Thrust-related metamorphism in Carboniferous slates of southern Patagonia (South America): the fate of forearc successions

Abstract

The Devonian to early Carboniferous western margin of Patagonia (South America) includes a NW-SE-trending magmatic arc associated with a palaeo NE-dipping subduction zone. Along the Andean region of southern Patagonia, the Eastern Andean Metamorphic Complex (EAMC) developed in a forearc position, consisting of a succession of very low- to low-grade metaturbidite-metabasic rocks emplaced from the Devonian to Carboniferous periods. There are significant uncertainties surrounding this metamorphic complex, mainly related to the tectonosedimentary setting of the basin and subsequent conditions of deformation and metamorphism, which hinder our understanding of the orogenic architecture. To reveal the link between tectonics and metamorphism, we conducted a structural analysis and sampled metapelites to measure illite crystallinity along a regional structural cross-section in the EAMC. Our analysis reveals broadly Lower to Upper Anchizonal metamorphism which is roughly synchronous with deformation along northward-verging thrusts. These findings support the development of a forearc hyperextended basin that was subsequently closed during the Gondwanide orogeny (late Paleozoic), a model that reconciles previous proposals suggesting passive margin vs. back-arc basin models. This closure led to the emplacement of suprasubduction zone ophiolites and turbidites over the continent through landward migration of brittle-ductile reverse shear zones.