River Antecedence and the Onset of Rapid Exhumation in the Eastern Himalayan Syntaxis

Abstract:

The peripheral Himalayan foreland basin preserves a rich archive of Himalayan landscape dynamics. In particular, high-resolution geochronology and thermochronology of sedimentary units provides unique perspectives on the evolution of river drainage patterns and source exhumation rates when conventional bedrock applications may be limited. A case example is the rapid exhumation of the Namche Barwa metamorphic massif within the eastern Himalayan syntaxis. Here, thermo-mechanical feedbacks between erosion by the Yarlung River and uplift of a crustal-scale antiform may have locally sustained exhumation rates as high as 10 km/Ma through the Plio-Quaternary. However, the origins of these feedbacks remain difficult to constrain from the extremely young (<4 Ma) cooling histories observed in bedrock samples. To extend the record of landscape evolution, we quantify the depositional age, provenance and cooling histories of detrital minerals from Neogene sedimentary units proximal to the eastern syntaxis. Specifically, we combine magnetostratigraphy, detrital muscovite ⁴⁰Ar/³⁹Ar thermochronology, zircon fission track thermochronology and U-Pb geochronology to test the hypothesis that the development of a thermo-mechanical feedback is related to capture of the Yarlung River by a Himalayan tributary to the Brahmaputra River. Thermal modeling of thermochronologic lag-times indicates that extremely rapid exhumation rates (5-10 km/Myr) have persisted in the syntaxis since 5 Ma, following a dramatic rate increase in the Late Miocene. However, the presence of Gangdese-age zircons in older sedimentary units requires connection of the Yarlung and Brahmaputra Rivers by at least 11 Ma, predating the onset of rapid exhumation. These results suggest that capture of the Yarlung River was not a potential mechanism to initiate rapid exhumation of the Namche Barwa massif. Instead, we propose that Late Miocene tectonic uplift locally steepened the antecedent river channel, increasing rock exhumation rates and promoting a thermo-mechanical feedback localized within this area of the eastern syntaxis, but not the broader eastern Himalaya, in the Plio-Quaternary.