40Ar/39Ar ages of muscovites from modern Himalayan rivers: Himalayan evolution and the relative contribution of tectonics and climate

International audience40Ar/39Ar ages from detrital muscovites have been analyzed from sixmodern rivers in central and western Nepal; the size of the drainage basinsassociated with these samples ranges from a few square kilometers to>40,000 km2. These data, when combined with previously published agesof detritalmuscovites from other modern rivers in the region, suggest thata good correspondence between normalized age and normalized topography(the comparison of t* and z*) is rare, due to either nonuniform rates ofpassage through the ~400 °C isotherm or subsequent faulting in the drainagearea. The closure temperature of Ar in muscovite is perhaps too high tomake meaningful comparisons to modern topography in tectonic analysis ofactiveorogens.The distribution of 40Ar/39Ar ages from detrital muscovites from the Karnalibasin in western Nepal is much older than that for the Narayani basin in centralNepal. The Karnali muscovites, when combined with previously publishedmuscovites from the Siwalik Group in western Nepal and zircon fission trackages from modern and ancient samples from the region, suggest a thermalhistory for western Nepal consistent with vigorous tectonics (and attendanterosion) before the middle Miocene but a significant diminution in the rate oferosion since ca. 10 Ma.40Ar/39Ar ages of detrital muscovites from the Narayani basin in centralNepal suggest a markedly different history with an acceleration of the rate oferosion since ca. 10 Ma and reactivation of major faults; this is consistent withthe abundant bedrock data from the Narayani basin.The strong difference in the erosional history of the adjacent Karnali andNarayani basins, as evidenced by the 40Ar/39Ar ages from detrital muscovites,is not likely to have been due to variations in climate, but rather due to strainpartitioning within the Himalaya during and after the Miocene