地貌参数揭示的三危山构造活动性差异及其原因探讨

三危山断裂位于青藏高原北缘,沿三危山西北麓展布,全长约175 km。根据其几何特征,可以将断裂分为三段:西段(西水沟—树沟子段)、中段(树沟子—十工口子段)和东段(十工口子—双塔段),其构造活动强度与地貌发育程度各段存在明显差异。根据断层陡坎测量和区域年代对比,三危山断裂西段垂直滑动速率约0.1 mm/a,中东段则较低。为进一步量化三危山断裂对研究区地形地貌的影响,利用地貌因子(高程、坡度、河道陡峭指数值和河道坡降指数值)对三危山各段地貌特征进行定量化分析。结果显示三危山西段值远大于中东段,揭示三危山抬升速率西段大于中东段,反映出地貌形态对活动构造具有良好的响应。对比岩性、降雨及构造活动,分析认为三危山断裂分段活动是造成三危山差异抬升的主要原因,而鸣沙山断裂的新活动及巴兔山断裂对中东段变形量的吸收也加速了三危山西段的隆升。

Differential Tectonic Activities of Sanweishan Area Revealed by Geomorphic Parameters and Its Causes

The Sanweishan fault extends along the northwest margin of Sanweishan, with a total length of 175 km. On the basis of its geometric characteristics, the fault can be divided into three segments, namely, the western segment (Xishuigou-Shugouzi), the middle segment (Shugouzi-Shigongkouzi), and the eastern segment (Shigongkouzi-Shuangta). The tectonic activity intensity and geomorphological development degree of each segment are obviously different. On the basis of fault scarp measurement and regional chronological comparison, the vertical slip rate of the western segment of the Sanweishan fault is approximately 0.1 mm/a and that of the middle and eastern segments is relatively low. To further quantify the influence of the Sanweishan fault on the topography and geomorphology of the study area, we use the geomorphic factors (i.e., elevation, slope, channel steepness index, and stream gradient index) to quantitatively analyze the geomorphic features of each segment. Results show that the value of the western segment is larger than that of the middle and eastern segments, indicating that the uplift rate of the western segment is higher than that of the two other segments. This finding further indicates that the topography has a good response to tectonic activity. From the comparison between lithology, rainfall, and tectonic activity, we propose that the segmentation of the Sanweishan fault is the main reason for the differential uplift of each segment. Moreover, new activities of the Mingshashan fault accelerated the differential uplift of the west segment of the Sanweishan fault.