Late Cenozoic Himalayan Erosion Rates Revealed by Cosmogenic Isotopes in Foreland Sediments, Northern India

Abstract:

Most surface processes that transport sediment are influenced by climate. For example, more rainfall enhances runoff and stream capacity, and colder temperatures expand glaciers at the expense of rivers. Late Cenozoic cooling and glacial cycles during the Quaternary should thus have affected erosion of the Earth’s surface. But whether these changes were also associated with an overall increase of erosion rates is not clear.

Here, we assess the erosional response of the fluvial-dominated Yamuna catchment in the Garhwal Himalaya, northern India, to late Cenozoic cooling and Quaternary climatic oscillations. Our approach is to measure cosmogenic radionuclide (¹⁰Be) concentrations in fluvial sediments (n = 14) eroded from uplifting foreland deposits and compare them with modelled concentrations for different paleo-erosion rate scenarios. This approach differs from previous ones that determine paleo-erosion rates from ¹⁰Be concentrations in distinct samples from stratigraphic sections, and avoids misinterpreting short-term fluctuations in ¹⁰Be concentrations that are unrelated to erosion rates. We tested this approach in the Mohand Range in northwest India, where Miocene to Quaternary deposits of the paleo-Yamuna River are actively uplifting, and where a robust kinematic model and published stratigraphic age constraints exist.

Our model free parameters are the shortening rate across the Main Frontal Thrust (MFT) and the onset of shortening, within a known amount of total MFT slip (4-5 km). Preliminary results show that we can reproduce the measured ¹⁰Be concentrations best when Himalayan erosion rates were lower in the past than they are now, or have been increasing towards the present. Within uncertainties, the best-fit parameter combinations give shortening rates between 10 and 20 mm/yr, which is consistent with independent estimates from a nearby dated strath terrace and expected uplift rates based on channel steepness indices. Scenarios in which erosion rates are constant in time and the same as today, or were higher in the past and have decreased towards the present, yield both poorer fits with measured concentrations and unrealistically low shortening rates. We conclude that long-term erosion rates in the Yamuna catchment have most likely been lower than today or during the Quaternary.