Structural evolution of the igneous-metamorphic basement in the Estancia Tres Hermanas, northeast of the Deseado region, Santa Cruz

Abstract

The structural evolution of Paleozoic basement in northeast of the Deseado region (Southern Patagonia) is described in terms of two deformation stages occurred in the pre-Early Permian. In Late Silurian intense ductile deformation (D₁) occurred along three progressive phases. During a first F_1a phase with inhomogeneous strain coeval with medium grade metamorphism and synkinematic magmatism, penetrative foliation, P_1a folds and boudinage developed. During F_1b phase with predominant plane strain conditions, NNW-striking NE-vergent P_1b folds affected former structures. Later on some more granitic dikes intruded P_1b folds. During F_1c phase, and yet in medium grade metamorphic conditions, narrow NNW-striking dextral-sense ductile shear zones developed. During D₂ stage, F_2a phase consisted in a wrench regime coeval with low grade metamorphism, which created a dextral-sense fragile-ductile shear zone that juxtaposed two blocks of rocks from different tectonic settings. Later, SW-vergent thrusts and associated NNW-striking P₂ folds were developed in F_2b phase, probably in association with the former wrench regime. D₂ structures would be involved in the process of exhumation of the basement prior to Early Permian. Despite of representing different deformation conditions, structures developed in stages D₁ and D₂ have similar geometric and kinematic characteristics, both indicating shortening in a NNE-SSW or NE-SW direction. This suggests geodynamic conditions controlling the evolution of the Paleozoic orogen in Deseado region of southern Patagonia were the same between Late Silurian and Early Permian times.