5: Late Variscan mineralizing systems related to orogenic processes: The French Massif Central

The French Massif Central constitutes an exceptional study area due to the diversity of its metallic deposits, its internal position in the Variscan belt, and the abundance of available geological, geophysical and metallogenic data obtained within the GeoFrance 3D programme. The deposits, formed towards the end of the orogenic evolution, represent the economic products of two distinct mineralizing systems, a Au ± Sb hydrothermal system and a W ± Sn and rare-metals magmatic–hydrothermal system, which were simultaneously active during a short time span between ca. 310 and 300 Ma.Two types of gold deposit can be distinguished on the basis of their depth of emplacement: “deep-seated” gold deposits developed under lithostatic to hydrostatic pressure during rapid exhumation, and “shallow” gold deposits emplaced under hydrostatic pressure with no significant uplift.Deposits of W ± Sn and rare-metals were emplaced in the upper crust during final crystallization of specialized magmas after their rapid ascent, perhaps enhanced by simultaneous regional uplift. The gold-bearing systems are associated with a complex network of re-activated crustal-scale faults initially active during the period between 335 and 315 Ma. Normal motion along the faults, coeval with 335 to 315 Ma granite–migmatite domes, played a major role in the 3D distribution of the hydrothermal plumbing system. Gold and related metals were carried within huge hydrothermal cells, which reached ca. 100 km by 10 km in area, and 30 km in depth. In contrast, granites rich in magmatophile elements (W, Sn, rare-metals) generated smaller hydrothermal cells (10 km by 10 km in area, and < 6 km deep). Extraction of metals, by both deep-seated fluids and specialized magmas, occurred during granulitization of the lower crust at 300 ± 15 Ma. In the French Massif Central, the genesis of the two late Carboniferous mineralizing systems coincided with the end of syn-collisional extension and ended just before post-collisional extension.     

Impacts of a Ghg-Induced Climate Change on Crop Yields: Effects of Acceleration in Maturation, Moisture Stress and Optimal Temperature

The present study involves using the Canadian Climate Centre (CCC) climate change scenario to evaluate the impacts of a CO₂-induced climate change on agriculture in Québec and vicinity. Climate change using the CCC General Circulation Model (GCM) data are fed into a crop model (FAO) so as to gauge the changes in agroclimatic factors such as growing season length and growing degree days, and subsequently potential yield changes for a variety of cereal (C₃ and C₄), leguminous, oleaginous, vegetable and special crops, for twelve major agricultural regions in southern Québec. Our results show that depending upon the agricultural zone and crop type, yields may increase (ex. corn and sorghum by 20%) or decrease (ex. wheat and soybean by 20 to 30%). Also, these crop yield changes appear to be related to acceleration in maturation rates, mainly to change in moisture stress and to shifts in optimal thermal growth conditions. These possible shifts in agricultural production potentials would solicit the formulation of appropriate adaptation strategies.     

L’analyse morphologique spatialisée : apports d’une méthode à la détection des accidents,une nécessité dans l’approche hydrodynamique et karstologique des aquifères fissurés. Exemples dans le Nord de la France crayeux

Résumé

L’analyse morphologique spatialisée est une nouvelle méthode de détection des accidents utilisant les données morphométriques du relief (pente, orientation, altitude), extraites d’un modèle numérique de terrain précis par un programme informatique. Ce programme, en tenant compte des commandes morphogénétiques (lithologie, climat, chronologie), permet l’identification de modes morphométriques pour chaque type de morphologie et l’extraction des anomalies à ces modes. Certaines anomalies, après un traitement de hiérarchisation spatiale, sont interprétées comme des signatures tectoniques, l’analyse étant possible à l’échelle du versant. L’existence effective des accidents est validée à l’aide de méthodes de géologie structurale classiques. Par une meilleure définition de la position et de la géométrie des accidents, cette méthode peut permettre l’optimisation des études hydrodynamiques en aquifère fissuré, et dans le cas de réservoirs carbonatés, une approche spatiale de la karstification. Elle est appliquée ici à l’aquifère crayeux du Nord de la France, où, couplée à des enregistrements fins de données hydrogéologiques, elle permet l’identification de zones à transfert hydrodynamique rapide au droit d’accidents. Ces constatations conduisent à de nouvelles pistes de réflexions dans une région où la karstification n’est pas reconnue. © Elsevier, Paris     

Sublimation of the Martian CO2 Seasonal South Polar Cap

The polar condensation/sublimation of CO₂, that involve about one fourth of the atmosphere mass, is the major Martian climatic cycle. Early observations in visible and thermal infrared have shown that the sublimation of the Seasonal South Polar Cap (SSPC) is not symmetric around the geographic South Pole.Here we use observations by OMEGA/Mars Express in the near-infrared to detect unambiguously the presence of CO₂ at the surface, and to estimate albedo. Second, we estimate the sublimation of CO₂ released in the atmosphere and show that there is a two-step process. From Ls=180° to 220°, the sublimation is nearly symmetric with a slight advantage for the cryptic region. After Ls=220° the anti-cryptic region sublimation is stronger. Those two phases are not balanced such that there is 22% ± 9 more mass the anti-cryptic region, arguing for more snow precipitation. We compare those results with the MOLA height measurements. Finally we discuss implications for the Martian atmosphere about general circulation and gas tracers, e.g. Ar.     

