Abstract

The Andes Cordillera of central Chile is one of the most active mountain ranges on the Earth, where tectonics and volcanism strongly imprint its Quaternary landscape evolution, and superficial processes, such as catastrophic mass movements, modulate it. This article addresses the genesis and flow mechanisms of a massive rock avalanche deposit in the Yeso River catchment within the Maipo River Andean watershed in front of Santiago, through geomorphological and geological analyses. The deposit has the characteristics of a huge rock avalanche (> 10(8) m(3)) with the Mount Marmolejo, an ancient volcano reaching up to 6100 m a.s.l., as a source area, from which horizontal and vertical travel distances of 20.5 km and 3475 m, respectively, can be estimated until the maximum extent of the deposit recognisable in the field. Our results suggest that the flow that generated this deposit responded more fluidly than what is hypothesised for a rock avalanche. We propose that the avalanche incorporated part of the glaciers of the Mount Marmolejo that collapsed during this event leading to an avalanche flow. This occurred after 2447 +/- 95 cal. years BP according to radiocarbon results from peat buried by the avalanche deposit. It may have been triggered by shallow seismicity nucleated along active faults present in the study area, even though the rock avalanche may have also been favoured by hydrothermal alteration and tectonised rocks, as well as by steep morphologies caused by glacial erosion affecting the Mount Marmolejo, in the context of a warming regional climate during the late Holocene.