Decomposition and Evolution of Intracontinental Strike‐Slip Faults in Eastern Tibetan Plateau

Abstract:Little attention had been paid to the intracontinental strike‐slip faults of the Tibetan Plateau. Since the discovery of the Longriba fault using re‐measured GPS data in 2003, an increasing amount of attention has been paid to this neglected fault. The local relief and transverse swath profile show that the Longriba fault is the boundary line that separates the high and flat tomography of the Tibet plateau from the high and precipitous tomography of Orogen. In addition, GPS data shows that the Longriba fault is the boundary line where the migratory direction of the Bayan Har block changed from eastward to southeastward. The GPS data shows that the Longriba fault is the boundary fault of the sub‐blocks of the eastern Bayan Har block We built three‐dimensional models containing the Longriba fault and the middle segment of the Longmenshan fault, across the Bayan Har block and the Sichuan Basin. A nonlinear finite element method was used to simulate the fault behavior and the block deformation of the Eastern Tibetan Plateau. The results show that the low resistivity and low velocity layer acts as a detachment layer, which causes the overlying blocks to move southeastward. The detachment layer also controls the vertical and horizontal deformation of the rigid Bayan Har block and leads to accumulation strain on the edge of the layer where the Longmenshan thrust is located. After a sufficient amount of strain has been accumulated on the Longmenshan fault, a large earthquake occurs, such as the 2008 Wenchuan earthquake. The strike slip activity of the Longriba fault, which is above the low resistivity and low velocity layer, partitions the lateral displacements of the Bayan Har block and adjusts the direction of motion of the Bayan Har block, from the eastward moving Ahba sub‐block in the west to southeastward moving Longmenshan sub‐block in the east. Four models with different depths to the Longriba fault were constructed: (1) a shallow fault with a depth of only 4 km, (2) a deeper fault that is half as deep as the Longmenshan fault, (3) a deep fault that is 2 km shallower than the low resistivity and low velocity layer, and (4) a fault that is as deep as the low resistivity and low velocity layer. The activity and influence of the Longriba fault with different development stage under this tectonic system were shown: in one Earthquake recurrence period, the rupture region of the fault increases with the depth of the fault, and the lateral slip partition by the fault also changes with the fault depth. It suggests that the Longriba fault is a newly generated fault that developed after the quick uplift in Late Cenozoic along this tectonic setting and gradually extended from the northwest to southeast. The calculations provide the characteristic of block deformation and fault behaviors of intra‐continental strike‐slip fault and major boundary thrust faults in the eastern margin of the Tibet plateau. Although the low resistivity and low velocity layer controls the deformation of the Bayan Har block and the uplift of the Longmenshan thrust, the partition of the Longriba fault has an important influence on the intra‐plate deformation and modern geomorphic evolution.