The Iglesia basin in the southern Central Andes: A record of backarc extension before wedge-top deposition in a foreland basin

The Iglesia basin hosts a key sedimentary archive of the evolution of the southern Central Andes. This basin is classically interpreted as recording the transition from an outer- to an inner-wedge-top (piggyback basin) depozone in the early Bermejo foreland basin system. In this study, we reevaluate the tectonic evolution of the Iglesia basin based on the interpretation of 2D seismic reflection data, structural geology, zircon U-Pb geo-chronology, and provenance analysis. We report an extensional event that initiated the basin opening. Our results indicate normal faulting development and synextensional deposition from the latest Oligocene (?)-earliest Miocene to middle Miocene. Since the middle Miocene, extensional activity experienced a progressive decrease in changing to shortening and basin inversion between the middle-late Miocene. The latter stage coincided with the full development of Pampean flat-slab subduction indicating maximum plate coupling at that time. Our findings challenge the idea of uninterrupted Paleogene to Neogene flexural subsidence related to the Andean orogenic-load in the backarc area. Therefore, the results of this study have important consequences for the early evolution of the Bermejo foreland basin and ultimately, for the growth of the southern Central Andes.