| The eastern part of South China is located in the south of the Qinling-Dabie Mountains,with Xuefeng Mountain as the boundary,the west side is a relatively stable Yangtze Craton,and the eastern part is a part of the Cathaysia Plate and the Yangtze Plate,which are structurally deformed violently.It is also known as the Huaxia Complex Mountainous Area.It was formed through the superposition of the intracontinental orogeny in caledonian period,Indosinian movement and composite continental margin structure of western Pacific in Meso-Cenozoic.It was extensively subjected to tectonic deformation,metamorphism and a number of intense magmatic activities.The continental shelf of the Southeastern China,is also a part of the South China Land Block,which is located at the converged and conjoined part of the Asia-European plate,the India-Australian plate and the Pacific plate,and is affected by the joint action of the Tethys and Pacific tectonic domains.There are many research results on the dee resolution p structure of the South China continent,but they are either limited to the velocity structure of the crust scale or the results of large-scale tomography with the insufficient resolution.Results on the high resolution of the upper mantle velocity structure is few,which leading to some key scientific issues related to the deep structure of South China that have not been solved so far.For example,the tectonic framework of the South China continent,the deep dynamics of the lithospheric thinning in South China,and the origin and evolution of the South China Sea and its relationship with the South China Block.Therefore,it is imperative for the high-resolution wave velocity structure information of the upper mantle to understand the deep dynamics process in South China and promote the development of the South China continental structure and dynamics.Two imaging methods are used in this paper:(1)Using finite difference travel time tomography(Fdtomo)to image three-dimensional P-wave and S-wave velocity structures of the upper mantle in the eastern part of South China based on 200 mobile network data in the eastern part of South China.(2)Based on the data of 95 permaanent stations,using the seismic triplication simulation method to perform one-dimensional P-wave imaging on the mantle transition zone below the eastern coast of South China.1,The spatial distribution of crust-scale velocity corresponds well with the surface tectonic units.The basin corresponds to low-speed anomalies,such as the Jianghan basin and the Hefei basin,which suggest that the basin area covers thicker sediments.The orogenic belt corresponds to high-speed anomalies,such as the Qinling-Dabie orogenic belt and Wuyi orogenic belts,which suggest that base rocks are uplifted or bulged.2,The velocity structure of the shallow mantle(33km-123km)shows the NNE tectonism pattern,which is arranged by high and low-velocity bands from west to east;It is coordinated with the tectonic environment controlled by the ancient Pacific subduction system since the Mesozoic in eastern China.3,The most prominent feature of the mid-deep mantle(203 km-283 km)is that the Yangtze block(including the Dabie orogenic belt)exhibits relatively high-speed anomalies,while the cathaysia block generally exhibits low-speed anomalies.In the planimetric map of 203 km,the Jiangshao suture zone defines the northern boundary of the contiguous low-velocity anomaly located in the Cathaysia block.The velocity value of the Jiangnan orogenic belt is between the high velocity of Yangtze and the low velocity anomalies of Huaxia,showing a transitional character.4,Cmparing the planimetric maps of 13km-83km,163 km and 203km-283km,the distribution of velocity anomaly also appears to show the following features:The 163km depth is an important mantle structure boundary,and the large range of high and low-velocity anomalies in this depth was broken and then reorganized.Taking 163km depth as the reference plane,the velocity structure in the mantle above and below the 163 km were reversed.The symmetric axis also rotates about 45°.The velocity anomaly is anti-symmetrical in NW-SE direction at?83 km.While the velocity distributon(the thickness of undamaged cratonic lithosphere)is antisymmetric in the NS direction in the depth range of 203 km-283km.5,The high-speed anomaly below the Dabie Mountains slopes from north to south.This direction is contrary to the former studies that the Yangtze craton has opposite polarity to the direction of the subduction of North China plate.6,There is a high velocity anomaly with a thickness of 215 km-225 km in the mantle transition zone below the continental shelf of the Southeast China and the 660 km discontinuity sinks 5-15 km.These results in our research support that the lithospheric deformation(stretching and thinning)in the eastern part of South China since the Mesozoic era was mainly controlled by the subduction and retreat processes of the Pacific plate.The convection reconstruction of the upper mantle caused by the plate fault and the dehydration of the internal plate accumulation in the mantle transition zone,together constituting a complex and unique dynamic mechanism.Unlike North China,large-scale craton lithosphere delamination may not yet have occurred in the eastern part of South China,and it may be related to the fact that the Cathaysia block is affected by multi-stage structural events and the uncompleted craton process.We interpret the high velocity anomaly in the MTZ to be mianly caused by the high angle diving of oceanic plate,followed by the breaking of fragments which fell into the MTZ,or/and by the unstable thick continental lithosphere dropping into the MTZ,sequentially or almost simultaneously.The result provides new evidence for a hidden subduction zone that now exists beneath the northeastern South China Sea.We postulated that the upwelling of the asthenosphere is the triggering mechanism for the continental margin of Southern China turning from an active margin to a passive margin. |
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