HR: 16:30h
AN: T34B-03    [Abstracts]
TI: Erosion rates and processes across two trans Himalayan transects in central Nepal
AU: * Heimsath, A M
EM: Arjun.Heimsath@Dartmouth.edu
AF: Dartmouth College, Department of Earth Sciences 6105 Fairchild Hall, Hanover, NH 03755 United States
AU: Wobus, C
EM: cwobus@mit.edu
AF: MIT, Dept. of Earth, Atmosphere and Planetary Sciences, Cambridge, MA 02139 United States
AB: Quantifying erosion rates across mountain ranges is difficult. Constraining long and short term erosion rates for the Himalaya is especially important as we attempt to quantify the degree of coupling among climate, tectonics, and surficial erosion in the quintessential collisional orogen. Conclusions drawn from recent studies in adjacent drainage basins across the higher Himalayan Range in central Nepal are inconsistent in their interpretations of the coupling between climate and long-term exhumation inferred from low-temperature thermochronometers. These differing interpretations underscore the need for additional erosion rate data representing timescales approaching the decadal timescales of our precipitation records. Here we present new results from an extensive study using in situ produced cosmogenic nuclides in both of these adjacent drainages: the Marsyandi and Burhi Gandaki Rivers. Average erosion rates determined from nuclide concentrations in detrital sands from first and second order catchments show similar patterns for both transects. Specifically, erosion rates increase from roughly 1-2 mm/yr where monsoon precipitation is under 2 m, to peak rates of about 2 and 7 mm/yr for the Marsyandi and Burhi Gandaki, respectively, corresponding to the region of highest precipitation. Both basins show a decline in average erosion rates and a corresponding decline in precipitation across the "rain shadow" of the main Himalayan crest. We suggest that the observed pattern of erosion rates may be largely controlled by climate, but is modulated by the locus of active structures in each drainage. Measurements of nuclide concentrations from ridge crest samples for the Marsyandi transect show little change in erosion rates with the same range of precipitation, suggesting a process-based discontinuity between ridge crest and side slopes. These ridge crest rates, averaging under 0.5 mm/yr, show a similar pattern to, but are significantly lower than, the spatially constant apatite fission track ages predicting long-term exhumation rates of about 2-5 mm/yr. Overall, these data support work correlating decadal records of climate with long term inferences of exhumation and erosion.
DE: 8107 Continental neotectonics
DE: 1719 Hydrology
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1625 Geomorphology and weathering (1824, 1886)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting