Évolution des mouvements verticaux néogènes de la chaîne du Rif (Nord-Maroc) : apports d’une analyse structurale et thermochronologique

The Rif chain (Northern Morocco) belongs, with the Betic Cordillera (Southern Spain), to the westernmost part of the Alpine belt. This sector is one of the main targets for the comprehension of vertical movements linked to orogenic processes. Despite the numerous investigations lead on the Pliocene uplifted deposits in the Rif Chain, the timing and amplitude of the vertical motions subsequent to the period of cooling have never been constrained. This work aims to discuss the geodynamical processes involved in the vertical motion of the Rif margin since the Neogene. A comparison between the vertical movements determined in the internal zones and estimated in the flyschs domain (South of Jebha fault), has been realized on the basis of AHe data. Correlations reported between the vertical movements and the evolution of the stress field highlighted the linkage between the main stages of vertical motion and extension period with a radial pattern. 1-The latest exhumation stages of the deep-seated metamorphic rocks, from 12-10 km of depth, were triggered by the radial extension of these units. The collapse extended from the low grade metamorphism (Greenschists facies) to the brittle strata during the Aquitano-Burdigalian times. This extension is expressed in the superficial units, whereas deepest parts and sectors at the front of the Rif chain underwent a compression. 2-A second uplift stage proceeds at a very slow raising pace and is associated to radial extension in throughout the internal zones since the Lower Pliocene. Le flyschs sector was uplifted faster since the Late Miocene.La chaîne du Rif (Nord du Maroc) constitue, avec les Cordillères des Bétiques (Sud de l’Espagne), la terminaison occidentale de la ceinture alpine. C’est un secteur clé pour la compréhension des mouvements verticaux associés aux processus orogéniques. Jusqu’à présent, aucune étude n’a permis de contraindre les mouvements verticaux postérieurs au Miocène inférieur, malgré la présence de dépôts marins pliocènes surélevés. L’objectif de ce travail est d’apporter des arguments permettant de discuter des mécanismes géochronologiques et cinématiques impliqués dans les mouvements verticaux de la marge du Rif depuis le Néogène. Une comparaison des déplacements identifiés dans les zones internes, a pu être effectuée, avec ceux estimés dans le domaine des flyschs (Sud de l’accident de Jebha), grâce aux données AHe. Les corrélations entre mouvements verticaux et évolution du champ de déformation mettent en évidence le lien existant entre les périodes de mouvements verticaux majeurs et les phases d’extension radiale. 1-Les derniers stades d’exhumation des unités métamorphiques, depuis près de 12-10 km de profondeur, résultent d’un stade d’extension radiale affectant ces formations. Le domaine de collapse s’étend depuis le faciès Schiste Vert jusqu’au domaine fragile durant l’Aquitano-Burdigalien. Les mouvements extensifs sont limités aux parties superficielles de l’orogène tandis que les parties plus profondes et situées en avant-chaîne sont en compression. 2-Une période de surrection lente associée à de l’extension radiale, affecte l’intégralité des zones internes, au moins depuis le Pliocène inférieur. La zone des flyschs se surélève à plus grande vitesse

Neogene vertical movements evolution in the Rif chain (North-Morocco) : insights from structural and thermochronological analysis

