The Velocity Structure Of The Crust And Mantle Beneath The East Margin Of The Tibetan Plateau

The eastern margin of the Tibetan Plateau locates at the junction of Qiangtang Block, Songpan-Ganzi Fold Belt and South China Block, where the tectonics are complex and geological activities are strong. Although many scholars studied this region, there are many disputes existing about the tectonic mechanisms of it. From2011to2013, The Institute of Geophysics in China Earthquake Administration with other institutions implemented the Project of ChinaArray in this region and set up350broadband temporary seismic stations with the station-to-station distance about30km. Using the unprecedented high-density distribution of seismic stations, we conducted the research of ambient noise tomography and teleseismic P-wave tomography in the eastern margin of the Tibetan Plateau.The ambient noise tomography has been developing rapidly since2004and the crucial theory basement of this method is the extraction of surface-wave Empirical Green’s function from long-time continuous waveforms. We collected continuous waveforms of557seismic stations in the eastern margin of the Tibetan Plateau, and after careful data preprocessing, calculated more than50thousand empirical Green functions using improved ambient noise data processing procedure. Use CPS (Computer Programs in Seismology) software to measure13,972Rayleigh wave group dispersion curves with application of empirical Green functions with SNR(Signal-Noise Ratio) bigger than10. To ensure the reliability and improve the efficiency of the inversion, we use Cluster Analysis technique to select dispersion curves.6203dispersion curves are used in the inversion eventually and Rayleigh-wave group velocities at periods6-48S are got.According to images of the Rayleigh-wave group velocity distribution at periods6-48S, this thesis summarizes the characteristic of group-velocity distributions. It found:(1) Sichuan Basin shows obvious low-velocity anomaly between periods6S and14S and the distribution of low-velocity anomaly is not uniform. At periods30-48S, weak high-velocity anomaly exists below Sichuan Basin and in the south of western basin it is lower than in the north.(2) At periods6-48S, high-velocity anomaly exists below Panzhihua and its surrounding area.(3) From the period of24S, the Nanpanjiang basin appears as high velocity and in the northwest its border coincides with Nanpanjiang.In combination with geological and other geophysical information of the Tibetan Plateau, this thesis explains these phenomena and discusses their tectonic implications.(1) Relative to the nearby Songpan-Garze fold belt, Qiangtang block, and other blocks, the Sichuan Basin has younger and thicker sediments. This leads to low velocity anomaly in the Sichuan Basin. Furthermore, the low velocity anomaly sketches out the boundaries of sediments of Sichuan Basin. The complex sedimentary history of Sichuan basin resultes in uneven distribution of sediment thickness in the basin, and the distribution of low-velocity anomaly reveals the lateral variation of sediments thickness. According to crustal thicknesses of the Sichuan Basin and its surrounding area, high-velocity anomaly at long periods in Sichuan Basin is caused by its thin crust thickness.(2) Inverted S-wave velocity profiles in the northwest and southwest of Sichuan Basin shows the low-velocity anomaly in the southwest is due to the low S-wave velocity in the lower crust and upper mantle. We speculate hot mantle material beneath the east of the Tibetan Plateau maybe erodes the lithospherical mantle of the Sichuan Basin. The reason also explains the differences of Poisson ratios, heat flow values and tectonic-activity intensity between north side and south side of Chengdu.(3) Geological and geochemical studies have shown that the Emeishan Large Igneous Province was caused by magmatism associated with the mantle plume. The distribution of mafic and ultrmafic rocks are centered at Miyi and the thickness at the center is biggest. We speculate this high-velocity anomaly is probably caused by the intrusion of mafic-ultramafic rocks in Emeishan magmatic activity. This high-velocity anomaly with strong medium strength impedes the material extrusion of Chuanxibei Subblock to some extent, bringing about the enormous difference of topography in both sides of Lijiang-Xiaojinhe Fault.(4) The inverted S wave velocity profile uncovers that the velocity of Nanpanjiang Basin is higher than that of places to the northwest of the basin from upper-middle crust to upper mantle. In combination with the crustal motion pattern, we infer that this high velocity, strong strength Nanpanjiang Block plays a part in the obstruction of southeastward material extrusion of the Tibetan Plateau.The teleseismic body-wave tomography is a common approach to study the structure of deep earth, and its results have high resolution in the vertical profiles. In this research, we collected111718P-wave traveltimes of888tele-earthquakes recorded by516seismic temporary and permanent stations in the study area. Using the highest density of seismic station distribution up to now, we got the fine P-wave velocity structure with application of FMTT technique. FMTT uses wave-front tracking method during the forward calculation and subspace inversion method for the inversion of travel-time residuals. Compared with commonly used traditional ray tracing method in the forward calculation and LSQR algorithm in the inversion, wave-front tracking method is stable and able to track complex phases; subspace inversion method requires less memory and can handle multiple parameters at the same time. To verify the reliability of the final result and resolving power, this research conducts a recovery test. Results shows that except the amplitude of anomaly is different, the morphologies of anomaly are basically the same in the original model and the recovered model, which confirms the reliability of the method. The anomaly of at the size of about0.5°in the original model can be recoverd in the recovery test result, which gives the confidence of the resolution of0.5°×0.5°.According to the resulted P wave-velocity structure, we have some conclusions about the tectonic mechanisms of the east margin of the Tibetan Plateau.(1) The result shows that high-velocity anomaly exists above50km in Panzhihua and its surrounding area, which we conclude caused by the intrusion of mafic and ultramafic magmas during the Permain-Triassic magmatism activity creating the Emeishan Large Igneous Province. At the same time, we infer the rising channel of magmas may situate beneath Panzhihua.(2) Furthermore, this research provides the changes of P-wave velocity with the depth under Sichuan Basin, that is above100km it is very high and at the depth of150km, it is weakly high. Due to the Sichuan basin being formed on the base of the Yangtze platform in the Proterozoic, and the current characteristics of surface heat flow distribution similar to the typical craton, the cold and dense lithospere of Sichuan Basin is the reason of high velocity.(3) At the profile of28°N, the high-velocity anomaly beneath Sichuan Basin is thick in the east and thin in the west. The low-velocity anomaly beneath the Chuandian rhombic block is deep towards the Sichuan Basin. Referring to the crust thickness of eastern Tibetan Plateau and Sichuan Basin, we educe the eastward transportation of the hot material of asthenosphere of the eastern Tibetan Plateau is blocked by the cold, dense and hard lithosphere of Sichuan Basin, and then changes the direction to flow downward. This downward material flow erodes the base of the lithosphere of the Sichuan Basin, bringing about the weak high-velocity anomaly.(4) The result gives the image of low-velocity anomaly deep to400km beneath Tengchong Volcano, and high-velocity anomaly below it being nearly flat at the depth of between400km and600km. We deduce that the high-velocity anomaly is the subducted Indian plate and the dehydration of the subducting slab leads to the upwelling of hot mantle material, which may rise to the crust through faults in the lithosphere, making the formation of volcanoes and hydrothermal places. Referring to the shape of the high-velocity anomaly, we think that the front edge of the subducting slab reaches to105°E and the depth of400-600km.According to the discussion of the P-wave velocity and Rayleigh wave group velocity distribution, the research gives some explanations of mechanisms of block interactions between the Tibetan Plateau and South China Block, which increases our knowledge about the Tibetan Plateau.