Cosmic ray exposure ages for formation of perched alluvial terraces and for abandonment of an alluvial/debris-flow fan on opposite sides of the Tangtse Valley (the outflow at the northwest end of Lake Panggong, which is in the Karakorum Range of Western Tibet) provide evidence of a humid period at ∼11.5 to ∼7 ka. This is consistent with other regional records and supports a controversial chronology for the sedimentary record from Lake Panggong. Fan abandonment appears to have occurred at ∼11.5 ka as the climate presumably became more humid in response to initiation of enhanced monsoonal circulation, consistent with previously reported onset of humid conditions in a sedimentary record from the easternmost basin of the lake. In contrast, the terraces did not form until about 7 ka with downcutting of the transverse valley by overflow from Lake Panggong. This lag can be explained in light of the bathymetry of Lake Panggong; the modern lake consists of five basins separated by shallow sills, and outflow through the Tangtse Valley could not occur until the water level was substantially above its present level. The easternmost basin receives the inflow of the major rivers feeding the lake, making its chemistry highly sensitive to changes in precipitation. However, sustained wet conditions are required to fill the basins to the west to the sill depth necessary for overflow through the Tangtse Valley and resultant downcutting and terrace formation.
Bibliographical noteFunding Information:
Funding for this project was provided by the National Science Foundation, Division of Earth Sciences (Grants EAR-9705841 and EAR-9706502 to E.T.B. and P.M., respectively). Discussions with Françoise Gasse, Doug Ricketts, Hugh Sinclair, and John Swenson and thoughtful reviews by Devendra Lal and Adrian Harvey improved this study considerably. We thank Jacques Lestringuez and Dominique Deboffle for their expertise in AMS measurements and Jason Agnich for skilled assistance in sample preparation. Tandétron operation is supported by CNRS, IN2P3, and CEA.
- Cosmogenic nuclides
- Debris flow
- Fluvial features