The origin of the San Jorge Gulf Basin in the context of the Mesozoic-Cenozoic evolution of Patagonia

Abstract The retroarc region of central Patagonia recorded three contractional stages (Late Triassic, Late Cretaceous, and Miocene) coincident with eastward broadening of arc magmatism. Inboard arc migration may be linked to shallowing of the subducted slab, subduction erosion of forearc regions, or a combination of both. Contractional episodes were followed by slab rollback, producing a series of extensional depocenters and magmatic belts across Patagonia at ~170-130 Ma, ~55-22 Ma and ~5 Ma. These rollback events weakened Patagonian crust through fracturing, mantle upwelling, and magmatic injection, favoring inception and propagation of subsequent contractional episodes. Phases of slab rollback and retroarc extension are reflected in eHf and eNd isotopic trends, with more juvenile trajectories corresponding to mantle upwelling into a broad asthenospheric wedge during slab retreat. Periods of crustal shortening are recorded by evolved eHf and eNd isotopic trajectories, demonstrating modified mantle sources. Mesozoic-Cenozoic extensional basins of Patagonia, such as the Chubut Basin, Canadon Asfalto Basin, Rio Mayo-Aysen Basin, Pilcaniyeu and Auca Pan-El Maiten magmatic belts/volcanogenic basins, and the Traiguen Basin were generated during episodes of slab rollback. In contrast, the extension-related San Jorge Gulf Basin constitutes an east-west-trending anomaly developed in an interval of the Cretaceous when the rest of western Patagonia experienced shortening. During this time, crustal evolution trends shifted from juvenile to more evolved and the arc expanded toward the continental interior most likely under a flat-slab regime. The San Jorge Gulf Basin is linked to Neocomian extensional structures in an intracratonic basin later influenced by compressional stresses linked to subduction zone in the west and incipient opening of the south Atlantic to east.