MIOCENE EXHUMATION OF THE HIMALAYAN METAMORPHIC CORE: NEW THERMOCHRONOLOGIC CONSTRAINTS FROM THE LANGTANG VALLEY, NEPAL

Ten new ⁴⁰Ar/³⁹Ar biotite samples from a steep transect in the Langtang valley of central Nepal define a kinked age-elevation profile that suggests a late-Miocene increase in exhumation rates within the metamorphic core of the Himalaya. Age-elevation gradients of <<0.1 mm/yr from ~13-21 Ma and roughly 0.5 mm/yr from ~8-10 Ma can be interpreted in terms of apparent erosion rates if vertical exhumation is assumed. Regardless of whether or not this assumption is valid, the kinked profile implies either an acceleration of average erosion rates or a change in exhumation pathway in late Miocene time. An increase in exhumation rates in the late Miocene is consistent with a number of models for the tectonics of central Nepal, including brittle reactivation of the Main Central Thrust (MCT) zone; development of new thrust faults in the immediate footwall of the MCT; or late-Miocene growth of a thrust duplex in the Lesser Himalaya. Late Miocene exhumation rates inferred from our data are also consistent with those obtained from late Miocene-early Pliocene samples further west in the Marsyandi valley, suggesting regionally consistent exhumation at ca. 0.5 mm/yr through the Miocene-Pliocene interval. However, all of the exhumation rates estimated from this study are substantially slower than Plio-Pleistocene rates inferred from young (ca. 2.5 Ma) muscovites and lower-temperature thermochronometers in the region. These data place important new constraints on the tectonothermal evolution of the central Nepalese Himalaya.