Discovery of active faults in the southern Beishan area, NW China: Implications for regional tectonics

On the north side of the Hexi Corridor, two active faults, which belong to two different fault systems, have been discovered on the southern margin of Beishan, namely the Jiujing-Bantan fault and the Ebomiao fault. The NE-trending (40°~50°) Jiujing-Bantan fault with a NW-trending dip angle of 60°~70° is ~28 km long and ~55 km away from Yumen City. It consists of 4 branches and presents a complex Y-shaped distribution, controlling the development of the two late Pleistocene basins on its west side. The nearly EW-trending Ebomiao fault with a NW-trending dip angle of 60°~80° is ~18 km long and ~50 km away from Jinta County. Base on results from the satellite image interpretation, offset geomorphological survey, trench excavation and optical luminescence dating, we discovered a series of ridges, gullies and terraces offset by the Jiujing-Bantan fault which has been active since ~20 ka ago mostly with normal left-lateral strike-slips. The northward thrusting of the Ebomiao fault formed a clear linear scarp and offset the gullies with left-lateral strike-slips. This fault has been active since ~30 ka ago, mainly with reverse left-lateral strike-slips. The neotectonic activities of these two faults evidences that the long-range strain transmission from the northern margin of the Tibetan Plateau has entered into the southern margin of the Beishan orogenic belt since the late Cenozoic.