On the Dynamics of Plate Tilting An Analytical and Numerical Approach ÖMER FARUK BODUR A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy School of Geosciences Faculty of Science The University of Sydney 2020 “On the Dynamics of Plate Tilting, An Analytical and Numerical Approach” © 2020 - Ömer Faruk Bodur SUPERVISORS Assoc. Prof. Patrice F. Rey Prof. Gregory A. Houseman Prof. Dietmar Müller TIME FRAME September 2016 — October 2019 Sydney, NSW 2006, Australia DECLARATION I declare that this thesis is less than 80,000 words in length and the intellectual content of this thesis is the product of my own work which has not been submitted for a higher degree at any other university or institution. I also certify that all the assistance received in preparing this thesis and sources have been acknowledged. Ömer Faruk Bodur 22nd Jun. 2020 PREFACE This thesis consists of four articles, the first article presented in Chapter 2 has been published by EGU-Solid Earth with open-access. Other chapters are prepared for submission. The thesis starts with an introductory chapter that retrospectively expands on the subject and briefly gives the aims of the thesis. The findings are presented in detail in the following chapters, and they form a coherent whole in the Discussion and Conclusion where I review their significance along with reflections about future studies. No ethical approval was required during the completion of this study. ii ACKNOWLEDGEMENTS To begin with, I wish to express my deep gratitude to my advisor, Patrice Rey, one of the kindest people I know to this point in my life. In three years, I learned a lot from him. He became my role model in matters not only to do with science, but of in life. Now, I better understand why he gave me the freedom to make all the mistakes I made. I want to thank Gregory Houseman to whom I am deeply indebted. He patiently listened to me whenever I asked questions, and shared an enormous amount of his time with me in discussing geodynamic problems that I could never address without his support. I am also grateful to Dietmar Müller, who did his very best to support my progress. His presence has always given me energy and motivation. Since I stepped into this wonderful Australian continent on a very hot day on 30th August 2016, walked up the stairs of the Gothic-style School of Geosciences building, was assigned a desk and a comfy chair to brood in, I met my colleagues: Sarah, Rakib, Sabin, Nick, Xuesong, Wenchao, Luke, Gilles, Amy, Rhiannon, Amanda and Carmen to which I am thankful for their friendship. Later, my list of wonderful friends increased after I got to know Chang, Xianzhi, Annie, Calvin and Maelis. I am especially very thankful to Chang and his family for their support and encouragement. I am greatly indebted to MingMing for giving me motivation and tremendous support. I would like to thank Matthew da Silva for opening his house to me and sharing his ideas on life, also helping me to improve my knowledge of English grammar. I also would like to thank Cumhur, Hadi, Haluk and my other friends in Sydney, as they made me feel like I am at home. I am thankful to the members of Basin Genesis Hub at The University of Melbourne for their technical support on Underworld geodynamics modelling code. I would like to thank Equinor, Oil Search and Chevron for their financial support. My flirtation with literature and my love of the beauty in the details of everyday life coloured my Ph.D. life here in Sydney. Of course, I had difficulties, at times, but my romantic iii environment was good enough to overcome my struggles. I was very lucky that whenever I needed, the Quadrangle of The University of Sydney was always there for me to open my heart and listen my stories, the writings of Karl Ove Knausgård, Anton Chekhov, Dostoevsky, Samuel N. Kramer and Marcel Proust were in my rucksack to improve my understanding of the people and life, my audio playlist, including songs by Sezen Aksu, Harry Gregson, Anouar Brahem, Yilmaz Erdogan ready to play and help me to relax, the ducks of the Victoria Park were next door to fill me with peace as I watch their unceasing feeding. At times, I couldn’t help myself watching the ibis bird, or “the bin chicken”, searching for food in desperation among people’s leftovers, and taking break from time to time in the treetops. I hope that I will be filming their daily lives one day. I was very lucky to experience the lively suburbs of this city that I found great enjoyment walking through. It was a great pleasure to walk to the city centre, stop by the Australian War Memorial in Hyde Park where I pay my respect to the history of this wonderful country and my hometown, and end up at the Opera House, making my favourite circle, breath enough air filled