Crustal thickness and upper mantle velocities in the Tibetan Plateau region from the inversion of regional Pnl waveforms: evidence for a thick upper mantle lid beneath southern Tibet

The crustal thickness and upper mantle velocity variation in Tibet and surrounding regions was mapped through the inversion of 130 individual long-period Pnl waveforms. Crustal thicknesses in the Tibetan Plateau, Karakoram, and Hindu Kush regions range from 63 to 72 km and Pn velocities are 0.10-0.20 km/s faster beneath these regions than those beneath northern India. The upper mantle structure of east India appears to be similar to the southern half of Tibet (100-km-thick positive gradient zone). Both the increase in Pn velocity from India into Tibet and the evidence for a thick upper mantle lid beneath Tibet are consistent with the model in which Indian lithosphere has underthrust southern Tibet. The structure beneath northern Tibet cannot be resolved, and thus underthrusting beyond central Tibet cannot be constrained. The rapid increase in upper mantle velocity from northern India into Tibet can be explained by pressure increase alone, produced by double-thick crust, and indicates that the Indian lithosphere, which underlies Tibet is out of thermal equilibrium. -from Authors