Les « Porphyroı̈des » de Bretagne méridionale : une unité de HP–BT dans la chaı̂ne hercynienneEvidence for HP–LT Hercynian metamorphism within the ‘Porphyroı̈des’ of South Brittany (France)

The so-called ‘Porphyro??des’ Unit of South Brittany is located below units marked by an early HP–LT event (blueschists from the Île de Groix) and above units marked by Upper Carboniferous HT metamorphism. P–T estimates in the Porphyro??des (at Belle-Île-en-Mer), using the phengite–chlorite thermobarometer, indicate pressures around 8 kbar and temperatures of 350–400 °C. These new estimates suggest that the early tectonic history of the ‘Porphyro??des’ should be linked to that of the overlying Blueschists, and imply a much larger areal extension of the HP–LT domain in this part of the Hercynian Belt. To cite this article: F. Le Hébel et al., C. R. Geoscience 334 (2002) 205–211.     

Contrasting trace element geochemistry in two American and French salt marshes

Trace metals were analyzed in the bulk fine phase of two contrasting salt marsh cores on the North Atlantic east (Baie de l'Aiguillon, LaRochelle, France) and west (Lewes, DE, USA) coasts. The purpose is to gauge how the different clay mineral sources and surface sedimentary diagenetic conditions affect the respective trace element geochemistries. It has been established that there is considerably more clay mineral diagenesis at the zone of redox transition in the Delaware core than in the l'Aiguillon core. The Delaware core shows unweathered glacial sedimentary material directly deposited from the continental shelf whereas the l'Aiguillon core is formed from soil clays deposited after river and estuarine transport. Although there is strong Mn redox depletion in the Delaware salt marsh core, and enrichment in the French one at l'Aiguillon, transition elements seem to follow iron content in both cores. The trace pollutant elements Pb, Sn and Zn show pronounced depth profiles in both North Atlantic cores. The Pb, Sn and Zn appear to be carried by clay sources into the French marshes, while appearing to be largely deposited by atmospheric processes as well into the Delaware marsh.Upon deposition, the Zn appears to become decoupled from lead, which is probably more strongly fixed on clays. The Sn seems to be both air transported, and carried as well on non-clay phases contaminated by industrial organo-tins, including antifouling bottom paints. Copper while uniformly enriched in France, probably from use in regional vineyards, appears enriched only near the top at Delaware, suggesting diagenetic recycling in the more abundant peat in the surface. The Cs and Li appear to be tracers for specific soil-derived clay sources in the French core source materials. These observations suggest that clay is a very important carrier in fixing trace elements for historical records as salt marsh sediments.     

Palaeozoic oolitic ironstone of the French Armorican Massif: a chemical and structural trap for orogenic base metal–As–Sb–Au mineralisation during Hercynian strike-slip deformation

In the Saint-Aubin-des-Chateaux quarry (Armorican Hercynian belt, western France), an epigenetic hydrothermal alteration affects an oolitic ironstone layer intercalated within the Lower Ordovician Grès armoricain Formation. The hydrothermal overprint produced pervasive and massive sulphidation with stratoid pyritised lenticular bodies within the oolitic ironstone layer. These sulphide lenses are spatially associated with strike-slip faults and extend laterally from them. After the massive sulphidation stage (Fe–As, stage 1), subsequent fracturing allowed the deposition of base metals (stage 2) and Pb–Sb–Au (stage 3) parageneses in veins. The dominant brittle structures are vertical extension veins, conjugate shear veins and strike-slip faults of various orders. All these structures are filled with the same paragenetic sequence. Deformation analysis allows the identification of structures that developed incrementally via right-lateral simple shear compatible with bulk strain affecting the Central Armorican Domain. Each increment corresponds to a fracture set filled with specific parageneses. Successive hydrothermal pulses reflect clockwise rotation of the horizontal shortening direction. Geothermometry on chlorite and arsenopyrite shows an input of hot hydrothermal fluids (maximum of 390–350°C) during the main sulphide stage 1. The subsequent stages present a marked temperature drop (300–275°C). Lead isotopes suggest that the lead source is similar for all hydrothermal stages and corresponds to the underlying Neo-Proterozoic basement. Lead isotope data, relative ages of deformation and comparison with neighbouring deposits suggest that large-scale fluid pulses occurred during the whole Hercynian orogeny rather than pulses restricted to the late Hercynian period. The vicinity of the Hercynian internal domain appears as a key control for deformation and fluid flow in the oolitic ironstones, which acted as a chemical and structural trap for the hydrothermal fluids. The epigenetic mineralisation of Saint-Aubin-des-Chateaux appears to be very similar to epigenetic sulphidation described in banded iron formation-hosted gold deposits.     

VISIONS OF LA GÉOGRAPHIE HUMAINE IN TWENTIETH‐CENTURY FRANCE

ABSTRACT. In the first half of the twentieth century French academic geography did not present a unified front: The “outsiders” Jean Brunhes and Pierre Deffontaines offered a vision of la géographie humaine that did not conform to the one proclaimed by Vidal de La Blache and his disciples. After sketching the careers of Brunhes and Deffontaines, this essay explores the network of connections between the academic editor Deffontaines and the contributors to the Geographie Humaine series published by Gaston Gallímard. As committed Christians, Brunhes and Deffontaines emphasized the realm of ideas, values, and decision making, as well as their tangible realization and visual expression in cultural landscapes. Both men saw the discipline of geography as a means of celebrating the diversity of the world and the popularization of geography as a way of promoting international understanding and tolerance. Their inclusivity and interaction with other academic disciplines are relevant to current practitioners of cultural geography.     

