Tectonic Control on large Rockslides and Significance for Landscape Evolution of the Southern Central Andes (32-34 °S)

An inventory of large (> 0.5 km3) rockslides has been developed for the Chilean sector of the southern central Andes (32-34° S) through field mapping, aerial photo interpretation and satellite image analysis along a swath 200 km long and 75 km wide that comprises the central and western Cordillera Principal. Previous work has indicated two main controls for the development of large landslides in this area: river incision and tectonic oversteepening. Evidence for the incision control is present in some of the high-discharge, actively incising trunk streams, but the relevance of fluvial erosion is minor in small catchments with low relief and low discharge. The lack of accurate mapping of the distribution of both large rockslides and regional faults has precluded a rationale for tectonic oversteepening. The landslides mapped in this study follow the same geographical pattern than fault zones recently mapped providing strng support for the tectonic control. Even though more geochronological data is needed to constrain the age of the landslides, relative chronology support the working hypothesis that there is continuum of reverse fault activity in the boundary Cordillera Principal / Central Depression (western boundary of inverted Oligo-Miocene Abanico Basin) from the Pliocene to at least Middle Pleistocene. In addition, a fringe of Late Pleistocene to Holocene landslides appear to be concentrated in the eastern boundary of the inverted Abanico Basin. Two major landslide-dammed lakes (Portillo and Laguna Negra) appear in the above mentioned area, indicating a probable Late Holocene age for the events responsible of the slides. This, in addition to recently published seismic data on shallow earthquakes (Barrientos et al 2004) and to the activation of the Cortaderal Slide by the Las Melosas earthquake in 1958, strongly support that the above mentoned area is a zone of active tectonics. Area-frequency distribution analysis (Malamud et al 2004) shows that between Upper Pliocene-Holocene times, areas in the western and eastern boundaries of the inverted Abanico basin might have local landscape lowering rates in the order of 1-10 mm/y due to increased sediment yield by large scale rockslides.