Étude gravimétrique de la ceinture archéenne de roches vertes de Kuhmo (Finlande orientale)

The Kuhmo greenstone belt (Eastern Finland) is mainly formed by mafic volcanic rocks and metasediments. Although the geological observations indicate that this belt appears as a supracrustal synform overlying a gneissic basement, its deep structures are actually unknown. The present gravity study intends to provide a better understanding of these structures. Interpretation of gravity anomalies of the Kuhmo belt, using two-dimensional modelling suggests that the structures of the belt are controlled by granitoids diapirs. Most of the calculated models show that the belt displays in section pocket-like shapes (purses) terminated by a gully-shape close to the surface. This particular geometry corroborates the hypothesis of a gravitational formation with the heavy volcanic material sinking in a light sialic crust. To cite this article: M. Jaffal et al., C. R. Geoscience 335 (2003).     

Sismique très haute résolution 3D : une nouvelle méthode d'imagerie des sols superficielsVery High Resolution 3D seismic: a new imaging tool for sub-bottom profiling

This note presents the first results of the development of a 3D Very High Resolution seismic method. Particular attention was paid to the design of an operational system to be in agreement with the expected goals in term of acquisition and processing. A set of three examples, collected in various geological environments, is presented. To cite this article: T. Marsset et al., C. R. Geoscience 334 (2002) 403–408.     

Preliminary optical and radiocarbon age determinations for Upper Pleistocene alluvial sediments in the southern Anti Atlas Mountains,MoroccoDatation des sédiments alluviaux du Pléistocène supérieur du versant sud de l'Anti-Atlas,Maroc

This paper presents the first results of a ¹⁴C and optical luminescence dating project on extensive alluvial silt formations in the southern Anti Atlas Mountains in southern Morocco. The silts comprise three members: basal silts, an inter-silt fluvial gravel member and a top silt member and are incised by the rivers. Top silt dates are: 11 340–11 940, 12 670–13 130 cal. yr BP, and 10.6±0.7 ka (OSL). OSL dates for the inter-silt gravels are 29.6±2.0 ka and for the bottom silts is 45.0±3.7 ka. The sediments correlate with Soltanian stratigraphies known elsewhere in Morocco. To cite this article: M. Thorp et al., C. R. Geoscience 334 (2002) 903–908.     

Les rhyolites hyperalcalines (pantellérites) du lac Tchad. Composition et signification tectonomagmatiqueThe peralkaline rhyolites (pantellerites) of the Lake Chad. Composition and tectonomagmatic significance

Remnants of rhyolite lava-dome and of alkaline microgranite extrusions emerge, as inselbergs, in the alluvial plain, south of Lake Chad. The peralkaline, arfvedsonite- and acmite-bearing rhyolites are determined as pantellerites. They can be related to a tectonomagmatic stage, dated of the Late Cretaceous, of the western central Africa rift system. They may constitute the NNE extension of a N50° to 30° elongated succession of small alkaline complexes, from the Gulf of Guinea to the North-Cameroon, which initiated the magmatic activity of the Cameroon Line. To cite this article: J.-P. Vicat et al., C. R. Geoscience 334 (2002) 885–891.     

Dévitrification et altération supergène d'une rhyolite (bassin du Paraná, Brésil) : mécanismes et bilans géochimiques

The weathering of a rhyolitic lava flow of the Paraná Basin (Brazil) developed, on few centimeters only, simplified petrographic features. The initial meteoric fluid pass-way is marked by celadonite-to-nontronite transformation. The second step is the complete dissolution of plagioclases, and the last one is the massive precipitation of halloysite. The geochemical mass balances according to the alteration features demonstrate the progressive leaching of major and trace elements fractionated by plagioclases, glass and opaques, and the sorption of Cu>As>Sb in the halloysite matrix. To cite this article: M. Bernard et al., C. R. Geoscience 336 (2004).     

