牛首山-罗山断裂带北段柳木高断裂第四纪活动特征

牛首山-罗山断裂带是青藏高原东北缘弧形断裂系最外缘断裂带，自南向北由固原断裂、罗山东麓断裂、牛首山断裂及三关口断裂组成。通过遥感解译、野外调查及探槽揭露等方法对牛首山断裂北段柳木高断裂第四纪几何学、运动学特征进行了研究，并通过断层截切地层关系及年代学测试，限定了该断裂第四纪演化过程及全新世的地震事件。研究结果表明，柳木高断裂上新世至晚更新世自南西向北东逆冲，晚更新世至全新世左行走滑逆冲，表现为正花状构造，而全新世则发生了正倾滑运动。全新世期间，1690±30 yr BP（公元320-415）之后发生了一次古地震事件，推测柳木高断裂可能是公元876年青铜峡南6.5级地震的发震断裂。柳木高断裂第四纪早期活动特征与固原断裂、罗山东麓断裂及牛首山断裂一致，是青藏高原北东向持续扩展引起的，而全新世的正倾滑运动可能与银川地堑的伸展作用有关。 

QUATERNARY ACTIVE CHARACTERISTICS OF THE LIUMUGAO FAULT IN THE NORTHERN SEGMENT OF THE NIUSHOUSHAN-LUOSHAN FAULT

The Niushoushan-Luoshan fault is located in the northeastern margin of the arcuate structure zone in the northeastern Tibetan Plateau, consisting of Guyuan, Eastern Luoshan, Niushoushan and Sanguankou faults from south to north. In this study, we defined the geometry and kinematics, and evolution of the Liumugao fault, which is the northern segment of the Niushoushan fault, during the Quaternary earthquakes through remote sensing interpretations, field observations and trench investigations, and bracketed the occurrence of Holocene earthquakes according to the analyses of cut-off relationship and age dating. We concluded that, the fault was dominated by NE-directed thrusting during Pliocene to late Pleistocene, followed by sinistrallateral torsional movement during late Pleistocene to pre-Holocene, which presented as positive flower structure, and normal faulting during Holocene. And the paleo-earthquake occurred after 1690±30 BP (Cal AD 320~415). The Liumugao fault might be the seismogenic fault of the M_w 6.5 south Qingtongxia earthquake in A.D. 876.The kinematic characteristic of Liumugao fault during Pliocene to late Pleistocene is consistent with the Guyuan, Eastern Luoshan and Niushoushan faults, which might result from the ongoing northeastward movement of the Tibetan Plateau. The Holocene normal faulting might be controlled by the extension of Yinchuan graben.