Late orogenic carboniferous extensions in the Variscan French Massif Central

International audienceThe Variscan French Massif Central experienced two successive stages of extension from Middle Carboniferous to Early Permian. In the northern Massif Central, the first stage began in the late Visean, immediately after nappe stacking, and is well recorded by Namurian-Westphalian synkinematic plutonism. The Middle Carboniferous leucogranites widespread in the NW Massif Central (Limousin and Sioule area) were emplaced within a crust extending along a NE-SW direction. At the same time, the hanging wall or "Guéret extensional allochton" moved toward the SE. Several examples of the synextensional plutonism are also recognized in central Limousin: Saint Mathieu dome, La Porcherie, and Cornil leucogranites. These examples illustrate the relationship between granite emplacement and crustal scale deformation characterized by NW-SE stretching and NE-SW shortening. In the central and southern Massif Central (Cévennes, Châtaigneraie, and Margeride areas), plutonism is dominantly granodioritic and exhibits the same structural features: NW-SE maximum stretching and overturning to the SE. Middle Carboniferous (Namurian-Westphalian) extension was parallel to the Variscan belt both in the Massif Central and southern Armorican area. This extensional regime was active from the late Visean in the north, while compression dominated in the southernmost domains (Montagne Noire and Pyrenées). The second extensional stage occurred from Late Carboniferous to Early Permian. This event was responsible for the opening of intramontane coal basins, brittle deformation in the upper crust, and ductile normal faulting localized on the margin of cordierite granite-migmatite domes. Data from the coal basins show that the half-graben is the dominant structural style, except for basins located along submeridianal left-lateral faults which have pull-apart geometries. Late Carboniferous extension occurred along the NE-SW direction. The NE-SW maximum stretching direction can be found in the whole Massif Central but is more developed in the eastern part. The extensional direction is transverse to the general trend of the belt, and top-to-the-NE shearing is dominant. Correlations of these two extension directions with neighboring Variscan massifs are discussed

Transpressional tectonics and Carboniferous magmatism in the Limousin, Massif Central, France: Structural and ⁴⁰Ar/³⁹Ar investigations

New structural, microstructural, and 40Ar/39 Ar data from the NW Massif Central (France) provide additional constraints on the timing and tectonic setting of late Variscan granite magmatism. Previous studies had emphasized the role of late orogenic extension in the emplacement of granite plutons in the Limousin region. In contrast, the new data set is consistent with syntectonic emplacement of magma in a dextral simple shear active from 350 to 300 Ma in a transpressional regime. As an alternative hypothesis to late orogenic extension, we propose that magmas migrated into tensional bridges between active P shears associated with a lithospheric shear zone comparable to a pop-up structure. The Galician region, in the western end of the Ibero-Armorican tectonic arc, exhibits major left-lateral ductile shear zones which can be interpreted as conjugate structures to the Limousin and Armorican shear zones. Copyright 2007 by the American Geophysical Union

Des petits jardins dans un « grand jardin »

Set within the town and its suburbs, surrounded by road facilities and lined up with the Orly airport air routes, the territory of the small gardens studied here covers 90 hectares and is situated about 15 kms South of Paris, at the end of the Hurepoix Plateau; this territory managed to keep its qualities thanks to a constant and strong political will. This landscape is preserved, but not intact however, for its occupation has changed. If you look closely, you will see uncultivated plots, ageing fruit trees, traces of old orchards, but also signs of furtive, subsistence or pleasure gardening. The real value of the area studied here lies in its non-permanent and undetermined character. It is a welcoming place from an environmental, agro-economical and social point of view. The saltus of the piedmont plain of the Rocher at Saulx-les-Chartreux is a mixed third -party area between agriculture, gardens and nature, a half-urban half-rural territory, a living heritage which is shared and needs to be cultivated

L’enseignement primaire en Afrique Occidentale Française

Quenardel Pierre. L’enseignement primaire en Afrique Occidentale Française . In: Bulletin de psychologie, tome 5 n°2, 1951. pp. 101-103

L’autocorrection en binôme homogène d’une évaluation formative favorise-t-elle l’assimilation et l’accommodation des compétences. « Effectuer des calculs littéraux ou numériques » et « Faire un schéma adapté » ?

