Rock type, precipitation, and the steepness of Himalayan threshold hillslopes

Abstract

Abstract Studies on the dynamic coupling between tectonics, climate, and erosion at the margins of the Tibetan Plateau entail the notion of widely occurring threshold hillslopes along the southern Himalayan front. Here we show that differences in major lithological units are sufficient to explain trends in topographical relief and mean slope gradients across the eastern Nepal Himalaya. This lithological control is manifest in modal slope gradients that serve as proxies of peak rock-mass strength, and remain strikingly invariant per rock type on both sides of the Main Central Thrust (MCT) despite comparably high amounts of monsoon precipitation. We infer that lithology rather precipitation patterns plays an important though hitherto largely neglected role in modulating threshold hillslope steepness. A nonlinear relationship between mean slope gradients and relief further indicates that significant steepening of threshold hillslopes through relief increases is more readily facilitated in low-relief terrain. Most evidence of large (>10 8 m 3 ) bedrock landslides in the Himalaya clusters where topographical relief >2 km, and slope gradients are above average. This provides the mechanism necessary for rapidly lowering drainage divides and limiting hillslope heights, were the steepness of threshold hillslopes to decline via relief destruction.