40AR/39AR DETRITAL MINERAL THERMOCHRONOLOGY IN ACTIVE FLUVIAL SYSTEMS

Some of the most powerful tools available for establishing erosional histories in active orogenic systems are the ⁴⁰Ar/³⁹Ar, (U-Th)/He, and fission-track cooling ages that can be obtained for detrital minerals from stream channels and young river terraces. The ⁴⁰Ar/³⁹Ar method is well-suited to such studies; the micas (particularly muscovite) are relatively resistant to grain-size communition and alteration during erosion and transport, and yield information pertaining to the cooling of source regions through the ~375-325 C temperature range. Modern laser microprobe systems permit the routine determination of sufficiently large numbers of single-grain dates for a statistically significant characterization of the age population in most detrital samples.

Two applications of this technique will be reviewed here. The first is a thought experiment regarding the value of ⁴⁰Ar/³⁹Ar thermochronometry for rapid reconnaissance characterization of the thermal history of large regions; such studies may prove critical to efforts such as the U.S. National Science Foundation's Earthscope initiative, where the goal is large-scale studies of the lithosphere. The second focus of the presentation will be the results of detrital ⁴⁰Ar/³⁹Ar thermochronometry of modern sediments from rivers draining the southern flank of the central Nepalese Himalaya. We show how such data have been used both to model catchment-wide erosion rates, and to monitor the kinematics of recent deformation.