La chaîne du Rif (Nord du Maroc) constitue, avec les Cordillères des Bétiques (Sud de l’Espagne), la terminaison occidentale de la ceinture alpine. C’est un secteur clé pour la compréhension des mouvements verticaux associés aux processus orogéniques. Jusqu’à présent, aucune étude n’a permis de contraindre les mouvements verticaux postérieurs au Miocène inférieur, malgré la présence de dépôts marins pliocènes surélevés. L’objectif de ce travail est d’apporter des arguments permettant de discuter des mécanismes géochronologiques et cinématiques impliqués dans les mouvements verticaux de la marge du Rif depuis le Néogène. Une comparaison des déplacements identifiés dans les zones internes, a pu être effectuée, avec ceux estimés dans le domaine des flyschs (Sud de l’accident de Jebha), grâce aux données AHe. Les corrélations entre mouvements verticaux et évolution du champ de déformation mettent en évidence le lien existant entre les périodes de mouvements verticaux majeurs et les phases d’extension radiale. 1-Les derniers stades d’exhumation des unités métamorphiques, depuis près de 12-10 km de profondeur, résultent d’un stade d’extension radiale affectant ces formations. Le domaine de collapse s’étend depuis le faciès Schiste Vert jusqu’au domaine fragile durant l’Aquitano-Burdigalien. Les mouvements extensifs sont limités aux parties superficielles de l’orogène tandis que les parties plus profondes et situées en avant-chaîne sont en compression. 2-Une période de surrection lente associée à de l’extension radiale, affecte l’intégralité des zones internes, au moins depuis le Pliocène inférieur. La zone des flyschs se surélève à plus grande vitesse.The Rif chain (Northern Morocco) belongs, with the Betic Cordillera (Southern Spain), to the westernmost part of the Alpine belt. This sector is one of the main targets for the comprehension of vertical movements linked to orogenic processes. Despite the numerous investigations lead on the Pliocene uplifted deposits in the Rif Chain, the timing and amplitude of the vertical motions subsequent to the period of cooling have never been constrained. This work aims to discuss the geodynamical processes involved in the vertical motion of the Rif margin since the Neogene. A comparison between the vertical movements determined in the internal zones and estimated in the flyschs domain (South of Jebha fault), has been realized on the basis of AHe data. Correlations reported between the vertical movements and the evolution of the stress field highlighted the linkage between the main stages of vertical motion and extension period with a radial pattern. 1-The latest exhumation stages of the deep-seated metamorphic rocks, from 12-10 km of depth, were triggered by the radial extension of these units. The collapse extended from the low grade metamorphism (Greenschists facies) to the brittle strata during the Aquitano-Burdigalian times. This extension is expressed in the superficial units, whereas deepest parts and sectors at the front of the Rif chain underwent a compression. 2-A second uplift stage proceeds at a very slow raising pace and is associated to radial extension in throughout the internal zones since the Lower Pliocene. Le flyschs sector was uplifted faster since the Late Miocene

Lower Sebtides Neogene exhumationand uplift in Ceuta and Cabo Negro areas(Internal Rif, Northern Morocco) : insightsfrom medium- and low-temperaturethermochronology

International audienceThe Rif Chain (Northern Morocco) belongs, with the Betic Cordillera(Southern Spain), to the western termination of the Alpine belt. Deepestunits of the Rif Internal Zones (i.e. Lower Sebtides) outcrop in three areas, from North to South : Ceuta, Cabo Negro and Beni Bousera.The quick exhumation of these units during the Late Oligocene-EarlyMiocene is related with a crustal thinning and the Alboran Sea rifting.Despite Beni Bousera massif has been well investigated, no data areavailable in Ceuta and Cabo Negro areas. This study aims to constrainthe last steps of cooling and exhumation in the Lower Sebtides usingmedium- and low-temperature thermochronological data. Combinationof Ar-Ar on micas, fission tracks and (U-Th)/He on apatites (respectivelyAFT and AHe) from high-grade rocks allows understanding thermalhistory between 450-350°C and surface temperatures. These datasuggest a similar cooling evolution of the Ceuta and Cabo Negro units.Ages obtained with all these methods are comprised in the same interval,between 22 and 18 Ma. These data record an almost instantaneouscooling (between 50 and 200°C/Ma) associated to final exhumation afterthe Oligo-Miocene thermal event affecting these units at shallowdepths (10 km). Low-temperature data suggest that Ceuta and CaboNegro units cooled at surface temperatures before Beni Bousera ones.Since the end of Burdigalian, the Rif internal zones vertical movementsdid not exceed 1km, even during the Messinian Salinity. AFT and AHeages in the whole Sebtides are also slightly older than these obtainedin their Spanish equivalent (Alpujarrides). The Gibraltar arc is dividedinto several independent crustal blocks separated by major fault zonesperpendicular to the coast that could have induced independent verticalmovements during the entire Neogen

Exhumation of the Ronda peridotite during hyper‐extension: New structural and thermal constraints from the Nieves Unit (western Betic Cordillera, Spain)

