A mega-rock avalanche in a glacial valley of the Argentinean Central Andes (32_ S).

MOREIRAS S.M.

Delhi

Congreso; 36th IGC; 2020

The Plata rock avalanche was identified by relict deposits covering an approximately 64,000 square km area with an estimated volume of 32 millions of cubic meters. Debris material moved down slope 26 km descending topographically 4,400 meters in high. A low angle friction was estimated from these parameters (H/D~0.17). This monolithic deposit greenish-grayish in color presents more than 98% of grey sandstones, conglomerates and phyllites of the Carboniferous El Plata formation. Greater boulders reach 3 m in diameter, while mean diameter is 0.50 m. Matrix content varies from 20 to 40% corresponding to middle sand in grain size.Stratigraphically, the rock avalanche deposit is covering fine alluvial levels in different locations that have been dated 37.3 ±2.5 ka and 39.7±3.9 ka, respectively. Besides, chaotic deposit is overlaid by a sandy alluvial deposit 35.9±2.9 ka in age. Triggering mechanism could be related to neotectonic activity of the Carrera Fault system. Furthermore, the study area was geomorphologically modified after this huge event. A glacier completely disappeared from the headwater of this basin favoring the generation of rock glaciers.