Investigation of prehistoric landslide dams in the central Andes of Chile and Argentina

Large volume landslides are common features in the high-topography Andes of central Chile and Argentina (31°-35°S). Many of these landslide deposits, mostly dating from Late Pleistocene to Holocene and related to a combination of deglaciation effects, seismicity and climate pattern changes, were large enough to block the glacial valleys and form landslide dams, some of which remain holding lakes until today. In this paper, we review and investigate some conspicuous landslide dams deposits from prehistoric times in the Andes Main Range in Chile and Argentina and the Andes Frontal Range in Argentina. A bibliographic review is followed by remote sensing analysis to obtain the main morphometric parameters of the landslides and the dams, complemented with field investigations in some of them. The landslide dams related with the rock slope failures of Portillo (1.1 km³, ~13 ka), Salto del Soldado (0.2 km³, 14- 9 Ka) Mesón Alto (4.3 km³, 4.7 Ka), Cortaderas-San Nicolás (2.4 km³, Holocene) and the Pangal complex (0.3 km³ , 40-11 ka) in the Chilean Western slope and the Laguna Blanca (81 hm³, ~12 ka), Horcones (0.35 km³, ~11-8 Ka), Negro (~18 Ka), Amarillo (~8Ka), and Laguna Atuel (56 hm³,~3 Ka) landslides in the Argentinean Eastern slope are described. Eldest rock avalanches associated with a seismic triggering mechanism that generate dammed lakes were previously studied in the Central Argentinean Andes such as Tigre Dormido (1.7 km³, ~46 Ka), Placetas Amarillas (1.6 km³, ~150 ka), and Piedras Blancas (0.89 km³, ~130 Ka). In these cases, the fine lake sequences reach 30 m in thickness but no outburst terraces were identified downstream, hardly supporting their catastrophic drainage.

In both Chile and Argentina, most preserved damming collapses correspond in general to huge rock avalanches, even though some slides and debris flows also blocked narrow valleys. In general, studied ancient dams are isolated in mountain remote areas but they show evidences of catastrophic ruptures that emphases their importance in the cascade hazard scenarios. While the large majority of identified dams in the region are thousands of years old, some historic cases show that these processes still occur and pose a geological hazard. Given the large area that can be affected by outburst floods downstream the usually remote landslide sites, potentially impacting villages as well as mining, energy, and transportation infrastructure, these geohazards must be considered in risk reduction strategies in the Andean region.