From Adria- to Africa-driven orogenesis: Evidence from porphyroblasts in the Betic Cordillera, Spain

In order to unravel the kinematic evolution of the Betic–Rif orogen, we investigated the geometry of porphyroblast inclusion trails in the Nevado-Filabride Complex, a high-pressure unit situated at the base of the Betic nappe stack. A large number of quantitative orientation data for these microstructures, and their mutual overprinting relationships, reveal two age groups of inclusion trails in the study area with distinctive geographic trends. Coupled to a comprehensive compilation of field data in the study area, these microstructures are shown to coherently witness the development of early NE–SW trending structures and fabrics, followed by younger NW–SE trending ones, whereby the former experienced clockwise reorientations. A major change in the direction of bulk crustal shortening from NW–SE to NE–SW is inferred from these data, and tentatively linked to the plate-tectonic history of the Mediterranean region. Paleomagnetic data indicate that, prior to about 35 Ma, NW–SE convergence between Adria, Eurasia and Iberia produced orogenesis along a belt bordering “Greater Iberia” (including Sardinian and Corsica) and probably continuous in the Alps (Briançonnais domain). The roughly NE–SW trend of this precursor appears to be preserved within porphyroblasts that grew during this period, presumably due to the partitioning of deformation around them. Africa–Iberia relative motion had been mainly transcurrent until 35 Ma (Azores transform), but then rapidly changed to head-on NNE–SSW convergence. This gave rise to a second set of NW–SE trending structures, which accompanied deformation of the precursor orogen into several oroclines (Western Alps, Gibraltar and Calabria Arcs). Orogenic collapse and crustal extension initiated shortly after this transition in tectonic regime suggesting a causal relationship. We propose that the waning of compressive stresses transmitted by Adria and their substitution by a near-perpendicular maximum stress direction resulting from Africa–Eurasia convergence could have enhanced the sinking of a thick lithospheric root or roll-back of a subducted slab, which in turn would have provoked crustal extension and basin formation.