A multi-geochronologic approach to identify the focus of erosion in the Kosi Basin, Nepal

Abstract

The Himalayan orogen represents a convergent tectonic setting in which two continental tectonic plates collided, forcing material upwards and forming a chain of high elevation peaks. This setting is also host to the Indian Summer Monsoon, which hammers the region with heavy seasonal rainfall. The respective roles of tectonics and climate in shaping this dramatic landscape remain a subject of debate. Our research approaches this gap in understanding by investigating sediment routing in Nepal's Kosi basin. The Kosi watershed is a well-suited basin to study this phenomenon, as it captures many of the key geologic structures and climatic conditions that are characteristic of the region. We sampled modern rivers to understand the sediment profile in the Kosi basin. We then utilized two radioactive dating methods to fingerprint sediment grains and understand their origin in the watershed. Detrital zircon U-Pb geochronology dates when a crystal formed and utilizes large sample sizes to match detrital age spectra to known bedrock profiles. Alternatively, (U-Th)/He low-temperature thermochronology dates when an individual zircon grain cooled to the surface. These detrital cooling dates are in turn matched to areas known to have cooled at similar times. We applied both methods to our modern river samples. Using our new geochronological and thermochronological datasets, we completed two statistical analyses to pinpoint accelerated areas of erosion: inverse monte-carlo mixture modeling and multi-dimensional scaling nearest-neighbor analysis. Our results suggest that each major geologic unit and sub-basin in the watershed contributes to the sediment load in the Kosi River to some extent. Furthermore, U-Pb mixing model results and young helium cooling ages both highlight an over representation of grains in the center of the watershed, by latitude. Integrating our datasets, these results suggest that erosion is focused north of the dominant precipitation band and south of the largest glacial cover. We propose that these findings support a model of landscape evolution in which structural geometry at depth, specifically duplexing along a midcrustal ramp, plays a critical role in shaping surface topography.