--> Cenozoic Evolution of the Magallanes-Austral Basin and Patagonian Fold-Thrust Belt: A Tale of Inheritance and Sediment Recycling

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Cenozoic Evolution of the Magallanes-Austral Basin and Patagonian Fold-Thrust Belt: A Tale of Inheritance and Sediment Recycling

Abstract

The Cenozoic Magallanes-Austral basin chronicles Andean orogenesis and foreland sedimentation during major plate reorganization surrounding southern South America. Key events such as episodes of Andean thrusting, middle Cenozoic opening of Drake Passage, and the subduction of oceanic spreading centers beneath Patagonia affected foreland basin development atop previously thinned lithosphere. The resulting stratigraphy is unique along the Andean foreland and lends insight into how narrow orogens evolve along complex plate boundaries. We present a synthesis of published and new sedimentological, geo-thermochronological, and structural data from the Cenozoic Magallanes Basin (50-52°S). The Upper Cretaceous retroarc depocenter was uplifted during Cenozoic deformation and eastward shift of foredeep subsidence. Kinematic reconstructions show a change from Late Cretaceous-Paleogene thin-skinned deformation to early Miocene reactivation of reverse faults. Basinwide disconformities and the paucity of Paleocene strata suggest that the Paleogene foreland was characterized by laterally discontinuous subaerial exposure, localized subsidence, and deposition of sediment derived from the widening thrust-belt. Published and new detrital thermochronology and vitrinite reflectance data from Maastrichtian-Eocene strata suggest Eocene basin heating interpreted as the result of high heat flow, burial heating by sediment overburden, and stratigraphically controlled flow of geothermal fluids. Deposition resumed in the middle Eocene with deltaic and marginal marine sedimentation within an embayed foreland depocenter. The Oligocene and Miocene basin received fluvial and shallow marine sediments from the arc and recycled clastic sources in the west. Detrital zircon U-Pb geochronology suggests a late Oligocene onset of fluvial deposition – younger than previously considered and compatible with deep (∼5 km) thrust-belt exhumation and tectonically driven coarse-grained sedimentation. Neogene provenance signals show a reduction in Upper Jurassic sources and a larger component of recycled detritus from the Cretaceous depocenter, suggesting isolation of Jurassic thrust sheets and eastward shift of the drainage divide. Major foreland sedimentation ceased during subduction of the Chile Ridge and regional foreland uplift at ca. 18 Ma. These datasets highlight tectonic inheritance and evolving plate boundaries as major controls on basin history during phases of Andean tectonic activity.