HR: 16:45h
AN: H44A-04    [Abstracts]
TI: Mega-landslides in eastern Tibet: Implications for landscape and river profile evolution, and the interpretation of tectonics from topography
AU: * Ouimet, W B
EM: wouimet@mit.edu
AF: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139
AU: Whipple, K X
EM: kxw@mit.edu
AF: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139
AB: Field work within the Yalong and Dadu Rivers, two major tributaries of the Yangtze River that dissect the eastern margin of the Tibetan Plateau, indicates that a high frequency of large, deep-seated landslide events, both modern and ancestral, have led to extensive and prolonged river damming throughout each of these deep river gorges. These observations highlight a strong feedback between hillslope processes and channel morphology that is prevalent throughout this landscape. The eastern margin of the Tibetan plateau is a rapidly evolving landscape adjusting to regional and localized surface uplift, climate changes, and large-scale river re-organization through river capture. The impact of mega-landslides introduces another important influence contributing to the transient state of river profiles in the region, further complicating the interpretation of profiles extracted from topographic data and the application of river incision models. The role of mega-landslides must be considered in trying to understand how river profiles can be used to highlight important aspects of the tectonic and geomorphic evolution of the plateau margin. Landslides exert a first order control on river profile and valley morphology, and often lead to the formation of strath terraces and bedrock gorges unrelated to long-term bedrock river incision rates. Profile knick-points are consistently related to stretches of river dominated by boulder deposits delivered to the channel by large landslides. Landslide deposits typically armor the bed and force river channels to steepen and narrow. When particularly large, landslides dam the valley and trigger upstream aggradation. River incision sets the pace of landscape lowering, but in eastern Tibet, landslides appear to be significantly inhibiting river incision. We are currently analyzing samples collected in the field to gather age information for various landslides and gain a better understanding of the impact, past and present, of mega-landslides in eastern Tibet river gorges. We are dating various features related to impoundment, such as lake sediments, fill terraces, in-place landslide boulders, using three quaternary dating methods: cosmogenic radio nuclides (CRN), optical stimulated luminescence (OSL), and carbon 14 (14C). This will help in determining the age, and perhaps duration, of individual landslide dam interruptions. Preliminary data indicates that landslide deposits can linger within the fluvial system, inhibiting bedrock incision, for $>$40,000 years.
DE: 9320 Asia
DE: 8107 Continental neotectonics
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting