The Geomorphic Response of the Passive Continental Margin of Northern Namibia to Gondwana Break-Up and Global Scale Tectonics Dissertation zur Erlangung des Doktorgrades der Mathematisch-Naturwissenschaftlichen Fakult¨aten der Georg-August-Universit¨at zu G¨ottingen vorgelegt von Matthias Johannes Raab aus G¨ottingen G¨ottingen 2001 D7 Referent: Prof. Dr. K. Weber Korreferent: Dr. R.W. Brown Tag der m¨undlichen Pr¨ufung: 21.06.2001 Contents Acknowledgments iv 1 Introduction 1 1.1Aim................................. 1 1.2 Purpose and Scope . ........................ 1 1.3 Outline of this Thesis ........................ 3 2 Geology, Geomorphology and Tectonics 5 2.1 Introduction . ........................ 5 2.2 Geology ............................... 6 2.2.1 Overview . ........................ 6 2.2.2 Origin of Major Lineaments . ............. 8 2.2.3 The Karoo Igneous Province . ............. 9 2.2.4 The Paran´a-Etendeka Province . ............. 10 2.2.5 The Damaraland Igneous Province ............. 11 2.2.6 Cretaceous Dyke Swarms ................. 13 2.2.7 Gondwana Fragmentation ................. 14 2.2.8 Offshore Basins . ................. 16 2.2.9 Post Break-Up Geology . ................. 17 2.3 Geomorphology . ........................ 18 2.3.1 Quantifying Geomorphic Development . ...... 18 2.3.2 Morphology of the Margin ................. 19 2.3.3 Evolution of High Elevated Margins . ...... 20 2.4 Tectonics . ........................ 23 3 Fission Track Thermochronology 25 3.1 Introduction . ........................ 25 3.2 Fission Track Data . ........................ 29 3.2.1 Track Formation . ................. 29 3.2.2 Track Annealing and the Effects of Temperature . 31 3.2.3 Compositional Affects on Annealing . ...... 32 3.3 Concepts behind Age Determination . ............. 34 3.3.1 Methods . ........................ 34 i 3.3.2 Age Equations . ................. 35 3.3.3 Error Calculation . ................. 39 3.3.4 Calculation of Uranium Content . ............. 40 3.4 Deriving Thermal Histories from Fission Track Data . ...... 41 3.4.1 Presentation of Fission Track Data ............. 41 3.4.2 Vertical Profiles . ................. 43 3.4.3 Thermal Modelling . ................. 46 4 Fission Track Results from Vertical Relief Profiles 48 4.1 Introduction . ........................ 48 4.2 Thermal and Numerical Modelling ................. 50 4.3 Calculation of Palaeogeothermal Gradient and Denudation . 50 4.4 The Brandberg Profile ........................ 51 4.4.1 Fission Track Results . ................. 54 4.4.2 Discussion . ........................ 56 4.4.3 Summary . ........................ 61 4.5 The Okenyenya Profile . ................. 63 4.5.1 Fission Track Results . ................. 63 4.5.2 Discussion . ........................ 64 4.5.3 Summary . ........................ 68 4.6 The Windhoekgraben Profile . ................. 71 4.6.1 Fission Track Results . ................. 71 4.6.2 Discussion . ........................ 71 4.6.3 Summary . ........................ 75 4.7 Conclusions . ........................ 77 5 Late Cretaceous Reactivation of Major Shear Zones in Northern Namibia 79 5.1 Introduction . ........................ 79 5.2 Methodology . ........................ 83 5.3 Previous Fission Track Work . ................. 83 5.4 Fission Track Results ........................ 84 5.5 Thermal Modelling . ........................ 89 5.6 Constraining Reactivation and Denudation ............. 89 5.7 Discussion . ........................ 91 5.8 Conclusions . ........................ 95 6 Denudation Chronology of Northern Namibia 100 6.1 Introduction . ........................100 6.2 Previous Work in Low Temperature Thermochronology ......101 6.3 Quantifying Long-Term Denudation . .............102 6.4 Denudation Chronology . .................103 6.5 Summary . ........................116 ii 7 Concluding Statements and Ideas for Future Work 117 Appendix 120 A Applied Statistics 120 A.1 Introduction . ........................120 A.2 The Chi-Square Test ........................120 A.3 Method of Least Squares . .................121 A.4 Weighted Mean . ........................122 B Sample Preparation and Experimental Conditions 124 B.1 Introduction . ........................124 B.2 Sample Preparation . ........................125 B.3 Experimental Conditions . .................126 B.3.1 Fission Track Dating . .................126 B.3.2 Track Length Measurements . .............127 B.3.3 Calibration . ........................127 B.3.4 Microprobe Analysis . .................128 C Fission Track Data Files 131 C.1 Sample Localities and Descriptions .................132 C.2 Analytical Results . ........................135 C.3 Raw Data Files . ........................141 D GMT-Scripts 222 D.1 Introduction . ........................222 