Geological and geomorphologic evidence of late Quaternary activity in the middle segment of the Hanxia－Dahuanggou fault in the northern margin of Qilian Shan

The Hanxia-Dahuanggou fault (HDF) is a thrust boundary fault in the western part of the north margin of Qilian Shan. No reliable evidence of its late Quaternary activity has been found, which is inconsistent with other parts of the strongly active Northern Qilian Thrust Fault Zone (NQTFZ). Therefore, finding out the latest activity of the HDF holds profound significance for revealing the extrusion expansion mode and deformation mechanism of the NQTFZ, as well as the seismic mechanism, earthquake prevention and disaster reduction in this area. Based on high-resolution satellite image interpretation, field investigation, unmanned aircraft vehicle photogrammetry, and dating of faulted strata and geomorphologic surface, this paper studies the geomorphic features, late Quaternary activity and seismic risk in the middle segment of the Hanxia－Dahuanggou fault (MHDF). According to the newly discovered fault section at Tianshengjuan and the age of the faulted T₁ terrace at Dafenggou, it is concluded that the active eras of the MHDF is the middle-late Holocene, and the latest tectonic activity occurred since 1970±290 a. The research shows that the MHDF is not only still active in the late Quaternary, but also the latest structural deformation has extended to the frontal Yumen fold-thrust belt and the secondary thrust system between them. Moreover, the MHDF has the potential for strong earthquakes of magnitude 6.5~7.1 or higher in the future, and the late Quaternary activity and seismic risk of the HDF deserve attention and in-depth study.