Focused erosion at the core of the Greater Caucasus: Sediment generation and dispersal from Mt. Elbrus to the Caspian Sea

The highest peaks in Europe are found in the central Greater Caucasus, an orogenic belt characterized by extremely active tectonic and sedimentary processes. In this rugged mountain region, we have investigated sand generation and erosion patterns by using high-resolution petrographic and heavy-mineral data on river sands complemented by detailed morphometric analysis of 9 major mountain branches of the Terek River. Also presented is an extensive review of sediment-transport data available in the Russian literature on glacial-lake outburst floods and rainfall-triggered debris flows, which owing to global warming and glacier retreat have afflicted the Greater Caucasus with increasing frequency in the last 50 years. The accurate definition of mineralogical signatures characterizing detritus derived from six end-member volcanic, metamorphic basement, and sedimentary/metasedimentary rock units allowed us to calculate the relative contribution of each to the sand load, with particular precision for the Baksan River draining Mt. Elbrus. Results clearly indicate that volcanic and basement rocks supply a disproportionate amount of sand relatively to their outcrop area, testifying to strongly focused erosion in the high-relief axial part of the range. River channels here display extreme steepness (ksn up to 200–300) and low concavity indices (θ 0.15 - 0.35) typical of thalwegs shaped by debris flows. Average erosion rates reach as high as 2–4 mm/a, which is explained by the frequency of catastrophic mass-transport events able to mobilize in one single episode between 106 and 107 tons of detritus, corresponding to 10–50 mm of erosion distributed across the catchment upstream. On 20 September 2002, the Kolka “glacier disaster” mobilized 2 × 108 tons of debris, corresponding to ≥ 400 mm of erosion in the upper Genaldon valley. To quantify the intrinsic capacity of different bedrock types to generate sand isolated from such a strong focused-erosion effect, we used the normalized steepness index ksn as a proxy of erosion rate and thus redefined the sand generation index as SGI*= [Formula presented] [Formula presented] [Formula presented]. More realistic values are thus obtained, although far more comprehensive data on sediment load than those presently available are required to discriminate robustly between the effect of focused-erosion versus lithological control on sand generation.