Evolution du rift du Massif central : distribution spatio-temporelle du volcanisme

International audienceThe Massif Central area is the largest magmatic province of the West-European Rift system.The spa­tial-temporal distribution of Tertiary-Quaternary volcanism in the Massif Central, France, shows that three magmatic phases can be defined, each of them characterized by different volumes and different locations. The first event, termed the pre-rift magmatic event, is very scarce and restricted to the north of the Massif Central. It is suggested that this could result from lithospheric bending of the European lithosphere ahead of the incipient Alpine chain during the Pa­leocene. The second event, termed the rift-related magmatic event, is located in the north of the Massif Central only and is spatially connected with zones of high crustal thinning (i.e. the Limagne graben). It immediately follows Oligo­cene graben formation and associated sedimentation, and is represented by more than 200 scattered monogenic edifices. This second event can be attributed to partial melting as a consequence of lithospheric thinning that affected the north of the Massif Central during the rifting event. The lack of volcanism in the south during the same period of time is probably related to the very slight lithospheric thinning during the Oligocene. The third event, termed the major magmatic event, started first in the South in the upper Miocene at about 15 Ma, well after the end of the sedimentation. lt is unrelated to any extensional event. This major magmatic event reached the North of the Massif Central at about 3.5 Ma, following a pause in volcanism of about 6 Ma after the rift-related magmatic event. These two episodes of the ma­jor magmatic event are spatially and temporally associated with the two main periods of uplift, suggesting a common origin of volcanism and uplift processes. The major magmatic event can be attributed to late thermal erosion of the base of the lithosphere above a mantle diapir, as suggested by seismic tomography data. This general magmatic evolution drawn from data at the Massif Central scale may apply to the Eger graben as well, as the three magmatic events described in this study (pre-rift magmatic event, rifting event and post-Miocene volcanic event) are also reported in the literature. This suggests that a single cause should explain the formation of the entire western European rift surroun­ding the Alpine mountain belt.Le rift Ouest-européen correspond à un épisode d'extension lithosphérique qui s'est produit de )'Eocène supérieur jusqu'au Miocène inférieur. La direction d'extension est globalement perpendiculaire au front de la chaîne alpine et s'exprime, d'est en ouest, par la formation du graben de l'Eger, du graben du Rhin et des fossés d'effondrement du Massif central. Le Massif central est la plus importante province magmatique liée à cet épisode de rifting. Le volcanisme de cette province peut être séparé en trois épisodes successifs. 1. Episode de magmatisme pré-rift. Cet épisode correspond à 15 localités répertoriées presque exclusivement dans le nord du Massif central et datées du Paléocène à la fin de !'Eocène. 2. Episode de magmatisme syn-rift. La sédimentation oligocène s'est effectuée à un niveau proche de celui de la mer et pratiquement sans manifestation volcanique. A l'échelle du rift (i.e. de la Bresse à la Limagne), l'extension symétrique de l'Eocène supérieur à l'Oligocène moyen est devenue asymétrique à partir de l'Oligocène supérieur. L'épisode de magmatisme a débuté à ! 