Mid-crustal deformation in a continental margin orogen: structural evolution and timing of the Famatinian Orogeny, NW Argentina

Abstract

New structural data from a mid-crustal segment in the Eastern Sierras Pampeanas, coupled with geochronological methods and P–T estimates, reveal polyphase contractional deformation and metamorphism during the Famatinian Orogeny over a long period of time. Peaks of metamorphic monazite and zircon ages are recorded at c. 500 Ma, between 484 and 465 Ma and at c. 440 Ma. Between 484 and 465 Ma the region attained high-temperature (HT) low-pressure (LP) conditions that resulted in widespread partial melting (peak at c. 470 Ma). A contractional phase occurred during this event, as suggested by syn-anatectic structures, as well as folding at subsolidus conditions. Renewed contraction under subsolidus conditions is evidenced by reverse ductile shearing and folding at c. 440 Ma. Thrusting along the La Chilca Shear Zone caused metamorphic inversion. The consistent orientation of folds in the Quebrada del Molle Metamorphic Complex, El Portezuelo Metamorphic–Igneous Complex and the La Chilca Shear Zone as well as WSW-directed thrusting at the later shear zone indicate uniform WSW–ENE-directed shortening. This late deformation records the Ocloyic tectonic phase and brought the Famatinian Orogeny to an end in the Late Ordovician to Early Silurian.