Crustal and mantle velocity models of southern Tibet from finite frequency tomography

Using traveltimes of teleseismic body waves recorded by several temporary local seismic arrays, we carried out finite‐frequency tomographic inversions to image the three‐dimensional velocity structure beneath southern Tibet to examine the roles of the upper mantle in the formation of the Tibetan Plateau. The results reveal a region of relatively high P and S wave velocity anomalies extending from the uppermost mantle to at least 200 km depth beneath the Higher Himalaya. We interpret this high‐velocity anomaly as the underthrusting Indian mantle lithosphere. There is a strong low P and S wave velocity anomaly that extends from the lower crust to at least 200 km depth beneath the Yadong‐Gulu rift, suggesting that rifting in southern Tibet is probably a process that involves the entire lithosphere. Intermediate‐depth earthquakes in southern Tibet are located at the top of an anomalous feature in the mantle with a low Vp, a high Vs, and a low Vp/Vs ratio. One possible explanation for this unusual velocity anomaly is the ongoing granulite‐eclogite transformation. Together with the compressional stress from the collision, eclogitization and the associated negative buoyancy force offer a plausible mechanism that causes the subduction of the Indian mantle lithosphere beneath the Higher Himalaya. Our tomographic model and the observation of north‐dipping lineations in the upper mantle suggest that the Indian mantle lithosphere has been broken laterally in the direction perpendicular to the convergence beneath the north‐south trending rifts and subducted in a progressive, piecewise and subparallel fashion with the current one beneath the Higher Himalaya.