| The Qinghai-Tibet plateau is a typical plate convergence boundary zone which is located in the transition zone between the Indian plate and the Eurasian plate.The tectonic deformation of the Qinghai-Tibet plateau is caused by the continuous extrusion of the Eurasian plate northward of the Indian plate under the action of continental tectonics since the Phanerozoic era.As the highest,largest and most active one of the plateaus,whose unique and complex topography and internal structure change has been received great attention of scientists all over the world,there has never been a unified conclusion about the mechanism of its uplift,the mode of material transport,and the dynamics of its origin.The study of the Qinghai-Tibet plateau can help us understand the earth science problems of tectonic evolution such as geodynamics and plate theory.Northeast plateau of Qinghai-Tibet as the mainland of the Qinghai-Tibet plateau expansion movement of the frontier area,studying the crust-mantle structure and tectonic deformation ways is key to understand the Qinghai-Tibet plateau uplift mechanism,extension deformation and continental dynamics.The seismic surface waveform data of 390 events from 2013 to 2016 were obtained from 674 mobile stations located on the northeast plateau of the Qinghai-Tibet by China digital seismic network.The method is the use of multi-channel waveform crosscorrelation technology within the region based on eikonal equation to get 20-150 s Rayleigh wave apparent phase velocity and group velocity maps,the grid is 0.4°x0.4°,and then get structure maps based on Helmholtz equation of phase velocity and group velocity is related to the frequency of the limited frequency effect correction stations velocity profile at the bottom of the structure.A one-dimensional s-wave velocity structure inversion is performed on the dispersion curve obtained from each grid point,and a three-dimensional velocity structure model is established in the research area.Combining previous research results in this region,analyzing and inferring the distribution characteristics of each block area velocity structure and causes,and material migration channel,and earthquake distribution is closely related to the velocity structure.The results are as follows:The s-wave velocity structure imaging results obtained from the combined inversion of surface wave phase velocity and group velocity can be used to visually observe the stratigraphic classification and structural information of crust and mantle: High and low speed line from the Qilian mountains to the Longmen mountain in the north-south seismic belt is pretty clear,the difference of velocity on both sides is apparent,which confirms the Qinghai-Tibet plateau as a low-speed weak block extrusion process in the north against by hard Alax block and Ordos block,and weak medium broken happened by the resistence of Yangtze block;Within the scope of the crust and upper mantle,along the Yinchuan-Linhe remain the state of low speed distribution near the fault zone,and increasing with depth distribution scope expanding gradually at low speed,which may be due to block Ordos' s resistence happened in the process of extrusion formation breakdown from the surface to the mantle,heat material in the mantle upwelling along the crack leading the low speed result.In the middle and lower crustal depth,the low velocity distribution of Qinghai-Tibet plateau is connected with the low velocity distribution of Yinchuan-Linhe fault area.The velocity structure of the Alax block below the depth of 150 km is quite different from that of the surroundings blocks,which presents obvious relatively low speed and belongs to the unstable block.The dense array detection experiments make us obtain the result of the higher resolution of velocity profile than ever,the imaging result provides the more subtle characteristics of small block and the 3-D velocity structure model of the material for the whole northeast plateau of Tibet area migration and tectonic evolution provides a new theory support. |
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