北祁连山和东昆仑山的地貌特征对比及其对构造抬升的指示意义

东昆仑山和北祁连山是青藏高原东北部的两条重要边界山脉, 其构造活动特征是理解高原形成与扩展的关键基础。目前研究认为, 北祁连山第四纪以来构造抬升活跃, 而东昆仑山北坡第四纪以来构造抬升是否活跃还存在争议。针对这一问题, 我们从地貌指标的角度出发, 以两地地貌形态特征为主要研究对象, 通过提取形态特征指标: 面积-高程积分、山前弯曲度、河流陡峭指数、裂点以及河流纵剖面形态, 对东昆仑山北坡和北祁连山北坡进行对比分析。结果显示, 东昆仑山北坡的面积-高程积分以及河流陡峭指数均小于祁连山北坡, 山前弯曲度大于祁连山北坡。两地河流纵剖面形态也有较大差别, 具体表现为东昆仑山北坡流域的河流纵剖面形态多为平滑下凹形态且鲜有裂点, 而祁连山北坡流域的河流纵剖面形态较平直且裂点较多。且通过对裂点成因的讨论, 东昆仑山北坡河流的裂点是由岩石抗侵蚀能力差异导致的, 而非构造成因的裂点。所有证据都表明构造活动可能是导致两山脉之间地貌指数差异的主要因素。此外, 两个区域的河流阶地特征和流域侵蚀速率特征也表明, 东昆仑山北坡的逆冲断裂现已处于不活跃状态, 指示北祁连山是高原东北部正在活动的主要边界。

Comparison of geomorphic characteristics between the Northern Qilian Shan and Eastern Kunlun Shan and its indications for tectonic uplift

The Eastern Kunlun Shan(Shan=Mountain in Chinese)and the Northern Qilian Shan are two important geomorphic boundaries in the northeastern Tibetan Plateau, and their tectonic characteristics are the key basis for understanding the uplift and extension of the plateau. Current researches suggest that the Northern Qilian Shan has been actively uplifting since the Quaternary, while the uplifting status of the north flank of the Eastern Kunlun Shan is still controversial. In this work, we will answer this question from the perspective of geomorphological indicators along these two mountain ranges. After extracting morphological feature indicators, including hypsometric Integral, mountain front sinuosity ratios, channel steepness index, knickpoints and channel longitudinal profiles, from the north flank of the Eastern Kunlun Shan and the north flank of the Northern Qilian Shan, we conduct a comparative analysis and a field work on typical river valleys. The results show that both the hypsometric Integral and channel steepness index on the north flank of the Eastern Kunlun Shan are smaller than those on the north flank of the Northern Qilian Shan, and the mountain front sinuosity ratio is also greater on the Eastern Kunlun Shan. The differences in the geomorphological features of the two ranges are also reflected in the morphology of the channel longitudinal profiles. Specifically, longitudinal profiles on the Eastern Kunlun Shan are mostly smooth and concave with few knickpoints, while the longitudinal profiles on the Northern Qilian Shan are relatively straight and have knickpoints. By comparing the lithology with the knickpoint, we find that the distribution for knickpoints on the north flank of the Eastern Kunlun Shan are all corresponding to the change of lithology with different erosion resistance, suggesting strong control of rock resistance on the river gradient. The relatively lower annual precipitation on the Eastern Kunlun Shan is apparently not the reason for its lower hypsometric integral and lower steepness index. Thus, all lines of evidence indicate that tectonic activity is probably the main factor in charge for the difference in geomorphic indexes between the two ranges. Moreover, the characteristics of river terraces and catchment erosion rates in the two ranges also indicate that the thrust faults on the northern edge of the Eastern Kunlun Shan are now in an inactive state. To sum up, all of these analysis results show that the uplift rate along the north flank of the Eastern Kunlun Shan is much lower than that along the north flank of the Northern Qilian Shan, and no traces of active tectonic deformation are found in the north flank of the Eastern Kunlun Shan, indicating that the Northern Qilian Shan are the main active boundary along the northeastern Tibetan Plateau.