SGER Proposal: Fluvial Incision Rates in the Three Gorges of the Yangtze River

中国四川省青藏高原东部最引人注目的景观之一是长江支流正在积极地切割四川盆地和川东褶皱带（与中国南北两大板块碰撞有关的三叠纪-侏罗纪褶皱带）的中生代基岩。这个切口是由飞行的斯特拉特（基岩）阶地数百米以上的所有主要支流和长江的主干延伸近500公里的高原外侧。中国的初步数据表明，所有这些阶地序列的年龄是第四纪，这意味着相对快速，最近的切割（数百米/百万年）。究竟是什么驱动了这个切口--它是否反映了扬子江的基准面变化，还是褶皱带和四川盆地下方活跃的岩石抬升的结果--这个问题基本上是未知的。如果区域阶地的相关性是可信的，那么景观中最高的阶地在下游的海拔上向褶皱带方向增加。很明显，一些有趣的事情正在发生--要么是河流改道，要么是阶地剖面因区域变形而弯曲，要么是阶地不相关。PI建议检查长江沿着通过三峡地区的河流下切的时间和速率，以努力开始测试这些假设。三峡地区（位于四川盆地东部的褶皱带）代表了关键地点，原因如下：o三峡以东，长江过渡到沿海洪泛平原上的沉积体系。基岩下切仅限于该点上游河段。通过三峡的河段有效地为整个上游盆地设定了当地基准面。三峡地区包含最高的河流阶地，可能反映了最大的切割率。河流附近有许多洞穴系统，为捕获河流沉积物提供了机会。PI建议通过利用宇宙成因产生的同位素的放射性衰变确定河流附近洞穴中河流沉积物的年龄的技术来建立河流切割年代学（格兰杰等人，1997年）。更广泛的影响这项提议将支持美中联合实地考察该地区，促进两个团体之间的知识交流。特别是，这项研究将使中国同事了解一种新的、正在发展的测定洞穴中河流沉积物年龄的技术。此外，这项工作的结果将是第一个辐射测定确定的时间和三峡开发速度的估计。

AbstractIntellectual MeritOne of the most striking aspects of the landscape east of the Tibetan Plateau, in China's Sichuan Province, is that tributaries of the Yangtze river are actively incising through Mesozoic bedrock of the Sichuan basin and the Eastern Sichuan Fold Belt (a Triassic-Jurassic fold belt related to the collision between the North and South China cratons). This incision is recorded by flights of strath (bedrock) terraces hundreds of meters above all of the major tributaries and the main stem of the Yangtze that extend nearly 500 km outboard of the plateau. Preliminary Chinese data suggest that all of these terrace sequences are Quaternary in age, implying relatively rapid, recent incision (hundreds of meters/m.y.). The question of what is driving this incision - whether it reflects a base-level change on the Yangtze or whether it is the result of active rock uplift beneath the fold belt and Sichuan basin - is essentially unknown. If regional terrace correlations are to be believed, the highest terraces in the landscape increase in elevation downstream, toward the fold belt. Clearly something interesting is going on - either the river has reversed course, the terrace profiles have been warped by regional deformation, or the terraces are miscorrelated. The PI proposes to examine the timing and rates of fluvial incision along the Yangtze River through the Three Gorges region in an effort to begin to test these hypotheses. The Three Gorges region (located in the fold belt, east of the Sichuan basin) represents the key locality for the following reasons:o East of the Three Gorges, the Yangtze transitions to a depositional system on the coastal floodplain. Bedrock incision is confined to the reaches upstream of this point. o The reach through the Gorges effectively sets the local base-level for the entire upstream basin.o The Gorges region contains the highest flights of fluvial terraces, presumably reflecting the greatest rates of incision.o The Three Gorges themselves are developed in limestone-cored anticlines. Numerous cave systems are developed adjacent to the river, affording the opportunity to trap fluvial sediment.The PI proposes to establish a chronology of river incision by exploiting a technique for determining the age of fluvial sediment in caves adjacent to the river from the radioactive decay of cosmogenically produced isotopes (Granger et al., 1997). Broader ImpactsThis proposal will support a joint American - Chinese field expedition to the region, fostering intellectual exchange between the two groups. In particular, this research will expose Chinese colleagues to a new and developing technique for dating fluvial sediments in caves. In addition, the results of this work will be the first radiometrically-determined estimates of the timing and rates of development of the Three Gorges.