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

Abstract

Intellectual Merit

One 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 Impacts

This 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.