A Dual-Geochronologic and Thermochronologic Detrital Approach to Identify the Focus of Erosion in the Kosi Basin, Nepal

The Kosi River watershed captures the major lithotectonic units, structures, and characteristic climatic conditions found along the Himalaya. Therefore, this setting provides a useful location to investigate the respective influences of tectonics and climate in eroding bedrock and driving landscape evolution. Using detrital zircon U-Pb geochronology and low-temperature (U-Th)/He thermochronology, we analyzed modern river sands from the Kosi River and its major tributaries to identify the spatial focus of erosion. Zircon U-Pb geochronological ages from river samples reflect input from all lithotectonic sequences. Six of eight detrital distributions have a dominant age range that is characteristic of the Lesser Himalayan Sequence bedrock age distribution (1700–2000 Ma). The furthest downstream sample, thereby the most integrated, has prominent age ranges from 20–50 Ma (5%), 400–430 Ma (3%), 540–570 Ma (4%), 1070–1120 Ma (11%), 1790–1830 Ma (5%), and 2450–2480 Ma (2%). (U-Th)/He low-temperature cooling ages (n = 100) range from 1.2 ± 0.1 to 15.9 ± 0.4 Ma, apart from one grain (55.2 ± 0.7 Ma). The median detrital cooling age is 4.4 Ma, compared to a median bedrock cooling age of 7.6 Ma. Detrital zircon (U-Th)/He ages <6 Ma indicate erosion from central latitudes and mid-elevations in the watershed. Our results suggest that peak erosion occurs north of the dominant precipitation band and south of glaciated high peaks. We interpret these data to support a landscape evolution whereby duplexing along a midcrustal ramp at depth plays a critical role in driving rock uplift and erosion.