GEOLOGY OF THE BORBOLLÓN RIVER AREA IN THE NORTHERN MALARGÜE FOLD AND THRUST BELT, MAIN CORDILLERA, MENDOZA PROVINCE, ARGENTINA

The study area is located in west central Mendoza province, adjacent to the international border with Chile, between 34°15' and 34°30 LS and 69°45' and 70° LW. It is part of the northern end of the Malargüe fold and thrust belt, in the transition between the flat-slab and normal subduction segments, being a key area to understand the relationship between these two sectors of the Andes with different tectonic behaviours. The stratigraphy of the area corresponds to the Neuquen basin Middle Jurassic to Lower Cretaceous fill, showing an alternation of marine and continental episodes. During the Cenozoic, the area was heavily affected by Andean tectonism. As a result, a complex structure was developed, and the Mesozoic succession was intruded and covered by Tertiary volcanics. The main structures are a series of east-verging thrusts and associated folds with trends varying from N to NE. Most of these thrusts present a detachment level in the evaporites of the Auquilco Formation. A secondary detachment occurs within the black pelites of the Vaca Muerta Formation in some of the NE thrusts. A broad, kilometer-scale anticline with NNE trend located in the eastern part of the study area exposes in its core the oldest, pre-Auquilco units. The activity of the Andean volcanic arc in the region can be divided in two main stages. The first consisted in the pre-tectonic intrusion of dikes and sills of dacitic composition. The second stage involved the post-tectonic eruptions of the Listado-Bayo and Picos del Barroso volcanic centers.The structural style described for the study area is very similar to the one found to the north in the Andes at this longitude. This suggests that the development of the structure is approximately contemporaneous in both areas, and previous to the Miocene shallowing of the subducted slab north of 33° LS.