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ROYAL HOLLOWAY UNIVERSITY OF LONDON DOCTORAL THESIS Numerical modelling of the craton destruction process, with a type-example from the North China Craton Author: Supervisors: Liang Liu Prof. Jason P. Morgan Prof. Martin A. Menzies Prof. Yigang Xu A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy April 10, 2018 Department of Earth Sciences Royal Holloway University of London Egham, United Kingdom iii Declaration of Authorship I, Liang Liu hereby declare that this thesis and the work presented in it is entirely my own. Where I have consulted the work of others, this is always clearly stated. Signed: Date: 2018/04/10 v Learning without thought is labor lost; thought without learning is perilous. ------ Confucius vii Abstract: The phenomenon of cratonic destruction has attracted attentions among numerical modelers. However, existing modelling work appears to lack enough constraints from geological and geophysical observations. To better combine the observations with the modelling approach, and to further understand the destruction of the North China Craton (NCC), I firstly reviewed current knowledge about the evolution of the North China Craton, and then outlined the strength and weakness associated with classic delamination and convective erosion models. By integrating new observational data and 2-D numerical modeling results, I put forward a two-stage model to account for the destruction of the NCC, that is, cratonic keel delamination followed by localized convective erosion and lithospheric extension. As revealed in the evolution of the NCC, its destruction appears to be accompanied by the inversion of a pre-existing North China Cratonic Basin (NCCB), which is partly preserved in southeastern Ordos. This >300 Myr subsidence of the NCCB can most probably be explained by assuming a pre- existing dense keel for the eastern NCC (ENCC), consistent with the petrological and geochemical observations from mantle xenoliths/xenocrysts hosted in kimberlite/basalt in the ENCC. As the bottom was dense, cratonic keel stability would have only been sustained by the buoyancy of the overlying lithospheric portion. Hereafter, if some weak intra-cratonic regions had existed above the dense bottom and were reactivated, intra-cratonic decoupling may finally occur, leading to keel delamination. Coincidentally, some seismically visible Mid-Lithospheric Discontinuity Layers (MLDLs) exist at depths of ~80-100 km within modern cratons, including the remaining portions of the NCC and the Wyoming Craton (WC),. Interestingly, these depths are also where the relic lithospheric bottom appears to remain beneath viii the keelless sub-regions of the eastern NCC and the WC. Because the MLDLs are suggested to be regions of preferential accumulation of metasomatic minerals and/or anomalously wet (>1000 ppm) peridotites (both of which would lead to a relatively weak rheology), we accordingly propose that the weak MLDLs may have been the intra-cratonic weak zone, which allowed delamination of the dense cratonic keels of the NCC and the WC. This delamination model is studied in detail using 2-D analytical and numerical modelling, and the modelling results may help constrain the MLDL rheology needed for explaining the keel delamination rate estimated on the NCC and the WC. One can speculate that the delaminated cratonic keels may still exist or stagnate within the upper mantle. This suggestion may explain some, if not all, of the high velocity bodies beneath North China and Iowa, USA (which was the location caped by the Wyoming Craton before the Laramide Orogeny). After keel delamination along the MLDLs, a protracted (ca. 50-100 Myr) period of small-scale convective erosion and/or lithospheric extension thinned the remaining ~90 km thick lithosphere and continuously reworked the former cratonic lithospheric mantle beneath the Moho. As indicated in the modeling results, this second stage evolution may explain the episodic magmatism and crustal deformation associated with the destruction of the NCC. ix Acknowledgements This thesis was supported by the joint PhD program between the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (GIGCAS) and Royal Holloway University of London (RHUL), and CAS Priority B Strategic project (XDB-18000000). First, I would like to acknowledge my lead supervisor Prof. Jason P. Morgan. Prof. Morgan proposed the ideas for my Ph.D. researches and formats of publications. Jason is always patient in explaining the basic grammars and structures on scientific writing and helped me to modify my drafts many… many… times. He was willing to discuss with me about my numerical results and helped me to dig out the geophysical meanings of my experiments. Second, I would like to acknowledge my second supervisor Prof. Martin Menzies and his wife Les. Prof. Menzies is one of the pioneer geologists studying craton destruction. He spent lots of time helping me to correct my writings and to think about geology. To have the chance to study his outlooks on life, career and values is one of my most significant achievements during the 3.5-year Ph.D. study in the U.K. I deeply appreciate the happy time in their house, and their kindly company during the Chinese Spring New Year. I would definitely like to acknowledge my supervisor Prof. Yigang Xu and his wife Lin Lin. Prof. Xu is also one of the earliest geologists studying craton destruction. He provided me with the chance to study in the RHUL, to know geodynamics modeling, and to think about better modeling strategy. He and his wife encouraged me to bestir myself during the most difficult time I ever encountered, and they helped me to re-gain my confidence to continue my research. Thanks to Miguel and Albert. Thank you both on helping me to learn the prototype code and sharing coding strategy. I also like to say thank you to Bin Sheng, Yajun Li, Wanpeng Li, Shuo Zhang, Zhi Wang, Xiao Zhang, Xiangming Tao, Hanbing Li, Yizhuo Sun, Ning An, and x Hong Chen. Thank you for your company in abroad. I always remember the time we spent in the Sports Center, Great Winsor Park and Magic Wok. Finally, but equally importantly, I would like to acknowledge my parents Liu Xianhou & Lv Li and my wife Zhu Ming. I know I spent <40 days with you in the past 3.5 yrs. Thank you for your supporting and understanding. You are always my power source! xi Contents Declaration of Authorship .....................................................................................iii Abstract: ............................................................................................................... vii Acknowledgements ............................................................................................... ix Contents ................................................................................................................ xi List of Figures .....................................................................................................xiii List of Tables ........................................................................................................ xv Chapter 1: Introduction .......................................................................................... 1 1. Destruction of the North China Craton. ..................................................... 1 2. Existing problems and objectives of this study .......................................... 5 3. Outline of the thesis ................................................................................... 6 References: ..................................................................................................... 8 Chapter 2: Sedimentary evidence for a pre-destruction dense keel beneath the Eastern North China Craton ............................................................................ 12 Abstract: ....................................................................................................... 13 1. Introduction .............................................................................................. 14 2. Review of the NCC’s evolution before destruction ................................. 16 3. Sedimentary cover on the NCC ............................................................... 23 4. A partly failed North China Cratonic Basin ............................................. 28 5. How did a cratonic basin form upon the NCC? ....................................... 29 6. A simple numerical model of the NCCB subsidence ............................... 35 7. Discussion ................................................................................................ 40 8 Conclusions ............................................................................................... 41 Appendix: ..................................................................................................... 42 References: ................................................................................................... 48 Chapter 3. Craton Destruction Part I: Cratonic Keel Delamination a weak Mid- Lithospheric Discontinuity layer ..................................................................... 53 1. Introduction .............................................................................................. 56 2. Review of Previous Scenarios for Keel Removal .................................... 63 3. Theoretical Analysis ................................................................................. 67 4. 2-D Numerical Experiments ...................................................................