GEOMORPHIC EVIDENCE FOR THE LATE CENOZOIC STRIKE-SLIPPING AND THRUSTING IN LONGMEN MOUNTAIN AT THE EASTERN MARGIN OF THE TIBETAN PLATEAU
Li Yong~① Zhou Rongjun~② A.L.Densmore~③ M.A.Ellis~④(①State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation,Chengdu University of Technology,Chengdu610059;②Institute of Earthquake Engineering,Seismological Bureau of Sichuan Province,Chengdu610041;③Institute of Geology,Department of Earth Sciences,ETH Zentrum,CH-8092 Zürich Switzerland;④Center for Earthquake Research and Information,( )University of Memphis,3890 Central Avenue,Memphis,TN 38152 USA)
Geomorphic evidence for active tectonics in the Longmen Mountain at the eastern margin of the Tibetan Plateau has been studied in this paper.We have surveyed some typical geomorphic markers in Wenchuan-Maowen Fault,Beichuan Fault,Pengguan Fault,and Dayi Fault,including terrace offsets,scarps,fault-controlled saddles,dextral shutter ridges,dextral channel offsets,grabens,shatter belts, and pull-apart basins.Electron spin resonance(ESR) ages and thermoluminescence(TL) ages using silty sand taken from below the surface of sediments have been obtained.According to these data,we calculated the rate of thrusting and strike-slipping,and the results indicate that the Cenozoic tectonic shortening at the margin of the Tibetan Plateau is minor with the rate of thrusting smaller than 1.1mm/a and the rate of strike-slipping smaller than 1.46mm/a,and the Longmen Mountain is a zone of NNE-directed dextral shear with extensive strike-slip faulting and secondary thrusting with slip-dip ratio of 6(∶)1～1.3(∶)1.From NW to SE,the thrust component becomes smaller,but strike-slip component becomes bigger.However,this does not explain the presence of the abrupt topographic escarpment at the margin in the absence of major shortening.We thus infer that some escarpment reliefs are due to the effects of erosional unloading and strike-slipping.Based on comparison of sedimentary evidence,tectonic evidence,geomorphic evidence and paleomagnetism between the Cenozoic and Mesozoic,we believe that the direction of strike-slip faulting has been rewersed from SSW-directed sinistral strike-slip in Mesozoic to NNE-directed dextral strike-slip in the Cenozoic.
【Fund】： 国家自然科学基金项目(批准号:40372084);; 教育部优秀青年教师资助计划项目;; 四川省重点学科建设项目(批准号:SZD0408)资助
【CateGory Index】： P553
【CateGory Index】： P553