DEFORMATION OF THE PUNCOVISCANA FORMATION IN SIERRA NOGALITO, ARGENTINA

Sierra del Nogalito in northern Sierras Pampeanas, NW Argentina, preserves a series of banded phyllites that have been traditionally correlated, based on composition and regional trend, with rocks of the Tommotian-Vendian Puncoviscana Formation. The turbiditic sediments of the Puncoviscana basin were deposited during the Precambrian-Cambrian on the proto-Pacific western margin of Gondwana and chevron folded during the mid Cambrian Pampean Orogeny. In Sierra del Nogalito, we identify second order “S”, “M”, and “Z” folds that affect the compositionally banded phyllites and are likely related to first order chevron folds. Intrusion of quartz rich veins, subparallel and at high angle with respect to the compositional banding, postdate the chevron folding. Subsequent shear deformation overprinted both chevron folds and quartz veins resulting in the formation of a strong planar biotitic foliation. In the chevron limbs, shear planes are characteristically subparallel to the compositional banding and the hinges of the planes intersect the compositional banding at high angle. During shearing, quartz veins were transformed into kinematic indicators (sigma grains) and asymmetric folds that indicate a top to the east sense of shear. In this area, phyllites and the overlying Tertiary sediments were affected by compressional brittle deformation that resulted in fragmentation and tilting of basement blocks, reverse faults, thrusts and associated folds, and conjugate jointing. The orientation of these structures suggests an E-W direction of compression. Based on the deformation style and structural grain, we associate the chevron folds in Nogalito with the Pampean deformation decribed in anchizone and greenschist-grade Puncoviscana Formation to the north. Although younger brittle deformation obliterates shear structures modifying the original shear sense, we can relate them to other N-S trending Ordovician and Devonian shear zones that indicate a tectonic transport to the East in basement blocks of northern Sierras Pampeanas. These shear zones are very likely related to the accretion of terrains to the west of the research area during the lower Paleozoic (Famatinian Terrane, Precordillera). The youngest E-W compression indicated by brittle structures is probably related to the Andean deformation.