Abstract

The Meson Alto chaotic deposit, located in the Main Cordillera at about 33 degrees 40'S, is an important landform with a volume of ca. 4.5 km(3) unconsolidated material deposited downstream of the Yeso Dam in the Yeso Valley, Rio Maipo drainage basin. Historical work related this large deposit to a glacial origin whereas later on, it was assigned to a megalandslide that originated in the Cerro Meson Alto Massif. First results of integrated fieldwork along with petrographic and geochemical laboratory work on granitoid blocks from five different portions of the deposit, compared with the major outcropping intrusive units in the neighbourhood ( La Gloria Pluton, Cerro Meson Alto Massif and Cerro Aparejo Intrusion) point to a landslide origin of the surface blocks. The results suggest that granitoid fragments of the deposit most likely belong to the Cerro Meson Alto Massif, the proposed source of the rock avalanche. However, morphometric parameters and field observations support the idea of a rock avalanche deposited on top of glacial material. Therefore, the Meson Alto deposit should be assigned to a composite origin. Confirmation of a post-glacial, large volume rock avalanche in a strategic area for existent infrastructure for Santiago water supply and ongoing energy projects is fundamental for a correct hazard and risk assessment of the region.