Apport de l'imagerie satellitaire radar pour l'exploration géologique en zones arides

The potential of radar imagery in geological exploration was investigated at a study site in Mauritania (Akjoujt region). Compared with optical images, the results obtained show how radar imagery can help not only in detecting surface geological structures such as dykes and veins, but also mapping subsurface structures beneath a shallow layer of sand (palaeochannels). The mapping potential was found to be much better at long wavelengths than at short ones (L-band, compared with C- and X-band). As for optical images, their contribution is much more limited in the mapping of surface geological structures, and inappropriate for detecting subsurface structures. We conclude that spatial remote sensing enables the improvement of existing geological maps and the optimization of cartographic surveying. To cite this article: N. Baghdadi et al., C. R. Geoscience 337 (2005).     

Preuves de la discordance de l'Ordovicien supérieur dans la zone axiale des Pyrénées : exemple du dôme de la Garonne (Espagne,France)

The geological mapping of the Garonne Dome had already shown an angular unconformity between the Upper Ordovician and the underlying series. This paper presents data from a nearby location, Planelh deth Pas Estret, southeast of the Garonne Dome, where the unconformity can be seen at outcrop scale. The unconformity angles amount to up to 20°. This unconformity is interpreted as a result of an extensional event of pre-Upper Ordovician age. To cite this article: J. Garc??a-Sansegundo et al., C. R. Geoscience 336 (2004).     

Mineralogy of metallic phases in sulphide mattes from primary lead smelting

The mattes resulting from lead smelting have been studied in order to determine the distribution of heavy metals and metalloids in primary phases. The chemical analysis (EPMA) revealed the presence of various metals (Pb, Sb, Cu), sulphides (galena, wurtzite, pyrrhotite, bornite, digenite, cubanite), arsenides (koutekite, löllingite) and other complex intermetallic compounds. Extreme saturation of the initial matte melt in heavy metals and metalloids as well as a relatively rapid cooling regime are responsible for numerous elemental substitutions in the crystal structures of all the involved phases. To cite this article: V. Ettler, Z. Johan, C. R. Geoscience 335 (2003).     

Les ensembles fluviatiles néogènes du bassin subandin d'Équateur et implications dynamiquesThe Neogene fluvial systems of the Ecuadorian foreland basin and dynamic inferences

A sedimentological study of the Neogene continental infill of the Subandean foreland basin of Ecuador led us to define an evolution of the fluvial system from an alluvial plain to an alluvial fan with an increasing slope in the same time as the drainage changed from mostly longitudinal to transverse. Combined with the data presently available on palaeotopography, exhumation, tectonic evolution and geomorphology, these results enable us to infer that, in contrast with the other Subandean foreland basins of Bolivia and Peru, the progradation of the Neogene alluvial fans proceeded by an overall expansion, associated with a relatively small tectonic shortening and not as a result of the development of successive thrust-related depocentres. This also indicates that the surrection of the Cordillera progressed in Ecuador throughout the Neogene. To cite this article: F. Christophoul et al., C. R. Geoscience 334 (2002) 1029–1037.     

Subsidence et effondrements le long du littoral jordanien de la mer Morte : apports de la gravimétrie et de l'interférométrie radar différentielle

The Dead Sea shore is affected by major subsidence and sinkholes hazards due to the decrease of the sea level. The frequency of resulting accidents increased during the last four decades. Those phenomena could be at the origin of the catastrophic destruction of a major salt evaporation pond on 22 March 2000. In this paper, we show the main results of eight years of research in gravimetry and radar interferometry devoted to identify potentially hazardous areas, at different scales along the Jordanian Dead Sea coast, from the metric scale (gravimetric approach) to the kilometric one (interferometric approach). To cite this article: D. Closson et al., C. R. Geoscience 335 (2003).     

How reliable are growth strata in interpreting short-term (10 s to 100 s ka) growth structures kinematics?

