Study Of The Crust And Mantle Structure And The Constraint On Deep Processes In The Lower Yangtze River Region And Its Adjacent Areas

The Lower Yangtze River Region is located in the east of Yangtze River Plate. During the Mesozoic,this region underwent a series of intracontinental orogenesis and formed abundant polymetallic deposits,which is the ideal place to study evolution of continent and deep metallogenic mechanism. This thesis applies seismic tomography and SKS/SKKS split in Lower Yangtze River Region and its adjacent areas and aimly to achieve two goals: (1) describe velocity structure of crust and mantle and trajectory of upper mantle materials in the Lower Yangtze River Region and its adjacent areas,finding if there are phenomena such as thinning of lithosphere, upwelling of asthenosphere and subduction of plate;(2) discuss the deep dynamic processes of large-scale tectonic-magmatic activities in the Lower Yangtze River Region and its adjacent areas during the Mesozoic.The result of seismic tomography shows:(1) the velocity structure in shallow depth is consistent well with the tectonic units of surface, basins and orogenic belts are characterized by low and high velocity anomaly, respectively, reflecting the thickness of the sedimentary layer is thick in basins and thin in orogenic belts;(2) there exists low velocity anomaly in upper mantle and mantle transition zone, this thesis believes the low velocity anomaly in upper mantle is the upwelling of asthenosphere and caused by the detachment of lithosphere and the low velocity anomaly in mantle transition zone is the upwelling of hot materials from mantle transition zone to upper mantle and caused by the retention and dehydration of subducting Pacific Plate in mantle transition zone,therefore, this thesis also believes the upwelling of asthenosphere which has the trend that gradually shallower from the south(west) to the north(east) in upper mantle beneath the Lower Yangtze River Region is interrelated with the temporal and spatial patterns of diagenesis and mineralization in this region; (3) there exists high velocity anomalies in upper mantle and mantle transition zone, this thesis explains high velocity anomalies in upper mantle are the detached lithosphere, the detached Yangtze Plate, the thinned root of the Yangtze Plate and the mixture of detached lithosphere and detached Yangtze Plate and the high velocity anomaly in mantle transition zone is the subduction of Pacific Plate,these anomalies prove that there indeed exist phenomena such as thinning and detaching of lithosphere and subduction of plate beneath the Lower Yangtze River Region and its adjacent areas.The result of SKS/SKKS split shows:(1)the anisotropic directions in the Lower Yangtze River Region are consistent well with the trend of this region, which may be caused by the upwelling of asthenosphere that has the trend gradually shallower from the south(west) to the north(east) in upper mantle;(2) the anisotropic directions near Sulu Orogen belt are NEE-SWW in the east and NWW-SEE in the west, respectively,reflecting this region experienced subduction of plate and thinning of lithosphere in the Mesozoic Period,the materials in upper mantle in the east and the west of Sulu Orogen belt produced different flow deformation under different stresses;(3)the anisotropic directions in Dabie Orogen belt are consistent well with the trend of faults, reflecting this region experienced collision and subduction of plate in the Mesozoic Period, the materials in upper mantle of Dabie Orogen belt were accumulated by extrusion stress and resulted anisotropic directions that along faults.Combining results of seismic tomography and SKS/SKKS split in our thesis and a lot of researches of other scholars, we proposed a model of deep dynamic processes of large-scale tectonic-magmatic activities in the Lower Yangtze River Region and its adjacent areas during the Mesozoic, this model suggests that the detachment of lithosphere is the deep mechanism of the thinning of lithosphere and upwelling of asthenosphere, which is also the main reason caused diagenesis and mineralization in the Lower Yangtze River Region.