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

SGER提案：长江三峡河流下切率

• 批准号: 0329404
• 负责人: Eric Kirby
• 金额: $ 1.44万
• 依托单位: Pennsylvania State Univ University Park
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2003
• 资助国家: 美国
• 起止时间: 2003-05-15 至 2005-04-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0329404&HistoricalAwards=false
• 关键词: SGER; Proposal; Fluvial; Incision; Rates

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.

这一切口是由位于所有主要支流和长江干流之上数百米的河床（基岩）梯田所记录的，这些支流和干流向高原外延伸了近500公里。中国的初步资料表明，所有这些阶地序列都是第四纪的，这意味着相对快速，最近的切割（数百米/米）。究竟是什么驱动了这个切口——它是反映了扬子江的基准面变化，还是褶皱带和四川盆地下活跃的岩石隆升的结果——基本上是未知的。如果区域阶地相关性是可信的，那么景观中最高的阶地在向褶皱带方向的下游高程增加。很明显，一些有趣的事情正在发生——要么是河流改道了，要么是阶地的轮廓被区域变形扭曲了，要么是阶地的对比错了。该项目建议研究长江沿岸流经三峡地区的河流切割的时间和速率，以开始测试这些假设。三峡地区（位于四川盆地东部褶皱带）是关键位置，其原因如下：(1)三峡以东，长江向沿海洪泛平原沉积体系过渡；基岩切口局限于该点的上游。o通过三峡的河段有效地为整个上游盆地设定了局部基准面。o三峡地区有最高的河流阶地，可能反映了最大的切割速率。o三峡本身发育在以石灰岩为核心的背斜上。河流附近形成了许多洞穴系统，为捕获河流沉积物提供了机会。PI建议利用一种技术，通过宇宙成因产生的同位素的放射性衰变来确定河流附近洞穴中河流沉积物的年龄，从而建立河流切割的年表（Granger等，1997年）。更广泛的影响这项提议将支持美中两国联合对该地区进行实地考察，促进两国之间的知识交流。特别是，这项研究将使中国同事接触到一种新的和正在发展的技术，用于测定洞穴中河流沉积物的年代。此外，这项工作的结果将是对三峡大坝发展时间和速度的第一次辐射测定估计。