Metamorphic and tectonic evolution of Ceuta peninsula (Internal Rif): new interpretation in the framework of arc and back arc evolution

International audienceIn the last twenty years, various geophysical investigations have established that the Western Mediterranean opened in a subduction context as a back arc domain. In the Alboran basin the dip of the subduction plane is eastwards or southeastwards depending of considered models. If the geological records of back-arc opening are well-known, the arc-related tectonic and petrologic evolutions are still poorly documented. In order to decipher these markers, we focalised structural, petrological and thermo-chronological studies on the Ceuta peninsula located in the Rif belt, on the western part of the Gibraltar arc to the North of Morocco. The present-day tectonic pile is constituted by: (1) the upper Ceuta unit, composed of High Pressure and High Temperature metapelites retromorphosed under Amphibolite-facies condition, with Ultra-High Pressure relicts, and pyrigarnite and spinel bearing peridotites boudins at its base, (2) the lower Monte Hacho unit, with orthogneisses metamorphosed under Amphibolite-facies conditions. Structural analysis indicates a polyphase tectonic evolution: (1) an earlier deformation phase only observed in the UHP metapelites and characterized by a steep S1 foliation plane, (2) a main deformation phase associated to a pervasive gently dipping S2 foliation plane bearing a L2 stretching lineation and synschistose folds whose axes are parallel to L2 and (3) a late deformation phase which developed S3 foliation plane and L3 stretching lineation coeval with development of narrow normal ductile shear zones. A zone of increasing deformation, several dozen meters wide, is identified as a major ductile shear zone involving the peridotitic lenses at the base of the metapelites of the Ceuta unit and overlaying this upper unit on top of the orthogneisses of the Monte Hacho lower unit. The attitude of mylonitic foliation and stretching and mineral lineations as well as the numerous shear sense indicators observed in the shear zone are consistent with a thrusting toward the NE. Furthermore, biotite-sillimanite bearing S2 foliation affecting the whole of crustal rocks is contemporaneous with the movement on this main ductile thrusting. We combined garnet-biotite and GASP thermo-barometers with thermodynamic modelling (Theriak-Domino) in order to constrain pressure and temperature conditions of D2 and D3 tectono-metamorphic events. P-T conditions of D2 deformation are in the range 7-10kbar and 770-820°C and are compatible with syn-tectonic partial melting. D3 deformation event occurred at 1-7kbar and 400-550°C. These metamorphic conditions reflect abnormally high geothermal gradients during both shortening and thinning and are clearly compatible with the thermal evolution recognized in continental arcs. Preliminary U-Th-Pb (monazite, zircon and xenotime) and previous Ar39/Ar40 (micas) analyses, furnished similar ages around 21 Ma for D2 and D3 events, suggesting a very fast transition from arc to back-arc dynamics

Miocene crustal extension following thrust tectonic in the Lower Sebtides units (internal Rif, Ceuta Peninsula, Spain): Implication for the geodynamic evolution of the Alboran domain

International audienceIn Western Mediterranean, the Rif belt in Morocco is part of the Gibraltar Arc built during the Tertiary in the framework of Eurasia-Africa convergence. The structural and metamorphic evolution of the internal units of this belt as well as their timing, crucial to constrain the geodynamic evolution of the Alboran Sea, is still largely debated.Our study on the Ceuta Peninsula (Northern Rif) provides new structural, petrological and geochronological data (U-Th-Pb, Ar-Ar), which allow to precise the tectono-metamorphic evolution of the Lower Sebtides metamorphic units with:(1) a syn-metamorphic thrusting event developed under granulite facies conditions (7–10 kbar and 780–820 °C). A major thrust zone, the Ceuta Shear Zone, drove the emplacement of metapelites and peridotitic lenses from the Ceuta Upper Unit over the orthogneisses of the Monte Hacho Lower Unit. This compressional event ended during the Upper Oligocene.(2) an extensional event developed at the boundary between amphibolite and greenschist facies conditions (400–550 °C and 1–3 kbar). During this event, the Ceuta Shear Zone has been reactivated as a normal fault. Normal ductile shear zones contributed to the final exhumation of the metamorphic units during the Early Miocene.We propose that the compressional event is related to the formation of an orogenic wedge located in the upper plate, in a backward position, of the subduction zone driving the geodynamic evolution of the Alboran domain. In this context, the episode of lithospheric thinning could be related to the opening of the Alboran basin in a back-arc position.Furthermore, unlike the previous models proposed for the Rif belt, the tectonic coupling between mantle peridotites and crustal metamorphic rocks occurred in Ceuta Peninsula at a depth of 20–30 km under high temperature conditions, before the extensional event, and thus cannot be related to the back-arc extension

