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The Lithosphere of the East African Rift and Plateau (Afar-Ethiopia-Turkana): Insights from Integrated 3-D Density Modelling

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The Lithosphere of the East African Rift and Plateau (Afar-Ethiopia-Turkana): Insights from Integrated 3-D Density Modelling The Lithosphere of the East African Rift and Plateau (Afar-Ethiopia-Turkana): Insights from Integrated 3-D Density Modelling Dissertation Zur Erlangung des Grades eines Doktors der Naturwissenschaften an der Mathematisch-Naturwissenschaftlichen Fakultät der Christian-Albrechts-Universität zu Kiel, Germany Vorgelegt von Girma Woldetinsae aus Addis Ababa, Äthiopien Kiel 2005 Referent : Prof. Dr. H.-J. Götze Korreferenten: Prof. Dr. Ron Hackney Tag der mündlichen Prüfung: 12. Juli 2005 Zum Druck genehmigt: 24. August 2005 Table of Contents page List of figures ii List of tables iii Important abbreviations iv Acknowledgements v Abstract vi Extended abstract (in German) viii 1.0 Introduction 1.1 Overview 1 aims and Scope of the Study 2.0 Topography, geological and tectonic settings 2.1 Topography 5 2.2 Geological and tectonic setting 7 3.0 Constraining information and data 3.1 Regional heat flow 10 3.2 Constraints from lithospheric scale seismic studies 12 3.2.1 Seismic studies 12 3.2.2 Global shear wave structures 13 3.3 Density determination 15 3.3.1 Velocity to density conversion 15 3.3.2 Measured densities 18 3.3.3 Density calculations using chemical composition 20 4.0 Potential field database 4.1 Aeromagnetic data coverage 26 4.2 Gravity database 27 4.2.1 Source of data and coverage 28 4.2.2 Data evaluation re processing and reduction 28 4.2.3 Gravity data gaps and alternative solutions 36 Merging Satellite gravity data (GRACE) 4.2.4 Free air anomaly and Bouguer gravity 38 4.2.5 Curvature of Bouguer gravity 39 4.2.6 Geoid 42 5.0 Isostasy and isostatic state of the area 5.1 Methods, analysis and models 45 5.2 Dynamic topography and dynamic compensations 47 5.3 Interpretation of isostatic and “iso-dynamic” models 48 5.3.1 Isostatic regional field 50 5.3.2 Isostatic residual field 56 Precambrian domains Rift system 5.3.3 Comparison of different isostatic models 60 i 6.0 Integrated three-dimensional density modelling 6.1 Forward modelling in gravity interpretation 63 6.2 Three dimensional modelling using the functionality of GIS 65 6.3 The initial model and general assumptions 67 The reference model 6.4 Interpretation of modelling results 70 6.4.1 Crustal models and rift structures 72 6.4.2 Vertical and Horizontal cross-sections (Earth models) 74 Vertical cross-sections Southern Afar-MER-Turkana (rift axial) Nubia-MER-Somalia (rift perpendicular) Horizontal cross sections 6.4.3 Volume and mass of bodies and load maps 80 6.4.4 Deep lithospheric structure 85 6.4.5 Model sensitivity (density) 89 6.5 Rigidity of the East African lithosphere 91 7.0 Discussion and conclusions 7.1 Discussions Why rifts are important 100 Extensions and rift models 100 Moho and residual Bouguer gravity 102 Melts and plumes 108 Depth to the top of basement 111 7.2 Summary conclusions 113 7.3 Further work and recommendation 115 References List of figures page Fig. 1 Regional set up of the study area 1 Fig. 1.1 Detail location map of the study area 3 Fig. 2.1 Topography 8 Fig. 3 Regional geology and tectonic set up 11 Fig. 3.1 Shear wave tomography 14 Fig. 3.2 Measured densities in Main Ethiopian Rift (MER) 19 Fig. 3.3 Location map for chemical compositions 21 Fig. 3.4 Density calculated from chemical composition data Turkana/S.Eth Rift 22 Fig. 3.5 Measured and calculated velocity and density models in MER 23 Fig. 3.6 Density calculated from chemical composition of MER lavas 24 Fig. 4 Aeromagnetic coverage of Ethiopia 26 Fig. 4.1 Gravity stations acquired for the study 30 Fig. 4.2 Data validation and adjustment using DBGRAV software 33 Fig. 4.3 Bouguer anomaly map 35 Fig. 4.4 Merged satellite and land data 37 ii Fig. 4.5 Definition of curvature 40 Fig. 4.6 Bouguer curvature attributes 41 Fig. 4.7 Geoid from EGM96 model 43 Fig. 5 Isostatic models 49 Fig. 5.1 Isostatic residual field 51 Fig. 5.2 Isostatic residual field with dynamic topography correction (Grand) 54 Fig. 5.3 Isostatic residual field with dynamic topography correction (Slab) 55 Fig. 5.4 Histogram of isostatic residual field 53 Fig. 5.5 Summary of calculated Mohos 61 Fig. 6 Construction of IGMAS 3-D model 65 Fig. 6.1 Flow chart for general process of 3-D model 67 Fig. 6.2 Initial and reference model 69 Fig. 6.3 Measured and modelled Bouguer anomaly 71 Fig. 6.4 N-S variation of crustal/upper mantle structures 73 Fig. 6.5 Rift axial and rift perpendicular profiles (Earth models) 75 Fig. 6.6 Simplified schematic model of EARS 76 Fig. 6.7 Shallow depth slices (20-30 km) 78 Fig. 6.8 Horizontal map at a depth of 39 km 79 Fig. 6.9 Thickness