The Study Of 3-D Structure Of P-wave Velocity And Anisotropy Beneath East-central China

There are complex structures in eastern-central China,and a series of special structures and geological phenomena have made the region a hot spot of international research.The middle and lower of the Yangtze River metallogenic belt,which is located in the center of the research area,is the most important polymetallic mineral resource base in the east of China.The deep dynamics of the metallogenic belt and the formation mechanism of metallogenic magma have been debated by the academic circles for a long time.In this paper,479 teleseismic events recorded by 321 stations included China seismic network and temporary seismic arrays of the study area are used,and 32728 Pwave arrival-time data are obtained by improved multi-channel cross correlation method(MMCC).After the test of the checkboard,the 1°× 1°longitude and latitude grid is used horizontally,and the grid is set at the depth of 50,100,200,300,400,500,600,700 and 800 km.The P-wave velocity structure and anisotropic horizontal and vertical section images are retrieved by the method of tomography,in which the anisotropy includes azimuth and radial anisotropy.The conclusions are as follows:(1)Through the analysis of velocity anomalies,it is found that there are "two highs and one low" velocity anomalies beneath the region of middle and lower of the Yangtze River.Based on the relevant geological data,it is considered that the shallow highvelocity anomalies reflect the existing lithosphere,while the high-velocity anomalies in the bottom of the upper mantle and the mantle transition zone are interpreted as the detached lithosphere and the subducted Pacific plate,respectively.While the large-scale low-velocity anomalies in the upper mantle are upwelling Hydrous hot material.(2)According to the anisotropic structure,the azimuthal fast velocity direction in the upper mantle of North China plate is NW-SE,which is basically consistent with the absolute plate motion,indicating that the origin of anisotropy in this area is related to mantle flow.The subducted Pacific plate has subducted into the mantle transition zone of the middle and lower Yangtze Block,which blocked the upwelling of mantle material under it.At the same time,in the same depth,a large area of low-velocity material in the Cathaysia Block present NNE of azimuthal fast velocity direction and mainly in the direction of ascending velocity,and after reaching the upper mantle,it migrates northward to the Yangtze Block,and the material migration track is basically consistent with the direction of anisotropy.(3)Combining our tomographic images with the previous geological,geophysical and geochemical results,we infer that the deep dynamic process in the study area is as follows: in the Triassic,the collision and fusion between the North China Craton and the South China plate occurred,and the lithosphere in eastern China had began to thicken.In the Middle Jurassic,the lithosphere in eastern China continued to thicken under the control of the NW subduction and compression of the Pacific Plate;later,the Yangtze Block detached,the lithosphere mantle broke,the lithosphere stretched and thinned,and large asthenosphere materials was upwelling.During the late Jurassic and early Cretaceous,the lithosphere in the lower Yangtze area was further disintegrated,and asthenosphere thermal material umderplated the lower crust then rose along the fault to the shallow for diagenesis and mineralization.