'Oligocène supérieur et s'est principalement développé au Miocèneinfërieur, pendant une quinzaine de millions d'années. Il est spatialement lié aux zones d'amincissement crustal maximal(fossé de la Limagne) et est absent de la partie sud du Massif central où les récentes données géophysiques montrentque l'amincissement crustal est négligeable. 3. Episode de magmatisme majeur: Cet épisode a démarré au sud du Massif central près de 15 Ma d'années après la fin de la sédimentation oligocène. C'est l'épisode majeur avec le développement des grandes provinces magmatiques du Cantal, du Velay ou de l'Aubrac. Une reprise plus tardive du volcanisme se produit dans la partie nord du Massif central, près de 6 Ma après la fin de l'épisode précédent dit syn-rift. Dans son ensemble, cet épisode majeur est caractérisé par deux pics d'activité: de 9 à 6,5 Ma uniquement au sud du Massif central, puis de 3,5 à 0,5 Ma tant au nord qu'au sud du Massif central.L'analyse du MNT permet de montrer que le nord du Massif central présente un champ de structures dominé par des failles nord-sud. Il s'agit de failles d'âge varisque réactivées en faille normale pendant l'extension et la création des fossés d'effondrement. L'étude des profils d'équilibre des rivières au passage de certaines failles ainsi que l'âge des coulées de lave actuellement en position de reliefs inversés montrent que le soulèvement dans cette partie nord a débuté il y a environ 3 Ma et se poursuit actuellement. Au sud, le MNT révèle un champ de failles dominant orienté N 135°E, souligné en particulier par des alignements volcaniques tels l'Aubrac, le Velay ou le Dcves. Cette direction majeure a été acquise pendant le soulèvement de la partie sud qui a débuté il y a environ 10 Ma, bien avant le soulèvement plus récent de la partie nord. Ce soulèvement s'est ralenti entre 5,5 Ma et 3-3,5 Ma pour redevenir très actif depuis cette période jusqu'à l'actuel, comme dans la partie nord. L'épisode magmatique pré-rift, extrêmement limité en volume, est attribué à une flexure de la lithosphère européenne au moment des premières compressions alpines pendant le Paléocène. Cette flexure de la lithosphère est encore apparente à l'échelle de la France grâce aux données de géophysique ou de géomorphologie dans le Massif central el le Morvan. L'épisode magmatique syn-rift, restreint au nord du Massif central. est clairement associé aux zones d'amincissement crustal maximal résultant principalement de l'extension asymétrique E-W oligocène supérieur à miocène inférieur. Cet amincissement lithosphérique permet d'expliquer, par décompression du manteau, le faible taux de fusion partielle nécessaire pour rendre compte du volcanisme, localisé au nord. de !'Oligocène supérieur au Miocène inférieur. L'orientation N-S dominante observée sur le MNT est un héritage de cette période d'extension E-W où les failles sub-méridiennes d'âge varisque ont été réactivées. L'épisode magmatique majeur est caractérisé par deux pics d'activité qui sont synchrones des deux périodes de soulèvement maximal: au sud vers 10-5,5 Ma, puis au nord et au sud à partir de 3-3,5 Ma. Au sud. la radiographie de la croûte et de la limite lithosphère-asthénosphère obtenue par sismique classique et tomographie sismique montre que le manteau lithosphérique a subi un très fort amincissement tandis que l'épaisseur de la croûte est quasi-normale. L"anomalie thermique définie sous la lithosphère dans cette partie sud témoigne alors d'une érosion thermique de la base de la lithosphère, responsable du soulèvement isostatique et du premier pic d'activité magmatique. Un second épisode d'érosion thermique à la base de la lithosphère. plus diffus mais réparti du nord au sud, expliquerait la seconde période de soulèvement isostatique ainsi que le second pic d'activité magmatique enregistrés au nord et au sud à partir de 3.5 Ma

