Complicated crustal deformation beneath the NE margin of the Tibetan plateau and its adjacent areas revealed by multi-station receiver-function gathering

We use a multi-station stacking method to analyze receiver function data recorded by a large-scale dense seismic array covering the northeastern (NE) margin of the Tibetan plateau and its adjacent regions to estimate crustal thickness (H), Vp/Vs ratio (κ) and crustal anisotropy (fast polarization direction φ and splitting time τ). For each station of the array, we gathered all the receiver functions recorded by the station and its nearby stations located inside a circle with a radius of 0.5°. We applied the H−κ stacking technique and the joint inversion scheme to a total of 654 station clusters to measure the (H,κ) and (φ, τ), respectively. The measured Moho depth varies from a peak value of 67 km beneath the northern-central plateau to 39-45 km beneath the surrounding blocks in the northeast. The front of the depressed Moho beneath the plateau margin exhibits a complicated geometry, which suggests that when the Tibetan plateau encroached upon the surrounding terranes, the front expanded unevenly depending on the strength of the encountering terranes. The thickened crust beneath the margin also has a very low Vp/Vs ratio, indicating that the crust is composed largely of felsic minerals. More than one third of the station clusters (221) have a splitting time significantly larger than 0.2 s. The average splitting times of the 221 measurements is 0.68 s, which is comparable to those measured from the SKS/SKKS (XKS) phases (0.94 s). The fast directions estimated from the Moho Ps and XKS phases recorded by the station clusters within the margin are very similar, and are also approximately parallel to the strikes of major faults, sutures and thrust fronts in the area. The low Vp/Vs ratio, together with the observed fast polarization directions from the Moho Ps and XKS phases, suggests that shortening of the entire lithosphere orthogonal to the compressional direction is likely the main cause for the observed crustal thickening occurring at the margin. We also found that the Moho Ps and XKS recorded by stations inside the western Hetao basin and the Yinchuan basin present very different anisotropy, which may suggest that the crust and mantle beneath the two basins have very different deformation patterns.