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 ~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.