2.5-Dimensional tomography of uppermost mantle beneath Sichuan-Yunnan and surrounding regions

We examine the Pn and Sn travel time data with long and short epicentral distance ranges separately and obtain 2.5-dimensional tomographic models for the uppermost mantle velocity and the Vp/Vs ratio beneath the Sichuan-Yunnan and adjacent regions. The tomographic models show that the average Vp and Vs of the upper layer are approximately 8.10. km/s and 4.60. km/s and those of the deeper layer are 8.19. km/s and 4.65. km/s, respectively; the average velocity ratio is 1.76 under the study area. A comparison of the Pn and Sn velocity distribution reveals that the high- and low-velocity anomalies are consistent. High velocity and low Vp/Vs ratios exist in the Sichuan Basin and low velocity and high ratios distribute in the Myanmar-Yunnan, Songpan-Ganzi, and Hainan regions, respectively. Our models provide more evidence for the subduction of the Indian plate beneath the Myanmar-Yunnan region and the existence of the hot material upwelling beneath the Hainan region. In our velocity model, there is a significant difference between the southern and the northern parts of the Yunnan region (i.e. north and south of 26° N); we infer that these regions are controlled by different dynamic processes. The velocity distributions at different depths are mainly similar in most of the regions of the study area, and the differences between the upper layer and the deeper layer beneath the Yunnan region indicate that the heat flow from the upper mantle has complex characteristics.