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

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

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

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

DEM-based model for reconstructing volcano's morphology from primary volcanic landforms

International audienceVolumes of magma intruded in and emitted by volcanoes through time can be estimated by reconstruction of vol-cano's morphology and time sequence. Classical approaches for quantifying magma volumes on active volcanoes are based on the difference between pre-and post-eruption digital elevation models (DEM), but this kind of approach needs the pre-eruptive surfaces to be available. For old and eroded volcanoes these surfaces are poorly constrained. However, because the geometrical form of many volcanic edifices exhibits a remarkable symmetry we propose, here, a new approach using primary volcanic landforms in order to estimate the amount of the both erupted and eroded material and to locate eruptive centers. A large fraction of composite volcanoes have near constant slope on their flanks and a form that is concave upwards near their summits. But many phenomena can lead to non-symetrical edifices and complex morphologies can result, for example from parasitic centers of volcanism on the flanks, from alternation of short effusive and explosive construction phases, from flank or caldera collapses, or from glacial and other types of erosion. In this study we propose that, on the first order approximation, complex morphologies can be modeled by piling regular cones. In this model, cones centers and slopes are derived by fitting primary volcanic landform with a linear function :elevation=f(distance from center). Such an approach allows to estimate both errors on location of the eruptive center and on the volume of the resulting cones. This model can then be used for quantifying volume of erupted and eroded material, and for quantifying catastrophic events as giant landslides or flank collapse. This approach is tested on four different active volcanoes : Mount Mayon (Philippines), Mount Fuji (Japan), Mount Etna (Sicily) and Mount Teide (Canary Island) to estimate errors in volume between modeled and actual edifices. It is then used on volcanoes of La Réunion hotspot to reconstruct the Piton des Neiges and Piton de la Fournaise volcanoes at its different stages of growing

Basaltic calderas: Collapse dynamics, edifice deformation, and variations of magma withdrawal

International audienceThe incremental caldera collapses of Fernandina (1968), Miyakejima (2000), and Piton de la Fournaise (2007) are analyzed in order to understand the collapse dynamics in basaltic setting and the associated edifice deformation. For each caldera, the collapse dynamics is assessed through the evolution of the (1) time interval T between two successive collapse increments, (2) amount of vertical displacement during each collapse increment, and (3) magma outflow rate during the whole collapse caldera process. We show from the evolution of T that Piton de la Fournaise and Fernandina were characterized by a similar collapse dynamics, despite large differences in the caldera geometry and the duration of the whole collapse caldera process. This evolution significantly differs from that of Miyakejima where T strongly fluctuated throughout the whole collapse process. Quantification of the piston vertical displacements enables us to determine the magma outflow rates between each collapse increment. Displacement data (tiltmeter and/or GPS) for Piton de la Fournaise and Miyakejima are used to constrain the edifice overall deformation and the edifice deformation rates. These data reveal that both volcanoes experienced edifice inflation once the piston collapsed into the magma chamber. Such a deformation, which lasts during the first collapse increments only, is interpreted as the result of larger volume of piston intruded in the magma chamber than magma withdrawn before each collapse increment. Once the effect of the collapsing rock column vanishes, edifice deflates. We also determine for each caldera the critical amount of magma evacuated before collapse initiation and compare it to analog models. The significant differences between models and nature are explained by the occurrence of preexisting weak zones in nature, i.e., the ring faults, that are not taken into account in analog models. Finally, we show that T at Piton de la Fournaise and Fernandina was equally controlled by the frictional resistance along the ring faults and the magma outflow rate. In addition to these two parameters, the collapse dynamics of Miyakejima was also influenced by variations of the magma bulk modulus, which changed after the influx of deep gas-rich magma into the collapse-related magma chamber. Altogether, our results show that the dynamics of caldera collapse in basaltic volcanoes proceeds in two phases: Phase 1, starting with the first collapse, is characterized by the largest collapse amplitude, an incremental edifice inflation, and a step-by-step increase of the rate of magma outflow. Phase 2 shows a rapid decrease of the magma discharge rate to a low level concomitant with the continuous edifice deflation. If deep magma is injected into the magma chamber, as at Miyakejima, an additional phase occurs (phase 3)

Geology and morphostructural evolution of Piton de la Fournaise

International audienceThe morphology of Piton de la Fournaise volcano results from the succession of construction, destruction and deformation processes that occurred since at least 530 ka. The chaotic surface of the gently dipping submarine flanks indicates that volcaniclastic deposits related to massive flank landslides and erosion cover most of the submarine flanks. Only a few seamounts like Cône Elianne and the submarine continuation of the rift zones are built by lava flows. In the subaerial domain, Piton de la Fournaise exhibits deeply incised canyons evidencing intense erosion and eastward verging scarps whose origin is still controversial. The different interpretations invoking flank landslides and/or summit collapse calderas are summarized. Geological data indicate a twofold construction of Piton de la Fournaise. Between 530 and 60 kyrs, the volcanic centre located in the current Plaine des Sables led to the building of the western part of the massif. The volcanic centre migrated eastwards to its current location, possibly at 60–40 kyrs. Then Piton de la Fournaise experienced caldera collapses and recurrent phreatomagmatic eruptions especially between 4880 and 2340 yr BP as evidenced by the Bellecombe ash deposit. Most of the recent volcanic activity is now currently focused restricted inside the Enclos Fouqué caldera where lava flow accumulation and rare explosive events built the 400-m-high Central Cone

Volcans de la Chaîne des Puys (Massif Central, France) : point sur la chronologie Vasset-Kilian-Pariou-Chopine

ThermoluminescenceInternational audienceLa compilation des datations radiocarbone de bois carbonisés par leurs déferlantes basales, complétée par des observations téphrochronologiques, permet d'avancer que le puy Chopine, il y a environ 9700 ans, a précédé le Vasset et le Kilian, tous deux péné-contemporains, vers 9400-9300 ans. Les produits du Nouveau Pariou sont recouverts par ceux d'un volcan trachytique, probablement le Kilian. Sous les produits explosifs initiaux du Nouveau Pariou (faciès "Traversin"), les trachytes à amphibole qui avaient été attribués au Kilian, sont vraisemblablement une forme méconnue des trachytes de la phase acide du Pariou lui-même. L'ordre chronologique des éruptions serait donc : Chopine/Pariou/?Vasset?/Kilian, la position du Vasset, hypothétique, restant à confirme

The 2007 eruptions and caldera collapse of the Piton de la Fournaise volcano (La Réunion Island) from tilt analysis at a single very broadband seismic station

International audienceSeismic records from La Réunion Island very broadband Geoscope station are investigated to constrain the link between the 2007 eruptive sequence and the related caldera collapse of the Piton de la Fournaise volcano. Tilt estimated from seismic records reveals that the three 2007 eruptions belong to a single inflation-deflation cycle. Tilt trend indicates that the small-volume summit eruption of 18 February occurred during a phase of continuous inflation that started in January 2007. Inflation decelerated 24 days before a second short-lived, small-volume eruption on 30 March, almost simultaneous with a sudden, large-scale deflation of the volcano. Deflation rate, which had stabilized at relatively low level, increased anew on 1 April while no magma was erupted, followed on 2 April by a major distal eruption and on 5 April by a summit caldera collapse. Long-term tilt variation suggests that the 2007 eruptive succession was triggered by a deep magma input

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