of Tertiary volcanics (WEP) 82 Fig. 6.10 MT results near Lake Tana 83 Fig. 6.11 Crustal structure along 12°N latitude 84 Fig. 6.12 Cross-sections in Afar and Ethiopian plateau 86 Fig. 6.13 Deep lithospheric structure 88 Fig. 6.14 Model sensistivity 90 Fig. 6.15 Flow chart for elastic thickness calculation 93 Fig. 6.16 Elastic thickness estimates 95 Fig. 6.17 Elastic thickness estimates 96 Fig. 6.18 Elastic thickness estimates 97 Fig. 7 Moho map of EARS 104 Fig. 7.1 Bouguer gravity field of the rift 106 Fig. 7.2 Residual Bouguer gravity 108 Fig. 7.3 3-D view of Moho above anomalous lithosphere 110 Fig. 7.4 Basement topography 112 List of tables page 1. Calculated density using Sobolev and Babyeko (1994) 16-17 2. Summary of density measurements in NMER 18 3. Averaged density from the Neoproterzoic rocks of western Ethiopia 19-20 4. Composite density log measurements from Eastern Sudan 20 5. Density and elastic parameters from Doffen axial volcanoe 25 6. Comparison of IGSN71 and Potsdam stations in Ethiopia 31 7. Estimated error of different surveys 31 8. Volumes and masses of modelled bodies 80 9. Allowable density variations of modelled units 89 10. Density contrasts beneath the rifts 105 iii Important abbreviations AAMP:African Magnetic Mapping Project ANS: Arabo Nubin Shield APM: Absolute Plate Motion BGI: Buerau Gravimetric International CARS: Central Africa Rift System CASZ: Central Africa Shear Zone CMER: Central Main Ethiopian Rift EARS: East African Rift System EAGLE: Ethiopia Afar Geoscientific Lithospheric Experiment EAO: East African Orogen EEP: Eastern Ethiopian Plateau GBR: Gofa Basin and Range GRACE: Gravity Recovery and Climate Experiment GSE: Geological Survey of Ethiopia. IGF: International Gravity Formula IGSN71: International Gravity Standardization Net 71 IGMAS: Interactive Gravity and Magnetic Analysis System KRISP: Kenya Rift International Seismic Project KR: Kenya Rift MER: Main Ethiopian Rift NMER: North Main Ethiopian Rift MB: Mozambique Belt WEP: Western Ethiopian Plateau iv Acknowledgement I had very successful years in Germany. Truthfully, without the support of many institutions and individuals my dream would have remained in air. This work was started at Freie Universität Berlin, Institut für Geowissenschaften and completed at the Christian Albrecht Universität zu Kiel, Institut für Geowissenschaften, Geophysik with the support of Katholischer Akademischer Ausländer-Dienst (KAAD). I am very much thankful to KAAD for financing my stay in Germany. I am very grateful to my advisor Prof. Dr. H.-J Götze for introducing to me the wide range of applications of gravity field and three dimensional modelling, useful suggestions, support and constant encouragement throughout the work. I am very much indebted to the reviewer of this work, Prof. Dr. Ron Hackney for his valuable suggestions and critical comments, which have greatly improved the work. I would also like to thank all the members of the PhD commission: Priv.-Doz. Dr. Bohlen, Prof. Dr. Böning, Prof. Dr. Grotemeyer, Prof. Dr. Janle, and Prof. Dr. Rabbel. I wish to thank the Geological Survey of Ethiopia (GSE) for permission to use the data sets from Ethiopia and for allowing me to remain absent for such an extended period in order to complete the work. Many thanks are due to Mamushet Zewge, Ketsela Tadesse and all staff members in the GSE and Ministry of Mines for their support. GETECH and the Sudan Geological Authority and BGI are acknowledged for the gravity data sets. I would like to thank Prof. R.G. Keller for allowing to use the recent EAGLE velocity data from the Main Ethiopian Rift, Dr. Jeron Ritesma for sending the tomographic data sets of EARS, Dr. Lithgow-Berteloni for the dynamic topographic model data of Africa, Dr. Andy Nyblade and Mullugeta Dugda for sharing their recent results and Dr. Tanya Furman for chemical compositions of Main Ethiopian Rift lavas. I would like to extend very sincere thanks to all academic and technical staff of the Institut für Geowissenschaften in Berlin and Kiel for their friendship and support. C. Prezzi and A. Tassara, are thanked for their interest in my work and many useful discussions. Very special thanks go to Dr. Sabine Schmidt for helping me to use the compute programs. Without the use of her programs this work would have not been realised. I am very much grateful to Tolla Gillo and Amelework Kassa in Berlin for their constant support and encouragement and in Addis to my mother, brothers and sisters. Many thanks Etenesh for your encouragement. v Abstract The area encompassing the Eastern branch of the East African Rift System (EARS: Afar- Ethiopia-Turkana) and associated plateaux is an ideal region to investigate extension and magmatism associated with rupturing continental lithosphere. Ethiopia covers an important part of the EARS.
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