Réaimantations syn-cinématiques au front de la chaîne varisque franco-belge : apports à la cinématique des structures plissées-faillées

De nouvelles données paléomagnétiques ont été acquises dans le massif des Ardennes (N France-S de la Belgique), au front septentrional de la chaîne varisque, afin d’établir des contraintes temporelles sur les déformations plissées- faillées et mettre en évidence d’éventuelles rotations autour d’axes horizontaux associées à l’existence de virgations majeures le long de la chaîne. Les calcaires dévoniens et carbonifères présentent deux composantes de réaimantation portées par de la magnétite : une composante de relativement basse température enregistrée très probablement au cours des dernières phases de plissement et une composante de haute température acquise au tout début des déformations plissées. Des réaimantations post-pli et syn-pli sont également observables dans les grès rouges du Dévonien inférieur à hématite. L’âge des réaimantations, établi à partir des inclinaisons caractéristiques mesurées comparées aux courbes de référence pour les zones stables du Continent des Vieux Grès Rouges, suggère que le premier épisode régional de réaimantation résulte de l’enfouissement des séries sous l’épais bassin molassique d’avant-pays d’âge Namurien-Westphalien et que le second soit enregistré lors de l’activation finale du front de chaîne (Stéphanien d’après les données d’inclinaison) lors de la reprise hors-séquence des chevauchements majeurs. Indépendamment de l’âge des réaimantations, les directions des vecteurs paléomagnétiques sont principalement contrôlées par l’orientation des structures. Des rotations horaires d’une amplitude maximale de 40o sont localisées le long des virgations des structures chevauchantes. Les zones, présentant l’orientation classique du segment ardennais, ne montrent aucune rotation par rapport aux domaines stables du continent des Vieux Grès Rouges. Il est suggéré que la torsion des structures se développent au sein de zones transpressives dextres mises en place aux limites latérales d’écailles chevauchantes majeures, ces dernières étant probablement contrôlées par des discontinuités profondes héritées de la géométrie du bassin rhéno-hercynien. La chaîne de chevauchements des Ardennes n’a ainsi pas subi de rotation d’ensemble par rapport au continent des Vieux Grès Rouges postérieurement au Namurien, conservant l’orientation initiale de la paléo-marge continentale

Hydrogeological conceptual model of andesitic watersheds revealed by high-resolution heliborne geophysics

We conducted a multidisciplinary study at the watershed scale of an andesitic-type volcanic island in order to better characterize the hydrogeological functioning of aquifers and to better evaluate groundwater resources. A heliborne Time Domain ElectroMagnetic (TDEM) survey was conducted over Martinique in order to investigate underground volcanic structures and lithology, characterized by high lateral and vertical geological variability and resulting in a very high heterogeneity of their hydrogeological characteristics. Correlations were made on three adjacent watersheds between resistivity data along flight lines and geological and hydrogeological data from 51 boreholes and 24 springs, showing that the younger the formations, the higher their resistivity. Correlation between resistivity, geology and transmissivity data of three aquifers is attested: within the interval 10–100&thinsp;ohm&thinsp;m and within a range of 1 to 5.5&thinsp;Ma, the older the formation, the lower its resistivity, and the older the formation, the higher its transmissivity. Moreover, we demonstrate that the main geological structures lead to preferential flow circulations and that hydrogeological watersheds can differ from topographical watersheds. The consequence is that, even if the topographical watershed is small, underground flows from an adjacent watershed can add significant amounts of water to such a catchment. This effect is amplified when lava domes and their roots are situated upstream, as they present very high hydraulic conductivity leading to deep preferential groundwater flow circulations. We also reveal, unlike basaltic-type volcanic islands, that hydraulic conductivity increases with age in this andesitic-type volcanic island. This trend is interpreted as the consequence of tectonic fracturing associated with earthquakes in this subduction zone, related to andesitic volcanic islands. Finally, our approach allows characterization in detail of the hydrogeological functioning and identification of the properties of the main aquifer and aquitard units, leading to the proposition of a hydrogeological conceptual model at the watershed scale. This working scale seems particularly suitable due to the complexity of edifices, with heterogeneous geological formations presenting high lateral and vertical variability. Moreover, our study offers new guidelines for accurate correlations between resistivity, geology and hydraulic conductivity for volcanic islands. Finally, our results will also help stakeholders toward a better management of water resources.</p

The use of a combined structural, stable isotope and fluid inclusion study to constrain the kinematic history at the northern Variscan front zone (Bettrechies, northern France)

A detailed structural, stable isotope and fluid inclusion study of distinct vein generations has been performed in a kilometre-scale syncline to correlate the temperature-pressure conditions of Vein formation with specific deformation episodes. The structural relationship of the Veins with cleavage, folds, faults and with one another, allows the identification of pre-, syn- and post-Variscan carbonate vein generations. The stable isotope composition of the vein calcites and dolomites is very similar to that of the surrounding limestones and dolostones, respectively, This indicates that the cements were precipitated from fluids buffered by the host-rock and implies that the temperature-pressure characteristics of the fluids reflect the thermal history of the deformation history within the syncline. Microthermometric data of primary fluid inclusions were used to estimate the temperature-pressure conditions at precipitation time, and thus of the deformation. Pressure-corrected trapping temperatures for the veins show a temperature evolution from pre-Variscan (less than or equal to 310 degrees C) to Variscan (260-200 degrees C) and post-Variscan (75 degrees C). This evolution indicates that the syncline developed not at a specific depth, but rather as an active, progressive deformation process during transport along the northern Variscan thrust front. (C) 2000 Elsevier Science Ltd. All rights reserved.status: publishe

Geometry and kinematics of the Boulonnais fold-and-thrust belt (N France) : implications for the dynamics of the Northern Variscan thrust front

International audienc

Cerium and neodymium co-precipitation in molten chloride by wet argon sparging

International audienc

3‐D Structure of the Variscan Thrust Front in Northern France: New Insights From Seismic Reflection Profiles

International audienceIn NW Europe, the late Carboniferous Variscan collision between the Laurussia and the Armorica-Gondwana continental blocks led to the development of a crustal-scale north-verging thrust system along the southern Laurussian margin. In northern France, the 3-D geometry and kinematics of the Variscan deformation front have been investigated on the basis of reprocessing and interpreting 532 km of industry-level seismic reflection profiles. This extensive seismic imaging provides new constraints on the structural and kinematic features of the orogenic front. It particularly emphasizes the localization of displacement along the main frontal thrust zone that accommodated more than 50 km of total displacement of the allochthonous units above the foreland. It also highlights the induced large underthrusting of the foreland basin below the frontal thrust zone, and its truncation in a general out-of-sequence mode of thrust propagation. We built structural maps that led to better delineating a major NW-SE lateral ramp along the main frontal thrust. The Mid-Upper Devonian series within the flexured foreland were deformed at depth by N060–080° trending and N110–130° trending syn-sedimentary normal faults that led to their south-to southwestward thickening. These pre-existing structures along the margin have exerted a primary control on the ensuing dynamics and geometry of the Northern Variscan Front by localizing both the frontal and lateral ramps during thrust wedge growth