Abstract

A landslide cluster located in the Pangal river valley, Cachapoal river basin, in the Andes of central Chile is investigated. Deposits of five rock avalanche deposits of volumes varying from 1.5 to 150 million cubic metres are distinguished by detailed geomorphological mapping and dating. The landslides were originated in volcanic rocks affected by hydrothermal alteration. The largest three avalanche deposits, formerly considered as a single deposit, were dated using Terrestrial Cosmogenic Nuclide methods. The results show three distinct ages from Late Pleistocene to Holocene, confirming the hypothesis of separate events. The presence of clay minerals detected by X-ray diffraction in matrix samples confirm the presence of hydrothermal alteration in the landslide source areas detected by satellite image multispectral analyses, suggesting an important control of such alteration in the landslide generation. The present river morphology and outcrops of lacustrine deposits and outburst terraces evidence that the largest rock avalanches, with deposit thicknesses of up to about 100 m, have dammed the valley to be later eroded by the Pangal river. The repetition of large volume landslides in the valley and their potential of river damming should be considered as a hazard that may endanger critical infrastructure and human settlements downstream. This case study is an example of potential hazards of large slope collapses and related river damming in the Chilean Andes that should be addressed in more depth along with further investigations on the landslide controlling factors and failure mechanisms in the region.