The crust and upper mantle structure beneath Yunnan from joint inversion of receiver functions and Rayleigh wave dispersion data

In this study, we apply joint inversion of receiver function and Rayleigh wave dispersion (phase and group) velocities data to estimate a shear wave velocity structure for the Yunnan and its adjacent region. Our joint inversion models indicate strong lateral variations in crustal thickness in this area, which increase gradually from 32 km in SE to 56 km in NW of Yunnan. The gradual change in crustal thickness matches with the gradual topography change over this region. Our models also show a uniform crust–upper mantle S-velocity structure across the region, with a 8–22 km thick upper crust (Vs < 3.4–3.5 km/s), overlying a gradational middle-to-lower crust with S-wave velocities up to 3.5–4.1 km/s at Moho depth. The thickness of middle-to-lower crust is correlated with the Moho depth beneath the seismic stations. There exists low-velocity zone in middle-to-lower crust beneath the Yunnan region, especially in the western Yunnan. It is possible that the thick crust in this study area is mainly caused by a more ductile flow in the middle-to-lower crust. The Sn velocities are slightly higher beneath the southern Yangtze block and Southeast Yunnan fold belt (4.35–4.75 km/s) than the other block (4.15–4.35 km/s). This low Sn velocity beneath western Yunnan and Panxi rift may imply that there has been thermal disturbance to the uppermost mantle away from deep structures in this region. The Vs in lower crust and upper mantle are different across the Red-river fault, which imply that Red-river fault is one main boundary fault. Combining results from other geophysical investigations, we conclude that both the Simao terrane and Baoshan–Tengchong terrane tectonically belong to the Gangwana plate.