INTEGRATING PETROGRAPHIC DATA WITH DETRITAL-MINERAL INVESTIGATIONS OF EROSION OVER MULTIPLE TIMESCALES IN THE TSANGPO-SIANG-BRAHMAPUTRA RIVER SYSTEM, NE INDIAN HIMALAYA

Patterns of erosion across an active orogen can vary significantly over small areas in response to regional tectonic or climatic variability. Detrital-mineral investigations using such techniques as 10Be cosmogenic-radionuclide measurements and low-temperature thermochronology are useful tools for mapping catchment-averaged erosion and exhumation rates over millenial to million-year timescales. In the right context, petrographic analysis of sampled sediment may add useful perspective to the interpretation of detrital-mineral data, particularly for small, high-relief catchments experiencing large-magnitude, low-frequency events such as landslides. Petrographic analysis can help validate crucial assumptions implicit in such detrital-mineral investigations and may offer an independent means to intepret provenace and map modern erosion patterns.This research investigates how petrographic data may serve as a useful aide in interpeting detrital-mineral data as well as the limits of its application. We focus on the Siang River, part of the Tsangpo-Brahmaputra river system downstream of the Eastern Himalayan Syntaxis, in tributary catchments draining the litholgically diverse Lesser Himalayan Sequence across a high-relief, tectonically active region. We present a petrographic study of modern sediments complimenting current 10Be cosmogenic radionuclide and 40Ar/39Ar thermochronologic analyses sampled from the same area.