s c y c l e from t h o l e i i t i c to a l k a l i b a s a l t magma g e n e r a t i o n w i l l t a k e p l a c e w h i c h can be c o r r e l a t e d to the f i e l d e v i d e n c e o f abundances and s p a t i a l d i s t r i b u t i o n s " a l o n g the mid-ocean r i d g e . Gast (1971) on the b a s i s o f r a r e e a r t h d i s t r i b u t i o n p a t t e r n s i n t h o l e i i t e s i n c o n t r a s t to a l k a l i b a s a l t s s u g g e s t e d 31 ( l i k e Green and Ringwood, 1967, from e x p e r i m e n t a l d a t a on b a s a l t s ) t h a t o c e a n i c ' t h o l e i i t e s may be the r e s u l t o f e x t e n s i v e m e l t i n g o f the p a r e n t m a t e r i a l , w h i l e a l k a l i b a s a l t s r e p r e s e n t e d a l e s s e r degree o f m e l t i n g . Yoder (1971), r e p o r t i n g p r o c e e d i n g s o f a meeting on the P e t r o l o g i c I m p l i c a t i o n s o f P l a t e T e c t o n i c s , s t a t e d t h a t " P r o d u c t i o n o f magma beneath the r i d g e may r e s u l t from the ( c o n v e c t i v e ) r i s e (from a h i g h - t e m p e r a t u r e , h i g h - p r e s s u r e environment) o f s o l i d g a r n e t p e r i d o t i t e w i t h o u t l o s s o f heat i n t o a lo w e r p r e s s u r e e n v i r o n -ment where m e l t i n g can b e g i n . G e n e r a t i o n o f the d i f f e r e n t types o f magma appears t o depend on s e v e r a l f a c t o r s ( i ) the c o m p o s i t i o n o f the p a r e n t a l m a t e r i a l ( i i ) the depth at which the magma i s s e p a r a t e d from the p a r e n t a l m a t e r i a l , ( i i i ) the degree o f p a r t i a l m e l t i n g a c h i e v e d ( i y ) the p h y s i o c h e m i c a l c o n s t r a i n t s t h a t l e a d to r e p e t i t i v e magma types (v) the water c o n t e n t s t o r e d i n hydrous phases and ( v i ) the e x t e n t o f f r a c t i o n a t i o n en r o u t e t o the s u r f a c e . " 32 CHAPTER THREE SHIPBORNE AND LABORATORY INVESTIGATIONS SHIPBORNE OPERATIONS I n t r o d u c t i o n Work i n the a r e a was c a r r i e d out d u r i n g two c r u i s e s i n the summer o f 1970, b o t h aboard CNAV ENDEAVOUR ( F i g u r e 22E); the f i r s t from May 25 t o June 14 and the second from J u l y 13 t o August 5. The second c r u i s e was Phase V I I o f the HUDSON '70 p r o j e c t ( a l s o c a l l e d BI 69-050) d u r i n g which the CSS HUDSON c i r c u m n a v i g a t e d the Am e r i c a s . D u r i n g Phase V I I ( a l s o c a l l e d BI 69-050), the CSS HUDSON and CNAV ENDEAVOUR worked t o g e t h e r on a s e i s m i c r e f r a c t i o n experiment o f f Queen C h a r l o t t e I s l a n d s , and i n d e p e n d e n t l y on a g e o l o g i c a l r e c o n n a i s s a n c e o f the r e g i o n ( S r i v a s t a v a e t a l , 1971). Continuous S e i s m i c R e f l e c t i o n P r o f i l i n g CSP p r o f i l e s IOUBC 70 -16-15-*21 , 23 and 26 were r e c o r d e d i n the area d u r i n g the f i r s t c r u i s e , w h i l e p r o f i l e s EN 70-025-1, 2,3,9,10,11 were r e c o r d e d on the second c r u i s e . P r o f i l e s BI 69-050- Hudson 3 and 6 were o b t a i n e d aboard the CSS HUDSON by Keen and B a r r e t t . B a t h y m e t r i c p r o f i l e s o f these l i n e s were a l s o r e c o r d e d . These p r o f i l e s o f sea bottom and a c o u s t i c a l r e f l e c t o r s below i t were f i l t e r e d and r e c o r d e d on a wet paper G i f f t G r a p h i c Recorder except f o r the two Hudson p r o f i l e s which were r e c o r d e d b o t h on an EPC dry paper r e c o r d e r and on an A l d e n wet paper Pr e -c i s i o n G r a p h i c Recorder. B o l t a i r g u n s were used as sound source (Model 600B, 5-40 c u . i n . at 1400 - 2000 p s i . on CNAV ENDEAVOUR and a 300 c u . i n . B o l t a i r g u n on CSS HUDSON). The r e t u r n i n g 34 echoes were r e c e i v e d on a l i n e a r a r r a y o f 20 hydrophones (Geospace M-7) towed 200 f e e t a f t . (For t e c h n i c a l d e t a i l s , see T i f f i n , 1969). Dredging A p i p e dredge was used d u r i n g the f i r s t c r u i s e (when o n l y one h a u l , IOUBC 70-16-12D was made i n the area) and c h a i n bag dredges ( F i g u r e 22) s u p p l i e d by B e d f o r d I n s t i t u t e d u r i n g the second c r u i s e . Each s i t e was s e l e c t e d w i t h the a i d o f a bathy-m e t r i c c h a r t (Mammerickx, 1969, S p e c i a l Chart No. 1, S c r i p p s I n s t i t u t e o f Oceanography), echosounder p r o f i l e s and CSP p r o f i l e s . The dredge was l o w e r e d from the s h i p ' s bow, and when the metered w i r e l e n g t h i n d i c a t e d the dredge to be on the bottom, the s h i p moved s l o w l y a s t e r n , p a y i n g out a d d i t i o n a l w i r e i n amounts o f a p p r o x i m a t e l y 50% o f water depth to p e r m i t d r e d g i n g w i t h w i r e angle r a n g i n g from 0° to 45°. At a l l s i t e s , the dredge was p l a c e d low t o p o g r a p h i c a l l y and drawn u p h i l l . O ther O p e r a t i o n s P r o f i l e s o f the t o t a l i n t e n s i t y o f the e a r t h ' s magnetic f i e l d were r e c o r d e d d u r i n g the f i r s t c r u i s e . The i n s t r u m e n t used was a V a r i a n p r o t o n p r e c i s i o n magnetometer. Heat f l o w measurements were o b t a i n e d by L i s t e r and Davis aboard the CSS HUDSON d u r i n g Phase V I I Hudson 70 ( F i g u r e 1 2 ) . N a v i g a t i o n was by b a t h y m e t r i c c h a r t , Loran A, c e l e s -t i a l f i x e s , dead r e c k o n i n g and S a t e l l i t e N a v i g a t o r . LABORATORY METHODS C.S.P. A n a l y s i s T i f f i n (1969) d i s c u s s e d i n d e t a i l the CSP r e c o r d i n g t e c h n i q u e and the a n a l y s i s o f CSP r e c o r d s . S i d e by s i d e r e c o r d -35 i n g o f each s h o t o f the a i r g u n ( f r e q u e n c y ^ 1 shot/5 sec.) and i t s subsequent echoes produced a p r o f i l e o f the seabottom and subbottom s t r u c t u r e s . A c o u s t i c r e f l e c t i o n s o c c u r at changes of a c o u s t i c impedance ( i . e . changes o f the p r o d u c t o f d e n s i t y and c o m p r e s s i o n a l wave v e l o c i t y ) . G r a i n s i z e , m i n e r a l composi-t i o n , p o r o s i t y , degree o f l i t h i f i c a t i o n , depth o f b u r i a l as w e l l as o t h e r f a c t o r s a f f e c t the e l a s t i c p r o p e r t i e s o f m a t e r i a l s upon wh i c h a c o u s t i c impedance depends. S i n c e a c o u s t i c impedance i s i n t i m a t e l y r e l a t e d to geology and s i n c e CSP r e c o r d s r e f l e c t changes i n t h i s impedance, g e o l o g i c a l i n t e r p r e t a t i o n s o f CSP r e c o r d s can be made, though s e v e r a l problems must be overcome. F i r s t , w i t h o u t d e c o n v o l u t i o n , the d u r a t i o n o f the o u t g o i n g soundwave tended to reduce r e s o l u t i o n on the CSP r e c o r d . The w i d t h o f the r e t u r n i n g echo was found by measuring the w i d t h of the band o f p a r a l l e l l i n e s r e p r e s e n t i n g the s e a f l o o r o r where p o s s i b l e , the a i r - w a t e r i n t e r f a c e . Subbottom r e f l e c t o r s were a l s o r e c o r d e d as bands o f l i n e s r a t h e r than a d i s c r e e t and sharp i n t e r f a c e . The f i r s t o f these l i n e s c o r r e s p o n d e d t o the t r u e two-way t r a v e l time to the r e f l e c t o r . The o t h e r l i n e s tended to obscure echoes from u n d e r l y i n g c l o s e l y spaced r e f l e c -t o r s . Thus, f o r example, i n IOUBC 17-16-23, i t i s not known whether the f i r s t subbottom r e f l e c t o r o u t c r o p s on the s e a f l o o r o r n o t , due to the w i d t h o f the band o f l i n e s r e p r e s e n t i n g the s e a f l o o r o b s c u r i n g the f i r s t subbottom r e f l e c t o r . Second, soundwaves r e f l e c t e d from the seabottom are not o n l y p i c k e d up by hydrophones, but may be r e f l e c t e d back a g a i n by the a i r - w a t e r i n t e r f a c e to the seabottom and r e t u r n e d a g a i n and a g a i n from the s e a f l o o r . Such r e v e r b e r a t i o n produced 36 m u l t i p l e s on the r e c o r d which because of t h e i r g r e a t e r energy, sometimes obscured subbottom r e f l e c t o r s as, f o r example, i n the p r o f i l e s across the c o n t i n e n t a l s l o p e . T h i r d , p o i n t r e f l e c t o r s tend to produce hyperbolae on the r e c o r d . Such hyperbolae are common where topography i s rough and i r r e g u l a r , where steep slopes e x i s t near the shi p ' s t r a c k , or where sources i n t e r n a l to the sediment (as f o r example, o c c a s i o n a l l a r g e boulders) are pr e s e n t . See, f o r example, the records c r o s s i n g the steep and i r r e g u l a r s l o p e s o f the Dellwood Seamounts, Hyperbolae o f t e n have ' t a i l s ' which obscure true s l o p e s . Even i f a ship does not pass d i r e c t l y over a p i n n a c l e or steep s l o p e , but to one side of i t , a h y p e r b o l i c p a t t e r n may s t i l l appear on the r e c o r d as a side echo above or below the s e a f l o o r s u r f a c e depending on the p o s i t i o n of the o b j e c t producing the s i d e echo. Side echoes are shown on s e v e r a l p r o f i l e s i n c l u d -i n g IOUBC 70-16-15, IOUBC 70-16-21A and EN 70-025-9, the l a s t having a subbottom side echo. I d e n t i f i c a t i o n of r e f l e c t o r s as specific rock types cannot be made on the b a s i s of s e i s m i c evidence o n l y . A d d i t i o n a l data i n the form of s e a f l o o r and core samples are necessary. However, from past e x p e r i e n c e , the general g e o l o g i c a l nature of the m a t e r i a l t r a v e r s e d may be surmised from the r e c o r d c h a r a c t e r i t s e l f . Thus the broad d i s t i n c t i o n between v o l c a n i c basement and sediments i s made on the records by s e v e r a l c h a r a c t e r i s t i c s which i n c l u d e the abundance of hyperbolae from p o i n t r e f l e c t o r s of v o l c a n i c topography and l a c k of p e n e t r a t i o n of sound energy i n t o the v o l c a n i c s . D i f f e r e n t r e f l e c t o r s w i t h i n the sediments may be due to degree of l i t h i f i c a t i o n , or changes i n l i t h o l o g y . 37 F a u l t s , f o l d s , d i s c o n t i n u i t i e s and o t h e r s t r u c t u r e s c o u l d be d e t e r m i n e d from 1) l a t e r a l c o n t i n u i t y o f r e f l e c t o r s 2) l a t e r a l change i n c h a r a c t e r o f r e f l e c t o r s 3) a t t i t u d e o f r e f l e c t o r s w i t h r e s p e c t t o o v e r l y i n g and n e i g h b o u r i n g r e f l e c t o r s 4) n a t u r e o f the upper c o n t a c t s o f a sequence of r e f l e c t o r s 5) t h i c k n e s s o f u n i t s , b o t h i n d i v i d u a l l y and i n sequence I t s h o u l d be n o t e d t h a t f a u l t s can o n l y show apparent d i p - s l i p movement on s e i s m i c r e c o r d s and, on the b a s i s o f a s i n g l e p r o f i l e , i t i s i m p o s s i b l e to t e l l the type o f f a u l t o r i t s a t t i t u d e , as i t i s a l s o i m p o s s i b l e t o t e l l the a t t i t u d e o f any s t r u c t u r e from o n l y one p r o f i l e . P e t r o g r a p h y Each sample i n a dredge h a u l was c h i p p e d and c l a s s i f i e d i n t o one o f s e v e r a l groups on a b a s i s o f hand-specimen d e s c r i p t i o n o f t e x t u r e and m i n e r a l o g y o f p h e n o c r y s t s . T h i n s e c t i o n s were then made of two o r t h r e e samples from each group and these f u r -t h e r d e s c r i b e d and c l a s s i f i e d , m a i n l y on a t e x t u r a l b a s i s . The o b s e r v e d t e x t u r e s a l l grade i n t o one a n o t h e r . F i g u r e 33 summar-i s e s t h e s e t e x t u r e s and r e l a t e s them to r a t e o f c o o l i n g . Photo-graphs were t a k e n o f a l l major t e x t u r a l f e a t u r e s o b s e r v e d . X-ray d i f f r a c t i o n a n a l y s i s on a s e l e c t few b a s a l t samples a i d e d i n i d e n t i f i c a t i o n o f minor m i n e r a l phases and d e t e r m i n e d the f o s t e r i t e c o n t e n t o f the o l i v i n e s . X-ray d i f f r a c -t i o n work was a l s o done on mud sample EN 70-025-70-72, manganese and i r o n c r u s t s and n o d u l e s , h y a l o c l a s t i t e s , and a sandstone sample IOUBC 70-16-12D-3 m a i n l y f o r i d e n t i f i c a t i o n o f the con-t a i n e d c l a y m i n e r a l s . 38 C h e m i c a l A n a l y s e s T h r e e b a s a l t s a m p l e s were s e n t t o J a p a n f o r a c c u r a t e w e t - c h e m i c a l d e t e r m i n a t i o n s o f m a j o r o x i d e s ( s e e P e t r o l o g y ) a n d t h e s e , t o g e t h e r w i t h a USGS b a s a l t s t a n d a r d a n d l o c a l s t a n d a r d s p r o v i d e d by F l e t c h e r (1971 - p e r s o n a l c o m m u n i c a t i o n ) , were u s e d as s t a n d a r d s f o r w h o l e r o c k c h e m i c a l a n a l y s e s b y a t o m i c a b s o r p -t i o n s p e c t r o p h o t o m e t r y . F o r d e t e r m i n a t i o n o f t h e m a j o r o x i d e s , t h e m e thod u s e d was s l i g h t l y m o d i f i e d a f t e r t h a t by M e d l i n e t a l ( 1 9 6 9 ) , F l e t c h e r (1971 - p e r s o n a l c o m m u n i c a t i o n ) and B a r r (1971 -p e r s o n a l c o m m u n i c a t i o n ) 0.1 gm. o f -100 mesh s a m p l e was m i x e d w i t h 0.8 gm. l i t h i u m m e t a b o r a t e a nd f u s e d (10 -15 m i n . a t 1000° C) i n a p r e i g n i t e d g r a p h i t e c r u c i b l e T h i s was t h e n d i s s o l v e d i n 50 m l . o f 5% n i t r i c a c i d a i d e d b y s t i r r i n g f o r a b o u t h a l f an h o u r w i t h a m a g n e t i c s t i r r e r . The s a m p l e s o l u t i o n was t h e n d i l u t e d t o 1000 y g / m l . f o r d e t e r m i n a t i o n o f T i 0 2 and MnO, t h e s o l u t i o n c o n t a i n i n g L a n t h a n u m i n r a t i o 1:9. F o r t h e d e t e r m i n a t i o n o f S i 0 2 , A 1 2 0 3 and K 2 0 , a s a m p l e s o l u t i o n c o n c e n t r a t i o n o f 400 y g / m l . was u s e d , t h i s s o l u t i o n c o n t a i n i n g c a e s i u m i n t h e . r a t i o o f 1:9. A s a m p l e s o l u t i o n c o n c e n t r a t i o n o f 50 y g / m l . a g a i n c o n t a i n i n g t h e same r a t i o o f L a n t h a n u m was u s e d f o r t h e d e t e r m i n -a t i o n o f CaO and t o t a l i r o n F e 2 0 3 , w h i l e a c o n c e n t r a t i o n o f 5 y g / m l . o f s a m p l e s o l u t i o n c o n t a i n i n g b o t h L a n t h a n u m and c a e s i u m ( e a c h i n r a t i o 1:9) was u s e d f o r d e t e r m i n a t i o n o f MgO and N a 2 0 . The a d d i t i o n o f L a n t h a n u m and c a e s i u m was t o c o r r e c t t h e i n t e r -f e r e n c e by o t h e r o x i d e s , d u r i n g a s p i r a t i o n . The o x i d e s t a n d a r d s o l u t i o n s u s e d w ere o b t a i n e d f r o m F l e t c h e r (1971 - p e r s o n a l c o m m u n i c a t i o n ) , e x c e p t t h o s e o f a l u m -i n u m a n d , s i l i c o n , w h i c h w e r e made f r o m r e c i p e s t a k e n f r o m 39 Langmyhr and Paus (1968) . Sample and s t a n d a r d s o l u t i o n s were then a s p i r a t e d i n an a i r - a c e t y l e n e flame e x c e p t f o r A 1 2 0 3 , S i 0 2 and T i 0 2 which were a s p i r a t e d i n a n i t r o u s o x i d e - a c e t y l e n e f l a m e , the s t a n d a r d s o l u t i o n s d i l u t e d t o v a r i o u s c o n c e n t r a t i o n s so t h a t absorbance v a l u e s when sample s o l u t i o n was a s p i r a t e d l a y between two c l o s e l y -spaced v a l u e s o f absorbance r e a d i n g when s t a n d a r d s o l u t i o n s o f d i f f e r e n t c o n c e n t r a t i o n s were a s p i r a t e d . Assuming a s t r a i g h t l i n e between the two s t a n d a r d absorbance r e a d i n g s , the c o n c e n t r a -t i o n C ( i n ug/ml.) o f the o x i d e i n the sample s o l u t i o n b e i n g a s p i r a t e d i s C = A + K E -Ei x 1 E 2 - E l where A = c o n c e n t r a t i o n o f lower s t a n d a r d i n g/ml. K = d i f f e r e n c e i n c o n c e n t r a t i o n s between upper and lower s t a n d a r d s E x = absorbance r e a d i n g f o r sample s o l u t i o n E 2 = absorbance r e a d i n g f o r upper s t a n d a r d El = absorbance r e a d i n g f o r l o w e r s t a n d a r d The p e r c e n t a g e by weight o f the o x i d e i n the ro c k i s then C/Co x 100 where Co i s the c o n c e n t r a t i o n ( i n ug/ml.) o f the sample s o l u t i o n used. For each o x i d e , a s p i r a t i o n s were done f o u r times and the a r i t h m e t i c mean c a l c u l a t e d . D u p l i c a t e a n a l y s e s were done i n some c a s e s . The c o n c e n t r a t i o n C o f the o x i d e i n the sample s o l u t i o n c o u l d a l s o be found g r a p h i c a l l y assuming a s t r a i g h t l i n e between absorbance r e a d i n g f o r upper and lo w e r s t a n d a r d s o l u t i o n s and t h i s was done i n some c a s e s . Trace element c o n c e n t r a t i o n s were det e r m i n e d i n o n l y a v e r y few samples, m a i n l y f o r economic p u r p o s e s . A 0.5 gm o f -100 mesh sample was d i s s o l v e d i n 25 ml. o f 1.5 molar hydro-40 c h l o r i c a c i d a c c o r d i n g to the method of F l e t c h e r (1970 - p e r s o n a l communication) and a f t e r e v a p o r a t i o n to dryness, mixed with 3 ml. water, 2 ml, n i t r i c a c i d , 0.5 ml. p e r c h l o r i c a c i d and 5 ml. hydro-f l u o r i c a c i d . T h i s s o l u t i o n ( c o n c e n t r a t i o n 20 mg/ml.) or when nece s s a r y a d i l u t e d p o r t i o n o f i t , was then a s p i r a t e d t o g e t h e r with standard s o l u t i o n s p r o v i d e d by F l e t c h e r (1971 - p e r s o n a l communication) i n the a i r - a c e t y l e n e flame of an atomic a b s o r p t i o n u n i t and c o n c e n t r a t i o n s o f Fe, Mn, Co, N i , Cu, Pb, Zn were determined. In the d e t e r m i n a t i o n o f Co, Ni and Pb, a hydrogen lamp was used i n a d d i t i o n to the element lamp to c o r r e c t f o r background a b s o r p t i o n . The c o n c e n t r a t i o n s o f the elements were c a l c u l a t e d g r a p h i c a l l y and d u p l i c a t e analyses showed r e s u l t s to be r e p r o d u c i b l e . In b a s a l t s , water was determined as f o l l o w s : 0.2500 - 0.3000 gm sample powder was mixed i n a hard g l a s s t e s t tube w i t h p r e i g n i t e d sodium t u n g s t a t e f l u x e q u i v a l e n t to s i x times weight of sample. A weighed p i e c e o f f i l t e r paper ( p r e v i o u s l y weighed i n a weighing b o t t l e ) was p l a c e d i n the top of the tube which was wrapped i n a wet paper towel around the upper p a r t . The sample and tube were then heated f o r about f i v e to ten min-u t e s , the r e l e a s e d water vapour being condensed and c o l l e c t e d i n the f i l t e r paper at the top of the tube. The f i l t e r paper was then t r a n s f e r r e d to the weighing b o t t l e and reweighed to give the amount of water r e l e a s e d . A d e t e r m i n a t i o n w i t h f l u x o n l y gave a n e g l i g i b l e i n c r e a s e i n weight of f i l t e r paper. D u p l i c a t e analyses were made on a l l samples. 41 CHAPTER FOUR SEISMIC DATA INTRODUCTION The s e i s m i c d a t a o b t a i n e d d u r i n g t h i s s t u d y are p r e - . s e n t e d i n P l a t e s I. to X I I I , w hich comprise photographs o f the o r i g i n a l r e c o r d e d p r o f i l e s and the g e o l o g i c a l i n t e r p r e t a t i o n i n the form o f l i n e d r a w i n g s . In a d d i t i o n , p r o f i l e s o f . t h e t o t a l i n t e n s i t y o f e a r t h ' s magnetic f i e l d are shown w i t h the CSP l i n e s from C r u i s e IOUBC 70-16. T r a c k s o f the s u r v e y l i n e s are i n d i c a t e d on F i g u r e 23 (see page 137) . E i g h t e e n p r o f i l e s r e p r e s e n t i n g 513 n a u t i c a l m i l e s (951 km) t r a v e r s e the a r e a . Because o f the d i f f e r e n c e between v e r t i c a l and h o r i z o n t a l s c a l e , a n g l e s on the p r o f i l e s are g r e a t l y d i s t o r t e d . Low a n g l e s are e x a g g e r a t e d h a v i n g the appearance of s t e e p s l o p e s , w h i l e s t e e p e r s l o p e ang-l e s are compressed though s t i l l b e i n g e x a g g e r a t e d . Nomographs on the l i n e drawings show the e x a g g e r a t i o n . The v e r t i c a l e x a g g e r a t i o n v a r i e s from r e c o r d to r e c o r d and sometimes v a r i e s o v e r the l e n g t h o f a s i n g l e r e c o r d ; t h i s v a r i a t i o n b e i n g caused m a i n l y by changes i n h o r i z o n t a l s c a l e due t o changes i n s h i p ' s speed. The v e r t i c a l e x a g g e r a t i o n and nomographs are c a l c u l a t e d f o r a sound v e l o c i t y i n water o f 1.5 km/sec. S i n c e sound v e l o c i t y v a r i e s i n d i f f e r e n t . t y p e s o f sediment and i n b e d r o c k , the npmo-: graphs p r o v i d e d s h o u l d s t r i c t l y be used t o measure s e a f l o o r s l o p e s o n l y . The v e l o c i t y o f sound i n u n c o n s o l i d a t e d wet s e d i - . ment v a r i e s between about 1.5 km/sec and 2.2 km/sec.. A key to the symbols used i n the l i n e - d r a w n i n t e r p r e t a t i o n s i s , g i v e n i n F i g u r e 50. 42 MAJOR TECTONIC AND PHYSIOGRAPHIC FEATURES The d a t a o b t a i n e d i n the p r o f i l e s o f P l a t e s I - X I I I are p r e s e n t e d w i t h r e f e r e n c e to the t e c t o n i c and p h y s i o g r a p h i c f e a t u r e s o f the a r e a . Appendix A l i s t s the t e c t o n i c and p h y s i o -g r a p h i c f e a t u r e s ( d i s c u s s e d f o l l o w i n g ) , the p r o f i l e s on which they are l o c a t e d and b r i e f remarks about the f e a t u r e s as seen on the r e s p e c t i v e p r o f i l e . E x p l o r e r S p r e a d i n g Zone The E x p l o r e r S p r e a d i n g Zone i s shown on o n l y one p r o -f i l e - IOUBC 70-16-15 ( P l a t e I ) . A magnetic h i g h (>56400 gammas) o c c u r s o v e r t h i s b a s i n which c o n t a i n s o n l y a t h i n l a y e r o f s e d i -ment d r a p i n g a v o l c a n i c basement but h a v i n g the e f f e c t o f smooth-i n g t h i s basement topography. Normal f a u l t i n g seems t o be p r e s e n t a f f e c t i n g b o t h t h i n sediment c o v e r and v o l c a n i c basement. Dellwood Seamount Range The magnetic p a t t e r n o v e r the De l l w o o d Seamount Range i s b a s i c a l l y s i m i l a r to the t o p o g r a p h i c p r o f i l e but i s s h i f t e d a l i t t l e t o the s o u t h e a s t o f the t o p o g r a p h i c p r o f i l e due p r o b a b l y to the magnetic i n c l i n a t i o n i n the a r e a . P r o f i l e IOUBC 70-16-15 and o t h e r p r o f i l e s (Appendix A) show t h i s range t o c o n s i s t m a i n l y o f rugged v o l c a n i c peaks w i t h sediment f i l l i n g p o c k e t s between them i n p l a c e s up t o 0.2 s e e s , t h i c k . A v e r y t h i n s e d i -ment l a y e r may c o v e r most o f the Seamount Range, but i s not r e s o l v a b l e by the l o n g p u l s e o f the a i r g u n . S o u t h e a s t D e l l w o o d K n o l l s The Southeast D e llwood K n o l l s i s a s e r i e s o f v o l c a n i c r i d g e s and peaks c o v e r e d w i t h sediments o f v a r y i n g t h i c k n e s s i n most areas g r e a t e r than 0.1 sees t h i c k . CSP l i n e IQUBC 70-16-20 w h i c h t r a v e r s e s t h i s s eamount i n a NW d i r e c t i o n shows t h e summit t o be r e l a t i v e l y f l a t and c o v e r e d w i t h s e d i m e n t . T h i s must i n d i c a t e t h a t i t i s r e l a t i v e l y o l d as c o m p a r e d , f o r e x a m p l e , w i t h t h e N o r t h w e s t K n o l l s . The s t e e p n e s s o f t h e s o u t h w e s t (EN 7 0 - 0 2 5 - 9 , P l a t e X) and n o r t h w e s t (IOUBC 7 0 - 1 6 - 2 0 , P l a t e IV) s l o p e s (>15°) p r e s u m a b l y i n d i c a t e f a u l t i n g a l o n g t h e s e s l o p e s . The s o u t h e a s t s l o p e (IOUBC 70-16-20) a l s o seems t o be f a u l t e d . N o r t h w e s t D e l l w o o d K n o l l s CSP l i n e IOUBC 70-16-16 ( P l a t e I I - BC) t r e n d s n o r t h -e a s t a c r o s s t h e n o r t h w e s t s l o p e o f N o r t h w e s t D e l l w o o d K n o l l s . T h i s l i n e c r o s s e s a v e r y s t e e p f a u l t (>30°) w h i c h p r e s u m a b l y t r e n d s a l o n g t h e s i d e o f a s m a l l c h a n n e l h a v i n g i t s h e a d i n t h i s s e a m ount and a c t i n g as a t r i b u t a r y o f t h e S c O t t C h a n n e l . CSP l i n e IOUBC 70-16-20 as w e l l as o t h e r l i n e s ( s e e A p p e n d i x A) show o n l y r e l a t i v e l y l i t t l e (<0.1 s e e s ) s e d i m e n t on t h e - s u m m i t o f t h i s s e a m o u n t . T h i s l i n e a l s o shows t h e c o n t r a s t i n g n a t u r e o f s e d i m e n t c o v e r and v o l c a n i c t o p o g r a p h y o f t h e D e l l w o o d K n o l l s . D e l l w o o d S p r e a d i n g Zone The name " D e l l w o o d S p r e a d i n g Z o n e " i s h e r e g i v e n t o t h e c h a n n e l b e t w e e n t h e D e l l w o o d K n o l l s . CSP p r o f i l e s IOUBC 70-16-20 ( P l a t e IV) and EN 70-025-3 ( P l a t e V) show t h a t s e d i m e n - . t a r y m a t e r i a l a b o u t 0.4 s e c . t h i c k i n t h e c h a n n e l i s d i s t u r b e d by n o r m a l f a u l t i n g . M o s t o f t h i s s e d i m e n t i s b e l i e v e d t o h a v e b e e n t r a n s p o r t e d t o t h e a r e a by t u r b i d i t y c u r r e n t s and i c e r a f t i n g d u r i n g t h e P l e i s t o c e n e ( s e e D i s c u s s i o n ) . The m a g n e t i c p r o f i l e o f P l a t e I V shows, h i g h e s t i n t e n s i t y o v e r D e l l w o o d K n o l l s SE and n o t o v e r t h e c h a n n e l b e t w e e n t h e k n o l l s as w o u l d be e x p e c t e d i f t h i s c h a n n e l w e re t h e s i t e o f a s p r e a d i n g segment ( s e e D i s c u s s i o n ) . 44 S c o t t Channel . In the a r e a o f s t u d y , the S c o t t Channel can be d i v i d e d i n t o two s e c t i o n s : the n o r t h e a s t s e c t i o n l y i n g between the n o r t h e a s t s l o p e s o f the Dellwood K n o l l s , and t h e c o n t i n e n t a l s l o p e and t h e n o r t h w e s t s e c t i o n l y i n g n o r t h w e s t o f t h e Dellwood Seamounts and K n o l l s . The n o r t h e a s t s e c t i o n c o n t a i n s the.Queen C h a r l o t t e F a u l t Zone (see n e x t s e c t i o n ) . Basement crops out,on the s o u t h -west s l o p e - o f t h i s c h a n n e l and here produces a magnetic h i g h (see P l a t e I I - CD).. Other p r o f i l e s a c r o s s t h i s p a r t o f the c h a n n e l show slump and s l i d e s t r u c t u r e s , e s p e c i a l l y , a l o n g t h e : n o r t h e a s t s l o p e s o f the c h a n n e l at the base o f the c o n t i n e n t a l s l o p e , These slump and s l i d e s t r u c t u r e s are i n t e r p r e t e d as-such by comparison w i t h s i m i l a r f e a t u r e s o b t a i n e d by Rona and C l a y (1967) and Emery et a l (1970) on CSP p r o f i l e s o f the c o n t i n -e n t a l r i s e o f f E a s t e r n N o r t h A m e r i c a . Sediment t h i c k n e s s , v a r i e s i n the S c o t t , C h a n n e l r e a c h i n g about 1.5 s e e s , i n P l a t e X I I I . : The n o r t h w e s t s e c t i o n o f the S c o t t Channel i s shown i n P l a t e V. Here subbottom r e f l e c t o r s i n d i c a t e a m i g r a t i n g c h a n n e l , the p r e s e n t c h a n n e l l y i n g a.few k i l o m e t e r s to the s o u t h -e a s t o f an e a r l i e r c h a n n e l . Such a . s i t u a t i o n would be e x p e c t e d as a r e s u l t o f t u r b i d i t y c u r r e n t s f l o w i n g o f f the c o n t i n e n t a l s l o p e o f the N o r t h American p l a t e (assumed h e l d s t a t i o n a r y ) onto the n o r t h w e s t moving P a c i f i c P l a t e . A f a u l t - z o n e e x i s t s below the s o u t h e a s t , s l o p e s o f t h i s c h a n n e l and may be the same as t h a t e n c o u n t e r e d on l i n e IOUBC 70-16-16 ( P l a t e I I - BC) t r e n d i n g a l o n g a t r i b u t a r y o f the S c o t t Channel and t r a n s e c t i n g the n o r t h -west s l o p e o f the .Northwest Dellwood K n o l l s . Beneath the s o u t h -45 e a s t s l o p e s o f t h e S c o t t C h a n n e l t h i s f a u l t - z o n e i s b u r i e d by more r e c e n t s e d i m e n t s t r a n s p o r t e d . a l o n g t h e S c o t t C h a n n e l . Slump s t r u c t u r e s o c c u r on t h e n o r t h w e s t s i d e -of. t h e c h a n n e l i n t h i s a r e a and t h e d i s t u r b e d n a t u r e o f u n d e r l y i n g s e d i m e n t s may a l s o be due t o s l u m p a n d : c u t - a n d - f i l l . S e d i m e n t s i n t h i s n o r t h w e s t p a r t o f t h e c h a n n e l a r e more t h a n 0.6 s e e s . . t h i c k . The Queen C h a r l o t t e F a u l t Zone The Queen C h a r l o t t e F a u l t Zone i s shown i n P l a t e s V I I I and X I I I . The f a u l t i n g s e e n i n P l a t e VI (HH') may a l s o be p a r t o f t h i s f a u l t z o n e . The s e d i m e n t , t h i c k n e s s i n t h i s f a u l t zone r e a c h e s 1.5 sees.,.o£ w h i c h t h e l o w e r s t r a t a a r e f a u l t e d , . s h o w i n g a p p a r e n t d i p - s l i p movements. F a u l t s r e a c h t h e s u r f a c e o n l y w h e re more r e c e n t s e d i m e n t s have b e e n c u t i n t o b y c h a n n e l s . In< P l a t e V I I I (MN), t h e f a u l t i n g ( e x c e p t f o r t h a t on t h e c o n t i n e n t a l s l o p e ) i s m a i n l y on t h e s o u t h w e s t s i d e o f t h e d e e p e s t p a r t o f t h e c h a n n e l , w h e r e a s i n P l a t e X I I I , t h e , f a u l t i n g i s on t h e n o r t h e a s t , s i d e . I t i s n o t e w o r t h y t h a t l i n e s w h i c h c r o s s t h e c h a n n e l t o t h e s o u t h w e s t o f p r o f i l e HH' ( P l a t e V I ) do not show t h e f a u l t z o n e , p r e s u m a b l y b e c a u s e t h e Queen C h a r l o t t e f a u l t , b e i n g a t r a n s -f o r m f a u l t , d i e s o u t i n a s o u t h w e s t d i r e c t i o n ( s e e D i s c u s s i o n ) . The Queen C h a r l o t t e f a u l t zone may n o t i n c l u d e t h e a p p a r e n t d i p - s l i p f a u l t s o f t h e c o n t i n e n t a l s l o p e s o f f Queen. C h a r l o t t e S o u n d . C o n t i n e n t a l S l o p e The s e i s m i c p r o f i l e s w h i c h t r a v e r s e t h e s l o p e and s h e l f edge o f f Queen C h a r l o t t e , S o u n d show s e v e r a l f e a t u r e s , o f i n t e r e s t , ( i ) U p p e r c o n t i n e n t a l s l o p e s e d i m e n t s d i p p i n g g e n t l y (^2°) s e a w a r d a r e t r u n c a t e d by, f a u l t s a l o n g t h e c o n t i n e n t a l s l o p e . 46 ( i i ) S t r u c t u r a l f e a t u r e s , which may be basement h i g h s , a n t i -c l i n e s , or i n t r u s i o n s l i e beneath the upper and lower slopes and dam sediments behind them. Such t e c t o n i c dams are s i m i l a r to those d e s c r i b e d by Emery (1969) beneath the slopes of many c o n t i n e n t a l margins. ( i i i ) Lower slope sediments are deformed, appearing crumpled (see P l a t e V I I I ) . ( i v ) Slump s t r u c t u r e s occur along the lower c o n t i n e n t a l slope advancing onto the n o r t h e a s t slopes of the S c o t t Channel. Paul Revere Ridge Paul Revere Ridge i s a complex s t r u c t u r e comprising sediments, i n c l u d i n g i n d u r a t e d t u r b i d i t e s , and b a s a l t s (see P e t r o l o g y ) . The southwest slope i s a f a u l t s c a r p , though at the n o r t h e r n end the f a u l t zone extends over most o f the r i d g e (Plate II I - E F ) . I t i s d i f f i c u l t to t r a c e the v o l c a n i c basement i n a n o r t h e a s t d i r e c t i o n beneath the Winona Basin because of l a c k of p e n e t r a t i o n of sound waves and the very f a u l t e d nature of the r i d g e and adjacent p a r t of the Winona B a s i n . This severe f a u l t -i n g i s not seen i n P l a t e VII (KK'), presumably i n d i c a t i n g that f a u l t s die out southwards. A magnetic low occurs over the r i d g e . In the southern p a r t , the r i d g e has the form of a cuesta with sediments a p p a r e n t l y d i p p i n g very s l i g h t l y (^1°) n o r t h e a s t . Winona Ba s i n The southern p a r t of Winona Ba s i n i s adjacent to Paul Revere Ridge. The b a s i n sediments show severe f a u l t i n g near the northwest end of the r i d g e (Plate VII - J J ' ) . Only one p r o f i l e ( P l a t e XII - LL') t r a v e r s e s the n o r t h e r n p a r t of the b a s i n (other-wise known as the S c o t t Islands F a u l t Zone - Couch, 1969 and others) , 47 showing a s e r i e s o f f o l d e d and f a u l t e d s e d i m e n t s , more than 3 see s , t h i c k . Couch (1969) i n t e r p r e t e d the n e g a t i v e f r e e - a i r g r a v i t y anomaly o f more than 150 mgals over the S c o t t I s l a n d f a u l t zone as the e x p r e s s i o n o f a graben f i l l e d w i t h two 3 km t h i c k sediment s e c t i o n s o f average d e n s i t i e s 2.0 gm/cm3 and 2.4 gm/cm3. The f o l d i n g and f a u l t i n g o f the sediments i n d i c a t e some component o f compres s i o n a f f e c t i n g the a r e a (see D i s c u s s i o n ) . The a t t i t u d e o f these sediments n e a r the base o f the c o n t i n e n t a l s l o p e cannot be de t e r m i n e d from the r e c o r d which i s poor i n t h i s a r e a . No sharp c o n t a c t e x i s t s between the v o l c a n i c Dellwood K n o l l s and the Winona B a s i n sediments a l o n g t h i s l i n e and may i n d i c a t e t h a t t h i s c o n t a c t c o n s i s t s o f i n t e r b e d d e d sediments and v o l c a n i c s . (See T i f f i n and Cameron, 1972). The most r e c e n t sediments i n the a r e a are not a f f e c t e d by the f o l d i n g and f a u l t -i n g and are h o r i z o n t a l . - .. Revere - Dellwood- F a u l t Zone-The Revere - Dellwood F a u l t Zone t r e n d s a l o n g the s o u t h -west s l o p e s o f P a u l Revere Ridge and So u t h e a s t Dellwood K n o l l s , and extends i n t o the b a s i n between the Dellwood Seamount Range and the Del l w o o d K n o l l s . A l l p r o f i l e s c r o s s i n g t h i s b a s i n s o u t h -west o f , but i n c l u d i n g , W ( P l a t e X I I I ) show some degree o f f a u l t i n g which i s most i n t e n s e a l o n g p r o f i l e EF ( P l a t e I I I ) and d i e s out bo t h i n n o r t h w e s t and s o u t h e a s t d i r e c t i o n s . Thus, f o r example, i t i s absent from p r o f i l e BC ( P l a t e I I ) and on p r o f i l e KK' ( P l a t e V I I ) i t i s r e s t r i c t e d to the southwest s l o p e o f P a u l Revere Ridge. The st e e p n a t u r e o f the southwest s l o p e o f the S o u t h e a s t Dellwood K n o l l s i n p r o f i l e RS ( P l a t e X) i s presum-a b l y due to f a u l t i n g a l o n g t h i s s l o p e . The sediment i n the b a s i n w to 2.0 3.0 4.0 5.0 j5 N a u t i c a l M i l e s _J.0 K i l o m e t e r s I I V O L C A N I C S 4 I f 16 H- 6 21B H-3 18 9 19 C.S.P . LINE 16 H- 6 21B H- 3 18 9 19 DEPTH TO SEDIMENT (SEC. ) 3.38 3. 33 3.33 3. 33 3.33 3. 41 3.44 DEPTH TO BASEMENT (SEC. ) 3.63 3. 90 4.00 4. 33 3.70 3. 63 3.60 CO FIGURE 24 P r o f i l e ZZ" a l o n g b a s i n between the Dellwood Seamount Range and Dellwood . K n o l l s , c o n s t r u c t e d from d a t a o b t a i n e d from p r o f i l e s w h i c h c r o s s the b a s i n . 49 between Dellwood Seamount Range and Dellwood K n o l l s i s t h i c k e s t (^1 sec. LL' P l a t e X I I ) a t end o f c h a n n e l t r e n d i n g between the Dellwood K n o l l s where the basement i s a l s o deepest ( F i g u r e 24) and d e c r e a s e s i n t h i c k n e s s toward b o t h the n o r t h w e s t and s o u t h -e a s t ( p r o f i l e BC - P l a t e I I - shows t h i c k n e s s o f l e s s than 0.2 s e c ) . Such a sediment d i s t r i b u t i o n would r e s u l t from ponding i n the deepest p a r t o f the b a s i n by sediments ( b e l i e v e d to be m a i n l y t u r b i d i t e s and i c e r a f t e d d e b r i s ) t r a n s p o r t e d from the c o n t i n e n t a l s h e l f and s l o p e a c r o s s the S c o t t Channel a l o n g the c h a n n e l between the Dellwood K n o l l s and i n t o the b a s i n between the Dellwood Seamount Range and Dellwood K n o l l s . F u r t h e r t r a n s -p o r t i n a southwest d i r e c t i o n i s b l o c k e d i n t h i s a r e a by the Dellwood Seamount Range. Other F e a t u r e s P r o f i l e V V ( P l a t e X I I I ) shows the sediment between the S c o t t Seamount Range and the Dellwood Seamount Range i s about 0.5 s e c . t h i c k , w h i l e t o the southwest o f the S c o t t Seamount Range, the sediment i s about 0.64 s e e s , t h i c k , presumably s i n c e i t i s n e a r e r t o the S c o t t Channel. T h i s t h i c k sequence o f sediments i s b e l i e v e d t o comprise m a i n l y t u r b i d i t e s t r a n s p o r t e d to the a r e a v i a the S c o t t Channel. A l s o o f i n t e r e s t i n p r o f i l e V V i s the s t e p l i k e a s c e n t o f the o c e a n i c basement l a y e r i n a n o r t h e a s t d i r e c t i o n - a f e a t u r e which l e d Ewing e t a l (1968) t o p o s t u l a t e a s h o r t s p r e a d i n g segment i n the Dellwood Region (see D i s c u s s i o n ) . The a s c e n t o f the o c e a n i c basement l a y e r i s the r e v e r s e o f the normal d i p o f o c e a n i c basement toward the c o n t i n e n t . 5/H 50H &0R£D6£ EM 70-Ott-io 3»DR£DG£ EN70~OE5'3i, 7*DREDGE EN 70-025-lj, 8*DREPCE EN 70-025-% 9-CREDGE EN 70-025-% -51' IZ'DREDGE IOUBC 70-16-fy •' JOIOES omu SITE M*ITT. 40 KM. NAUTICAL kH.ES DREDGE STATIONS l - l CORE STATION • FIGURE 25 51 CHAPTER FIVE ANALYSIS OF DREDGE HALLS AND CORE Figure 25 shows the l o c a t i o n of s i x dredge hauls o b t a i n e d d u r i n g the summer of 1970 and one 500 metre hole d r i l l e d by JOIDES from the d r i l l ship GLOMAR CHALLENGER durin g the sum-mer of 1971. These recovered samples p r o v i d e the only data on the rock types of the Dellwood Seamount Area and although they are from so few recovery s i t e s , c e r t a i n g e n e r a l i s a t i o n s can be made as to d i s t r i b u t i o n of major rock types; t e x t u r a l and chem-i c a l d i f f e r e n c e s between b a s a l t types; t h i c k n e s s and d i s t r i b u t i o n of manganese c o a t i n g ; amount of weathering; and the r e l a t i o n s h i p between these o b s e r v a t i o n s and s e a f l o o r s p r e a d i n g . PETROGRAPHY OF BASALTS The absence of m i n e r a l o g i c a l v a r i a t i o n among samples from any one dredge h a u l , and the a n g u l a r i t y and rough s u r f a c e t e x t u r e of most of the b a s a l t samples suggest that they were not t r a n s p o r t e d f a r from t h e i r source areas and may be fragments from a t a l u s s l o p e . A catalogue d e s c r i p t i o n of the rocks recovered d u r i n g the summer c r u i s e EN 70-025 i s the s u b j e c t of a volume by Thomlinson, B e r t r a n d and Chase ( i n p r e s s ) . EN 70-025-2D A l l the b a s a l t i c fragments recovered from t h i s dredge ha u l taken from the Northwest Dellwood K n o l l s (Figure 26 and CSP l i n e IOUBC 70-16-20, P l a t e IV) are fragments of p i l l o w s and are d e s c r i b e d as g l o m e r o p o r p h y r i t i c o l i v i n e b a s a l t s (Figure 27B). C h a r a c t e r i s t i c p i l l o w f r a c t u r i n g with curved g l a s s y outer sur-face i s common (Figure 26A). . The b a s a l t fragments are a l l 52 B - Glotneroporphyritic olivine - plagioclase basalt. Specimen NO. 3T 70-025-2D-1. FIGUR3 26 Basaltic fra^jnents from the Northwest Dellwood I'nolls. Dredge Haul NO. Si 70-025-2D. S3 v e s i c u l a r w i t h v e s i c l e s g e n e r a l l y <1. mm i n d i a m e t e r . Both i n hand specimen ( F i g u r e 26B) and i n t h i n s e c t i o n ( F i g u r e 2 7 ) , the r o c k i s g l o m e r o p o r p h y r i t i c w i t h p h e n o c r y s t s c o m p r i s i n g m a i n l y c l u s t e r s o f i n t e r p e n e t r a t i n g p i a g i o c l a s e l a t h s (AnsH-ss) and/or o l i v i n e ( F 0 9 0 ) , the p e r c e n t a g e p l a g i o c l a s e to o l i v i n e i n the p h e n o c r y s t s v a r y i n g c o n s i d e r a b l y ( F i g u r e s 2 7A and B ) . S i n g l e p h e n o c r y s t s o f o l i v i n e o r p l a g i o c l a s e a l s o o c c u r . The o l i v i n e s are e u h e d r a l to a n h e d r a l i n shape, most b e i n g sub-h e d r a l . Some o f the a n h e d r a l o l i v i n e s show r e a c t i o n rims and a l t e r a t i o n a l o n g c r a c k s and may c o n t a i n opaque i n c l u s i o n s ( F i g u r e 27B). L o c a l l y zoned p l a g i o c l a s e s a l s o o c c u r ( F i g u r e 27C). The groundmass o f these r o c k s i s h y a l o p i l i t i c , c o n s i s t -i n g o f an abundance o f p l a g i o c l a s e m i c r o l i t e s (An ,f 5 _ 60 ) , o l i v i n e , c l i n o p y r o x e n e and opaqiies ( m a i n l y m a g n e t i t e ) l y i n g i n a m a t r i x o f b a s a l t i c g l a s s p a r t l y a l t e r e d and d e v i t r i f i e d . The c l i n o p y -roxene o c c u r s i n g r a i n s and a l s o i n fans and sheaves growing out from p l a g i o c l a s e m i c r o l i t e s ; t h i s l a t t e r h a b i t o f the c l i n o p y -roxene b e l i e v e d due t o d e v i t r i f i c a t i o n o f the b a s a l t i c g l a s s . The fans and sheaves encompass p r e v i o u s l y c r y s t a l l i s e d ore min-e r a l s . These p r e v i o u s l y c r y s t a l l i s e d ores a l s o o c c u r w i t h i n some o l i v i n e p h e n o c r y s t s . Some o f the groundmass p l a g i o c l a s e i s s a u s s u r i t i z e d y i e l d i n g e p i d o t e - g r o u p m i n e r a l s . C h e m i c a l l y , the r o c k s from t h i s dredge h a u l c o n t a i n a s m a l l amount of, n o r m a t i v e n e p h e l i n e 0,91, 20% o l i v i n e , and >17% A 1 2 0 3 and thus are c l a s s i f i e d as h i g h - a l u m i n a o l i v i n e b a s a l t . A complete c h e m i c a l a n a l y s i s i n Ta b l e II shows t h a t , w h i l e the ro c k c o n t a i n s a l i t t l e n o r m a t i v e n e p h e l i n e , i t i s l o w , i n a l k a l i s and i s thus i n t e r m e d i a t e between s a t u r a t e d o l i v i n e t h o l e i i t e and 54 Plagioclase (Bytownite An-8S) phenocryst in glomeroporphyritic basalt. The grounduass is hyalopilitic. Specimen NO.EH 70-025-2D-1. (X50). Olivine and zoned, plagioclase phenocrysts in glomeroporphyritic basalt. The groundmass is hyalopilitic. Specimen NO.EN 70-025-2D-8. (X50). 27 Photomicrographs of olivine - plagioclase gloaieroporphyritio basalt from the Northwest Dellwood T'nolls. 55 a l k a l i o l i v i n e b a s a l t . (See Ch e m i s t r y . ) The l a r g e d i s c r e p a n c y between p l a g i o c l a s e and o l i v i n e p h e n o c r y s t c o m p o s i t i o n and norma-t i v e p l a g i o c l a s e and o l i v i n e c o m p o s i t i o n may i n d i c a t e t h a t the p h e n o c r y s t s are i n f a c t x e n o c r y s t s . EN,70-025-3D T h i s dredge h a u l was taken from a f a u l t s c a r p a l o n g the S o u t h e a s t Dellwood K n o l l s ( F i g u r e 25 and CSP l i n e IOUBC 70-16-20, P l a t e I V ) . The b a s a l t fragments are a l t e r e d but the o r i g i n a l t e x t u r e , i s p r e s e r v e d enough to be c l a s s i f i e d i n t o two groups depending on the t e x t u r e o f the groundmass, namely a h y a l o p i l i t i c b a s a l t and an i n t e r s e r t a l b a s a l t ( F i g u r e s 28A and B ) . A l l the b a s a l t samples r e c o v e r e d are v e r y v e s i c u l a r ( F i g u r e 28D), v e s i c l e s r a n g i n g from about 0.2mm t o 5 mm i n d i a m e t e r and c o a t e d w i t h manganese o x i d e up to 50mm t h i c k on one specimen ( F i g u r e 28C). Some o f the fragments are p o r p h y r i t i c ( F i g u r e 28D). The h y a l o p i l i t i c b a s a l t i s p o r p h y r i t i c c o n t a i n i n g about 5% p l a g i o c l a s e p h e n o c r y s t s ( A n 9 2 ) i n a h y a l o p i l i t i c ground-mass o f s a u s s u r i t i z e d p l a g i o c l a s e m i c r o l i t e s (Anj+o-so)* o l i v i n e , c l i n o p y r o x e n e and m a g n e t i t e l y i n g i n a t a c h y l y t e m a t r i x . Occa-s i o n a l m i c r o p h e n o c r y s t s o f o l i v i n e a l s o o c c u r . The i n t e r s e r t a l o l i v i n e b a s a l t c o n t a i n s l e s s pheno-c r y s t s but much more g r a n u l a r c l i n o p y r o x e n e w i t h o n l y i n t e r s t i -t i a l t a c h y l y t e ( F i g u r e 28B). S a u s s u r i t i z e d p l a g i o c l a s e m i c r o l i t e s are common and the rock a l s o c o n t a i n s o l i v i n e as m i c r o p h e n o c r y s t s and i n the groundmass. The p o s i t i o n o f the .130 peak on an X-ray d i f f r a c t i o n p a t t e r n shows the o l i v i n e m i c r o p h e n o c r y s t s t o be about Fogg. M a g n e t i t e i s the major opaque m i n e r a l . The h i g h F e 2 0 3 c o n t e n t (>2.0V), and h i g h Fe 20 3/FeO 56 A - Anor l-hite phenocryst (Au-92) i n C - Thiol: ferro-anao, arxese crust (~50ffim) hyalopilitic: basalt. (X50) on weathered intersertal basalt. Specimen No. 3N70 - 025 - 3D - 1 Speciifeen No. SR 70 - 025 - 3D - 4. 3 - Phocondcrocraph o f intersertal basalt. D - P o r p h y r i t i c and / e s i c u l a r basa l t s Specimen " o . 3! 70 - 025 - 3D - 3. 'X50) as seen i n hand specimen. F10TJRS.28 Basaltr; from the Southeast DelP./ood Knolls. Bred- e Pau l "o.3T TO-025-3D. 57 TABLE I I Chemical and Normative c o m p o s i t i o n s o f some b a s a l t i c r o c k fragments from the Dellwood Seamount A r e a . SAMPLE NO. EN 70-025-2D-1 EN 70-025-3D-1 EN 70-025-8D-121 ROCK TYPE Hig h A l - 0 1 . - A l t e r e d O l i v i n e B a s a l t 1 V e s i c u l a r B a s a l t T h o l e i i t e OXIDE WEIGHT % WEIGHT % WE" IGHT S i 0 2 47 .34 50 .04 49 .70 Ti 0 2 1 .24 2 .37 1 .58 A I 2 O 3 17 .65 16 .29 15 .91 F e 2 0 3 1 .19 2 .70 1 . 78 FeO 7 .70 7 .09 7 .78 MgO 10 .45 5 .28 7 .14 MnO 0 .14 0 .15 0 .16 CaO 11 .39 11 .77 12 .50 Na 20 2 .64 3 .49 3 .11 K 2 0 0 .16 0 .52 0 .23 P2O5 0 .06 0 .19 0 .11 CO 2 0 .14- 0 .05 0 .15 H 20 0 .38 0 . 53 0 .39 H 20- 0 .24 0 .28 0 .20 TOTAL 100 .72 100 . 75 100 .74 MINERAL Quartz - - — - — N e p h e l i n e 0 .91 - -O r t h o c l a s e 0 .95 3 .07 1 .36 A l b i t e 20 .67 29 .53 26 .32 A n o r t h i t e 35 .84 27 .25 28 .77 Wo 8 .10 12 . 35 13 .18 D i o p s i d e En 5 .25 7 .55 7 .86 Fs 2 . 29 4 4 .64 En Hypersthene p s -1 1 .97 .07 0 0 .83 .49 O l i v i n e ^° 14 .56 2 .55 6 .37 Fa 7 .01 1 .53 4 .15 M a g n e t i t e 1 .73 3 .91 2 .58 I l m e n i t e 2 .36 4 . 50 3 .00 A p a t i t e 0 .14 0 .44 0 .25 C a l c i t e 0 .32 0 .11 0 .34 Water 0 .38 0 .53 0 . 39 TOTAL 100 .48 100 .47 100 .54 NOTE: The chemical compositions quoted above were determined by Japan A n a l y t i c a l C h e m i s t r y Research I n s t i t u t e . Glomeroporphyritic o l i v i n e - plagioclase basalt of F i g u r e s 27 A and B. 58 (>0.3 H e k i n i a n , 1971 and o t h e r s ) i n d i c a t e a l t e r a t i o n by c h e m i c a l w e a t h e r i n g o f the r o c k . The ro c k c o n t a i n s no n o r m a t i v e nephe-l i n e (Table I I ) and the r e l a t i v e l y h i g h c o n t e n t s o f Na 20 and K 2 0 as w e l l as the low MgO c o n t e n t may be the r e s u l t o f c h e m i c a l a l -t e r a t i o n . However, the T i 0 2 c o n t e n t i s b e l i e v e d t o remain con-s t a n t d u r i n g w e a t h e r i n g ( H e k i n i a n , 1971) and i t s h i g h c o n t e n t i n t h i s r o c k may i n d i c a t e t h a t the r o c k i s i n t e r m e d i a t e between t h o l e i i t e and a l k a l i b a s a l t . The A l 2 0 3 c o n t e n t , a l s o b e l i e v e d t o remain c o n s t a n t d u r i n g w e a t h e r i n g ( H e k i n i a n , 1971"), i s f a i r l y h i g h >16%. The d i s c r e p a n c y between the p l a g i o c l a s e p h e n o c r y s t and o l i v i n e m i c r o p h e n o c r y s t c o m p o s i t i o n s and t h e i r r e s p e c t i v e n o r m a t i v e c o m p o s i t i o n s i n d i c a t e s these p h e n o c r y s t s may be xeno-c r y s t s . D e l l wood Seamount Range B a s a l t i c fragments from Dredge Hauls EN 70-025-7D', EN 70-025-8D and EN 70-025-9D ( F i g u r e 25 and CSP l i n e lOuBC 70-16-15, P l a t e I) have s e v e r a l f e a t u r e s i n common. Most o f the fragments are p a r t s o f p i l l o w b a s a l t s ( F i g u r e 29) and show c h a r a c -t e r i s t i c g l a s s y c u r v e d s u r f a c e , r a d i a l f r a c t u r i n g and v e s i c u l a r i t y . A few fragments do not show these f e a t u r e s and are here termed b l o c k l a v a s ( F i g u r e 34). From t h i n s e c t i o n d e s c r i p t i o n s , t h r e e main t e x t u r a l t y pes can be d i s t i n g u i s h e d i n the p i l l o w fragments o f t h e . D e l l -wood Seamount.Range. They are ( i ) H y a l o p i l i t i c . O l i v i n e B a s a l t ( i i ) I n t e r s e r t a l O l i v i n e B a s a l t ( i i i ) S u b o p h i t i c O l i v i n e B a s a l t These t h r e e types grade i n t o one an o t h e r even w i t h i n 59 A - Characteristic fracturing i n pillo'.v lavas. C - Rounded top surface and radial fracturing i n pillow fragments. 3 - Pounded top surface of pillow fragments. D - ("lassy surface of pillow fra; tnents. FIGTRE 29 Characteristic features of basaltic pillow fragments froc the Dellwood Eean.ount Range. . 60 the same ro c k fragment ( F i g u r e 32) and are d i s t i n g u i s h e d m a i n l y on the c o n d i t i o n o f the t a c h y l y t e m e s o s t a s i s , the amount o f t a c h y l y t e and the h a b i t o f the c l i n o p y r o x e n e whether f e a t h e r y , f i b r o u s or g r a n u l a r . In hand specimen, the p i l l o w fragments are g e n e r a l l y , s i m i l a r showing s p a r s e p l a g i o c l a s e An 8o - 9o r a r e r o l i v i n e pheno-c r y s t s and weathered rims u s u a l l y c h i l l e d on the upper s u r f a c e . The p h e n o c r y s t s may o c c u r i n c l u s t e r s o r as s i n g l e c r y s t a l s . Most o f the fragments are s l i g h t l y m a g n etic. In t h i n s e c t i o n , s e v e r a l t e x t u r a l f e a t u r e s d i s t i n g u i s h the m i n e r a l o g i c a l l y s i m i l a r b a s a l t s . W i t h i n the group o f h y a l o -p i l i t i c b a s a l t s , the main d i s t i n g u i s h i n g f e a t u r e s are the h a b i t o f the c l i n o p y r o x e n e and the e x t e n t of d e v i t r i f i c a t i o n o f the t a c h y l y t e . The f e a t h e r y and f i b r o u s pyroxene grow out i n fans and sheaves from p l a g i o c l a s e m i c r o l i t e s and e n c l o s e . p r e v i o u s l y c r y s t a l l i s e d opaque m i n e r a l s ( m a i n l y m a g n e t i t e ) . T h i s h a b i t o f pyroxene i s b e l i e v e d due to d e v i t r i f i c a t i o n and d e u t e r i c a l t e r a -t i o n o f t a c h y l y t e ; an e x p l a n a t i o n a l s o o f f e r e d by M u i r and T i l l e y (1964) f o r s i m i l a r f e a t u r e s o f MAR1 b a s a l t s . F i g u r e s 30B and 3.1C show t h i s f i b r o u s h a b i t . In some r o c k s , the h y a l o p i l i t i c , b a s a l t : m a y have a narrow zone showing v a r i o l i t i c t e x t u r e ( F i g u r e 31B). M i c r o p h e n o c r y s t s o f p l a g i o c l a s e l a t h s (Anso -6o)» and o l i -v i n e ( F i g u r e 30B) which i s l o c a l l y s k e l e t a l o c c u r i n v a r y i n g amounts i n the h y a l o p i l i t i c groundmass c o m p r i s i n g p l a g i o c l a s e m i c r o l i t e s ( A n ^ - s s ) , f e a t h e r y t o f i b r o u s c l i n o p y r o x e n e , o l i v i n e , g r a n u l a r c l i n o p y r o x e n e and m a g n e t i t e , l y i n g i n a m a t r i x o f t a c h y -l y t e . Some of the p l a g i o c l a s e . m i c r o l i t e s are s a u s s u r i t i z e d and l U i d - A t l a n t i c Ridge 61 A - Bytownite (An-8 i) phenocryst i n C hyalopilitic basalt. Specimen No. EN 7 0 - 0 2 5 ~ JD - 1 3 . B - Hyalopilitic basalt with feathery D to fibrou:; groundmass clinopyroxene Specimen No. EN 70^ )25-7D-ll. - Lath-shaped plagioclase, skeletal olivine and occasional clinopyroxene microphenocrysts i n intersertal basalt. Specimen No.EN 70-025-7D-1A. - Olivine and plagioclase microphenocryst in intersertal basalt. Specimen No. EN 70-025-7D-6. FIGURE 30 Photomicrographs of basalts from the Dellwood Secu.ount Range. Dredge Raul No. SN 7 0 - 0 2 5 - 7D. (NOTIFICATION 5 ' 3 X ) 62 A - Bytownite (An-87)—Olivine nd. j r o - B - Su b v a r i o l i t i c Basalt. phenocryst i n h y a l o p i l i t i c basalt. Specimen No. 3J 70-025-9D-2. Specimen No. 3T 70 -025-9D-1. C - F y a l o p i l i t i c to i n t e r s e r t a l b a s a l t D - Plagioclase microphenocryst i n with f i b r o u s clinopyroxene. fine-drained i n t e r s e r t a l basalt. Specimen No. 3T 7 0 - 0 2 5 - 9 D - 2 2 . Specimen No. M 70-025-8D-4. (MAGNIFICATION 50X ) FIGURE j l Fhotomicro;graphs of basalts from the Dellwood Seamount Ran.e. Dredge P a u l Nos. BI 70-025-8D and BI 70-O25-9D 63 some o f the h y a l o p i l i t i c b a s a l t s are a l s o g l o m e r o p o r p h y r i t i c ( F i g u r e 31A). The i n t e r s e r t a l b a s a l t i s d i s t i n g u i s h e d from the h y a l o p i l i t i c b a s a l t m a i n l y i n the l e s s e r amount o f t a c h y l y t e , t h i s b e i n g r e s t r i c t e d t o i n t e r s t i c e s i n the groundmass of the i n t e r s e r t a l b a s a l t . M i n e r a l o g y i s s i m i l a r t o the h y a l o p i l i t i c b a s a l t but c l i n o p y r o x e n e a l s o o c c u r s as m i c r o p h e n o c r y s t s ( F i g u r e 30C). In the groundmass, the c l i n o p y r o x e n e i s m o s t l y g r a n u l a r ( F i g u r e s 31D and 30C), i n p l a c e s f i b r o u s ( F i g u r e 31C), but never f e a t h e r y . S k e l e t a l o l i v i n e ( F i g u r e 30D) i s much more common i n the i n t e r s e r t a l b a s a l t as m i c r o p h e n o c r y s t s . Some o f the i n t e r -s e r t a l b a s a l t s are v e r y f i n e - g r a i n e d ( F i g u r e 31D) and may c o n t a i n m i c r o p h e n o c r y s t s of p l a g i o c l a s e and o l i v i n e i n c l u s t e r s o r as s i n g l e c r y s t a l s . The s u b o p h i t i c s t a g e ( i n w h i c h t h e r e i s no g l a s s and i n w h i c h the pyroxene i s l a t h shaped, but s t i l l l a r g e enough t o b e g i n to e n c l o s e the p l a g i o c l a s e l a t h s ) was not o f t e n e n c o u n t e r e d i n the samples s t u d i e d under the m i c r o s c o p e . One example o f a s u b o p h i t i c t e x t u r e i n a p i l l o w b a s a l t fragment i s shown, i n F i g -u r e s 32E and F. A l l the photographs shown i n F i g u r e 32 were t a k e n of one t h i n s e c t i o n from sample EN 70-025-8D-6 and shows how d r a s t i c the t e x t u r e s can change w i t h i n 3 cms. o f the s u r f a c e o f a p i l l o w . Going from the d e v i t r i f i e d g l a s s y s u r f a c e t o the i n t e r i o r o f the p i l l o w , the t e x t u r e changes g r a d a t i o n a l l y from g l a s s y w i t h a few m i c r o l i t e s and s p a r s e p h e n o c r y s t s t h r o u g h h y a l o -p i l o t i c and i n t e r s e r t a l t o s u b o p h i t i c . P h e n o c r y s t s and v e s i c l e o c c u r .throughout. The p h e n o c r y s t s are m a i n l y o l i v i n e and p l a g i o -c l a s e i n c l u s t e r s o r as s i n g l e c r y s t a l s . 6 4 A - L e v i t r i f i e d g l a s s y s u r f a c e , ( p l a n e l i g h t X 5 0 ) 3 - O l i v i n e and p l a g i o c l a s e p h e n o c r y s t s i n d e v i t r i f i e d g l a s s y groundmass. ( X - N i c o l s x50) C - O l i v i n e and p l a g i o c l a s e p h e n o c r y s t s i n h y a l o p i l i t i c grourxlmass. ( p l a n e l i g h t X50) D - I n t e r s e r t a l t e x t u r e , ( p l a n e l i g h t X 5 0 ) E - P l a g i o c l a s e p h e n o c r y s t s i n sub- F - P l a g i o c l a s e p h e n o c r y s t s i n s u b o p h i t i c o p h i t i c groundmass. ( p l a n e l i g h t X50) grourylmass. ( X - N i c o l e x50) FIGURE 32 P h o t o m i c r o g r a p h s s h o v i n g changes i n t e x t u r e w i t h d e p t h i n t o p i l l o w f r a g m e n t . Specimen NO. EfT 70-025-8D-6. . H y a l o p i l i t i c + f e a t h e r y cpx. ± g r a n u l a r cpx. H y a l o p i l i t i c + f i b r o u s t o a c c i c u l a r cpx. G l a s s y - * — H y a l o p i l i t i c w i t h no cpx. . H y a l o p i l i t i c + g r a n u l a r cpx. + I-l u rH a rH (J •rH c o crj o r* CS —'-Or— S u b o p h i t i c I h t e r s e r t a l ^ _ _ i J n t e r g r a n u l a r _ P i l l o w m a r g i n -Decrease i n r a t e of c o o l i n g - - P i l l o w c o r e R=20cm. Time (T) r e q u i r e d f o r c o n d u c t i o n c o o l i n g t h r o u g h the i n t e r v a l 1200°C t o 900°C a t a depth Z beneath the s u r f a c e o f a b a s a l t sphere ( i d e a l i z e d p i l l o w ) " o f r a d i u s R. Three cases are shown f o r p i l l o w s o f r a d i u s 5, 10,, 20 cm. Constant t h e r m a l d i f f u s i v i t y o f 0.007 and a s u r f a c e temperature of 0°C a r e assumed, ( a f t e r M a r s h a l l and Cox, 1971). 10 15 Z (cm.) 20 FIGURE 3 3 Observed groundmass t e x t u r e s i n b a s a l t s from the Dellwood Seamount A r e a and t h e i r r e l a t i o n s h i p t o c o o l i n g r a t e and p i l l o w s i z e . 66 I t i s b e l i e v e d t h a t the t e x t u r e s e n c o u n t e r e d i n these p i l l o w fragments depend on the p o r t i o n o f the p i l l o w t h a t was examined under t h i n s e c t i o n , the r a t e o f c o o l i n g o f the p i l l o w as a whole and the r a t e o f c o o l i n g o f t h a t p o r t i o n o f the p i l l o w w h i c h was s e c t i o n e d . T h i s r a t e o f c o o l i n g depends on p i l l o w s i z e , ( M a r s h a l l and Cox, 1971) and F i g u r e 33 shows the r e l a t i o n s h i p between r a t e o f c o o l i n g , s i z e o f p i l l o w and b a s a l t t e x t u r e . Some fragments r e c o v e r e d from the Dellwood Seamount Range showed no f e a t u r e s c h a r a c t e r i s t i c o f p i l l o w l a v a s ( F i g u r e 3 4 ) . In hand specimen, they are n o t i c e a b l y - c o a r s e r g r a i n e d t han p i l l o w f r a g m e n t s , and a l l have a weathered r i m . ,In t h i n s e c t i o n , the r o c k s are almost h o l o c r y s t a . i l i n e (except i n sample IOUBC 70-16-12D-1 d e s c r i b e d below) w i t h e i t h e r a d i a b a s i c ( i n t e r -s t i t i a l g r a n u l a r c l i n o p y r o x e n e ) as i n F i g u r e 34B or s u b o p h i t i c ( w i t h l a t h - s h a p e d c l i n o p y r o x e n e b e g i n n i n g t o e n c l o s e the p l a g i o -c l a s e [ A n 5 0 _ 6 0 ] l a t h s ) t e x t u r e as i n F i g u r e s 34D and E. The r o c k s may be p o r p h y r i t i c w i t h p h e n o c r y s t s o f o l i v i n e F o 9 0 ( F i g -ure 34B) o r p l a g i o c l a s e A n 8 0 . O l i v i n e a l s o o c c u r s i n the ground-mass and as m i c r o p h e n o c r y s t s . The r o c k s are magnetic and ma g n e t i t e i s the common opaque m i n e r a l . Most fragments are a l s o f i n e l y , v e s i c u l a r . These b l o c k l a v a s show the.same m i n e r a l o g y and pheno-c r y s t s as the p i l l o w l a v a s and may be the lo w e r p a r t s or more s l o w l y c r y s t a l l i s e d p a r t s o f the - same f l o w s t h a t produced the p i l l o w l a v a s d e s c r i b e d above. The v e r y c a l c i c n a t u r e o f t h e , p l a g i o c l a s e p h e n o c r y s t s ( A n s o - 9 0 ) a n d m a g n e s i a - r i c h n a t u r e o f the o l i v i n e p h e n o c r y s t s and m i c r o p h e n o c r y s t s ( F o g o ) when compared w i t h the A n o r t h i t e con-67 C M S A - Diabasic b a s a l t . Specimen NO. EN 70-025-8D-85. 3 - Photomicrograph showing o l i v i n e pl.eno-c r y s t i - i n d i a b a s i c b a s a l t . Specimen NO. EN 70-025-8D-85. (] 50 INCHES 4 5 C M : C - Plagioclase phenocrysts i n su b o p h i t i c b a s a l t . Specimen NO. EN 70-025-7D-69. D - Photomicrograph of subophitic - Photomicrograph of subop h i t i c basalt. basalt. Specimen No.iiJN 70-025-7D-69. Specimen NO.EN 70-O25-7D-69. (plane light }50 ) (X-Nicols X50) FIGURE 34 Holocrystalline black lavas from the Dellwood Seamount Range. 68 t e n t o f the groundmass and m i c r o p h e n o c r y s t p l a g i o c l a s e ( A n i + s . g n ) and the n o r m a t i v e p l a g i o c l a s e ( A n s s ) and o l i v i n e (Fogn) composi-t i o n (Table I I ) i n d i c a t e s t h a t the p l a g i o c l a s e p h e n o c r y s t s and o l i v i n e p h e n o c r y s t s and m i c r o p h e n o c r y s t s o f the p i l l o w b a s a l t s and b l o c k l a v a s o f the D e l lwood Seamount a r e a may i n f a c t be x e n o c r y s t s . Only one b a s a l t fragment l a r g e enough t o be t h i n s e c -t i o n e d and o t h e r w i s e a n a l y s e d was o b t a i n e d from dredge h a u l IOUBC 70-16-12D. In hand specimen, t h i s r o c k i s a p o r p h y r i t i c p l a g i o c l a s e - o l i v i n e b a s a l t ( F i g u r e 35A) w i t h l a r g e zoned p l a -g i o c l a s e ( A n 8 o - 8 5 ) p h e n o c r y s t s (< 5 cm., i n diameter) and l e s s abundant and s m a l l e r o l i v i n e p h e n o c r y s t s (Fosa) , many showing e v i d e n c e o f p a r t i a l r e s o r p t i o n ( F i g u r e 35B) i n a weathered f i n e l y v e s i c u l a r glassy.groundmass. On the s u r f a c e o f the f r a g -ment, the p l a g i o c l a s e p h e n o c r y s t s weather out making the r o c k l o o k l i k e a b r e c c i a . T h i n s e c t i o n shows the l a r g e r p l a g i o c l a s e s c o n t a i n i n g opaque i n c l u s i o n s (presumably m a g n e t i t e and brown b a s a l t i c g l a s s ) o u t l i n i n g t w i n n i n g and growth zones ( s u b p a r a l l e l to the c r y s t a l f a c e s ) as shown i n F i g u r e s 35B and C; s m a l l e r , o l i v i n e p h e n o c r y s t s up t o 1 cm. i n l a r g e s t d i m e n s i o n ; and p l a g i o -c l a s e and o l i v i n e m i c r o p h e n o c r y s t s i n a f i n e l y v e s i c u l a r ground-mass o f p l a g i o c l a s e and perhaps o t h e r m i n e r a l s i n a m e s o s t a s i s o f a l t e r e d b a s a l t i c g l a s s ( F i g u r e 35D). The l a r g e r p l a g i o c l a s e s are e u h e d r a l t o . s u b h e d r a l , the o l i v i n e s are s u b h e d r a l t o a n h e d r a l and most show p a r t i a l r e s o r p t i o n ( F i g u r e 35B). Some o f the p l a g i o c l a s e m i c r o p h e n o c r y s t s are c o m p o s i t i o n a l l y zoned. A l l the l a r g e r p h e n o c r y s t s are i n t e n s e l y f r a c t u r e d and v e r y f r e s h as compared to the h i g h l y weathered m a t r i x w h e r e - a l l the v e s i c l e s B - B y t o w n i t e phenocryst with opaques D - Weathered v e s i c u l a r , l a s s y o u t l i n i n g g r o w t h zones "md a p a r t i a l l y groundmass o f p o r p h y r i t i c r e s o r b e d olivine (Fo-83) p h e n o c r y s t i n p l a g i o c l a s e b a s a l t . ( X 5 0 ) p o r p h y r i t i c p l a g i o c l a s e b a s a l t . (X50) FIGURE 35 P o r p h y r i t i c p l a g i o c l a s e basalt 'IOUBC 70-16-12D-1) f r o m t h e D e l l w o o d Seamount Range. 70 are l i n e d w i t h f i b r o p a l a g o n i t e ( F i g u r e 35D). The opaque i n c l u s i o n s i n the l a r g e p l a g i o c l a s e s are c o n c e n t r a t e d i n random areas o f the p l a g i o c l a s e , o u t l i n i n g e i t h e r t w i n n i n g o r growth zones p a r a l l e l t o the c r y s t a l o u t l i n e . C o m p o s i t i o n a l z o n i n g i s g e n e r a l l y r a r e . Most o f the opaques are e u h e d r a l to s u b h e d r a l i n shape, due t o t h e i r p o s i t i o n a l o n g t w i n p l a n e s . S e v e r a l p l a g i o c l a s e p h e n o c r y s t s from the o t h e r dredge h a u l s o f the Dellwood Seamount Range a l s o c o n t a i n opaque i n c l u s i o n s o u t l i n i n g growth zones and t w i n n i n g . T h i s v e r y p o r p h y r i t i c p l a g i o c l a s e i s b e l i e v e d t o r e p r e -s e n t i n i t s p h e n o c r y s t s a semi-cumulate m a i n l y o f p l a g i o c l a s e and l e s s e r o l i v i n e which was b e i n g formed near the bottom o f a magma chamber, but a change i n c o n d i t i o n s caused these l a r g e p h e n o c r y s t s t o be e x t r u d e d w i t h the magma which r a p i d l y c o o l e d to g i v e the g l a s s y (hut l a t e r a l t e r e d ) groundmass. CHEMISTRY OF BASALTS Chemical c o m p o s i t i o n s o f 14 b a s a l t specimens from the Dellwood Seamount Area are shown on Table I I I . These can be compared w i t h s e l e c t e d a n a l y s e s from known o c e a n i c r i d g e s , i s l a n d s and seamounts shown on Table IV. A v i s u a l comparison o f the t a b l e s shows the Dellwood Seamount Area b a s a l t s t o be t r a n s i t i o n -a l between t h o l e i i t e s and a l k a l i b a s a l t s h a v i n g more a l k a l i s , b o t h Na 20 and K 2 0 and l e s s MgO, than o c e a n i c t h o l e i i t e s from s p r e a d i n g c e n t r e s i n the A t l a n t i c , I n d i a n and P a c i f i c oceans but g e n e r a l l y . l e s s a l k a l i s e s p e c i a l l y K 2 0 , than the a l k a l i b a s a l t s from seamounts and o c e a n i c i s l a n d s . 71 TAB LI: III CllliMICAI. COMPOSITION' OF BASALTS FROM Till: DLLLWOOI) SEAMOUNT ARF.A SAMPLE NUMBER AND ROCK TYPE LOCATION CHEMICAL COMPOS ITION IN WT. \ S i 0 2 T i 0 2 A l 2 0 3 F e 2 0 3 FeO MnO KgO CaO Na 20 K 20 l l 2 0 TOTAL ! EN 70-02S-2D-1 . Gloneropo r p h y r i t i c "Ol-Plag. B a s a l t Northwest Dellwood K n o l l s 47.34 1.24 17.65 1.19 7.70 0.14 10 .45 11.39 2 .64 0.16 0.62 100.52 i 1 EN-70-025-3D-1 H y a l o p i l i t i c Southeast Dellwood K n o l l s 50.04 2.37 16.29 2.70 7.09 0.15 5.28 11.77 3.49 0.52 0.81 100.51 EN 70-02S-8D-121 I n t e r s e r t a l -D i a b a s i c B a s a l t Dellwood Seamount Range 49.70 1.58 15.91 1.78 A 7.78 ... C .16 7.14 12.50 3.11 0.23 0.59 100.48 EN 70-025-2D-8 G l o r . e r o p o r p h y r i t i c O l - P l a g . B a s a l t Northwest Dellwood K n o l l s 46.1 ±1.0 1 .24 ±0.08 16.3 ±0.2. Fe as Fej.Oj 9.2±0.3 0.12 + .01 8.76 ± .13 11.0 ± .1 3.2 + .1 0.19 ± .02 0.8 ± .01 96.9 EN 70-025-3D-2 I n t e r s e r t a l B a s a l t (weathered) Southeast Dellwood K n o l l s 47.1 ±1.0 2.23 ± .08 14.3 i .2 10.2±0.2 0.11 ± .01 5.54 + .04 10.6 ±•2. 3.4 + .1 0.5 ± .1 1.6 ±0.3 9S.6 EN 70-025-7D-25 I n t e r s e r t a l B a s a l t Dellwood Seamount Range 47.9 ±1.8 1.80 ± .07 14.7 ±0-3 10.3+0.2 0.13 ± .01 6.88 ± .07 10.7 t O . l 3.2 ± .2 0.33 + .05 0.5 ±0.1 96.4 EN 70-02S*7-D~38 H y a l o p i l i t i c B a s a l t Dellwood Seamount Range 46.6 ±1.0 1.38 ±.08 15.7 ±0.3 9.1+0.1 0.12 ± .01 6.70 ± .05 11.4 + .3 3.1 ± .1 0.16 ± .03 0.9 + 0.2 • 95.2 EN 70-02S-8D-4 I n t e r s e r t a l B a s a l t Dellwood Seamount Range 48.0 ±1.0 1.40 ±.07 14.6 ±.4 9 . 3 ±0 .1 0.13 ±.01 6 .94 ±.09 L1.7 ±.2 3.2 ±.1 0.19 ±.02 0.7 ±0.1 '96.1 EN 70-02S-8D-6 S u p o p h i t i c B a s a l t Dellwood Seamount Range 48.9 ±1.0 1.35 ± .04 15.4 ±.2 9.6±0.1 0.12 ±.01 6 .75 ±.05 L1.6 ±.1 3.3 ± .1 0.17 ±.02 1.0 ±0.2' 98.2 EN 70-025-8D-25 D i a b a s i c B a s a l t Dellwood Seamount Range 50.2 ±1.3 1.50 ± .07 13.7 ±•4 8.9±0.1 0.13 ±.01 6.40 + .03 11.9 ± .1 3.3 ±.1 0.22 ±.03 0.6 ±0.1 98.8 EN 70-025-9D-1 H y a l o p i l i t i c B a s a l t Dellwood Seamount Range 46.9 ±2.2 1.5 + .1 14.7 ±.2 9.8+0.2 0.13 + .01 6.94 + .09 11.5 + .2' 4.5 ± .1 0.16 + .03 1.3 + 0.2 97.4 EN 70-02S-9D-13 H y a l o p i l i t i c B a s a l t Dellwood Seamount Range 46.4 ±.5 1.48 ±.08 14.6 ±.2 9.6+0.3 0.13 ±.01 6.40 + .06 11.9 ±.1 3.3 ± .1 0.18 ±.03 1.4 ±0.2 95.4 EN 70-025-9D-33 H y a l o p i l i t i c Dellwood Seamount Range 47.9 ±3.0 43.5 ±3.0 1.53 ±.03 0.4 ±.1 14.3 ±.1 9.7±0.2 0.12 ±.01 6.2 ± .1 11.5 + .1 3.2 ± .1 0.23 + .02 1.9 ±0.3 96.6 IOUBC 70-16-12D-1 P o r p h y r i t i c Plag - 6 l . B a s a l t Dellwood Seamount Range i25.5 3.4±0.3 0.04 ±.01 4.3 ±.1 15.3 ±.5 2.2 ± .2 0.11 ±.02 2.0 + 0.3 96.8 * Analyses done by Japan A n a l y t i c a l Research Chemistry I n s t i t u t e (See Table I I ) . A l l e r r o r s shown are maximum d e v i a t i o n s from the nean and are not r e l a t e d to accuracy of the estimates. 72 T A n U IV CIILMICAL COMPOSITIONS OF HAS ALT IC HOCKS OF KNOWN SKAMOUNTS, OCLANIC ISLANDS S OCEANIC RIDGES SAMPLE NO. AND BASALT TYPE LOCATION CHEMICAL COMPOSITION KT. t SiOj T i 0 2 AljOj [Fe 2 0j ! FcO M/iO MgO CaO NajO ! K,0 i I 11,0 ! P,0< 1. A l k a l i Basalt Bowio Seamount 48.0 2.43 18.S l . S 7.5 0.1S S.O B.S 4.0 1 1.6 0.6 0.07 2. A l k a l i Basal t Bowie Seamount 45.2 J . l 16.4 0.6 . 10.3 0.18 6.9 8.1 3.8 2.4 0.6 0.71 3. A l k a l i Basal t Bowie Seamount 4S.33 J.S3 1S.0 1.81 10.64 0.23 6.77 8.S 4.3 2.36 0.21 0.73 4. A l k a l i O l i v i n e Basal t Po lu lu Se r i e s , Kohala, Hawaii •47.98 J.S3 IS.32 2.49 8.86 0.12 6.16 10.28 3.56 1.08 0.87 0.96 0.22 0.27 5. T h o l e i i t e (Avg. of 10 basal ts ) Koolau Se r i e s , Oahu, Hawaii I S0.4S ! 2.33 j 14 .94 3.38 7.SS 0.08 7.67 9.17 2.84 0.3S 6. T h o l e i i t e (Avg. of 24 basal ts ) Mauna Loa, Hawaii SO.42 2.97 11.62 2.71 9.07 0.10 10.11 9.74 2.09 0.39 0.27 7. T h o l e i i t e (Avg. of 4 analyses) Carlsberg Ridge, Indian Ocean SI.9 1.8 1S.S 3.6 6.4 7.3 9.S 3.9 0.1 8. High A l - O l T h o l e i -i t e (Avg. 2 anal . ) Mid- • A t l a n t i c Ridge 48.2 0.7 17.3 1.2 8.6 10.2 11.3 2.4 0.1 9. O l i v i n e T h o l e i i t e (Avg. of 3 anal . ) Mid-A t l a n t i c Ridge SO.6 1.1 16.2 1.6 1 .. -1 7.8 8.7 11.3 2.6 0.1 10. T h o l e i i t e (Avg. of 7 analyses) M i d - A t l a n t i c Ridge near 30°N. 48.61 1.S4 IS.31 A l l Fe as Fe,0« 10.61 7.68 10.87 2.63 0.20 11. T h o l e i i t e (Avg. of 7 analyses) Juan de Fuca Ridge 49.82 49.62 2.12 1.S2 13.77 IS.78 12.66 6.49 7.29 7.00 10.90 11.78 2.73 2.42 2.S7 2.81 0.12 0.1S 12. T h o l e i i t e (Avg. of 7 analyses) Gorda Ridge . 9.90 13. T h o l e i i t e (Avg. of 3 analyses) East P a c i f i c Rise 49.17 1.S1 16.23 9.3S 11.83 11.SO 0.12 0.09 0.40 . 14. T h o l e i i t e Explorer Seamount 48.10 1.2S 18.30 9.SO 8.30 Rc ferences 1. I l e r i c r , 1970 2. l i e n o r , 1970 3. Engel and l ingel , 1963 4. Macdonald, 1949 5. h'vntvorth and Winche l l , 1947; Turner and Vcrhoogen, 1960 6. Macdonald, 19-19; Turner and Vcrhoogen, 1960 7. N i c h o l l s c t a l , 1964 8. N i c h o l l s c f 1964 9. N i c h o l l s c_t aT, 1964 • 10. Average obtained from analyses quoted by Kay e_t al , 1970 )1 . Average obtained from analyses quoted by Kay o_t aT, 1970 12. Average obtained from analyses quoted by Kay ct^  aT, 1970 13. Average obtained from analyses quoted by Kay ct aT, 1970 14. Kay c t a l ^ 1970 " 73 S i l i c a and a l u m i n a cannot be d i r e c t l y compared because the atomic a b s o r p t i o n a n a l y s e s o f these two o x i d e s are c o n s i s t - , e n t l y l o w e r t h a n t h e i r quoted v a l u e s f o r s t a n d a r d s a n a l y s e d by t h e , a u t h o r . A comparison between samples a n a l y s e d by c l a s s i c a l wet c h e m i c a l methods by Japan A n a l y t i c a l Research C h e m i s t r y I n s t i t u t e ( t h e . f i r s t 3 a n a l y s e s o f T a b l e III) and those a n a l y s e d by,the a u t h o r show s t r i k i n g d i f f e r e n c e i n the wt. % A 1 2 0 3 . For example, sample EN 70-025-2D-1 c o n t a i n i n g .17,65% A 1 2 0 3 gave o n l y 16.3% by atomic a b s o r p t i o n ( i . e . an e r r o r o f about 7.5%), w h i l e sample EN 70-025-3D-1 c o n t a i n i n g 16.29% A 1 2 0 3 gave a v a l u e o f o n l y 14.6% (an e r r o r o f about 1 0 % ) . A USGS s t a n d a r d b a s a l t c o n t a i n i n g 13.65% A 1 2 0 3 gave o n l y 12.85% al u m i n a ( i . e . an e r r o r o f about 6%) when a n a l y s e d by the a u t h o r . Thus, i t seems from these comparisons t h a t the atomic a b s o r p t i o n a n a l y s e s o f a l u m i n a a r e low by about 6-10%. The s i l i c a a n a l y s e s s i m i l a r l y p r o v e d to be c o n s i s t e n t l y low, but o n l y by 1-2 wt. % (an e r r o r of 2-4%). These low S i 0 2 and A 1 2 0 3 v a l u e s p r o b a b l y account f o r the low t o t a l s shown i n the l a s t column o f Table I I I . Because of the i n a c c u r a c y o f S i 0 2 and A 1 2 0 3 a n a l y s e s and no F e 20 3/FeO r a t i o s , , n o r m a t i v e c o m p o s i t i o n s were not c a l c u l a t e d f o r the r o c k s a n a l y s e d by the a u t h o r . The a n a l y s e s o f a l l the o t h e r elements are b e l i e v e d to be a c c u r a t e w i t h i n the quoted e r r o r s , and can be used i n i n t e r p r e t i n g the r e s u l t s . The e r r a t i c n a t u r e o f the S i 0 2 and A l 2 0 3 a n a l y s e s shows i n the s i l i c a v a r i a t i o n diagrams of F i g u r e 36. S t i l l , the f o l l o w - ; i n g p e r t i n e n t o b s e r v a t i o n s can be made: ( i ) Sample EN 70-025-2D-1 w i t h 47.34% S i 0 2 c o n t a i n s more magnesium than i r o n . T h i s i s to be e x p e c t e d , s i n c e t h i s 74 25 20 FIGURE 3 6 S i l i c a V a r i a t i o n Diagrams f o r b a s a l t s o f the De l l w o o d Seamount A r e a . + A1 20 3 v CaO A T o t a l I r o n as F e 2 0 5 a MgO - T o t a l A l k a l i s = Na 20 + K 20 o T i 0 2 15 A 1 2 0 3 to w Q i—i X o 12 CaO 10 T o t a l I r o n as Fe203 MgO Na 20 + K 20 8^ TiO, 40 I WT % .STT.TfA 4 5 -•Boundary between ..•^"H'awaiian T h o l e i i t e s and , A l k a l i B a s a l t s on t o t a l A l k a l i s vs S i l i c a p l o t . 75 specimen i s a g l o m e r o p o r p h y r i t i c p l a g i o c l a s e - o l i v i n e b a s a l t w i t h o l i v i n e p h e n o c r y s t s h a v i n g a h i g h f o s t e r i t e c o n t e n t ( F o 9 0 ) . ( i i ) The semi - cumulate IOUBC 70-16-12.D-1 ( F i g u r e 35A) does not form p a r t o f any d i f f e r e n t i a t i o n t r e n d shown by the v a r i a t i o n diagram. T h i s rock i s u l t r a b a s i c (43.5% S i 0 2 ) h i g h i n a l u m i n a , and l i m e and v e r y low i n MgO and FeO w i t h MgO >FeO. Such a c o m p o s i t i o n i s s u p p o r t e d by the m i n e r a l o g y as t h i s r o c k c o n t a i n s m a i n l y p l a g i o c l a s e ( A n ^ ) w i t h l e s s e r o l i v i n e F o 8 3 . ( i i i ) P l o t o f t o t a l a l k a l i vs s i l i c a shows a wide s c a t t e r and the Hawa i i a n Boundary l i n e BB' d i v i d e s the b a s a l t s i n t o t h o l e i i t i c (below the l i n e ) and a l k a l i t y p e s . ( i v ) The v e r y wide s c a t t e r i n the a l u m i n a v s . s i l i c a p l o t i s m a i n l y due t o the too low e s t i m a t e s o f A I 2 O 3 by the atomic a b s o r p t i o n method. The t r a n s i t i o n a l t o a l k a l i n e n a t u r e o f the b a s a l t s o f the Dellwood Seamount Are a i s shown more c o n v i n c i n g l y on F i g u r e , 37, which i s an AFM diagram where the p e r c e n t a g e A 1 2 0 3 and S i 0 2 are n ot c o n s i d e r e d . Here the p l o t o f b a s a l t s ( e x c ept f o r the unu s u a l p l a g i o c l a s e semi-cumulate IOUBC 70-16-12D-1) f o l l o w more c l o s e l y the a l k a l i b a s a l t d i f f e r e n t i a t i o n t r e n d . (The d i f f e r e n -t i a t i o n t r e n d s shown i n F i g u r e 40 were ta k e n from Macdonald and K a t s u r a , 1964, F i g u r e 5.) Kuno (1968) p o i n t e d out t h a t w h i l e the s i l i c a v a r i a -t i o n diagram has the d i s a d v a n t a g e o f e m p h a s i z i n g the l a t e s t a g e of f r a c t i o n a t i o n , the SI ( S o l i d i f i c a t i o n Index) v a r i a t i o n diagram can be used to show e a r l y and midd l e s t a g e s o f f r a c t i o n a t i o n as w e l l , s i n c e SI i s the p e r c e n t a g e MgO i n the AFM diagram, and here i n any f r a c t i o n a t i o n t r e n d p l o t t e d , c o m p o s i t i o n o f the magma 76 Dellwood Seamount Area ° B a s a l t s . o IOUBC 70-16-12D-1 77 FIGURE 38 SI V a r i a t i o n Diagrans f o r b a s a l t s of the D i U u o o d Searaouiit Area. CaO \ . _ _ I o t a l _ Iron as-FeO 1 MgO 5L Tin, JS 2S NaaO —•—" ' 0.5 0.4 0.3 0.2 0.1 K.a • --45 S 1 0 j _ _ ; ' • A l j O . IS 10 I 30 4 S 40 35 Mgo-100 /(Mgo • FeO ( t o t a l ) • Na,0 • K aO) - S o l i d i f i c a t i o n Index ( S I ) . 25 78 i n v a r i a b l y changes so as t o decrease the SI v a l u e a s ' f r a c t i o n a t i o n proceeds., F i g u r e 38 shows SI v a r i a t i o n diagrams f o r the b a s a l t s o f the Dellwood Seamount A r e a . S e v e r a l o b s e r v a t i o n s can be made from these 1 diagrams. ( i ) There i s much l e s s s c a t t e r than on the s i l i c a v a r i a t i o n diagrams and t r e n d s can.be drawn. ( i i ) The p l a g i o c l a s e cumulate a g a i n does not l i e on any o f the d i f f e r e n t i a t i o n t r e n d s shown. ( i i i ) Of p r i m a r y importance i s the o b s e r v a t i o n t h a t r o c k s o f the Northwest Dellwood K n o l l s are the l e a s t d i f f e r e n t i a t e d , w h ich i s t o be e x p e c t e d i f the k n o l l s are p r o x i m a l to a s p r e a d i n g zone. These have SI v a l u e s o f 44.9 and 39.2. Rocks o f the D e l l -wood Seamount Range have SI v a l u e s between 30 and 35 and o l d r o c k s , from a f a u l t s c a r p on the S o u t h e a s t Dellwood K n o l l s have SI v a l u e s around 26. T h i s low SI v a l u e f o r the r e l a t i v e l y . o l d r o c k s o f the Southeast Dellwood K n o l l s may, r e f l e c t age and degree o f weather-i n g r a t h e r than amount o f d i f f e r e n t i a t i o n . ( i v ) In o n l y two specimens shown i n F i g u r e 41 - those w i t h SI v a l u e s o f 31.5 (EN 70-025-7D-25) and 26.6 (EN„70 - 025 - 3D-2),. FeO >CaO presumably r e p r e s e n t i n g a more i r o n - r i c h groundmass c l i n o p y r o x e n e phase i n these r o c k s r e l a t i v e to the o t h e r s . (v) The d i f f e r e n t i a t i o n t r e n d s shown by these SI v a r i - ; a t i o n diagrams are r e a s o n a b l e and show no s u r p r i s e s and are g e n e r a l l y what would be e x p e c t e d from a f r a c t i o n a t i o n p r o c e s s . The g e n e r a l s l o p e s o f the t r e n d s o f the o x i d e s ( i . e . whether 79 p o s i t i v e o r n e g a t i v e ) agree w i t h those shown by Kuno (1968) . HYALOCLASTITE BRECCIAS H y a l o c l a s t i t e b r e c c i a s ( S i l v e s t r i , 1963) o r p a l a g o n i t e t u f f s (Nayudu, 1964) were r e c o v e r e d from two dredge h a u l s (one' fragment from EN 70-025-2D and s e v e r a l from EN 70-025-9D). The fragments from the l a t t e r h a u l were a l l s i m i l a r to each o t h e r , but d i f f e r e n t i n hand-specimen and X-ray a n a l y s i s from the one specimen from EN 70-025-2D. The h y a l o c l a s t i t e fragment r e c o v e r e d from dredge h a u l EN 70-025-2D can b e s t be d e s c r i b e d as a p a l a g o n i t e t u f f b r e c c i a ( F i g u r e 39A) c o n t a i n i n g b a s a l t i c g l a s s f r a g m e n t s ; f r a c t u r e d and s h a t t e r e d fragments o f g l a s s y p i l l o w s sometimes rounded by p a l a g o n i t i z a t i o n o f the g l a s s y s k i n , . b u t more o f t e n a n g u l a r ; and o c c a s i o n a l p l a g i o c l a s e g r a i n s i n a green to dark brown m a t r i x . The whole, f r a g m e n t , i s c o v e r e d w i t h a t h i n r i n d o f manganese -i r o n o x i d e s ( w h i c h p e n e t r a t e s i n t o the fragment i n some p a r t s . The i n d i v i d u a l g l a s s y b a s a l t i c fragments v a r y c o n s i d e r a b l y i n s i z e from m i c r o s c o p i c t o p i e c e s 2.5 cm. i n l a r g e s t d i m e n s i o n ; t h e s e l a r g e r fragments b e i n g rounded due to p a l a g o n i t i z a t i o n . The fragment i s o n l y l o o s e l y c o n s o l i d a t e d , t e n d i n g to crumble when d r i e d , the l a r g e r g l a s s and g l a s s y b a s a l t fragments r e s t i n g i n a m a t r i x o f p a l a g o n i t e and c l a y m i n e r a l s , shown by X-ray d i f f r a c t i o n t o be m a i n l y m o n t m o r i l l o n i t e . The h y a l o c l a s t i t e fragments r e c o v e r e d from dredge h a u l EN 70-025-9D are n e a r l y a l l f l a t , some w i t h a t h i n ferromanganese c r u s t on,one s u r f a c e o n l y ( F i g u r e 39C). The main d i f f e r e n c e s , from the fragment a l r e a d y d i s c u s s e d i n c l u d e : A - Eyaloclastite Breccia. Specimen No. EN 70-025-2D-24. B - Photomicrograph of hyaloclastite breccia showing pala r;onitized glass shards in a dark ferruginous matrix, (plane l i g h t 750) Specimen No. EN 70-025-9D-27. C - Hyaloclastite Breccia fragments fro.i. Dredge Haul No.EN 70-O25-9D. IGURE 39 Hyaloclastite Breccia fragments from the Dellwood Seamount Area. 81 TABLE V Chemical C o m p o s i t i o n o£ H y a l o c l a s t i t e B r e c c i a s from the Dellwood Seamount Area OXIDES CHEMICAL COMPOSITION IN WT. EN 70-025-2D-24 EN 70-025-9D-27 S i 0 2 T i 0 2 A 1 2 0 3 A l l I r o n as F e 2 0 3 MnO MgO CaO Na 20 K 20 H 20, e t c . 1 Cu N i Co Pb Zn 37.2 ±1.0 0.6 ±0.1 7.7 ±0.5 18 . 23±0.05 0.16 + 0 .01 5.56±0 .11 3.85±0 .09 1.8 ±0.1 1.4 ±0.1 17.8 ±0.5 0 .0045 0.0085 0.0025 0 .0315 0 .0150 46.7 ±0.7 0.7 ±0.1 10 .6 ±0.6 11.08±0.05 0 .49±0 .01 4 .44±0 .12 2 .64±0.10 4.2 ±0.1 1.9 ±0.1 12.5 ±0.5 0 .0060 0 .0163 0 .0040 0 .0000 0.0175 NOTES: E r r o r s quoted above are maximum d e v i a t i o n s from the mean. ! H 2 0 , e t c . r e p r e s e n t s v o l a t i l e s l o s t on h e a t i n g f o r 24 hours at 700° C. 82 ( i ) The ferromanganese c r u s t on one s u r f a c e (upper s u r f a c e ) o n l y . ( i i ) The absence o f l a r g e rounded f r a c t u r e d and s h a t -t e r e d g l a s s y p i l l o w fragments w i t h p a l a g o n i t i z e d r i m s . I n s t e a d , a l l the fragments are s u b m i c r o s c o p i c t o m i c r o s c o p i c c o m p r i s i n g m a i n l y p a l a g o n i t i z e d g l a s s s h a r d s . . ( i i i ) The f l a t shape o f the frag m e n t s . ( i v ) M i n e r a l o g y : T h i n s e c t i o n ( F i g u r e 39B) and X-ray d i f f r a c t i o n a n a l y s i s shows the h y a l o c l a s t i t e s at l o c a l i t y EN 70-025 -9D to c o n t a i n y e l l o w - b r o w n p a l a g o n i t i z e d g l a s s . s h a r d s , q u a r t z , and p l a g i o c l a s e i n a green m a t r i x of p a l a g o n i t e and c l a y m i n e r a l s m a i n l y m o n t m o r i l l o n i t e but a l s o c h l o r i t e and i l l i t e . (v) C h e m i s t r y : Table VI shows d i f f e r e n t p e r c e n t a g e by w e i g h t i n almost e v e r y o x i d e a n a l y z e d f o r the two types o f h y a l o c l a s t i t e s , r e f l e c t i n g the d i f f e r e n t m i n e r a l o g y and perhaps a l s o d i f f e r e n t o r i g i n ( d i s c u s s e d f o l l o w i n g ) . Sample EN-70-025-2D-24 c o n t a i n s no q u a r t z , but perhaps more c a l c i c p l a g i o c l a s e ; c o n t a i n s l e s s s i l i c a and more c a l c i u m and i s low i n a l k a l i s . Sample EN 70-025-9D-27 c o n t a i n s q u a r t z (perhaps d e t r i t a l ) , and more p l a g i o c l a s e (some o f which may be d e t r i t a l ) , c l a y m i n e r a l s , s i l i c a , a l u m i n a and a l k a l i i s . The f o r m a t i o n o f h y a l o c l a s t i t e b r e c c i a s i s g e n e t i c a l l y r e l a t e d t o submarine v o l c a n i c e r u p t i o n s . S i l v e s t r i (1963) summarised t h i s r e l a t i o n as f o l l o w s : " Immediately a f t e r the opening o f a f i s s u r e , the seawater p e n e t r a t e s i n t o i t , comes i n t o c o n t a c t w i t h the r i s i n g magma and c h i l l s i t . Thus, a s o l i d c r u s t i s formed, but i s r a p i d l y b r o k e n t o p i e c e s by the upward p u s h i n g magma which i s r e p e a t e d l y c h i l l e d and . s h a t t e r e d .... The e r u p t i n g magma may p a r t i a l l y , i n -t r u d e between the sea bottom and t h i s i n i t i a l 83 ( h y a l o c l a s t i t e 1 ) b r e c c i a l i k e a s i l l , but more o f t e n i t w i l l p e r f o r a t e the c o v e r i n g b r e c c i a , p u s h i n g i t a s i d e and f l o w i n g out ( f o r m i n g p i l l o w l a v a s sometimes w i t h h y a l o c l a s t i t e b r e c c i a between the p i l l o w s l ) . . . . At the s u r f a c e o f the l a v a , g l a s s y fragments accumu-l a t e t o b u i l d up h y a l o c l a s t i t e s which may d e r i v e a l s o from the s u r f a c e o f the l a v a f l o w i t s e l f , as w e l l as from the g l a s s y c r u s t s o f the growing p i l l o w s . " The p a l a g o n i t i z a t i o n o f the b r e c c i a s i s a secondary p r o c e s s due to c h e m i c a l a l t e r a t i o n by r e a c t i o n between seawater and hot l a v a and t o h y d r o t h e r m a l a c t i v a t i o n by h e a t e d seawater w h i c h has p e n e t r a t e d i n t o the h y a l o c l a s t i t e s and t h e r e mixed w i t h gases r e l e a s e d by the l a v a . The c l a y m i n e r a l s are a l a t e r a l t e r a -t i o n phase i n the c o n t i n u i n g h y d r a t i o n o f the b a s a l t i c g l a s s shards by sea w a t e r . Because o f i t s low S i 0 2 c o n t e n t , absence o f d e t r i t a l m i n e r a l s , and h i g h CaO c o n t e n t , the sample r e c o v e r e d from EN 70-025-2D i s b e l i e v e d t o be p a r t o f an i n i t i a l o r i n t e r p i l l o w h y a l o -c l a s t i t e b r e c c i a , w h i l e samples r e c o v e r e d from dredge h a u l EN 70-025-9D are p a r t o f the h y a l o c l a s t i t e b r e c c i a formed above the l a v a f l o w perhaps i n p a r t as a pavement c o v e r (ferromanganese c o a t i n g on the upper s u r f a c e o n l y ) . D e t r i t a l q u a r t z and f e l d s p a r and perhaps a l s o p l a n k t o n i c organisms c o u l d t h e n e a s i l y have been added to t h i s h y a l o c l a s t i t e b r e c c i a pavement. CHEMICAL WEATHERING AND ALTERATION OF BASALTIC FRAGMENTS In hand specimen, the v a r y i n g degrees o f w e a t h e r i n g are shown by c o l o u r changes from core to r i m ( F i g u r e 4 0 ) . A weathered fragment ( F i g u r e 40B) would thus show a h i g h l y a l t e r e d y e l l o w to red-brown l a y e r i m m e d i a t e l y below t h e , o u t e r manganese c o a t i n g , Statements i n p a r e n t h e s e s do n o t form p a r t o f the q u o t e , but are added by the a u t h o r . 