on the south face of Aconcagua mountain and in the Late Pleistocene to Early Holocene mass movements Cl terrestrial cosmogenic nuclide dating suggests 36

The morphology, sedimentology and mineralogy of deposits that previously had been associated with glacial advances (the Penitentes, Horcones and Almacenes drifts) were reinvestigated and dated using the terrestrial cosmogenic nuclide (TCN) Cl. These results indicate that the deposits previously associated with the Horcones and Almacenes drifts are actually deposits of a rock slope failure from the southern face of Aconcagua mountain forming a debris–ice avalanche that were deposited 10 490 + 1120 years ago, while the deposits previously associated with the Penitentes drift is a rock avalanche from the Mario Ardito valley that deposited in the Las Cuevas valley 11 220 + 2020 years ago. Earlier in the Late Pleistocene a further rock–ice avalanche sourced from Aconcagua mountain and deposited in the Las Cuevas valley, predating related lake sediments with a calibrated C age of 14 798–13 886 years and travertine deposits with a U-series age of 24 200 + 2000 years. In addition, three further rock-avalanche deposits were dated that sourced from Tolosa mountain, having Cl mean ages of 14 740 + 1950 years, 12 090 + 1550 years and 9030 + 1410 years. No deposits of massive rock slope failures were found in those parts of the valleys that date younger, suggesting that climatic conditions at the transition from the Late Pleistocene to the Holocene, that were different from today’s, caused the slopes to fail. Alternatively, the rock slope failures could have been seismically triggered. We suggest that the slope failures at the southern face of Aconcagua mountain have caused or contributed to a reorganization of glacial ice flow from Aconcagua mountain that might ultimately be the cause of the surging behaviour of the Horcones Inferior glacier today. Our results indicate that the glacial stratigraphy of this part of the Central Andes is still poorly understood and requires detailed mapping and dating. Supplementary material: Sample coordinates, sample porosity and density, Cl nuclide composition and geochemical composition are available at http://www.geolsoc.org.uk/SUP18753. The impact of climate warming on slope stability, mainly through thaw of alpine permafrost and debuttressing of glacially oversteepened, unstable rock slopes due to glacier retreat, has been the subject of much discussion (Abele 1974; Evans & Clague 1994; Noetzli et al. 2007; Huggel et al. 2010; Clague et al. 2012; Fischer et al. 2012). Except for the historic period, however, this causative relationship can only be demonstrated by dating large numbers of rock slope failures, although care has to be taken as both climate-related conditioning and seismic activity can result in multiple rock slope failures in a given region. In general multiple rock slope failures that occurred at the same time in the past are interpreted as indicators for palaeoseismic events, especially if additional independent indicators for such events exist (e.g. Adams 1981; Schuster et al. 1992; Moreiras 2006; Hermanns & Niedermann 2011). On the other hand, temporal clustering of rock slope failures coinciding or following climatic changes are commonly interpreted as an indication that stability conditions of rock slopes are primarily linked to climate change. Statistically representative examples have been published from Norway, the European Alps and Scotland (e.g. Prager et al. 2008; Soldati et al. 2004; Hermanns & Longva 2012; Ostermann & Sanders 2012; Ballantyne & Stone 2013, respectively). In the Andes the first systematic regional inventory of ages of rock slope failures was presented by Hermanns et al. (2000) for NW Argentina. Previously 25 of 55 mapped deposits had been dated; this dataset was extended by Hermanns & Schellenberger (2008) to 33 dated deposits. None of the mapped rock slope failures had a source on glaciated slopes, even though alpine glaciers reached down to 4300 m above sea-level (asl) in the easternmost From: Sepúlveda, S. A., Giambiagi, L. B., Moreiras, S. M., Pinto, L., Tunik, M., Hoke, G. D. & Farı́as, M. (eds) Geodynamic Processes in the Andes of Central Chile and Argentina. Geological Society, London, Special Publications, 399, http://dx.doi.org/10.1144/SP399.19 # The Geological Society of London 2014. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics by guest on May 25, 2014 http://sp.lyellcollection.org/ Downloaded from