Large landslides database along the Central Western Andes (15º - 20º S): constraints on mass-movement development and implications on relief evolution

The western flank of the central Andes presents an exceptional concentration of large paleo-landslides (v> 100*10⁶ m³), most of which being well-preserved morphologies due to low erosion and weathering related to the hyper-arid climate of the Atacama Desert since the Miocene. First order questions are pending about the triggering of those mass-movements, their dynamics, their locations and their roles on the Andean relief evolution. Previous studies included geomorphological analysis and few dating on individual landslides (e.g. in Peru: Margirier et al., 2015; Crosta et al., 2014; Zerathe et al., 2017; Delgado et al., 2020; e.g. in Chile: Strasser and Schlunegger , 2005; Pinto et al., 2008; Crosta et al., 2017). Preliminary regional mapping have been attempted in Peru (Geocatmin-INGEMMET and Audin & Bechir 2006) and in Chile (Matther et al., 2014 and Crosta et al., 2014).

Here we proposed a new and exhaustive mapping of large landslides of the Western Andes updating and homogenizing the previous works. The considered area locates between latitude 15° and 20°S, from the coast to the mean elevation of the Altiplano (~5000 m a.s.l). The landslide mapping was done by using Google Earth and DEMs (TanDEM-X and Pléiades). We mapped polygons (surface area > 0.1 km²) corresponding to destructured areas and strictly including the evidence of major landslide scarps (cliffs, unusual slope-breaks, etc.) and its sliding mass (offset lithology, boulders fields, etc.).

We identified more than 700 landslides, distributed into three main typologies: (1) deep-seated rockslide (DSR) showing “in-mass” displacement; (2) rock-avalanche (RA) with typical granular-flow morphologies (e.g. levees, boulders fields) and (3) destabilizations associated with both dynamics. This GIS database allows statistical analysis and interpretations crossing the landslide distribution and typologies versus relief properties, geology-lithology, long-term uplift, dating, etc. Preliminary analysis of this database shows that spatial distribution of mass-movements is not homogeneous. Instead, we observed cluster of mass-movements following the main valleys or canyons. They mainly located at elevation between 1500 and 2000 m a.s.l. Interestingly, the largest landslides (surface area > 50 km²) are disconnected to fluvial incision. They occurred within interfluve areas. Few of the largest landslides cover alone more than 30 % of the total cumulated landslide area in this region and, on their own, might contribute at a first order to the relief erosion.