ARCHITECTURE OF CRUSTAL-SCALE DUCTILE SHEAR ZONE SYSTEMS IN A HOT OROGEN: THE ORDOVICIAN FAMATINIAN OROGEN, NW ARGENTINA

Extensive Ordovician magmatism and high-T metamorphism occurred during the Famatinian orogeny as result of subduction along the continental margin of Gondwana. In NW Argentina, throughout the Famatinian arc, many ductile shear zones developed at deep- to upper-crustal levels. A synthesis of the main features of more than 40 crustal-scale ductile shear zones provides a summary of the geometry, kinematics, timing and conditions of the shear zone systems in the orogen. Most of the shear zones (~ 75%) have minimum widths > 0.5 km, rarely reaching widths of 25 km, and ~ 90% of the shear zones exhibit lengths higher than 5 km (~ 65% between 10 and 40 km). West-vergent shear zones show a predominant N/NNW strike (C planes) and typically moderate to low east dips in the fore and back arcs. Steep doubly-vergent ductile shear zones developed in the granitic-dominated core of the orogen. Locally shears located east of this plutonic core are folded. We interpret the steepening and folding of shears in the core of the orogen to reflect the replacement of ~80% of host rock by N-striking, steeply dipping plutonic sheets resulting in formation of a strong anisotropy and mass balance driven vertical crustal thickening. Analyses of stretching lineations and kinematic indicators indicate asymmetric, west-vergent displacements for for-arc and back-arc shear zones and greater symmetric crustal shortening and thickening in the arc core. Recrystallization of quartz and micas occur in all shear zones and commonly plagioclase and, when present, amphibole suggesting deformation temperatures typically around 450 than 550°C. Recrystallization of K-feldspar and rarely garnet indicate higher T’s in deep and/or shears in the core of the orogen. Late lower T, brittle slip overprinting ductile shear is common. Shear zones initiated during magmatism and partial melting (495-450 Ma) continued well after magmatism shut down (Silurian) and are discordantly intruded by Upper Devonian-Lower Carboniferous plutons (385-330 Ma). This dramatic orogen-scale shear system resulted from asymmetrical deep to shallow crustal shortening and crustal thickening along a convergent margin during and after magmatism and crustal melting in the Famatinian orogeny.