The Characteristics Of Quaternary Activities Of SN Normal Fault And Strike Sip Fault,Central Tibet

The Tibetan Plateau currently experiences N-S compression and E-W extension.A series of～NS-trending rifts with NE-or NW-trending(south and north of the Bangong-Nujiang suture[BNS],respectively),conjugate strike-slip faults are present in the interior of the plateau,considered to absorb E-W extension in central Tibet.However,how exactly they absorb deformation,how exactly normal and strike-slip faults are related;and what are their slip rates remain poorly constrained,which further restricts our understanding of the deformation processes of central Tibet,as well as of regional seismicity.The Riganpei Co fault and Norma Co graben are located near the southern margin of the Qiangtang block in central Tibet,with clear,large-scale evidence of Quaternary activity.In this paper,we determine the exact geometry of the Riganpei Co fault and Norma Co graben boundary normal fault,as well their late Quaternary activity.We also suggest that the Mw6.4 Nima earthquake on July 23,2020 occurred on the Yibug Caka graben.We measured fault offsets based on interpretation of high-resolution satellite images,Li DAR,differential GPS,and UAV surveys,we dated the surfaces using U-series dating,and determined the deep structure of the fault using Ground Penetrating Radar.(1)The left-lateral Riganpei Co fault strikes NEE 60°～80° for ～360 km,with a long-term cumulative horizontal displacement of ～14-15 km.Its late Pleistocene slip rate is about 0.25±0.05 mm/yr,which is significantly lower than the slip rates along the dextral faults south of the BNS.This implies that the deformation mechanisms are most likely different on each side of the BNS.(2)The Norma Co graben strikes ～NE40° and the active faults along its west side strike NE20-50°.The cumulative vertical offsets on the fault range from 1 to 13 m,with possible co-seismic offsets of ～ 1-2 m.We infer that the late Quaternary vertical slip rate is0.13+0.02/-0.01 mm/yr,the extension rate is 0.05+0.02/-0.03mm/yr.(3)The 2020 Nima Mw6.4 earthquake occurred along the Yibug Caka graben,and the seismogenic fault was most likely the normal fault bounding its eastern side.Under the N-S compression context of the Tibetan Plateau,the material of the central Qiangtang block is extruded eastward along the boundary of this large-scale active fault zone,while the western end experiences E-W extension,thus causing the earthquake.(4)We systematically summarize and analyze the geometric distribution characteristics and Quaternary activity of the NS-trending rifts and strike slip faults present in the Lhasa and Qiangtang blocks,and found that: in the Lhasa block,NS-trending rifts are highly active with high slip rates,resulting from divergent thrusting along the curved Himalayan arc.Deformation in the western Qiangtang block by contrast,is distributed on a series of normal and strike slip faults with much lower slip rates.Eastern Qiangtang however acts as a rigid block extruded southeastward along major active strike-slip faults.