Implications of paleomagnetic results from the Permian Rodez basin for the late Variscan tectonics in the southern French massif Central.

A paleomagnetic study has been carried out on three sedimentary formations of the Permian Rodez basin in the southern France. Two of them yield paleomagnetic poles of Saxonian and Thuringian age showing counterclockwise rotation of moderate amplitude, during or after the Thuringian deposition. For the French Massif Central, contrary to its stable southern (Lodève basin) and eastern (Largentière basin) borders, on its southwestern border, in a large area including the Rodez, Saint-Affrique and perhaps Brive basins suffered rotations due to the extensional tectonics during the Late Variscan period

Evidence and effects of fluid circulation on organic matter in intramontane coalfields (Massif Central, France)

Recent evidence for a Late Carboniferous hydrothermal event responsible for Au-As mineralization within the Variscan belt of the French Massif Central adds a supplementary episode to the already rather complex thermal history of this area. To better understand this history, 45 coal samples from various sites in the Massif Central were studied petrographically (reflectance analysis) and geochemically (Rock-Eval pyrolysis). The results of this study suggest that the studied coal was buried to 1500 m and that the coalification took place within 25 Ma, probably ending at the boundary between the Early and Late Permian (marked by the Saalic orogeny). Two thermal end-members basins were identified: (i) the Carboniferous of Bosmoreau-les-Mines (Limousin) and West Graissessac (Montagne Noire) showing geothermal paleoflow values between 150 and 180 mW m−2, and (ii) the Stephanian of Argentat and Détroit de Rodez (SW Massif Central) with values estimated at between 100 and 120 mW m−2. By plotting the Tmax and Ro values on a diagram, the samples were grouped into two populations, the first showing a positive correlation between Ro and Tmax and the second with higher Tmax values than expected after Ro values. Selected samples of the second group are also characterized by a high Oxygen Index (OI) that increases with Tmax. These divergence between Ro and Tmax associated with a high OI may be the result of the circulation of slightly oxidizing hot fluids subsequent to coalification. The other kind of Ro-Tmax divergence seems to be linked to local, particularly high thermal activity, especially in Graissessac and Bosmoreau basins. It is interpreted as being due to a difference in response of these two maturity indicators, respectively to the intensity and duration of the thermal events (e.g., short-lived hydrothermal circulation and thermal domes of regional extent)

Cycles sédimentaires du carbonifère supérieur (Stéphanien B) du bassin de Neffies (Sud-est de la Montagne Noire, Hérault) et comparaison avec le Houiller de Saint-Rome du Tarn, Bassin de Sainte Affrique (Aveyron).

Mémoire HS n° 13 - Géologie Alpine : Le détritisme dans le Sud-Est de la France - Colloque Association des Géologues du Sud-est - Grenoble 11-12 décembre 1986Deux coupes de terrains sont décrites dans le Stéphanien des collines des Geissières et du Bousquet, immédiatement au nord de Neffiès, près de Pézenas (Hérault). Elles montrent deux cycles sédimentaires distincts

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

Infection de prothèse vasculaire artérielle profonde (particularités de la prise en charge médicale)

POITIERS-BU Médecine pharmacie (861942103) / SudocPARIS-BIUM (751062103) / SudocSudocFranceF

Erroneous coal maturity assessment caused by low temperature oxidation.

Previous work carried out on different outcrop coals from the French Massif Central revealed abnormally high Tmax values, which initially were attributed to high temperature oxidising fluids. This increase in Tmax, which was exclusively observed for medium to low volatile bituminous coals (Rr≥1.5%), was accompanied by a very clear exponential-like decrease of the hydrogen index (HI) together with an increase in the oxygen index (OI). With the aim of checking the possibility that this rise in Tmax could simply be caused by weathering, a few mature coal samples (Rr>1.4%) were experimentally oxidised at rather low temperatures (110, 125 and 150 °C) and for durations of up to 6.5 month. Then the oxidation residues were subjected to Rock–Eval 6 pyrolysis and to vitrinite reflectance measurements. These experiments effectively reproduce the geochemical changes displayed by the naturally altered coals, thus supporting the hypothesis that the previously studied natural alteration process could have been ultimately caused by weathering. The observed artificial oxidation path could be divided into two stages: (i) a strong decrease in HI and an increase in OI and (ii) a constant HI value and a slow rise in OI. The increase in Tmax seems related to the transition between the first and the second stages. This behaviour of HI and OI during artificial oxidation suggests that these two Rock–Eval derived indices can be practically used to detect a moderate and even a low oxidation degree. Finally, if one ignores the constancy of the vitrinite reflectance values all along the oxidation path, the decrease of HI and the concomitant increase of Tmax mimic a thermal evolution pathway

Erroneous maturity assessment given by Tmax and HI Rock-Eval parameters on highly weathered coals.

Two sets of mature coals have been collected at outcrop on two dipping coal seams in the Alès Carboniferous coalfield (Massif Central, France). Visual examination, petrographic observations, and Rock-Eval pyrolysis studies unequivocally suggest that these coals have been weathered, the intensity of this alteration increasing progressively towards the soil. According to the reflectance (Rr) values of 1.7–1.8%, these coals have reached low volatile bituminous rank. In contrast to the reflectance, which does not exhibit any marked variation along the studied sections, Tmax, although classically considered as a rank parameter, increases progressively with decreasing distance from the surface. This Tmax increase is accompanied by a decrease in the hydrogen index (HI) and with an increase in oxygen index (OI). All these changes are undoubtedly consequential to weathering. Thus in the present case, Tmax must not be considered as a maturity indicator, but as an oxidation parameter like OI and the O/C atomic ratio. This conclusion is also supported by the positive correlation between Tmax and proportion of oxidised vitrinites (i.e. bearing oxidation marks) provided by maceral analysis. This peculiar behaviour of Tmax in response to the weathering of mature coals is in total contrast with its invariance during low-mature coal alteration. This finding obliges us to moderate or even to revise the conclusions of a previous work on mature coals sampled at outcrop [Int. J. Coal Geol. 44 (2000) 49] where higher Tmax than expected from Rr values, were supposed to have been necessarily caused by warm (and oxidizing) fluids

From late Visean to Stephanian: pinpointing a two-stage basinal evolution in the Variscan belt. A case study from the Bosmoreau basin (French Massif Central) and its geodynamic implications

International audiencePost-convergence evolution of the Variscan belt is characterized by the development of intramontane coal-bearing basins containing volcano-sedimentary successions. In the French Massif Central, K––Ar ages on clay particles from fine-grained sediments of the Bosmoreau basin (Limousin area), help pinpoint the evolution of the basin. In the lower part of the sedimentary pile, illite in a siltstone underlying a volcanic layer previously dated at 332±4 Ma by the U––Pb method on zircon, yields a consistent K––Ar age of ca. 340 Ma. Upward in the sedimentary succession, illite yields Stephanian K––Ar ages, which can be combined to provide a mean deposition age of 296.5±3.5 Ma. The Bosmoreau basin, albeit mainly filled with Stephanian deposits, was initiated during the late Visean, i.e. ca. 30 Ma earlier than inferred from biostratigraphical constraints. During the Stephanian, the same structure was reactivated and late Visean deposits were eroded and subsequently blanketed by thick clastic sediments. These results emphasise a two-stage evolution for the Bosmoreau basin, which is closely related to extensional tectonics identified on basement country rocks, and they are used to propose a geodynamic evolution of the studied area