The Driving Forces of the Large-Scale Deformation in the India-Eurasia Collision Region: Joint Modeling of Lithosphere and Mantle Dynamics

Abstract:

The origin of the large-scale deformation of the India-Eurasia collision zone has been pursued for more than 4 decades. However, the driving forces for the largest area of continental deformation zone on earth have not been entirely resolved; the source of such driving forces remains enigmatic. One reason could be that the driving forces have to be sufficiently large to overcome the resistance of the Tibetan Plateau, created by excess gravitational potential energy (GPE) over a long time span. Another reason is that seismic experiments carried out in the Tibetan Plateau, due to the harsh natural conditions, are fewer, making it challenging to resolve high-resolution seismic structure beneath Tibet. We address this issue of driving forces in this deformation zone by quantifying the primary contributions to the lithospheric stress field. We take into account effects of topography and shallow lithosphere structure, as well as tractions originating from deeper mantle convection, in order to calculate model estimates of the total lithosphere stresses. We evaluate recent published global seismic tomographic models (P-wave, S-wave, and geodynamic models) and select a tomographic model which, when used in the semi-analytical mantle circulation model HC (Hager and O'Connell, 1981; Milner et al., 2009), provides a best fit to observations of geoid, surface motions, strain rates, and stress orientations. We use the joint modeling of lithosphere and mantle dynamics approach of Ghosh and Holt (2012) to compute the full lithosphere stresses, except that we use HC for the circulation model, which can only handle radial viscosity variations. After using the selected seismic tomographic model of SAW642AN (Panning and Romanowicz, 2006) to compute the global lithosphere stresses, we refine the calculated stresses in the India-Eurasia collision zone. Our results show that both the driving stresses from mantle convection and GPE differences contribute to the deviatoric stress field in this area. The mantle convection influence, consisting of significant downwelling of negative density buoyancies beneath Tibet and Indian Plate, contributes to the NE-SW compressional stresses, whereas the GPE differences contribute the North-South rifts and the clockwise rotation of motions around the Eastern Himalayan Syntaxis.