Drainage divide asymmetry as an indicator of large-scale landscape transience on the Southeast Tibetan Plateau

Abstract

Landscapes are sculpted by a complex response of surface processes to external forcings, such as climate and tectonics. Several major stream captures have been documented on the Southeast Tibetan Plateau, leading to the hypothesis that the region experiences exceptionally high rates of drainage reorganization driven by horizontal shortening and propagating uplift. Here we determine the prevalence, intensity, and spatial patterns of ongoing drainage reorganization on the Southeast Tibetan Plateau and evaluate the relative time scales of this transience by comparing drainage divide asymmetry for four geomorphic metrics that operate at different spatial and temporal scales. Specifically, we evaluate drainage divide asymmetry in catchment-restricted topographic relief, hillslope gradient, normalized channel steepness (ksn), and χ. ksn and χ are both precipitation-corrected to account for the strong precipitation gradient across the region. We calculate the migration direction and Scherler & Schwanghart (2020)’s divide asymmetry index (DAI) in each metric for drainage divides across the entire region in order to analyze how well the asymmetry in these metrics agree along divides and where consistent divide movement is inferred. We find a high incidence of strongly asymmetric divides in all metrics across the entire Southeast Tibetan Plateau. While the magnitude of asymmetry varies significantly between metrics, a majority of divides agree on divide migration direction across all metrics. Divides with higher magnitudes of asymmetry are more likely to agree on migration direction across multiple metrics. While χ agrees least often with the other metrics on migration direction, it agrees on direction >90% of the time when low DAI divides are excluded. We also establish that disagreement in predicted divide migration directions between χ and the other geomorphic metrics can be interpreted as evidence of localized variations in tectonic uplift or erodibility, glacial alteration, or recent lateral stream capture. Our work confirms the high incidence of drainage reorganization across the Southeast Tibetan Plateau and highlights both transient and stable areas in the landscape with unprecedented resolution. In addition, we propose how to combine geomorphic metrics to ascertain how drainage divides migrate across different timescales and identify local deviations in tectonic uplift and erodibility.