Late Quaternary Activity Characteristics Of The Beida Shan Fault In The Southern Margin Of The Alxa Block,Western China

The southern Alxa Block is located at the crustal front of the northern Tibetan Plateau.It was considered a stable intracontinental block with weak tectonic activity,and little research has been done on this area.In recent years,a series of studies have shown that there are several active faults in the southern margin of the Alxa block.However,the tectonic deformation model of these faults is still controversial.Therefore,studying the Late Quaternary activity characteristics of these faults in this area will help to understand the Late Quaternary tectonic deformation model of the Alxa Block and the response process to the northward growth of the Tibetan Plateau.The Beida Shan Fault is the clearest fault trace in the fault system on the southern margin of the Alax Block,but the research degree of this fault is low at present.What are the geometric and kinematic characteristics of Beida Shan fault?What is the late Quaternary slip rate of the fault?Is its rate coordinated with other fault slip rates in the southern margin of Alxa?What are the Late Quaternary activity characteristics and paleoearthquake history of the Beida Shan Fault?In order to answer these questions,this paper mainly studies the late Quaternary tectonic activity characteristics and the history of earthquake rupture in Quaternary alluvium in the west of Beida Shan’s bedrock by means of remote sensing image interpretation,field geological investigation and Quaternary geochronology.The conclusions are as follows:（1）Based on the remote sensing image data,according to the surface shape,relative height,development density and cutting depth of the alluvial fan,the alluvial fan widely developed in the southern margin of Beidashan can be divided into four stages:Fan0,Fan1,Fan2 and Fan3,respectively.¹⁰Be cosmogenic nuclides determine that the abandoned ages of Fan2 and Fan3 are 26～36ka and 85ka,corresponding to deep-sea oxygen isotope stages MIS3 and MIS5a,and the warm climate indicates that the formation of late Quaternary alluvial fans in the study area is mainly affected by climate.（2）The Beida Shan Fault is an EW-trending fault that is about 100km long.The investigation area of this study is the fault segment about 55km long in the west section of the Beida Shan Fault.According to the geometric distribution,we can divide the fault into three segments:Langwashan segment,Jiapiquanshan segment and Baigedashan segment.The Langwashan segment is about 10km,and the faults are mainly distributed along the boundary between the bedrock and the alluvial fan in front of the piedmont,mainly sinistral strike-slip,with a thrust component.The length of Jiapiquanshan segment is about 30km,which is divided into south and north branch faults.The north branch fault is mainly developed in the bedrock,showing a series of offset gullies and ridges.The southern branch fault offseted the multi-stage alluvial fan in front of the mountain,forming offfset landforms of different scales.The length of the Baigedashan segment is about 15km,and the western fault is distributed along the boundary between bedrock and Quaternary strata,showing fault scarps of tens of centimeters,small compressional uplifts and a series of sinistral offset gullies,the eastern fault enters the bedrock,which is mainly characterized by linear fault valleys,rare offset gullies and weak fault activity.At the same time,the latest surface rupture zone with a length of at least 22km has been found along the Jiapiquanshan segment and the Baigedashan segment,showing a fault scarp of tens of centimeters and a small gullies offseted about1m～2m.Combined with the offset landforms characteristics of each segment,it shows that the Beida Shan Fault is a left-lateral strike-slip fault with thrust component in Holocene.（3）Four well-preserved offset landforms along the Beida Shan Fault are selected.By using UAV-Sf M photogrammetry and ¹⁰Be cosmogenic radionuclide exposure dating,we estimated that the late Quaternary left-lateral strike-slip rate of the Beida Shan Fault is 0.4～1.4mm/a.（4）We identified and measured the left-lateral displacements along the Beida Shan Fault,and obtained 181 left-lateral horizontal displacements.The cumulative offset probability density（COPD）analysis of the displacement of less than 18m shows that the displacement distribution has obvious clustering characteristics.There are four COPD peaks of 1.6m,3.6m,6.1m and 9.2m,it represents at least 4 paleoseismic events on the Beida Shan fault.At the same time,three fault profiles along the fault revealed1,2 and 3 paleoseismic events respectively.（5）Both GPS rate and geological slip rate show a linear decay in the rate of crustal shortening from the Qilian Shan to the Alax Block.The shortening of Alxa block may be gradually absorbed by a series of mountain fold deformation.Our results support that the tectonic deformation model of the fault system in the southern margin of the Alxa Block is the compression and oblique slip model of the northward extension of the Tibetan Plateau.