International audienceThe Ronda peridotite (Betic Cordillera, Southern Spain) is the largest alpine-type peridotite massif worldwide. Yet, the emplacement mechanism of these mantle rocks is still a highly debated topic. In this study, we aim at better constraining their context of exhumation by focusing on the Nieves Unit, which is mostly composed of Mesozoic metasediments displaying HT metamorphism and deformation along the contact with peridotite. The topmost parts of the peridotites and the bottom parts of the Nieves Unit share the same planar/linear fabric parallel to the contact accompanied by component of sinistral kinematic indicators. Raman Spectroscopy on Carbonaceous Material geothermometry then shows a contraction of peak-temperature conditions within metasediments of the Nieves Unit, as well as a northeastward decrease along strike, highlighting a strong thinning of the unit and a significant obliquity of the isograds to the contact with the peridotites. Local magnetite ores and ophicalcite bodies are finally observed nearby the contact, suggesting a hyper-extension context for the mantle exhumation. Indeed, the Nieves Unit, the Ronda peridotite and the Jubrique Unit, where a strong thinning has also been documented, are all in contact at a triple junction between the western contact of the peridotite and the Nieves/Jubrique contact. Altogether, these features support the idea that the western shear zone limiting the peridotites from the Nieves and Jubrique units is a tilted large-scale extensional detachment that has exhumed mantle rocks from beneath the continental basement (Jubrique Unit) and its Mesozoic cover (Nieves Unit) during an episode of hyper-extension

Geodynamic evolution of the western Mediterranean basin since the Late Cretaceous

International audienceThe western Mediterranean basin results from the convergence between Africa and Eurasia and from related interactions between an undetermined number of macroplates and microplates during the Late Cretaceous. Various and very different models proposed during the last thirty years attempted to explain the geodynamic evolution of this area (e.g. Carminati et al., 2012; Schettino and Turco, 2011; Handy et al., 2010; Jolivet et al., 2006). However, none of those models are totally satisfactory, especially when regarding the Gibraltar arc region. The western Mediterranean constitutes a unique laboratory to study interactions between surface deformations and crustal and mantle processes (slab roll-back, slab break-off, delamination, etc.). The goal of this study is to understand how these deep processes are coupled to crustal evolution during the collision between Africa and Eurasia. In this context, two different approaches will be undertaken. Firstly, using GPlates software (Boyden et al., 2011), kinematic reconstructions will be performed based on stratigraphic, metamorphic, magmatic, structural and paleomagnetic data. These reconstructions will be made from the Present to the Late Cretaceous and using the principle of rigid polygons which enables deformation of areas between polygons. Realizing these reconstructions backward allows to be free from any preconceived geodynamic model. The principle of rigid polygons helps avoiding problems due to rough approximations linked to rigid blocks. Secondly, obtained results will be used to constrain 4D numerical modelling (space and time) of the western Mediterranean subduction zone evolution since the Late Cretaceous. The influence of different primordial parameters (rheological and thermal stratification of the upper and lower plates, convergence rates, presence of weak zones, etc.) will be tested. The results will be compared to natural data (surface velocity field, thermal anomalies, temporal and spatial evolution of the deformation, magmatism distribution, basin geometry) in order to understand the main processes influencing the subduction zone dynamics. The preliminary results of this study and working perspectives will be presented

Extensional emplacement of the Ronda peridotite as recorded in the Dorsale unit

International audienceThe Betic-Rif orogen forms the westernmost part of the Alpine orogenic system and results from the closure of the Tethys Ocean between Africa and the Iberian Peninsula. Subduction and crustal thickening leading to the formation of high-pressure and low-temperature (HP/LT) rocks were followed by a late-orogenic extensional stage in an overall still convergent setting. Indeed, plate-kinematic reconstructions reveal a continuous convergence from Late Cretaceous times to the present day and characterized by slow convergence rates. If this large-scale scenario is now broadly admitted, some first order questions remain opened. Among these questions, the timing and kinematics of the emplacement of the Ronda or Beni Bousera peridotite massifs remain particularly unclear. Due to the numerous published early Miocene ages, the emplacement of these massifs is classically considered as a very fast event that occurred just after a large-scale high-temperature metamorphic known in a large part of the Betics and part of the Rif. In most scenarios, the peridotite bodies are emplaced by overthrusting onto the continental crust within a compressional context but other models emphasize extensional context in the Alboran back-arc rift. Based on detailed field observations along the contact between the Ronda peridotites and the high-temperature continental basement and the Dorsale Unit, as well as and Raman spectroscopy on carbonaceous material and Ar/Ar dating on phlogopite, we reconsider this discussion. Indeed, our results bring new constrains on the geometry and the thermal structure of the narrow metamorphic zone in the Dorsale unit. We are considering that the peridotite - dorsale - crustal unit contact and the formation of these marbles occurred during an hyper-extension event leading to the partial exhumation of the mantle body in the upper crust. In fact, we can observe in this area a strongly localized ductile deformation with stretching lineations parallel to the contact and peridotite in contact with the Dorsale carbonates in the NE and with the Paleozoic basement in the SW. Ar/Ar ages on phlogopites from the contact marbles show that the HT-LP metamorphism and thus the extension lasted until the Early Miocene, when the massif and a large part of the Alpujarride were already exhumed. Published evidence of reworked peridotites suggests that exhumation had started before, in the Mesozoic or even the Paleozoic. At least two extensional events are thus recorded, a first during Mesozoic or Paleozoic times, linked with the rifting of the Tethys Ocean, and second during back-arc extension during the late Oligocene until the Early Miocene, before final exhumation during the renewal of crustal thickening. This study is part of the Orogen Project, an academic-industrial collaboration between CNRS, BRGM and Total

