LAUSR

Abstract Superposed structural fabrics in the easternmost Fuegian Andes reveal two distinct, non-coaxial deformation phases across the transition from the orogenic core to the thrust-fold belt. Each phase is characterized by different metamorphic conditions and consistently different orientations, which allow the structural correlation between the orogenic core and the internal thrust-fold belt. The first deformation phase was coeval with regional metamorphism reaching upper greenschist facies, and featured simple shear deformation of the basement (Paleozoic and Jurassic) and cover (Cretaceous) in the top of the underthrusting South American plate. The second phase developed during collision of the orogenic wedge with the Rio Chico Arch, a promontory in the underthrusting plate; this phase was characterized by thrust sheet emplacement and formation of a crustal duplex, with rock uplift and consequent low to very-low grade metamorphism. Buttressing against the Rio Chico Arch is responsible for the change in shortening orientation that distinguishes both phases.