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A Geophysical Study of the Romanche Transform and Surrounding Areas of the Mid-Atlantic Ridge

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A Geophysical Study of the Romanche Transform and Surrounding Areas of the Mid-Atlantic Ridge A GEOPHYSICAL STUDY OF THE ROMANCHE TRANSFORM AND SURROUNDING AREAS OF THE MID-ATLANTIC RIDGE by MARTIN VAUGHAN THOMAS A thesis submitted in candidature for the degree of Doctor of Philosophy of the University of London Department of Geological Sciences University College of London September 1989 ProQuest Number: 10797802 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a com plete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest ProQuest 10797802 Published by ProQuest LLC(2018). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States C ode Microform Edition © ProQuest LLC. ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106- 1346 2 ABSTRACT Using a combination of GLORIA long range side-scan sonar, bathymetry, seismic reflection profiles, free-air gravity and total magnetic field data, this study looks at the morphology and structure of an area centred on the Romanche Transform in the Equatorial Atlantic. It also includes portions of both the active and inactive sections of the Saint Paul Fracture Zone, together with adjacent seafloor areas. The main tectonic and structural features are mapped from GLORIA sonographs. In particular, sonar records enable precise mapping of the plate boundary separating the African and South American plates. The identification of the Romanche Transform Fault allows a direct comparison between the direction of local plate motion and that predicted by global plate tectonic models. The overall trend of the Romanche Transform o Fault is about 3 north of that predicted from the recent global models. Gravity data, after correction for topography, sediment thickness and a crustal layer of average thickness and density, show regional anomalies of up to lOOmgal across the Romanche Fracture Zone. This phenomenon is attributed to the different ages and hence contrasting thermal and density structures of juxtaposed lithospheric blocks in fracture zones. These regional fields are isolated and removed using the method of upward continuation. Residual anomalies are modelled by local changes in the thickness and density of crust and reductions in upper mantle density using the results of 3 seismic refraction experiments over fracture zones as constraints on possible solutions. Modelling indicates a lowering of crustal and, probably, upper mantle density beneath the Romanche Transform Valley. It also suggests that the crustal, and upper mantle structure, is generally different beneath the Saint Paul Fracture Zone fracture valleys and Romanche Transform Valley. Factors affecting magnetic anomalies associated with fracture zones located within the study area are examined. Two-dimensional modelling of total field magnetic profiles indicates that the intensity of magnetisation of crustal source rocks is inhomogeneous within the Romanche Fracture Zone. This modelling also indicates that crustal source rocks are weakly magnetised within the southern fracture valley of the Saint Paul Fracture Zone. 4 ACKNOWLEDGEMENTS In presenting this thesis I should like to thank my supervisor, Dr. E.J.W. Jones, for his guidance and support. My thanks also goes to Dr. R.C. Searle of the Institute of Oceanographic Sciences (I.O.S.) for advice on the interpretation of GLORIA sonar records. I also thank the officers and crew of the RRS Discovery cruise 142, and the technicians of Research Vessel Services in Barry and the members of staff at I.O.S. Wormley who participated on this cruise? which provided the bulk of the data used in this thesis. Finally, I should like to thank my mother for her support. 5 CONTENTS Title Page 1 Abstract 2 Acknowledgements 4 Table of Contents 5 List of Tables 13 List of Figures 14 List of Plates 20 Chapter 1 INTRODUCTION 21 1.1 Characteristics of oceanic fracture zones 21 1.1.1 Fracture zones and plate tectonics 21 1.1.2 Mapping oceanic fracture zones from 24 the sea surface 1.1.3 Terminology used for the description 25 of fracture zones 1.1.4 Large scale tectonic patterns within 29 and in the vicinity of fracture zones 1.1.5 Variation in morphology of transform 32 6 zones with spreading axis offset and differences in spreading rate 1.1.6 Detailed observations from manned 37 submersibles and deep-towed cameras 1.1.7 Petrology of rocks from fracture zones 39 1.1.8 Evidence for abnormal crustal and 40 upper mantle structure from seismic refraction experiments 1.1.9 Gravity anomalies over fracture zones 44 1.1.10 The magnetic signature of fracture 46 zones 1.2 Fracture zone offsets in the Equatorial 47 Atlantic 1.3 Area of investigation 50 1.4 Objectives of study 51 Chapter 2 DATA ACQUISITION AND REDUCTION 52 2.1 Data sources and