Upper mantle velocity structure beneath the Tibetan Plateau from Pn travel time tomography

We inverted 1510 P arrival times from regional distances (333-1600 km), in and around the Tibetan Plateau to map the lateral velocity variation within the uppermost mantle. Previous studies have placed first-order constraints on upper mantle velocities but relied on data recorded almost exclusively at stations outside of the plateau. We improve resolution by using 40 events recorded at stations within the Tibetan Plateau. We combine these data with observations obtained from the International Seismological Centre (ISC) to extend our coverage by including Pn arrivals from 85 additional plateau events, relocated in previous studies, and recorded at stations in and around the Tibetan Plateau. We use synthetic travel time data to evaluate the resolution of our data set. The observations provide good resolution to about 1 degrees over most of the plateau and surrounding regions. Our results show average Pn velocities that are about 3% lower in the northern plateau relative to the southern plateau. These variations correlate well with major tectonic features and previous geophysical observations. In the Qiangtang terrane of the northern plateau, an area known to be inefficient for Sn propagation, Pn is slow relative to both the plateau south of the Banggong-Nujiang suture and the tectonically stable Tarim basin north of the plateau. This is strong evidence for the existence of partial melt within the uppermost mantle beneath the northern Tibetan Plateau. However, when laboratory estimates of relationships between temperature, velocity, and attenuation are applied, a relatively small temperature variation (240 degrees to 370 degrees C) is required to explain our Pn velocity observations. When combined with geochemical constraints from volcanics in the northern plateau, our results strongly suggest that the mantle lid is intact beneath the northern plateau. This result would preclude tectonic models involving wholesale delamination of the mantle lithosphere in the northern Tibetan Plateau.