The Indo-Asian continental collision: A 3-D viscous model

- We develop a 3-D power-law viscous fluid FEM model with fault elements.
- We simulate large deformation during Indo-Asia collision using the FEM model.
- We explore the role of rheology, faults and boundary resistance in orogeny.
- A new explanation is shown for the apparent vertical coherent motion in the Tibet.

The large-scale physical process of the Indo-Asian continental collision and the formation of the Himalayan-Tibetan Plateau have been simulated in various viscous thin-sheet models, but the thin-sheet simplification also kept some important issues from being fully explored. Among these issues are the role of strike-slip fault zones in facilitating large-scale lateral translation of lithospheric blocks (the escaping tectonics) during the collision, and the speculated lateral flow of the ductile middle-lower crust under the Tibetan Plateau. Here we present a fully three-dimensional finite element model to simulate the Indo-Asian continental collision. The model includes major boundary faults to simulate the escaping tectonics, and vertically variable rheological structures to model lower crustal flow. The collisional process is constrained by the history of the Indo-Eurasian plate convergence and the present crustal thickness and topography of the Tibetan Plateau. Our results indicate that the restrictive boundaries of the Tibetan Plateau, including the rigid Tarim and South China blocks, largely control the spatiotemporal patterns of crustal deformation in the collision zone. As the Indian indenter moves toward the Tarim block, higher strain rates and topography developed in the western part of the collision zone than in the eastern part, causing the northward migration of the deformation front to gradually change to eastward migration in the past 10-20 Myr, broadly consistent with the initiation of widespread E-W extension in the Plateau. These restrictive boundary blocks also force the crustal and mantle materials in the collision zone to flow coherently, hence providing an alternative explanation for the apparently vertically coherent deformation in Tibet. Assuming that the crust weakens as it thickens, our model predicts the lateral expansion of the Tibetan Plateau, an important feature of the Tibetan tectonics that is missing in previous models with constant rheology.