TECTONIC EVOLUTION AND GEOLOGICAL HISTORY OF THE COLINA AND VOLCÁN RIVER VALLEYS IN THE HIGH ANDES OF SANTIAGO, CHILE

The eastern border of the Principal Cordillera in the Andes of Central Chile is characterized by a complex tectonics evolution, in which the development of the Aconcagua Fold and Thrust Belt is one of the main processes responsible for its modern configuration. This has motivated investigators, but despite the efforts, scientific knowledge has been insufficient to provide understanding of this structure and its role in the Andes. Between May 28 and April 7 2017, the course of Field Geology II of the University of Chile mapped this area at the scale of 1:50,000, with the objective to determine the tectonic evolution and geological history of the Colina and Volcán river valleys in the high Andes of Santiago. As a result, five lithostratigraphic units which correspond to La Engorda, El Cabrerío, Las Arenas, El Morado and Baños Morales which have been described in literature as Río Colina (González 1963), Río Damas (Klohn 1960), Lo Valdés (González 1963), Colimapu (Klohn 1960) and Abanico (Aguirre 1960) formations. These correspond to marine and continental sedimentary series. Quaternary units were defined, which are constituted by landslides, fluvial, alluvial and glacial deposits disposed in the valleys. Two different Quaternary volcanic units where described, corresponding to the recent volcanic products of San José Volcano, as well as a Plutonic unit. A cross section across the study area revealed the configuration of the Aconcagua Fold and Thrust Belt, characterized as an east-vergent thin-skin structure. Seven first order geological structures were defined; four of them correspond to east-vergent faults (Cerro Vega, Catedral, Arriero, and Nieves Negras faults), one of them corresponds to a west-vergent fault system (Sistema de Falla Puntiagudo) and two of them to west-vergent folds (Pliegue Andrade and Cerro Amarillo). Second order structures were also described corresponding to anticlines and inverse faults. It is proposed an east-vergent propagation of deformation with a minimal shortening estimated as 34 kms, equivalent to 53% of the initial configuration. These structures evidence a complex history of tectonic activity and can explain the modern configuration of the geological units defined in this work that together with exogenous erosive processes, modify the Earth's surface in this portion of the Andes.