No abstract

During the Neogene, the Western Mediterranean subduction-related orogen developed under differing modes and senses of subduction, resulting in the formation of the Apennines, Maghrebides, Rif, and Betics. In this work, we present the Neogene kinematic evolution of the Rif, based on literature data and new results from structural-stratigraphic analyses and biostratigraphic investigations carried out in the External Zone of the Rif. We analyzed three stratigraphic sections: Dar Zhirou, Saf Lahmame, and Seguedla. The results of these analyses allow us to reconstruct a wide Tortonian-Messinian wedge-top basin in the Tanger-Al Manzla area, when the leading edge of the Rif was at the frontal thrust of the Prerif. The presence of this wedge-top basin points to a Tortonian-Messinian compressional deformation that affected a wide area of the Rif, including the Prerif, Mesorif, and Intrarif. The late Miocene compressional deformation that affected the Mesorif and Intrarif occurred as out-of-sequence thrusting, due to renewed compressional tectonics in the internal zone of the chain triggered by the collision of the Rif accretionary wedge with the North African Margin. In the Neogene evolution of the Western Mediterranean subduction-related orogen, the late Miocene was a major pulse in the interplay between African and European plates, as evidenced by the increase in migration rates of some segments of the circum-Mediterranean fold-and-thrust belt, the out-of-sequence thrusting in the Rif, as well as the occurrence of late Miocene imbricate thrust structures in oceanic fracture zones of the African Plate (Central Atlantic Ocean).

During the Tertiary evolution of the Western Mediterranean subduction system, a migrating foreland basin system developed between the Maghrebian orogenic belt and the adjacent African Craton. However, a comprehensive reconstruction of the foreland basin systems of the Rif Chain is still missing. By integrating field observations with quantitative biostratigraphic data from calcareous nannofossil assemblages, sandstone composition, and detrital zircon U-Pb geochronology from selected stratigraphic successions, we reconstruct the foreland basin system that developed in the early Miocene in front of the growing Rif orogen. The successions analyzed are representative of (1) the classical “Numidian Facies” from the Intrarifian Tanger Unit and (2) the Numidian-like deposits (mixed successions) of the “Mérinides Facies” from the “Maghrebian Flysch Basin” and the “Beliounis Facies” from the Predorsalian Unit. Our petrographic analyses and detrital zircon U-Pb ages show that the quartzarenites of the “Numidian Facies” originated from the African Craton, whereas the sublitharenites and feldspathic litharenites from the Mérinides and Beliounis Facies originated from a cratonic area and the exhuming Rif Chain. Our biostratigraphic analyses suggest a simultaneous arrival of the quartz grains in the Numidian, Mérinides, and Beliounis deposits, which indicates that their deposition occurred at ∼1 m.y. (ca. 20−19 Ma, early Burdigalian) and allows us to delineate the early Burdigalian foreland basin system of the Rif Chain. The foreland depozone received the “Numidian Facies,” the foredeep-hosted ∼2000 m of the “Mérinides Facies” and the Beni Ider Flysch, whereas the wedge-top depozone was characterized by deposition of the “Beliounis Facies.” The Numidian Sandstones and the Numidian-like deposits analyzed in Morocco show the same age as similar deposits from Algeria, Tunisia, and Sicily, which suggests a comparable early Burdigalian tectono-sedimentary evolution along the southern branch of the Western Mediterranean subduction-related orogen.

<p>The Rif chain is located on the western edge of the Apennine-Maghrebian Chain. It is classically subdivided into three main superimposed tectonic domains: The Internal, Flysch Basin, and the External domains. The latter regroups three sub-domains: Intrarif, Mesorif, and Prerif. The present work is based on new geological mapping with lithostratigraphic logging and sampling for geochemical analysis from Lower-Middle Jurassic sedimentary successions belonging to the Prerif sub-domain. Four stratigraphic sections have been analyzed (Jbel Zerhoune, Dhar Nsour I and II, and Jbel Outita section). The Lower Jurassic successions are mainly characterized by alternating marls with limestones, bioclastic limestone, and calcareous sandstones, whereas the Middle Jurassic ones are mainly represented by calcareous sandstones and bioclastic limestones, both alternating with marly intervals.</p><p>The collected fifteen samples have been examined using Laser Ablation–Inductively Coupled Plasma–Mass Spectrometry (LA-ICP-MS) and X-Ray Fluorescent (XRF) for Elemental geochemical analysis to provide new insights about provenance, source rocks, paleoweathering, mechanical sorting/recycling, and geodynamic setting.</p><p>The elemental geochemistry (Major, Trace, and Rare Earth Elements) reveals that major oxides concentrations (SiO<sub>2</sub>, Al<sub>2</sub>O<sub>3</sub>, MgO, and K<sub>2</sub>O) are relatively close to the Post-Archean Australian Shales (PAAS) except for MnO and Na<sub>2</sub>O, which show a depletion trend, while Cao and Sr display an enrichment, also Zr, Hf and REEs show low concentrations and indicate weak recycling processes. In order to characterize the provenance, we used a combination of the Cr/V vs. Y/Ni plot, the Cr/Th vs. Th/Sc plot, and the elemental ratios of provenance (La/Sc, Th/Sc, Th/Co, Th/Cr, and Cr/Th). Various discriminant diagrams were used to reveal the paleoweathering intensity, sorting, and maturity of sediment during their source to sink fate. The Chemical Index of Alteration (CIA) shows values varying from 49.09 % to 77.01 %. Th/Sc versus Zr/Sc ratios and the Al2O3-Zr-TiO2 plots have been used to assess the sorting related to the fractionation of sediment during their transport. This is also corroborated by the relatively high values of the compositional variability index (ICV) of Jbel Zerhoune and Dhar Nsour (I and II) and Jbel Outita (ICV<1 and ICV>1, respectively). We also used multidimensional discriminant function diagrams to characterize the geodynamics setting on the analyzed samples.</p><p>The main expected results and related interpretations reveal that the provenance of the lower-Middle Jurassic sediments mainly indicates a supply from felsic source rock areas as attested by prevalent enrichment in Sr and probably from a minor mafic supply. The chemical index of alteration indicates a low to moderate degree of source area weathering. The Th/Sc versus Zr/Sc ratios and the Al2O3-Zr-TiO2 recycling plots, and the depletion of Hf and Zr reflect poor mechanical sorting and recycling processes which are confirmed by the high ICV values indicating that almost all samples are immature first-cycle sediments with unweathered detrital minerals. The analyzed sediments have been deposited within a passive margin controlled by a rifting/drifting geodynamic evolution of the northern African margin during the Early-Middle Jurassic.</p>