The internal = northern zones of the Maghrebide belt includes allochthonous massifs that define, together with those from the Betics, Sicily and Calabria, a disrupted 'Alkapeca' terrane. In the Rif transect, taking advantage of new thermobarometric and geochronologic studies, we recognize a metamorphic complex with a mostly non-Alpine upper plate (Dorsale, Ghomarides-Malaguides), and a lower plate (Sebtides-Alpujarrides) affected by HP-LT, then HT-LP Alpine metamorphism. Some eastern Rif allochthons (Temsamane area) display MP metamorphism and could represent either Sebtide inliers or slivers from the African paleomargin. The Sebtide-Alpujarride late HT event also affects the bottom of the Ghomaride-Malaguide complex and can be related to the onset of the Alboran Sea rifting at ∼25-22 Ma, whereas their HP event is ascribed to an earlier (∼30-25 Ma?), north-to northeast-dipping subduction event. A review of the Algerian literature allows us to infer a similar structure and evolution for the Kabylides. The Alkapeca disruption is classically explained by back-arc extension above the retreating subduction of the Ligurian-Maghrebian oceanic lithosphere. The question of whether the latter subduction followed an earlier and opposite Alpine-Betic (Nevado-Filabride) subduction or not, remains open to discussion.
The Betic‐Rif orogen forms the western termination of the Alpine orogenic system in the Mediterranean region. The precise timing, structural evolution, and distribution of high‐grade metamorphic units (Alpine versus pre‐Alpine) in the inner zones of the orogen (Alboran Domain) remain controversial issues. In this paper we report occurrence of distinct generations of peraluminous granitic bodies intruded within Beni Bousera peridotites and their amphibolite‐to‐granulite facies envelope, in the core of the Alboran Domain of the Rif chain (northern Morocco). These granitic bodies are central to the reconstruction of the high‐grade evolution of the Alboran Domain because they provide first‐order structural markers to assess the P‐T‐t deformation history of the high‐grade terranes. Here we document the petrography and structural relationships with the host rocks and constrain the timing of granite emplacement using laser ablation–inductively coupled plasma–mass spectrometry U‐Pb zircon and/or monazite dating, complemented by 40Ar/39Ar dating. The results indicate that granite emplacement occurred in two major episodes of anatectic magmatism, during the Hercynian (circa 300 Ma) and Alpine (circa 22 Ma) periods, respectively. These data (1) provide conclusive evidence for an important phase of Hercynian magmatism and high‐grade metamorphism in the Alboran Domain and (2) permit a revaluation of the significance of the high‐grade early Miocene event documented in the Alboran Domain in terms of a late stage, thermal pulse that reworked a polymetamorphic (Hercynian and Alpine) nappe pile. These results provide new constraints for construction of a feasible tectonometamorphic model for the Alpine evolution of the western Mediterranean.
In the Rif (northern Morocco) and the Western Betics (southern Spain), the Alboran Domain forms a complex stack of metamorphic nappes including mantle peridotites (Beni Bousera and Ronda). We present in this paper new temperature data obtained in the Alboran Domain based on Raman spectroscopy of carbonaceous material (RSCM thermometry). In the lower metamorphic nappes of the Alboran Domain (lower Sebtides-Alpuja´rrides) temperature ranges from > 640°C at the base of the metapelitic sequence to 500°C at the top. The relationships between field isotherms and nappe structure show that peak temperatures were reached during strong ductile thinning of these nappes whereas they partly postdate this main episode in the Rif. In the upper nappes of the Alboran Domain (Ghomarides-Mala´guides), generally supposed to be only weakly metamorphosed, temperatures range from 500°C at their base down to < 330°C at the top. This temperature gradient is consistent with progressive Cenozoic resetting of K-Ar and 40 Ar-39 Ar ages. These nappes were thus affected by a significant thermal metamorphism, and the available age data in the underlying Sebtides-Alpuja´rrides show that this metamorphism is related to the metamorphic evolution of the whole Alboran Domain during the Late Oligocene-Early Miocene. Such thermal structure and metamorphic evolution can be explained by generalized extension in the whole Alboran Domain crustal sequence. At a larger scale, the present thermal structure of the Alboran Domain is roughly spatially consistent around the Beni Bousera peridotites in the Rif, but much more affected by late brittle tectonics around the Ronda peridotites in the Western Betics. Therefore, on the basis of the observed thermal structure, the metamorphic evolution of the Alboran Domain can be interpreted as the result of the ascent of hot mantle units contemporaneous with thinning of the whole lithosphere during an Oligo-Miocene extensional event. The resulting structure has however been dismembered by late brittle tectonics in the Western Betics.
In the Alboran domain, two crustal‐thickening late‐orogenic extension cycles are superposed. The importance of the late Alpine thinning of the Alpujarride‐Sebtide crustal section on top of the Beni Bousera peridotites is discussed here in metamorphic petrological terms. The Alpine metamorphism operated first under a HP–LT gradient, and reached the eclogite facies in the Permian–Triassic phyllites, before retrogression under a high geothermal gradient. A contrasting, higher temperature metamorphism characterizes the pre‐Permian section, reaching the HP‐granulite facies at the bottom of the crustal section. By reference to the western European setting, the granulites relate to the Hercynian orogeny, as supported by the isotopic ages of the enclosed, armoured monazite crystals. Thus, thinning of an overthickened crust might have occurred there during the late Hercynian extension and Tethyan opening, before being reactivated during the late Oligocene.
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