A megalandslide in the Northern Caucasus foredeep (Uspenskoye, Russia): Geomorphology, possible mechanism and age constraints

An extremely large landslide, ~ 2.8 km3 in volume and 32 km2 in area, has been detected on the right bank of the Kuban River in the Northern Caucasus foredeep (Russian Federation). The megalandslide has a very gently inclined slip surface (< 5°) and originated in a low-gradient landscape formed by weak, nearly horizontally inclined Miocene limestones overlying clay-rich strata. It is one of the largest documented slope failures in the world originating in low-gradient settings outside of high-relief mountain regions. Field analysis of 22 outcrops within the landslide body and interpretations of a 3″ SRTM digital elevation model and satellite images indicate at least two stages of emplacement. It started as a catastrophic rotational blockslide and turned into an earthflow in the distal part of the landslide. The secondary phase is attributed to a steepening of the landslide toe due to lateral erosion of the Kuban River. As a consequence, multiple rotational collapses of the distal part of the megalandslide occurred. OSL and AMS radiocarbon dating of deformed soil and loess overlying the landslide body suggest possible formation of the megalandslide between ~ 13 and 35 ka BP. Although the trigger of the megalandslide is difficult to establish, we hypothesise that during the slightly moister interval of the Last Glacial, it may have been triggered by an earthquake associated with the rupture of the Late Quaternary Kazminskiy Fault nearby. Fragmented and weakened material from the megalandslide has recently been subjected to intensive denudation by numerous shallow landslides, gully erosion and subsequent aggradation of material on valley floors.