Cosmogenic Nuclide Tracking of Sediment Recycling From a Frontal Siwalik Range in the Northwestern Himalaya

The Himalayan orogen exports millions of tons of sediment annually to the Indo-Gangetic foreland basin, derived from both hinterland and foreland fold-thrust belts (FTB). Although erosion rates in the hinterland are well-constrained, erosion rates in the foreland FTB and, by extension, the sediment flux have remained poorly constrained. Here, we quantified erosion rates and sediment flux from the Mohand Range in the northwestern Himalaya by modeling and measuring the cosmogenic radionuclide (CRN) 10Be and 26Al concentrations in modern fluvial sediments. Our model uses local geological and geophysical constraints and accounts for CRN inheritance and sediment recycling, which enables us to determine the relative contributions of the hinterland and foreland FTB sources to the CRN budget of the proximal foreland deposits. Our model predictions closely match measured concentrations for a crustal shortening rate across the Mohand Range of 8.0 +/- 0.5 mm yr-1 (i.e., approximately 50% of the total shortening across the Himalaya at this longitude) since 0.75-0.06+0.02 $0.7{5}_{-0.06}<^>{+0.02}$ Ma. This shortening implies a spatial gradient in erosion rates ranging from 0.42 +/- 0.03 to 4.92 +/- 0.34 mm yr-1, controlled by the geometry of the underlying structure. This erosion pattern corresponds to an annual sediment recycling of similar to 2.0 megatons from the Mohand Range to the downstream Yamuna foreland. Converted to sediment fluxes per unit width along the Himalaya, the foreland FTB accounts for similar to 21% +/- 5% of the total flux entering the foreland. Because these sediments have lower 10Be concentrations than hinterland-derived sediment, they would lead to similar to 14% overestimation of 10Be-derived erosion rates, based on Yamuna sediments in the proximal foreland. Cosmogenic radionuclides such as beryllium-10 (10Be) are important tools for tracking erosion processes at the Earth's surface. However, they are challenging to apply when sediment remains temporarily stored for millions of years before being re-eroded. We call this sediment recycling. We quantified sediment recycling in the Mohand Range, at the foothills of the northwestern Himalaya, based on the present-day 10Be concentration in fluvial sediments recycled from tectonically uplifted older foreland deposits. We do this by modeling how the 10Be concentrations change as sediment is first eroded in the Himalayan source region, then deposited in the foreland, and finally eroded again. By comparing modeled and measured concentrations, we find that erosion rates in the Mohand Range vary from 0.42 +/- 0.03 to 4.92 +/- 0.34 mm yr-1, corresponding to about 2.0 megatons of sediment recycling from this range to the downstream Yamuna foreland. Because the 10Be concentration in recycled sediment is lower than that in sediment eroded from the high Himalaya, admixing high Himalayan sediments with recycled material may result in erosion rates that are higher than the actual erosion rates in the high Himalaya. Novel approach for tracking cosmogenic nuclide accumulation in fluvial sediments through each step of a fold-thrust belt sediment routing system10Be-derived erosion rates and sediment flux from the Mohand Range in the northwestern HimalayaOnset of Main Frontal Thrust motion and rate of crustal shortening across the Mohand Range constrained by modeling 10Be concentrations in present-day river sediments