coverage 52 2.2 "Discovery" cruise 142 (1983) - Dakar 57 (Senegal) to Dakar: instrumentation and data processing 2.2.1 Navigation 57 2.2.2 GLORIA sonar 57 7 2.2.3 Precision echo-sounding (P.E.S) 64 2.2.4 Seismic reflection profiling (S.R.P) 65 2.2.5 Measurement and reduction of gravity 65 data 2.2.6 Acquisition of total field magnetic 68 intensity data 2.3 Reduction of total field magnetic intensity 68 values to magnetic anomalies 2.3.1 Cross-over analysis 69 Chapter 3 MORPHOLOGY OF THE ROMANCHE TRANSFORM 80 ZONE AND SURROUNDING AREAS FROM GLORIA SONOGRAPHS, BATHYMETRY AND SEISMIC REFLECTION PROFILES 3.1 Introduction 80 3.2 Interpretation of GLORIA sonographs 82 3.2.1 Factors affecting character of return 82 signal 3.2.2 Identification of seafloor features 87 3.3 Data presentation 90 3.3.1 Bathymetric compilation 90 3.3.2 Summary chart of main tectonic, 90 volcanic and topographic features 3.3.3 Sediment distribution chart 91 3.4 The Romanche Fracture Zone 91 3.4.1 The transform valley 91 3.4.2 Shallow topography flanking the 97 transform valley 3.5 The eastern Romanche ridge-transform 102 intersection and plate boundary between the Romanche and Chain Fracture Zones 3.6 Spreading centre between the Romanche and 109 Saint Paul Fracture Zones and the ridge- transform intersections 3.7 The Saint Paul Fracture Zone 119 3.7.1 The Saint Paul Transform Zone 119 3.7.2 The inactive eastern extension of the 124 Saint Paul Fracture Zone 3.8 Seafloor areas between the major fracture 125 zones 3.8.1 Region between the Romanche Fracture 125 Zone and the eastern inactive Saint Paul Fracture Zone 3.8.2 Region north of the Saint Paul 126 Fracture Zone 3.8.3 Region south of the Romanche Fracture 126 Zone 9 3,8.4 Discussion of the oblique trough and 133 other oblique features south of the Romanche Fracture Zone 3.9 Plate motions in the Equatorial Atlantic 136 Chapter 4 INVESTIGATION OF CRUSTAL AND UPPER MANTLE 139 STRUCTURE BENEATH THE ROMANCHE AND SAINT PAUL FRACTURE ZONES FROM FREE-AIR GRAVITY ANOMALIES 4.1 Free-air gravity anomaly map 139 4.2 Factors influencing free-air gravity anomalies 141 over fracture zones 4.2.1 Isolation of gravitational effects 147 due to variations in crustal and upper mantle structure 4.3 The gravitational edge-effect across fracture 156 zones and its removal 4.3.1 Upward continuation: outline theory 161 4.4 Upward continuation of observed free-air 165 gravity profiles 4.5 Two-dimensional modelling of local residuals 180 4.5.1 The seismic velocity and density 181 structure of oceanic crust as observed from seismic refraction experiments conducted over fracture zones in the Atlantic 10 4.5.2 Upper mantle density 186 4.5.3 Constraints on the values of crustal 187 and upper mantle densities used for modelling 4.5.4 Discussion of modelled results 188 Chapter 5 ANALYSIS OF MAGNETIC PROFILES CROSSING 220 THE ROMANCHE AND SAINT PAUL FRACTURE ZONES 5.1 Total field intensity magnetic anomaly map 220 5.2 Assumption used in magnetic modelling 222 5.3 Source of marine magnetic anomalies 225 5.4 Investigation of seafloor spreading anomalies 232 and anomalies associated with fracture zones within the study area 5.4.1 Seafloor spreading anomalies 232 5.4.2 Fracture zone anomalies 237 5.5 Comparison of multi-layer and single-layer 251 sources for magnetic anomalies 5.6 Predictive modelling of magnetic anomalies 258 across the Romanche and Saint Paul Fracture Zones 5.6.1 Results 259 11 5.7 Iterative modelling of magnetic anomalies 271 across the Romanche and Saint Paul Fracture Zones 5.7.1 Forward modelling solutions 273 5.8 Discussion of results of magnetic modelling 281 5.8.1 Romanche Fracture Zone 281 5.8.2 Saint Paul Fracture Zone 283 Chapter 6 SUMMARY AND CONCLUSIONS 285 6.1 Morphology and tectonic features 285 6.2 Studies of free-air gravity anomalies 290 6.3 Studies of total field magnetic anomalies 292 Oiscusscon cf ohyerrccterix afxi. ftzs o fe ZIH a 6 S Fbfrurc2_ Xh ^ Appendix 1 NAVIGATION USING SATELLITE FIXES AND 295 TWO-COMPONENT ELECTROMAGNETIC LOGS DURING RRS "DISCOVERY" CRUISE 142 Al.l Satellite fixes 295 A1.2 Dead reckoning between satellite fixes 296 Appendix 2 SCREEN CURSOR ROUTINE FOR INTERACTIVE 298 TWO-DIMENSIONAL GRAVITY MODELLING A2.1 Introduction and basic requirements of 298 program 12 A2.2 Instructions for operating CURG 29 9 A2.2.1 Input file data 299 A2.2.2 Running the program 302 A2.2.3 The control screen 303 A2.2.4 Using the interactive screen display 303 A2.3 CURG: Program listing 311 Appendix 3 SCREEN CURSOR ROUTINE FOR INTERACTIVE 342 TWO-DIMENSIONAL MODELLING A3.1 Introduction and basic requirements of 342 program A3.2 Instructions for operating CURM 343 A3.2.1 Input file data 343 A3.2.2 Running the program 343 A3.2.3 The control screen 346 A3.2.4 Using the interactive screen display 346 A3.3 CURM: Program listing 354 References 378 13 LIST OF TABLES 2.1 Data sources and types.
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