Tectonic history of the Calcatapul Formation, Chubut province, Argentina, and the “Gastre fault system”

Abstract

Field mapping, structural analysis, and U–Pb dating of rocks in the western part of the North Patagonian Massif (Argentina) yield a revised interpretation of structural evolution in the area of the ‘Gastre fault system’ that previously has been interpreted as an important dextral lineament. The Calcatapul Formation predominantly consists of metapyroclastic rocks and does not represent mylonites with a dextral sense of shear. The succession can be assigned broadly to the Early Paleozoic (?Silurian–Devonian). The Yancamil granite, dated here as Permian, intruded the Calcatapul Formation. Both units were affected by a first deformation under ∼NE–SW compression, accompanied by a lower greenschist facies metamorphism. This Permian (–?Triassic) event took place prior to the intrusions of the Triassic Lipetrén Formation. The second deformation, with comparable compression, affected the Calcatapul Formation and Yancamil granite, as well as injected dykes of the Lipetrén Formation. Minor mylonite strips in the Lipetrén Formation intrusions display different orientations and senses of shear and do not fit with a large-scale dextral shear zone. NW–SE-striking fracture zones in this sector of the North Patagonian Massif can be interpreted also as the result of downfaulting instead of large-scale strike-slip fault lines. These observations from the Gastre area do not support the existence of the dextral Gastre fault system as a large-scale, intracontinental structural element that traverses extra-Andean Patagonia.