Response of alluvial fans to climatic changes and fault activities in the north front of South Yongchang Mountains, Northeast margin of Tibet Plateau

Abstract: The South Yongchang Mountains are located in the east part of the Hexi Corridor and controlled by the Fengle fault in the north. The Fengle fault, an important one of the Qilian Mountains thrust belt on the northern east margin of the Tibet Plateau, stretches from the Xida River in the west to the Xiying River in the east. Two surfaces of the Late Quaternary alluvial fans are cut off by the fault and formed fault scarps. Unmanned aerial vehicle (UAV) scanning was implemented to generate the Digital Elevation Model (DEM) for the alluvial fans in the Dujiatuan village, Zhoujia village and the terraces of Xiying River in mountain estuary. Linear regressions were performed both on hanging wall and foot wall of the fault to obtain the offsets. Loess samples, collected from T1, the top of gravel layer of the Dujiatuan village alluvial fan and Dongda river terrace T3, were dated by stimulated luminescence (OSL), and ages of 19.30±2.45ka and 27.40±2.47ka are gained. Combining with published dating ages, the ages of the 2 alluvial fans in the north front of the South Yongchang Mountains are suggested to be 19.3~16.3ka and 29.9~27.4ka, respectively, formed during the transition from cold to warm climate. Based on the offsets and dating results, we obtained that since the last stage of the Late Pleistocene, the vertical slip rate of the Fengle fault in the Dujiatuan village, Zhoujia village and Xiying river estuary are 0.65~0.82, 0.60~0.76 and 1.99~2.56mm/a respectively.