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 overalongperiodoftime.Peaksofmetamorphicmonaziteandzirconagesarerecordedatc.500 Ma,between484and465 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 evidencedbyreverseductileshearingandfoldingatc.440 Ma.ThrustingalongtheLaChilcaShearZonecausedmetamorphic 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-directedshortening.ThislatedeformationrecordstheOcloyictectonicphaseandbroughttheFamatinianOrogeny to an end in the Late Ordovician to Early Silurian.