STRUCTURAL ANALYSIS OF A SHEAR ZONE IN TACUIL (PROVINCE OF SALTA), NW ARGENTINA

The Eastern Sierras Pampeanas of northwestern Argentina preserve N-S and NW-SE trending shear belts that locally affect a suite of predominantly Ordovician plutonic rocks and their metasedimentary Late Neoproterozoic to early Paleozoic basement host rocks. In Tacuil (province of Salta), a transect through one of the NW-SE shear zones displays markedly different styles of deformation, which are localized along discrete shear bands, mostly due to differences in lithology. This shear zone is generally characterized by the presence of mylonites, shear bands, and shear lenses. In undeformed areas, a circa 472 Ma porphyritic granite with abundant host-rock xenoliths is intruded by a series of pegmatitic and aplitic bodies of mostly felsic and intermediate compositions. This sequence of plutonic intrusions transitions into discrete greenschist-grade 5 to 20 meters thick proto- to ultramylonitic deformation bands. The presence of pegmatites and dikes controlled the localization of strain, which is shown as deformational structures along their borders. The shear zone foliation generally strikes NW-SE and dips to the SW or NE. Throughout the shear zone, down-dip SW or NE trending mineral lineations characterize the mylonites and ultramylonites. Sigma, and delta kinematic indicators, and S/C fabrics show a normal East over West movement. The mylonitic foliation and lineation have been affected by a younger episode of deformation characterized by millimetric-scale crenulations, and asymmetric kink and box-folds with predominant N-S and NNW-SSE trending axis that generally indicate tectonic transport to the west. These structures were subsequently overprinted by a system of N-S reverse faults, and more recently, by a set of NNW-SSE extensional fractures. We attribute these younger structures to effects of the Andean Orogeny. Based on correlation with shear zones in adjacent regions, the Tacuil mylonites are probably related to the Famatinian orogeny, a Middle to Late Ordovician deformational event that involved the accretion of major tectonic terranes to the western proto-Andean margin of Gondwana. Subsequent younger angular folds and brittle structures are very likely related to compression during the Upper Tertiary Andean orogeny that culminated with an extensional regime in this area.