Extensional emplacement of the Ronda peridotite as recorded in the Dorsale unit

International audienceThe Betic-Rif orogen forms the westernmost part of the Alpine orogenic system and results from the closure of the Tethys Ocean between Africa and the Iberian Peninsula. Subduction and crustal thickening leading to the formation of high-pressure and low-temperature (HP/LT) rocks were followed by a late-orogenic extensional stage in an overall still convergent setting. Indeed, plate-kinematic reconstructions reveal a continuous convergence from Late Cretaceous times to the present day and characterized by slow convergence rates. If this large-scale scenario is now broadly admitted, some first order questions remain opened. Among these questions, the timing and kinematics of the emplacement of the Ronda or Beni Bousera peridotite massifs remain particularly unclear. Due to the numerous published early Miocene ages, the emplacement of these massifs is classically considered as a very fast event that occurred just after a large-scale high-temperature metamorphic known in a large part of the Betics and part of the Rif. In most scenarios, the peridotite bodies are emplaced by overthrusting onto the continental crust within a compressional context but other models emphasize extensional context in the Alboran back-arc rift. Based on detailed field observations along the contact between the Ronda peridotites and the high-temperature continental basement and the Dorsale Unit, as well as and Raman spectroscopy on carbonaceous material and Ar/Ar dating on phlogopite, we reconsider this discussion. Indeed, our results bring new constrains on the geometry and the thermal structure of the narrow metamorphic zone in the Dorsale unit. We are considering that the peridotite - dorsale - crustal unit contact and the formation of these marbles occurred during an hyper-extension event leading to the partial exhumation of the mantle body in the upper crust. In fact, we can observe in this area a strongly localized ductile deformation with stretching lineations parallel to the contact and peridotite in contact with the Dorsale carbonates in the NE and with the Paleozoic basement in the SW. Ar/Ar ages on phlogopites from the contact marbles show that the HT-LP metamorphism and thus the extension lasted until the Early Miocene, when the massif and a large part of the Alpujarride were already exhumed. Published evidence of reworked peridotites suggests that exhumation had started before, in the Mesozoic or even the Paleozoic. At least two extensional events are thus recorded, a first during Mesozoic or Paleozoic times, linked with the rifting of the Tethys Ocean, and second during back-arc extension during the late Oligocene until the Early Miocene, before final exhumation during the renewal of crustal thickening. This study is part of the Orogen Project, an academic-industrial collaboration between CNRS, BRGM and Total

New field evidence for the emplacement of the Ronda peridotite

International audienceThe Betic-Rif orogen forms the westernmost part of the Alpine orogenic system and results from the closure of the Tethys Ocean between Africa and the Iberian Peninsula. Subduction and crustal thickening leading to the formation of high-pressure and low-temperature (HP/LT) metamorphic complexes were followed by a late-orogenic extension stage in an overall convergent setting. Plate kinematic reconstructions indeed reveal a continuous convergence between Africa and Eurasia from Late Cretaceous times currently characterized by slow convergence rates that add in complex ways with body forces stored during crustal thickening stages and subsequently released during crustal thinning. If this large-scale scenario is now broadly admitted, some first order questions remain opened. Among these questions, the timing and kinematics of the emplacement of the Ronda or Beni Bousera peridotite massifs remain particularly unclear. Due to the numerous published early Miocene ages, the emplacement of the Ronda or the Beni Bousera massifs is classically considered a very fast event before the high-temperature event. In this scenario, peridotite bodies are emplaced by overthrusting onto the continental crust within a compressional context. Based on new detailed field observations along the contact between the Ronda peridotites and the high-temperature continental basement and high-temperature marbles of the Dorsale Unit, as well as a metamorphic petrology approach, we reconsider this interpretation. We argue that this contact could instead be an early detachment, possibly active during the Mesozoic or before. A few old ages found in the western part of the chain could indeed be linked with such an episode of extreme thinning. This event is consistent with the opening of the Tethyan Ocean and associated with oceanization in the eastern part of the chain. In this work, we will argue for an emplacement as old as the Triassic, at least, thus much older than the Miocene thrusting event. This study is part of the Orogen Project, an academic-industrial collaboration between CNRS, BRGM and Total

