晚第四纪长江源区水系演化对断裂活动和气候变化的响应  

Late Quaternary drainage evolution and its response to fault activity and climatic change in the source region of the Yangtze River

作　　者：于洋 王先彦[1] 李正晨 李一泉[1] 鹿化煜[1] YU Yang;WANG Xianyan;LI Zhengchen;LI Yiquan;LU Huayu(School of Geography and Ocean Science,Nanjing University,Nanjing,210023,Jiangsu)

机构地区：[1]南京大学地理与海洋科学学院,江苏南京210023

出　　处：《第四纪研究》2022年第3期880-898,共19页Quaternary Sciences

基　　金：国家自然科学基金项目(批准号:42021001和41971005);科技部第二次青藏高原综合科学考察研究项目(批准号:2019QZKK0205)共同资助。

摘　　要：河流沉积/侵蚀过程和水系演化对构造活动、气候变化和侵蚀基准面的响应过程与机制是流域地貌的重要科学问题。长江源地区河流受高寒气候和走滑断层引起的局部多种构造活动样式的影响,是河流地貌过程对气候变化和构造活动响应的典型案例。文章通过野外考察、地貌指数(陡峭指数(k_(sn))、Chi(χ)、Gilbert指数、河谷宽高比(Vf))的计算、沉积学和地形分析,探讨了长江源水系演化对构造活动和气候变化的响应过程。通天河和澜沧江支流从上游相对年老的谷地(北西-南东向低k_(sn)值、高Vf值的宽谷)转变为下游相对年轻的谷地(南西-北东向高k_(sn)值、低Vf值的陡峭峡谷),表明南西-北东向河流对北西-南东向河流的袭夺。通天河和澜沧江支流上游地区显示的高χ值、低起伏地形向下游方向快速转变为低χ值、高起伏地形,也指示澜沧江和通天河向上游溯源侵蚀、袭夺分水岭地区原有老年期水系。局部分布的北西-南东向河谷(风口)、一系列间隔分布低洼地内河流沉积物和初步光释光年测年结果(13.3ka和>115ka)表明该地区在晚第四纪(可能在暖期)发育一条南东流向的古河流。随后断层走滑运动导致的局部构造抬升将河流分割成相互独立的内流湖。后期通天河和澜沧江干流(局部侵蚀基准面)侵蚀下降导致南西-北东向支流溯源侵蚀,袭夺内流湖;水系由树枝状演化为倒勾状。区域构造活动和局部侵蚀基准面变化是控制该地区水系演化的主要因素。晚第四纪气候变化引起河流堆积/侵蚀过程的改变也对水系的连通/中断有一定的影响,后期需要进一步的研究。Tectonic,climate and base level control the process of fluvial erosion and deposition and drainage evolution,but how the interactions of these factors affecting fluvial evolution on the Tibetan Plateau is still unclear. The upper reaches of the Yangtze River and Lancang River, which are located in the eastern Tibetan Plateau, are characterized by the diverse tectonic activities( regional strike slip movement,but locally subsiding and uplift),barbed drainages,and alternated broad valleys and steep gorges. Here is an ideal place to study the fluvial process and drainage evolution as a response to tectonic activity and climatic fluctuation,due to the special drainage patterns and tectonic styles. On the basis of field investigation,calculation of geomorphologic indexes( ksn,Chi( ),Gilbert and Vf),topographic and sedimentary analyses and OSL dating,the evolution of drainage patterns and landscape in this region since Late Quaternary is reconstructed. Accordingly,we assess the contribution of different factors( tectonic activity,climatic change,lithology and base level) to the drainage evolution. The upper reaches of the tributaries of the Tongtian River and Lancang River show NW-SE-trended broad valleys( lower steepness,higher Vf),higher  and lower relief,while the lower reaches are SW-NE-trended V-shaped valleys( higher steepness,lower Vf),lower  and higher relief. This indicates that the SW-NE-trended rivers erode headward and capture the original NW-SE-trended drainage in the watershed. The sedimentary structure of sediment sequences in the separated depressions and the OSL ages( 13. 3 ka and > 115 ka) show a southeast flow dendritic drainage on a low relief landscape,which could developed mainly during interglacial condition at Late Quaternary.Then,the spatially diverse local uplift and subsidence as a result of strike slip fault activity,impeded the southeast flow of rivers,and separated the valley into isolated endorheic lakes. The rapid headward erosion as a result of decreasing local base level led t

关 键 词：水系演化 侵蚀基准面 构造活动 长江源地区 晚第四纪 

分 类 号：P315[天文地球—地震学] P231[天文地球—固体地球物理学]