Fluid flow and polymetallic sulfide mineralization in the Kettara shear zone (Jebilet Massif, Variscan Belt, Morocco)

The Kettara shear zone is a regional wrench shear zone within the Jebilet massif of Western Morocco, part of the Variscan orogenic belt. This massif is characterized by bimodal magmatism, largely intrusive, and by a number of polymetallic massive sulfide deposits. A syntectonic mafic-ultramafic intrusion and an adjacent, deformed pyrrhotite-rich massive sulfide deposit are located within a ‘compressional jog’ of the shear zone. Hydrothermal alteration in both the intrusion and the wall rocks adjacent to the deposit is characterized by syntectonic replacement processes leading to formation of chlorite-schists and quartz ± calcite veins. Fluid inclusions in mineralized (pyrrhotite-bearing) quartz veins from the wall rocks adjacent to the deposit and in veins associated with chlorite-schists within the intrusion indicate a prevalence of H2O-CO2-CH4-N2 and H2O-salt fluid systems. In the mineralized veins the fluid shows reducing conditions, with gas dominated by CH4 and N2 and salinities around 7.5 wt% NaCl, whereas in the chlorite shear zones fluid is CO2 dominated and salinities are higher than 23 wt% NaCl. Hydrogen and oxygen isotopic compositions of chlorite and quartz are similar and demonstrate involvement of metamorphic water in both the deposit and the intrusion. The data are consistent with a regional metamorphic fluid flow through the Kettara shear zone. The migrating metamorphic fluids were reduced in the organic matter-rich host rocks leading to deposition of sulfides in the mineralized veins. There are two possible hypotheses for the origin of these mineralized veins: either they were formed during deformation and remobilization of a syn-sedimentary massive sulfide deposit, or they were formed synchronously with the sulfide deposit during development of the Kettara shear zone

Commentaire à la note intitulée Synergie entre la télédétection multispectrale et les données de terrain pour la conception d'un nouveau modèle géodynamique d'ouverture du bassin paléozoïque des Jebilet centrales (Maroc), de A. El Harti et al.

International audienc

Etude métamorphique des micaschistes et gneiss de l'unité de Filali du Massif de Beni Bousera (Rif interne, Maroc)

Les péridotites du massif de Béni Bousera sont, avec les péridotites de Ronda, des péridotites sous-continentales appartenant aux domaines internes des chaînes Alpines du Rif et des Bétiques, respectivement. Leurs exhumations et leurs insertions dans la croûte se font dans un contexte de recul de la subduction Alpine vers le Sud et l'Ouest durant l'Oligo-Miocéne. Pour étudier l'amincissement crustal dans ce contexte d'extension arrière-arc, lié à la mise en place de ces péridotites, nous avons étudié les conditions P-T associées à des différents épisodes déformationels dans leur encaissant (Sébtides inférieures, unité de Filali). Cette unité présente une évolution tectonométamorphique polyphasée. En s'approchant du massif des péridotites, une première schistosité (S1) est associée à des paragenèses allant de grenat-biotite; grenat-staurotide-disthène-biotite à grenat-biotite-feldspath potassique ± disthène. La deuxième schistosité (S2) est associée à des paragenèses à biotite-andalousite loin des péridotites et sillimanite-biotite prés des péridotites. Le grade métamorphique de terrain augmente vers les péridotites pour les deux schistosités. En utilisant les pseudosections calculées dans le système MnO-Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-FeO3 (MnNCKFMASHTO) avec le logiciel THERMOCALC, nous montrons que les échantillons ont enregistrés des chemins P-T horaires, dominés par une décompression entre S1 et S2. Les conditions P-T associées à S1 varient entre 5,4 kbar / 550°C et 8,5 kbar / 620°C, et représentent donc un gradient métamorphique de type barrovien. Les conditions P-T associées à la schistosité S2 évoluent entre 2,5 kbar / 550°C et 4 kbar / 700°C et sont caractéristiques des gradients de haute température / basse pression de type Abukuma. Dans notre interprétation, les deux épisodes tectonométamorphiques sont d'âge alpin. Le développement de la schistosité S1 résulte de l'épaississement crustal dû à la subduction Alpine entrainant le métamorphisme HP-BT dans les Bétiques (unité Alpujarides) et le Rif (unité des Sebtides). Le deuxième épisode est associé à l'amincissement crustal dû à l'étirement de la croûte dans un contexte d'extension arrière-arc Oligo-Miocène. Nous discuterons de l'amplitude de cet amincissement et nous le comparerons à celui observé dans les unités équivalentes des chaînes Bétiques

