Late Quaternary Activity Of Liu-leng Mountain North Fault

The Shanxi Graben System(SGS),as the east edge of the Ordos block,locates spatially on the central axis structure of north China region.There is a big debate that the dynamics background in this region is under the control of either the north-east expansion of the Qinghai Tibet plateau or the westward subduction of the Pacific plate,even or combined actions of both.Rupture characteristics and slip rates of main fault zones provide kinematic constraints for the understanding of the deformation dynamics,also base evidences for building kinetic model of the Shanxi Graden System.Slip rate of a fault represents the average level of faulting activities during a long time,also reflects the accumulation of strain energy accumulation along the fault.Therefore,how to determine accurately the slip rates of a fault is essential to assess seismic risk.The Liu-Leng-Shan Northern Piedmont Fualt(LLNF)is a controlled fault in the northern region of the Shanxi Graben System,and it is significant to investigate the Late quaternary activity characteristics and rupture history for identifying the kinetic model of SGS and accurate evaluation of assess seismic risk.Compared with other faults,LLNF is lack of detailed researches in fault geometry,paleoearthquakes and activity characteristics.We carried out detailed researches on geometry,kinematics and paleoearthquakes on LLNF through interpretation of high-resolution remote sensing satellite image,field investigation,displaced-landform measurements and dating techniques,and the following important insights have been obtained.(1)Through the high resolution imagery interpretation and field investigations,the LLMF is a normal fault,extending for more than 140 km.This fault has a N70-80 ° E trend,dipping NNW with the angle of 60-80 °.According to the geometric characteristics,LLMF is divided into five discontinuous oblique-arranged segments ranging from 10 to 30 km in length,including Beimazhuang-Xiayukou section(A),Mayukou-Xiquanbao section(B),Dayukou-Quchangcheng section(C),Dushanbao-Xiangcaogou section(D)and Longmazhuang-Xinbao section(E).The geological sections at Beimazhuang and Jijiayao shows that the fault traces are coincident with the boundary lines between the quaternary strata and prequaternary bedrocks,which indicates that LLMF is an inherited fault,and has not migrated toward the basin.(2)The LLNF has activated strongly since late Pleistocene,with vertical uplift movements which produced various of displaced landforms including linear fault scarps,bedrock fault surfaces and offset streams.Based on detailed topographic measurements of six typical displaced landforms,we obtained several vertical slip rates of the LLNF zone since late quaternary.These slip rates range from 0.18 mm/a to 0.63 mm/a,with the average value of 0.3 mm/a.Our result is agreed with the vertical slip-rate obtained from geological body offset since 3Ma.(3)Three geologic sections were cleaned on the west,middle and east sections of the LLNF,named by the Beimazhuang site,the Xujiabao site and the Jijiayao site,respectively.According to the analysis results from topographic features,colluvial wedge series,and relationships between strata and fault traces,we identified 4 paleoearthquake events with maximum co-seismic vertical offset of 130 cm at the Beimazhuang site,6 events at the Xujiabao site,and 3 events at the Jijiayao site.