HR: 08:45h
AN: T21E-04    [Abstracts]
TI: Influence of topographic evolution and faulting on detrital thermochronometer ages: Application to the Nepalese Himalaya
AU: * Whipp, D M
EM: dwhipp@umich.edu
AF: Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109, United States
AU: Ehlers, T A
AF: Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109, United States
AB: Detrital thermochronology is an increasingly popular technique for determining catchment-wide, long-term denudation histories in mountainous regions. However, in the rapidly eroding Himalayan front detrital age data interpretation is complex and it remains unclear how tectonic and topographic variations affect age distributions in river sediments and syn-tectonic basin deposits. In this study, we evaluate how temporal variations in topographic evolution and thrust faulting influence detrital grain-age distributions measured in modern river sediments. We utilize a 3D thermokinematic numerical model with evolving topography to investigate the effects of thrust faulting and relief change on synoptic probability density functions of various thermochronometer systems. Model inputs include fault kinematics, magnitude and timing of relief changes, thermal boundary conditions, and material properties. Model topography and tectonics are based on the Marsyandi River region in central Nepal over the last 20 My. Thrust motion is partitioned between the Main Frontal Thrust and Main Central Thrust at rates of 0.2-14.5 and 0-4.1 mm/y, respectively, generating denudation rates of 0.1-5 mm/y. Topographic relief is varied by ±50% at various times during the simulation. Results show that detrital grain-age distributions from modern rivers have a strong sensitivity to the denudation rate dictated by the fault kinematics, but are not sensitive to fault slip partitioning without relief change. However, relief changes produce age distributions that are sensitive to the fault kinematics, especially in higher-temperature thermochronometers (e.g., muscovite ⁴⁰Ar/³⁹Ar). Predicted thermochronometer age ranges also vary with relief change, generating wider age ranges with increasing relief and narrower ranges with decreasing relief. In contrast, the primary peak ages are not sensitive to relief changes at rapid exhumation rates, but secondary age peaks may develop. Taken together, these results suggest: (a) Modern river samples are sensitive to changes in relief, but this signal will be obfuscated in lag times calculated from syn-tectonic basin deposits, and (b) in rapidly eroding regions, higher temperature detrital thermochronometer samples have the greatest sensitivity to topographic evolution.
DE: 1140 Thermochronology
DE: 1815 Erosion
DE: 1847 Modeling
DE: 8175 Tectonics and landscape evolution
DE: 9320 Asia
SC: Tectonophysics [T]
MN: 2007 Fall Meeting