| The middle reaches of the Yangtze River and surrounding region is an important area which has experienced complex geological processes,is home to various resources and is prone to geological disasters.This region inherits important past geological processes as it's located in the geotectonic domain superimposed by Tethys and the Pacific Ocean.This region is also an important area for studying the dynamics of the basin-mountain coupling and dynamics of the Jianghan Basin and its surrounding mountain,the collision mode of the South China plate and North China plate,the exhumation mechanism of the Ultrahigh pressure(UHP)metamorphic rocks.Geologists have carried out a number of studies in this region and have achieved a good understanding of the tectonic evolution.However,there are still many ambiguities in the shear wave velocity and radial anisotropy structure of the study region.This thesis uses three-component continuous waveform data obtained from the China National Seismic Network,the regional seismic network and transportable seismic array recorded by 142 stations located in the middle reaches of the Yangtze River.Using these seismic data,this thesis adopts ambient noise tomography and teleseismic two-plane-wave tomography techniques to obtain the group and phase velocity maps of Rayleigh waves and Love waves and Rayleigh wave H/V.Then,a Bayesian-Monte Carlo method is used to jointly invert the Rayleigh/Love wave group/phase velocity dispersion curves and Rayleigh wave H/V to construct the crust and upper mantle shear wave velocity and crust radial anisotropic structure of the study region.These models provide geophysical constraints on the understanding of dynamic mechanism of the study region.The main works and results of this thesis are as follows:(1)The group and phase velocity map of Rayleigh/Love waves and Rayleigh wave H/V map of the study region.Based on the three-component continuous seismic waveform data,the thesis obtained the Rayleigh/Love group velocity maps at 8~30 second periods,the Rayleigh/Love phase velocity maps at 8~80 second periods and Rayleigh wave H/V maps at 8~30 second periods,by using ambient noise tomography combined with teleseismic surface wave tomography and Rayleigh wave H/V measurement method.(2)The crust and upper mantle shear wave velocity and radial anisotropy structure.Jointly inverting the group and phase velocity dispersion curves of Rayleigh/Love wave and Rayleigh wave H/V,the thesis constructed the crust and upper mantle shear velocity structures and the crust radial anisotropy structure.In the upper crust(< 10 km),Various basins are characterized by lower shear velocity anomalies and positive radial anisotropy.The northwestern part of the Daba Mountain,the Wuling Uplift and surrounding region and the Eastern Qinling-Dabie orogenic belt are characterized by higher shear velocities and weak negative anisotropy.In the middle crust(15-25 km),the difference of the higher shear velocities between the northwest and southeast wings of Daba Mountain and Daba Mountain becomes smaller.The thesis interprets the high velocities of the east and the west as two mechanically strong blocks which could lead to the uplift of Daba Mountain and the eastward expansion of the Qinghai-Tibet Plateau and the westernward of Pacific.In the lower crust and upper mantle(25-50km),the shear velocities show that the crust of Yangtze plate is characterized by "V" type thinning.In the lithospheric mantle(50-80km),It is characterized by high shear wave velocities in the western part of China and low shear wave velocity in the northern part of China,suggesting that the thickness of the lithosphere in the eastern China is about 70-80 km.(3)The basin-mountain coupling relationship between the Jianghan Basin and the surrounding orogeny.The shear wave velocity structure and radial anisotropy structure of the east-west section and the north-south section of the Jianghan Basin are significantly different,revealing different synergistic evolution processes between the basin and mountain.In the east-west direction,the region is controlled by the Indian-Eurasian collision and the post-arc expansion caused by the westward subduction of the Pacific plate.In the north-south direction,the Jianghan Basin is located in the middle of the Yangtze plate,which is sandwiched between the Jiangnan orogenic belt and the Qinling-Dabie orogenic belt.Since the mid-Mesozoic era,this region experienced complex dynamics of the sea-continental intersection,continental intersection superimposed basin,and the piece of the north Yangtze plate and south Yangtze plate.The depth of the Moho gradually increases from the Jiangnan orogenic belt to the Jianghan Basin,which may be caused by the superposition of the Philippine plate and the Pacific plate.The depth of the Moho beneath the Shangdan fault reveals the ancient suture between the Yangtze craton and the North China craton.(4)The significant difference of radial anisotropy structure in the crust between the region of the Three Gorges area and Jianghan Basin.Two weak negative radial anisotropy structures are present beneath the Badong-Zigui-Yichang area,one extending from the lower crust at the edge of the Sichuan Basin to the upper crust of the Badong-Zigui,and the other changing from positive anisotropy in the lower crust of the Jianghan Basin to weak negative anisotropy and extending to the upper crust of Badong-Zigui area.The radial anisotropy changes from negative to positive in the middle crust(depth of 10-20 km)and lower crust beneath the Jianghan Basin.The differences in radial anisotropic structures between the Three Gorges and Jianghan basins may be caused by tectonic thermal events or magma upwelling.(5)Tectonic evolution of the Daba Mountain and surrounding areas.Compared with the Hannan-Micang mountain uplift and the Huangling anticline,the Daba Mountain is characterized by lower velocities in the middle and lower crust,which suggests that this mountain is formed by the intracontinental orogeny by the hard Hannan-Micang mountain uplift,Huangling anticline,and the Qinling orogenic belt since the late Mesozoic.It is also an important area that has blocked the expansion of the Qinghai-Tibet Plateau to the northeast and east since the Cenozoic.The Hannan-Micang Mountain,Daba Mountain and Huangling anticline-Jianghan Basin have two uplifted areas,suggesting that the eastward extension of the Qinghai-Tibet Plateau is strongly blocked by the Hannan-Micang Mountain.The Moho depth and the shear wave velocity heterogeneities of the mantle beneath the Daba Mountain are different from those beneath the Hannan-Micang Mountain,which implies that the eastward expanding of the Qinghai-Tibet Plateau did not completely traverse the Daba Mountain,and the deformation of lithospheric block extrusion and the thickening of the crust increase thickness may evolve with each other.(6)The "V" thinning mode of the thickness of the sedimentary layer and the depth of the Moho.The thesis reveals a "V" thinning mode of the thickness of the sedimentary layer and the Moho depth,while the position of the Moho thinning and the position of the sediment layer thinning are inconsistent.This difference may be due to the retreat of the Pacific subduction plate and the eastward expansion of the Qinghai-Tibet Plateau,which may cause the lithospheric mantle and the upper-middle crust in the study area to undergo different tectonic stress superposition effects. |
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