P and S wave velocity structure of the uppermost mantle beneath Sichuan-Yunnan

Sichuan-Yunnan lies to the southeast of Tibetan Plateau. As an important place for the migration of crustal material in Tibetan Plateau, this area has complex tectonic belts, strong tectonic activities, and frequent earthquakes. We build three-dimensional seismic velocity models of its uppermost mantle to provide more information for its structure and evolution.

Researchers have done a lot to study the velocity structures beneath this region, but the 3-D P and S wave velocity models with high resolution for the uppermost mantle haven't been obtained. In this research, we use a large amount of seismic data to build more accurate velocity models for the uppermost mantle. During the inversion, the initial velocity models come from Litho1.0, and the Moho interface is extracted from Crust 1.0. Over 25000 Sg and Sn wave traveltimes between 1995 and 2019 are used to joint invert for the S wave velocity model basing on a method called Fast Marching TOMOgraphy. We also use more than 60000 Pg and Pn wave traveltimes to obtain the P wave velocity model. The resolution of the new models can reach 0.1°Χ0.1° in some regions, they thus can reveal more detailed structures of the uppermost mantle.

The S and P wave velocity models show strong heterogeneity in the uppermost mantle. The patterns of S and P wave velocities are generally consistent, and have a good correspondence with the distribution of the crustal blocks. For example, Sichuan Basin, which is part of Yangtze platform, has high S and P wave velocities, while the Songpan-Ganzi fold belt, who has more tectonic activities, is mainly featured by low velocity distribution. The low velocity patterns around the Songpan-Ganzi fold belt are closely related to the crustal stress distribution in this region, they can be caused by the high temperature anomalies, which are mainly due to the extrusion in the region. Some low velocity zones have a good correspondence with the distribution of volcanoes in the west of Yunnan. These low velocity regions indicate that the magma in the crust may extends to the uppermost mantle, and the volcanoes are related to the eastward movement of the Indian plate.