Abstract: The northern margin of the Qinghai-Tibet Plateau is currently the leading edge of uplift and expansion of the plateau. Over the years, a lot of research has been carried out on the deformation and evolution of the northeastern margin of the Qinghai-Tibet Plateau, and many ideas have been put forward, but there are also many disputes. The Altyn Tagh Fault constitutes the northern boundary of the Qinghai-Tibet Plateau, and there are two active faults on the north side of the Altyn Tagh Fault, named Sanweishan Fault with NEE strike and Nanjieshan Fault with EW strike. Especially, studies on the geometric and kinematic parameters of Sanweishan Fault since the Late Quaternary, which is nearly parallel with the Altyn Tagn Fault, are of great significance for understanding the deformation transfer and distribution in the northwestward extension of the Qinghai-Tibet Plateau. Therefore, interpretation of the fault landforms and statistical analysis of the horizontal displacement on the Sanweishan Fault and its newly discovered western extension are carried out in this paper. We believe that the Sanweishan Fault is an important branch of the eastern section of the Altyn Tagh fault zone. It is located at the front edge of the northwestern Qinghai-Tibet Plateau and is a left-lateral strike-slip and thrust active fault. Based on the interpretation of satellite imagery and microgeomorphology field investigation of Sanweishan main fault and its western segments, it's been found that the Sanweishan main fault constitutes the contact boundary between the Sanweishan Mountain and the alluvial fans. In the bedrock interior and on the north side of the Mogao Grottoes, there are also some branch faults distributed nearly parallel to the main fault. The main fault is about 150km long, striking 65°, mainly dipping SE with dip angles from 50° to 70°. The main fault can be divided into three segments in the spatial geometric distribution:the western segment(Xizhuigou-Dongshuigou, I), which is about 35km long, the middle segment(Dongshuigou-Shigongkouzi, Ⅱ), about 65km long, and the east segment(Shigongkouzi-Shuangta, Ⅲ), about 50km long. The above three segments are arranged in the left or right stepovers.
In the west of Mingshashan, it's been found that the fault scarps are distributed near Danghe Reservoir and Yangguan Town in the west of Minshashan Mountain, and we thought those scarps are the westward extension of the main Sanweishan Fault. Along the main fault and its western extension, the different levels of water system(including gullies and rills)and ridges have been offset synchronously, forming a series of fault micro-geomorphology. The scale of the offset water system is proportional to the horizontal displacement. The frequency statistical analysis of the horizontal displacement shows that the displacement has obvious grouping characteristics, which are divided into 6 groups, and the corresponding peaks are 3.4m, 6.7m, 11.4m, 15m, 22m and 26m, respectively. Among them, 3.4m represents the coseismic displacement of the latest ancient earthquake event, and the larger displacement peak represents the accumulation of coseismic displacements of multi-paleoearthquake events. This kind of displacement characterized by approximately equal interval increase indicates that the Sanweishan Fault has experienced multiple characteristic earthquakes since the Late Quaternary and has the possibility of occurrence of earthquakes greater than magnitude 7. The distribution of displacement and structural transformation of the end of the fault indicate that Sanweishan Fault is an "Altyn Tagh Fault"in its infancy. The activities of Sanweishan Fault and its accompanying mountain uplift are the result of the transpression of the northern margin of the Qinghai-Tibet Plateau, representing one of the growth patterns of the northern margin of the plateau.

Key words: northern margin of Qinghai-Tibetan plateau, Sanweishan Fault, westward extension, displacement grouping, horizontal movement characteristics and mechanism

摘要： 三危山断裂为阿尔金断裂带东段的重要分支，位于青藏高原北缘NW向扩展的前缘位置，是一条左旋走滑兼逆冲的活动断裂。经卫片解译和野外实际调查发现，三危山主断裂穿过鸣沙山后，向W至党河水库及阳关镇附近以断层陡坎的形式出露地表。沿主断裂及其西延部分，断裂左旋断错了不同级别的水系（冲沟和纹沟）和山脊，形成一系列断错微地貌，断错水系的规模与水平位移量呈正相关。水平位移量的频率统计分析显示位移量存在明显的分级特征，可分为6级，对应的峰值分别为3.4m、6.7m、11.4m、15m、22m和26m，各级之间的增量为3～4m。其中，3.4m代表了最新一次古地震事件的同震位移量，而更大的位移量峰值则代表了多次古地震事件累积的同震位移量。这种位移量近似等间隔增加的特征，表明三危山断裂在晚第四纪以来存在多期次的地震活动，具有特征型地震的活动特点，具有发生7级以上地震的可能性。三危山断裂的位错量分布和端部的构造转换等特征，显示其可能是一条处于幼年期的"阿尔金断裂"，该断裂的活动及其伴随的山体隆升是青藏高原北缘整体压剪作用的结果，代表了高原北缘的一种生长方式。

关键词: 青藏高原北缘, 三危山断裂, 断裂西延, 位移量分级, 水平运动特征与机制