ALBERT

Description: The Roquebrune-Cap Martin basin (RCMB), developed along the eastern rim of the Nice arc, represents an exclusive sedimentary marker constraining the timing of the deformation in the Nice arc (southern Subalpine chain) during Miocene times. Structural and sedimentological analyses as well as 3D geometrical modeling of the RCMB revealed morphological, structural and sedimentological features characterizing an active tectonic control of the sedimentary infills and the basin development. Structural and microstructural analyses along the eastern boundary of the Nice arc evidenced a N-S left-lateral strike-slip ‘en echelon’ faults system named Mont Gros-St Agnès Castillon relayed by the Biancon E-W thrusts and sheets. The formation of the RCMB appears to be genetically linked to these strike-slip ‘en échelon’ faults. Such characteristics include the presence of the Mont Gros strike-slip fault structural high relief bounding the RCMB to the West, the West-East asymmetry of the sedimentary infill with a laterally transition facies from breccias directly below the fault relief to conglomerates and sandstones in the central part of the basin and the presence of mass wasting in all structural levels of the basin. The onset and the evolution of the basin were driven by transpresssive tectonics, generating a deep and narrow tectonic depression, bounded by steep tectonically controlled slopes. The transpresssive character of the eastern Nice arc boundary where the syn-tectonic RCMB is hosted, accommodate a general southward translation of the Nice arc in response to a N-S shortening regime. The sedimentological and previous paleontological analyses suggest that the activity of the eastern Nice arc transpresssive boundary generating the RCMB and thus the southward motion of the Nice arc, started during the Early Miocene (Aquitanian), continuing through the Late Miocene (Tortonian). The style and the timing of the syn-sedimentary deformation of the Nice arc is coherent in space and time with the one affecting the Digne and Castellane arc.

Description: We document the occurrence of micro-diorite magmatic sills, with magmatic enclaves, in the Ceuta Peninsula within metapelites from the Lower Sebtides units (Internal Rif). All magmatic rocks show a primary magmatic mineralogy and geochemical signature diagnostic for high-K calc-alkaline to shoshonitic island arc magmatism. Moreover, these rocks are significantly affected by secondary metamorphic transformations under greenschist- to amphibolite-facies conditions, regionally dated at c. 21 Ma. Geometric relationships between the sills and the main regional foliation, developed under intermediate-pressure granulite-facies conditions at c. 28 Ma, demonstrate that the sills emplaced during the late stage of this main tectonic event. New U–Pb in situ analyses of monazite performed on the micro-diorite sills provide an age of 20.64 ± 0.19 Ma, coherent with this chronological framework and interpreted as the age of greenschist-facies re-equilibration. The discovery of pre-Miocene high-K calc-alkaline to shoshonitic arc-related magmatism is clearly consistent with the subduction context proposed for the Alboran Basin evolution, according to geophysical investigations. In this framework, the Lower Sebtides units could be considered as part of the upper plate of the subduction system, while the Upper Sebtides must be regarded as the lower subducted plate.