New structural data and P-T estimates of syn-deformational assemblages within the Beni Bousera peridotites and their crustal envelope are used to explain their Alpine exhumation. The Beni Bousera peridotites occur as thin sheets within high grade crustal units of the lower Sebtides (inner Rif, Morocco) and are composed of weakly deformed spinel lherzolite in the core of the massif and garnet-spinel mylonite at the rim. The main foliation trajectories in both the peridotites and overlying crustal units show systematic rotation towards their mutual contact, indicating a kilometer-scale top to the NW shearing with a dextral component along this crust/mantle contact. Widespread top to the NW shear criteria within the crustal units overlying the peridotite support this feature. Available ages constrain the development of the main foliation in both the peridotites and crustal rocks between 25 and 20 Ma. New P-T data from the peridotites show that deformation occurs during decompression from approximate to 22 kbar, 1050 degrees C to approximate to 9-15 kbar, 800 degrees C. As a consequence, exhumation of the Beni Bousera peridotites takes place during the Oligo-Miocene lithosphere thinning in the footwall of a lithospheric extensional shear zone. The exceptional preservation of garnet within the mylonitic peridotites results from rapid cooling of the border of the massif due to the juxtaposition with colder crustal rocks along this shear zone. Uplifting of the hot mantle rocks simultaneously induces high temperature metamorphism in the overlying crustal units. These new findings allow us to reconstruct the deformation history of the Beni Bousera region and the Alboran domain in the framework of the western Mediterranean geodynamics during the last 40 Myrs.. (C) 2011 Lavoisier SAS. All rights reserve

Le plutonisme bimodal des Jebilet centrales est daté à 330,5 ^+0,68 _−0,83 Ma par la méthode UPb sur zircons. Cet âge, identique à celui des plutons syntectoniques de granodiorite à cordiérite des Jebilet, date également la phase tectonométamorphique majeure locale. Le plutonisme syntectonique des Jebilet, constitué de suites tholéiitiques, alcalines et calco-alcalines peralumineuses, est très diversifié. La mise en place des magmas est favorisée par l'extension locale induite par le coulissage le long de la limite ouest des bassins carbonifères de la Meseta marocaine. Les Jebilet constituent un bon exemple d'activation de sources magmatiques multiples durant un épisode de convergence continentale et de déformation transcurrente.

Les plutons d'âge Jurassique moyen du Haut Atlas central, mis en place dans des rides anticlinales de direction N45°, sont caractérisés par des faciès pétrographiques très variés : roches basiques (troctolites), roches intermédiaires (diorites, monzodiorites), roches évoluées (syénites), et un faciès hétérogène issu du mélange entre les magmas évolués et les magmas intermédiaires. L'étude minéralogique et chimique montre le caractère transitionnel de cette série magmatique Jurassique et l'intervention d'un contaminant continental à un stade tardif de la cristallisation fractionnée.

Deciphering Variscan versus Alpine history in the Internal Rif system is a key to constrain the tectonic evolution of the Alboran domain and hence the geodynamics of the western Mediterranean system during the Cenozoic. This study focuses on the evolution of the metamorphic envelope of the Beni Bousera massif and its relation to the underlying peridotites. Combining structural geology, metamorphic petrology, and laser ablation inductively coupled plasma mass spectrometry U-Th-Pb dating of monazite, this study contributes to the understanding of the tectonic history of the western Internal Rif. The regional foliation (S2) is characterized by low pressure–high temperature (LP-HT) mineral assemblages and obliterates a former foliation (S1) developed along a Barrovian (medium pressure–medium temperature, MP-MT) metamorphic gradient. The dating of some metamorphic monazite grains from a micaschist and a migmatitic gneiss demonstrates that the crustal envelope of the peridotite recorded two distinct tectonometamorphic episodes. Data from monazite inclusions in S1 garnet suggest that the first event, D1, is older than 250–170 Ma and likely related to the Variscan collision, in agreement with the Barrovian type of the metamorphic gradient. The second event, D2, is Alpine in age (at circa 21 Ma) and corresponds to a strong lithosphere thinning allowing subsequent subcontinental mantle exhumation. Such a tectonic context provides an explanation for the LP-HT metamorphic gradient that is recorded in the regional foliation of the western Betic-Rif system. This extension is probably related to a subduction slab rollback in the western end of the Mediterranean realm during the Oligo-Miocene times. No evidence for a Tertiary high pressure/low temperature metamorphism has been identified in the studied area.