A 3-D finite element model for lithospheric deformation during the Indo-Asian collision

The Indo-Asian collision in the past 50-70 Ma has led to strong lithospheric deformation in the Himalayan- Tibetan plateau and the surrounding regions. Previous models using the viscous thin sheet approximation has illustrated some of the basic physics of the collisional tectonics, but the two-dimensional representation and the lack of major fault zones in the viscous thin sheet model have also left many important details of the collisonal tectonics uncertain. We have developed a fully three-dimensional (3-D) finite element model to calculate the finite strain of the lithospheric deformation during the Indo-Asian collision. Treating the deforming lithosphere as a 3-D viscous layer, our model considers both vertical and lateral variations of the lithospheric rheology. The model also includes major faults, which are simulated as narrow weak zones. We conducted a suite of forward models to explore the impacts of the 3-D rheological structure, tectonics boundary conditions, and the fault zones on the tectonic evolution of the Himalayan-Tibetan Plateau. We found that 1) the collision was mainly accommodated by crustal thickening during the early stage of the collision, but lateral crustal flow and escaping of the Asian continental blocks have gradually become more important as the plateau rises; 2) the vertical coherence of lithospheric deformation within the Tibetan Plateau results mainly from the tectonic boundary conditions, hence is not indicative of whether the crust is mechanically coupled with the mantle or not; and 3) the boundary faults around the Tibetan Plateau could be the natural products of lateral contrasts of lithospheric rheology during the lithospheric deformation.