Abstract: In southern mountainous area of Lanzhou city, there are 4 large-scale regional fault zones, which have been active since Late Pleistocene. They include the active Northern marginal fault zone of Maxianshan Mountains(F₁), the active Southern marginal fault zone of Maxianshan Mountains(F₂), the active Southern marginal fault zone of Xinglongshan Mountains(F₃)and the active Northern marginal fault zone of Xinglongshan Mountains(F₄). They are assigned to the Maxianshan Xinglongshan Mountains active fault system. Among the 4 fault zones, the Northern marginal fault zone of Maxianshan Mountains has the strongest activity. The fault initiates from Neiguan~ying faulted basin in the east, passing through Miaowan, Yangzhai, Yinshan, and after converging with the Southern marginal fault zone of Xinglongshan Mountains at Moyunguan it runs along Tianjiagou, Hutan, Guanshan, Xianshuigou, and terminates at Bapanxia Gorge of the Yellow River in the west. The fault is generally striking N60°W, having a total length of about 115 5km.The nearest distance from Lanzhou city to the fault is only about 4km, so that the recent activity and the seismic potential of the fault zone play an important role in the seismic design and the safety measures for the Lanzhou City. The geometric feature of the Northern marginal fault zone of Maxianshan Mountains is relatively simple, and it can be divided into 4 subsidiary segments according to the branching, bending, and discontinuous step over of the fault, as well as the difference of recent activity. These subsidiary fault segments are called the Neiguanying(F_1-1), Maxianshan(F_1-2), Qidaoliang(F_1-3)and Wusushan(F_1-4)segments, respectively. The results of geological mapping of the active faults on the scale of 1/50000 indicate that the Northern marginal fault zone of Maxianshan Mountains is a long standing, south dipping thrust fault, which has become a left lateral strike slip fault with dip slip component since Middle Pleistocene. Apart from this segment, the other segments are Holocene active fault zones. Among them, the Wusushan segment is also dominated by thrust movement but is dipping to the north. It is only the Maxianshan segment that displays obvious left lateral strike slip movement with normal component, resulting in a series of offset landforms, such as offset ridge, hills, valley and terrace. The largest offset may reach up to several hundred meters, while the smallest only about several meters. The amounts of horizontal displacements along the fault are concentrated mainly at 10～ 30m, 95～ 105m and 140～ 160m, reflecting that the faults are dominated mainly by stick-slip movement. At the same time, a series of fault scarps are developed along the fault zone, and the height of the scarp at the first level terrace is about 1～1.5m. The standard offset of the second level terrace is observed at Quanshenmiao gully as about 49m, and that of the first level terrace is observed at the eastern branch of Shitougou gully as about 25m. According to the ages of the terraces, it is estimated that the average horizontal slip rate of the fault since Late Pleistocene is about 3.73mm/a.

Key words: active fault zone, slip rate, recent activity, Lanzhou City, Maxianshan Mountains

摘要： 通过详细地质填图研究获得了兰州马衔山北缘断裂带的几何学、新活动性、断错微地貌及滑动速率等的定量资料.马衔山北缘断裂为一条历史悠久的区域性活动断裂,其几何形态较简单,总体走向N60°W,全长约115.5km.根据断裂分叉、拐弯及不连续阶区等几何特征和活动性的差异,大致可以将该断裂划分为4条次级断层段,自东向西分别称为内官营段、马衔山段、七道梁段和雾宿山段.断裂的新活动具分段性,其中的内官营段为具逆断特性的晚更新世活动段,而马衔山段、七道梁段和雾宿山段均为全新世活动段.该断裂早期具逆断特性,大约自中更新世以来以左旋走滑为主兼具倾滑运动分量,形成了一系列山脊、冲沟、阶地等的左旋断错,断距几米至几百米.沿断裂带的水平位移具分组现象,可能反映了断裂的黏滑活动特性.根据Ⅰ,Ⅱ级阶地的左旋断错值及其年代,得到了马衔山北缘断裂带晚更新世以来的平均水平滑动速率约为3.73mm/a.同时沿断裂带还保存有断崖和断坎等,其中Ⅰ级阶地上的断坎高1～1.5m左右.

关键词: 活动断裂带, 滑动速率, 新活动, 兰州, 马衔山