The formation of the West European Rift; a new model as exemplified by the Massif Central area

International audienceIn this paper, we use mainly field data from the Massif Central area, which have been presented in a com­panion paper (Michon and Merle, 2001), to discuss the origin and the evolution of the West European Rift system. It is shown that the tectonic event in the Tertiary is two-stage. The overall geological evolution reveal a tectonic paradoxe as the first stage strongly suggests passive rifting, whereas the second stage displays the first stage of active rifting. ln the North, crustal thinning, graben formation and sedimentation at sea level without volcanism during the Lower Oligo­cene, followed by scattered volcanism in a thinned area during Upper Oligocene and Lower Miocene, represent the classical evolution of a rift resulting from extensional stresses within the lithosphere (i.e. passive rifting). In the South, thinning of the lithospheric mantle associated with doming and volcanism in the Upper Miocene, together with the lack of crustal thinning, may be easily interpreted in terms of the first stage of active rifting due to the ascent of a mantle plume. This active rifting process would have been inhibited before stretching of the crust, as asthenospheric rise associated with uplift and volcanism are the only tectonic events observed. The diachronism of these two events is emphasized by two clearly distinct orientations of crustal thinning in the north and mantle lithospheric thinning in the south. To understand this tectonic paradox, a new model is discussed taking into account the Tertiary evolution of the Alpine chain. lt is shown that the formation of a deep lithospheric root may have important mechanical consequences on the adjacent lithosphere. The downward gravitational force acting on the descending slab may induce coeval exten­sion in the surrounding lithosphere. This could trigger graben formation and laguno-marine sedimentation at sea level followed by volcanism as expected for passive rifting. Concurrently, the descending lithospheric flow induces a flow pattern in the asthenosphere which can bring up hot mantle to the base of the adjacent lithosphere. Slow thermal ero­sion of the base of the lithosphere may lead to a late-stage volcanism and uplift as expected for active rifting

Discussion on "Evolution of the European Cenozoic Rift System: interaction of the Alpine and Pyrenean orogens with their foreland lithosphere" by P. Dèzes, S.M. Schmid and P.A. Ziegler, Tectonophysics 389 (2004) 1–33

International audienceThe evolution and origin of the European Cenozoic Rift System (ECRIS) is a matter of debate for several decades (e.g., Tapponnier, 1977; Bergerat, 1987; Ziegler, 1992; Michon et al., 2003). This rift system was characterized by the development of several grabens in the Pyrenean and Alpine forelands and by a magmatic activity starting at the K/T transition. Dèzes et al. (2004) propose an additional reappraisal and interpret the ECRIS formation and the associated volcanism as resulting from the Alpine and Pyrenean collision and the emplacement of a mantle plume at depth below western Europe. Our remarks on this paper will be focused on three different topics which make the final conclusions of Dèzes et al. (i.e., origin of the extension in the ECRIS) highly questionable

Mode of lithospheric extension: Conceptual models from analogue modeling

International audienceComparison of analogue experiments at crustal and lithospheric scale provides essential information concerning the mode of deformation during lithospheric extension. This study shows that during extension, lithospheric deformation is controlled by the development of shear zones in the ductile parts. At lithospheric scale, the global deformation is initiated by the rupture of the brittle mantle lithosphere. This failure generates the formation of conjugate and opposite shear zones in the lower crust and the ductile mantle lithosphere. The analysis of the internal strain of the ductile layers suggests that the two opposite shear zones located below the asymmetric graben in the lower crust and the ductile mantle lithosphere prevail. Experiments show that from a similar initial stage, the relative predominance of these shear zones originates two different modes of deformation. If the crustal shear zone prevails, a major detachment-like structure crosscuts the whole lithosphere and controls its thinning. In this model named the simple shear mode, the resulting geometry shows that crustal and lithospheric thinning are laterally shifted. If the mantle shear zone predominates, the lithospheric thinning is induced by the coeval activity of the two main shear zones. This process called the necking mode leads to the vertical superposition of crustal and mantle lithospheric thinning. Applied to natural laboratories (West European rift, Red Sea rift and North Atlantic), this conceptual model allows a plausible explanation of the different geometries and evolutions described in these provinces. The North Atlantic and the Red Sea rift systems may result from a simple shear mode, whereas the necking mode may explain part of the evolution of the West European rift especially in the Massif Central and the Eger graben

La place d'un intervenant extérieur à l'école, le CPIE Pays-Gersois, dans la pratique et l'apprentissage de la démarche d'investigation

