Three-dimensional numerical modelling of subduction/collision and lithospheric deformation
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Abstract
One of the most striking features of plate tectonics and lithospheric deformation is the India-Asia collision zone. It is no surprise then that understanding the formation and evolution of the abnormally thick and high Himalaya-Tibet region has been the focus of many tectonic and numerical models. While some of these models have successfully illustrated some of the basic physics of continental collision, none can simultaneously represent active processes such as subduction, underthrusting, delamination, channel flow or extrusion, which are thought to be important during continental convergence, since these mechanisms require the lithosphere to interact with the underlying mantle. Integrated 3-D models of lithosphere and mantle dynamics are needed to overcome these limitations.
In this thesis, I perform systematic 3-D numerical simulations using the code LaMEM, and combine the numerical results with insights from semi-analytical models and scaling analysis to explore some fundamental aspects of continental collision and mountain-building dynamics in an India-Asia collision framework. I applied the models to investigate (i) how subduction and collision affect mountain-building processes and how large topographic plateaus can form in an integrated lithospheric and upper-mantle scale model, (ii) appropriate numerical and theoretical techniques for studying lithospheric deformation at convergent margins, and (iii) how the shape and convergence of Greater India affected the subsequent tectonic evolution of central and SE Asia.
Obtaining anomalously high topographic amplitudes has been a challenge in previous 3-D collision models because contrasting processes simultaneously affect the development of topography. On one hand, continental collision promotes topography build-up through indentation. On the other hand, it also leads to slab break-off and lateral extrusion of material, which act to lower the topography. In a first part of the thesis (Chapter 3), 3-D results suggest that slab pull alone is insufficient to generate high topography, and that external forcing and the presence of heterogeneous strong blocks, such as the Tarim Basin, are necessary to create and shape anomalously high topographic fronts and plateaus. Moreover, different modes of surface expression are predicted in continental collision models, thus improving our understanding of how mountain-belts are formed and sustained.
In the second part, I discuss a common numerical problem of the marker-in-cell method for staggered grids, that of conservative advection of markers (Chapter 2), and I test the effect of rheological approximations on mantle and lithosphere dynamics in a geometrically simplified model setup of subduction/collision (Chapter 4). The model results exhibit a wide range of behaviors depending on the rheological law employed: from linear viscous to temperature-dependent visco-elasto-plastic rheology that takes into account both diffusion and dislocation creep. These two studies demonstrate that the choice of rheology or numerical techniques can radically alter slab dynamics and topography evolution. A combined effort of improving numerical methods and understanding dynamics of complex systems will be needed for future studies.
Finally, in Chapter 5, I present a new type of 3-D forward models to investigate how the India-Eurasia convergence in the last 120 Ma has been accommodated by subduction in the Neo-Tethys, and how the shape of Greater India affects collision dynamics. The results show the spontaneous formation and merging of a double subduction system, which resulted in a stable intra-oceanic subduction with a trench-trench-trench triple junction. The collision dynamics is controlled by upper plate parameters and the size and shape of the Greater India extension, which in turn controls the timing of collision and deformation pattern.
Overall, this work demonstrates the importance of coupling lithosphere and mantle dynamics for the study of convergent margins, and represents fundamental progress on understanding the formation of mountain belts.