Abstract
The timing and process of exhumation of the subcontinental peridotites of the Gibraltar Arc (Ronda, Beni Bousera) have been discussed extensively over the last decades. In this work, we contribute to this debate through the first mapping, structural and petrological analyses, and SHRIMP U-Th-Pb dating of high-grade marbles that crop out around the Beni Bousera antiform of the Alpujarrides-Sebtides units of northern Rif (Morocco). These marbles, here termed the Beni Bousera marbles (BBMs), instead of being intercalations in the granulitic envelope (kinzigites) of the Beni Bousera peridotites, as previously described, form minor, dismembered units within a ∼30 to 300 m thick mylonitic contact between the kinzigites and the overlying gneisses of the Filali Unit (Filali–Beni Bousera Shear Zone, FBBSZ). They display silicate-rich dolomitic marbles, sandy-conglomeratic calcareous marbles and thinly bedded marble with interleaved biotite-rich schists. An unconformable contact, either of stratigraphic or tectonic origin, with the underlying kinzigites, is observed locally. Pebbles or detrital grains include K-feldspar, quartz, almandine garnet and zircon. Peak mineral assemblages consist of forsterite, Mg-Al-spinel, geikielite (MgTiO3), phlogopite and accessory zirconolite, baddeleyite and srilankite in dolomite marble, as well as K-feldspar, scapolite, diopside, titanite and accessory graphite and zircon in calcite marble. These assemblages characterize peak HT-LP metamorphic conditions close to 700–750 °C, ≤4.5 kbar. The FBBSZ includes minor ductile thrusts that determine kinzigite horses or slivers carried NW-ward over the marbles. Within the latter, NNE-trending folds are conspicuous. Brittle, northward-dipping normal faults crosscut the FBBSZ ductile structures. Detrital cores of zircon from the BBMs yield two U-Th-Pb age clusters of ∼270 Ma and ∼340 Ma, whereas their rims yield ∼21 Ma ages. Correlations with comparable settings in other West Mediterranean Alpine belts are discussed. The BBMs compare with the Triassic carbonates deposited over the crustal units of the Alpujarrides-Sebtides. The assumed Triassic protoliths may have been deposited onto the kinzigites or carried as extensional allochthons over a detachment in the Early Jurassic during the incipient formation of the Alboran Domain continental margin. Thus, it is concluded that the Beni Bousera mantle rocks were exhumed to a shallow depth during early rifting events responsible for the birth of the Maghrebian Tethys.
Highlights
At the westernmost tip of the West Mediterranean Alpine belts, the Gibraltar Arc is famous for its large massifs of subcontinental peridotites, up to ∼5 km thick, i.e., the Ronda massifs in the northern (Spanish) branch of the arc, and the Beni Bousera massif in the southern (Moroccan) branch (Fig. 1), both topped by a granulitic cap a few hundred meters thick
We report new field and laboratory data involving detail mapping, structural analysis, petrological studies, and SHRIMP (Sensitive High-Resolution Ion Microprobe) dating of zircon grains from marble outcrops scattered around the Beni Bousera massif
The variation in thickness observed at the different outcrops can be largely attributed to the tectonic deformation that affected the marbles in the Filali–Beni Bousera Shear Zone (FBBSZ)
Summary
At the westernmost tip of the West Mediterranean Alpine belts, the Gibraltar Arc is famous for its large massifs of subcontinental peridotites, up to ∼5 km thick, i.e., the Ronda massifs in the northern (Spanish) branch of the arc, and the Beni Bousera massif in the southern (Moroccan) branch (Fig. 1), both topped by a granulitic cap a few hundred meters thick These massifs are included in a complex of crustal nappes, i.e., the Alpujarrides (Spain)–Sebtides (Morocco) Complex, which, together with the underlying NevadoFilabrides of Spain and the overlying Malaguides (Spain)– Ghomarides (Morocco) nappes, constitutes the Alboran Domain (see reviews in Chalouan et al, 2008; Jabaloy Sánchez et al, 2019a, 2019b). In a pioneering paper, Kornprobst and Vielzeuf (1984) compared the Ronda-Beni Bousera lherzolites with those of the Pyrenees (Fig. 2A) and emphasized the major role of extension . Michard et al (1991, 1997) compared the Ronda-Beni Bousera peridotites with those of Ivrea in the Western Alps, whose earliest exhumation is related to Permian-Jurassic extensional tectonics (Brodie et al, 1989; Vavra et al, 1999)
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