Late Cenozoic tectonic deformation of the Haiyuan fault zone in the northeastern margin of the Tibetan Plateau

Haiyuan fault zone, which is the boundary fault zone of the northeastern margin of the Tibetan Plateau, provides a laboratory for studying how the plateau has grown northeastward, owing to its plentiful tectonic deformational styles in the Cenozoic time. The analysis results of the apatite fission-track thermochronology from the South Huashan in the hanging-wall of the Haiyuan fault and the seismic reflection sections across the Haiyuan fault zone suggested that the fault had undergone northeastward thrusting with the following left-lateral strike-slip motion since the Late Cenozoic time. In the first phase, the initial northeastern thrusting along the Haiyuan fault occurred at 12±3 Ma and caused rapid uplift of the ranges and flexural subsidence in the hanging-wall and foot-wall of the fault, respectively; at the same time, the thrusting also separated the huge Cenozoic sedimentary basin in the NE margin of the Tibetan Plateau. This event apparently suggested that the outward growth of the Tibetan Plateau had propagated into the present-day NE margin of the plateau and made the Haiyuan fault as the newest part of the plateau since ~12 Ma. The second phase deformation along the Haiyuan fault initiated at ~5.4 Ma and was characterized by gradual increase in left-lateral strike-slip rate. The component of left-slip displacement along the Haiyuan fault was transferred into eastward crustal shortening in the Liupan Shan and Madong Shan region. This phase of deformation may have resulted from the late period of the northeastward convergence in the NE margin of the Tibetan plateau, which led to the eastward motion of materials in the NE Tibetan Plateau along the main boundary faults.