Basin-averaged erosion rates from the uplifting Siwalik Hills in northwest India

The Siwalik Hills are an important component of the Himalayan orogen and are considered to be a key example of steady state topography, where landscape form can be used to infer tectonic patterns associated with the Himalayan Frontal Thrust. Previous work shows that rock uplift rates in the Siwaliks correlate with variations in channel steepness, local-scale relief, and even stream-power based estimates of fluvial incision. However, so far quantitative assessment of landscape change has been limited to estimates of incision rates from fluvial terraces. Here, we use cosmogenic radionuclide (10Be) concentrations in quartz from river sands to determine basin-averaged erosion rates from ~20 catchments (3-58 km2 in size and 285-568 m in total relief) draining the uplifting Mohand and Chandigarh folds at the northwest Himalayan front. Our erosion-rate estimates are 0.9-2.3 mm/yr (± ~10-30%) on the southern flank of the Mohand range compared to 0.3-0.5 mm/yr on the northern flank. This contrast is consistent with an inferred difference in mean rock uplift rate between both flanks from a balanced cross section. Two nested samples from southern-flank catchment midpoints have erosion rates of 0.8-1.6 mm/yr that are 60-70% of the total catchment-averaged rates. Erosion-rate estimates from the Chandigarh anticline are 0.6-1.4 mm/yr. Estimated rock uplift rates are at least 2-3 times faster than the erosion rates in both ranges. Apparent ages from our samples, as well as two bedrock exposure ages near the drainage divide, range from 350 to 3,000 years. We see moderate to weak correlations between erosion rate and various geomorphic parameters (basin size, relief, mean basin slope, channel steepness, hillslope relief). However, slow (≤0.5 mm/yr) versus fast (≥1 mm/yr) eroding basins appear to be separated by a threshold in mean basin slope (~11 degrees) and hillslope relief (~35 m). These observations suggest that (a) spatial variations in basin-averaged erosion rates and mean rock uplift rate are related, (b) basin-scale erosion rates averaged over century-to-millennial timescales are lower than estimated rock uplift rates, and (c) hillslope gradients above a threshold appear to cause more efficient lowering of basin-wide elevations. Our results show some relationship between erosion and uplift but question whether these Siwalik Hills are in topographic steady state at all scales.;