青藏高原北缘三危山断裂晚更新世活动特征

ACTIVE CHARACTERISTICS OF THE SANWEISHAN FAULT IN THE NORTHERN MARGIN OF THE TIBETAN PLATEAU DURING LATE PLEISTOCENE

三危山断裂位于青藏高原北缘,属于阿尔金断裂带向NW扩展的分支断裂,其最新的构造活动反映了青藏高原北部地区的构造演化及地震活动特征。文中通过遥感影像解译、野外实地调查和地质填图,对该断裂晚第四纪构造活动特征进行了研究。结果表明,三危山断裂发育于三危山西北麓,长约175km,断裂以左旋走滑为主,兼有逆断层性质,局部表现出正断层特征。其构造活动的地貌表现形式主要有:基岩陡坎、断层沟槽以及山包、冲沟左旋等。古地震探槽开挖揭示三危山断裂主要断错晚更新世地层,在距今(40.3±5.2)^(42.1±3.9)ka有过1次古地震活动,为1条晚更新世活动断裂。

The Sanweishan fault is located in the northern margin of the Tibetan plateau.It is a branch of the Altyn Tagh fault zone which extends to the northwest.A detailed study on Late Quaternary activity characteristics of the Sanwei Shan Fault can help understanding the strain distribution of the Altyn Tagh fault zone and regional seismic activity and northward growth of the Tibetan plateau.Previous research on this fault is insufficient and its activity is a controversial issue.Based on satellite images interpretation,field investigations and geological mapping,this study attempts to characterize this feature,especially its activity during Late Quaternary.Trench excavation and sample dating permit to address this issue,including determination of paleoseismic events along this fault.The results show that the Sanweishan fault is a large-scale active structure.It starts from the Shuangta reservoir in the east,extending southward by Shigongkouzi,Lucaogou,and Shugouzi,terminates south of Xishuigou,with a length of 175km.The fault trends in NEE,dipping SE at angles 50°～70°.It is characterized by left-lateral strike-slip with a component of thrust and local normal faulting.According to the geometry,the fault can be divided into three segments,i.e.Shuangta-Shigongkouzi,Shigongkouzi-Shugouzi and Shugouzi-Xishuigou from east to west,looking like a left-or right-step pattern.Plenty of offset fault landforms appear along the Sanweishan Fault,including ridges,left-lateral strike-slip gullies,fault scarps,and fault grooves.The trench study at the middle and eastern segments of the fault shows its activity during Late Pleistocene,evidenced by displaced strata of this epoch.Identification marks of the paleoearthquakes and sample dating reveal one paleoearthquake that occurred at(40.3±5.2)～(42.1±3.9)ka.