High-frequency stratigraphic cycles (10 s to 100 s ka) often show, at a specific location, an alternation of ‘dynamic’ (proximal-energetic), and ‘non-dynamic’ (distal-pelagic) processes with time. When sedimentation is syn-deformation, these processes tend respectively to fill-up tectonically-induced topography or to drape it. As a consequence, growth strata are alternatively thickened and isopach across the growth structure. High-resolution kinematic studies of growth structures (folds and faults), which assume that sedimentation always fills up topographies (‘fill-to-the-top’ model), may therefore mistake sedimentary cyclicity for tectonic cyclicity. We address this problem with one example of growth anticline in the Spanish Pyrenees, and we discuss the fill-to-the-top model. To cite this article: S. Castelltort et al., C. R. Geoscience 336 (2004).     

Mesures in situ par infiltrométrie des propriétés hydrodynamiques des sols de Mnasra (Maroc)

The aim of this work is the field hydraulic characterisation of Mnasra soils in northern Morocco, which represents an essential step to study the hydraulic and chemical transports through the vadose zone. We have used a tension infiltrometer associated with a transient axisymmetric infiltration method to determine the hydraulic conductivity, which reduces the duration of measurements. This allows us to characterise a large area with many measurements. Parameters of the characteristic functions K(h) and θ(h) are estimated for six different soils belonging to two geomorphologically different domains: a sandy zone and an alluvial plain. To cite this article: K. Tamoh, A. Maslouhi, C. R. Geoscience 336 (2004).     

Homogénéisation et validation des corrections de terrain gravimétriques jusqu'à la distance de 167 km sur l'ensemble de la FranceUpdating and validating the French gravity terrain corrections out to a distance of 167 km

For regional and national study purposes, there is a high need for updating the terrain corrections (TC) in the French gravity database. We have recomputed the TC for all the French gravity stations from 50 m out to a distance of 167 km. We compute the TC with a flat-top-prism algorithm and three DEM with grid spacing of 50, 250 and 1000 m, used in the zones 53 m/3 km, 3 km/10 km et 10 km/167 km, respectively. Analysing the DEM/station Δz and comparing our results to the ones previously obtained in the Alps area, we estimate the accuracy of our TC to be better than 1 mGal. To cite this article: G. Martelet et al., C. R. Geoscience 334 (2002) 449–454.     

Définition de la perméabilité équivalente des massifs fracturés par des méthodes d'homogénéisationDefinition of the permeability of fractured rock masses by homogenisation methods

In the methods used for determination of the hydraulic behaviour at large scale of fractured rock masses, based on the simulation of flow in a fracture network, the mean flux and the mean pressure gradient in the network are not rigorously determined. A method is given for deriving these quantities, in a heterogeneous permeable block, from pressure and flux values on the boundary of the block. A block conductivity tensor is then defined, based on the condition of linear variation of the pressure on the boundary of the block. It is shown that this conductivity tensor is symmetric and positive-definite. An example of application to a model of fractured medium is given. To cite this article: A. Pouya, A. Courtois, C. R. Geoscience 334 (2002) 975–979.     

Analyse isotopique de l'air piégé dans la glace pour quantifier les variations de température

Isotopic measurements in polar ice core have shown a succession of rapid warming periods during the last glacial period over Greenland. However, this method underestimates the surface temperature variations. A new method based on gas thermal diffusion in the firn manages to quantify surface temperature variations through associated isotopic fractionations. We developed a method to extract air from the ice and to perform isotopic measurements to reduce analytical uncertainties to 0.006 and 0.020$\text{‰}$ for δ¹⁵N and δ⁴⁰Ar. It led to a 16±1.5 °C surface temperature variation during a rapid warming ($\text{?70}\text{000}$ yr). To cite this article: A. Landais et al., C. R. Geoscience 336 (2004).     

Anisotropie verticale de la perméabilité de l'horizon fissuré des aquifères de socle : concordance avec la structure géologique des profils d'altération

Pumping tests carried out in the fissured layer of a granitic hard-rock aquifer, interpreted at the observation wells by means of the analytical solution of Neuman and at the pumping wells with that of Gringarten show the existence of a strong vertical anisotropy of this layer of the aquifer; the horizontal permeability is clearly and systematically higher than the vertical one. These results agree perfectly with the geological observations, the fissured layer of the weathered granite profile showing the existence of many sub-horizontal fissures. It confirms that, within the fissured layer, the permeability of sub-horizontal fissures due to the weathering process dominates over that of sub-vertical fissures of tectonic origin. To cite this article: J.-C. Maréchal et al., C. R. Geoscience 335 (2003).To cite this article: J.-C. Maréchal et al., C. R. Geoscience 335 (2003).     

