Seismic tomographic imaging of the crust and upper mantle beneath the northeastern edge of the Qinghai-Xizang Plateau and the Ordos area

Combining the data recorded by the portable broadband seismic array across the northeastern edge of the Qinghai-Xizang Plateau and the Ordos area and the travel time data of the regional seismic network, we investigate the P wave velocity structure of the crust and upper mantle down to 400km depth along a profile. Our results show that the structure of the crust and upper mantle along the profile has obviously laterally inhomogeneities, which occur not only between different blocks, but also within block. Along the profile can be identified obvious difference between the velocity structures of the upper mantle beneath the northeastern Qinghai-Xizang and Ordos plateau. The former has a lower averaged P wave velocity than the global model, and the latter is higher than the global model. This is consistent with tectonic activities in these two blocks. Between the northeastern Qinghai-Xizang and Ordos plateau there is a transitional zone of about 200km wide, which has indicators on the surface are Lanzhou and Haiyuan, respectively. In the upper mantle this transitional zone appears as a sloping strip with about 45° toward the eastern direction. The main features of the transition zone appear as a mixture of high-velocity and low-velocity media. We can also identify the differences of the upper mantle velocity structures beneath the Qaidam basin and the Qilian block within the northeastern Qinghai-Xizańg. This appears mainly as a low average velocity in the Qaidam block and a high average velocity in the Qilian block, respectively. The difference between them is up to about 8%. A low-velocity intrusive body is located on the top of the upper mantle beneath Zeko, Lanzhou and the Haiyuan area. This is in accord with the results by the receiver function method.