Basal shear stress below elastic crust of the Tibetan plateau inferred from three-dimensional finite element modeling

Abstract:

Associated with northward convergence of the India continent, the surface motion of the Tibetan plateau, documented mainly by dense geodetic GPS measurements, changes greatly both on magnitude and on direction in different tectonic units. The most remarkable discordance of surface motion is around the eastern Himalayan syntaxis, where GPS velocity field is rotated gradually to oppositional direction near the southeastern Tibetan plateau with respect to the northward convergence of the India continent. Such a velocity field could be result from lateral boundary conditions, since the strength of lithosphere is probably weaker in the Tibetan plateau than in the surrounding regions. However, whether the surface motion of the Tibetan plateau is affected by basal shear at base of the elastic crust, that could exist if the coupling condition between the elastic and the viscous crust were changed, is unclear. Here, we developed a large-scale three-dimensional finite element model to explore the possible existence of basal shear below the Tibetan plateau and the surrounding regions. In the model, the lateral boundaries are specified with far-field boundary condition; the blocks surrounding the Tibetan plateaulike the Tarim, the Ordos, and the South China are treat as rigid blocks; and the mean thickness of elastic crust is assumed about 25km. Then, the magnitude and distribution of basal shear stress is automatically searchedin numerical calculation to fit surface (GPS) motion of the Tibetan plateau. We find that to better fit surface motion of the Tibetan plateau, negligible basal shear stress on the base of elastic crust is needed below majority of the western and the central Tibetan plateau; Whereas, around the eastern and the southeastern Tibetan plateau, especially between the Xianshuhestrike-slip fault and the eastern Himalayan syntaxis, at least ~1.5-3.0 Mpaof basal shear stress is needed to cause rotational surface motion as GPS measurements documented. This suggests that the state of mechanical couplingbetween the elastic and the viscous crust exist significant difference below the eastern and the western Tibetan plateau. In addition, the predicted magnitude and distribution of basal shear stress explains the north-south trended normal faults diffusively developed mainly around the western Tibetan plateau.