Synergie entre la télédétection multispectrale et les données de terrain pour la conception d'un nouveau modèle géodynamique d'ouverture du bassin paléozoïque des Jebilet centrales (Maroc)

The geodynamic model of the Palaeozoic basin opening of central Jebilet has been unknown before this study in spite of the abundance in the geological studies carried out in the studied sector using conventional methods. This is due to the scarcity of the key beds and synsedimentary structures. Using the synergy between the image data of the Landsat satellite TM sensor and the ground data, we have highlighted, herein, new structural data allowing the design of a new model of the Palaeozoic basin opening of central Jebilet. This opening could have been made according to dextral submeridian transverse faults with the individualization of subequatorial normal faults. To cite this article: A. El Harti et al., C. R. Geoscience 336 (2004).     

Cartographie de corps stériles sous couverture quaternaire par méthode de résistivités électriques dans le gisement phosphaté de Sidi Chennane (Maroc)Sterile bodies mapping under Quaternary cover using resistivity-sounding method in the phosphatic bearing of Sidi Chennane (Morocco)

In the Ouled Abdoun sedimentary basin (Morocco), the phosphatic series is composed of regular interbedded phosphatic and marly limestone layers. Some phosphatic deposits in this basin show sterile bodies causing two kinds of problems: (1) as they are hard, compact and masked by a Quaternary cover, they disturb the exploitation in some yards and give bad reserve calculations; (2) even the use of wells and mechanical boreholes did not evidence their delimitation. Therefore, electric prospecting method has been used to evidence their geometrical shape. Petrographical and geometrical studies on these sterile bodies allowed us to choose the appropriate geophysical method to map them. The electrical resistivity survey that we used in the Sidi Chennane area shows that this technique is a good tool to contour these sterile bodies. This method is now considered as useful to the mining engineers to get round them during the exploitation. To cite this article: A. Kchikach et al., C. R. Geoscience 334 (2002) 379–386.     

Étude expérimentale des transformations de phase dans un gradient thermique : application au granite de Soultz-sous-Forêts,France

The circulation of an injected fluid in the deep granite of the Soultz-sous-Forêts HDR (‘Hot Dry Rock’) site causes important textural and mineralogical modifications. In order to determine the potential crystallization sequences along the fluid pathway, several experiments under thermal gradient were conducted at 600 bar (between 300 and 200 °C). After 40 days, the observed sequences of the newly formed products along the thermal gradient are always as follows, from high to low temperatures: quartz, feldspars, and finally interstratified clays, this implying a significant mobility of aluminium in the solution. To cite this article: A. Baldeyrou et al., C. R. Geoscience 335 (2003).     

Hydrogeological investigation in the Mekong Delta around Ho-Chi-Minh City (South Vietnam) by electric tomographyInvestigation hydrogéologique dans le delta du Mékong,autour de la ville de Ho-Chi-Minh (Sud-Vietnam), par tomographie électrique

In the Mekong Delta (South Vietnam), the agglomeration of Ho-Chi-Minh (HCM) City, with more than 5 million inhabitants, is confronted with a dramatic shortage of fresh water supply because of the pollution of several aquifers at different depths. The electric tomography, obtained by concurrent inversion of two complementary geoelectrical methods, the Vertical Electrical Sounding (VES) and the Magneto-Telluric Sounding (MTS), turned out to be very efficient to provide a complete electrical image of the underground from the surface until about 800 m depth. This methodology constitutes a very cheap guide for the evaluation of the quality of the groundwater resources in the vast alluvial plain of the Mekong Delta. To cite this article: V.N. Pham et al., C. R. Geoscience 334 (2002) 733–740.