Mid- to late Holocene environmental and climatic changes in New Caledonia, southwest tropical Pacific, inferred from the littoral plain Gouaro-Déva

Multiproxy analysis of three littoral cores from western New Caledonia supports the hypothesis that the main controlling factors of environmental changes are sea-level change, ENSO variability and extra-tropical phenomena, such as the Medieval Warm Period (MWP) marked by a tendency for La Niña-like conditions in the tropical Pacific. The record starts during the late Holocene sea-level rise when the terrestrial vegetation indicated wet and cool conditions. The site was a coastal bay definitely transformed into a freshwater swamp at around 3400 cal yr BP, after the rapid drawdown of sea level to its current level. Sediments and foraminiferal assemblages indicated subsequent episodes of freshwater infillings, emersion or very high-energy conditions, likely related to climatic changes and mostly controlled by ENSO variability. Between 2750 and 2000 cal yr BP, relatively dry and cool climate prevailed, while wetter conditions predominated between ca. 1800 and 900 cal yr BP. The Rhizophoraceae peak between ca. 1080 and 750 cal yr BP, coeval with the MWP, may indicate a global phenomenon. Microcharcoal particles present throughout the record increased after 1500 cal yr BP, suggesting an anthropogenic source. From ca. 750 cal yr BP the appearance of current type of vegetation marks the human impact.     

Inverse Modelling for Identification of Multiple-Point Releases from Atmospheric Concentration Measurements

An inverse modelling methodology is proposed for the identification of multiple-point sources releasing similar tracer, in which influences from the various emissions are merged in each detector’s measurement. The identification is addressed from a limited merged set of atmospheric concentration measurements. The methodology is blended with the natural information provided by the geometry of the monitoring network in terms of the weight functions that interpret the associated visibility/illumination of the monitoring network. The release parameters, locations and intensities of the multiple-point sources are estimated by minimizing the objective function within the least squares framework. The methodology has been successfully applied to identify the two- and three- point simultaneous emissions from synthetic measurements generated by the model without noise or with controlled noise artificially added, and from pseudo-real measurements generated from the Indian Institute of Technology low wind diffusion experiment by combining several of single-point release runs. With the synthetic measurements, all the release parameters are retrieved exactly as those prescribed in all the runs. With the pseudo-real measurements, the release locations are retrieved with an average error of 13 m and intensities are estimated on an average within a factor of 1.5. In a sensitivity analysis, it is shown that the incurred errors in the retrieval of the two- and three-point sources with the pseudo-real data correspond to the 10–30 % Gaussian distributed random noise in the observations. Theoretical and computational comparisons are given between the weighted and non-weighted classical formulations. In addition, an alternative strategy is proposed in order to reduce the computational time required in the source estimation.     

Basic concepts of the crystallization from aqueous solutions: The example of aluminum oxy(hydroxi)des and aluminosilicates

This overview features the chemical background on condensation phenomena of the aluminum cation in aqueous solution. The formation of polycationic molecular clusters and nanosized solid phases of aluminum oxy(hydroxi)des is interpreted with illustrative mechanisms, building a bridge between solution chemistry and solid-state chemistry. The formation of the main structural types of aluminosilicates (zeolites, clays, imogolite) is also illustrated through the aqueous chemistry of aluminum and silicates.     

The Ufa indenter: stratigraphic and geophysic evidences for an actual indentation of the Southern Urals by the East European craton

Study of the altitudes of the lowest part of the Upper Cretaceous–Eocene and Aktschagylian–Quaternary stratigraphic ensembles known on the western slope of the Southern Urals evidences the existence of an East–West elongated dome which follows the N53° latitude. This ridge is superimposed at depth with the remnants of the Sernovodsk–Abdulino Aulacogen and with the Belaya tear fault, which support the existence of a recent rejuvenation of these old structures. North of these disruptions the Southern Urals display a clear bent towards the East. Detailed microstructural studies show that this curvature is associated with a typical stress pattern which suggests the existence of an indentation of the fold belt by the East European craton. The hypothesis of an Ufa indenter is not supported by an equivalent East–West deep fault north of the bend. However, a long N100° magnetic anomaly, interpreted as a shear zone, suggests that the indenter is a reality. Quaternary uplift and crustal thickening at its front as well as seismological data support our interpretation. It is not stressed that the curvature of the Urals observed at 56° latitude results solely from this recent indentation. It is only assumed that the actual indentation is rejuvenating a former unevenness which existed before in the East European craton. Study of the inner part of the indenter shows that this type of structure is not necessarily rigid and undeformed. Some of the structures described on the URSEIS deep seismic line could be much younger than previously expected.