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SEISMIC TOMOGRAPHY CONSTRAINTS ON RECONSTRUCTING THE PHILIPPINE SEA PLATE AND ITS MARGIN A Dissertation by LINA HANDAYANI Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY December 2004 Major Subject: Geophysics SEISMIC TOMOGRAPHY CONSTRAINTS ON RECONSTRUCTING THE PHILIPPINE SEA PLATE AND ITS MARGIN A Dissertation by LINA HANDAYANI Submitted to Texas A&M University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Approved as to style and content by: Thomas W. C. Hilde Mark E. Everett (Chair of Committee) (Member) Richard L. Gibson David W. Sparks (Member) (Member) William R. Bryant Richard L. Carlson (Member) (Head of Department) December 2004 Major Subject: Geophysics iii ABSTRACT Seismic Tomography Constraints on Reconstructing the Philippine Sea Plate and Its Margin. (December 2004) Lina Handayani, B.S., Institut Teknologi Bandung; M.S., Texas A&M University Chair of Advisory Committee: Dr. Thomas W.C. Hilde The Philippine Sea Plate has been surrounded by subduction zones throughout Cenozoic time due to the convergence of the Eurasian, Pacific and Indian-Australian plates. Existing Philippine Sea Plate reconstructions have been made based primarily on magnetic lineations produced by seafloor spreading, rock magnetism and geology of the Philippine Sea Plate. This dissertation employs seismic tomography model to constraint the reconstruction of the Philippine Sea Plate. Recent seismic tomography studies show the distribution of high velocity anomalies in the mantle of the Western Pacific, and that they represent subducted slabs. Using these recent tomography data, distribution maps of subducted slabs in the mantle beneath and surrounding the Philippine Sea Plate have been constructed which show that the mantle anomalies can be related to the various subduction zones bounding the Philippine Sea Plate. The high velocity mantle anomalies are clearly coincident with Wadati- Benioff zones in the upper mantle. The lower mantle anomalies, although distributed in the “transition zone” (500-1000 km) as stagnant slabs in some cases, can clearly be iv mapped as continuations of upper mantle subduction zones. Reconstructing the subduction of the slabs now in the mantle best fits Philippine Sea Plate reconstructions that involve the minimal or simplest rotations. Northward movement of the Philippine Sea Plate, WNW subduction of the Pacific Plate since Eocene time (~50 Ma), and northward subduction of the Indian/Australian Plate along Indonesia best explain the subducted slab mantle anomalies. The origin of the eastern plate boundary was a transform zone that evolved into a subduction zone a few million years before the Pacific Plate changed its movement. In addition, the initiation of this subduction zone might possibly be one of the triggers of the Pacific Plate motion changes. The 90o rotation of the Philippine Sea Plate including southward plate subduction at its northern boundary proposed in the reconstruction by Hall (2002) is not supported by seismic tomography evidence for slab distribution in the mantle beneath the Philippine Sea region. A hypothesis of minimal rotation of the Philippine Sea Plate, supported by the seismic tomography, guides the reconstruction model presented. v ACKNOWLEDGEMENTS I would like to express my sincere thanks to Dr. Thomas Hilde, chairman of my advisory committee, for his expert guidance and unceasing support throughout all stages of this study. I am also grateful to the other members of my committee: Dr. Mark Everett, Dr. David Sparks, Dr. Rick Gibson, and Dr. William Bryant, for all suggestions and critical reviews during the course of this research. I benefited from the fruitful discussion with Dr. Sparks on the mantle and subducting slab properties, with Dr. Everett and Dr. Gibson on the basic of seismology and seismic tomography in particular. I am deeply indebted to Dr. Sri Widiyantoro from the Department of Geophysics and Meteorology, Institut Teknologi Bandung, for the tomography model he has provided, without which my work is not possible. I would also like to thank the Department of Geology and Geophysics of Texas A&M University for the financial support I received since the start of my doctorate study until the end of my stay through the teaching assistantship and several fellowships. I could not go through this process without the never ending support and encouragement from my dear husband and the endless love and patience of my daughters that has brightened my way through. vi TABLE OF CONTENTS Page ABSTRACT...................................................................................................... iii ACKNOWLEDGEMENTS.............................................................................. v TABLE OF CONTENTS.................................................................................. vi LIST OF FIGURES .......................................................................................... viii LIST OF TABLES............................................................................................ xi CHAPTER I INTRODUCTION ............................................................................... 1 Background.......................................................................................... 1 Motivation............................................................................................ 3 Research Objective .............................................................................. 5 II PREVIOUS STUDIES......................................................................... 7 Tectonic Setting ................................................................................... 7 Previous Reconstructions..................................................................... 19 Seismic Tomography in Geodynamic Studies..................................... 23 Geoid Anomalies over the Philippine Sea Plate .................................. 28 III TOMOGRAPHY DATA ANALYSIS ................................................ 31 Tomography Models............................................................................ 31 Cross Sections of Slab Images............................................................. 36 Japan Subduction Zone.................................................................. 41 Izu-Bonin Subduction Zone........................................................... 41 Ryukyu Subduction Zone ............................................................. 42 Mariana Subduction Zone.............................................................. 43 Philippine Subduction Zone........................................................... 43 Java Subduction Zone.................................................................... 44 Others............................................................................................. 45 Shallow Subducting Slab .................................................................... 46 vii CHAPTER Page Tomography and Seismicity Maps ...................................................... 47 Three Dimensional Slab Models.......................................................... 51 Subducted Slab Distribution Map........................................................ 55 IV SUBDUCTED SLAB ANALYSIS...................................................... 57 Phase Change....................................................................................... 57 Rheological Structure of Subducted Slabs........................................... 60 Trench Migration ................................................................................. 61 Horizontal Mantle Flow....................................................................... 64 Density Distribution............................................................................. 65 Sinking Rate of Descending Slab ........................................................ 68 V SUBDUCTED SLABS BENEATH THE PHILIPPINE SEA PLATE 72 The Age of Subducted Slabs................................................................ 72 Relocation of Past Subduction Zones .................................................. 81 VI DISCUSSION...................................................................................... 86 Tomography Constraints and Tectonic Implications........................... 86 Philippine Sea Plate Rotation......................................................... 86 Pacific Plate and Indian-Australian Plate Subduction ................... 89 Izu-Bonin – Mariana Subduction Zone.......................................... 92 Philippine Subduction Zone and Southern Philippine Sea Plate ... 95 Reconstruction ..................................................................................... 98 Reconstruction at 55 Ma ................................................................ 100 Reconstruction at 40 Ma ................................................................ 105 Reconstruction at 30 Ma ................................................................ 105 Reconstruction at 25 Ma ................................................................ 108 Reconstruction at 15 Ma ................................................................ 110 Reconstruction at 5 Ma .................................................................. 113 Present Tectonics ........................................................................... 113 VII CONCLUSIONS...............................................................................
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