New field evidence for the emplacement of the Ronda peridotite

International audienceThe Betic-Rif orogen forms the westernmost part of the Alpine orogenic system and results from the closure of the Tethys Ocean between Africa and the Iberian Peninsula. Subduction and crustal thickening leading to the formation of high-pressure and low-temperature (HP/LT) metamorphic complexes were followed by a late-orogenic extension stage in an overall convergent setting. Plate kinematic reconstructions indeed reveal a continuous convergence between Africa and Eurasia from Late Cretaceous times currently characterized by slow convergence rates that add in complex ways with body forces stored during crustal thickening stages and subsequently released during crustal thinning. If this large-scale scenario is now broadly admitted, some first order questions remain opened. Among these questions, the timing and kinematics of the emplacement of the Ronda or Beni Bousera peridotite massifs remain particularly unclear. Due to the numerous published early Miocene ages, the emplacement of the Ronda or the Beni Bousera massifs is classically considered a very fast event before the high-temperature event. In this scenario, peridotite bodies are emplaced by overthrusting onto the continental crust within a compressional context. Based on new detailed field observations along the contact between the Ronda peridotites and the high-temperature continental basement and high-temperature marbles of the Dorsale Unit, as well as a metamorphic petrology approach, we reconsider this interpretation. We argue that this contact could instead be an early detachment, possibly active during the Mesozoic or before. A few old ages found in the western part of the chain could indeed be linked with such an episode of extreme thinning. This event is consistent with the opening of the Tethyan Ocean and associated with oceanization in the eastern part of the chain. In this work, we will argue for an emplacement as old as the Triassic, at least, thus much older than the Miocene thrusting event. This study is part of the Orogen Project, an academic-industrial collaboration between CNRS, BRGM and Total

Geodynamic evolution of the western Mediterranean basin since the Late Cretaceous

International audienceThe western Mediterranean basin results from the convergence between Africa and Eurasia and from related interactions between an undetermined number of macroplates and microplates during the Late Cretaceous. Various and very different models proposed during the last thirty years attempted to explain the geodynamic evolution of this area (e.g. Carminati et al., 2012; Schettino and Turco, 2011; Handy et al., 2010; Jolivet et al., 2006). However, none of those models are totally satisfactory, especially when regarding the Gibraltar arc region. The western Mediterranean constitutes a unique laboratory to study interactions between surface deformations and crustal and mantle processes (slab roll-back, slab break-off, delamination, etc.). The goal of this study is to understand how these deep processes are coupled to crustal evolution during the collision between Africa and Eurasia. In this context, two different approaches will be undertaken. Firstly, using GPlates software (Boyden et al., 2011), kinematic reconstructions will be performed based on stratigraphic, metamorphic, magmatic, structural and paleomagnetic data. These reconstructions will be made from the Present to the Late Cretaceous and using the principle of rigid polygons which enables deformation of areas between polygons. Realizing these reconstructions backward allows to be free from any preconceived geodynamic model. The principle of rigid polygons helps avoiding problems due to rough approximations linked to rigid blocks. Secondly, obtained results will be used to constrain 4D numerical modelling (space and time) of the western Mediterranean subduction zone evolution since the Late Cretaceous. The influence of different primordial parameters (rheological and thermal stratification of the upper and lower plates, convergence rates, presence of weak zones, etc.) will be tested. The results will be compared to natural data (surface velocity field, thermal anomalies, temporal and spatial evolution of the deformation, magmatism distribution, basin geometry) in order to understand the main processes influencing the subduction zone dynamics. The preliminary results of this study and working perspectives will be presented