La démarche d'investigation est préconisée dans les programmes pour l'école élémentaire de 2008 dans les termes suivants : « Les connaissances et les compétences seront acquises dans le cadre d'une démarche d'investigation qui développe la curiosité, la créativité, l'esprit critique et l'intérêt pour le progrès scientifique et technique. » Cette démarche et son enseignement étant placés au cœur de l'apprentissage des sciences expérimentales et technologiques, la question posée ici est de savoir si un partenaire tel que le CPIE peut permettre aux élèves de la pratiquer et de l'acquérir dans le cadre des interventions faites en classe. Après une présentation de cette démarche et du partenaire CPIE Pays-Gersois, l'étude s'articule autour de trois outils : un questionnaire auprès d'enseignants ayant accueilli les animations du CPIE, un entretien mené auprès des trois animateurs permanents de cette structure et l'observation de séances en classe

How summit calderas collapse on basaltic volcanoes: New insights from the April 2007 caldera collapse of Piton de la Fournaise volcano

International audienceIn April 2007, Piton de la Fournaise volcano experienced a caldera collapse during its largest historical eruption. We present here a structural analysis both of the caldera and the surrounding area, and precise GPS data recorded with a dense GPS network specifically dedicated to the analysis of deformation related to the summit collapse structures. Despite a collapse of more than 300 m in the central zone, the geometry of the new caldera is similar in map view to that of the pre-existing collapsed structure, which was formed from the coalescence of several pit craters. The caldera shows an asymmetric inner geometry with sub-vertical walls in the NW quadrant and steep scarps composed of inward tilted blocks in the southern half. The presence of preserved polished surfaces on the lower part of the sub-vertical scarp indicates that it corresponds to the caldera north-western ring fault. The April 2007 caldera collapse led to the development and the reactivation of concentric fractures on the caldera rim, mostly along the southern limit of the caldera. GPS data show that fractures result from radial extensional stresses that are restricted within the first tens of meters of the caldera edge. GPS data also reveal that the caldera collapse was coeval with a centripetal deflation, whose magnitude is largest along the southern half of the caldera. The displacements recorded by GPS result from both a general deflation, due to magma withdrawal from Piton de la Fournaise's summit magma chamber, and additional local effects related to the caldera collapse. Comparison of the caldera collapses at Piton de la Fournaise, Miyakejima and Fernandina reveals striking similarities, with cyclic seismic signals accompanying small-scale deflation–inflation cycles. This strongly suggests a common mode of collapse. Hence, we propose a unifying model of caldera collapse in basaltic setting, in which the inward deflation due to magma withdrawal from the magma chamber prevents the collapse of the caldera roof until the gravitational stress acting on the rock column above the magma chamber exceeds the shear strength along pre-existing ring faults. The downward displacement stops when the pressure increase into the magma chamber is able to again sustain the rock column. The succession of (1) inward deflation that prevents the collapse, (2) collapse due to gravitational stress and (3) stopping of the downward motion is repeated many times. The frequency of the cycles is influenced by the rate of magma withdrawal and by the amount of intrusion of magma along the ring faults

The Cenozoic evolution of the Roer Valley Rift System integrated at a European scale

International audienceThe Roer Valley Rift System (RVRS) is located between the West European rift and the North Sea rift system. During the Cenozoic, the RVRS was characterized by several periods of subsidence and inversion, which are linked to the evolution of the adjacent rift systems. Combination of subsidence analysis and results from the analysis of thickness distributions and fault systems allows the determination of the Cenozoic evolution and quantification of the subsidence. During the Early Paleocene, the RVRS was inverted (Laramide phase). The backstripping method shows that the RVRS was subsequently mainly affected by two periods of subsidence, during the Late Paleocene and the Oligocene–Quaternary time intervals, separated by an inversion phase during the Late Eocene. During the Oligocene and Miocene periods, the thickness of the sediments and the distribution of the active faults reveal a radical rotation of the direction of extension by about 70–80j (counter clockwise). Integration of these results at a European scale indicates that the Late Paleocene subsidence was related to the evolution of the North Sea basins, whereas the Oligocene–Quaternary subsidence is connected to the West European rift evolution. The distribution of the inverted provinces also shows that the Early Paleocene inversion (Laramide phase) has affected the whole European crust, whereas the Late Eocene inversion was restricted to the southern North Sea basins and the Channel area. Finally, comparison of these deformations in the European crust with the evolution of the Alpine chain suggests that the formation of the Alps has controlled the evolution of the European crust since the beginning of the Cenozoic

