Tomographic Study On The Crustal And Upper Mantle Structure Beneath The South-North Seismic Zone

Imaging the crustal and upper mantle structure is an effective way to research the deep structure of the Earth and geodynamic problem. The South-North Seismic Zone is the transitional belt between the Tibetan plateau and its adjacent blocks, where the geological structure is quite complicated and the seismic activity is very high. Imaging the crustal and upper mantle structure beneath the South-North Seismic Zone is beneficial to deepen the understanding of geodynamic problem of the interaction between the Tibetan plateau and its adjacent blocks. The purpose of this thesis is to obtain the structure of the crustal and upper mantle beneath the South-North Seismic Zone. We use teleseismic P-wave receiver function to estimate the Moho depth and Poisson’s radio in the South-North Seismic Zone, apply traveltime seismic tomography to invert the P-wave velocity structure in the southern South-North Seismic Zone, utilize the ambient noise data to calculate the S-wave velocity in the middle South-North Seismic Zone, and employ seismic surface wave data to gain the phase velocity of Rayleigh wave and Love wave and the S-wave velocity in the South-North Seismic Zone. By analyzing and discussing the obtained results, this thesis gets several main conclusions as follow:(1) The distribution of Moho depth beneath the South-North Seismic Zone presents gradually deepening trend from east to west, which correlates well with the trend of surface topography. The thinnest crustal thickness (-27km) in the southern Yangtze craton and the thickest crustal thickness is estimated in the western Songpan-Ganzi terrane. The estimation of Poisson’s ratio is an important seismological method to measure the mineral composition of the crust. The distribution of Poisson’s ratio in this thesis shows the significantly lateral heterogeneity. Low Poisson’s ratios (0.231) appear in the Yangtze craton, which denotes the curst may be more felsic in composition. Intermediate values of Poisson’s ratio (0.24≤υ≤0.27) exhibit in the Ordos block, the Qilian orogen, the Qinling-Dabie orogen, the Songpan-Ganzi terrane and the Sichuan basin. High Poisson’s ratios (0.287) are found in the Chuandian rhombic block, which suggests the crust may be dominantly mafic in composition. Crustal Poisson’s ratios are greater than0.30in the northern Chuandian rhombic block. Taking into account other geophysical parameters, such as the low resistivity, the low S wave velocity and the high heat flow, these higher Poisson’s ratios imply the crust in this region is likely to indicate the presence of locally partial melting.(2) The P-wave velocity structure of shallow crust in the southern South-North Seismic Zone presents banded feature, which is similar to the surrounding fault strike of Chuandian rhombic block. The low velocity anomaly extending to about400km depth is observed in the Tengchong volcanic area. The obviously high velocity anomaly is found beneath the Sichuan basin and the Yangtze craton, and this anomaly is also observed by the previous studies with seismic tomography. The stronger low velocity anomaly is located at the depth of50km beneath Chuandian rhombic block, and the area distributing the low velocity anomaly is consistent with that distributing the high Poisson’s ratio. Therefore, the locally partial melting inferred by Poisson’s ratio anomaly may locate in the mid-lower crust.(3) Demarcated by the Longmenshan fault belt, the east and the west distributions of S-wave velocity structure in the middle South-North Seismic Zone suggest the completely opposite characteristics at different depth. In shallow depth, the Sichuan basin displays low S-wave velocity due to thick sedimentary deposits, while the eastern Tibetan plateau presents high S-wave velocity, which attributes to the shallow depth of bedrock. In deep depth, the S-wave velocity beneath Sichuan basin is higher than that under the eastern Tibetan plateau. Moreover, the eastern Tibetan plateau is filled with the obviously low velocity layer in the mid-lower crust.(4) Beneath the South-North Seismic Zone, the distributions of Rayleigh wave and Love wave phase velocity at different periods show the different features, and the S-wave velocities at different depths also present different characteristics. At short period10s, the phase velocity of Rayleigh wave and Love wave under the Sichuan basin with the thick sedimentary deposits is lower than that beneath the eastern Tibetan plateau with the crystalline rocks, which agrees with the distribution of S-wave velocity at depth10km. At periods from20to40s, the phase velocities of Rayleigh wave and Love wave mainly reflect the structure in the mid-lower crust. Bordered by the Qilian orogen, Longmenshan fault belt, Anninghe fault belt and Xiaojiang fault belt, the eastern Tibetan plateau, to the west, displays lower phase velocity, while Alashan block, Ordos block, Sichuan basin and Yangtze craton, to the east, show higher velocities. In the mid-lower crust, the significantly low S-wave velocity anomaly exhibits beneath Songpan-Ganzi block and Chuandian rhombic block. At depths from90to120km, the distinctly S-wave velocity anomaly is observed between the Himalayas east structural knot and Xianshuihe fault belt, which may be related to that the Indian lithosphere plate has subducted down to the southeastern Tibetan plateau along the Himalayas east structural knot. Beneath the Chuandian rhombic block, the mid-lower crust shows low velocity, while upper mantle presents high velocity. We think that this distribution of velocity anomaly provides the necessary condition of the crust-mantle geodynamic decoupling.(5) In the mid-lower crust of Songpan-Ganzi block and Chuandian rhombic block, combining with our results of the low P-wave velocity, the low phase velocity of Rayleigh wave and Love wave and the low S-wave velocity, and other people’s results of the low P-wave and S-wave velocities, the high heat flow and the low resistivity observed by different geophysic methods, we infer that these anomalies could be the seismological evidence of the mid-lower crustal flow existing in the mid-lower crust of Songpan-Ganzi block and Chuandian rhombic block. Affected by the collision between the Indian and the Eurasian plates, the crustal materials of the Tibetan plateau escape in an easterly direction, then are resisted by the rigid Sichuan basin and redirected into northeastward and southeastward, which causes the crustal deformation of the eastern Tibetan plateau and the interaction between the Tibetan plateau and its adjacent blocks, results the high seismic activity in this area and induces, such as the Wenchuan and Lushan earthquake, the strong earthquakes.