The Merinchal antimoniferous district (French Massif Central)

International audienceThis article presents the Merinchal antimoniferous district (French Massif Central) from the historical and metallogenic points of view. This small district has been mined in the early 20th century. Subsequent characterization of mineralized indexes occurred afterwards in the 1960's and 1980's. We briefly describe its history and the mining works carried out. Finally, we add to the previously known minerals of this deposit (mainly berthierite and stibnite) some species not mentioned yet (dyscrasite, pyrargyrite, tetrahedrite-freibergite). The goal of the present study is to present and illustrate the textural relationships between mineral

Carte géologique de la France au 1/50.000è, feuille de ST-SULPICE-DES-CHAMPS (n°666), Bureau de recherches géologiques et minières, Orléans.

Bureau de recherches géologiques et minières, Orléans

Fammenian-Tournaisian dextral ductile shear in the French Variscan belt

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Evaluation of subsidence hazard in mantled karst setting: a case study from Val d'Orléans (France)

International audienceSoil subsidence/collapse is a major geohazard occurring in karst region. It occurs as suffosion or dropout sinkholes developing in the soft cover. Less frequently it corresponds to a breakdown of karst void ceiling (i.e. collapse sinkhole). This hazard can cause significant engineering challenges. Therefore decision-makers require the elaboration of methodologies for reliable predictions of such hazards (e.g., karst subsidence susceptibility and hazards maps, early-warning monitoring systems). A methodological framework was developed to evaluate relevant conditioning factors favouring subsidence (Perrin et al. submitted) and then to combine these factors to produce karst subsidence susceptibility maps. This approach was applied to a mantled karst area south of Paris (Val d'Orléans). Results show the significant roles of the overburden lithology (presence/absence of low-permeability layer) and of the karst aquifer piezometric surface position within the overburden. In parallel, an experimental site has been setup to improve the understanding of key processes leading to subsidence/collapse and includes piezometers for measurements of water levels and physico-chemical parameters in both the alluvial and karst aquifers as well as surface deformation monitoring. Results should help in designing monitoring systems to anticipate occurrence of subsidence/collaps

Carboniferous magmatism related to progressive pull-apart opening in the western French Massif Central

International audienceThe Variscan continental collision induced the development of large crustal melting in the western French Massif Central, accompanied by emplacement and deformation of syn- to post-tectonic granites spatially related to normal and strike slip faulting. Our study focuses on the regional deformation and shear zone patterns in the Millevaches massif, one of the largest magmatic area of the French Massif Central. In this massif, the syn-tectonic intrusions are related i) to the dextral wrenching along the Treignac-Pradines shear zones and the Creuse faults system, and ii) to the coeval extension along the N000°–N020° normal faults on the western edge of the Millevaches massif (Bourganeuf and Argentat faults). The analysis of deformation and kinematics correlated to new datations of granites allow us to propose a pull-apart model to explain the tectono-magmatic evolution of this part of the Variscan belt from 350 Ma to 325 Ma. At that time, these granites intruded a “pull-apart” system bounded by two major N140°–160° dextral strike-slip zones operating in the middle continental crust during a bulk N020° regional shortening.From 325 Ma to 320 Ma, a clockwise rotation of the regional shortening axis was responsible for the late reactivation of the N020° eastern Millevaches tectonic border as a dextral fault system (Felletin-Ambrugeat fault system). This NE-SW shortening displaced the N140°–160° Creuse fault system and induced a reverse motion along the northern border of the Millevaches massif (St-Michel-de-Veisse fault). This Visean tectono-magmatic event induced the late exhumation of the Millevaches massif with respect to surrounding units and favoured the widespread granite emplacement in this part of the Variscan belt

Notice explicative de la carte géologique de St-Sulpice-des-Champs (n°666) au 1/50.000.

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Notice explicative de la carte géologique d'Aubusson (n°667) au 1/50.000.

Bureau de recherches géologique en minières, Orléans

Characterization of wrench tectonics from dating of syn- to post-magmatism in the north-western French Massif Central

This work establishes the relative timing of pluton emplacement and regional deformation from new dating and structural data. (1) Monazite and (2) zircon dating show Tournaisian ages for the Guéret granites [Aulon granite 352 ± 5 Ma (1), 351 ± 5 Ma (2

Characterization of wrench tectonics from dating of syn- to post-magmatism in the north-western French Massif Central.

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Notice explicative de la carte géologique de Felletin (n°691) au 1/50.000

Bureau de recherches géologique en minières, Orléan