THE TRANSITION FROM EXTENSIONAL RIFT BASIN TO COMPRESSIONAL RETRO-ARC FORELAND BASIN IN THE SOUTHERNMOST ANDES (54.5°S): NEW PROVENANCE DATA FROM BAHíA BROOKES AND SENO OTWAY

In the southernmost Andes, the Cretaceous inversion of a late Jurassic extensional basin called the Rocas Verdes created the Cretaceous-Neogene Magallanes retro-arc foreland basin between the South American craton and an active volcanic arc. To test previous tectonic models of rift basin inversion and its relationship to foreland sedimentation, we examined Lower and Upper Cretaceous sedimentary units that record the transition from back arc basin to retro-arc foreland basin in two previously unstudied parts of the basin, Bahía Brookes and Seno Otway. We characterize changes in source areas during this transition using provenance data, including detrital zircon age spectra, that provide constraints on the uplift and denudation of rock units in the internal part of the orogen. Petrography of pre and post inversion sediments, the Zapata and Punta Barrosa Formations, respectively, in both study areas suggest that these sediments are compositionally less mature than those previously studied in parts of the basin farther north. Two kilometers of measured section that are exposed in thrust slices in each locality reveal sedimentary structures and compositions that support a submarine fan depositional setting for both units. We observed a unique conglomeratic composition at the transition in Bahía Brookes, whereas a medium-grained thick-bedded sandstone marks the transition in Seno Otway. Diverse paleocurrent data support a model of an elongate back arc submarine fan. Preliminary detrital zircon analyses within the transition zone yield ages of ~88 Ma to ~82 Ma, which are younger than previously reported ages for these units. Our analysis of detrital modes from both units indicates a provenance of a proximal volcanic arc and sedimentary sources, which contrast with the abundant metamorphic lithics reported for these units farther north. These data support an interpretation whereby the southernmost Andean orogen began as a submarine thrust wedge that mostly deformed rocks infilling the Rocas Verdes rift basin behind an active late Cretaceous arc, and that significant uplift and erosion of pre-rift basement schists and Upper Jurassic volcanic rock did not occur until after ~82 Ma.