The first structural phase of the Aconcagua folds and thrust belt: Importance of the early Miocene pehuenche phase

Abstract

The examination of the Alto Vacas section (High Cordillera of Mendoza) shows the importance of the primordial unconformity between the Miocene volcanic complex of Aconcagua and the frontal tectonic slices of the Principal Cordillera fold and thrust belt. This setting, widely observed in the region, points out that the main orogenic phase at this latitude is related to the Early Miocene pehuenche phase, and therefore is more important the later reactivation linked to the late Miocene quechua phase. The synorogenic deposits associated with this large tectonosedimentary event are linked with the fanglomerates of Penitentes type, which are neatly different from the volcanic agglomerates of Santa María type, which are formed by the subsequent dynamic eruptions clearly linked to the Aconcagua Volcanic Complex. Two key sections, one immediately to the north of Puente del Inca (El Barquito section), and the other in the headwaters of Los Indios creek (eastern flank of Cerrada cordon), show the unconformable relationship between the Santa María Agglomerates and the Penitentes Conglomerates, indicating a polyphasic deformation. Within a more regional framework, the tectogenetic importance of the Pehuenche phase is confirmed by recent geochemical evidence of retroarc crustal thickening observed in the Oligocene-Miocene boundary.