Paleozoic deformation in the Sierras de Cordoba and Sierra de Las Minas, eastern Sierras Pampeanas, Argentina

Abstract

Basement orthogneisses, paragneisses, and migmatites in the Sierras de Córdoba of the eastern Sierras Pampeanas of central Argentina represent remnants of a Cambrian arc and accretionary prism that initially formed above a subduction zone along the early Cambrian margin of Gondwana. These basement rocks contain many high strain zones that record major events in the tectonic evolution of the western margin of Gondwana during the latest Proterozoic–middle Paleozoic. Initial orthogonal shortening and chevron folding of the accretionary prism rocks occurred prior to a high temperature/low pressure, relatively static metamorphism and migmatization event during 509–525 Ma, simultaneous with the Pampean orogeny. Localized deformation along a narrow zone of dextral transpression occurred late in the Pampean cycle. After peak metamorphism, the gneisses and migmatites deformed on east-dipping, greenschist-grade, mylonite-, ultramylonite-, and pseudotachylyte-bearing zones. Kinematic data from a selection of these shear zones include field data, microstructural shear sense indicators, and quartz C-axis fabric asymmetry. Almost all show west-directed, dip-slip reverse fault movement, but most do not achieve major crustal shortening. One major ultramylonite zone in western Sierras de Cordoba may represent a major tectonic boundary with the adjacent Sierra de San Luis rocks. This high strain zone is intruded by the Devonian-aged Achala batholith. Other age constraints include a pseudotachylyte vein that has been dated by 40Ar/ 39Ar method as mid-Silurian and Ordovician I-type plutonic rocks in the Sierra de Las Minas to the west that are deformed into greenschist-grade protomylonite and pseudotachylyte-bearing zones. Our data are consistent with late Ordovician to mid-Devonian orthogonal deformation throughout the Sierras de Córdoba, related to the emplacement of the Precordillera and Chilenia terranes to the west.