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.

ABSTRACT Integration of geochronological analyses of in situ zircons with the petrological study of their microstructural growth microdomains (through petrography and phase diagram modeling) yields a more precise interpretation and understanding of any geological process than the study of ages from separate zircons. This is essential especially for those rocks affected by polymetamorphic histories. We present new in situ U-Pb zircon dating from metapelitic granulites exposed in the contact with the ultramafic massif of Beni Bousera (northern Morocco, Western Mediterranean). Geochronological data scatter from Paleoproterozoic (1508 ± 23 Ma) to Miocene (22.9 ± 0.7 Ma), though the majority of ages range between 100 and 400 Ma, with four main peak ages clustering around 390–319, 286–264, 193, and 105 Ma. The Middle Permian (ca. 286–264 Ma) and earliest Miocene (22.9 ± 0.7 Ma) ages are constrained from a thermobarometric point of view: The former ages represent the formation of the zircons at depth (higher pressure), and the latter ages represent the exhumation process (lower pressure). Combining these petrological results with previous geophysical and geodynamical studies from the literature in the area, we propose a four-step geodynamic model for the Beni Bousera orogenic peridotite since the end of the Paleozoic (i.e., Middle Permian) to the present, supporting an evolution linked to a subduction-related scenario since Late Cretaceous times.