The Yangtze,the longest river in Asia,was hypothesized to be assembled through a series of Cenozoic capture events,such as the reversal of middle Yangtze River and the capture of upper Yangtze River,but the history remains largely unknown.Here,we present new geomorphic observations in the structural context of the eastern Sicbuan basin,namely the eastern Sichuan fold belt,and identify an important drainage divide along the "midline" of this arc-shape fold belt.Based on longitudinal profile analysis,we find that the river capture events more likely occurred in the syncline valleys of low-relief landscape.Our results yield a new perspective on middle Yangtze River reversal,and we propose that the "midline" drainage divide,rather than the Three Gorges,was the starting site of middle Yangtze River reversal.In this manner,the reversal could have been accomplished by a sequence of river reversal over range-parallel segments in syncline valleys with less impact on the pre-existing drainage system in eastern Sichuan basin.Moreover,the Cretaceous-Cenozoic sedimentary records are investigated in western Jianghan basin,east of the Three Gorges,in order to provide timing constraint for middle Yangtze River reversal.The result indicates that the initial capture and reversal might start in Middle Eocene when the basin was filled by eastwardflowing,upward-coarsening braided river sequence of Cheyanghe Formation.The through-going Yangtze River might have been formed during Miocene time,accompanied by incision of the Huangling Granite and deposition of the Yangtze Gravels.%万里长江的形成经历了数次大规模的河流袭夺事件,现今的长江中游河段(川江)曾经作为古长江的西支,自东向西汇入南流的古金沙江,后来由于不断的反向才使其与古长江的东支,即现今的长江下游得以贯通.针对这一河流演化模式,学术界普遍将长江三峡的黄陵背斜视为古长江东支和西支的分水岭,然而却一直未能获得有效的地貌证据,以致于无法解释具体的河流袭夺和反向过程.为此,本文从川东-湘鄂西弧形断褶带的构造地貌特征入手,深入分析了不同区段长江及其支流与构造线的交切关系,提出古长江东支和西支的分水岭应位于弧形带的“中线”附近的向斜谷地内,而并非前人认为的黄陵背斜.通过对向斜谷地内正交型水系样式下的河流袭夺案例分析,结合先进的河流纵剖面分析技术,揭示出由于基准面下降而造成的纵向河段被袭夺并最终反向的机制.另外,根据江汉盆地西缘(宜昌地区)的白垩纪-新生代地层的沉积学和物源分析,进一步限定了长江中游的袭夺和反向开始于始新世,直到中新世才形成现今贯通的长江.
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