Présence de Paléocène marin dans les Grands Causses (France)

In the Grands Causses, incised valleys, lapies, fissures and sinkholes inherited from successive polyphase karstifications were filled by Palaeocene marine sediments overall assigned to the P1c–P3 interval (Upper Danian–Lower Selandian). These sediments are distributed into three detritic facies, generated by extensional tectonics controlling karstic and erosional processes. Upper Cretaceous marine fossils known within these facies are interpreted as reworked from hypothetically pellicular deposits. The probable palaeogeographic connection with the Pyrenean Palaeocene ‘Breccia trough’ supposes the presence of a SE–NW ‘ria’ running across the continental areas of Lower Languedoc and draining towards the northwest the marine waters of the Palaeocene transgression as far as the Rodez region. To cite this article: B. Peybernès et al., C. R. Geoscience 335 (2003).     

Le volcanisme Pliocène inférieur de Maguelone (région de Montpellier,Hérault)

The volcanism of the Maguelone Island indicates a surtseyan volcanism. It contains petrographical and mineralogical characteristics of coastal phreatic volcanism. Its discharges are interbedded in the marine or lagoon Pliocene context. Rodents fauna and high-stand in the curve of Vail indicate a period between 4.2 and 3.8 Myr. The geographical pattern of its markers suggests a new reading and dating of the Pliocene sequences, particularly known from the scarcity of chronological indications. To cite this article: P. Ambert, C. R. Geoscience 335 (2003).     

Characterization and correlation of Upper Jurassic (Oxfordian) bentonite deposits in the Paris Basin and the Subalpine Basin, France

Abstract Explosive volcanic activity is recorded in the Upper Jurassic of the Paris Basin and the Subalpine Basin of France by the identification of five bentonite horizons. These layers occur in Lower Oxfordian (cordatum ammonite zone) to Middle Oxfordian (plicatilis zone) clays and silty clays deposited in outer platform environments. In the Paris Basin, a thick bentonite (10–15 cm), identified in boreholes and in outcrop, is dominated by dioctahedral smectite (95%) with trace amounts of kaolinite, illite and chlorite. In contrast, five bentonites identified in the Subalpine Basin, where burial diagenesis and fluid circulation were more important, are composed of a mixture of kaolinite and regular or random illite/smectite mixed-layer clays in variable proportions, indicating a K-bentonite. In the Subalpine Basin, a 2–15 cm thick bentonite underlain by a layer affected by sulphate–carbonate mineralization can be correlated over 2000 km². Euhedral zircon, apatite and biotite crystals have been identified in all the bentonites. The geochemical composition of the bentonites in both basins is characterized by high concentrations of Hf, Nb, Pb, Ta, Th, Ti, U, Y, Zr and low concentrations of Cr, Cs and Rb. Biostratigraphical and geochemical data suggest that the thick bentonite in the Paris Basin correlates with the thickest bentonite in the Subalpine Basin, located 400 km to the south. These horizons indicate that significant explosive volcanic events occurred during the Middle Oxfordian and provide potential long-distance isochronous marker beds. Immobile element discrimination diagrams and rare-earth element characteristics indicate that the original ash compositions of the thickest bentonites correspond to a trachyandesitic source from a within-plate alkaline series that was probably related to North Atlantic rifting.     

Mise en évidence d'un décrochement ductile senestre dans l'ı̂le de Porquerolles (Var,France) et ses implications sur la tectonique du massif des Maures au Paléozoı̈que supérieur

A new geological map of the Palaeozoic units from the Porquerolles Island provides evidence for two main units joined along a north–south-trending shear zone: a western unit of schist, sandstone and microconglomerates, an eastern unit of schist including calcareous rocks, metabasite and various types of mineralisations. The shear zone has experienced a main strike-slip, sinistral movement in a ductile regime, and a late westward normal movement from ductile to brittle regimes. It crosscuts an early thrust fault, marked by penetrative syn-metamorphic deformation and the abundance of quartz rods. According to geometrical relationships, the regional conic fold is interpreted as a large-scale dragging of foliation related to strike-slip faulting. Combined folding and wrenching point out a main transpressional tectonics related to the continental collision dated Middle Visean in the central and eastern parts of the Variscan Maures massif. To cite this article: J.-P. Bellot, C. R. Geoscience 336 (2004).     

L'escarpement de faille de la Têt est-il le résultat de la tectonique active Plio-Pléistocène ou d'une exhumation Pléistocène ?

The Têt Fault looks like an active fault. Its activity, attested for Pliocene, remains discussed for Pleistocene. We propose a new scenario of evolution for the Têt Fault, highlighting the importance of Pleistocene exhumation processes. It is based on a relationship between morphology and basin filling. During Pleistocene, the fault activity is weak and vertical displacements seem to have no considerable impact on the drainage features. This study concludes that Pliocene offset may also be reduced: the fault activity generating relief is anterior to Pliocene. To cite this article: J.-M. Carozza, S. Baize, C. R. Geoscience 336 (2004).     

Découverte d'un flux géothermique anormal au puy de Monténard (chaı̂ne des Puys,France) : résultats préliminaires

Preliminary soil-temperature measurements over 16 months indicate an important geothermal flux of at least 0.4 W m^?2 on the summit of a scoria cone in the Cha??ne des Puys (France). This anomaly can be explained by a residual hydrothermal flux that has begun when the maar and the overlying cone were formed around 30 000 years ago. To cite this article: M. Aubert, C. R. Geoscience 336 (2004).