COUPLED DETRITAL ZIRCON U/PB-HE THERMOCHRONOLOGIC RECORD OF THRUST BELT UNROOFING IN THE MAGALLANES BASIN, PATAGONIAN ANDES

Coupled detrital zircon U/Pb geochronology and (U-Th)/He thermochronology of sandstones collected from Maastrichtian-Lower Miocene Magallanes Basin strata, southernmost Chile and Argentina, reveal significant changes in sediment provenance and the source unroofing history of the Patagonian fold-thrust belt near 51°30’S. The Magallanes Basin developed in the retroarc foreland position of the advancing thrust belt beginning in Late Cretaceous time. Kinematic reconstruction of deformation at this latitude recognizes a phase of Campanian-Oligocene shortening of the thrust belt, although in detail the timing and magnitude of this deformational phase is poorly resolved. Detrital zircon samples were collected from two sites along the eastern flanks of the Patagonian fold-thrust belt. The two stratigraphically lowest samples were deposited on a southward-prograding shelf in the axial foreland system of the Late Maastrichtian Dorotea Formation. These samples exhibit U/Pb age distributions with dominant Cretaceous grains (80-110 Ma) derived from the Southern Patagonian Batholith, a distinct Late Jurassic peak (145-155 Ma), and few Paleozoic and Precambrian grains. In contrast, samples from the Lower Oligocene-Miocene Rio Turbio and Santa Cruz Formations, which constitute sediments shed eastward from the thrust belt, exhibit drastically different distributions. These samples contain suppressed Cretaceous peaks, minor late Jurassic and Paleozoic grains, and large Paleogene (40-25 Ma) and Neogene (17-20 Ma) populations. The overall detrital populations reflect (1) a departure by Early Oligocene time from the dominant Cretaceous age peaks that persisted throughout the Upper Cretaceous basin record, (2) significant differences in provenance age and proportion in Early Oligocene–Miocene samples, suggesting along-strike variability in sources and uplift of the Cretaceous foredeep basin fill (3) persistence of sediment dispersal systems that connected the foreland basin to the magmatic arc. We combine provenance data with (U-Th)/He cooling ages from the same zircons to constrain the spatial variations in timing and rates of source exhumation and inferred thrust-related unroofing. Bedrock zircon (U-Th)/He data from the thrust belt indicate early Miocene unroofing of the sub-Andean thrust belt.