Lithospheric thickness, thinning, subduction, and interaction with the asthenosphere beneath China from the joint inversion of seismic S-wave train fits and Rayleigh-wave dispersion curves

A joint inversion method of simultaneously inverting regional multimode surface waveforms and fundamental-mode surface wave dispersion curves is used to better constrain a 3-D S-wave velocity model for the upper mantle beneath China. Thousands of broadband vertical-component seismograms were processed, and ∼ 16,500 Rayleigh-wave group-velocity measurements were retrieved at a period of 20 s, with fewer measurements at shorter and longer periods. In addition, 4947 regional Rayleigh waveforms were obtained for paths passing mainly through the Chinese mainland. The joint inversion of this dataset provides new S-wave velocity structures at upper-mantle depths down to 400 km. In particular, the new model is resolved to greater depths than the approximately 200 km depth obtainable in most fundamental-mode surface-wave tomographic studies, and is better resolved at shallow lithospheric depths than that possible in teleseismic body-wave tomographic studies. Because the upper-mantle S-wave velocity is controlled mainly by temperature rather than by composition, thermal structures estimated from the tomographic S-wave velocity model are used to estimate the thickness of the lithosphere beneath China.The results indicate that in eastern China, the North China craton and the Yangtze craton are characterized by pronounced variations in lithospheric thickness, with the lithosphere thinning from west to east. The underlying low-velocity layer and deeper-level high velocities provide important evidence in support of the idea that lithospheric thinning beneath eastern China is related to the westward subduction of oceanic lithosphere to the east and related thermal and dehydration processes. Beneath western China, thickened lithosphere is imaged beneath the Qinghai-Tibetan Plateau, except for beneath Qiangtang in the northern part of the plateau, which is underlain by low velocities. The present model generally supports the interpretation of low velocities beneath Qiangtang as mantle material squeezed out by convergence between the Indian and Asian lithospheres. Moreover, two separate segments of the subducting Indian lithosphere are imaged, indicating that the deeper portion of the subducting slab may have started to break off from the shallower portion as early as 20 Ma.