Velocities and propagation characteristics of Pn and Sn beneath the Himalayan arc and Tibetan plateau: Possible evidence for underthrusting of Indian continental lithosphere beneath Tibet

New seismological observations on velocities and propagation characteristics of Pn and Sn waves beneath Tibet can be interpreted, although not uniquely, to indicate the shallow-angle underthrusting of the Greater Indian continental lithosphere beneath the Tibetan plateau. This is inferred from available geological and geophysical data and is now supported by new seismological observations. The most significant observation is that high-frequency Sn waves (shear waves that travel in the mantle lithosphere) propagate efficiently in the uppermost mantle beneath the Tibetan plateau, except beneath the north-central part of Tibet (the Chang Thang terrane). Furthermore, Sn propagates efficiently across the stable block of the Tarim basin, along the Tien Shan and Himalayan Mountains, and across the Indian shield. Apparent velocities of Pn and Sn waves that traverse the uppermost mantle beneath the Tibetan plateau are 8.42 and 4.73 km/s, respectively. These velocities are very similar to those beneath the Himalayan Mountains and the Indian shield. The efficient propagation and the velocities of Pn and Sn waves beneath Tibet are also similar to those commonly observed beneath shield and stable continental regions. These results are consistent with the underthrusting model and do not favor the class of alternative models in which the Tibetan plateau is formed by shortening and thickening of hot crustal and uppermost mantle material in response to the convergence of India and Asia.