Timing of Alpine Orogeny and Postorogenic Extension in the Alboran Domain, Inner Rif Chain, Morocco

The variety of temporal and structural constraints on the Alpine tectonometamorphic signature of the metamorphic core of the Betic-Rif orogen (Alboran Domain) has supported a long-lasting debate regarding the Alpine tectonic and geodynamic evolution of the Western Mediterranean region. Uncertainty still exists on the timing and tectonic significance of (a) the Alpine orogenic construction; (b) exhumation of the deep roots of the orogen; and (c) transition from orogenic shortening to crustal extension. In this study, we address these major geological issues by focusing on the lower-grade units of the Alboran Domain (Upper Sebtides and Ghomarides) exposed in the Rif belt of northern Morocco. Through a multidisciplinary approach that integrates mesostructural and microstructural investigations with X-ray diffraction, quantitative mineral chemistry, and 40Ar/39Ar geochronology, a 20 Ma long tectonic history is reconstructed, which involves burial of the tectonic units at depth (late Eocene) and postorogenic exhumation under brittle conditions in the upper crust (early Miocene). We document a Priabonian (∼37-34 Ma) D1/M1 progressive compressional deformation, during the formation of a SW-verging orogenic wedge (present coordinates), accreted toward the Africa plate. Brittle extensional detachment tectonics operated during the Burdigalian (∼18-17 Ma), controlling the thinning of the previously structured Alboran Domain nappe stack and the final exhumation of the Alpine orogenic units. We propose that transition from orogenic build-up to collapse in the hinterland of the Betic-Rif orogen occurred when the retreat of the Apennine-Maghrebian subduction was efficient to drive transition from shortening to extension in the back-arc domain of the western termination of the Apennine-Maghrebian subduction zone.