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Gravity and Structures of the Crust and Subcrust in the Northeast Pacific Ocean West of Washington and British Columbia

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Gravity and Structures of the Crust and Subcrust in the Northeast Pacific Ocean West of Washington and British Columbia AN ABSTRACT OF THE THESIS OF RICHARD WILLIAM COUCH for the DOCTOR OF PHILOSOPHY (Name) (Degree) in GEOPHYSICS presented on J I9 (Major) DateY Title: GRAVITY AND STRUCTURES OF THE CRUST AND SUB- CRUST IN THE NORTHEAST PACIFIC OCEAN WEST OF WASHINGTON AN RITISH COLUMBIA Redacted for Privacy Abstract approved: Gravity measurements made with LaCoste-Romberg surface ship gravity meter S-9, along 20, 000 ku-i of track line, off Washing-. ton an& British Columbia indicate the region is approximately in isostatic equilibrium.Free-air anomalies over the region average near zero except along the base of the continental slope.A -80 to -100 mgalfree-air anomaly occurs west of the Queen Charlotte Islands over the partially sediment-filled Queen Charlotte trough. A free-air anomaly greater than -l50mgal occurs over the Scott Is lands fracture zone off the north end of Vancouver Island.The fracture zone, 170 to 240 km long, exhibits minor bathymetric relief and is interpreted as a graben-like structure filled with approximately 6 km of sediments.Five crustal and subcrustal cross sections consistent with seismic refraction and gravity data indicate that the Mohorovicic (Moho) discontinuity is at depths of approximately 25 to 28 km in the Insular Belt of British Columbia and 19 km in western Washington.In the areas of the Cascadia Abyssal Plain and continental rise off British Columbia, the com-. bination of near-zero gravity anomalies and thin crust indicates that mantle densities are less than under the adjacent continent and Tufts and Alaskan abyssal plains.Still lower densities occur in the upper mantle immediately beneath the Juan de Fuca and Explorer ridges where Moho depths are less than 7 km. Gravity and Structures of the Crust and Subcrust in the Northeast Pacific Ocean West of Washington and British Columbia by Richard William Couch A TNESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy June 1969 APPROVED: Redacted for Privacy Professor of Oceanogphy in charge of major Redacted for Privacy Chairm\an of the Departr4ient of Oceanography Redacted for Privacy Dean of Graduate School Date thesis is presented Typed by Gwendolyn Hansen for Richard William Couch AC KNOW LEDGEMENTS This research was performed under the direction of Dr. Peter Dehlinger, Professor of Geophysics at Oregon State University.I am grateful to Dr. Dehlinger for providing theopportunity to do this research, for his many suggestions in the analysis of the data and for editing the manuscript. I am grateful to Messrs. E. Robey Banks and Michael Gemperle for their assistance in making gravity measurements at sea and in applying computer operations to gravity problems.Their willingness and ability to help were a constant encouragement. Appreciation is also extended to Dr. John N. Gallagher for assistance in making gravity measurements at sea and for many helpful discussions about the research. Professor Gunnar Bodvarsson and Professor Tjeerd H. van Andel reviewed the thesis and made many helpfulcomments. This thesis reflects the patience and encouragement of Carol, my wife. During my tenure as a graduate student, I was the recipient of a most appreciated National Aeronautics andSpace Administration fellowship. This research was supported by the Office of Naval Research under contracts Nonr 1286(09) and (10).For this assistance I am very grateful. TABLE OF CONTENTS Page INTRODUCTION 1 HISTORICAL DEVELOPMENT 3 THEORETICAL CONSIDERATIONS 6 PREVIOUS WORK 9 Physiography 9 Geology 12 Geophysics 14 Seismic Refraction and Reflection Measurements 14 Earthquake Observations 19 Heat Flow 25 Magnetics 26 Gravity 31 NEW GRAVITY DATA 36 Surface-Ship Gravity Measurements 36 Reduction of Gravity Data 41 Distribution of Gravity Measurements 46 The Free-Air Gravity Anomaly Maps 48 INTERPRETATION OF THE FREE-AIR GRAVITY ANOMALY MAPS 51 Gravity Anomaly Trends 51 Isostatic Equilibrium in the Northeast Pacific Ocean 52 Fjords of the British Columbia Coast 54 Gravity Anomaly Trends in Dixon Entrance, Hecate Strait and Queen Charlotte Sound 55 The Continental Shelf Off Washington and Vancouver Island 58 The Randsenken Effect Along the Continental Margin of Washington and British Columbia 60 The Queen Charlotte Trough 63 A Postulated East-West Fracture Zone Near 51° N Latitude 65 Page The Scott Islands Fracture Zone 67 Seamounts 69 The Explorer Trough 72 The Cascadia Abyssal Plain 82 CRUSTAL AND SUBCRUSTAL CROSS SECTIONS ACROSS THE CONTINENTAL MARGINS OF WASHINGTON AND BRITISH COLUMBIA 84 Construction of the Crustal and Subcrustal Cross Sections 84 Sources of Error in the Cross Sections 88 The Central Washington Cross Section 90 The South Vancouver Island Cross Section 96 The Scott Islands Cross Section 102 The Moresby Island Cross Section 109 The Dixon Entrance Cross Section 115 The Continental Margin Structures off Washington and British Columbia 119 CONCLUSIONS 122 REGIONAL TECTONICS 124 BIBLIOGRAPHY 133 APPENDICES 148 Appendix 1.The Reduction of Gravity Measurements 148 Appendix 2.Supplementary Data 161 Appendix 3.Two Dimensional Gravity Computations 171 LIST OF FIGURES Figure Page 1.Physiographic diagram of the northeast Pacific Ocean off Washington and British Columbia. 