HR: 16:45h
AN: T34B-04    [Abstracts]
TI: Thermal and Mechanical Modeling of Bedrock Cooling Ages in the Nepalese Himalaya
AU: Brewer, I
EM: Ian.Brewer@stos.co.nz
AF: Shell Todd Oil Services Ltd, 167 Devon St West, New Plymouth, 4620 New Zealand
AU: * Burbank, D W
EM: burbank@crustal.ucsb.edu
AF: Dept. of Geological Sciences, University of California, Santa Barbara, CA 63106 United States
AB: We introduce a new method to use detrital mineral cooling ages in conjunction with a digital elevation model (DEM) to test numerical models of collisional orogens. We apply this methodology to the Marsyandi valley in the central Nepalese Himalaya, where we use a 2D kinematic-and-thermal model to predict variations in bedrock cooling ages along an averaged structural and topographic transect within the Himalaya. The model is based upon a simple ramp-and-flat style decollement, representing the Main Himalayan Thrust and is constrained by the INDEPTH transect, surface geology, seismicity, and geomorphology. The range is assumed to be in a topographic steady state, such that all rock advected to the surface is eroded. The 2D kinematic-and-thermal model in the plane of advection is extrapolated laterally (parallel to the range) to calculate the 3D distribution of cooling ages predicted for actual Himalayan topography. The modeled detrital cooling-age signal results from convolving the distribution of cooling ages within a catchment with the rate at which individual points are eroding and with the distribution of the mineral used as the thermochronometer. Predicted distributions of cooling ages are compared with detrital $^{40}$Ar/$^{39}$Ar muscovite data to assess varying tectonic scenarios. Model results, assuming that the Main Boundary Thrust represents the surface expression of the Main Himalayan Thrust, illustrate that the distribution of detrital cooling ages is sensitive to how the observed 20 mm/yr of Indo-Tibetan convergence is partitioned: the best-fit model assigns 5 mm/yr to overthrusting by the Asian plate and 15 mm/yr to underthrusting by the Indian plate. A trade off exists, however, between ramp geometry and the best-fit rate. A model using the approximate present position of the Main Central Thrust to represent the surface expression of the Main Himalayan Thrust provides a better fit to the observed data and supports evidence for recent activity on the Main Central Thrust.
DE: 8102 Continental contractional orogenic belts
DE: 8105 Continental margins and sedimentary basins
DE: 8107 Continental neotectonics
DE: 1815 Erosion and sedimentation
DE: 1886 Weathering (1625)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting