Delineation of lithosphere structure and characterization of the Moho geometry under the Himalaya–Karakoram–Tibet collision zone using surface wave tomography

Group velocities for a period range 6–60 s of the fundamental mode of Rayleigh wave passing across the Himalaya–Karakoram–Tibet orogen are used to delineate the structure of upper lithosphere using the data of 35 broadband seismic stations. 2D tomography velocity maps of group velocities are obtained at grids of 1° separation. Redefined local dispersion curves are inverted nonlinearly to obtain 1D velocity models and construct a 3D image of the S-wave structure down to 90 km depth. Moho discontinuity is correlated with c. 4.0 km s^-1 S-wave velocity. The results depict a NE-dipping trend of Moho depth from c. 40 km beneath the frontal part of the Himalaya and up to c. 70–80 km beneath the collision zone before descending to c. 40 km beneath the Tarim basin. The study also reveals thick deposits of sediments in the Indo-Gangetic plains and the Tarim basin. A broad low-velocity zone at mid-crustal depth in the western Tibetan plateau, Karakoram region and the surface collision part of India–Eurasia tectonic plates is interpreted as the effect of partial melting and/or presence of aqueous fluid. The high velocities in the southern deeper part indicate that the lower crust and uppermost mantle of the Indian plate are dense and cold.