PARTITIONING OF THE ANDEAN RETROARC FORELAND BASIN DURING CRETACEOUS-PALEOGENE FLAT-SLAB SUBDUCTION IN NORTHERN PATAGONIA, ARGENTINA: STRATIGRAPHIC ARCHITECTURE, PROVENANCE, AND SEDIMENT ROUTING

A Cretaceous-Paleogene clastic succession in the retroarc foreland of northern Patagonia (42-46°S, Argentina) recorded the inception of Andean mountain building and transition from contiguous to broken foreland basin conditions. Progressive shallowing of an east-dipping subducting slab is the favored mechanism of foreland partitioning, consistent with ~100-200 km of cratonward (eastward) migration of arc magmatism. Although previously reported growth strata help define basement-involved uplift, the precise structural timing and consequences for sediment routing and stratigraphic architecture remain poorly understood. In this study, a coupled geochronologic and provenance approach benefits from distinguishable sediment source regions and syndepositional arc volcanism. We present new results from detrital zircon U-Pb geochronology, sandstone petrography, and paleoflow analysis to demonstrate a distinct change in provenance from west-derived Andean sources to north- and east-derived basement sources at the transition to flat-slab subduction. Furthermore, the depositional systems of mid-Cretaceous to Paleocene strata (Chubut Group, Paso del Sapo Formation, and Lefipán Formation) within the broader north Patagonian foreland basin are poorly defined and incompletely described. To document the resultant stratigraphic architecture of flat-slab subduction regimes, we define six facies associations across a network of measured sections in the Paso del Sapo and Lefipán formations. These include tide-dominated prodelta and delta front, estuarine tidal bar, estuarine distributary channel, estuarine tidal flat, and shoreface environments. We report a new maximum depositional age for the basal Paso del Sapo Formation of 72.1 ± 0.8 Ma. We interpret distinct source age populations from detrital zircon U-Pb age spectra as Andean (~173-75 Ma), North Patagonian Massif (~530-185 Ma), and North Patagonian Precambrian basement (~2000-500) Ma.