HR: 0800h
AN: H31A-1279    [Abstracts]
TI: Long and short-term erosion and river incision on the eastern margin of the Tibetan plateau, a transient landscape
AU: * Ouimet, W B
EM: wouimet@mit.edu
AF: Massachusetts Institute of Technology, Department of Earth, Atmospheric and Planetary Sciences, Cambridge, MA 02139
AU: Whipple, K X
EM: kxw@mit.edu
AF: Massachusetts Institute of Technology, Department of Earth, Atmospheric and Planetary Sciences, Cambridge, MA 02139
AU: Royden, L H
EM: lhroyden@mit.edu
AF: Massachusetts Institute of Technology, Department of Earth, Atmospheric and Planetary Sciences, Cambridge, MA 02139
AB: The eastern margin of the Tibetan plateau is a transient landscape, characterized by deep river gorges cutting into an uplifted, low-relief relict landscape. Rivers in the region are adjusting to regional and localized surface uplift, variable sediment flux from climate changes, large-scale river re-organization through river capture, and the effects of persistent, large, landslide dams. As a result of these factors, river channels display transient morphologies, with many abrupt changes in channel slope, as well as variable channel and valley widths, and frequent alternations between steep, incising bedrock reaches and gentle, depositional, alluvial reaches. In addition, longitudinal profiles deviate from smoothly concave-up profile forms, with frequent knickpoints and gradual changes in profile convexity. Spatially variable rates of erosion and river incision characterize this transient landscape. Published thermochronologic studies show that river incision began 9-13 million years ago and long-term denudation rates for the Dadu river gorge are between 0.25 and 0.5 mm/yr. Measured concentrations of 10Be in quartz extracted from stream sediments indicate that short-term (10⁴-10⁵ yr) basin-wide erosion rates for the high elevation, relict landscape are low, between 0.024 and 0.036 mm/yr, while erosion rates for the lower elevation, dissected landscape are much higher, between 0.230 and 1.31 mm/yr. Meanwhile, preliminary incision rates calculated from 10Be exposure ages on fluvially polished bedrock straths adjacent to modern river channels range from 0.8 to 7 mm/yr, suggesting that river channels are capable of incising at rates much faster than long-term rates. Additional bedrock samples collected fall 2003 and 2004 within the Yangtze, Yalong and Dadu river gorges will allow us to further constrain rates of long-term river incision throughout the eastern margin of the Tibetan plateau. These samples are from three age-elevation transects and will be analyzed for (U-Th)/He thermochronology. The variable nature of erosion on both temporal and spatial scales that we see along the eastern margin is typical of transient landscapes. However, the persistence of slow erosion rates and patches of high elevation, relict landscape despite rapid incision and intense fluvial dissection by trunk streams at lower elevations for at least the last 8 to 10 million years suggests a much longer transient response than predicted by the well-known stream power river incision models, especially given the wet climate of the region.
DE: 1140 Thermochronology
DE: 1150 Cosmogenic-nuclide exposure dating (4918)
DE: 1815 Erosion
DE: 8175 Tectonics and landscape evolution
DE: 9320 Asia
SC: Hydrology [H]
MN: Fall Meeting 2005