Metamorphic and tectonic evolution of Ceuta peninsula (Internal Rif): new interpretation in the framework of arc and back arc evolution

International audienceIn the last twenty years, various geophysical investigations have established that the Western Mediterranean opened in a subduction context as a back arc domain. In the Alboran basin the dip of the subduction plane is eastwards or southeastwards depending of considered models. If the geological records of back-arc opening are well-known, the arc-related tectonic and petrologic evolutions are still poorly documented. In order to decipher these markers, we focalised structural, petrological and thermo-chronological studies on the Ceuta peninsula located in the Rif belt, on the western part of the Gibraltar arc to the North of Morocco. The present-day tectonic pile is constituted by: (1) the upper Ceuta unit, composed of High Pressure and High Temperature metapelites retromorphosed under Amphibolite-facies condition, with Ultra-High Pressure relicts, and pyrigarnite and spinel bearing peridotites boudins at its base, (2) the lower Monte Hacho unit, with orthogneisses metamorphosed under Amphibolite-facies conditions. Structural analysis indicates a polyphase tectonic evolution: (1) an earlier deformation phase only observed in the UHP metapelites and characterized by a steep S1 foliation plane, (2) a main deformation phase associated to a pervasive gently dipping S2 foliation plane bearing a L2 stretching lineation and synschistose folds whose axes are parallel to L2 and (3) a late deformation phase which developed S3 foliation plane and L3 stretching lineation coeval with development of narrow normal ductile shear zones. A zone of increasing deformation, several dozen meters wide, is identified as a major ductile shear zone involving the peridotitic lenses at the base of the metapelites of the Ceuta unit and overlaying this upper unit on top of the orthogneisses of the Monte Hacho lower unit. The attitude of mylonitic foliation and stretching and mineral lineations as well as the numerous shear sense indicators observed in the shear zone are consistent with a thrusting toward the NE. Furthermore, biotite-sillimanite bearing S2 foliation affecting the whole of crustal rocks is contemporaneous with the movement on this main ductile thrusting. We combined garnet-biotite and GASP thermo-barometers with thermodynamic modelling (Theriak-Domino) in order to constrain pressure and temperature conditions of D2 and D3 tectono-metamorphic events. P-T conditions of D2 deformation are in the range 7-10kbar and 770-820°C and are compatible with syn-tectonic partial melting. D3 deformation event occurred at 1-7kbar and 400-550°C. These metamorphic conditions reflect abnormally high geothermal gradients during both shortening and thinning and are clearly compatible with the thermal evolution recognized in continental arcs. Preliminary U-Th-Pb (monazite, zircon and xenotime) and previous Ar39/Ar40 (micas) analyses, furnished similar ages around 21 Ma for D2 and D3 events, suggesting a very fast transition from arc to back-arc dynamics

Metamorphic and tectonic evolution of Ceuta peninsula (Internal Rif): new interpretation in the framework of arc and back arc evolution

International audienceIn the last twenty years, various geophysical investigations have established that the Western Mediterranean opened in a subduction context as a back arc domain. In the Alboran basin the dip of the subduction plane is eastwards or southeastwards depending of considered models. If the geological records of back-arc opening are well-known, the arc-related tectonic and petrologic evolutions are still poorly documented. In order to decipher these markers, we focalised structural, petrological and thermo-chronological studies on the Ceuta peninsula located in the Rif belt, on the western part of the Gibraltar arc to the North of Morocco. The present-day tectonic pile is constituted by: (1) the upper Ceuta unit, composed of High Pressure and High Temperature metapelites retromorphosed under Amphibolite-facies condition, with Ultra-High Pressure relicts, and pyrigarnite and spinel bearing peridotites boudins at its base, (2) the lower Monte Hacho unit, with orthogneisses metamorphosed under Amphibolite-facies conditions. Structural analysis indicates a polyphase tectonic evolution: (1) an earlier deformation phase only observed in the UHP metapelites and characterized by a steep S1 foliation plane, (2) a main deformation phase associated to a pervasive gently dipping S2 foliation plane bearing a L2 stretching lineation and synschistose folds whose axes are parallel to L2 and (3) a late deformation phase which developed S3 foliation plane and L3 stretching lineation coeval with development of narrow normal ductile shear zones. A zone of increasing deformation, several dozen meters wide, is identified as a major ductile shear zone involving the peridotitic lenses at the base of the metapelites of the Ceuta unit and overlaying this upper unit on top of the orthogneisses of the Monte Hacho lower unit. The attitude of mylonitic foliation and stretching and mineral lineations as well as the numerous shear sense indicators observed in the shear zone are consistent with a thrusting toward the NE. Furthermore, biotite-sillimanite bearing S2 foliation affecting the whole of crustal rocks is contemporaneous with the movement on this main ductile thrusting. We combined garnet-biotite and GASP thermo-barometers with thermodynamic modelling (Theriak-Domino) in order to constrain pressure and temperature conditions of D2 and D3 tectono-metamorphic events. P-T conditions of D2 deformation are in the range 7-10kbar and 770-820°C and are compatible with syn-tectonic partial melting. D3 deformation event occurred at 1-7kbar and 400-550°C. These metamorphic conditions reflect abnormally high geothermal gradients during both shortening and thinning and are clearly compatible with the thermal evolution recognized in continental arcs. Preliminary U-Th-Pb (monazite, zircon and xenotime) and previous Ar39/Ar40 (micas) analyses, furnished similar ages around 21 Ma for D2 and D3 events, suggesting a very fast transition from arc to back-arc dynamics

La montagne, laboratoire du changement climatique

La montagne est un territoire d’exception, réservoir de multiples ressources (bois, minerais, eau, énergie, neige, etc.), mais qui doit composer avec des handicaps naturels (pente, altitude et climat). Elle s’est ainsi construite au fil de son histoire une place spécifique au sein de l’Aménagement du territoire (RTM, Plan neige, loi « Montagne »). La question du changement climatique s’ajoute aujourd'hui aux réflexions et aux pratiques d’aménagement. Il faut alors s’interroger sur les implications de la rencontre entre un objet aux spécificités marquées et un phénomène générique, dont la représentation découle d’une appréhension globale du climat (réchauffement planétaire ou global warming) à travers des modèles prospectifs sans finesses dans leurs projections locales. L’objectif de ce numéro thématique de la RGA est d’explorer cette rencontre selon quatre domaines particuliers : écosystèmes de montagne, agropastoralisme, forêt et sylviculture, risques – naturels et économiques. Il s’agit dans ce cadre de tester l’hypothèse selon laquelle, dans un contexte marqué par le spectre du changement climatique, le droit à l’expérimentation défendu par les élus montagnards prend une dimension nouvelle, faisant de la montagne un laboratoire de l’Aménagement du territoire. Mountain areas are exceptional environments, endowed with a wide variety of natural resources (wood, minerals, water, energy, snow, etc.), but at the same time having to deal with a number of natural handicaps (slope, altitude and climate). Over the years, mountain areas have thus come to occupy a special place in spatial planning and development (RTM (Mountain Terrain Restoration programme), Plan neige (winter resort development programme), Loi Montagne (Mountain Act)). A further concern today for planners and decision-makers is the question of climate change. It is therefore important to examine the implications of the interaction between an object, the mountain area, with its distinctive characteristics and a generic phenomenon, the representation of which stems from a global understanding of climate (global warming) obtained through general prospective models, models that need to be more refined in their projections at the local level. The objective of this special issue of the RGA is to explore this interaction in terms of four special areas: mountain ecosystems, agropastoralism, forests and forestry, and natural and economic risks. This framework provides the opportunity to test the hypothesis that in a context haunted by the spectre of climate change, the right to experimentation defended by the elected representatives of mountain areas takes on a new dimension, making mountain areas a laboratory for spatial planning

Patience, prudence et petits pas, A la recherche du sens du travail social et médico-social.: L'exemple des Maisons de la métropole du territoire de Vénissieux - Saint-Fons

International audienceCet ouvrage de "sociologie dessinée" est le fruit d'une recherche-action mené par le Collectif Métis qui a rassemblé et fait dialoguer pendant deux ans une équipe de sociologues, un illustrateur, des praticiens du travail social et des agents d'encadrement

Learning from the past to build the future governance of groundwater use in agriculture

International audienceThe use of groundwater is increasing worldwide, particularly in agriculture. This leads to pumping races (from which the poorest farmers are often excluded), environmental disasters and the degradation of groundwater quality. Based on discussions between scientists and operational experts in two workshops held in 2018 and 2020, this paper, after taking stock of the dynamics and motivations of groundwater use in agriculture, reviews the solutions most commonly proposed, in particular by public authorities, to regulate the access to and use of this resource and to limit its overexploitation. These (often optimistic) solutions generally combine regulatory or economic instruments, or indirect measures linking water to other issues, and mechanisms based on the participation of all users. However, they rarely question the intensive agricultural systems driving groundwater demand and fail to recognise the multifunctional nature of groundwater. To overcome these hurdles we draw up possible ways forward for policy makers and resource users to develop negotiated solutions