Three dimensional P-wave velocity structure underneath the southeastern margin of the Tibetan Plateau and the deep tectonic significance

The deep velocity structure underneath the southeastern margin of the Tibetan Plateau is the key to understand the tectonic evolution of the Tibetan Plateau. In this study, 145,254 relative travel-time residuals were measured from the records of 1360 teleseismic events using the waveform correlation method, we determined the threedimensional (3-D) P-wave velocity structures in the upper mantle using a large-scale dense seismic array. The western boundary of the Yangtze Craton extends from the Longmenshan fault zone in the north to the Red-River fault in the south, our imaging results show that in a depth range of 50-150 km, obvious differences in physical properties of the medium exist across the Longmenshan fault zone. The Red-River fault and the Xiaojiang fault are the western and eastern boundaries of Chuan-Dian diamond block, respectively. Low-velocity anomalies are visible from the north of the Red-River fault to the east of the Xiaojiang fault and their intersection region. The high-velocity anomaly below the Sichuan Basin is thinning westward, with the thickness of 350 km below eastern Sichuan Basin. We obtained a low-velocity anomaly extending 400 km below the Tengchong volcanic field. A high-velocity anomaly, extending down to the mantle transition zone, is thought to be related to the subduction of the Indian slab. Our results also reveal an obvious low-velocity anomaly exists to the south of 26 degrees N, we speculate that the flow of the upper mantle material beneath the Tibetan Plateau affects the velocity structure in this region.