HR: 16:00h
AN: T34B-01    [Abstracts]
TI: Cooling and Denudation History of the Marsyandi Drainage in the Central Nepalese Himalaya From Apatite Fission Track and (U-Th)/He Analyses
AU: * Blythe, A E
EM: blythe@usc.edu
AF: University of Southern California, Department of Earth Sciences, Los Angeles, CA 90089 United States
AU: Burbank, D W
AF: University of California, Department of Geological Sciences, Santa Barbara, CA 93106 United States
AU: Schmidt, K
AF: Lewis-Clark State College, Division of Natural Sciences, Lewiston, ID 83501 United States
AU: Putkonen, J
AF: University of Washington, Quaternary Research Center, Seattle, WA 98195 United States
AB: Apatite fission track and (U-Th)/He analyses document the cooling history and the links between climate and tectonic processes of the Marsyandi river catchment in the central Nepal Himalaya. The Marsyandi river originates in the Tibetan Plateau and traverses the High Himalaya to the east of Annapurna, crossing the Tibetan detachment and the Main Central Thrust. We have obtained $>$80 apatite fission track (AFT) and (U-Th)/He ages from samples collected along the Marsyandi drainage, and along ridges on either side of the river. The youthfulness of this orogen is pushing the limits of both techniques. AFT ages range from $>$3 to 0$\pm$0.2 Ma, and He ages from 0.7 to 0.3 Ma, with AFT and He ages at river level of 0.4 and 0.3 Ma, respectively. Samples were collected along ridge lines that ascend away from the river. Although ages in general increase with increasing elevation away from the valley bottom, striking geographic variation in the total age range occurs: several ridges have maximum ages of 1 to 0.8 Ma at 2 km above the valley bottom, whereas only one ridge crest (in the southwest of the field area) displays ages as old as $\sim$ 2 Ma. We suspect that there is a structural boundary between this ridge and others to the north and east. With these data, we calculate cooling and infer denudation rates over the last 2 Ma. Cooling and denudation rates are uniformly high ($\geq$1 km/m.y.) from 2 to 0.8 Ma, but they appear to have accelerated at 0.8 Ma to 3 km/m.y. Denudation rates from 0.3 Ma until the present are calculated by assuming (appropriately) high FT and He closure temperatures of 140 and 90$\deg$C, and a geothermal gradient of 50 to 100$\deg$C/km, indicating rates of 3 to 8 km/m.y. The thermochronologic data are being used in conjunction with geomorphic and climate data to evaluate the mechanisms controlling the physiography of one of the world's major mountain belts.
DE: 8102 Continental contractional orogenic belts
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting