TIMING AND KINEMATICS OF INTRA-ARC DEFORMATION IN THE MIDDLE CRUST OF THE FAMATINA ARC

The Famatina arc of northwest Argentina is a Cambro-Ordovician continental arc formed along the western margin of Gondwana. Previous workers have generally considered the Famatina arc as a static arc that experienced little deformation. In the Sierra de Ramaditas, we recognized a middle-crustal shear zone developed within rocks of the Famatina arc. The region preserves metapelitic Kfs-Grt-Bt±Crd±Sil migmatite, intermediate-felsic orthogneiss, lesser amphibolite, and Grt-Scp-Cpx calc-silicates. Zircon U-Pb ages from migmatitic leucosomes and monazite U-Pb and garnet Lu-Hf ages from migmatitic melansomes date granulite facies metamorphism at 460-455 Ma.

These units are variably deformed within the Ramaditas shear zone, a ≥500 m wide, NNE-striking and steeply dipping mylonite zone. Moderately plunging mineral lineations are defined by stretched quartz and aligned sillimanite. Kinematic indicators in outcrop and thin section record dominantly dextral, east-side-up displacement across the shear zone. Zircon U-Pb ages from deformed tonalite suggest the thrust component of deformation may have begun by 449 Ma. Monazite U-Pb ages of 426 Ma imply continued deformation until that time and cross-cutting dike of 418 Ma brackets the final age of deformation.

The results suggest that some component of regional shortening was accommodated within the Famatina arc. Early shortening on the Ramaditas shear zone began after terrane accretion to the west and later oblique displacement may have overlapped with sinistral oblique convergence on lower-crustal shear zones within the Valle Fertil fault zone.