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. The 130 paths crisscross the Tibetan Plateau, Hindu Kush, Karakoram, southwest China, Burma, and northern India regions. We regionalized these areas on the basis of topographic and geologic expression and determined the average upper mantle velocity and crustal thickness value for each block or region through a linear least squares inversion of the 130 parameter values. Crustal thicknesses in the Tibetan plateau, Karakoram, and Hundu 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. We observed impulsive first arrivals (evidence for positive upper mantle gradients) on seismograms recorded at distances between 1000 and 1400 km. The paths of these waves crossed portions of Tibet, the Hindu Kush, and east India regions. Simple modeling of the long-period Pn waveform and amplitude requires fairly high upper mantle gradients of 0.18--0.25 km/s per 100 km beneath the Pamir-Hindu Kush region and requires an upper mantle lid in excess of 100 km thickness. The southern half of Tibet also requires a lid of about 100 km thickness.

A thinner upper mantle lid (50 km) provided a poor match to the Tibetan observations, even with a very high gradient of 0.2 km/s per 100 km. 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 is which Indian lithosphere has underthrust southern Tibet. Due to poor path coverage, 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 the pressure increase alone, produced by the double-thick crust, and indicates that the Indian lithosphere, which underlies Tibet, is out of thermal equilibrium. ¿ American Geophysical Union 1990