Lateral growth of the Altyn Tagh strike-slip fault at the north margin of the Qinghai-Tibet Plateau: Late Cenozoic strike-slip faults and the crustal stability in the Beishan area, Gansu

Analysis of remote sensing images and field geological features of the Beishan area indicates that the Altyn Tagh fault, Sanweishan-Shuangta fault, Daquan fault and Hongliuhe fault are distributed from southeast to northwest in the Beishan area. The faults are nearly parallel and all left-lateral strike-slip faults. They have similar distribution characteristics and all strike NE40-50o. Secondary dendritic branch faults are developed at the end of the fault system. Granites or blocks intercalated within the branch faults rotate clockwisely, forming “Domino” structure. It is the main structural style of the Beishan area. Analysis of sediments in fault valleys and ESR dating of fault gouge show that the Sanweishan-Shuangta fault formed in the late Pliocene, while the Daquan fault in the early Pleistocene (1.5-1.2 Ma), and that the activities of the branch faults occurred in the late Pleistocene (after 400 ka). Geometric and chronological studies of faults in the Beishan area indicate that the Altyn Tagh fault has propagated in a NW direction since the Late Cenozoic, which is the important growth manner of the Altyn Tagh strike-slip boundary. The special strike-slip structural association style in the Beishan area caused the structural deformation in the area to accumulate energy in a particular fault with difficulty, while the displacement amounts in several secondary faults negligible. So this area is the “most stable seismically active area.