Study Of The Present Crustal Deformation Characteristics Of The Southwest Margin Of Ordos Block Were Using GPS

The southwestern margin of Ordos is located at the intersection of the three major structural units of the Ordos block,Alashan block,and the northeastern margin of the Tibet Plateau.Within this area,an arc-shaped tectonic zone with unique kinematics has developed.Therefore,the study of the current crustal deformation characteristics in the southwestern margin of Ordos can not only explore the structural deformation model of the southwestern margin of Ordos and its neighboring regions,but also further enhance understanding of the conversion relationship between the four major structural intersections of the Ordos block,Alashan block,Tibet plateau and Qinling orogenic belt.Meanwhile,the southwestern margin of the Ordos in the northern section of the north-south seismic belt.Its tectonic activity is very strong.There have been many earthquakes of magnitude 8 and above in history.So,the risk of earthquakes should not be underestimated.In this study,we rely on geodesy,geophysics and other methods,comprehensively using the observation data of GPS stations to obtain the current 3D interseismic deformation field in the southwestern margin of Ordos.Further,the GPS velocity field obtained above is used as input data for the inversion model,combined with the collected geometrical kinematic parameters of active faults in the study area,and deep seismic exploration data,a back-slip block model is used to establish a threedimensional kinematics of faults in the study area.In the model,we inversely simulate the slip rate,locking coefficients and slip deficit of the main faults in the southwestern margin of Ordos.Furthermore,the present crustal deformation characteristics in the southwestern margin of Ordos are studied,and the regional dynamic mechanism of its structural transformation is discussed.At the same time,we discussed the elastic deformation around the fault caused by slip deficit,carried out seismic risk analysis,and circled the areas where the seismic risk is greater.The main conclusions are as follows:(1)According to the current GPS three-dimensional velocity field in the southwestern margin of Ordos and its neighboring areas,we found that the northwestern side of the Ordos block moves northward relative to the Ordos block,the Haiyuan fault and its surrounding area moved northeast relative to the Ordos block and the area on the southwestern margin of Ordos hardly moves relative to the Ordos block.The movement of Tibet Plateau to the northeast relative to the Ordos block shows that the northeastward movement of the Tibetan Plateau after the collision of the 50 Ma BP Indian plate and the Eurasian plate is still continuing.Meanwhile,it can be seen inside the South China block site very small rate of movement described relative to the South China block Ordos block and no significant horizontal movement.There are two reasons for speculation.One may be that the Ordos block and the South China block are moving as a whole.The second may be due to the extrusion of the Tibet Plateau,the hard Ordos block and the South China block are difficult to produce structural deformation,and the periphery of the block is driven by the extrusion to rotate the whole block.And the rotation rate of Ordos and South China blocks is about the same.(2)Based on the "back-slip block model",the three-dimensional distribution of the main fault blocking coefficient and slip deficit obtained by inversion with GPS velocity field constraints shows that the west section of Haiyuan fault,the middle west section of west Qinling fault and the northwest section of Liupanshan fault,Longriba fault south section and other areas are in a strong locked state,the seismic risk of the above areas is greater than other areas on the southwestern margin of Ordos.(3)We combined the back-slip block model with the lower crustal flow model,and discussed and analyzed the crustal deformation mechanism and geodynamic mechanism of the southwestern margin of Ordos as the two major structural transition areas between the Tibet Plateau and the Ordos block.The results of the discussion suggest that with the western Qinling as the boundary,the southern Tibetan Plateau and Ordos block tectonic transition area is more suitable for interpretation by the lower crustal flow model.The geometric distribution of the faults and topography in this area reflected the crustal deformation caused by the NE movement of the lower crustal flow.However,the northern Tibetan Plateau and Ordos block tectonic transition area is more suitable for the block model.In this area,the faults are dominated by larger basins.After the lower crust flow flows from the southwest,the block as a whole move,resulting in the current structural form of the southwestern margin of Ordos.