Impact of Drainage Basin Geology and Geomorphology on Detrital Thermochronometric Data from Modern River Sands: A Case Study in the Bhutan Himalaya

Abstract:

Detrital thermochronology has become an important tool to quantify the erosional history of mountainous regions. Despite an increasing number of studies utilizing detrital records, it remains unclear how the record of spatially variable erosion of upstream drainage basins is preserved in the thermochronologic signal contained in the sediments. This important spatiotemporal problem is a first-order unknown that limits the interpretation of the geological significance of the detrital signal. To improve our understanding of detrital records in terms of spatiotemporal erosion rates, we use a three-step approach to study modern fluvial sediments from the Bhutan Himalaya. First, based on a preferred tectonomorphic scenario extracted by inversion of in situ multi-thermochronological ages, we predict apatite fission-track (AFT) age distributions in 18 catchments using the Pecube software. Second, we compare AFT age distributions from modern sand bars collected at each catchment outlet to distributions extracted from Monte Carlo sampling of the predicted catchment ages. We find that observed and predicted age distributions are statistically equivalent for only ~75% of the catchments. Third, we calculate predicted detrital age distributions by scaling the prevalence of ages in the catchment in proportion to topographic and climatic metrics (e.g., local relief, steepness index, specific stream power weighted by precipitation rate) or landslide-driven erosion to quantify their effects and relationships to the observed detrital AFT age distributions. Preliminary results suggest erosion in proportion to the topographic metrics cannot reproduce the observed age distributions, but bedrock landsliding may provide sufficient age variability to reproduce the observations. Ongoing work is determining whether variable target mineral concentrations in bedrock geological units or non-uniform sediment sourcing from moraine- or glacier-covered regions can reproduce the observed ages.