Structures et métamorphisme des péridotites de Béni Bousera et de leur encaissant (Rif interne, Maroc): Implications géodynamiques durant l'extension arrière-arc à l'Oligo-Miocène

National audienc

Oligo-Miocene exhumation of the Beni-Bousera peridotite through a lithosphere-scale extensional shear zone

New structural data and P-T estimates of syn-deformational assemblages within the Beni Bousera peridotites and their crustal envelope are used to explain their Alpine exhumation. The Beni Bousera peridotites occur as thin sheets within high grade crustal units of the lower Sebtides (inner Rif, Morocco) and are composed of weakly deformed spinel lherzolite in the core of the massif and garnet-spinel mylonite at the rim. The main foliation trajectories in both the peridotites and overlying crustal units show systematic rotation towards their mutual contact, indicating a kilometer-scale top to the NW shearing with a dextral component along this crust/mantle contact. Widespread top to the NW shear criteria within the crustal units overlying the peridotite support this feature. Available ages constrain the development of the main foliation in both the peridotites and crustal rocks between 25 and 20 Ma. New P-T data from the peridotites show that deformation occurs during decompression from approximate to 22 kbar, 1050 degrees C to approximate to 9-15 kbar, 800 degrees C. As a consequence, exhumation of the Beni Bousera peridotites takes place during the Oligo-Miocene lithosphere thinning in the footwall of a lithospheric extensional shear zone. The exceptional preservation of garnet within the mylonitic peridotites results from rapid cooling of the border of the massif due to the juxtaposition with colder crustal rocks along this shear zone. Uplifting of the hot mantle rocks simultaneously induces high temperature metamorphism in the overlying crustal units. These new findings allow us to reconstruct the deformation history of the Beni Bousera region and the Alboran domain in the framework of the western Mediterranean geodynamics during the last 40 Myrs.. (C) 2011 Lavoisier SAS. All rights reserve

Does lithology control post-orogenic topography and rock erodibility? Insight from Anti-Atlas of Morocco

International audienc

Tectonic Evolution of the Western High Atlas of Morocco: Oblique Convergence, Reactivation, and Transpression

The High Atlas of Morocco is a double-vergent mountain belt developed by Cenozoic shortening and inversion of a Triassic-Jurassic rift. The structural setting, the morphometric features, and the patterns of exhumation through time and space change remarkably both along and across the strike. Here we combine structural data with revised thermochronological data to unravel the kinematic and evolution of the western High Atlas. Our results show that the structural grain of the western High Atlas is defined by two main groups of faults, namely, thrust and oblique-slip faults, which mainly strike subparallel from W-E to NE-SW. The slip direction of the thrust structures is NNW-SSE to NW-SE oriented, and the slip direction of the oblique-slip faults is WSW-ENE to NW-SE oriented. Pieces of thermochronological and geological evidence indicate that in the last ~10 Ma the exhumation rate increased during the activity thrusts and oblique-slip faults. The coexistence of these two fault systems also suggests partitioning of deformation under a transpressive regime. In the western High Atlas, we estimate a displacement of ~12 km on the frontal thrusts and of at least ~22 km on the axial oblique-slip structures. Thrusts and oblique-slip structures together result in a total cumulative displacement of ~25 km, which represents about half of the Africa-Eurasia convergence

Lithological control on erosional dynamics in a tectonically inactive mountain belt (Anti-Atlas, Morocco)

International audienc

Lithological control on erosional dynamics in a tectonically inactive mountain belt (Anti-Atlas, Morocco)

International audienc