Axisymmetric Tokamak Equilibria computed with a Predefined Safety Factor Profile as CHEASE input

The CHEASE code, which has been developed at the Swiss Plasma Center, generates an accurate reconstruction of toroidal magnetohydrodynamics equilibria by numerically solving the Grad-Shafranov equation. Having demonstrated its ability to achieve good convergence while remaining fast and very flexible, this code is now extensively used at different research facilities. From two input profiles, usually the pressure and the current density as well as specified boundary conditions, the code provides a complete equilibrium description which is essential for the study of tokamak plasmas. This project focuses on computing axisymmetric equilibria within ideal magnetohydrodynamics (MHD) by imposing a specific shape for the safety factor $q$. The Grad-Shafranov equation requires two free functions to be specified: one for the current density and one for the pressure (p=nT). Until now, the options in CHEASE allowed to give the pressure profile or its derivative on one hand and either $TT'$, $I^*$, $I_{\parallel}$, or $J_{\parallel}$ on the other hand. The safety factor profile was therefore a result of the computation of the equilibrium. However, this profile and its radial derivative are essential for stability and transport issues into the tokamak. The goal of the present project is thus to modify the code so as to be able to provide a safety factor profile as a CHEASE input. However, equilibria generated with the safety factor profile as input can easily lead to surface currents if strong variation of the derivative $\frac{dq}{d\rho}$ appears in the solution. It is therefore necessary to develop a method to avoid these problems, especially at the edge of the plasma. A further aim of this work is hence to demonstrate that there is no continuity problem within solutions computed with a q-profile as input. There are two ways of achieving this: one is to impose the current profile within one iteration to obtain the desired safety factor profile and the other is to impose the $TT'$ profile in the same way. The two solutions are explored in this project with the purpose to determine which one is most appropriate. Finally, the extended version of CHEASE developed in this master project is used to study the influence of safety factor profiles on the stability of tearing modes by solving the energy principle equation within the cylindrical approximation which depends directly on $q$, $q'$ and $q''

Approche expérimentale de la tectonique de l'Etna

International audienceL'importance relative de la tectonique régionale et de l'étalement gravitaire sur la stabilité de l'Etna sont étudiés à l'aide de modèles analogiques sable-silicone dimensionnés. Les résultats de cette modélisation montrent notamment que les structures actives observées sur le volcan sont la conséquence d'une interaction entre effets gravitaires et tectonique régionale. En particulier, les composantes décrochantes des failles observées à la fois sur le terrain et sur les modèles réduits sont liés à l'interaction entre le champ de contrainte gravitaire et le champ de contrainte extensif régional. Nous montrons également dans nos modèles que la Valle del Bove, structure majeure du flanc oriental de l'Etna, ne se forme que sous l'influence conjuguée de l'effondrement gravitaire du volcan et du retrait du panneau plongeant ionien suivant des failles majeures

Nodular posterior scleritis mimicking choroidal metastasis: a report of two cases

Posterior scleritis is a rare underdiagnosed condition that can potentially cause blindness. Its varied presentations lead to delayed or incorrect treatment. We present here the cases of two patients with nodular posterior scleritis mimicking a choroidal metastasis. Two female patients presented with a sudden unilateral visual loss associated with ocular pain. Fundus examination revealed temporomacular choroidal masses with exudative detachments that, due to angiographic presentation, were suggestive of choroidal metastasis. Systemic examinations were unremarkable. In the two cases, a local or general anti-inflammatory treatment led to the complete recovery of the lesions, which were, thus, considered nodular posterior scleritis. The diagnosis of nodular posterior scleritis has to be evoked in all patients presenting with a choroidal mass in fundus examination. It represents the principal curable differential diagnosis of malignant choroidal tumor