青藏高原P波速度层析成像与岩石圈结构

Using arrival data from seismic networks in western China, we inverted three-dimensional P-wave velocity structures of the Tibetan plateau and surrounding areas by the body-wave tomography technique. Based on the velocity variation and tectonic features in the crust and lithospheric mantle, our analysis focuses on the lower crust flow, mantle upwelling and lithospheric thinning, and their relations with Cenozoic volcanic activity and the rift system in southern Tibet. The results indicate that the Tibetan plateau is underlain by the low-velocity zones in middle and lower crust, which are distributed in the Lasa and Qiangtang blocks, enlarged to the Songpan-Garzê block at the lower crust depth. These low-velocity zones are not interconnected, instead separated by high-velocity zones. This implies that ductile deformation in the mid-lower crust is restricted in tectonically special regions and a large-scale crusal flow could not happen throughout the plateau. Likewise, the mantle upwelling seems not to occur widespread in the Tibetan plateau, only confined to the Qiangtang, Songpan-Garzê and eastern Himalayas, which had lead to the thinning of the lithospheric thickness and strong volcanic and magma activities there. Due to the mantle upwelling, which is caused by the subduction of the Indian continental lithosphere, heat flows probably intruded up into the crust. This effect has a significant impact on the formation of Cenozoic volcanic rocks in northern Tibet, the rift system in southern Tibet and the ductile deformation of the middle and lower crust.