Oligo-Miocene exhumation of the Beni-Bousera peridotite through a lithosphere-scale extensional shear zone

Abstract

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

Metamorphic and magmatic overprint of garnet pyroxenites from the Beni Bousera massif (northern Morocco): Petrography, mineral chemistry and thermobarometry

“…In addition to the occurrence of graphite pseudomorphs after diamond, thermobarometry data are consistent with very-high to high pressure and temperature conditions (Afiri et al, 2011;Gysi et al, 2011;Kornprobst et al, 1990;Pearson et al, 1995). For example, Kornprobst et al (1990) demonstrated that the Cpx-Gt associations in corundum-bearing garnet clinopyroxenites indicate temperatures as high as 1200-1350°C at 2 GPa.…”

Wealth of P–T–t information in medium-high grade metapelites: Example from the Jubrique Unit of the Betic Cordillera, S Spain

“…Such zircon ages, although somewhat younger (see Esteban et al, 2011a), were also reported from an orthogneiss of the Alpujarride complex [Zeck and Whitehouse, 1999: 285 ± 5 (2σ) Ma]. In spite of these Hercynian ages, at least the exhumation of the Ronda peridotite, as part of the subcontinental lithospheric mantle (Hidas et al, 2013, and references therein; refers also to the Beni Bousera peridotite massif), seems to be unequivocally an Early Miocene event, which was very fast (for the Beni Bousera peridotite massif: Afiri et al, 2011) and accompanied by a drastic thinning of the orogenic crust (Platt et al, 2003;Zeck, 1996). This could also explain the extensional tectonics in the Jubrique unit and the condensed crustal section as reported, for instance, by Balanyá et al (1993).…”

Extreme extension across Seram and Ambon, eastern Indonesia: evidence for Banda slab rollback

“…Both arcs have closeto-180 • curvatures with similar radii, can be explained in terms of slab rollback, show evidence for continental subduction (Morales et al, 1999), and have peridotites exposed along part of their length. The Beni Bousera peridotite (Rif Mountains, Morocco) is particularly analogous to Seram owing to its association with mylonitised kinzigites (aluminous garnet-sillimanite granulites) which have also been interpreted to form the hanging wall above sheared peridotites exhumed beneath a low-angle detachment (Afiri et al, 2011). Furthermore, the El Hoyazo region of the Betic Cordillera is one of very few other localities aside from Ambon where ambonites have been erupted, which similarly contain xenoliths of their granulitic protolith (Zeck, 1970;Zeck and Williams, 2002;Acosta-Vigil et al, 2010;Álvarez-Valero and Waters, 2010).…”