LARGE MASSIVE ROCK SLOPE FAILURES IN THE ARGENTINE AND CHILEAN ANDES BETWEEN 36 AND 38° S

In the deeply incised valleys of the Austral Central Andes 49 large massive rock slope failures with volumes of a few million to several billion cubicmeters were identified. About 85% of them are of the rock avalanche type, the others are related to rock-block slide or rotational slides. All slope failures occurred in Pliocene to Holocene stratified volcanic rocks of basaltic to andesitic composition, although other lithologies are also present in this part of the Andes. The break-aways formed predominantly in sequences of nearly horizontal attitude. However, frequent vertical joints are anisotropies which reduce rock strength. When linked to folded or faulted sequences, landslides are commonly associated with the horizontal panels of the structures. At those places where landslides occurred in inclined strata the dip is always towards the valley. Only two rock avalanches occurred in Pliocene to Quaternary volcanic edifices. In general, slope failures originated along the slope crests high above the valley bottoms. At least 90% of the landslide deposits dammed the valleys, in three cases reorganizing the drainage system. At present, 23 landslide dammed lakes exist. In most cases, landslide deposits obstructed valleys perpendicular to the strike of the valley. However, there are frequent examples of small lagoons within the hummocky morphology of the deposits or between the proximal rim of the deposit and the break-away zone. Rock avalanche deposits in narrow valleys occur as hummocky deposits projected far away from the break-away zone. In those cases where rock avalanche movement was not restricted by relief, deposits have a flat pancake-like morphology without any pronounced rims. As far as studied in detail slope failures are related to Neogene to Quaternary fault systems with dextral transpressive to transtensive dynamics. These faults constitute the northernmost known prolongation of the Liquiñe-Ofqui fault system and comprise various pull-apart basins. Several of the landslides directly form part of the filling of the transtensional depocenters. Due to the flat-lying nature of the collapsed rocks of the valley walls, the position of the break-away high above the bottom of the valley, and the regional relation to the active faults it is interpreted that most landslides formed during strong paleoearthquakes.