Abstract: The top of the piedmont alluvial fan has the characteristic of fan-shaped terrain and gradually descending terrain in the downstream direction. Faulting of various natures will result in different geomorphic features of alluvial fan surface. The variation of slope aspect and height of the pure sinistral fault scarp at the top of the alluvial fan is analyzed firstly under the three conditions, namely, the fault plane is vertical, the fault plane inclines toward the upper stream of the river, and the fault plane inclines toward the downstream of the river. We have also analyzed the variation of slope aspect and height of the fault scarps at the top of the alluvial fan under different fault inclination conditions of inverse sinistral strike-slip fault and the sinistral strike-slip normal fault. The seven geomorphic types we analyzed above cover the geomorphic features caused by the activity of strike-slip faults at the top of alluvial fans, which can help us to analyze the formation of the landforms. Based on drone-measured terrain data, Google satellite images and field investigations, we found that the Dongbielieke Fault, which strikes northeast-southwest and is located in the eastern margin of the Tacheng Basin, Xinjiang, almost vertically passes through the Ahebeidou River which develops from southeast to northwest. The direction of central axis of the alluvial fan at Ahebedou River is northwest, with a north-facing slope. The fault activity has caused the development of an uphill-facing scarp that has a height of~5.2m and a slope aspect facing southeast on the top of the alluvial fan at the Ahebiedou River section of the Dongbielieke Fault. And on the piedmont alluvial fan 1km away on both sides of the river bed, the sinistral fault scarps have a northwest-facing slope aspect and a height of 1~5m. The river terraces are divided into five levels, the T2 on the left bank, T4 on the right bank and T5 terraces on the left and right banks of Ahebeidou River were affected by fault activity. Sinistral offsets and southeast-facing fault scarps were developed on the geomorphic surface. By using DispCalc_Bathy_v2, a script based on Matlab, we get the offsets of the river terraces from the high-resolution DEM data obtained by using UAV photogrammetry technology. The sinistral horizontal offsets of T2 on the left bank, T4 on the right bank and T5 terraces on the left and right banks of Ahebeidou River are(10.1±0.2)m, (10.6±0.7)m, (29.1±0.2)m and(20.0±0.7)m, respectively. The vertical displacements are(1.5±0.1)m, (3.6±0.3)m, (4.7±0.2)m and(5.2±0.1)m, respectively. The asymmetrical development of terrains on both sides of the river is affected by topography and fault activity. The terraces on the lower elevation right bank of the river are misplaced into the channel by sinistral strike-slip faulting to receive more erosion, so the offsets we measured on the left bank of the river are more reliable than that on the right bank. Through field surveys, we found two fault outcrops, indicating that the fault plane is inclined to the southeast. The young river terrace T2 was effected by faulting and a uphill-facing scarp was developed, which indicates that the latest faulting was of sinistral strike-slip with a normal component, but the fault scarp's aspect changed twice within a short area of two kilometers, which is not consistent with the geomorphological type caused by the strike-slip faulting on the top of the alluvial fan as we previously analyzed. According to the landform features and the strike-slip fault geomorphic model, a model for the geomorphic surface development of the Ahebiedou River section is established. In this model, we think the Dongbielieke Fault was an inverse sinistral strike-slip fault after the formation of an older phase geomorphic surface S1 in the area. The early fault activity formed a northwest-facing fault scarp at S1, the height of the scarp is about 10m. Then the alluvial fan(Fan1)began to develop, and the material brought by the flowing water deposited and buried the fault scarp at the exit of piedmont, resulting in the disappearance of the existing fault scarp in the piedmont. Then the characteristic of fault changed into left-lateral strike-slip with a normal component. The activity of normal fault with the fault plane dipping to SE would form a fault scarp facing SE on the geomorphic surface. With the gradually cutting of the river, river terraces began to form on both sides of the river, and the corresponding geomorphic features were formed under the influence of fault activities. A fault scarp with a slope facing southeast formed at both banks of the river's mountain outlet with a height of about 5.2m through several fault activities, and sinistral horizontal offsets of river terraces increased at the same time. And the height of the pre-existing northwest-facing scarp 1~2km away from both banks of the river's mountain outlet decreased to about 5m, which can be observed in the field. The eventual geomorphic surface is characterized by the features of downhill-facing scarp-no scarp-uphill-facing scarp-no scarp-downhill-facing scarp from southeast to northeast.

Key words: strike-slip fault, alluvial fan, fault scarp, slope aspect of scarp, Tacheng Basin

摘要： 依据山前洪积扇顶部的扇形地形和向下游方向逐渐降低的地形特征, 文中首先分析了断层面直立、 向河流上游倾斜、 向河流下游倾斜3种条件下左旋走滑断层错动在洪积扇顶部形成的断层陡坎的坡向和高度变化。 其次, 分析了左旋逆走滑断层、 左旋正走滑断层在不同断层倾向条件下, 断层错动在洪积扇顶部形成的断层陡坎的坡向和高度变化。 利用无人机实测地形数据、 谷歌卫星影像, 结合野外地质地貌调查, 发现新疆塔城盆地东缘NE-SW走向的冬别列克断裂近垂直穿过了自SE-NW发育的阿合别斗河。 阿合别斗河处洪积扇的中轴线为NW向, 坡向朝N, 断层活动使其顶部发育了高约5.2m、 坡向SE的反向断层陡坎。 而在河床左、 右两岸各1km之外的山前洪积扇上, 断层陡坎为坡向NW的正向断层陡坎, 坎高1~5m不等。 阿合别斗河左岸T2、 右岸T4与左、 右两岸T5阶地的左旋水平位错量分别为(10.1±0.2)m、 (10.6±0.7)m、 (29.1±0.2)m、 (20.0±0.7)m, 垂直位错量分别为(1.5±0.1)m、 (3.6±0.3)m、 (4.7±0.2)m、 (5.2±0.1)m。 野外调查发现2处断层露头, 断层面均倾向SE。 根据阿合别斗河附近的断错地貌和走滑断层断错地貌模型, 认为冬别列克断层在地貌面S1形成后为左旋逆走滑性质, T5阶地面形成后断层的性质转变为左旋正走滑并多次活动, 形成了自SW-NE连续分布的正向断层陡坎—无陡坎—反向断层陡坎—无陡坎—正向断层陡坎的地貌现象。

关键词: 走滑断层, 洪积扇, 断层陡坎, 陡坎坡向, 塔城盆地