Receiver Function And Surface Wave Tomography Studies Of Southwest China

Southwest China is a collage of many small plates from different geological history, and it is an important segment during the Tethyan tectonic development. Meanwhile, this region plays an important role in the extrusion model of the Tibet plateau materials towards east, the crustal thickening model and the lower crust flow model. Therefore, it is significant to study the features of the crust and upper mantle beneath southwest China, to obtain a deep understanding of the Tethyan tectonic features and the tectonic activity mechanisms of the Qinghai-Tibet plateau. A large number of deep and large faults are distributed in this region. Since the1970s, a series of large earthquakes (M>7.0) happened here. The occurrence of large earthquakes has caused tremendous casualties and great damage to local society and economy. It is significant to carry out detailed studies on the deep structural features of southwest China, to know the regional geo-tectonics, crustal evolution, the cause of earthquakes and even the trend of strong earthquakes.In recent years, a number of large-scale mobile seismic array observations are carried out in southwest China by the China Earthquake Administration and the observation equipment is from the "ChinArray". More than900mobile and fixed broadband seismic stations are deployed in the study region, with an average station spacing of less than40km. In this dissertation, we will make full use of the observational data from the mobile and fixed seismic stations, including using the H-κ stacking technique of P-wave receiver functions to obtain the distribution of crustal thicknesses and Poisson’s ratios; using the3-D seismic migration image of P-wave receiver functions to obtain the spatial distribution of the velocity discontinuity of the upper mantle transition zone; using the seismic migration image of S-wave receiver functions to study the lateral variation in the lithospheric bottom interface; using surface wave tomography to obtain the Rayleigh wave phase velocity maps at periods up to100s, and using the joint inversion of receiver functions and surface waves to reveal the S-wave velocity structure stretching over important tectonic boundaries within200km in depth.Overall, the variation in the crustal thicknesses of southwest China is consistent with the tectonic background. The crustal thicknesses become thinner arc-shaped from-60km in the northwest towards the northeast, southeast and southwest, and the thinnest parts in the southeast and southwest are-30km. The average crustal thickness of the Songpan-Ganzi terrane is-60km. It becomes thinner severely to45km eastwards in Sichuan basin and becomes thinner more slowly towards southeast, indicating the Qinghai-Tibet plateau materials further escape towards southeast after they are prevented by the Sichuan basin.Intermediate and high Poisson’s ratios (σ≥0.26) are mainly distributed in the Songpan-Ganzi terrane, Panzhihua-Emeishan region, Sichuan basin and Tengchong terrane. In these regions, the faults are very dense and criss-cross, showing strong crustal activity features. Because high density, high velocity and high Poisson’s ratio correspond to more mafic compositions in the crust, the high density and higher P-wave velocity in southwestern Sichuan basin and Panzhihua-Emeishan region may be related to the Emeishan basalt distribution. The south part of the Xianshuihe-Xiaojiang fault penetrates northern Panzhihua-Emeishan region, and high Poisson’s ratio caused by partial melting in the lower crust is strengthened by the mafic in the intermediate and upper crust, causing the highest Poisson’s ratio to appear in this region. There are volcanoes in the Tengchong terrane; the Poisson’s ratios are up to0.32at station TNC.YN, implying melting or partial melting magma chambers exist there, which is consistent with the low velocities beneath this region. Low Poisson’s ratios (σ≤0.26) are mainly shown in the southeast part of the study region:Yangtze craton, Cathaysian block and Yunnan-Burma-Thailand block. After we exclude the impact of the sediment, higher Poisson’s ratios are shown beneath the stations near the Red River fault (up to0.28-0.30for part of the stations) compared to the surrounding stations. The velocities in the uppermost mantle are low and the velocity structure is similar to those at the crust-mantle transition zone, indicating the Red River fault is a deeply penetrating fault, and the higher Poisson’s ratios may be related to the partial melting at the bottom of the lower crust.The lithospheric thickness is up to250km in southwestern Sichuan basin, and gradually becomes thinner to200km in the central and southern parts, and further becomes thinner to-100km on the eastern edge and-130km on the northern edge. Double-layer low velocity interfaces exist within100km at the top of the upper mantle beneath the Yunnan-Burma-Thailand block and it is also enough to confirm that the lithospheric thickness in this region is less than100km. Double-layer low velocity interfaces also exist at the top of the upper mantle beneath the Cathaysian block, and its contact styles to the lithosphere of the Yangtze craton are different in the north and west:mutual integration may exist in the west while partition between shallow and deep part may exist in the north. Significant and large-scale low velocity interface exists at100km depth beneath the west of Qinling-Dabie tectonic belt and Weihe plain.The thickness of the upper mantle transition zone is basically greater than250km in the Yangtze craton (mainly including regions like the Sichuan basin and Panzhihua-Emeishan region), and less than250km in the Cathaysian block; it is close to the thickness of the Yangtze craton in thick area and is less than250km westwards. The thick area of the upper mantle transition zone corresponds to the high velocity bodies of P-wave velocity structure at500km depth, which may be related to the subduction materials.S-wave velocity structure beneath stations in typical regions:the S-wave velocities are lower at the top of the upper mantle from45km to90km beneath the Tengchong region. Double-layer low velocity model is shown beneath the Cathaysian block; the upper low velocity layer is thinner and is-60km at depth, while the lower low velocity layer is thicker and is-100-140km at depth. There is a low velocity layer in the upper mantle ranging80-120km at depth beneath the Songpan-Ganzi terrane. A higher velocity layer at the top of the upper mantle ranging50-170km at depth with a large lithosphere thickness is located beneath the Sichuan basin.