青藏高原周缘的河流纵剖面特征及其对构造地貌演化的启示
Longitudinal profile analysis of rivers draining edges of the Tibetan Plateau and its geomorphology implications
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摘要: 河流水系是改造地表地貌的主要外动力,其形态样式记录了造山带和高原的生长和演化。目前对于青藏高原周缘的河流剖面研究主要集中在南缘、东南缘及东缘。相比而言,对于流经高原其他边缘河流的纵剖面特征仍然缺乏分析,不同位置的高原边缘河流发育是否存在相似性或差异性及其内在原因尚未开展相关研究。选取青藏高原北缘西昆仑—阿尔金地区、东北缘祁连山地区、东南缘三江地区和南缘喜马拉雅等典型区域,通过开展河流地貌对比研究,利用河流纵剖面的关键地貌参数定量分析了高原周边地貌边界带的空间特征及相关的构造与气候因素。结果表明,在青藏高原周缘地貌边界带上,河流陡峭度指数最大值与区域地形起伏最大区域一致,河流流经高原边缘时陡峭度有显著升高,而在高原边缘两侧陡峭度均相对较低,且河流向高原内部的溯源侵蚀十分有限,而且河流裂点均出现在区域地形起伏较高的地貌部位。其中青藏高原北缘、东北缘与南缘,地形起伏最大的区域与构造边界所围限的高原边界一致,并大致对应最新构造活动的强隆升区,而青藏高原东南缘的高原边界较模糊,地形起伏度变化最明显的区域出现在雅砻逆冲断裂两侧,与前人提出的古高原边界一致,暗示该区河流裂点的形成可能与雅砻逆冲断裂的早期活动有关。基于研究结果认为,对于青藏高原这样大尺度或经历长时间构造演化过程的地貌单元,基于稳态假设下的河流形态分析高原隆升时间可能存在较大偏差。青藏高原东南缘的高海拔低起伏地貌面可能是在较平缓的内流水系作用下形成的,在随后的河流袭夺等过程中演变为现今的外流河状态。Abstract: River systems are the main external driving force to transform the plateau landscape,and their morphological patterns record the growth of orogenic belt. The information of regional tectonics and climate can be obtained by extracting river geomorphology. At present,the study of river profiles around the margins of the Tibetan Plateau is mainly focused on the southern,southeastern and eastern edges. The similarities and differences in the development of rivers at different locations of the plateau edge have not been studied. In this study,we selected the major rivers in northern margin(West Kunlun-Altyn),northeastern margin(Qilian Mountain range),and southeastern margin(Sanjiang),and southern margin(Himalaya Mountain Range)to conduct a comparative study of river geomorphology,and to obtain quantitative regional tectonic and climatic information through longitudinal river profile analysis. The results show that the steepness of the rivers is associated with relief of the region: it increases significantly when they flow through the plateau edges,while the steepness is relatively low on both sides of the plateau edge. The rivers flowing through the northern edge have a steep increase in slope at the plateau edge,while the rivers in the northeastern,southeastern and southern edges are relatively gentle. Taken together,the knickpoints are mainly located at the plateau edge,indicating that the river incision is confined to the plateau edge and the migration of the knickpoints is limited. For a geomorphic unit as large as the Qinghai-Tibet Plateau,it is biased to analyze the timing of plateau uplift by river morphology under the steady-state assumption. The high-elevation,low-relief surfaces on the southeastern edge of the Tibetan Plateau may have formed under the action of gentler in stream flow systems and evolved to their present-day outflow state during subsequent river raids and other processes.