Our professional dissertation deals with the effect of correcting a formative classroom assessment on learning (assimilation and accommodation process) of two essential skills in the Physical and Chemical Sciences discipline : "Performing literal or numerical calculations" and "Making A suitable scheme "? Indeed, evaluation and its correction must be an integral part of student learning. The bibliographic study led us to favor the self-correction in homogeneous binomial using a " criteria of success grid" and a "model copy" (for one of the teachers) during the remediation of the formative evaluation. Through a study of five classes of Second in three different secondary schools and using a summative evaluation, we showed that self-correction in pairs has had an overall more positive effect on the learning (assimilation and accommodation) of the two skills analyzed, than individual self-correction.Notre mémoire professionnel traite de l’effet de la correction d’une évaluation formative en classe sur l’apprentissage (processus d’assimilation et d’accommodation) de deux compétences essentielles à la discipline Sciences physiques et chimiques : « Effectuer des calculs littéraux ou numériques » et « Faire un schéma adapté » ? En effet, l'évaluation et sa correction doivent faire partie intégrante de l’apprentissage des élèves. L’étude bibliographique nous a conduit à privilégier l’autocorrection en binôme homogène à l’aide d'une « grille de critères de réussite » et d'une « copie type » (pour l'un des enseignants) lors de la remédiation de l'évaluation formative. À travers une étude de cinq classes de seconde dans trois lycées différents et à l’aide d’évaluations formative et sommative, nous avons montré que l’autocorrection en binôme a eu un effet globalement plus positif sur l’apprentissage (assimilation et accommodation) des deux compétences analysées, que l'autocorrection individuelle

