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ABSTRACT

The western Alboran peridotites crop out across the Strait of Gibraltar (western end of the Mediterranean) and are the largest worldwide exposure of subcontinental lithospheric mantle. The present study focuses on the Cenozoic part of the long and complex metamorphic-deformation history of the western Alboran peridotites. During the Cenozoic, continental lithosphere thinning in a back-arc setting occurred and allowed the extensional exhumation of subcontinental mantle from 70–90 km depth to shallow crustal levels. Continental rift inversion at 20 Ma then triggered the final crustal emplacement of the western Alboran peridotites: the Sierra Bermeja, Alpujata, and Carratraca peridotites, which constitute the Ronda peridotites in the Betics, and the Ceuta and Beni Bousera peridotites in the Rif. A compilation of ductile shear indicators, recorded along the crust-mantle extensional shear zone during lithosphere thinning, is used here to reconstruct the three-dimensional geometry of the Oligocene–Miocene continental rift. The western Alboran peridotite bodies were back-rotated in their initial position at 20 Ma using (1) paleomagnetic data and (2) structural constraints for an ~100 km west/southwestward displacement of the Alboran Domain. A consistent NNE-SSW shear direction is found with locally opposite sense of shear. Two-dimensional numerical models of continental rifting indicate that such opposite shearing at the Moho identifies the initial position of the rift axis. On these bases, we propose an oblique rift system elongated N-S, with several NW-SE rift axes connected by NNE-SSW transform faults. The western Alboran peridotites correspond thus to different segments of this oblique rift system. These findings are then tentatively compared to the position of (1) present-day early Miocene depocenters, and (2) onshore faults, possibly reactivating transform and axis faults of the former rift.

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