Pleistocene rock avalanche, damming, and secondary debris flow along the Cotahuasi river, Peru

Abstract

Landslides are among the most frequent and dangerous mass removal processes around the globe. They can be triggered by different phenomena such as earthquakes, extraordinary rains, glacier outbursts, volcanic activity, among others. In this study, we reconstruct the origin and potential cause of a rock avalanche that occurred in the Cotahuasi canyon in southern Peru. The head-scarp is a cirque carved during the Last Glacial Maximum and is formed by Alpabamba ignimbrites and Upper Barroso Formation volcanics. Slope failure removed a minimum volume of 1.13 km³ generating a dry rock avalanche that was immediately confined into the Cachana valley, where it traveled 11 km downstream before reaching the Cotahuasi river. There, it did run for another 3 km towards the opposite Mungui mountains with minimum speeds of 72 m/s. The transversal Mungui range forced the avalanche to laterally spread upstream and downstream along the Cotahuasi river, as well as upstream the Pampamarca tributary prior to its final stop. The resulting deposit (H/L = 0.16) developed hummocky topography and formed a 10 km long and ≤100 m high dam that impounded two temporary lakes. At their maximum capacity, these lakes could hold ~1.6 × 108 m³ of water prior to overtopping the dam. The dam-breakout generated a catastrophic flood (massive layer forming terraces) that traveled for tens of kilometers and likely as far as the sea. The debris flow deposit could be well-documented up to a distance of 20 km from the breach with the presence of 4 to 50-m-thick terraces with a minimum volume of 0.72 km³. After this distance, the deposit disappears, likely due to post-emplacement erosion by the river itself. Considering the missing volume at the head-scarp cirque (~3.85 km³) and the sum of the minimum volumes of the rock avalanche and debris flow deposits (~1.85 km³), circa forty percent of the material must have been removed by glacial and fluviatile activity since the time of its original emplacement.