Refined Tectonic Evolution of the Betic-Rif Orogen Through Integrated 3-D Microstructural Analysis and Sm-Nd Dating of Garnet Porphyroblasts

Abstract

High-resolution microstructural analysis of porphyroblast inclusion trails integrated with Sm-Nd garnet geochronology has provided new insight into the tectonic history of the Betic-Rif orogen. Three principal age groups of porphyroblasts are demonstrated with distinctly oriented inclusion-trails. Inclusion-trail curvature axes or “FIA” (Foliation Inflexion/Intersection Axes) are shown to represent “fossilized” crenulation axes from which a succession of different crustal shortening directions can be deduced. The regional consistency of microstructural orientations and their geometric relationship with multiple sets of macroscopic folds reveal the composite character of the Gibraltar Arc formed by a superposition of different folding directions and associated lineations. Bulk-garnet ages of 35–22 Ma obtained from five micaschist samples of the Alpujarride-Sebtide complex (ASC) and of 35–13 Ma from four micaschists of the Nevado-Filabride complex (NFC) allow to deduce NNE-SSW directed shortening in the Late Eocene changing to NW-SE shortening in the early Oligocene, alternating with suborthogonal NE-SW shortening during the Miocene. These directions can be related to a major swing in the direction of relative Africa-Iberia plate-motion known from kinematic modeling of magnetic seafloor anomalies, and subsequent dynamic interference between plate convergence and suborthogonal “tectonic escape” of the Alboran Domain. Coupled to previously established P-T-t paths, the new garnet ages support a common tectono-metamorphic evolution of the ASC and NFC as laterally equivalent orogenic domains until, in the Miocene, the second became re-buried under the first.