Holocene giant collapses in the Aconcagua Park, Argentina.

MOREIRAS, S.M.

Foz de Iguazu

Encuentro; The Meeting of the Americas; 2010

Landslides are major processes in the Aconcagua Park; nonetheless, this kind of processes has been rarely described being underestimated in the management of this natural reserve. Findings obtained from geomorphological studies carried out in the framework of research projects (PIP 2006-5759, PME 2006-02703 and SECYT 2009-2011) reveal the importance of these processes such as intend to be integrated in the park zonation. More than 400 events were identified in the landslide inventory map covering approximately 20% of the park area according to spatial analyses. Important rockfall are associated to Jurassic-Cretaceous rocks with high dipping impacting directly on campaign sectors such as Plaza Francia (4200 m asl) and Pampa de Leñas (3200 m asl). Along the Horcones valley, debris flow occurrence predominates mainly associated to alluvial cones and source zone of rock glaciers. As well, debris flows are commonly generated at the toe of glaciers by intense thawing during warm seasons. As well, temporal dammed lakes impounded by landslides have been frequently reported along the Vacas river. Minor processes are frequent at present, but giant rotational slides moving extraordinary quantity of debris material occurred in the past. Based on stratigraphic relations with Pleistocene moraines, a Holocene age is established for these deposits. Besides, they are located above 3,200 m asl. without any evidence of being eroded by Pleistocene glacier masses. Holocene occurrence of rock avalanches has been previously reported. Blocks measured on the surface of the Horcones deposit resulted 11,110±760 years and 8,170±1220 years (Fauqué et al. 2008a). The last catastrophic event dammed the Cuevas River valley generating an impounded lake with radiocarbon age of 11 ka (Espizúa, 1993) and cosmogenic age of 14.8±14 ka (Fauqué et al. 2008a). Likewise, cosmogenic datings of rupture planes generated on the western hillslope of the Tolosa peak gave 14,650±1,900 and 11,380±1,500 years, respectively, which matches with ages measured on block surfaces of 15.5±2.1 ka and 13.9±2.1 ka (Rosas et al., 2008). Even though a seismic triggering mechanism is proposed for the dated rock avalanches, giant rotacional slides identified in the Vacas river seem to be more related to debuttressing effect combined with land isostatic rebounding after Pleistocene glacier mass retreatment. Maybe greater permafrost degradation and thawing during the Holocene forced slope instability in the Central Andes as well. Following this idea, Wilson et al. (2008) proposed climate as the main conditioning factor of catastrophic geological processes in Las Cuevas valley. Hence, this study advertises about the natural hazard of Aconcagua park with an increasing tourism demands denoting the necessity of its zonation. Geomorphological parameters reveal that 89% of these events are active with very likely future reactivations amazing park activities.