Late Cenozoic activity of the Yumen fault in the western segment of the Hexi Corridor, NW China

There is a distinct steep morphologic steep slope in the central segment of the Yumen fault, where the principal fault is directly exposed at the surface, making pre-Cenozoic or Early Cenozoic strata in fault contact with Late Cenozoic loose sediments. This indicates that the central segment of the Yumen fault formed earlier, was active for a longer time and has a larger displacement amount, thus forming a giant fracture zone >200 m wide in the Binggoukou area. The main features of neotectonic activity in the western segment of the Yumen fault is that the principal fault is seldom exposed at the surface and in the main hidden. At the surface the asymmetric folds (Laojunmiao anticline) made up of Neogene strata are usually seen at the surface, belonging to a deformed “active anticline” closely related to the thrust fault. This suggests that the western segment of the fault formed later, was active for a shorter time and has a smaller displacement amount. Characteristically, the fault moves mainly by means of stick-slip deformation and less commonly by creep deformation. As a Holocen active one, the fault experienced tow neotectonic movements in the middle and late Holocene, i.e. at ~7.4 and ~3.2 ka BP respectively. The fault is also an earthquake structure presently, and its propagation and rupture at depth at the western end caused the Ms 5.9 Yumen earthquake in 2002.