Le Massif Central / The central massif

The French Massif Central - Recent works on the lithological composition, the structure and the geodynamic evolution (including magmatic and metamorphic petrology, tectonics, kinematics...) of the French Massif Central have completely renewed the idea that one could have of this region some fifteen years ago. We emphasize (1) large nappes, the roots of which are unknown and (2) the superimposition of pre-Devonian, Devonian (thrust tectonics) and Carboniferous (thrust and wrench tectonics) orogenic phases. In order to present the data and to discuss some of the proposed models, this chapter is presented in the form of regional monographies. They are completed by general elements of metallogeny. A short historical review summarizes the evolution of the ideas on the Massif Central. The presently discussed questions are : direction of transport and vergence of the nappes ; occurrence of inverted gradient of metamorphism ; tectonic reconstruction in the frame of a single (or of several) collision(s) associated with oceanic closure ; and geodynamic signification of the leptyno-amphibolic complexes. These questions are also discussed within each regional approaches. In the Limousin, five main units are described ; the lithological composition and the tectonometamorphic evolution of each are presented. The structure of the Tulle region represents a late antiform, the grading up of the metamorphic grade from the flanks to the central part of the structure being the result of the overthrusting of a (relatively) low-grade gneissic unit upon a high-grade one. The importance of one (or several) oceanic opening, Cambro-Ordovician in age, is discussed. The tectonolithological succession of the Northern Massif Central have some common features with the succession of the Limousin region. One of the originality of the northern region is the intrusion of basic rocks in the Boussac domain, during Late Devonian, after the main thrust tectonic phase. The succession of Carboniferous tectonic events is well documented in the La Marche-Combrailles Shear Zone, between the Plateau d'Aigurande and the Guéret granitic massif. The Sillon Houiller appears as a shear zone marked by sedimentary basins. To the West, a blastomylonitic zone have registered a sinistral ductile deformation of Westphalian age. To the East, the border fault of the coal-basins shows a brittle deformation of Stephanian age. The Paris Basin Magnetic Anomaly is located in the northern prolongation of the Sillon Houiller. The origin of this anomaly is still unknown. Southern and eastern Massif Central are presented as a model of continental collision. The southern external margin (Montagne Noire, Albigeois, Rouergue, Cévennes) registered a foreland sedimentation ; the basement of these sediments is not known and the deformations are weaker and younger than in the internal parts. An intermediate zone is composed of metamorphic frontal klippes (Najac-Carmaux, La Bessenoits, Lévézou, Marvejols, Tanargue and Vivarais). Part of the material of these klippes could be of oceanic origin. Internal crystalline nappes (Artense, Truyère, Haut-Ailier, Lyonnais, Morvan) yielded a more complex evolution, more or less comparable with that of the Limousin and the Northern Massif Central, and showing a plurifacial metamorphism. Metamorphic units are overlain by unconformable series (Brévenne, Beaujolais, Morvan, Bourbonnais) Mid-Devonian or younger in age. The presentation of the metallogeny of the Massif Central emphasizes the importance of the geodynamic events in the controls of the mineralizations. Ore deposits are numerous and varied (U, Ba, F, Sb, Sn, Au, W, Zn). They are of post-Visean age. The material involved in the metamorphic nappes of the Massif Central is Eocambrian or Cambro-Ordovician. The age of the deformation and (eventually) of the associated metamorphism span from Silurian (HP) to Permian. The main tectonic phase of the internal domain is of Devonian age ; it is associated with Barrowian metamorphism (MP-MT). Lower Carboniferous deformations are mainly dextral ductile wrenching with subordinate thrusting, when Upper Carboniferous deformations are represented mainly by sinistral ductile, then brittle, strike slip faults and thrusts.Les travaux récents sur la nature lithologique, la structure et l'évolution géodynamique (magmatique, tectonique, métamorphique ...) du Massif Centrai ont profondément modifié l'image que l'on se faisait de cette région il y a une quinzaine d'années. Les deux faits les plus marquants mis en évidence sont l'existence de nappes de charriages, dont on ne connaît pas la zone de racines, et la superposition de phases orogéniques pré-dévonienne (métamorphisme), dévonienne (tectonique tangentielle) et carbonifères (décro-chevauchements). Afin de présenter les faits et de discuter quelques-uns des modèles proposés, ce chapitre est traité après une introduction sous forme d'analyses régionales. Il est complété par des éléments de métallogénie. Dans l'introduction, un rapide historique permet de montrer l'évolution des idées sur l'ensemble du Massif Central. Les problèmes discutés actuellement concernent : la vergence de la chaîne et les directions de transport ; le métamorphisme inverse ; les hypothèses de reconstruction dans le cadre d'une collision (ou plusieurs) associée à la fermeture d'un océan ; et la signification géodynamique des complexes leptyno-amphiboliques. Ces questions sont également discutées dans les paragraphes traitant des différents domaines. En Limousin, cinq unités principales sont décrites. La nature des séquences et leur évolution tectonométamorphique sont précisées. La structure de la région de Tulle correspond ainsi à une antiforme tardive ; l'augmentation du métamorphisme vers le coeur de la structure traduit le passage d'une nappe gneissique à une autre (et non une augmentation progressive du métamorphisme). L'importance d'une (ou de plusieurs) ouverture océanique, probablement cambro-ordovicienne, est discutée. Dans le Nord du Massif Central, on retrouve une succession tectonolithologique comparable à celle du Limousin. Une des originalités de cette région tient à l'existence de roches basiques intrudées dans le domaine de Boussac à la fin du Dévonien, après la tectonique tangentielle majeure. La succession des événements tectoniques carbonifères est bien démontrée dans la zone de cisaillement de la Marche-Combrailles, entre le Plateau d'Aigurande et le massif de Guéret. Le Sillon Houiller apparaît comme une zone de décrochement jalonnée de bassins sédimentaires. On distingue, à l'Ouest, un couloir blastomylonitique à déformation ductile senestre westphalienne et, à l'Est, la faille bordière des bassins houillers, dont le jeu fragile est d'âge Stéphanien. L'Anomalie Magnétique du Bassin de Paris se situe dans le prolongement du Sillon Houiller. L'origine de cette anomalie n'est pas encore connue. On a choisi de présenter le Massif Central méridional et oriental sous la forme d'un modèle caractéristique d'une chaîne de collision continentale. La marge externe méridionale (Montagne Noire, Albigeois, Rouergue, Cévennes) enregistre une sédimentation d'avant-pays dont on ne connaît pas le socle et où les déformations sont relativement simples et jeunes. Une zone intermédiaire est constituée de klippes frontales métamorphiques (Najac-Carmaux, La Bessenoits, Lévézou, Marvejols, Tanargue et Vivarais) dont une partie du matériel pourrait être d'origine océanique. Les nappes cristallines internes (Artense, Truyère, Haut-Allier, Lyonnais, Morvan) ont subi une évolution plus complexe ; le métamorphisme y est systématiquement plurifacial ; on retrouve ici l'évolution décrite dans le Limousin et dans le Nord du Massif Central. Les unités métamorphiques sont recouvertes par des séries discordantes postérieures au Dévonien moyen (Brévenne, Beaujolais, Morvan, Bourbonnais). La présentation métallogénique du Massif Central souligne l'importance des événements géodynamiques dans le contrôle de l'apparition des gisements. Les minéralisations sont relativement nombreuses et variées (U, Ba, F, Sb, Sn, Zn, W, Au). Leur âge est post-Viséen. Le matériel impliqué dans les nappes métamorphiques du Massif Central est éocambrien ou cambro-ordovicien. L'âge des déformations, et (éventuellement) du métamorphisme qui les accompagnent, s'échelonne du Silurien (HP) au Permien. La phase majeure est dévonienne dans le domaine interne ; elle est accompagnée d'un métamorphisme de type barrowien (MP-MT). Dans le même domaine, les déformations carbonifères inférieures sont essentiellement décro-chevauchantes dextres et ductiles, alors que les déformations du Carbonifère supérieur, également décro-chevauchantes, sont généralement senestres, ductiles puis fragiles.Quenardel Jean-Michel, Santallier Danielle, Burg Jean-Pierre, Bril Hubert, Cathelineau Michel, Marignac Christian. Le Massif Central / The central massif. In: Sciences Géologiques. Bulletin, tome 44, n°1-2, 1991. Les massifs anciens de France - I, sous la direction de Alain Piqué. pp. 105-206

