La Necropoli de Misincinis Tavagnacco

Cette publication est consacrée à l’étude de l’importante nécropole à incinération de Misincinis, située à proximité du village de Paularo, dans la vallée du Chiarso qui entaille les premiers contreforts des Alpes friulanes au nord-est de la plaine du Pô. Datée de l’âge du Fer, elle présente la précieuse particularité d’avoir été entièrement fouillée et d’avoir suscité de nombreuses études sur son mobilier et son environnement. L’ouvrage présenté ici est constitué de trois volumes, le premier..

Analyse des objets à base cuivre protohistoriques de l’Ouest de la France : signature chimique de dépôts de haches à douille de type armoricain, de haches à talon de dépôts du Bronze moyen II et d’objets de dépôts de l’horizon de l’épée en langue de carpe du Bronze final IIIb

Un programme d’analyses d’objets à base cuivre a débuté en 2007 sur le Grand-Ouest de la France. Il est ciblé actuellement sur trois horizons chronologiques fondés sur les compositions métallographiques des objets : l’épée en langue de carpe attribuée au Bronze final IIIb, les haches à douille de type armoricain et les haches à talon produites au Bronze moyen II. Ces derniers objets ont été anciennement attribués au Bronze final, mais les découvertes récentes tendent à les rattacher au premie..

Physical modelling of chemical compaction, overpressure development, hydraulic fracturing and thrust detachments in organic-rich source rock

International audienceGeological evidence for overpressure is common worldwide, especially in petroleum-rich sedimentary basins. As a result of an increasing emphasis on unconventional resources, new data are becoming available for source rocks. Abnormally high values of pore fluid pressure are especially common within mature source rock, probably as a result of chemical compaction and increases in volume during hydrocarbon generation. To investigate processes of chemical compaction, overpressure development and hydraulic fracturing, we have developed new techniques of physical modelling in a closed system. During the early stages of our work, we built and deformed models in a small rectangular box (40 40 10 cm), which rested on an electric flatbed heater; but more recently, in order to accommodate large amounts of horizontal shortening, we used a wider box (77 75 10 cm). Models consisted of horizontal layers of two materials: (1) a mixture of equal initial volumes of silica powder and beeswax micro-spheres, representing source rock, and (2) pure silica powder, representing overburden. By submerging these materials in water, we avoided the high surface tensions, which otherwise develop within pores containing both air and liquids. Also we were able to measure pore fluid pressure in a model well. During heating, the basal temperature of the model surpassed the melting point of beeswax (w62 C), reaching a maximum of 90 C. To investigate tectonic contexts of compression or extension, we used a piston to apply horizontal displacements. In experiments where the piston was static, rapid melting led to vertical compaction of the source layer, under the weight of overburden, and to high fluid overpressure (lithostatic or greater). Crosssections of the models, after cooling, revealed that molten wax had migrated through pore space and into open hydraulic fractures (sills). Most of these sills were horizontal and their roofs bulged upwards, as far as the free surface, presumably in response to internal overpressure and loss of strength of the mixture.We also found that sills were less numerous towards the sides of the box, presumably as a result of boundary effects. In other experiments, in which the piston moved inward, causing compression of the model, sills also formed. However, these were thicker than in static models and some of them were subject to folding or faulting. For experiments, in which we imposed some horizontal shortening, before the wax had started to melt, fore-thrusts and back-thrusts developed across all of the layers near the piston, producing a high-angle prism. In contrast, as soon as the wax melted, overpressure developed within the source layer and a basal detachment appeared beneath it. As a result, thin-skinned thrusts propagated further into the model, producing a low-angle prism. In some experiments, bodies of wax formed imbricate zones within the source layer. Thus, in these experiments, it was the transformation, from solid wax to liquid wax, which led to chemical compaction, overpressure development and hydraulic fracturing, all within a closed system. According to the measurements of overpressure, load transfer was the main mechanism, but volume changes also contributed, producing supra-lithostatic overpressure and therefore tensile failure of the mixture

Time scales of regional circulation of saline fluids in continental aquifers (Armorican massif, Western France)

International audienceIn recent decades, saline fluids have been sampled worldwide at great depths in continental basements. Although some of them have been attributed to marine transgressions the mechanisms allowing their circulation is not understood. In this paper, we describe the horizontal and vertical distribution of moderately saline fluids (60 to 1400 mg L−1) sampled at depths ranging from 41 to 200 m in aquifers at the regional scale of the Armorican Massif (northwestern France). The horizontal and vertical distributions of high chloride concentrations are in good agreement with both the altitudinal and vertical limits and succession of the three major transgressions between the Mio-Pliocene and Pleistocene ages. The mean chloride concentration for each transgression area is exponentially related to the time spanned until present. It defines the potential laws of leaching of marine waters by fresh meteoric waters. The results of the Armorican aquifers provide the first observed constraints for the time scales of seawater circulation in the continental basement and the subsequent leaching by fresh meteoric waters. The general trend of increasing chloride concentration with depth and the time frame for the flushing process provide useful information to develop conceptual models of the paleo-functionning of Armorican aquifers

Transpressional tectonics and Carboniferous magmatism in the Limousin, Massif Central, France: Structural and ⁴⁰Ar/³⁹Ar investigations

New structural, microstructural, and 40Ar/39 Ar data from the NW Massif Central (France) provide additional constraints on the timing and tectonic setting of late Variscan granite magmatism. Previous studies had emphasized the role of late orogenic extension in the emplacement of granite plutons in the Limousin region. In contrast, the new data set is consistent with syntectonic emplacement of magma in a dextral simple shear active from 350 to 300 Ma in a transpressional regime. As an alternative hypothesis to late orogenic extension, we propose that magmas migrated into tensional bridges between active P shears associated with a lithospheric shear zone comparable to a pop-up structure. The Galician region, in the western end of the Ibero-Armorican tectonic arc, exhibits major left-lateral ductile shear zones which can be interpreted as conjugate structures to the Limousin and Armorican shear zones. Copyright 2007 by the American Geophysical Union

Subducting slabs: jellyfishes in the Earth mantle

International audienceThe constantly improving resolution of geophysical data, seismic tomography and seismicity in particular, shows that the lithosphere does not subduct as a slab of uniform thickness but is rather thinned in the upper mantle and thickened around the transition zone between the upper and lower mantle. This observation has traditionally been interpreted as evidence for the buckling and piling of slabs at the boundary between the upper and lower mantle, where a strong contrast in viscosity may exist and cause resistance to the penetration of slabs into the lower mantle. The distribution and character of seismicity reveal, however, that slabs undergo vertical extension in the upper mantle and compression near the transition zone. In this paper, we demonstrate that during the subduction process, the shape of low viscosity slabs (1 to 100 times more viscous than the surrounding mantle) evolves toward an inverted plume shape that we coin jellyfish. Results of a 3D numerical model show that the leading tip of slabs deform toward a rounded head skirted by lateral tentacles that emerge from the sides of the jellyfish head. The head is linked to the body of the subducting slab by a thin tail. A complete parametric study reveals that subducting slabs may achieve a variety of shapes, in good agreement with the diversity of natural slab shapes evidenced by seismic tomography. Our work also suggests that the slab to mantle viscosity ratio in the Earth is most likely to be lower than 100. However, the sensitivity of slab shapes to upper and lower mantle viscosities and densities, which remain poorly constrained by independent evidence, precludes any systematic deciphering of the observations

Contraintes internes dans des scories paléométallurgiques : quantification par photoélasticimétrie et par modélisation

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Les scories des métallurgies anciennes : des analogues anthropiques pour la connaissance du devenir des déchets vitrifiés actuels

International audienceLa vitrification est l’un des procédés de stabilisation de déchets industriels ou nucléaires. Aussi, il est important de prédire leur comportement à long terme. L’étude des analogues naturels et anthropiques permet de caractériser leur altération dans diverses conditions naturelles. Les scories sont une famille d’analogues pour lesquels les temps d’altération vont de 100 à 4000 ans. Cette période est intermédiaire entre les temps obtenus par les tests en laboratoire et par l’étude des analogues géologiques tels que les basaltes. L’étude des surfaces altérées permet de déterminer les mécanismes et le taux d’altération selon les conditions d’altération. Il est aussi possible de caractériser le comportement des éléments toxiques. Dans les scories paléosidérurgiques, les éléments toxiques sont généralement en faible concentration. Les sites sidérurgiques sont nombreux. On peut ainsi étudier l’effet du temps pour une composition donnée, mais également, l’effet de la composition chimique, ou du milieu, pour une même durée d’altération. Les scories polymétalliques possèdent également une matrice silicatée mais en plus, elles présentent des teneurs très importantes en éléments toxiques qui sont très nombreux (Pb, Zn, Co, Ni, As, Sb, Sn, Cu, Ba), souvent de l’ordre de plusieurs %. L’observation des scories sidérurgiques montre que l’altération de ces matériaux correspond aux premiers stades (formation de couches altérées amorphes et d’oxy-hydroxydes) observés sur les analogues naturels (qui présentent des stades plus avancés avec formation d’argiles et de zéolites). Le taux moyens d’altération varie de 20 à 180 µm/1000ans (valeur relativement faible comparée aux roches naturelles). Ce taux semble être corrélé à l’état de polymérisation du verre. Certains éléments polluants comme Pb et Zn ne sont pas retenus dans la couche altérée et sont relâchés dans l’environnement. D’autres peuvent être, comme Sb, plutôt immobilisés

Tin production in Brittany (France) : a rich area exploited since Bronze Age

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