Tectonic Deformation And Faults Interaction Since Late Quaternary In The West End Of Hexi Corridor

The western end of the Hexi Corridor is located at the intersection of the Qilian Mountains orogenic belt,the Alashan block and the Tarim block,including the Jiuxi Basin,the eastern end of Altyn Tagh fault(ATF)and Kuantan Shan area,where is also the foremost margin of the Tibetan Plateau.The area has been strongly deformed since the late Quaternary and present three sets of active structures striking NWW,NW and nearly EW.At present,there is still a lot of controversy about the tectonic deformation mechanism at the western end of the Hexi Corridor.One view holds that the tectonic deformation is mainly controlled by the structural transformation from strike-slip to thrust at the end of ATF,and another view suggests that it is majorly controlled by the eastward extension of the Qilian Shan.The above-mentioned differences are mainly due to the different understanding of the geometric and kinematic relationships between the different active structures.In this paper,we systematically determined the spatial relationship between the three sets of faults with different strikes and obtained the the quatitative active parameters since late Quaternary according to remote sensing survey,geological and geomorphological survey and dating method.On this basis,we discussed the mechanism of structural deformation at the west end of the Hexi Corridor.Mainly gains of this article as the following:The tail structure at the eastern end of the ATF is formed by the north Hongliuxia fault(NHLF),the Changshanling reverse fault(CSRF),the Hongliuxia fold(HF)and the Wuhuashan fold(WHF).The slip motion is transformed from strike-slip to thrusts,folds and the uplift along the fault.In the west of the NHLF,the strike-slip rate is ?1.0mm/a,and it decreases rapidly to ?0.3mm/a eastward,which is absorbed by the crustal shortening in the Hongliuxia area,and the horiztontal slip rate is close to zero in the Changshanling area.The western segment of Qilian Shan and its foreland area are thrust fault system with NWW strikes,consisting of Hanxia-Dahuanggou fault(HDF),Laojunmiao anticline-Yumen reverse fault zone(LYFZ),Baiyanghe anticline-Bainan reverse fault zone(BBFZ)and Huoshaogou fold(HSGF).The structural deformation is developed from thick-skin structures to thin-skin and the extension of faults takes the form forward propagation with backthrust.And it is found that three faults including Xinminpu fault and Lucaogou fault are the flexural-slip faults of Huoshaogou fold.Using the geomorphological deformation and chronological constraints of the Baiyanghe,we obtained the quantitative active parameters of the various structures in the western segement of the Qilian Shan and its foreland area.The total crustal shortening rate in the western Qilian Shan and its foreland area is ?2.35mm/a,while the shorten of the thurst forward-LYFZ and the foreland accounts for half and half.The North Kuantanshan fault zone is a group of NW-trending faults with dextral strike-slip and thrust activity,which consists of three near-parallel structures,namely the north Kuantanshan fault(NKTSF),the Huolushan-Xiatianjinwei fault(HXF)and the Huangtuyazi fold-Beishan fault(HBF).And based on fault geomorphology and chronological data,we obtained the active parmeters of each fault.We suggest this group of NW-trending dextral strike-slip faults,which developed in the central and western Qilian Shan and its foreland,is a product of plateau material extruding northwestwards.And they formed a conjugate deformation zone with ATF,coordinating the lateral extrusion of the material under the NNE direction of Qilianshan.The tectonic mechanism of the ATF extending northeast is explained as follows.The ATF constitutes the northwestern border of the Tibean Plateau,with a total length of 1500 km,linearly geometric,and an stability angle of ?55°with the main compressive stress axis in the region.However,in the process of its expansion,the angle between the tail structure and the main compressive stress axis increased to ?71°,which is not conducive to the occurrence of strike-slip.However,with the northwest extrusion of the material on northeast side of ATF and the dextral strike-slip of the NW-trending faults(such as North Kuantan Shan fault),the tail structure of the ATF will rotate counterclockwise.When the angle between the ATF and the compressive stress axis decrease to ?55°,which consistents with the direction of the maximum effective moment,the sinistral strike slip of the main fault occurred and the tail structure becomes part of the ATF.At this time,the ATF will continue to expand forward and form a new tail structure.The old tail structure and the NW-trending fault will be continuously remould by the new tectonic system.Therefore,the eastward extention of the ATF is realized by the NNW compression of the Qilian Shan and the dextral strike-slip of the NW-trending faults.The end of the ATF is 20 km in NNE direction from the latest deformation position of the Qilianshan thrust fault system(the northern margin of Kuantan Mountain and Heishan),so it is formed behind the Qilianshan foreland thrust structure and plays a coordinating role as a transfer fault.During the deformation process extending forward,the final attribution of multi-branch thrust faults at the tail end of the ATF is different.The branch fault on the north side rotates counterclockwise to first reach the direction of the main fault and becomes part of the main fault.The branch on the south side will be combined with the Qilianshan thrust faults under the lateral growth.As at present,the north Hongliuxia fault and LYFZ are growing laterally and eventually moving towards the union,repalcing the HDF as the new boundary fault on the southern margin of the Hexi Corridor.In short,during the late Quaternary under the NNE direction wedging of the Indian plate,the NEE crustal shortening occurred and the above three gourps of faults played different roles.The NWW-trending Qilian Shan thrust structure belt coordinated materils' s vertical movement,promoted crust thickening and surface uplift,the ATF fault coordinated material moved to north-east,and the NW-trending dextral strike-slip faults coordinated material's northwest lateral movment and the northeast expansion of ATF.