STRUCTURAL EVOLUTION OF THE PUNCOVISCANA FORMATION IN SIERRA DE QUILMES, NORTHERN SIERRAS PAMPEANAS, NW ARGENTINA

A NNE-striking shear zone crops out along the eastern border of the Sierra de Quilmes, in NW Argentina. Preliminary work between the localities of Quilmes and Santa Maria shows that the shear zone is at least 7 km wide, and it can be traced along strike for approximately 30 km between the two localities. The shear zone developed in the Puncoviscana Fm. (PF), a Neoproterozoic to Early Cambrian meta-turbiditic sequence of pelitic schists and banded psammites that were chevron folded on the western Gondwana margin during the mid-Cambrian Pampean Orogeny. Younger pegmatites intruded the PF prior to the development of the shear zone. Metasediments and pegmatites were metamorphosed under amphibolite-facies conditions synchronous with the shear-zone deformation and have been variably mylonitized, from proto to ultramylonites.

The shear zone is characterized by a strong, mostly NE-striking foliation and a pervasive NE-trending mineral lineation defined by oriented fibrolite and biotite, elongated polymineralic aggregates, and pressure shadows associated with porphyroclasts. Shear strain decreases to the W and SW, as indicated by the preservation of the chevron folded PF in W Sierra de Quilmes. Kinematic indicators (asymmetric mantled porphyroclasts, folds, and S-C fabrics) suggest a general N over S sense of shear. Based on similarities in trend and kinematics to other Ordovician/Silurian shear zones in N Sierras Pampeanas, we interpret this shear zone as related to a late stage of the Famatinian Orogeny.

The mylonitic foliation is locally overprinted by a centimetric to milimetric-scale crenulation, with axes at a low angle or subparallel to the mineral lineation. The crenulation is folded by centimetric to metric-scale open folds with NNW-trending axes. These folds, locally associated with thrusts, are also present in the PF, to the W of the shear zone in Sierra de Quilmes. These young structures share similar tectonic style and are coaxial with Andean thrust-related folds and faults that developed in Cretaceous and Tertiary sediments to the E of the shear zone. Our observations suggest that rocks in Sierra de Quilmes span a record of multiple deformation events, from the lower Paleozoic Pampean Orogeny to the youngest Andean Orogeny, during which thick-skinned deformation has been predominant.