Age constraints and paleoseismic implication of rock avalanches in the northern Patagonian Andes, Argentina

Landslides and rock avalanches are far more common features than once believed in the glacial valleys of the northern Patagonian Andes of Argentina. We describe three rock avalanches located in northwestern Neuquen province that generated landslide-dammed lakes. Cosmogenic radionuclide 36 Cl dating constrains their average ages as Late Pleistocene for the Lagunas de Varvarco rock ava- lanches (79.0 ± 3.0-63.0 ± 3.0 ka and 37.0 ± 1.8-29.0 ± 1.5 ka) and Holocene (2.2 ± 0.6 ka) for the Barrancas avalanche. The landslide dam originated by the Barrancas rock avalanche became unstable on December 29, 1914, causing a devastating outburst flood. These phenomena indicate rock volumes greater than 5 · 10 5 m 3 each, and the Varvarco Campos rock avalanche (3.51 · 10 9 m 3 ) is the largest rock avalanche recognized in Argentina. The rock avalanches described here occurred after the glacial retreat. The volume of the land- slides and the matching of these gravitational phenomena with current seismicity data, as well as other morphometric, lithologic, and structural characteristics, suggest that these rock avalanches likely were triggered by earthquakes. On the basis of empirical relations, we postulate that the Varvarco Campos rock avalanche testifies to a paleoearthquake of magnitude (M > 8), much larger than the events recorded by instrumental seismicity (M 6 5.8). � 2007 Elsevier Ltd. All rights reserved.