青藏高原东北缘马衔山夷平面特征指标的提取与分析

夷平面研究在地貌演化和新构造运动,特别是青藏高原隆升过程研究中占有极其重要的地位,准确界定夷平面分布范围是开展夷平面定量--半定量研究的重要基础.截至目前,前人已利用计算机图像处理技术和目视解译等方法开展了相关工作,并取得重要研究进展.然而,以往利用坡度高程指标提取夷平面时存在一定的随意性和主观性.青藏高原内部及其周边地区发育系列夷平面,其中高原东北缘马衔山地区较好的保存了两级夷平面.被夷平的地层为前寒武系马衔山群混合岩,山顶面上发育冰缘地貌,主夷平面波状起伏,上有岛山和晚新生代沉积,局部覆盖有40 m左右红粘土和30 m以上更新世黄土,并在局地残留3 m以上风化壳,是开展夷平面研究的理想地区.为探讨如何确定提取夷平面的最佳坡度高程指标,本文基于夷平面的特殊地貌几何形态,利用均值变点分析法和最小误差法确定了提取夷平面的最佳高程和坡度指标,并据此对马衔山地区的两级夷平面的分布范围进行提取和统计分析.最后结合剖面线,坡向分布图和区域地质资料探讨了夷平面的构造变形特征.研究发现马衔山地区两级夷平面最佳坡度指标均为12°,山顶面和主夷平面高程主要分布在3470~3640 m和2670~2870 m;统计分析表明山顶面面积约为6.4 km²,平均高程为3559 m,主夷平面面积约为15.5 km²,平均高程为2771 m;而夷平面的变形特征主要受控于区域断层.

Extraction and analysis of Maxianshan planation surfaces in northeastern margin of the Tibetan Plateau

Planation surfaces not only play a major role in revealing the geomorphological evolution, but also shed lights on the regional tectonic activities, especially in the fields of Tibetan Plateau uplift. Before we explain their tectonic significance, the critical step is to quantitatively or semi-quantitatively obtain the exact features of planation surfaces, such as their distribution, altitude and area. Till now, much progress has been achieved in extracting the planation surface characters via computer image processing and visual interpretation, but there is still some subjective and random when we choose the index of slope and elevation to extract the information of planation surfaces. Two extensive planation surfaces (the Summit and the Main surface) remain in the inner part of the Tibetan Plateau and its surrounding region. For example, in the Maxianshan mountains of NE Tibetan Plateau, two planation surfaces are identified as the Summit and Main surfaces of Tibet. Some typical periglacial landforms can be observed on the Summit surface, and the Main surface is characterized by the undulating flat topography and several monadnocks and covered by the red clay of 40 m in thickness and loess of 30 m in thickness. In addition, the weathered regolith of granite rocks reaches 3 m thick in some regions. Hence, the Maxianshan mountains are the ideal area for studies on the planation surface. In detail, ridge and valley profiles present the "S"-shaped curve based on our observation, and they are explained by the exponential model which can transform to the mean change point model. In order to obtain the exact altitude and slope parameters of Maxianshan planation surfaces, we chose the mean change point theory and the minimum error method to extract them. Our results show that the best slope parameter of both planation surfaces is 12°. Its reliability is also demonstrated by contrasting the results between our method, visual interpretation and field validation. Subsequently, the distribution and area of planation surfaces were extracted and analyzed. Statistical analyses suggest that the Summit surface covers a total area of about 6.4 km², with an average elevation of 3559 m and the Main surface has a total area of about 15.5 km², with an average elevation of 2771 m. Furthermore, we evaluate the deformation characters of the planation surfaces after integration of the slope profiles, slope aspect distribution and regional geology data. The conclusion supports that the deformation of Maxianshan planation surfaces is controlled by regional faults.