11 2.Location map of seismic refraction lines and heat flow stations off the coasts of Washington and British Columbia. 15 3.Earthquake epicenters in the Pacific Northwest. 21 4.Total magnetic anomaly map of the region west of Washington and British Columbia. 28 5.Map of gravity meter track lines off the coasts of Washington and British Columbia. 33 6.Free-air gravity anomaly map of the region off the west coast of Washington and British Columbia from the Columbia River to Cape Scott. 42 7.Free-air gravity anomaly map of the region off the west coast of British Columbia from Cape Cook to Dixon Entrance. 43 8.Bathymetric chart of the Explorer trough and Scott Islands fracture zone. 74 9.Bathymetric, gravity and magnetic profile across Explorer trough along line X-X'. 76 10.Bathymetric, gravity and magnetic profile across Explorer trough along line Y-Y'. 77 11.Bathymetric, gravity and magnetic profile across Explorer trough along line Z-Z'. 78 12.Location map of crustal and subcrustal cross sections. 85 13.Graph of the Nafe-Drake relation between compres- sional wave velocity and rock density. 87 Figure Page 14.Central Washington crustal and subcrustal cross section BB'. 91 15.South Vancouver Island crustal and subcrustal cross section CC'. 97 16.Scott Islands crustal and subcrustal cross section DD'. 101 17.Free-air gravity anomaly map of the Scott Islands fracture zone. 106 18.Total magnetic intensity map of the Scott Islands fracture zone. 10.7 19.Bathymetric, gravity and magnetic profile across the Scott Islandsfracture zone along line SS'. 108 20.Moresby Island crustal and subcrustal cross section EE'. 110 21.Total magnetic intensity map off Moresby Island. 113 22.Dixon Entrance crustal and subcrustal cross sectithI FF'. 116 23.Diagram illustrating equation parameters for com- puting gravity over a cone structure. 165 24.Polygon illustrating equation parameters for two- dimensional gravity computations. 171 LIST OF TABLES Table Page 1.Information on the gravity anomaly size and gravity station density in different physiographic provinces 47 2.Seamount load and compensation parameters .71 3.Seamount locations, dimensions and gravity 170 GRAVITY AND STRUCTURES OF THE CRUST AND SUBCRUST IN THE NORTHEAST PACIFIC OCEAN WEST OF WASHINGTON AND BRITISH COLUMBIA INTRODUCTION The northeast Pacific Ocean is an area of great interest to geologists and geophysicists because of its diverse structures, par- ticularly as these structures have global geotectonic implications. Descriptions and studi.es of various parts of this region have been made over a period of years. Bathymetric recordings showed a smooth floor, termed by Hurley (1960) the Cascadia Abyssal Plain, immediately west of the generally narrow continental shelf and steep continental slope off Washington and British Columbia. Between Tufts Abyssal Plain, an area of smooth sea floor approximately 300 miles west of Washington and British Columbia, and the Cascadia Abyssal Plain lies a region of abyssal hills and mountains.This region, first termed the Ridge and Trough Province by Menard (1955b),is now considered a seg- ment of the world-encircling mid-ocean ridge and rise system (Menard, 1960, 1964).In this same region Raff and Mason (1961) made magnetic measurements which showed the now classical long magnetic anomaly lineatilons.Vine and Mathews (1963) suggested that the magnetic anomalies, which are now found paralleling the mid- ocean ridge system in all oceans, represented a magnetic recording of sea floor spreading, a still controversial hypothesis first postulated by Hess (1962) and Dietz (1961).Only recently have suf- ficient data become available to permit a study of the structures and dynamics of this region. The surface of the solid earth west of Washington and British Columbia is covered by approximately 3. 5 km of water; consequently, direct observations of the physical and structural form of the earth in this region are limited.In this environment geophysical measure- ments provide valuable indirect observations which contribute greatly to the understanding of the earth beneath the sea. This study is an effort to provide a description of the earth's gravity field in the oceanic region contiguous with Washington and British Columbia and to relate these measurements to possible structures of the earth' s crust and subcrust.The initial part of this study is concerned with the construction of free-air gravity maps of the region which help to delineate and delimit the major structures of the region. The final portion of this study combines the gravity information with other geophysical data, such as seismic refraction measurements, to form hypothetical crust and subcrustal cross sec- tions of the major geologic structures, notably the Juan de Fuca Ridge- -a portion of the mid-ocean rise system- -and the continental margin region where the relatively thin oceanic crust adjoins the much thicker continental crust.
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