Famenno-Carboniferous (370-320 Ma) strike slip tectonics monitired by syn-kinematic plutons in the French Variscan belt (Massif Armoricain and French Massif central).

International audienceThe Variscan continental collision has led to the development of large strike-slip shear zones in western Europe. Our study focuses on the regional deformation and shear zone patterns in the Massif Armoricain and the French Massif Central. The synthesis of granite emplacement ages associated to granite deformation fields, allow us to propose a geodynamic model for the tectonic evolution of this part of the Variscan belt between 370 Ma – 320 Ma (Late Devonian – Namurian). After the first steps of the continental subduction-collision, leading to high temperature and anatexis associated with N-S shortening at 380-370 Ma (Frasnian to Famennian), the southern part of the Massif Armoricain and western part of French Massif Central underwent large dextral shearing along N100-N130 trending shear zones up to early Visean time. These large-scale displacements progressively decreased at around 350-340 Ma, during the first emplacements of biotite bearing granites (Moulins-les Aubiers-Gourgé massif and Guéret massif intrusions). During middle Visean times, the shortening axis direction rotated towards a NNE-SSW direction implying changes in the regional deformation field. The occurrence of N070-N100 sinistral and N110-N130 dextral conjugate shear zones within leucogranites are related to that time. Finally, new N150-N160 dextral shear zones appeared in middle to late Visean times: as for examples, the Parthenay and the Pradines shear zones in the SE Massif Armoricain and the Millevaches massif, respectively. These shear zones were conjugated to the sinistral N020 Sillon Houiller in the French Massif Central. They reflect large scale brittle continental indentation in the French Variscan belt during the middle to late Visean