Indian Crustal Front beneath Tibet Inferred from Seismic Anisotropy

Postcollisional downgoing of the Indian plate has absorbed large amounts of the Cenozoic India-Asia convergence and is a key process governing the Tibetan Plateau's evolution and growth; however, to date, whether the Indian slab is currently subducting or underthrusting is still controversial. Here, we present new constraints on the nature of seismic anisotropy within the crust of the eastern Lhasa terrane and the adjacent areas by exploiting the splitting phenomenon of P-to-S converted phases from the Moho. A crustal seismic anisotropy dataset in the main collisional belt of Tibet is then assembled by incorporating the newly obtained and published measurements. Our results highlight the systematic changes in preferred orientation of deep-crust anisotropic minerals along the perpendicular direction of the Himalayan arc, with a sharp northward transition from convergence-parallel to orogen-parallel directions occurring at similar to 450 to 500 km north of the Main Frontal thrust. The latter observation provides an estimate of the northern frontier of the Indian lower crust from the perspective of seismic anisotropy and indicates overall and long-distance underthrusting of the Indian plate under Tibet.