Extension tardi-orogénique et formation des bassins intracontinentaux: le bassin stéphanien des Cévennes

Résumé

Le bassin carbonifère des Cévennes, voisin du décrochement sénestre de Villefort, a été étudié en intégrant les données sédimentologiques, les données structurales du socle et du remplissage, ainsi que les données de la pétrologie métamorphique du socle. Le remplissage du bassin est contrôlé par des failles subméridiennes induisant une subsidence localisée en début d’ouverture. Cette subsidence devient plus régionale en fin d’histoire. Un grenat mangane- sifère est associé aux minéraux du faciès schistes verts, des blastes d’andalousite recoupent l’ensemble. Cette association minéralogique indique un gradient de température de 50°C/km. Ce gradient est semblable au gradient estimé dans le bassin sur la matière organique. La schistosité porte trois familles de linéation : 1- une famille à biotite-chlorite-quartz, orientée N45 ± 20° et associée à la mise en place des nappes cévenoles; 2- une famille à minéraux phylliteux dont l’orientation est située autour de N90 et associée à l’ouverture du bassin; 3- une famille à minéraux phylliteux orientée NO horizontale sur des plans de schistosité redressés près de la faille de Villefort. La schistosité régionale est affectée par des plis asymétriques, des bandes de cisaillement et des zones cata- clasées, le tout ayant une cinématique en faille normale vers l’Est. Les relations entre ces différentes structures suggèrent un continuum de déformation depuis des niveaux ductiles jusqu’à des niveaux fragiles. Le remplissage du bassin est affecté par des décrochements parallèles à la faille de Ville- fort. Le bassin des Cévennes s’est donc ouvert dans un contexte extensif est-ouest sur une croûte épaissie. La région a ensuite été reprise par des décrochements nord-sud dont celui de Villefort.     

Experiments and geochemical modelling of CO2 sequestration by olivine: Potential,quantification

Aqueous solutions equilibrated with supercritical CO₂ (150 °C and total pressure of 150 bar) were investigated in order to characterize their respective conditions of carbonation. Dissolution of olivine and subsequent precipitation of magnesite with a net consumption of CO₂ were expected. A quantified pure mineral phase (powders with different olivine grain diameter [20–80 μm], [80–125 μm], [125–200 μm] and [>200 μm]), and CO₂ (as dried ice) were placed in closed-batch reactors (soft Au tubes) in the presence of solutions. Different salinities (from 0 to 3400 mM) and different ratios of solution/solid (mineral phase) (from 0.1 to 10) were investigated. Experiments were performed over periods from 2 to 8 weeks. Final solid products were quantified by the Rock-Eval 6 technique, and identified using X-ray diffraction, Raman spectroscopy, electron microprobe and scanning electron microscopy. Gaseous compounds were quantified by a vacuum line equipped with a Toepler pump and identified and measured by gas chromatography (GC). Carbon mass balances were calculated.     

Calculating rates of syndepositional normal faulting in the western margin of the Mesozoic Subalpine Basin (south-east France)

A method is developed to quantify the rate of fault movement, with a very fine time-resolution, so that relevant histories of fault movements can be obtained. The study subject is a Triassic–Jurassic syndepositional normal fault located at the margin of an intracratonic deep basin, the Subalpine basin of south-eastern France. The fault has recently been identified and specifically investigated by a seismic survey along with drilling (Géologie profonde de la France Program). The investigation is based on correlation of time-lines on both sides of the structure through a period of about 70 Myr. Correlations have been made using variable approaches depending on the stratigraphic interval: recognition of laterally continuous marker-beds, biostratigraphic information and application of genetic stratigraphy concepts. In the case of biostratigraphic data, absolute ages are assigned to selected lines of correlation to determine time lengths and calculate velocities of fault movements. A specific backstripping procedure is established. The differential subsidence history between the two sites is restored not as a simple subtraction made after conventional backstripping on each site but as the sum of discrete differential subsidence increments calculated for each chronostratigraphic interval. The originality of the work lies in the completion of the supporting data base, implementation of high-resolution correlations within the large time-span of the investigation and development of a method to calculate the differential subsidence. Even though unassessable errors and uncertainties are still associated with the stratigraphic correlations, the backstripping procedure and the chronological control, the overall method offers a certain validity because the calculated and the observed differential subsidences are close. Beyond the obvious control on depositional geometries and localization of some reservoirs at the toe of the fault, the kinetic regime of the normal fault played an indirect but crucial part in the differential burial-related alteration of the reservoirs recorded on both sides of the fault. The high accuracy of the calculation has revealed that: (1) the growth pattern of the fault does not result from a continuous thermomechanical process but is composed of a series of rifting and sliding events related to gravity-driven extension; (2) the spectacular differential stratigraphic record on both sides of the fault is associated with fairly low values of the fault growth rate (maximum of 165 m Myr^?1). The method for measuring the growth of structures can be applied to any tectonic and sedimentary environment and offers a wide range of applications.