TESTING DRIVERS OF LARGE-SCALE DRAINAGE CAPTURE WITH LOW-TEMPERATURE THERMOCHRONOLOGY, SUTLEJ RIVER, NORTHWEST HIMALAYA

Determining plausible mechanisms, or combinations of mechanisms, for facilitating large-scale drainage capture is necessary to identify feedbacks associated with orogenesis and the relative importance of autogenic dynamics in redistributing eroded sediment in active orogens. Field evidence and recent landscape analyses suggest the Zhada basin of southern Tibet (~23,000 km² in drainage area) was captured by the trans-Himalayan Sutlej river between 266-399 ka, effectively doubling its drainage area within the orogen. This event initiated an ongoing transient landscape response that has increased the sediment supply to the Sutlej river by ~50% since capture. Here, we use a dataset of apatite (U-Th)/He (AHe) closure ages and a numerical channel incision model to test three hypothetical mechanisms that may have facilitated this large-scale capture event: Quaternary activity along the South Tibetan Detachment System (STDS), prior capture of the Spiti river, and preferential erosion of weak lithologies along active brittle faults. We use a 1D numerical incision model to simulate post-capture channel evolution under conditions specific to each mechanism or combination of mechanisms. We then identify simulations that produce results consistent with the recorded thermal history of rocks along the Sutlej by comparing best fit model results for each modeled scenario to independently-derived exhumation histories inferred from inverse modeling of sample ages from our AHe dataset. In the absence of direct field observations supporting Quaternary activity along the STDS, we prefer scenarios whereby capture is facilitated by autogenic mechanisms. This work is vital to improve our understanding of how orogenic systems evolve and whether Himalayan drainage reorganization is primarily a product of internal dynamics or tectonic forcings.