Imaging the colliding Indian and Asian lithospheric plates beneath Tibet

[ 1] A high-resolution image of the base of the lithosphere from S-to-P converted seismic waves revealed the collision architecture of the Indian and Asian continental plates beneath the Tibetan Plateau. The base of the Indian lithosphere dips northward from a depth of 160 km beneath the Himalayas to a depth of 220 km just south of the Bangong suture. The base of the Asian lithosphere is nearly horizontal at the depth of 160 - 180 km from central to northern Tibet. There is a vertical gap of about 50 km between Indian and Asian lithospheres. Our observation of a well-defined, thick lithosphere throughout the entire plateau is not consistent with models of wholesale convective instability of a thickened mantle lithosphere, which would predict a very thin Asian lithosphere. The hypothesized sequential southward subduction of Tibetan and Asian continental lithospheres leading to the growth of the Tibetan Plateau, if correct, cannot be occurring below similar to 180 km depth. Our results, along with available geological and geophysical data, strongly support that the plateau is predominantly formed by a relatively coherent north dipping subducted Indian continental lithosphere in the south, which presently can be traced to the middle of the plateau, and a south dipping subducted Asian lithosphere in the north at a shallower depth.