Abstract

FRACCHIA, D.  and  GIACOSA, R.. Structural evolution of the igneous-metamorphic basement in the Estancia Tres Hermanas, northeast of the Deseado region, Santa Cruz. Rev. Asoc. Geol. Argent. [online]. 2006, vol.61, n.1, pp.118-131. ISSN 0004-4822.

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.

Keywords : Patagonia; Comarca del Deseado; Paleozoic; Tectonics; Metamorfism.

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