Early Miocene andesite conglomerates in the Sierra de Famatina, broken foreland region of western Argentina, and documentation of magmatic broadening in the south Central Andes

A thick synorogenic megasequence, known as the Del Crestón Formation, crops out in the Famatina belt in western Argentina. Its coarsening upward arrangement, rotational syntectonic unconformities, and provenance analysis allowed considering this unit as an this unit was an Andean-related foreland succession, although its age and geological meaning have been largely controversial. It unconformably overlies Late Paleozoic (Paganzo Group) red beds and is covered by the Middle–Late Miocene Angulos Group in a faulted contact. No previous geochronologic data constrain this important synorogenic cycle bracketed between the Triassic and the late Neogene. This work provides a set of new ages for andesite boulders recorded in the lowermost volcanogenic conglomerate of the Del Crestón Formation. The andesite boulders provide evidence of backarc volcanism in the external foreland during the Early Miocene (ca. 17 Ma). This volcanic pulse, together with other evidence for Early Miocene volcanism in the Argentine Precordillera, support a previously unrecognized stage of magmatic broadening in the south Central Andes associated with the initiation of shallow subduction and changes in the plate convergence pattern at about 18 Ma. Major progressive unconformities and unroofing history from conglomerates indicate deformation at a proximal foreland depozone and basement involvement (broken foreland stage). This deformation could have occurred coevally with the deposition of distal facies in the Bermejo basin system to the south, suggesting markedly nonuniform Andean foreland development.