IMPORTANCE OF PREDECESSOR BASIN HISTORY ON PROVENANCE OF THE MAGALLANES RETROARC FORELAND BASIN, CHILE (50°S-52°S)

An integrated provenance analysis of the Upper Cretaceous Magallanes retroarc foreland basin of southern Chile (50°30’-52°S) provides new constraints on source area evolution, regional patterns of sediment dispersal, and depositional age. Over 450 new single-grain detrital-zircon U/Pb ages, which are integrated with sandstone petrographic and mudstone geochemical data, provide a comprehensive detrital record of the northern Magallanes foreland basin-filling succession (>4,000 m thick). Prominent peaks in detrital-zircon age distribution among the Punta Barrosa, Cerro Toro, Tres Pasos, and Dorotea Formations indicate that the incorporation and exhumation of Upper Jurassic rift-related igneous rocks (ca. 147-155 Ma) into the Andean fold-thrust belt was established in the Santonian (ca. 85 Ma) and was a significant source of detritus to the basin by the Maastrichtian (ca. 70 Ma). This exhumation record and inferred deformation of the Jurassic Patagonian thrust sheets improves age constraints of the less well known Cretaceous thrust belt history at this latitude. Sandstone compositional trends indicate an increase in volcanic and volcaniclastic grains upward through the basin fill corroborating the interpretation of an unroofing sequence. Detrital-zircon ages indicate that the Magallanes foredeep received young arc-derived detritus throughout its ca. 20 m.y. filling history, constraining the timing of basin-filling phases previously based only on biostratigraphy. Additionally, spatial patterns of detrital-zircon ages in the Tres Pasos and Dorotea Formations support interpretations that they are genetically linked depositional systems, thus demonstrating the utility of provenance indicators for evaluating stratigraphic relationships of diachronous lithostratigraphic units. This integrated provenance dataset highlights how the sedimentary fill of the Magallanes basin is unique among other retroarc foreland basins and from the well-studied Andean foreland basins further north, which is attributed to the nature of the predecessor rift and backarc basin.