甘肃北山南缘断裂的活动特征及其意义

The active characteristics and its significance of the southern margin fault of Beishan area in Gansu Province

在河西走廊北侧,北山南缘发现活动断裂,称之为北山南缘断裂。该断裂位于金塔县城北侧,距县城约50 km。断裂全长约18 km,整体呈近东西向,倾向北西,倾角60°~80°。该断裂晚第四纪以来的活动形成了一系列断错地貌,断裂走滑运动使得多条冲沟发生同步肘状水平位错,位错量4.5~6.2 m,平均位错量5.2 m。断裂的北向逆冲形成清晰的线性陡坎,陡坎落差0.2~1.4 m。探槽揭露了至少2次快速滑动事件,较老的一次发生于距今30 ka之前,较新的一次发生在距今8.4~3.3 ka。结合卫星影像、地貌和探槽等证据,认为该断裂在晚更新世和全新世有过活动,为一条以左旋走滑为主,兼有逆冲分量的活动断裂。该断裂的活动可能揭示了青藏高原东北缘的扩展已穿越河西走廊,进入北山地区。

Obiective:On the north side of the Hexi Corridor,southern edge of the Beishan area,an active fault was discovered and we named it as Beishan southern margin fault.However,the movement pattern and activity age of this fault are unknown.Meanwhile,the special location of this fault may be important for understanding and identifying the northern growth pattern of Qinghai-Xizang(Tibet) Plateau.Methods:Based on interpretation of satellite images,investigation of fault geomorphology,trench and OSL dating method,this study gave the activity characteristics of this fault.Results:A series of ruptured landforms were formed because of the fault activities since the Late Quaternary,including synchronous elbow displacement of gullies that cross the fault due to the left-lateral strike-slip movement of fault,the amount of displacement is 4.5 ~ 6.2 m,and the average value is 5.2 m.The northward thrust of the fault forms a clear linear fault scarp with height of 0.2 ~ 1.4 m.In addition,four trenches along the fault revealed at least two sudden quick displacement events,the older one occurred in 30 ka ago,and the newer one occurred between 8.4 ka and 3.3 ka.Conclusions:Beishan southern margin fault is located on the north side of Jinta County,about 50 km from the county seat.The length of the fault is about 18 km.Strike of fault is near east-west direction,dip is northwest,and the dip angle is 60°~ 80°.Combined with the evidence from satellite imagery,geomorphology and trenches,it is believed that the fault has been active in the Late Pleistocene and Holocene,and is an active fault with leftlateral strike-slip and thrust property.The activities of this fault may reveal that the extension of the northeastern margin of the Qinghai-Xizang(Tibet) Plateau has crossed the Hexi Corridor and entered the Beishan area.