Abstract: The Qilianshan north-edge thrust (QNT)is located at the boundary between the northern margin of the Qilianshan mountain and Hexi Corridor, with a length over 700km. The Minle-Damaying fault (MDF), trending NWW, is part of the eastern section of the QNT, cutting through the Minle and Wuwei Basins. Hexi Corridor is a region of intense seismic activities, where many large earthquakes have been documented in history, such as the M7.5 Gaotai earthquake in 180, M8.5 Haiyuan earthquake in 1920, M8.0 Gulang earthquake in 1927 and the M7.6 Changma earthquake in 1932. While, there is no seismic record on the MDF. The Dongda River flows across the MDF from south to north. One of the tributary of the Dongda River, Xie River, has very well preserved terraces (T₆-T₁)which were offset by the MDF. On these terraces, there is clear trace of scarps, of which the height increases from terraces T₃ to T₆, indicating an accumulation of offset with time. In order to acquire the cross-section of scarps, unmanned aerial vehicle (UAV)scanning was implemented. With a digital camera mounted on, the UAV scanned an area of 0.52km² and digital elevation model (DEM)was generated with an accuracy of 0.2m vertically. The Thompson's method was utilized to conduct linear regressions on both the hanging wall and foot wall of the fault. The difference between the intercepts of the regression lines with the vertical line going through the intersection of the scarp surface on the fault surface is considered as the vertical offset. Terraces from T₆ to T₃ are very well preserved where MFD intercepts the Xie river, while T₂ and T₁ are badly eroded at the same location. Utilizing the cross-sections extracted from high resolution DEM, we estimate that the vertical offsets of T₆-T₃ are 13.26~15.67m, 9.74~10.13m, 5.86~7.35m and 5.03~5.60m, respectively, with 95%confidence interval. From the offsets of terraces, at least 4 paleo-seismic events are indentified. Terraces were dated by the AMS ¹⁴ C dating, yielding ages (cal BP)of T₆-T₂ as (16 405±210)a, (111 975±21)a, (5 697.5±210)a, (4 470.5±54.5)a and (3 137.5±77.5)a. Liner regression was performed for the relation between the ages and the offsets of terraces, resulting in the average vertical slip rate of MDF since the formation of T₆ as 0.91 average v. As the dip of MDF is about 35°, the shortening rate is estimated to be (1.3±0.13)mm/a. This study provides important parameters for the analysis of seismic activity in heavily populated Minle and Yongchang areas.

Key words: Minle-Damaying fault, offset terraces of Xie River, fault scarp, vertical slip rate

摘要： 祁连山北缘冲断带位于祁连山北缘与走廊盆地群的交界处，其中民乐-大马营断裂位于冲断带的东段民乐盆地和武威盆地之中，是民乐盆地南缘断裂。东大河由北至南横穿民乐-大马营断裂东段。其上游的支流横穿断层，其中的斜河发育了形态完整的阶地（T₁—T₆），被断层错断并在各级阶地上形成了形迹明显的断层陡坎。各级阶地上的陡坎走向近EW，并且由老至新高度依次降低，反映了断层位移量的累积。为了获得陡坎的横剖面，利用无人机对被错断的阶地实施了0.52km²范围内的扫描，生成了高精度的数字高程模型（DEM）影像，并利用它提取了各级阶地的横剖面形态。根据Thompson等（2002）的模式对各级阶地的上、下盘面分别进行线性拟合，通过蒙特卡洛模拟得到T₆—T₃的位错量95%置信区间分别为13.26～15.67m、9.74～10.13m、5.86～7.35m和5.03～5.60m。为了得到河流阶地的年代数据，在各级阶地的河流相砾石层顶部进行¹⁴ C采样，此位置的¹⁴ C年龄代表了阶地离开水面的年龄。获得T₆—T₂的年龄（Cal BP）分别为（16 405±210）a、（11 197±45）a、（5 697.5±52.5）a、（4 470.5±54.5）a和（3 137.5±77.5）a。利用各级阶地上陡坎的位错量和年代数据，对民乐-大马营断裂的垂直滑动速率线性拟合的结果为（0.91±0.09）mm/a，是1条全新世活动断裂。为人口稠密的民乐和永昌地区地震危险性分析提供了重要的地震活动性参数。

关键词: 民乐-大马营断裂, 东大河阶地, 断层陡坎, 垂直滑动速率