Abstract
The Mesón Alto chaotic deposit, located in the Main Cordillera at about 33°40’S, is an important landform with a volume of ca. 4.5 km3 unconsolidated material deposited downstream of the Yeso Dam in the Yeso Valley, Río 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 Mesón 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 Mesón 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 Mesón 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 Mesón 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.
Highlights
The Mesón Alto sedimentary deposit is a ca. 4.5 km3 deposit located in the Yeso Valley (33.6oS, 70.1oW, ca. 2,500 m a.s.l.), immediately downstream of the Yeso water reservoir (Fig. 1)
The Mesón Alto chaotic deposit, located in the Main Cordillera at about 33°40’S, is an important landform with a volume of ca. 4.5 km3 unconsolidated material deposited downstream of the Yeso Dam in the Yeso Valley, Río 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 Mesón Alto Massif
According to these results and field observations, the Mesón Alto deposit seems to have been generated on the north-western slope of the Cerro Mesón Alto Massif (5,257 m a.s.l.), where the Cerro Mesón Alto monzogranite intrusive and andesitic rocks of Abanico
Summary
The Mesón Alto sedimentary deposit is a ca. 4.5 km deposit located in the Yeso Valley (33.6oS, 70.1oW, ca. 2,500 m a.s.l.), immediately downstream of the Yeso water reservoir (Fig. 1). It was originally assigned to glacial moraines formed by glaciers coming from the NNW (Marangunic and Thiele, 1971; Thiele, 1980), but later identified as a large rock slide and rock avalanche (Abele, 1984; Moreno et al, 1991) generated in the Cerro Mesón Alto andesitic and granodioritic massif (5,257 m a.s.l.), located immediately to the east of the deposit New insights on the origin of the Mesón Alto deposit, Yeso Valley, central Chile. Antinao and Gosse (2009) identified the deposit as a megalandslide and dated it at 4.5-4.7 ka using the cosmogenic radionuclide 10Be isotope, in a single point of the deposit, which indicates that at least part of it is definitively post-glacial They suggested as a possible controlling factor for the slope instability the Laguna Negra Fault that crosses the area (Fig. 1).
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