Polyphase structural evolution in the northeastern segment of the North Patagonian Massif (southern Argentina)

Abstract

Structural analyses in the northeastern segment of the North Patagonian Massif (southern Argentina) show that the simply deformed and metamorphosed phyllitic succession of the Late Precambrian–Cambrian El Jagüelito Formation has been intruded by Ordovician granitoids that are not ductilely deformed. The unconformable cover of the Silurian–Lower Devonian Sierra Grande Formation suggests that the Early Paleozoic Famatinian deformation of western Argentina did not affect this sector of the North Patagonian Massif. The ∼NW–SE compression of this succession led to the formation of open fold structures combined with high-angle reverse and sinistral strike–slip faults. Deformation interfered with the cooling of the Laguna Medina granitoids and is assigned here to the Late Paleozoic interval (probably Permian). A comparable mechanism is assumed for the metamorphism in the Sierras Australes fold-and-thrust belt north of Patagonia. The ∼NE–SW compression in the area west of Mina Gonzalito led to the formation of mylonites in the Peñas Blancas and La Laguna granites. It is suggested that ductile deformation is Permian in age and took place along important shear horizons. On a regional scale, it is comparable to that of the Cerro de Los Viejos granite (La Pampa Province) and the Sierras Australes fold-and-thrust belt (Buenos Aires Province) north of the inferred suture between Patagonia and Gondwana South America. This suggests that, on both sides of the boundary, intense compression took place during the same Gondwanide period and that extra-Andean Patagonia collided with Gondwana South America. The deformation in the Sierra Grande area is interpreted as a second-stage event during the Gondwanide deformational and magmatic history.