甘肃北山南部活动断裂的发现及其区域构造意义

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

在河西走廊北侧、北山南缘新发现属于不同断裂系统的两条晚第四纪活动断裂,分别称之为旧井-板滩断裂和俄博庙断裂。其中,旧井-板滩断裂长约28 km,距玉门市约55 km,由4条分支断裂组成,呈复杂的“Y”字形分布。总体走向北东40°~50°,倾向北西,倾角60°~70°,控制了西侧两个晚新生代盆地的发育。俄博庙断裂长约18 km,距金塔县城约50 km,走向近东西,倾向北西,倾角60°~80°。根据卫星影像解译、断错地貌调查、探槽开挖和光释光测年结果,旧井-板滩断裂断错了一系列山脊、冲沟和阶地,在距今约2万年以来有过活动,以正左旋走滑为主;俄博庙断裂北向逆冲形成清晰的线性陡坎,并左旋断错了冲沟,在距今约3万年以来有过活动,以逆左旋走滑为主。以上两条断裂的新构造活动揭示了青藏高原北缘晚新生代以来的远程应变传递已经进入北山造山带南缘。

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.