Late Quaternary Activity Of The Longxian-Qishan-Mazhao Fault

Longxian—Baoji faults zone is one of the margins of northeastern edge of Tibet Plateau and southwestern margin of the Ordos block.Longxian—Qishan—Mazhao fault is the largest and most active one in Longxian—Baoji faults zone.The characteristics of its Late Quaternary tectonic activity and geometrical structure are of great significance for understanding the northeastward growing effects of Tibet Plateau,the seismic deformation and tectonic deformation pattern of the southwestern margin of the Ordos block and the evolution of Weihe Basin.In this study,Based on the interpretation of high—resolution satellite remote sensing images and analysis of DEM(1:50000),combined with the fieldwork,the geometric distribution of fault was determined.Through geological profile filling,drilling data collection and analysis,and sUAV—based photogrammetry,Optically Stimulated Luminescence dating,the activity Longxian—Qishan—Mazhao fault has defined.The main conclusions are as follows:(1)Longxian — Qishan — Mazhao fault striking 300 ° ~315 °,starting from Caobi town in Qianyang Loess Plateau,passing through Sha'ao,Zaozigou,Tianjiagou,fault scarp of the Second Loess Plateau between Disi and Gushui county,Yabai town to Mazhao town in the north of Qinling piedmont,extending about 130 km.(2)According to the segemental signs of strike—slip faults,such as the fracture zone and fault intersection,late quaternary activity of faults.Longxian — Qishan — Mazhao fault was divided into three segments.(1)Northwest-striking Caobi—Tianjiagou segment extending about 61 km,it consist of three sinstral left—stepped faults.This segment is left—lateral fault.Caobi—Tianjiagou segment can be divided into two left stepped form subsegments,since intersects with eastern Qianyang fault,they are Caobi—Sha'ao segment and Zaozigou—Tianjiagou segment,there is a 2.9km*2.1km stepover between them.(2)Northwest-striking Disi—Gushui segment extending about 30 km along side the loess scarp.It is a sinistral fault with normal component.Longxian — Qishan — Mazhao fault intersects with the Beishan fault at the north of Qishan county,and the decreasion of slip rate,which are taken as signs of segmentation of Caobi—Tianjiagou segment and Disi—Gushui segment.(3)Jiangzhang — Mazhao segment extending about 37 km from south boundary of loess plateau in Fufeng to Qinling piedmont.It is a blind fault.Longxian — Qishan — Mazhao fault intersects with the Weihe fault at the Gushui village,and activity changed,which are taken as signs of segmentation of Disi—Gushui segment and Jiangzhang — Mazhao segment.(3)Longxian — Qishan — Mazhao fault is a left—lateral strike—slip active fault with normal component since late Pleistocene epoch.Caobi—Tianjiagou segment has the strongest left—lateral strike—slip activity,the maximum left—lateral offset of gullies which across active faults reached kilometer level.By surveying horizontal displacement of terrace and OSL dating,the slip rate was acquired.Thay are 1.34±0.29mm/a since late Pleistocene in Caobi—Tianjiagou segment and 0.87±0.09mm/a since late Pleistocene in Disi—Gushui segment.(4)The SE-NW tension of Weihe Basin absorbed 0.35mm/a velocity which is equal to the decreasion of slip-rate(about 0.37mm/a)between Caobi—Tianjiagou segment and Disi—Gushui segment.