HR: 10:50h
AN: T52A-03 INVITED     [Abstracts]
TI: The Detrital Record of Himalayan Orogenesis
AU: * Burbank, D W
EM: burbank@crustal.ucsb.edu
AF: Dept. of Geological Sciences, University of California, Santa Barbara, CA 93106 United States
AU: Brewer, I
EM: Ian.Brewer@stos.co.nz
AF: Shell Todd Oil Services Ltd, 167 Devon St West, New Plymouth, 000000 New Zealand
AU: Hodges, K
EM: kvhodges@mit.edu
AF: Dept. of Earth, Atmospheric, and Dept. of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139 United States
AB: Recent studies in foreland basins have yielded increasingly detailed detrital records of orogenesis. Along with more traditional petrographic and geochemical analysis, single-crystal dating and isotopic analyses of detrital minerals allow both more refined reconstructions of source areas, erosion rates, and structural evolution within the hinterland, as well as testing of concepts, such as orogenic steady state. Whereas analysis of the detrital record has thus been enhanced in recent decades, the way in which that record is produced has received much less attention. What are the spatial variations in erosion rates within the hinterland? How are these manifested in the detrital record in the foreland? How does the detrital signal in major transverse rivers evolve as it passes through the hinterland? We have focused on these questions through a study of Ar/Ar dating of detrital muscovite in the Marsyandi River of central Nepal. By dating samples from both tributaries and the main stem, we track the downstream evolution of the detrital muscovite signal across the Himalaya. Subsequently, we try to interpret the data with a numeric model that makes two simplifying assumptions: erosional and topographic steady state prevail within each catchment; and a sample's signal is a high-fidelity indication of the true dispersion of bedrock cooling ages in the catchment. If topographic relief is $<$6 km and the erosion rate is $<$3 mm/yr, then muscovite bedrock-cooling ages are approximately a function of erosion rate and height above the closure isotherm. Consequently, the downstream evolution of the detrital signal should be a direct function of the erosion rate in each tributary catchment, its catchment area and hypsometry, and the proportion of the target mineral (muscovite, in this study) in the contributed sediment. Other factors, such as spatial and temporal stability of the detrital signal, or downstream attrition of the target mineral, also need to be addressed. We reconstruct $>$2-fold variations in erosion rates across the orogenic belt, from the Tethyan realm in the north to the Lesser Himalaya in the south. The bulk of the detrital muscovite signal at the mouth of the Marsyandi appears to derive from $<$30% of the catchment. Our mixing models of tributary with main-stem samples emphasize the major impact that the fraction of muscovite has on the calculated erosion rates. This suggests that the often-used assumption in detrital studies of a uniform distribution of a target mineral (e.g. muscovite, zircon, apatite) in the source area needs to be evaluated. Erosion rates are most rapid on the southern flank of the Himalaya, where enhanced rates may be associated either with active deformation in the region of the MCT and/ or with intense monsoonal precipitation.
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