84 A - Intensely weathered basalt fragments. Basalt fragments with weathered zones restricted to the rim. 3 - Weathered zones along fractures i n basalt fra, iuents. B - Moderately weathered basalt fragmen" Specimen NO.HJ 70-O2;~-9B- 22. FIGURE 40 Weathered zones in basalts of the Bell wood Seamount Area. 85 then a y e l l o w - g r e e n l a y e r f o l l o w e d by a g r e e n i s h - b l a c k and b l a c k l a y e r s and f i n a l l y a grey c o r e . I n t e n s e l y weathered fragments may l a c k the grey core ( F i g u r e 40A) and s l i g h t l y weathered f r a g -ments may l a c k the red-brown o r y e l l o w i n t e n s e l y weathered s u r -f a c e l a y e r and show o n l y the b l a c k or g r e e n i s h b l a c k l a y e r s s u r r o u n d i n g a c o r e . Weathering i s m a i n l y r e s t r i c t e d , however, to the r i m o f fragments and may show the v a r i o u s c o l o u r changes w i t h i n 2 cm. o f the s u r f a c e , the weathered r i m s u r r o u n d i n g the r e l a t i v e l y f r e s h l a r g e grey core ( F i g u r e 40C). W i t h i n the.wea-, t h e r e d r i m , c o l o u r changes may be s h a r p , but are o f t e n g r a d a t i o n a l . A sharp c o n t a c t , however, o f t e n e x i s t s between weathered r i m and f r e s h i n t e r i o r . In f r a c t u r e d f r a g m e n t s , w e a t h e r i n g extends f a r i n t o the i n t e r i o r o f the fragment a l o n g f r a c t u r e p l a n e s and shows the c o l o u r z o n a t i o n on the f r a c t u r e p l a n e i t s e l f p a r a l l e l to the r i m o f the fragment ( F i g u r e 40B). The c o l o u r changes r e f l e c t the v a r y i n g amounts o f m a i n l y p a l a g o n i t e , but a l s o h e m a t i t e , l i m o n i t e , and c l a y m i n e r a l s w hich r e s u l t from h y d r a t i o n o f b a s a l t i c g l a s s by sea w a t e r . The o u t e r y e l l o w l a y e r i s m a i n l y the p a l a g o n i t i z e d g l a s s y s u r f a c e o f the p i l l o w . The red-brown zone c o n t a i n s h e m a t i t e , l i m o n i t e and p a l a g o n i t e a l t e r a t i o n p r o d u c t s and the g r e e n i s h b l a c k and b l a c k l a y e r s c o n t a i n m a i n l y p a l a g o n i t e , but a l s o i r o n o x i d e s , as a l t e r a t i o n p r o d u c t s . W e a t h e r i n g a f f e c t s m a i n l y the t a c h y l y t e . m e s o s t a s i s c o n v e r t i n g i t t o p a l a g o n i t e , i r o n o x i d e s and c l a y m i n e r a l s . In h i g h l y weathered zones, v e s i c l e s are l i n e d and f i l l e d w i t h p a l a g o n i t e and i r o n o x i d e s . In the l e s s weathered p a r t s o f the specimens, c h e m i c a l a l t e r a t i o n i s , when p r e s e n t , s a u s s u r i t i z a t i o n o f p l a g i o c l a s e m i c r o l a t h s (due to d e u t e r i c 86 a l t e r a t i o n ) and c h e m i c a l w e a t h e r i n g i s m a i n l y p a l a g o n i t i z a t i o n o f the t a c h y l y t e v e s i c l e l i n i n g s and o f the i n t e r s t i t i a l t a c h y -l y t e i n the groundmass around the v e s i c l e s . P a l a g o n i t e may o c c u r as the i n i t i a l h y d r a t e d phase i n the g r a d u a l c h e m i c a l w e a t h e r i n g o f b a s a l t i c g l a s s by.sea w a t e r , c l a y m i n e r a l s b e i n g l a t e r p h ases; or p a l a g o n i t e may be due to s y n g e n e t i c h y d r a t i o n and a l t e r a t i o n ( d e u t e r i c a l t e r a t i o n ) . Most of the p a l a g o n i t e i n the samples s t u d i e d i s b e l i e v e d due t o chem-i c a l a l t e r a t i o n by sea water s i n c e i t s d i s t r i b u t i o n i n d i c a t e s t h a t i t i n c r e a s e s i n amount by r e p l a c i n g the b a s a l t i c g l a s s i n w a r d from the o u t e r s u r f a c e and from the w a l l s o f c r a c k s and v e s i c l e s . Replacement p r o b a b l y c o n t i n u e s u n t i l p r e c i p i t a t e d manganese and i r o n o x i d e s form enough of a c r u s t o ver the b a s a l t fragment to p r e v e n t f u r t h e r c o n t a c t between sea water and b a s a l t i c g l a s s . W e a t h e r i n g i s a l s o r e f l e c t e d i n c h e m i c a l c o m p o s i t i o n o f the b a s a l t s as r e p o r t e d by H e k i n i a n (1971). Deeply weathered r o c k s show an F e 20 3:FeO r a t i o >0.3; i n c r e a s e i n H 20, K 20 and F e 2 0 3 and a decrease i n MgO and FeO compared to f r e s h r o c k s . However, T i 0 2 and A 1 2 0 3 show no d e t e c t a b l e change. H a r t (1970) mentioned t h a t exchange between sea water and o c e a n i c t h o l e i i t e s c o u l d produce c h e m i c a l c o m p o s i t i o n s which are s i m i l a r to a l k a l i b a s a l t s . The t r a n s i t i o n a l t o a l k a l i n a t u r e o f the b a s a l t s o f the D e l l w o o d Seamounts may t h e r e f o r e be due at. l e a s t i n p a r t to w e a t h e r i n g by sea w a t e r . VESICLE LININGS AND AMYGDULES Two types o f v e s i c l e l i n i n g s and amygdules o c c u r commonly FIGURE 41 V e s i c l e l i n i n g s and amygdules i n b a s a l t s from the Dellwood Seamount A r e a . A - C a l c i t e l i n i n g v e s i c l e i n h y a l o p i l i t i c b a s a l t . Specimen NO. EN 70-025-7D-11. (X - N i c o l s X50) B - C a l c i t e f i l l i n g v e s i c l e i n h y a l o p i l i t i c b a s a l t . Specimen NO. EN 70-02.5 -7D-1. (X - N i c o l s X50) C - S e g r e g a t i o n v e s i c l e s i n i n t e r s e r t a l b a s a l t . Specimen NO. EN 70-025-3D-4. (P l a n e l i g h t X50) D - P a l a g o n i t e l i n i n g s u b s i d i a r y s u r f a c e o f s e g r e g a t i o n v e s i c l e whose p r i n c i p a l s u r f a c e i s l i n e d w i t h t a c h y l y t e . Specimen NO. EN 70-025-8D-1. (P l a n e l i g h t X50) E - F i b r o - p a l a g o n i t e l i n i n g s e g r e g a t i o n v e s i c l e s i n i n t e r s e r t a l b a s a l t . Specimen NO. EN 70-025-7D-6. (X - N i c o l s XSO) F - P a l a g o n i t e l i n i n g s u b s i d i a r y s u r f a c e o f s e g r e g a t i o n v e s i c l e whose p r i n c i p a l - s u r f a c e i s l i n e d w i t h d e v i t r i f i e d t a c h y l y t e . Specimen NO. EN 70-025-7D-11. (P l a n e l i g h t X50) 87 FIGURE 41 Vesicle linings and an.ygdules i n basalts froa. the Dellwood Seamount Area. (MAGNIPICATION X 5 0 ) 88 i n the b a s a l t i c r o c k s o f the Dellwood Seamount A r e a , 1) C h e m i c a l l y p r e c i p i t a t e d l i n i n g s and f i l l i n g s ( F i g u r e s 41A and B) 2) S e g r e g a t i o n v e s i c l e s and t h e i r a l t e r a t i o n p r o d u c t s ( F i g u r e s 41C, D, E and F) The c h e m i c a l l y p r e c i p i t a t e d l i n i n g s and f i l l i n g s com-p r i s e red-brown h y d r a t e d i r o n o x i d e s ( m a i n l y h e m a t i t e and l i m o n i t e ) b l a c k manganese o x i d e s and s p a r r y c a l c i t e . Both i r o n and manganese o x i d e s o c c u r i n v e s i c l e s i n the weathered r i m near the s u r f a c e o f the fragment, t h e i r p r e c i p i t a t i o n a i d e d by l e a c h -i n g o f Mn and Fe and o t h e r elements from the l a v a by r e a c t i o n w i t h sea w a t e r . The f o r m a t i o n o f s p a r r y c a l c i t e may be due to r e a c t i o n o f sea water t r a p p e d i n the v e s i c l e s o f a c o o l i n g l a v a w i t h the l a v a . The s e g r e g a t i o n v e s i c l e s o b s e r v e d i n t h i n s e c t i o n ( F i g u r e 41) are s i m i l a r t o those d e s c r i b e d by Smith (1967) "as r e p r e s e n t i n g s o l i d i f i e d r e s i d u a l melt w h i c h moved i n t o e a r l y formed v e s i c l e s b e f o r e c o n s o l i d a t i o n o f the l a v a , " t h i s movement b e i n g due to "an i n c r e a s e i n c o n f i n i n g p r e s s u r e a p p l i e d t o the f l u i d w i t h i n the v e s i c l e s . P a r t i a l c r y s t a l l i s a t i o n o f a v e s i c u - , l a r l a v a f o l l o w e d by c o m p l e t i o n o f c r y s t a l l i s a t i o n a t a h i g h e r c o n f i n i n g p r e s s u r e , f o r example, a t a deeper marine e n v i r o n m e n t , c o u l d p r o d u c e " the s e g r e g a t i o n v e s i c l e s . The p r i n c i p a l s u r f a c e o f these s e g r e g a t i o n v e s i c l e s are thus l i n e d w i t h b a s a l t i c g l a s s ( t a c h y l y t e ) and s u b s i d i a r y s u r f a c e s may be l i n e d w i t h p a l a g o n i t e due t o a l t e r a t i o n o f the t a c h y l y t e ( F i g u r e s 41D and E) or p a l a g o -n i t e may f i l l the w h o l e . v e s i c l e . Most o f t h i s p a l a g o n i t e which l i n e s o r f i l l s v e s i c l e s o c c u r s as a g r e e n i s h , . a n i s o t r o p i c f i b r o u s low b i r e f r i n g e n t m i n e r a l s ( F i g u r e s 41D and E) d e s c r i b e d by Peacock 89 (1928) as f i b r o p a l a g o n i t e . The l e s s f i b r o u s y e l l o w - g r e e n i s o -t r o p i c v a r i e t y . w h i c h r e p l a c e s the i n t e r s t i t i a l g l a s s o f the groundmass o f these b a s a l t s was d e s c r i b e d by. Peacock as g e l p a l a g o n i t e . D e v i t r i f i c a t i o n o f the b a s a l t i c g l a s s i n s e g r e g a t i o n v e s i c l e s can cause m i c r o l i t e s and c r y s t a l l i t e s t o f i l l o r l i n e v e s i c l e s ( F i g u r e 4 1F). POST-VOLCANIC DEPOSITS Ferro-Manganese Nodules and C r u s t s F i g u r e 42 shows ferromanganese nodules and c r u s t s from the Dellwood'Seamount Range, w h i l e F i g u r e 28C shows a t h i c k manganese c r u s t (<_ 50 mm.) on an i n t e r s e r t a l b a s a l t from the Southe a s t Dellwood K n o l l s . The maximum t h i c k n e s s of.manganese c r u s t s g i v e some measure o f the r e l a t i v e age o f specimens from each dredge h a u l , t h i c k e r c r u s t s o c c u r r i n g on o l d e r r o c k s . Some rough e s t i m a t e s o f the age o f the r o c k s are shown i n Tab l e V I , based on e s t i m a t e d r a t e s o f growth, o f manganese c r u s t and nodules f o r the N o r t h P a c i f i c (Bender e t a l , 1970; Barnes and Dymond, 1967). For p e l a g i c n o d u l e s , the e s t i m a t e d growth r a t e i n the N o r t h P a c i f i c i s 2.5 to 3 mm/106 y r . F o r . n o d u l e s n e a r e r c o n t i n -e n t s j growth r a t e s would be much h i g h e r s i n c e the nodules and c r u s t s here would a l s o c o n t a i n c o n t i n e n t a l d e b r i s l i k e q u a r t z and c l a y , as i n the Dellwood Seamount A r e a . Thus, any e s t i m a t e based on a v a l u e o f 2.5 mm/106 y r . i s p r o b a b l y , a maximum e s t i m a t e o f age o f the r o c k . The maximum quoted r a t e o f growth of manganese n o d u l e s i s 40 mm/106 y r . (Barnes and Dymond, 1967) . That ferro-manganese c r u s t on r o c k s from the dredge h a u l from the S o u t h e a s t Dellwood K n o l l s i s t h i c k e r than the 90 manganese r i n d on r o c k s from the Northwest Dellwood K n o l l s i s o f s p e c i a l i n t e r e s t . (See D i s c u s s i o n . ) TABLE VI T h i c k n e s s o f Ferro-Manganese C o a t i n g and E s t i m a t e d Age Dredge Haul No and L o c a t i o n EN 70-025-2.D Northwest Dellwood K n o l l s EN 70-025-3D South e a s t Dellwood K n o l l s EN 70-025-7D S o u t h e a s t e r n end o f Dellwood Seamount Range EN 70-025-8D Dellwood Seamount Range EN 70-025-9D Dellwood Seamount Range IOUBC 70-16-12D N o r t h w e s t e r n end of. Dellwood Seamount Range Remarks on T h i c k n e s s o f Mn-Fe C o a t i n g Manganese R i n d <_ 1 mm. on 5 3 out of 54 specimens. The l a s t s p e c i -men has a c o a t i n g a v e r a g i n g 1 mm. but l o c a l l y r e a c h i n g 3 mm. where i t comprises Mn-Fe o x i d e s t o g e t h e r w i t h d e t r i t a l d e b r i s . M a i n l y a r i n d on g l a c i a l e r r a t i c s On b a s a l t f r a g m e n t s , i t i s 3-6 mm. t h i c k , b u t becomes 50 mm. t h i c k on p a r t s o f one specimen where a g a i n the Mn-Fe o x i d e s are a s s o c i a t e d w i t h d e t r i t u s . V a r i a b l e , up to 5 mm. t h i c k . V a r i a b l e , up to 12 mm. t h i c k , L a y e r e d c r u s t s up to 80 mm. t h i c k , and n o d u l e s . M a i n l y manganese r i n d and f r a g -ments o f a l a m i n a t e d , i r o n - r i c h d e p o s i t < t h i c k . E s t i m a t e d 1 Age 0.02+1 myr, 1+70 myr 0.1+2 myr 0.3+4.8 myr, 2+32 myr, The c r u s t , when w e l l d e v e l o p e d , i s t h i n l y l a y e r e d ( F i g u r e 42B) and the s u r f a c e may be p i t t e d and f u r r o w e d ( F i g u r e 42C), p r o b a b l y due t o s o l u t i o n o f the manganese. The nodules are l a y e r e d around a n u c l e u s commonly a b a s a l t o r h y a l o c l a s t i t e f r a g -Age e s t i m a t e d u s i n g maximum v a l u e s o f 40 mm/106 y r . and minimum v a l u e s o f 2.5 mm/106 y r . 91 - Ferron.anganese crust (~30mm thick) unconfoni.ably overlying a bedded Hyaloclastite Breccia. Specimen No.EN 70-025-9D-71. j C - Pitted and furrowed ferromanganese crust. FIGURE 42 Ferromanganese nodules and crusts from the Dellwood Seamount Range. TABLE VII ELEMENTAL CHEMICAL ANALYSES OF POST VOLCANIC DEPOSITS SAMPLE NO. ROCK TYPE LOCATION ELEMENT COMPOSITIONS IN WT. h Na Mg A l S i K Ca T i Mn Fe Co Cu Zn Pb MAXIMUM (Mero, 1964) PACIFIC OCEAN Mn-NODULES 4.7 2.4 6.9 20.1 3.1 4.4 1.7 41.1 26.6 2.3 2.0 1.6 0.08 0.36 MINIMUM (Mero, 1964) 1.5 1.0 0.8 1.3 0.3 0.8 0.11 8.2 2.4 0.014 0.16 0.028 0.04 0.02 AVERAGE (Mero, 1964) 2.6 1.7 2.9 9.4 0.8 1.9 0.67 24 .2 14.0 0.35 0.99 0.53 0.047 0.09 EN 70-025-9D-92 Mn-Nodule Dellwood Seamount Range 1.6 o.8 3.0 10 .7 0.7 1.8 0.23 14.5 7.75 0 .048 0.23 0.041 0.073 0.019 EN 70-025-3D-4 Mn-Fe Crust Southeast Dellwood KnolIs 1.4 1.8 2.2 8.1 r.i 1.2 0.16 22.1 6.13 0.0052 0.049 0.013 0.035 0.0028 IOUBC 70-16-12D-2 Iron Deposit Dellwood Seamount Range 1.5 0.7 1.0 8.6 0.3 1.3 0 2.05 28.5 0 0.006 0.0015 0.0076 0 EN 70-025-3D-22 Mn-impregnated Sediment Southeast Dellwood K n o l l s 3.0 1.7 3.9 16.8 1.0 2.9 0.19 9.0 7.2 0 .0015 0.006 0.0035 0.013 0 IOUBC 70-16-12D-3 Sandstone Dellwood Seamount Range 2.1 1.4 4.1 24 .1 1.5 3.4 . 0 .34 0.12 9.13 0.0012 0.002 0.0015 0.0053 0 Mn-Nodule ( G r i l l , 1968) J e r v i s I n l e t 0.95 1.82 1.7 ND1 0.98 1.33 0.096 32.72 5 .01 0.013 0.026 0.0055 1 0.0019 ND1 1ND - Not Determined 93 ment. In F i g u r e 42A, a t h i c k ferromanganese c r u s t , 35 mm. t h i c k , o c c u r s o v e r a d i p p i n g bedded h y a l o c l a s t i t e b r e c c i a . X-ray a n a l y s i s o f the ferromanganese c r u s t s shows t o d o r o k i t e (a h y d r a t e d manganese o x i d e ) as the p r i n c i p a l m i n e r a l o f manganese, but the c r u s t s a l s o c o n t a i n m o n t m o r i l l o n i t e , c h l o r -i t e , q u a r t z , p l a g i o c l a s e and g o e t h i t e . E l e m e n t a l a n a l y s e s . o f a i r - d r i e d p o s t - v o l c a n i c d e p o s i t s , i n c l u d i n g the ferro-manganese c r u s t s and n o d u l e s , are shown i n Table V I I . Maximum, minimum and average a n a l y s e s (Mero, 1964) as w e l l as one a n a l y s i s by G r i l l (1968) f o r a nodule from J e r v i s • I n l e t , B r i t i s h Columbia are a l s o shown f o r comparison. I r o n D e p o s i t S e v e r a l fragments o f a r e d t h i n l y l a y e r e d l o o s e l y , c o n s o l i d a t e d d e p o s i t ( F i g u r e 43) were r e c o v e r e d from dredge h a u l IOUBC 70-16-12D. The a d j a c e n t l a y e r s o f the s e fragments v a r y s l i g h t l y i n c o l o u r ; the l a y e r s are u n d u l a t i n g o r s l i g h t l y f o l d e d , p r o b a b l y due t o d e p o s i t i o n on an i r r e g u l a r bottom. T h i n s e c t i o n shows the o c c a s i o n a l r e l i c t d e t r i t a l heavy m i n e r a l s , e.g., sphene, but more i m p o r t a n t , shows a complex l a y e r e d t e x t u r e , some l a y e r s composed o f r e d ' r i b l i k e ' i s o t r o p i c m a t e r i a l r e s e m b l i n g s t a l a c -t i t e s ( F i g u r e 43B). X-ray d i f f r a c t i o n w i t h Cuka r a d i a t i o n shows the m a t e r i a l to be amorphous and the d e p o s i t i s thus b e l i e v e d to c o n s i s t m a i n l y o f a h y d r a t e d i r o n o x i d e ( e . g . , l i m o n i t e o r c r y p t o - , c r y s t a l l i n e g o e t h i t e ) . A p a r t i a l c h e m i c a l a n a l y s i s o f t h i s d e p o s i t (IOUBC 70-16-12D-2) i s g i v e n i n Table V I I I t o g e t h e r w i t h a n a l y s e s o f s i m i l a r d e p o s i t s from the South P a c i f i c by B o n a t t i and Joen s u u , (1966). An e l e m e n t a l c h e m i c a l c o m p o s i t i o n o f the r o c k i s g i v e n i n T a b l e V I I w i t h c h e m i c a l c o m p o s i t i o n s o f o t h e r P o s t V o l c a n -i c d e p o s i t s . 94 CMS A - Layered Iron Deposit. B -Photo..!crograph of iron deposit sho-,ving layers composed of r i b - l i k e limonite resembling stalactites, (plane light Jr0) FIGURE 43 Iron Deposit (I0U3G 70-16-12D-2) from the Dellwood Seamount Range. 95 TABLE V I I I P a r t i a l Chemical C o m p o s i t i o n o f Some Deep Sea I r o n t . % IOUBC 70-16-125-2 18.4 ±0.5 1.9 ±0.3 1.82+0.01 1.16±0.09 0.36±0.02 28.5 ±0.1 2 .05±0.05 62 0 15 NOTE - a i , a 2 , a3 a r e a n a l y s e s o f South P a c i f i c i r o n d e p o s i t s by B o n a t t i and Joensuu, 1966. > The o r i g i n o f t h i s fragment i s . b e l i e v e d to be s i m i -l a r to t h a t o f manganese c r u s t s , namely by c h e m i c a l d e p o s i t i o n a t the sediment-water i n t e r f a c e . The sour c e of the i r o n , l i k e the source o f the manganese, i s a problem. In s h a l l o w - w a t e r n o d u l e s ( e . g . , those from J e r v i s I n l e t , B r i t i s h C o l u m b i a ) , the manganese i s p r o b a b l y o b t a i n e d from c o n t i n e n t s by w e a t h e r i n g and o c e a n i c t r a n s p o r t . In the deep s e a , submarine v o l c a n i s m w i t h i t s emanations ( r i c h i n Fe and o t h e r e l e m e n t s ) , and the l e a c h i n g o f Fe and Mn from submarine b a s a l t s due to r e a c t i o n between sea wate r and submarine l a v a may be the prime s o u r c e s o f the i r o n and manganese. I r o n - r i c h d e p o s i t s c o n s i s t i n g m a i n l y o f c r y p t o -c r y s t a l l i n e g o e t h i t e have been r e p o r t e d from the South P a c i f i c by B o n a t t i and Joensuu (1966) , who b e l i e v e d t h a t "the d e p o s i t was formed by f r a c t i o n a l p r e c i p i t a t i o n o f Fe and Mn which had D e p o s i t s Oxide o r Sample and C o m p o s i t i o n i n Element a 2 a3 S i 0 2 17 .6 13 .9 12.4 A 1 2 0 3 1.0 1.0 1.0 CaO 2.6 2.3 2.7 MgO 1.5 0 . 85 0.80 K 20 0.39 0.45 0.38 Fe 32 .5 28.8 31 .1 Mn 1.94 2 .43 0.58 C o m p o s i t i o n i n ppm. N i 400 460 90 Co 35 120 32 Cu 74 120 60 96 been i n t r o d u c e d i n t o the bottom w a t e r by h y d r o t h e r m a l s o l u t i o n s o f v o l c a n i c o r i g i n and by l e a c h i n g o f deep sea b a s a l t i c l a v a s Such an o r i g i n i s a l s o a p p l i c a b l e t o the i r o n d e p o s i t o f the Dellwood Seamount Range a t l o c a l i t y IOUBC 70-16-12D where the v e r y weathered n a t u r e o f the groundmass and low i r o n c o n t e n t o f the a s s o c i a t e d b a s a l t s (sample IOUBC 70-16-12D-1) may i n d i - ' c a t e a d d i t i o n o f i r o n t o the bottom water by l e a c h i n g o f the b a s a l t i c r o c k s . SEDIMENTS L o o s e l y C o n s o l i d a t e d Sediments From dredge h a u l IOUBC 70-16-12D, s e v e r a l fragments o f a green l o o s e l y c o n s o l i d a t e d sandstone ( F i g u r e 44A) were r e c o v e r e d . In hand specimen, the o c c a s i o n a l g l a c i a l p e b b l e o r co b b l e i s found surrounded by a s a n d - s i z e d green m a t r i x . A v e r y t h i n manganese r i n d o c c u r s i n some p a r t s o f some fragments. T h i n s e c t i o n ( F i g u r e s 44C and D) shows the ro c k t o c o n t a i n forams and d e t r i t a l ( g l a c i a l ) s u b a n g u l a r to rounded q u a r t z , p l a g i o c l a s e and a l t e r e d rock fragments w i t h l e s s e r amounts o f h o r n b l e n d e , sphene, z i r c o n and e p i d o t e , a l l f l o a t i n g ( i . e . , w i t h l i t t l e o r no g r a i n to g r a i n c o n t a c t ) i n a green c l a y - r i c h m a t r i x shown by X-ray d i f f r a c t i o n to c o n s i s t o f c h l o r i t e , m o n t m o r i l l o n i t e and i l l i t e . An e l e m e n t a l c h e m i c a l c o m p o s i t i o n o f t h i s r o c k (IOUBC 70-16-12D-2) i s shown i n Table V I I wh i c h shows the ro c k to be v e r y i r o n r i c h . The sandstone fragments (IOUBC 70-.16-12D-2) are b e l i e v e d t o be a u t h i g e n i c , c o m p r i s i n g m a i n l y g l a c i a l d e t r i t u s C os D — Photomicrographs of authigenic sandstone showing angular to rounded quarts, plagioclase, forams and rock fragments i n a dark ferruginous matrix. Specimen NO. I0U3C 70-16-12D-3. (X50) FIGURE 44 Authigenic loosely consolidated sediu.ents from the Dellwood Seamount Area. 98 b e i n g c o n s o l i d a t e d and cemented i n p l a c e by a f e r r u g i n o u s cement which may be p a r t l y o f a v o l c a n i c o r i g i n , as i s the i r o n d e p o s i t from the same l o c a l i t y . Foram t e s t s found i n t h e s e sandstone fragments are a l l p e l a g i c and were added to the sediment by s e t t l i n g to the bottom a f t e r d e a t h . One fragment from dredge h a u l EN 70-025-3D ( F i g u r e 44B) i s a l s o a l o o s e l y c o n s o l i d a t e d s ediment, but c o n t a i n s more foram t e s t s , l e s s g l a c i a l l y d e r i v e d d e t r i t u s , and i s impregnated w i t h manganese o x i d e which may be t h e p r i n c i p a l cementing agent. D e t r i t a l s a n d - s i z e d g r a i n s i n c l u d e p l a g i o c l a s e , q u a r t z , h o r n -b l e n d e and the o c c a s i o n a l a l t e r e d r o c k f r a g m e n t s . More than 80% o f the f r a g m e n t s , however, c o n s i s t o f h y d r a t e d manganese and i r o n o x i d e s ( m a i n l y t o d o r o k i t e ) and c l a y m i n e r a l s ( m a i n l y i l l i t e , c h l o r i t e and m o n t m o r i l l o n i t e ) . Foram t e s t s are m a i n l y r e s t r i c t e d to the s u r f a c e o f the f r a g m e n t s . An e l e m e n t a l c h e m i c a l a n a l y s i s i s shown i n Table V I I I . Mud . The c h a i n - b a g dredges used d i d not r e c o v e r much mud and hence o n l y one sample (EN 70-025-7D-72) was examined. X-ray d i f f r a c t i o n showed i t t o c o n t a i n q u a r t z , p l a g i o c l a s e , c a l c i t e (forams) and c l a y m i n e r a l s , a g a i n m a i n l y c h l o r i t e , m o n t m o r i l l o n -i t e and i l l i t e . Most o f t h i s i s p r p b a b l y g l a c i a l l y d e r i v e d m a t e r i a l . G l a c i a l E r r a t i c s G l a c i a l e r r a t i c s were r e c o v e r e d from a l l dredge s i t e s e x cept EN 70-025-7D which i s f u r t h e s t away from a c o n t i n -e n t a l s o u r c e . The g l a c i a l e r r a t i c s v a r y i n s i z e , shape and c o m p o s i t i o n . They range from p e b b l e s t o b o u l d e r s >60 l b . i n S3 C D FIGURE 45 Glacial Erratics recovered from the Dellwood Seamount Area. 100 w e i g h t ( F i g u r e 4 5 E), they are s u b a n g u l a r to w e l l rounded, and are m a i n l y igneous o r metamorphic. Igneous r o c k s i n c l u d e g r a n i t e s , g r a n o d i o r i t e s , d i o r i t e s , a m y g d a l o i d a l b a s a l t s and d i a b a s e s . Metamorphic r o c k s i n c l u d e s c h i s t s and g n e i s s e s . F i g u r e 45 shows some o f the g l a c i a l e r r a t i c s o b t a i n e d i n dredge h a u l s from the Del l w o o d Seamount Ar e a and Table IX l i s t s the c o n s t i t u e n t r o c k t y p e s found. The rock t y p e s l i s t e d i n d i c a t e t h a t the Coast Mountains o f B r i t i s h Columbia i s the p r o b a b l e s o u r c e a r e a . TABLE IX G l a c i a l E r r a t i c s from the Dellwood Seamount A r e a Dredge Number and Number o f Specimens ROCK TYPE Igneous V o l c a n i c EN 70- EN 70- EN 70- EN 70- IOUBC 70 025-2D 025-3D 025-8D 025-9D 12-12D. Igneous g P l u t o n i c 8 4 4 Metamorphic 2 1 1 1 4 Sedimentary - - - - 1 U n d i f f e r e n t i a t e d * - '- * * - S e v e r a l p e b b l e s and c o b b l e s , m a i n l y i g n e o u s . NOTE - There were no g l a c i a l e r r a t i c s r e c o v e r e d from EN 70-025-7D j Most o f the g l a c i a l e r r a t i c s are c o v e r e d w i t h a manga-. Inese r i n d , a t l e a s t i n p a r t s . Some a l s o show g l a c i a l s t r i a t i o n s . JOIDES D r i l l H ole No. 177 Table XI shows a l o g type d e s c r i p t i o n o f a core d r i l l e d by. JOIDES on the n o r t h w e s t end o f P a u l Revere Ridge.. The age and t o t a l t h i c k n e s s o f the sediment column are not e w o r t h y and g i v e some i d e a o f s e d i m e n t a t i o n r a t e s . These average about 4 101 TABLE X D e s c r i p t i o n of cor e l o c a t e d a t 50 28.18'N. 130 12.30'W. on the n o r t h w e s t end of the P a u l Revere Ridge a t water depth of 2006m. ( a f t e r Kulm and Von Huene,19 71). Depth i n cor e j u n i t j T h i c k n e s s i D e s c r i p t i o n o f u n i t (meters) | No., o f u n i t J 100 -rO-6 200 300 400 500 j G e o l o g i c a l , -age ; 62 • O l i v e - g r a y to dark j Upper I g r e e n i s h - g r a y s i l t y muds. P l e i s t o c e n e Upper 4 i T h i n , o c c a s i o n a l l y graded sand and s i l t t u r b i d i t e s i n t e r b e d d e d w i t h h ard f i s s i l e muds. Lower P l e i s t o c e n e 128 1-9 0i 1 r 3 I 32 — 2-2 2 r 4-N a n n o f o s s i l and c a r b o n a t e . ] Upper r i c h mud. I P l i o c e n e 156 W e l l - d e f i n e d graded sand ' Lower and s i l t t u r b i d i t e s . I P l i o c e n e -3 ~3"8". 6 ' 68 I^TBas-aTt -Si41- r D a r k - o l i v e N a n n o f o s s i l muds w i t h no sand o r s i l t beds. Lower P l i o c e n e -4-Si f >56 5071 I n d u r a t e d f i n e g r a i n e d m o d e r a t e l y s o r t e d massive ' Lower f a n o r c o n t i n e n t a l r i s e | P l i o c e n e sediments a l t e r n a t i n g w i t h h e m i p e l a g i c s e d i m e n t s . 102 cm/1000 y r . f o r the P l i o - P l e i s t o c e n e p e r i o d and about 10 cm/1000 y r . f o r the P l e i s t o c e n e . The d e s c r i p t i o n of the sedimentary u n i t s i n d i c a t e d e p o s i t i o n by t u r b i d i t y c u r r e n t s , and, periods of quiescence during which only p e l a g i c muds were deposited. This probably r e f l e c t s the di s t u r b e d h i s t o r y of the c o r e - s i t e , i n the middle of a f a u l t zone. I t i s b e l i e v e d that sedimentation rates i n the area of the proposed Dellwood Spreading Segment i s much greater (see Discussion) because i t i s at the base of t h e . c o n t i n -e n t a l slope and thus much nearer to a c o n t i n e n t a l source than the Paul Revere Ridge, and also because i t i s t o p o g r a p h i c a l l y much lower than the Paul Revere Ridge. 103 CHAPTER SIX DISCUSSION THE RELATIONSHIP OF THE DELLWOOD SEAMOUNT AREA TO THE JUAN DE FUCA PLATE The existence of the Explorer Spreading Zone has been established (Ewing et a l , 1968; Morgan, 1968; Atwater, 1970 and others). This spreading zone terminates at the Revere - Dellwood Fault Zone (Thomlinson, 1971, personal communication and present study), along the southwest slope of the Paul Revere Ridge. The Queen Charlotte right l a t e r a l transform f a u l t trends along the continental slope o f f Queen Charlotte Islands (Chase and T i f f i n , in press) and into the northeastern section of the Scott Channel off Queen Charlotte Sound (MN - Plate VIII and VV -Plate XIII). Faulting also occurs on the lower slope and i n the Winona Basin further south o f f Vancouver Island but this f a u l t i n g may be a previously active more southerly extension of the Queen Charlotte f a u l t or may be due to subduction of the Juan de Fuca Plate in the region, The problem then i s what kind of boundary exists between the P a c i f i c and Juan de Fuca Plates in the region between the Explorer Spreading Zone and the Queen Charlotte transform f a u l t . Only three types of plate boundaries are presently recognised - transform faults,.spreading centres, and subduction zones. The bathymetry, vector geometry, po s i t i o n and orienta-tion of known spreading centres and transform faul t s of the Juan de Fuca - P a c i f i c Plate boundary and the d i s t r i b u t i o n of sea-, mounts i n the area a l l seem to preclude either a subduction zone or transform f a u l t trending northeast from the Dellwood Seamount 104 Range to the s o u t h e r n end o f the Queen C h a r l o t t e F a u l t o f f Queen C h a r l o t t e Sound, as the type o f boundary between the P a c i f i c and Juan de Fuca P l a t e s i n the a r e a . Two o t h e r p o s s i b i l i t i e s e x i s t as f o l l o w s : 1. E i t h e r the Queen C h a r l o t t e F a u l t Zone becomes v e r y d i f f u s e i n the a r e a , making the boundary between the N o r t h American and P a c i f i c P l a t e s v e r y complex i n the Dellwood Seamount A r e a , the boundary c o m p r i s i n g a s e r i e s o f v e r y s m a l l s p r e a d i n g c e n t r e s (too s m a l l to be d e l i n e a t e d by the magnetic a n o m a l i e s ) j o i n e d by s m a l l t r a n s f o r m f a u l t s . T h i s i s s u p p o r t e d by the d i f f u s e d i s t r i b u t i o n o f earthquake e p i c e n t r e s i n the a r e a ( F i g u r e 2 ) , However, t h e > l o c a t i o n s o f the e p i c e n t r e s are not v e r y a c c u r a t e , each l o c a t i o n h a v i n g a l a r g e c i r c l e o f e r r o r {>_ 10 km r a d i u s , T o b i n and Sy k e s , 1968); o r 2. A w e l l - d e f i n e d s p r e a d i n g c e n t r e l i e s w i t h i n the Dellwood Seamount Area c o n n e c t i n g the E x p l o r e r S p r e a d i n g segment v i a the Revere - Dellwood T r a n s f o r m F a u l t t o the Queen C h a r l o t t e Trans-form F a u l t . From the b a t h y m e t r y , the most o b v i o u s l o c a t i o n f o r t h i s s p r e a d i n g c e n t r e would be the channel between the Dellwood K n o l l s , h e r e a f t e r c a l l e d the Dellwood Channel. I f the Dellwood K n o l l s do mark the s i t e o f a s p r e a d i n g c e n t r e - the Dellwood Channel - t h e n s e v e r a l p o i n t s and conse-quences o f t h i s h y p o t h e s i s must be c o n s i d e r e d as f o l l o w s : . 1. The Dellwood Seamount Area may mark the t r i p l e j u n c t i o n between the P a c i f i c , N o r t h A m e r i c a n , and Juan de Fuca P l a t e s . T h i s would be the t r i p l e p o i n t where the Dellwood S p r e a d i n g Seg-ment, 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 the s u b d u c t i o n zone o f the Juan de Fuca P l a t e a l l meet.. T h i s s u b d u c t i o n zone 105 i s r e p r e s e n t e d i n the ar e a by the S c o t t I s l a n d s F a u l t Zone. On the o t h e r hand, s u b d u c t i o n may not p r e s e n t l y be a c t i v e and Juan de Fuca P l a t e i n t h i s r e g i o n may be f u s e d t o , and moving w i t h , the N o r t h American p l a t e - i n which case the t r i p l e p o i n t i s somewhere t o the so u t h and the Dellwood S p r e a d i n g Zone would r e p r e s e n t the boundary between the P a c i f i c and N o r t h American P l a t e s , i n the a r e a . 2. The Queen C h a r l o t t e and r e l a t e d f a u l t s r e p r e s e n t a r i g h t l a t e r a l t r a n s f o r m f a u l t zone o f the r i d g e t o a r c type ( W i l s o n , 1965) j o i n i n g the A l e u t i a n I s l a n d A r c to the E a s t P a c i f i c R i s e i n the Dellwood Region. T h i s t r a n s f o r m f a u l t ends q u i t e a b r u p t l y i n the n o r t h e a s t e r n s e c t i o n o f the S c o t t Channel as i s shown by 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 i n g i n the a r e a (see S e i s m i c Data -Queen C h a r l o t t e F a u l t ) . W h i l e p r o f i l e s MN ( P l a t e V I I I ) , W ( P l a t e X I I I ) and HH' ( P l a t e VI) show f a u l t i n g b e l i e v e d to be the s o u t h e r n end o f the Queen C h a r l o t t e F a u l t Zone, p r o f i l e s f u r t h e r t o the s o u t h , l i k e NO ( P l a t e I X ) , ST and TU ( P l a t e X I ) , do n o t show any such f a u l t i n g i n d i c a t i n g t h a t the Queen C h a r l o t t e F a u l t has d i e d out s o u t h e a s t o f the S c o t t Channel. F u r t h e r s o u t h i n the Winona B a s i n , however, the f o l d i n g and f a u l t i n g shown on p r o f i l e s J J ' ( P l a t e V I I ) and LL' ( P l a t e X I I ) i n d i c a t e compres-s i o n p r o b a b l y due to s u b d u c t i o n . S u b d u c t i o n may a l s o be the cause o f the c r u m p l i n g o f l o w e r s l o p e sediments shown on p r o f i l e MN ( P l a t e V I I I ) . The f a c t t h a t the topmost l a y e r s are undeformed i n the Winona B a s i n and t h a t many o f the f a u l t s i n t h i s b a s i n and i n the S c o t t Channel do not r e a c h the s u r f a c e , i n d i c a t e s t h a t s e d i m e n t a t i o n i s g o i n g on a t a much f a s t e r r a t e than d e f o r m a t i o n and t h a t r e l a t i v e l y q u i e t t e c t o n i c c o n d i t i o n s e x i s t e d i n the v e r y 106 r e c e n t p a s t . A l t e r n a t i v e l y , s t r i k e - s l i p motion may have been so much more i m p o r t a n t than d i p - s l i p motion 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 n the r e c e n t t e c t o n i c h i s t o r y o f the area', t h a t f a u l t i n g i n the a r e a i s u n r e c o g n i s a b l e i n a v e r t i c a l s e c t i o n s i n c e sediments on b o t h s i d e s o f the f a u l t are too s i m i -l a r i n a c o u s t i c impedance t o show the presence o f s t r i k e - s l i p d i s p l a c e m e n t on a CSP s e c t i o n . 3. 1 The Revere - Dellwood F a u l t Zone i s a r i g h t l a t e r a l t r a n s f o r m f a u l t o f the r i d g e to r i d g e t y p e , d y i n g out i n a n o r t h w e s t e r l y d i r e c t i o n near the southwest end o f , t h e Dellwood S p r e a d i n g Segment. T h i s i s a g a i n s u p p o r t e d by s e i s m i c d a t a which shows e v i d e n c e o f f a u l t i n g a l o n g the southwest s l o p e s o f P a u l Revere Ridge and the S o u t h e a s t Dellwood K n o l l s , and i n the b a s i n between the Dellwood Seamount Range and the Dellwood K n o l l s , h e r e a f t e r r e f e r r e d to as the D ellwood B a s i n . Such f a u l t i n g i s shown on p r o f i l e s DD' and EF ( P l a t e I I I ) , J J ' ( P l a t e V I I ) and RS ( P l a t e X ) . However, f a u l t i n g i s not o b s e r v e d on the southwest s l o p e o f the n o r t h w e s t Dellwood K n o l l s o r i n the b a s i n between the Dellwood K n o l l s and the Dellwood Seamount Range (the Dellwood B a s i n ) a l o n g l i n e IOUBC 70-16 -16 (BC - P l a t e I I ) . 4. The a r e a s h o u l d have h i g h heat f l o w . The two heat f l o w v a l u e s i n the a r e a ( F i g u r e 12) are 4.8 y c a l s / c m 2 - s e c i n the chan-n e l between the Dellwood K n o l l s and 4,0 y c a l s / c m 2 - s e c i n the b a s i n between the Dellwood K n o l l s and the Dellwood Seamount Range. These v a l u e s are a t l e a s t t w i c e the average f o r the Juan de Fuca P l a t e and g r e a t e r than t h r e e times the average f o r ocean b a s i n s (see L i t e r a t u r e Review - Chapter I I ) . The v a l u e s are a l s o g r e a t e r than t w i c e the average f o r o c e a n i c r i d g e s (1.82 y c a l s / c m 2 - s e c ) and 107 about t w i c e the average f o r the E a s t P a c i f i c R i s e (2.13 y c a l s / c m 2 -sec - Lee and Uyeda, 1965). A l s o note t h a t the heat f l o w v a l u e i s h i g h e r (4.8) w i t h i n the p r o p o s e d s p r e a d i n g zone than (4.0) to the s o u t h e a s t o f i t . The h i g h v a l u e o f 16.1 y c a l s / c m 2 - s e c i n the Explore* S p r e a d i n g Segment i s anomalous but b e l i e v e d due to heat produced b o t h by r i s i n g magma i n a s p r e a d i n g t r e n c h and f r i c t i o n a l h eat produced by movement a l o n g the Revere - Dellwood T r a n s f o r m - F a u l t . 5. U s u a l l y , a c t i v e s p r e a d i n g zones are c o v e r e d w i t h l i t t l e o r no s e d i m e n t s , b e i n g v e r y young and f a r from c o n t i n e n t s . The t h i c k sediment c o v e r i n p a r t s o f the Dellwood Seamount A r e a , and e s p e c i a l l y i n the channel between the Dellwood K n o l l s (the p r o - . posed s p r e a d i n g a r e a ) , can e a s i l y be e x p l a i n e d by r a p i d t r a n s p o r t and d e p o s i t i o n o f sediments by t u r b i d i t y c u r r e n t s and i c e . r a f t i n g from the nearby N o r t h American c o n t i n e n t d u r i n g the P l e i s t o c e n e . Such r a p i d d e p o s i t i o n has been c a l l e d upon by Moore (1970) to e x p l a i n the t h i c k sediments i n the Escanaba.Trough - a median v a l l e y 15 km. wide t h a t f o l l o w s the a x i s of the Gorda R i s e . Here the sediment c o v e r averages 500 metres t h i c k and i s c u t by s e v e r a l f a u l t s . A 390 metre core from t h i s t r o u g h comprised o n l y Q u a t e r n a r y sediments and Moore e s t i m a t e d t h a t the r a t e o f s e d i m e n t a t i o n was ca..350 cm/10 3 y r . 6. As i n the Escanaba Trough, so too i n the Dellwood C h a n n e l , the sediments and v o l c a n i c basement are c u t by s e v e r a l f a u l t s showing d i p - s l i p movements ( p r o f i l e s FG - P l a t e IV and PQ -P l a t e V ) . Normal f a u l t i n g i s c h a r a c t e r i s t i c o f s p r e a d i n g cen- , t r e s w i t h median v a l l e y s . . 7. B a s a l t Types i n the A r e a : Chemical a n a l y s e s (Table I I I ) and 108 d i f f e r e n t i a t i o n diagrams ( F i g u r e s 37 and 38) show t h a t the v o l -c a n i c s o f the Dellwood Seamount Are a are, i n g e n e r a l , i n t e r m e d i a t e between a l k a l i and t h o l e i i t e t y p e s . Comparing Table I I I w i t h T a b l e IV and T a b l e I shows tha.t the t o t a l a l k a l i v a l u e s f o r most o f the r o c k s a n a l y s e d are i n t e r m e d i a t e between v a l u e s f o r t h o l e i - ' i t e s and a l k a l i b a s a l t s . However, T i 0 2 v a l u e s , e x c e p t f o r those from EN 70-025-3D are low and t y p i c a l l y t h o l e i i t i c Whether t h e n , the i n t e r m e d i a t e n a t u r e o f the b a s a l t s i s t r u e , o r whether i t r e ; f l e e t s the degree o f w e a t h e r i n g ( H a r t , 1970), i s a problem. S i n c e , however, o n l y r e l a t i v e l y f r e s h p a r t s o f p i l l o w s were sample f o r c h e m i c a l a n a l y s i s ( e x c e p t f o r samples from EN 70-025-3D which, were a l l v e r y weathered) then t h e s e c h e m i c a l a n a l y s e s p r o b a b l y r e f l e c t the t r u e i n t e r m e d i a t e n a t u r e o f the b a s a l t s . T h i s i n t e r -mediate n a t u r e o f b a s a l t s , , h o w e v e r , . i s - n o t e s p e c i a l l y u n c h a r a c t e r -i s t i c o f s p r e a d i n g c e n t r e s , s i n c e M u i r and T i l l e y (1964) found t h a t most, o f the b a s a l t s they examined from the r i f t zone o f the M i d - A t l a n t i c Ridge were i n t e r m e d i a t e , though t y p i c a l t h o l e i i t e s a l s o e x i s t e d . N e v e r t h e l e s s , some more c o n c r e t e s u p p o r t f o r a s p r e a d -i n g c e n t r e i n the r e g i o n o f the Dellwood K n o l l s comes from the v a r i a t i o n diagrams o f F i g u r e 38, which show t h a t the l e a s t d i f f e r -e n t i a t e d r o c k s i n t h e . a r e a come from the Northwest Dellwood K n o l l s and have SI v a l u e s o f 44.9 and 39.2, w h i l e r o c k s from the D e l l -wood. Seamount Range have l o w e r SI v a l u e s ( 3 0 - 3 5 ) , i n d i c a t i n g t h a t they are more d i f f e r e n t i a t e d and thus f u r t h e r away from the r i d g e t h a t i n i t i a t e d them. I t would be e x p e c t e d t h a t the l e a s t d i f f e r -e n t i a t e d r o c k s i n the a r e a would be c l o s e s t to the s p r e a d i n g c e n t r e t h a t - p r o d u c e d them. A problem a r i s e s , ' h o w e v e r , when we 109 c o n s i d e r r o c k s from dredge h a u l EN 70-025-3D which have SI v a l u e s around 26, but s i n c e these r o c k s are v e r y weathered as shown b o t h by p e t r o g r a p h y and c h e m i c a l a n a l y s i s (Table 111) , the SI . v a l u e s may r e f l e c t the degree o f w e a t h e r i n g r a t h e r t h a n the o r i g -i n a l n a t u r e o f the b a s a l t . I t s h o u l d be n o t e d t h a t MgO c o n t e n t d e c r e a s e s w i t h i n c r e a s e d w e a t h e r i n g ( H e k i n i a n , 1971) and thus SI v a l u e s would decrease w i t h i n c r e a s e d w e a t h e r i n g . A problem a r i s e s i n . t h a t the degree o f w e a t h e r i n g i s p r o p o r t i o n a l to age, i n d i c a t i n g t h a t the r o c k s are r e l a t i v e l y o l d . However, these r o c k s from EN 70-025-3D were dredged from a f a u l t e d t a l u s s l o p e which.may have exposed the o l d e r r o c k s i n . t h e a r e a . . 8, P r o f i l e W ( P l a t e X I I I ) shows t h a t the o c e a n i c v o l c a n i c basement ( l a y e r 2) r i s e s toward the c o n t i n e n t , r e a c h i n g i t s s h a l l o w e s t l e v e l i n the Dellwood Seamount A r e a and then s u d d e n l y d i p s i n a n o r t h e a s t d i r e c t i o n i n the S c o t t Channel. T h i s s t e p -l i k e r i s e o f the o c e a n i c basement l a y e r toward the c o n t i n e n t i s the o p p o s i t e o f the normal d i p o f o c e a n i c basement toward the. c o n t i n e n t and t h i s f e a t u r e l e d Ewing et a l (1968) to p o s t u l a t e a s p r e a d i n g segment i n the Dellwood Seamount Area. The i n t e r p r e -t a t i o n o f R a f f and Mason's 1961 magnetic anomaly map by V i n e , 1968 ( F i g u r e 46) shows t h a t p r o f i l e V V ( P l a t e X I I I ) would t r a -v e r s e younger and younger o c e a n i c v o l c a n i c basement i n a n o r t h -e a s t e r l y d i r e c t i o n and thus even w i t h o u t s p r e a d i n g in. the Dellwood Seamount A r e a , one would e x p e c t a r i s e i n basement l a y e r i n a n o r t h e a s t e r l y d i r e c t i o n as o b s e r v e d on the CSP l i n e . There are thus two p o s s i b l e e x p l a n a t i o n s f o r the o b s e r v e d s t e p l i k e a s c e n t o f the basement. 9. V i n e ' s i n t e r p r e t a t i o n o f R a f f and Mason's magnetic anomaly I l l map ( F i g u r e 3) f o r the Dellwood Seamount A r e a i s shown i n F i g u r e 46, T h i s i n t e r p r e t a t i o n was made b e f o r e i t was known t h a t the Revere - Dellwood F a u l t Zone e x i s t e d and i s a t r a n s f o r m f a u l t as shown by t h i s study,. U s i n g t h i s d a t a , m o d i f i e d i n t e r p r e t a t i o n s o f the magnetic anomaly map are shown i n F i g u r e s 47 and 48 where the main d i f f e r e n c e i s the age o f the p o s i t i v e anomaly i n the Dellwood Seamount A r e a . I n V i n e ' s i n t e r p r e t a t i o n , t h i s i s anomaly 2 which i s g r e a t e r than 2 myr. o l d . I n the a u t h o r ' s i n t e r p r e t a t i o n , t h a t p a r t o f the anomaly t o the n o r t h e a s t o f the Revere - Dellwood F a u l t Zone i s anomaly 1, a l l o f which i s • l e s s t h a n 2 myr, o l d a c c o r d i n g to the anomaly time s c a l e shown on the f i g u r e s . • The youngest p a r t o f t h i s anomaly 1 i s b e l i e v e d to be l e s s than 700,000 y r . o l d . T h i s l a t t e r i n t e r p r e t a t i o n i s s u p p o r t e d by the e s t i m a t e d age o f r o c k s from the Northwest D e l l -wood K n o l l s as shown i n Table V I I . Thus the i n t e r p r e t a t i o n s i n F i g u r e s 47 and 48 sup p o r t the h y p o t h e s i s o f a s p r e a d i n g c e n t r e i n the Del l w o o d Seamount Area c o n n e c t i n g the E x p l o r e r S p r e a d i n g Segment but o f f s e t from i t by the Revere - Dellwood T r a n s f o r m f a u l t t o the Queen C h a r l o t t e F a u l t Zone. 10. I f the s p r e a d i n g c e n t r e i s the channel-between the Dellwood K n o l l s , then t h i s channel s h o u l d have h i g h h e a t f l o w , normal f a u l t i n g , young r o c k s on bo t h s i d e s o f the cha n n e l w i t h age i n c r e a s i n g w i t h d i s t a n c e away from the cha n n e l and s t r o n g e s t mag-n e t i c i n t e n s i t y o v e r the c h a n n e l . The presence o f . h i g h heat f l o w and normal f a u l t i n g has a l r e a d y been e s t a b l i s h e d (above). The magnetic anomaly i n t e r p r e t a t i o n e i t h e r by V i n e , o r i n t h i s p r e s -ent s t u d y , s u g g e s t s age i n c r e a s i n g w i t h d i s t a n c e on.the n o r t h w e s t side., o f the c h a n n e l , but more d e t a i l e d magnetic mapping i n the FIGURE 47 Interpretation of magnetic anomalies assuming the Revere - Dellwood fault zone oripinated as l e f t - l a t e r a l transcurrent f a u l t . FIGURE 48 I n t e r p r e t a t i o n o f m a g n e t i c a n o m a l i e s a s s u m i n g t h e p e l l w o o d , S p r e a d i n g C e n t r e o r i g i n a t e d i i i s i t u l e s s t h a n 2 myr. ago Winona B a s i n i s n e c e s s a r y t o d e l i n e a t e any l i n e a r magnetic anom-a l i e s t h a t may e x i s t on the s o u t h e a s t s i d e o f the Dellwood Channel. On the o t h e r - h a n d , magnetic anomalies t h a t may have e x i s t e d i n t h e . p a s t i n the a r e a o f the Winona B a s i n have s i n c e been subdued by the t h i c k sediment c o v e r . As shown on l i n e IOUBC-70-16-20, the s t r o n g e s t magnetic i n t e n s i t y i s n o t - o v e r the Dellwood C h a n n e l , b u t . s l i g h t l y t o the s o u t h e a s t o f i t . A g a i n on l i n e IOUBC-70-16-15, the magnetic h i g h s are d i s p l a c e d to the s o u t h e a s t o f the t o p o g r a p h i c h i g h s , but the g e n e r a l shape o f the t o t a l magnetic i n t e n s i t y p r o f i l e - i s s i m i l a r to the t o p o g r a p h i c p r o f i l e . T h i s s o u t h e a s t d i s p l a c e m e n t o f the magnetic anomaly r e l a t i v e t o the topography may be due to the magnetic i n c l i n a t i o n i n the a r e a ; the t o p o g r a p h i c e f f e c t ; , and the d i s t a n c e above the sea f l o o r at which the magnetic f i e l d was measured. Thus, on l i n e IOUBC 70-16-20, the magnetic h i g h to the s o u t h e a s t o f the Dellwood Channel may have been produced by r o c k s i n the c h a n n e l . 11. The e x t e n t o f the w e a t h e r i n g and t h i c k n e s s o f the manganese c o a t i n g on r o c k s from EN 70-025-3D suggests t h a t the S o u t h e a s t , D e l l w o o d K n o l l s are o l d e r than the Northwest Dellwood K n o l l s when they s h o u l d be about the same age i f they were produced by a s p r e a d i n g c e n t r e i n the Dellwood Channel. T h i s o l d e r age o f the S o u t h e a s t Dellwood K n o l l s i s s u p p o r t e d by the t h i c k e r s e d i -ment c o v e r and subdued topography shown on p r o f i l e s FG ( P l a t e IV) and RS ( P l a t e X ) . Dredge EN 70-025-3D i s from a f a u l t e d s l o p e , and t h i s f a u l t i n g may; have exposed much o l d e r r o c k s . .More de-t a i l e d d r e d g i n g i s needed t o e s t a b l i s h the youngest age o f these k n o l l s . 115 12. O r i g i n o f the Dellwood Seamounts and K n o l l s : Each of the seamounts o f the Dellwood Seamount Range was probably i n i t i a t e d near the a x i s of the E x p l o r e r Spreading Segment, but continued to grow as they moved with the spreading s e a f l o o r (Menard, 1969), perhaps due to t h e i r p o s i t i o n adjacent to a zone of weakness -the Revere - Dellwood fault, zone. Hence the s h a l l o w e s t (highest) seamount of t h i s range i s the one f u r t h e s t away from the E x p l o r e r Spreading c e n t r e . This o r i g i n i s a l s o supported by the estimated age of the recovered p i l l o w b a s a l t s from t h i s range which show t h a t age i n c r e a s e s with d i s t a n c e from the E x p l o r e r Spreading Seg-ment. There i s at present.no c o n t i n u a t i o n of the E x p l o r e r Spreading Zone i n the Winona Ba s i n southeast of the Dellwood K n o l l s . How, then, d i d the k n o l l s o r i g i n a t e ? Perhaps there was i n the past such a c o n t i n u a t i o n now destroyed by subduction, and the formation o f the Dellwood K n o l l s was r e l a t e d to t h i s c o n t i n u a t i o n i n much the same way as the Dellwood Seamount Range i s r e l a t e d to the E x p l o r e r Spreading Segment. I f so, then the Northwest Dellwood K n o l l s should be o l d e r than the southeast ones. As s t a t e d above, the sparse dredging on the k n o l l s i n d i -cate the o p p o s i t e . I f the k n o l l s were produced by a spreading zone i n the Dellwood Channel, they should be about the same age. Perhaps, then, the spreading zone trends somewhere in- a.northeast d i r e c t i o n along the middle of the northwest k n o l l s . T h i s c o u l d produce o l d e r k n o l l s to the southeast f u r t h e r away from the spread-i n g centre than the younger northwest k n o l l s . The magnetic anomaly p a t t e r n , however, seems to preclu d e t h i s p o s s i b i l i t y . Perhaps, then, as a l r e a d y mentioned, the rocks o b t a i n e d from EN 70-025-3D 116 come f r o m a f a u l t e d s l o p e t h a t e x p o s e d o l d r o c k s w h i c h a r e n o t r e p r e s e n t a t i v e o f t h e y o u n g e s t e x t r u s i v e s o f t h e S o u t h e a s t . D e l l - , wood K n o l l s . The e v i d e n c e p r e s e n t e d t h u s seems t o i n d i c a t e t h a t a s p r e a d i n g zone does- e x i s t w i t h i n t h e D e l l w o o d Seamount A r e a , b u t f u r t h e r w ork i s n e e d e d t o d e t e r m i n e i t s p r e c i s e n a t u r e and l o c a - , t i o n . One a d d i t i o n a l q u e s t i o n r e m a i n s - How and when was t h e D e l l w o o d S p r e a d i n g Segment f o r m e d ? C o u c h and H e i n r i c k s (1971) p r o p o s e d t h a t t h e p r e s e n t n o r t h w e s t P a c i f i c P l a t e m o t i o n was e s t a b l i s h e d 8-12 myr. ago.. T h i s i s s u p p o r t e d b y A t w a t e r (1970 - F i g u r e 1 6 ) , whose m o d e l o f c o n s t a n t m o t i o n s p r o d u c e s a r e a s o f u n a c c e p t a b l e c o n t i n e n t a l and o c e a n i c o v e r l a p w h i c h i n d i c a t e s " t h a t t h e d i r e c t i o n o f P a c i f i c - N o r t h A m e r i c a n m o t i o n (P^) p r o b a b l y s u f f e r e d a t l e a s t a m i n o r c h a n g e s o m e t i m e b e t w e e n 20 and , 4 myr. a g o . " A f t e r t h e P a c i f i c P l a t e m o t i o n was e s t a b l i s h e d , t h e n , a c c o r d i n g t o C o u c h and. H e i n r i c h s ".... t h e San A n d r e a s and Queen C h a r l o t t e - F a i r w e a t h e r F r a c t u r e Zones became a c t i v e t r a n s f o r m f a u l t s , p o s s i b l y ' a l o n g o l d l i n e s o f w e a k n e s s .... B e c a u s e o f t h e d i r e c -t i o n o f s e p a r a t i o n , t h e s h e a r e n e r g y i n v o l v e d , and t h e c u r v a t u r e o f t h e c o n t i n e n t a l m a r g i n , t h e J u a n de F u c a r i d g e b e g a n m i g r a t i n g n o r t h w e s t e r l y and r o t a t i n g by g r o w t h and f r a c t u r e t o w a r d an a l i g n m e n t n o r m a l t o t h e t r a n s f o r m f a u l t s y s t e m s as t h e P a c i f i c P l a t e p u l l e d s l o w l y away f r o m t h e c o n t i n e n t a l m a r g i n o f f V a n c o u v e r I s l a n d . I n i t i a l l y t h i s was accommodated by c r u s t a l e x t e n s i o n , b u t a f t e r 5 - 6 , m i l l i o n y e a r s , a new r i d g e - t h e E x p l o r e r R i d g e was r e q u i r e d and f o r m e d . I n t h e l a s t 1 myr. a d d i t i o n a l s h o r t s e g m e n t s o f r i d g e h a v e b e e n r e q u i r e d and f o r m e d n o r t h w e s t o f t h e E x p l o r e r R i d g e " ( F i g u r e 48) . ' Thus Couch and H e i n r i c h s a c c o u n t f o r t h e f o r m a t i o n o f t h e D e l l W o o d S p r e a d i n g Segment n o r t h w e s t o f t h e E x p l o r e r R i d g e . , I n A t w a t e r ' s ( 1 9 70) m o d e l s ( F i g u r e s 15 and 1 6 ) , " r o t a t i o n o f t h e 117 Juan de Fuca r i d g e toward an a l i g n m e n t normal t o the t r a n s f o r m f a u l t systems i s a c c o m p l i s h e d by merely a change i n the d i r e c t i o n o f u n d e r t h r u s t i n g o f the Juan de Fuca P l a t e , t h i s r o t a t i o n caus-i n g the f o r m a t i o n o f the E x p l o r e r and presu m a b l y , more r e c e n t l y , the Dellwood S p r e a d i n g Segments. T h i s r o t a t i o n o f the Juan de Fuca P l a t e i s shown by the fan-shaped p a t t e r n o f the magnetic a n o m a l i e s i n the a r e a . P e t e r and L a t t i m o r e (1969) and P e t e r and DeWald (1971) , u s i n g a magnetic anomaly and f r a c t u r e p a t t e r n o f P a v o n i (1966), p r o p o s e d a r e c o n s t r u c t i o n " i n which the Juan de Fuca and Gorda r i d g e s form a s i n g l e c o n t i n u o u s n o r t h - s o u t h t r e n d i n g f e a t u r e . " A c c o r d i n g t o them, f o l l o w i n g the development o f a NW-SE zone o f weakness, c l o c k w i s e r o t a t i o n caused by NE-SW compres s i o n caused l e f t - l a t e r a l t r a n s c u r r e n t f a u l t i n g ( s i n c e Anomaly 5 - i . e . l e s s t han 10 myr. ago) and d i s p l a c e m e n t o f p a r t s o f the once c o n t i n u -ous r i d g e c r e s t and magnetic anomalies i n the a r e a . The t r a n s -c u r r e n t f a u l t s , once i n i t i a l d i s p l a c e m e n t has taken p l a c e , a c t as r i g h t - l a t e r a l t r a n s f o r m f a u l t s due t o a c t i v e growth o f the i n d i v i d u a l segments o f r i d g e . I f the h y p o t h e s i s i s c o r r e c t , the Dellwood S p r e a d i n g Segment c o u l d a l s o have been s i m i l a r l y formed by l e f t - l a t e r a l t r a n s c u r r e n t motion from the E x p l o r e r Ridge a l o n g the Revere - Dellwood F a u l t and then the sense o f movement a l o n g i t changed t o d e x t r a l by c o n t i n u e d a c t i v e growth i n the r e g i o n o f the Dellwood K n o l l s ( F i g u r e 4 7 ) . 118 CHAPTER SEVEN SUMMARY AND CONCLUSIONS Continuous s e i s m i c p r o f i l i n g i n • t h e D e l l w o o d Seamount A r e a i n d i c a t e s t h a t : 1. The Revere - Dellwood F a u l t Zone i s a r i g h t l a t e r a l t r a n s -form f a u l t o f the r i d g e t o r i d g e t y p e . I t t r e n d s a l o n g the southwest s l o p e s o f P a u l Revere Ridge and So u t h e a s t Dellwood K n o l l s and i n t o the Dellwood B a s i n between the Dellwood Seamount. Range and Dellwood K n o l l s . I t d i e s out i n a n o r t h w e s t e r l y d i r e c -t i o n and i s n o t ob s e r v e d i n the n o r t h w e s t end o f t h i s D ellwood Bas i n . 2. The Queen C h a r l o t t e r i g h t l a t e r a l t r a n s f o r m f a u l t appears t o d i e out i n a s o u t h e a s t e r l y , d i r e c t i o n i n the r e g i o n o f the S c o t t Channel n e a r i t s s o u t h e a s t end. F u r t h e r t o the s o u t h e a s t , i t i s r e p l a c e d by S c o t t I s l a n d s F a u l t Zone and the Winona B a s i n . 3. The Winona B a s i n c o n t a i n s g r e a t e r than 3 seconds 3000 m.) o f f o l d e d and f a u l t e d s e d i m e n t s , the f o l d i n g and f a u l t i n g p r o b a b l y b e i n g due t o some component o f com p r e s s i o n i n the a r e a produced by s u b d u c t i o n o f the Juan de Fuca Plate„ T h i s s u b d u c t i o n may have ceased or.changed t o s t r i k e - s l i p f a u l t i n g i n the r e c e n t p a s t . 4. The E x p l o r e r S p r e a d i n g Segment, c o n t a i n i n g o n l y a t h i n veneer o f s e d i m e n t s , shows d i p - s l i p f a u l t i n g a f f e c t i n g b o t h sediment and v o l c a n i c basement. T h i s s p r e a d i n g segment appears t o end a b r u p t l y a t the P a u l Revere Ridge - the s i t e o f the Revere - Dellwood t r a n s f o r m f a u l t i n the a r e a . 5. Normal f a u l t i n g a f f e c t i n g b o t h sediments and v o l c a n i c base-ment e x i s t s i n the Dellwood Channel between the Dellwood K n o l l s . 119 T h i s c h a n n e l c o n t a i n s >0.4 s e e s . (400 m.) o f sediments b e l i e v e d t o be t r a n s p o r t e d m a i n l y by t u r b i d i t y . c u r r e n t s i n t o the a r e a d u r i n g the P l e i s t o c e n e . 6. The P a u l Revere Ridge i s a c o m p l e x l y f a u l t e d r i d g e c o m p r i s i n g P l i o c e n e and P l e i s t o c e n e sediments and b a s a l t i c s i l l s and f l o w s . A magnetic low o c c u r s over the n o r t h w e s t e r n end o f the r i d g e . 7. The l o w e r c o n t i n e n t a l s l o p e o f f Queen C h a r l o t t e Sound i s f a u l t e d and t h i s may be the n o r t h e r l y e x t e n s i o n o f the S c o t t I s l a n d F a u l t Zone and a s i t e o f p r e s e n t or r e c e n t l y ceased sub-d u c t i o n i n the a r e a . T e c t o n i c sediment t r a p s (? d i a p i r s ) o c c u r beneath the s l o p e . Slumping i s prominent on the l o w e r s l o p e and i n the S c o t t Channel. F a u l t i n g o c c u r s beneath the n o r t h w e s t e r n p g r t o f the S c o t t Channel. The f a u l t i n g seen i n the n o r t h e a s t e r n p a r t o f the S c o t t Channel i s b e l i e v e d t o be the s o u t h e r n . e n d o f 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. 8. The sediment c o v e r over much o f the S o u t h e a s t Dellwood K n o l l s has subdued the topography and i n d i c a t e s t h a t these k n o l l s may be r e l a t i v e l y o l d . From an a n a l y s i s o f the r o c k s dredged from the Dellwood Seamount A r e a , the f o l l o w i n g c o n c l u s i o n s can be drawn: A. Most:of the fragments are p i l l o w b a s a l t s i n t e r m e d i a t e i n c h a r a c t e r between t h o l e i i t e s and a l k a l i b a s a l t s . B. The m i n e r a l o g y o f the b a s a l t s remains e s s e n t i a l l y the same - b y t o w n i t e ( A n 8 0 - 9 o ) p h e n o c r y s t s ( p r o b a b l y x e n o c r y s t s ) ; l a b r a d o r i t e ( A n 5 o - 6 o ) m i c r o p h e n o c r y s t s ; a n d e s i n e - l a b r a d o r i t e (An^g.go) g r o u n d m a s s ' m i c r o l i t e s • o l i v i n e b o t h as p h e n o c r y s t s ( p o s s i b l y x e n o c r y s t s ) F o 9 q , and i n groundmass c l i n o p y r o x e n e h a v i n g a f e a t h e r y , f i b r o u s o r g r a n u l a r h a b i t and r e s t r i c t e d 120 t o t h e g r o u n d m a s s ; and t a c h y l y t e g l a s s . The amounts o f t h e v a r i o u s m i n e r a l s p r e s e n t , h o w e v e r , v a r y c o n s i d e r a b l y . I n t h e p i l l o w b a s a l t s , d i f f e r e n t g roundmass t e x t u r e s a r e e n c o u n t e r e d - g l a s s y t h r o u g h h y a l o p i l i t i c a nd i n t e r s e r t a l t o i n t e r g r a n u l a r and s u b o p h i t i c - d e p e n d i n g e s s e n t i a l l y , o n t h e r a t e o f c o o l i n g w h i c h f u r t h e r depends on s i z e o f p i l l o w and d e p t h b e l o w s u r f a c e o f p i l l o w a t w h i c h t e x t u r e was o b s e r v e d . B l o c k l a v a s o f s i m i l a r m i n e r a l o g y , b u t e s s e n t i a l l y h o l o c r y s -t a l l i n e w i t h d i a b a s i c o r s u b o p h i t i c t e x t u r e s , were a l s o r e c o v e r e d . The r o c k s o f t h e N o r t h w e s t . D e l l w o o d K n o l l s a r e t h e y o u n g e s t ; and l e a s t d i f f e r e n t i a t e d r o c k s i n t h e . a r e a and may r e p r e s e n t r e l a t i v e l y r e c e n t r i d g e t y p e e x t r u s i o n s . T h e s e r o c k s a r e a l s o g l o m e r o p o r p h y r i t i c o l i v i n e - p l a g i o c l a s e b a s a l t s w h i l e most o f t h e o t h e r r o c k s i n t h e a r e a c o n t a i n o n l y s p a r s e pheno-c r y s t s o f p l a g i o c l a s e ( b y t o w n i t e A n 8 o - 9 o ) a n d / o r o l i v i n e . The f e a t h e r y t o f i b r o u s c h a r a c t e r o f t h e c l i n o p y r o x e n e ' i n many o f . t h e s e r o c k s may be due t o d e v i t r i f i c a t i o n and d e u t e r i c a l t e r a t i o n o f b a s a l t i c g l a s s . D redge H a u l IOUBC 70-16-12D f r o m t h e n o r t h w e s t e r n m o s t s e a - . mount o f t h e D e l l w o o d Seamount Range c o n t a i n s a c u r i o u s a s s o r t m e n t o f r o c k t y p e s i n c l u d i n g ( i ) a p o r p h y r i t i c p l a g i o -c l a s e b a s a l t whose p h e n o c r y s t s w e re p r o b a b l y o r i g i n a l l y a c c u m u l a t i n g as a p l a g i o c l a s e s e m i c u m u l a t e , t h e p l a g i o c l a s e p h e n o c r y s t s c o n t a i n i n g o p a q u e s ( m a g n e t i t e ) and s e m i o p a q u e s . ( t a c h y l y t e ) o u t l i n i n g g r o w t h z o n e s and t w i n n i n g , ( i i ) an i r o n d e p o s i t c o n t a i n i n g g r e a t e r t h a n 28% F e , ( i i i ) an i r o n -r i c h a u t h i g e n i c sandstone and ( i v ) g l a c i a l e r r a t i c s . H. Ages e s t i m a t e d from t h i c k n e s s o f manganese c r u s t s , t o g e t h e r w i t h the topography o f the Dellwood Seamount Range, i n d i c a t e t h a t the seamounts o f t h i s range were p r o b a b l y a l l i n i t i a t e d n e a r the a x i s o f the E x p l o r e r S p r e a d i n g Segment but c o n t i n u e d to grow f o r d i f f e r e n t l e n g t h s o f time as they moved w i t h the s p r e a d i n g c r u s t . ' The t h i c k n e s s o f manganese c r u s t s a l s o i n d i c a t e t h a t the S o u t h e a s t . D e l l w o o d K n o l l s may be much o l d e r than the Northwest ones. I . Two c h e m i c a l l y - , p e t r o g r a p h i c a l l y and p r o b a b l y a l s o g e n e t i c a l l y d i f f e r e n t t y p e s o f h y a l o c l a s t i t e b r e c c i a s o c c u r i n the a r e a . B o t h , however,,have an i r o n - r i c h m a t r i x . The JOIDES - P a u l Revere Ridge Core, the slump f e a t u r e s shown on the s e i s m i c p r o f i l e s , and the r e c o v e r e d g l a c i a l e r r a t i c s i n d i c a t e t h a t t u r b i d i t y c u r r e n t s are the major agent o f sediment t r a n s p o r t i n the a r e a , i c e r a f t i n g b e i n g somewhat l e s s i m p o r t a n t and p e l a g i c s e d i m e n t a t i o n p r o b a b l y b e i n g i m p o r t a n t o n l y d u r i n g " q u i e t " p e r i o d s . The i r o n and manganese i n the ferromanganese n o d u l e s and c r u s t and i n the i r o n d e p o s i t had p r o b a b l y been added to the sea water (from which i t p r e c i p i t a t e d ) from s e v e r a l s o u r c e s - c o n t i n e n t a l w e a t h e r i n g and t r a n s p o r t a t i o n by r i v e r s , g l a c i e r s and t u r b i d i t y c u r r e n t s ; l e a c h i n g o f Fe and Mn, from o c e a n i c b a s a l t s d u r i n g p r o g r e s s i v e c h e m i c a l w e a t h e r i n g by sea w a t e r ; and from submarine v o l c a n i c emanations. The s e i s m i c and p e t r o l o g i c d a t a when combined w i t h o t h e r d a t a on heat f l o w , m a g n e t i c s , e t c . , l e a d s the a u t h o r t o c o n c l u d e t h a t a spreading,segment does e x i s t w i t h i n t h e , D e l l w o o d 122 Seamount Area, connecting the E x p l o r e r Spreading Segment v i a the Revere - Dellwood Transform F a u l t Zone to the Queen C h a r l o t t e , r i g h t l a t e r a l t r a n s f o r m f a u l t , as shown i n F i g u r e 49. The p r e c i s nature and l o c a t i o n of t h i s Dellwood Spreading Segment r e q u i r e s f u r t h e r continuous s e i s m i c p r o f i l i n g , d e t a i l e d magnetic and heat flow data and more d e t a i l e d dredging and subsequent accurate age estimates based on the f i s s i o n t r a c k d a t i n g method. The author b e l i e v e s there are two p o s s i b l e l o c a t i o n s f o r t h i s Dellwood Spreading Segment - e i t h e r ( i ) The channel between the Dellwood K n o l l s with i t s normal f a u l t i n g and h i g h heat flow, or ( i i ) Trending n o r t h e a s t along the centre of the Northwest Dellwood K n o l l s with i t s young, r e l a t i v e l y u n d i f f e r e n t i a t e d b a s a l t s . Figure 49 summarises the author's i n t e r p r e t a t i o n of major t e c t o n i c f e a t u r e s of the Dellwood Seamount Area. The f e a t u r e named the Scott I s l a n d Subduction Zone i s the Scott I s -lands F r a c t u r e Zone of Couch (1969),.and o t h e r s . I t s width i s u n c e r t a i n and may, i n f a c t , i n c l u d e more of the Winona Basin than i n d i c a t e d . The d i r e c t i o n of u n d e r t h r u s t i n g i s u n c e r t a i n , but i s b e l i e v e d to be c l o s e to a n o r t h e r l y d i r e c t i o n at about 2 cm/yr - t h i s based on estimated values of about 6 cm/yr. f o r P a c i f i c - American p l a t e motion and s l i g h t l y l e s s than 6 cm/yr. f o r P a c i f i c - Juan de Fuca p l a t e motion by Atwater (1970) , and o t h e r s . The shape of the E x p l o r e r Spreading Segment i s based on the magnetic anomaly p a t t e r n i n the area and the e x i s t e n c e of extremely h i g h heat flow and f r e s h b a s a l t s i n the l e f t arm of anomaly 1. i n the area. Spreading i s b e l i e v e d to have r e c e n t -124 l y ceased i n , the a r e a o f the r i g h t arm o f t h i s anomaly, but became a c t i v e i n the a r e a o f the l e f t arm perhaps a t about the time t h a t the Dellwood S p r e a d i n g Segment came i n t o e x i s t e n c e i n i t s , p r e s e n t l o c a t i o n . The Dellwood S p r e a d i n g Segment may have once been c o n t i n u o u s w i t h , t h e E x p l o r e r S p r e a d i n g Segment, but was o f f s e t from i t by l e f t - l a t e r a l t r a n s c u r r e n t . f a u l t i n g ( l e s s than 10 m. y r . ago and p r o b a b l y l e s s than 2 m. y r . ago) a l o n g the,Revere • Dellwood F a u l t Zone, due to r o t a t i o n o f the Juan de Fuca P l a t e caused by a change i n the d i r e c t i o n o f the p l a t e movement as e v i d e n c e d by,the fan-shaped p a t t e r n o f magnetic anomalies i n the a r e a . 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T i f f i n , 1971, G e o p h y s i c a l d a t a c o l l e c t e d d u r i n g Hudson 70 Phase V I I o f f B r i t i s h C olumbia, A t l . Oceanogr, Lab Data S e r i e s No. 1971-5D. S r i v a s t a v a , S.P. , D.L. B a r r e t t , C.E. Keen, K.S. Manchester, K.G. S h i h , D.L. T i f f i n , R.L. Chase, A.G. Thomlinson, E. E. D a v i s , C.R.B. L i s t e r , 1.971, P r e l i m i n a r y a n a l y s i s o f g e o p h y s i c a l measurements n o r t h o f the Juan de Fuca Ridge: Can. J . E a r t h S c i . , 8_, 1265-1281 . T a l w a n i , M., X. Le P i c h o n § J.R. H e i r t z l e r , 1965, E a s t P a c i f i c R i s e - The magnetic p a t t e r n and f r a c t u r e zones: S c i e n c e , 15JV 1109-1115. Thomlinson,.A.G., W.G. B e r t r a n d § R.L. Chase, 1972, Catalogue d e s c r i p t i o n o f r o c k s dredged d u r i n g Hudson 70 Phase V I I , A t l . Oceanogr. Lab Data S e r i e s ( i n p r e s s ) . 130 T i f f i n , f y . L . , 1968, Continuous S e i s m i c R e f l e c t i o n P r o f i l i n g i n S t r a i t o f G e o r g i a , Ph.D. T h e s i s , U n i v e r s i t y o f B r i t i s h Columbia. T i f f i n , D.L., J.W. Murray § B. Cameron, 1972, T e c t o n i c and d e p o s i t i o n a l h i s t o r y of the c o n t i n e n t a l margin.west o f Vancouver I s l a n d : Can. J , E a r t h S c i . ( i n p r e s s ) , T o b i n , D.G. § L.R. Sykes,,1968, S e i s m i c i t y and t e c t o n i c s o f the N o r t h e a s t P a c i f i c : J . Geophys. Res., 7_3, 3821-3846. V i n e , F . J . , 1968, M a g n e t i c anomalies a s s o c i a t e d w i t h mid-ocean r i d g e s i n The H i s t o r y o f the E a r t h ' s C r u s t - A symposium e d i t e d by.R.A. P h i n n e y , ; P r i n c e t o n U n i v e r s i t y P r e s s V i n e , F . J . § T.J. W i l s o n , 1965, M a g n e t i c anomalies over a young o c e a n i c r i d g e o f f V a n c o u v e r . I s l a n d : S c i e n c e , 150, 485-489. W e t m i l l e r , R.J., 1969, An Earthquake Swarm on the Queen C h a r l o t t e I s l a n d F r a c t u r e Zone: M.Sc. T h e s i s , U n i v e r s i t y o f B r i t i s h Columbia. W i l s o n , T . J . , 1965, Transform f a u l t s , o c e a n i c r i d g e s and magnetic anomalies southwest o f Vancouver I s l a n d : S c i e n c e , 150, 482-485. ~~ Yoder, H.S, § T.G. Sahama, 1957, O l i v i n e - X - r a y d e t e r m i n a t i o n c u r v e : Am. M i n e r a l o g i s t , 4_2 , 475-491. Yoder, H.S. § C E . T i l l e y , 1962, O r i g i n o f b a s a l t magmas:; an e x p e r i m e n t a l s t u d y o f n a t u r a l and s y n t h e t i c . r o c k systems:, J . P e t r o l . , 3_, 342-532. Yoder, H.S., 1971, P e t r o l o g i c i m p l i c a t i o n s o f p l a t e t e c t o n i c s : . S c i e n c e , . 1 7 3 , 464-466. APPENDIX A SUMMARY OF TECTONIC AND PHYSIOGRAPHIC FEATURES AND THEIR CHARACTERISTICS AS SHOWN ON C.S.P. TECTONIC AND PHYSIOGRAPHIC FEATURES PROFILE ON WHICH FEATURE IS LOCATED PLATE LETTER PROFILE CHARACTERISTICS OF FEATURE AS SHOWN ON PROFILE EXPLORER SPREADING ZONE DELLWOOD SEAMOUNT RANGE I AB IOUBC 70-16-15 I AB IOUBC 70-16-15 I I BC IOUBC 70-16-16 I I I DD1 IOUBC 70-16-18 VI II ' IOUBC 70-16-21B X RS 1 EN 70-125-9 X I I I W BI 69-050-HUDSON 6 Very t h i n sediment cover on v o l c a n i c basement. Apparent d i p - s l i p f a u l t i n g . Magnetic i n t e n s i t y p r o f i l e i s s i m i l a r to topographic p r o f i l e , but appears s h i f t e d to the southeast of the topographic p r o f i l e . A l l p r o f i l e s show the Range to be mainly v o l c a n i c peaks forming rugged topography w i t h sediment i n pockets between peaks reaching 0.2 sees, t h i c k . SOUTHEAST DELLWOOD KNOLLS NORTHWEST DELLWOOD KNOLLS I I I DD' IOUBC IV FG IOUBC IX NO EN 70 V PQ EN 70 X RS EN 70 XII LL' BI 69 II BC IOUBC IV FG IOUBC V I I I MN EN 70 V PQ EN 70 X I I I VI W* HH' BI 69 IOUBC •20 Sediment covers most o f seamount. R e l a t i v e l y f l a t topography due to sediment cover; f a u l t e d NW § SE s l o p e s , magnetic p r o f i l e shows hig h near NW slope o f k n o l l s . Sediment i n pockets between v o l c a n i c peaks smoothes out topography. Sediment i n pockets between v o l c a n i c peaks. F a u l t e d at SE end of l i n e . F a u l ted SW slope of k n o l l s , r e l a t i v e l y f l a t topography. R e l a t i v e l y f l a t topography. Steep f a u l t i n t e r s e c t i n g NW slope of k n o l l s , and o v e r l y -i n g sediment cover. P o s s i b l e f a u l t i n g near NE end of t h i p r o f i l e may be r e l a t e d to the Queen C h a r l o t t e , f a u l t zone. Note c o n t r a s t i n g topography and sediment d i s t r i b u t i o n r e l a t i v e to the Southeast Dellwood K n o l l s . Cross the southeastern slope o f K n o l l s , APPENDIX A continued TECTONIC AND PHYSIOGRAPHIC FEATURES PROFILE ON WHICH FEATURE IS LOCATED PLATE LETTER PROFILE DELLWOOD SPREADING ZONE IV V FG PQ IOUBC EN 70-70-16-20 •025-3 I I CD' IOUBC 70-16-17 V I I I MN EN 70-•025-1 VI HH' IOUBC 70-16-21A SCOTT CHANNEL X I I I W BI 69- •OSO-HUDSON IX XI XI NO ST TU EN EN EN 70-70-70-025-2 025-10 025-11 V PQ EN 70- 025-3 . V I I I MN EN 70- 025-1 QUEEN CHARLOTTE FAULT ZONE VI HH1 IOUBC 70-12-21A X I I I W BI 69- 050-HUDSON V I I I MN EN 70- 025-1 CONTINENTAL SLOPE IX NO EN 70- 025-2 X I I I X I I W LL' BI BI 69-69-OSO-HUDSON OSO-HUDSON CHARACTERISTICS OF FEATURE AS SHOWN ON PROFILE Magnetic high i s not over the zone, but to the southeast of i t . Both p r o f i l e s show sediments i n the zone of t h i c k n e s s ^ 0.4 sees, d i s t u r b e d by apparent d i p - s l i p f a u l t s . Basement i s not shown due to l a c k o f sound p e n e t r a t i o n . Outcrop o f oceanic v o l c a n i c basement l a y e r produces mag-n e t i c h i g h . Contains f a u l t s r e l a t e d to the Queen C h a r l o t t e f a u l t zone Contains f a u l t s r e l a t e d to the Queen C h a r l o t t e f a u l t zone Contains f a u l t s r e l a t e d to the Queen C h a r l o t t e f a u l t zone Sediment t h i c k n e s s ^ 1 . 5 sees. Slump and s l i d e s t r u c t u r e s on NE slopes of channel. No f a u l t i n g . Channel i n subbottom r e f l e c t o r s , sediment t h i c k n e s s > 0.8 sees., f a u l t s below SE s i d e of channel slump s t r u c t u r e s on NW s i d e of channel. Apparent d i p - s l i p f a u l t i n g on SW s i d e o f deepest p a r t of Scott Channel. Apparent d i p - s l i p f a u l t i n g . Apparent d i p - s l i p f a u l t i n g on NE s i d e of deepest p a r t of channel. Sediment t h i c k n e s s ^ 1.5 sec. T e c t o n i c dam below base of s l o p e ; slump and s l i d e s t r u c -t u r e s , t e c t o n i c dam below upper s l o p e , upper c o n t i n e n t a l slope sediments are g e n t l y s l o p i n g ^ 2 and t r u n c a t e d by c o n t i n e n t a l slope f a u l t s . T e c t o n i c dams act as sediment traps.. S i m i l a r sediment d i s t r i b u t i o n as p r e v i o u s p r o f i l e , except t e c t o n i c dam below base o f slope not d i s t i n g u i s h e d . Gently d i p p i n g c o n t i n e n t a l s h e l f sediments. .APPENDIX A continued TECTONIC AND PHYSIOGRAPHIC FEATURES PROFILE ON MUCH FEATURE IS LOCATED PLATE LETTER PROFILE PAUL REVERE RIDGE I I I EF IOUBC 70-16-19 VII J J ' IOUBC 70-16-23 VII KK' IOUBC 70-16-26 VII J J ' IOUBC 70-16-23 WINONA BASIN V KK' IOUBC 70-16-26 XII LL' BI 69-050-HUDSON 3 I I I DD IOUBC 70-16-18 REVERE - DELLWOOD FAULT ZONE AND BASIN BETWEEN DELLWOOD SEAMOUNT RANGE AND DELLWOOD KNOLLS I I I EF IOUBC 70-16-19 X RS 1 EN 70-025-9 VI HH' IOUBC 70-16-21A VI I I ' IOUBC 70-16-21B VII J J ' IOUBC 70-16-23 X I I I W BI 69-050-HUDSON 6 XII LL' BI 69-050-HUDSON 3 II BC IOUBC 70-16-16 ZZ* Figure 24 CHARACTERISTICS OF FEATURES AS SHOWN ON PROFILE Faulted sediments and sedimentary rock over rugged v o l -c a n i c basement. Magnetic low over Ridge. R e l a t i v e l y u n d i s t u r b e d sediments over rugged v o l c a n i c basement. Magnetic low over Ridge. Faulted sediments adjacent to Paul Revere Ridge. S e d i -ments t h i c k e n toward c o n t i n e n t . Folded sediments near NE end of l i n e . R e l a t i v e l y u n d i s t u r b e d sediments which t h i c k e n toward c o n t i n e n t . Folded and f a u l t e d sediments g r e a t e r than 3 sees, t h i c k . Upper l a y e r s of sediment are un d i s t u r b e d . I n t r u s i o n i n b a s i n presumably f a u l t bounded as there i s apparent displacement of r e f l e c t o r on both s i d e s o f i n t r u -s i o n . F a u l t i n g a l s o along SW slope of k n o l l . Shows width of f a u l t zone over Paul Revere Ridge. S e d i -ment t h i c k n e s s i n b a s i n ^ 0.16 sees. Faulted SW slope o f k n o l l i s only evidence o f Revere -Dellwood f a u l t . Sediment t h i c k n e s s i n b a s i n > 0.15 sees. Sediment t h i c k n e s s i n b a s i n ^0.4 sees. No evidence of f a u l t i n g . Sediment t h i c k n e s s i n b a s i n ^ 0.7 sees. No evidence of f a u l t i n g . F a u l t i n g along SW slope o f Paul Revere Ridge may not be r e l a t e d to that i n Winona B a s i n . F a u l t i n g i n area at base of slope o f Paul Revere Ridge where sediment < 0.15 sees, t h i c k . F a u l t i n g i n b a s i n between Dellwood K n o l l s and Dellwood Seamount Range where sediment t h i c k n e s s > 0.5 sees. Sediment t h i c k n e s s i n b a s i n * 1 sec. No Revere - Dellwood F a u l t . Sediment t h i c k n e s s i n b a s i n ^ 0.3 sees. Shows sediment t h i c k n e s s i n b a s i n and p r o f i l e o f v o l c a n i c basement and sediment-water i n t e r f a c e . APPENDIX A continued TECTONIC PHYSIOGRAPHIC FEATURES PROFILE ON WHICH FEATURE IS LOCATED PLATE LETTER PROFILE CHARACTERISTICS OF FEATURE AS SHOWN ON PROFILE OTHER FEATURES XIII BI 69-050-HUDSON 6 St e p l i k e ascent of basement l a y e r between area SW o f Scott Seamount Range and Queen Charlotte f a u l t zone. Thickness of sediment above basement l a y e r - t h i c k e r (0.64 sec.) southwest of Scott Seamount Range than between Scott Seamount Range and Dellwood Seamount Range where i t i s 0.5 sec. t h i c k . A P P E N D I X B D R E D G E H A U L R O C K D E S C R I P T I O N D R E D G E S I T E L O C A T I O N D E P T H ( M e t e r s ) B R I E F D E S C R I P T I O N A N D R E M A R K S • N . L a t i t u d e W . L o n g i t u d e G e o g r a p h i c F e a t u r e F r o m T o I O U B C 7 0 - 1 6 - 1 2 D 5 0 ° 4 5 . 7 ' 1 3 0 ° S 3 . 0 ' D e l l w o o d S e a m o u n t R a n g e N W S e a m o u n t 8 0 0 6 0 0 ^ 2 3 k g . t o t a l . S e v e r a l d i s t i n c t r o c k t y p e s . I O U B C 7 0 - 1 6 - 1 2 D - 1 , O n e l a r g e f r a g m e n t o f a p o r p l i y r i t i c p l a g i o c l a s e - o l i v i n e b a s a l t w i t h a v e r y w e a t h e r e d v e s i c u l a r -g r o u n d m a s s . I O U B C 7 0 - 1 6 - 1 2 D - 2 , S e v e r a l s m a l l f r a g -m e n t s o f a r e d l a m i n a t e d i r o n d e p o s i t . I O U B C 7 0 - 1 6 - 1 2 D - 3 , S e v e r a l s m a l l f r a g -m e n t s o f a g r e e n l o o s e l y c o n s o l i d a t e d s a n d s t o n e . G l a c i a l e r r a t i c s > v 9 k g . o f v a r i o u s s i z e s a n d r o c k t y p e s E N 7 0 - 0 2 5 - 1 D 5 0 ° S 2 . 1 ' 1 3 0 ° 3 5 . 1 ' N o r t h w e s t D e l l w o o d K n o l l s ... ... D r e d g e d i d n o t t o u c h b o t t o m . E N 7 0 - 0 2 5 - 2 D 5 0 ° S 3 . 7 ' 1 3 0 ° 3 6 . 6 ' N o r t h w e s t D e l l w o o d K n o l l s 2 0 0 0 1 6 0 0 ^ 1 1 4 k g . t o t a l . ^ 9 0 % , p i l l o w f r a g m e n t s o f g l o m e r o p o r -p h y r i t i c o l i v i n e - p l a g i o c l a s e v e s i c u l a r b a s a l t w i t h h y a l o p i l i t i c g r o u n d m a s s , m a n g a n e s e r i n d o n l y . O n e f r a g m e n t o f h y a l o c l a s t i t e b r e c c i a . O n e l a r g e g l a c i a l f r a g m e n t ^ 2 k g . a n d s e v e r a l p e b b l e - s i z e a n d c o b b l e - s i z e g l a c i a l e r r a t i c s . E N 7 0 - 0 2 5 - 3 D 5 0 ° 4 6 . 0 ' 1 3 0 ° 2 4 . 0 " S o u t h e a s t D e l l w o o d K n o l l s 2 1 0 0 1 7 0 0 ^ 3 0 k g . t o t a l . S e v e n l a r g e w e a t h e r e d v e s i c u l a r p i l l o w f r a g m e n t s , o n e w i t h t h i c k m a n g a n e s e c r u s t ^  S O m m . S e v e r a l g l a c i a l e r r a t i c s o f v a r i o u s s i z e s a n d r o c k t y p e s . O n e f r a g m e n t o f a m a n g a n e s e i m p r e g n a t e d l o o s e l y c o n s o l i d a t e d s e d i m e n t . A P P E N D I X B c o n t i n u e d ™ " D R E D G E S I T E L O C A T I O N D E P T H ( M e t e r s ) N . L a t i t u d e W . L o n g i t u d e G e o g r a p h i c F e a t u r e F r o m T o B R I E F D E S C R I P T I O N A N D R E M A R K S E N 7 0 - 0 2 5 - 7 D 5 0 ° 1 8 . 3 * 1 3 0 ° 2 5 . 3 ' D e l l w o o d S e a m o u n t R a n g e S o u t h e a s t e r n e n d 2 0 0 0 1 9 0 0 ^ 2 3 0 k g . t o t a l . M a i n l y v e s i c u l a r b a s a l t i c p i l l o w f r a g -m e n t s w i t h h y a l o p i l i t i c t o i n t e r s e r t a l t e x t u r e s . A f e w f r a g m e n t s o f b a s a l t i c b l o c k l a v a s w i t h h o l o c r y s t a l l i n e t e x t u r e s . A 0 . 5 k g . m u d s a m p l e . N o g l a c i a l e r r a t i c s . V a r i a b l e t h i c k n e s s o f m a n g a n e s e c r u s t ( < 5 m m . t h i c k ) . E N 7 0 - 0 2 S - 8 D 5 0 ° 2 8 . 0 ' 1 3 0 ° 3 2 . 5 ' D e l l w o o d S e a m o u n t R a n g e S E S e a m o u n t 1 6 0 0 1 3 0 0 ^ 2 7 5 k g . t o t a l . S e v e r a l f r a g m e n t s o f b a s a l t i c p i l l o w l a v a s w i t h h y a l o p i l i t i c t o s u b o p h i t i c t e x t u r e s a n d b l o c k l a v a s w i t h d i a b a s i c t e x t u r e s . F r a g m e n t s v a r y c o n s i d e r a b l y i n s i z e f r o m a b o u t 0 . 5 k g . t o 4 0 - k g . V a r i a b l e t h i c k n e s s o f m a n g a n e s e c r u s t ( < 1 2 m m . t h i c k ) . O n e v e r y l a r g e ( ^ 3 0 k g . ) a n d a f e w s m a l l e r g l a c i a l e r r a t i c s . E N 7 0 - 0 2 5 - 9 D 5 0 ° 3 7 . 6 ' 1 3 0 ° 4 8 . 6 ' D e l l w o o d S e a m o u n t R a n g e M i d d l e S e a m o u n t 1 9 0 0 1 6 0 0 ^ 1 8 5 k g . t o t a l . M a i n l y f r a g m e n t s o f b a s a l t i c p i l l o w l a v a s w i t h h y a l o p i l i t i c t o i n t e r s e r t a l t e x t u r e s . S e v e r a l f r a g m e n t s o f h y a l o c l a s t i t e b r e c c i a . S e v e r a l f r a g m e n t s o f m a n g a n e s e n o d u l e s . L a y e r e d m a n g a n e s e c r u s t s u p t o 8 0 m m . t h i c k . A f e w g l a c i a l e r r a t i c s . N O T E : A m o r e d e t a i l e d d e s c r i p t i o n o f t h e r e c o v e r e d r o c k s p e c i m e n s i s t h e s u b j e c t o f a v o l u m e b y T h o m l i n s o n , B e r t r a n d a n d C h a s e i n A t l a n t i c O c e a n o g r a p h i c L a b o r a t o r y D a t a S e r i e s R e p o r t s . FIGURE 23 CONTOUR INTEff.'AL ?00 l,'FTPF<: •  138 FIGURE 50 Key t o S e i s m i c P r o f i l e s P l a t e s I t o X I I I VOLCANIC BASEMENT, d a s h e d where u n c e r t a i n GOOD REFLECTOR I N SEDIMENTS MODERATE REFLECTOR IN SEDIMENTS POOR REFLECTOR I N SEDIMENTS FAULTS AFFECTING VOLCANIC BASEMENT FAULT IN SEDIMENTS SIDE ECHO • V TECTONIC DAM UNCERTAIN F 2 0 DELLWOOD KNOLLS (SE) L E N G T H OF LINE 3 0 NAUT. Ml . A V E R A G E S P E E D 4-6 K N O T S V E R T I C A L E X A G G E R A T I O N 12 3 J I O lOkm. DELLWOOD SPREADING ZONE DELLWOOD KNOLLS WW) -146 + 4-0 40 149 6-or LENGTH OF LINE 13-5 NAUT. Ml. AVERAGE SPEED 4-9 KNOTS VERTICAL EXAGGERATION 13 ! l' 10 km. LENGTH OF LINE 12-5 NAUT. Ml. AVERAGE SPEED 6-3 KNOTS VERTICAL EXAGGERATION 17 I I 0 10 km. AWARDS: Shell Trinidad Scholarship 1966-1969 U.B.C. Graduate Fellowship 1970-1971 This form i s to be completed by candidates for the Master's or higher degree and submitted to the University Library Special Collections D i v i s i o n with the th e s i s . 

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