with ocean flavour, and get my strength up to survive until the following Friday. On my way back home, I had happy moments when I stopped by Xian noodle for dinner, always having been offered the best seat available. There, I enjoyed the delicious hot noddles and pancakes with my friends. After submission of the thesis, another phase of my life initiated. The COVID-19 pandemic strongly impacted my life as it did to others, but I was very lucky that I was offered a postdoc position at the University of Wollongong, which helped me in transitioning from being a PhD student, in this chaotic period, to a different one that I just started exploring in my post-PhD life coming with many responsibilities. In that regard, MingMing played a big role in supporting my transition by sharing her life experience with me. Last, but not the least, I wish to express my heartfelt thanks to my parents for their tremendous support. I felt very safe whenever I heard their voices. During my journey, my much-loved sister and my brother always made me laugh, hence they deserve a big hug and, of course, nice gifts to be received soon! iv Returning Home, Ardahan, 2004 © Nuri Bilge Ceylan v ABSTRACT The Earth’s topography is dynamically evolving under the forcing of convective motions in its mantle. As tectonic plates override subducted slabs and rising plumes, they are pulled down and pushed up, hundreds of meters over durations of tens to hundreds of Myr, and over lateral length scales of thousands of kilometres. These vertical motions of plates impact on the evolution of sedimentary basins which are the Earth’s billion-year history tablets, with records of sea-level variations, climate changes, and evolution of species. They are also the major source for oil, gas, geothermal energy and water. To unravel the Earth’s geodynamic history and make better predictions about the Earth’s natural resources, it is of critical importance to understand the coupling between tectonic plates and the underlying mantle. In that regard, predictions for amplitude of dynamic topography derived from global mantle convection models don’t agree well with dynamic topography amplitudes derived from investigations based on sedimentary basins and ocean bathymetry (i.e. residual topography). Furthermore, the geodynamic models considering mantle density anomalies, hitherto, cannot explain the long-wavelength, episodic and rapid subsidence and uplift of sedimentary basins, such as the enigmatic Neogene tilting of India and rapid tilting events in Australia since the Late Cretaceous. In this thesis, first, I propose that, at shorter wavelengths less than 1,000 km, predictions for dynamic topography amplitude driven by mantle density anomalies can be improved by considering the non-Newtonian rheology of the upper mantle. In the second chapter, a torque-balance analysis of relative horizontal rigid plate motion suggests that basal shear stress, on its own, induces dynamic plate-scale tilting, producing elevation changes of a few tens of metres, comparable to eustatic sea-level changes. This finding motivates the further analysis of horizontal plate motion via 2D thermo-mechanical numerical experiments considering progressive transition in viscosity between the plate and underlying mantle. These numerical experiments show that horizontal plate motions are associated with gradients in basal vi shear stress and normal stress, the latter generally dominating the plate’s dynamic topography. This process could give insights into the Neogene tilting of India. In the following chapter, to estimate the tempo of this dynamic plate tilting, a suite of analytical models is presented by assuming that plate has constant viscosity and density. The results suggest that, after a change in plate motion, the associated basal stresses can tilt the plate in ~50 kyr for a stress anomaly wavelength of 3,000 km and a plate viscosity of 1023 Pa s. This rapid plate-tilting can explain the anomalous vertical motions of Australia that followed sudden plate accelerations or decelerations since the Late Cretaceous. In the last chapter, 2D numerical models simulating relative horizontal plate motion, but now including conjugate passive margins and a connecting ocean basin show that, during plate motion, the trailing edges of the plates are being compressed. This process could shed light on the post mid-Eocene episodic basin inversions documented along the south-east Australian margin. In summary, this thesis provides a broad perspective on the coupling between the plates and the underlying mantle. It predicts the tempo and magnitude of vertical motions of plates via a suite of analytical and numerical models and applies them to the rapid long-wavelength vertical motions of India and Australia.