Rock avalanching into a landslide-dammed lake causing multiple dam failure in Las Conchas valley (NW Argentina) - evidence from surface exposure dating and stratigraphic analyses

Generally landslide dams which exist for several hundreds to thousands of years are considered as stable. We show with an example from the Argentine Andes that such dams can exist for several thousands of years but still may fail catastrophically. Multiple rock avalanches impounded two lakes with surface areas of similar to 8 km(2) and similar to 600 km(2), respectively, in Las Conchas valley, NW Argentina. Surface exposure dating (SED) by Be-10 of the rock-avalanche deposits or landslide scars indicates that these landslides occurred at 15,300 +/- 2,000 yr and 13,550 +/- 900 yr. The dams were stable during a strong earthquake, as suggested by seismites within related lake sediments and by multiple coeval landslides in this region, which occurred at similar to 7.5 kyr. However, when a further rock-avalanche fell into the lower, smaller lake at 4,800 +/- 500 yr the dam downriver was destroyed, presumably by the resulting tsunami wave. The resulting flood also destroyed an additional rock-fall dam which had formed at similar to 5,630 yr C-14 cal BP 30 km downriver. The new dam formed by the second rock avalanche was eroded prior to 3,630 yr C-14 cal BP. This dam erosion coincides with an important climatic shift towards more humid conditions in the Central Andes. Our results show that instead of direct effects of strong seismicity on landslide dams, (1) landsliding into a landslide-dammed lake, (2) abrupt hydrological changes, and (3) climate change towards conditions related to enhanced run-off are processes which can produce failures of "quasi-stable" natural dams.