PRINCIPAL FEATURES OF RECENT ACTIVITY OF THE ACTIVE NORTHERN MARGINAL FAULT ZONE OF MAXIANSHAN MOUNTAINS, LANZHOU, GANSU PROVINCE

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 (F 1), the active Southern marginal fault zone of Maxianshan Mountains (F 2), the active Southern marginal fault zone of Xinglongshan Mountains (F 3) and the active Northern marginal fault zone of Xinglongshan Mountains (F 4). 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.