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Chapter One: Introduction ADDIS ABABA UNIVERSITY COLLEGE OF NATURAL SCIENCES SCHOOL OF EARTH SCIENCES GRAVITY STUDIES OF THE CRUSTAL STRUCTURES BENEATH THE NORTHERN MAIN ETHIOPIAN RIFT AND THE ADJOINING EASTERN PLATEAU FITHAT HABTEWELD KASSA A thesis submitted to the school of Graduate studies of Addis Ababa University in partial fulfillment of the requirement for the Degree of Masters of Science in Exploration Geophysics Submission date 2014 ADDIS ABABA UNIVERSITY COLLEGE OF NATURAL SCIENCES SCHOOL OF EARTH SCIENCES GEOPHYSICAL STUDIES TO MAP CRUSTAL STRUCTURES BENEATH BOTH THE MAGMATIC SEGMENTS IN THE NORTHERN MAIN ETHIOPIAN RIFT AND THE EASTERN BORDER FAULTS By Fithat Habteweld Kassa Approved by board of examiners: Dr. Seifu Kebede .................................................................................................................................................. (Chairman, Department Signature Graduate Committee) Dr. Tilahun Mammo ……………………………………………………………………………………………… Advisor Signature Dr. Tigstu Haile ……………………………………………………………………………………………… Internal examiner Signature Dr. Tesfaye Kidane ……………………………………………………………………………………………… External examiner Signature Dr. Mulugeta Alene ……………………………………………………………………………………………… Chairman Signature Acknowledgements My profound gratitude goes to my advisor Dr Tilahun Mammo, for the proper supervision, support and guidance he provided me throughout my research. His critical comments, fatherly approach and unreserved effort have given me the opportunity to explore more. Special thanks to Geological Survey of Ethiopia for giving relevant data to this work. At last I would like to express my deepest heart felt gratitude to my family. Their care and encouragement has played important role throughout my academic life. i Table of contents Page Acknowledgements ..................................................................................................................... i Table of contents ........................................................................................................................ ii List of Figures ........................................................................................................................... iv List of Tables .............................................................................................................................. v Acronyms .................................................................................................................................. vi Abstract .................................................................................................................................... vii CHAPTER ONE: INTRODUCTION ......................................................................................... 1 1.1 General Overview ............................................................................................................. 1 1.2 Location of the study area ................................................................................................. 3 1.3 Statement of the problem .................................................................................................. 3 1.4 Significance of the study ................................................................................................... 4 1.5 Objectives of the study ...................................................................................................... 5 1.5.1General objective: ........................................................................................................ 5 1.5.2 Specific objectives: ..................................................................................................... 5 1.6 Methodology ..................................................................................................................... 5 1.7 Structure of the Thesis ....................................................................................................... 6 CHAPTER TWO: GEOLOGICAL AND TECTONIC SETTING ............................................ 7 2.1 Geological and tectonic setting of the Main Ethiopian Rift .............................................. 7 2.2.2 Seismicity and distribution of current deformation .................................................. 16 CHAPTER THREE: GEOPHYSICAL METHOD .................................................................. 19 3.1 THE GRAVITY METHOD ............................................................................................ 19 3.1.1 Fundamental Principles ............................................................................................ 19 3.1.4 The Geoids and the Reference Ellipsoid .................................................................. 24 3.1.5 Gravity corrections ................................................................................................... 24 CHAPTER FOUR : DATA ACQUISITION AND REDUCTION .......................................... 30 4.1 Data acquisition ............................................................................................................... 30 ii 4.2 Data reduction ................................................................................................................. 31 4.2.1 Free Air Anomaly Map ............................................................................................. 32 4.2.2 Complete Bouger Anomaly map .............................................................................. 33 4.3 Separation of Regional and Residual Anomalies ............................................................ 36 4.3.1 Residual anomaly map obtained from second order polynomial fitting. ................. 36 4.3.2 Upward continued Complete Bouger Anomaly map ............................................... 37 CHAPTER FIVE: DATA ANALYSIS AND INTERPRETATION ........................................ 39 5.1 Qualitative interpretations ............................................................................................... 39 5.1.1 Vertical derivative map (VD) ................................................................................... 39 5.1.2 Horizontal gradient map ........................................................................................... 40 5.1.3 Analytic signal (AS) map ......................................................................................... 41 5.1.4 Tilt derivative ........................................................................................................... 42 5.1.5 3D Euler deconvolution ............................................................................................ 43 CHAPTER SIX: QUANTITATIVE INTERPRETATION AND DISCUSSION .................... 47 6.1 Quantitative interpretation............................................................................................... 47 6.1.1 Gravity forward modeling ........................................................................................ 47 6.1.2 Initial model .............................................................................................................. 47 6.2 Interpretation of the models ............................................................................................ 49 6.2.1 Model along profile AA’ .......................................................................................... 50 6.2.2 Model along profile BB’ ........................................................................................... 51 6.2.3 Model along profile CC’ ........................................................................................... 52 6.2.4 Model along profile DD’ .......................................................................................... 53 6.2.5 Model along profile EE’ ........................................................................................... 55 6.3 Discussion ....................................................................................................................... 56 CHAPTER SEVEN: CONCLUSIONS AND RECOMMENDATIONS ................................. 59 7.1 Conclusions ..................................................................................................................... 59 7.2 Recommendations ........................................................................................................... 60 References ................................................................................................................................. 61 iii List of Figures Figure 1.1 An overview of Ethiopian rift and NMER ................................................................ 1 Figure 1.2 Location map of the study area ................................................................................. 4 Figure 1.3 Flow chart of the methodology.................................................................................. 6 Figure 2.1 Digital elevation model of the study area .................................................................. 9 Figure 2.2 Major faults and magmatic segments of the Northern Main Ethiopian Rift ........... 11 Figure 2.3 Geological map of the study area ............................................................................ 14 Figure 2.4 Structural map of the Northern MER ..................................................................... 15 Figure 2.5 Seismicity distribution in the Northern Main Ethiopian Rift .................................. 17 Figure 3.1 Gravity force per unit mass acting on a mass, m on earth ....................................... 21 Figure 3.2 components of forces acting on mass, m on earth’s
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