Complex Structure beneath the Southeastern Tibetan Plateau from Teleseismic P-Wave Tomography

A high-resolution image of the deep structures in the southeastern Tibetan plateau and Yangtze block was revealed by teleseismic inversion based on the dense teleseismic data collected from the permanent seismic stations and a recently deployed temporary seismic array. The results show that low-velocity anomaly is imaged mainly in the eastern and southeastern Tibetan plateau (except the southern Panzhihua area) above 200 km depth in the upper mantle, whereas a high-velocity anomaly exists to its east especially beneath the Sichuan basin. We infer the cold crustal and upper mantle of the Sichuan basin prevents the extrusion of mantle materials of the Tibetan plateau, while the cold lithosphere of the western Yangtze block is eroded by the upper-mantle materials with high temperature from the extrusion of the Tibetan plateau. Under the Tengchong volcanic region, a low-velocity anomaly extends to 300 km depth. A high-velocity anomaly is shown beneath the region from the southern Sichuan to Yunnan in the mantle transition zone, and the anomaly reaches deeper to the lower mantle in the western Yunnan area. The widespread low velocity above the high velocity in the Yunnan region indicates the dehydration of the Indian plate may occur in the mantle transition zone, which causes fast-rising heat and lower melting temperature in the upper mantle. A small-scale, high-velocity anomaly in the southern Panzhihua region is probably related to the late Paleozoic mantle plume activity. It plays an important role in impeding the southward extrusion of the plateau materials, which might be one of the causes for the rapid change of the polarization direction of the fast shear waves around the area.