Tectonic evolution of the Betic–Rif arc: New constraints from 40Ar/39Ar dating on white micas in the Temsamane units (External Rif, northern Morocco)
, Negro, François, De Sigoyer Julia, Goffé, Bruno, Saddiqi, Omar, Villa, Igor M
The Betic–Rif orogen, at the western end of the Mediterranean, is a key region to improve our knowledge on the Africa–Eurasia convergence. The Temsamane units, in spite of their external position in the Rif (northern Morocco), underwent medium-pressure low-temperature (MP–LT) conditions (ca. 7–9 kbar; 330–430 °C). We propose a new tectonic and metamorphic evolution scenario for the Rif (southern) branch of the orogen on the basis of first 40Ar/39Ar dating on petrologically and structurally characterized white micas. Three groups of 40Ar/39Ar ages are observed: (1) Chattian or older Si-rich (highest-pressure) mica relics, (2) 15–12.5 Ma corresponding to the micas defining the foliation and (3) Messinian or younger late micas and alteration. We propose that the MP–LT metamorphic event in the External Rif is Oligocene in age, highlighting a subduction event during this period which could be almost contemporaneous with the burial of HP–LT units from the internal Rif (Alboran Domain). The exhumation of these units characterized by an intense E–W stretching and by top-to-the-west shear senses, is Middle to Late Miocene in age. We propose a correlation of tectonic and metamorphic events at the Betic–Rif arc scale. We argue that the exhumation of the external units of the Rif (1) is younger than that of the Alboran Domain (internal) unit of the Rif, and mirrors a different tectonic setting, but (2) strongly resembles to that documented in the lower Alboran Domain units of the Betics. We show that a regional E–W extension is recorded on both sides of the Betic–Rif arc during the Middle Miocene. This extension probably reflects back-arc deformation of an eastward dipping subduction that retreated westward during the Middle to Late Miocene in the Western Mediterranean.
Thermal structure of the Alboran Domain in the Rif (northern Morocco) and the Western Betics (southern Spain). Constraints from Raman spectroscopy of carbonaceous material
, Negro, François, Beyssac, Olivier, Goffé, B, Saddiqi, O, Bouybaouène, L
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–Alpujárrides) 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–Maláguides), 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 40Ar–39Ar ages. These nappes were thus affected by a significant thermal metamorphism, and the available age data in the underlying Sebtides–Alpujárrides 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.