Building the Tibetan orogenic plateau : the dominant role of faults

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, 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, models to study the mechanical behaviour in the lower crust of the upper crustal thrust faults observed along the Tibet eastern edge, have been made, based upon recent thermo-kinematic modelling of thermochronology data (Pitard et al., 2021). 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 (Pitard et al., 2023). 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. On the contrary, by using a model driven by an overpressure in the lower crust, we show that the obstacle halts the viscous lower crustal flow and generates a smooth exhumation gradient at the edge of the plateau, not observed in Eastern Tibet. Furthermore, in the Yalong margin region, the channel flow scenario has been related to a regional uplift of low-relief surfaces where exhumation is mainly driven by deeply entrenched river incision. By testing such scenario on the complete thermochronologic data set available for the Muli thrust, we show that incision alone is not reproducing the thermochronologic ages.

References

Pitard, P., A. Replumaz, M.-L. Chevalier, P.-H. Leloup, M. Bai, M.-P. Doin, C. Thieulot, X. Ou, M. Balvay, and H. Li (2023), Exhumation History Along the Muli Thrust—Implication for Crustal Thickening Mechanism in Eastern Tibet, GRL, 48, doi: 10.1029/2021GL093677

Pitard, P., Replumaz, A., Thieulot, C., & Doin, M.-P. (2023). Modeling deep rooted thrust mechanism of crustal thickening in Eastern Tibet. Geophysical Research Letters, 50, doi: 10.1029/2023GL104134