D.2 Contouring Fission Track Data . .................222 D.3 Generating Location Maps . .................223 D.4 Generating Age Dependent Location Map .............224 D.5 Generating Maps for Animations . .................225 E Data-CD 235 E.1 Contents ...............................235 References 236 iii Acknowledgments Writing a PhD-thesis is a huge undertaking and a very long process. It takes years of research and experience, involving a lot of happy days with splendid ideas and research results, and many less happy days with disappointments and scientific and financial setbacks. The path towards a PhD is a stony one, but luckily I was fortunate enough to have more people than I would ever have imagined to help, support and encourage me to reach the end of that path, and I would like to express my appreciation to all of them. First of all, I would like to thank Prof. Klaus Weber, my supervisor, for his constant support and his understanding. Throughout this research and several field campaigns in Namibia he made countless helpful, inspiring suggestions and observations. It was he who initiated this project with a funding from the Deutsche Forschungsgemeinschaft (DFG), no. WE488-48/1, and succeeded to keep my fundings, and consequently this research, running. Dr. Roderick Brown, my co-supervisor, supplied me with all the information, scripts and papers possible, and guided me through all the tasks of my fission track thesis. This enabled me to get a much better perspective on my own results. Without his input this thesis would have been only a fraction of what it is now. Over the previous two and a half years he has guided me, has provided profound advice, a car, fine dining and friendship. Dr. Roderick Brown is a member of the Australian Fission Track Research Group. I was fortunate enough to meet members of the group in Harare/Zimbabwe in 1997. All the group members are wonderful people and their intellectual input and support were fundamental for my research. Their unique contributions, their positive and encouraging influence on me, and their thirst for knowledge have had a huge impact on my work. Specifically, I thank Prof. Andrew Gleadow, the head of department at La Trobe University and, later at The University of Melbourne, for inviting me to Australia, and giving me the privilege to work with his group in Melbourne for more than two and a half years. He was immensely helpful and supportive, and through his invitation made it possible to continue, and ultimately carry out my research. I am also grateful to Dr. Barry Kohn, the managing director of the Fission Track Research Group. He always took the time to listen to my problems and ideas, and found ways for me to support my family financially. I greatly benefited from his numerous suggestions for my thesis, and for my first publication. Dr. Kerry Gallagher is to be thanked for modelling my data with Suntrax, iv converting them into a useful format (GMT readable) for me. His speedy return of a revised manuscript of mine, which was important for this thesis, was also very much appreciated. Dr. Hermione Cockburn’s expertise and constructive comments on special top- ics in this thesis were of great help. Dr. Asaf Raza shared his knowledge of the most sophisticated way of fission track analysis and preparation with me, and pro- vided many useful advice and strategies as well as friendly encouragement at all times. Dr. Paul O’Sullivan trained me to become a confident operator and gave me deep insight into the variability of counting. I would like to thank all three of them very much. I am also grateful to Eva Wynn who thoroughly proofread this thesis. During the last years she has become a very important and close friend to my family and of me. It is a pleasure knowing her. Furthermore I would like to thank David Belton, on the one hand, for his friendship and unending readiness to help; and, on the other, for great scientific discussions we engaged in. Special thanks, too, to Dr. Sara Vassolo and Ingo Bardenhagen for their friendship and support in Windhoek/Namibia during my fieldworks. We had a magnificent time, with interesting talks, good fun, good wine and good food. Anja B¨ohm, Ulrich Hilken, Jeannette and Dr. Thomas Becker, who accompa- nied the Brandberg expedition in 1996; my “Hiwis” in G¨ottingen, Helge Knieriem, Markus N¨ortemann, Christian Gross, Annett B¨uttner, Thomas Wink and Thomas Buchholtz, should all be mentioned, too. I thank them for their work and all the good and bad times we had together. Also, I am grateful to the Deutscher Akademischer Austauschdienst (DAAD). Their scholarship, which was awarded to me for the period from September 1998 until February 2000, was crucial for a successful start of this project in Australia. Finally, I