Building the Tibetan orogenic plateau : the role of thrust faults and the influence of erosion on the eastern edge.

Despite decades of controversy, our understanding of the formation of the Tibetan Plateau remains limited. The role of competing mechanisms, such as distributed crustal thickening versus lateral propagation of thrust faulting at crustal or lithospheric scales, is still poorly understood. Conceptual models explaining observations at the continental scale are based on hypotheses that are hard to reconcile, on the one hand buoyancy forces dominating with low influence of upper crustal faulting, on the other hand faults dominating by favour discrete propagation of rigid upper crustal thickening since the onset of collision at ~50 Ma. However, in view of the 3D nature and temporal complexity of the involved deformation processes, no numerical model taking into account the role of strike-slip faults in accommodating stepwise evolution of thrust faulting, as well as the interaction between the deep crust and the surface, has yet been implemented. Therefore, it remains difficult to test the mechanical and rheological consistency, and the ability to explain observations, of end-member conceptual models at the scale of the Tibetan Plateau.

In order to generate new insights in deformation modes in Tibet, I will present models to study the mechanical behaviour in the lower crust of the upper crustal thrust faults observed along the Tibet eastern edge, which setup is based upon recent thermo-kinematic modelling of thermochronology data (Pitard et al., 2021). During the PhD of Paul Pitard, in collaboration with Cédric Thieulot and Marie-Pierre Doin, we made schematic 2-D viscous models of thrusts embedded in the crust, to study eastern Tibet thrust activity in the building of the topography through time. We show that both the high viscosity upper crust in which the fault is embedded and more surprisingly the low viscosity lower crust with no fault, are driven toward the surface by the fault. This generates along the fault a parallel zonation of the vertical velocity field, with high velocities close to the fault, decreasing away from it, fitting well the rejuvenation of cooling ages observed toward the thrust of SE Tibet.

In order to explore the influence of erosion during the building of the plateau, I will also present thermo-kinematic modelling of thermochronology data along the Mekong River at the eastern edge of Tibet, including schematic erosion process (Ou et al., 2020). During the PhD of Xiong Ou, in collaboration with Pieter van der Beek, we estimated that the Mekong River incision, locally more than 2000m, is 25-30% of the total exhumation since 10 Ma. Strong differences in elevation and relief on both sides of the Mekong River are linked to strongly differing tectonic imprint, with high elevation low relief surfaces observed when tectonic imprint is low, in part due to glacial “buzzsaw-like” processes, and high elevation high relief massif observed when tectonic imprint is high and when glacial processes are not sufficient to erase the topography created.