Abstract: The Longxian—Qishan—Mazhao fault is the largest and most active one in the Longxian—Baoji fault zone. The characteristics of its Late Quaternary tectonic activity and geometrical structure are of great significance for understanding the seismic deformation and tectonic deformation pattern of the southwestern margin of the Ordos block. In this study,based on the interpretation of high-resolution satellite remote sensing images and analysis of DEM(1: 50000), combined with fieldwork, geometric distribution of the fault was determined. Longxian—Qishan—Mazhao fault strikes 300 °~315 °, starting from Caobi town, passing through Qianyang County, Qishan County, fault scarp of the Loess Plateau in the southwestern of Fufeng County, Yabai town to Mazhao town in the north of Qinling piedmont, extending about 130 km. Since the Late Pleistocene, the whole movement of left-lateral strike-slip has been characterized, and normal component has been found in some segments. According to the segmentary signs of strike-slip faults, such as the fracture zone, fault intersection, and separation, Longxian—Qishan—Mazhao fault was divided into three segments. From northwest to southeast, there are Caobi—Tianjiagou segment, Disi—Gushui segment and Jiangzhang—Mazhao segment respectively. Caobi—Tianjiagou segment can be divided into two left stepped form sub-segments, which are Caobi—Sha'ao segment and Zaozigou—Tianjiagou segment. Longxian—Qishan—Mazhao fault intersects with the Beishan fault at the north of Qishan county, which can be taken as an indicator of segmentation of Caobi—Tianjiagou segment and Disi—Gushui segment. There is a 3 km loss of fracture in this area. Longxian—Qishan—Mazhao fault intersects with the Weihe fault at the Gushui village, which can be taken as an indicator of segmentation of Disi—Gushui segment and Jiangzhang—Mazhao segment. Jiangzhang—Mazhao segment intersects with north piedmont of Qinling fault is a sign of segmentation.

Key words: Active fault, Geometric distribution, The Longxian—Baoji faults zone, Southwestern Ordos