Modèles de dépôt sous-glaciaires et dynamique de remplissage des vallées tunnel (exemple au Quaternaire (Bray, Irlande) et application à l'Ordovicien supérieur de l'Anti-Atlas (Alnif, Maroc))

Au cours du Paléozoïque inférieur, un changement climatique majeur a permis le développement d une calotte glaciaire sur paléocontinent Gondwana. Cette calotte a plusieurs fois recouvert la plateforme nord-gondwanienne au cours des différents épisodes d avancée et de recul des fronts glaciaires. Cette dynamique a permis le développement d un modèle glaciaire, qui inclut des surfaces d érosion majeures d extension régionale et des vallées glaciaires. Parmi ces vallées, les vallées tunnel font référence aux vallées glaciaires mises en évidence en Europe, et qui sont associées aux grandes glaciations quaternaires. Par définition, ces vallées regroupent des dépressions linéaires de plusieurs kilomètres de large et de plusieurs dizaines de kilomètres de long. Elles sont en générale profondes de quelques centaines de mètres et leur fond présentent des zones de surcreusement. Des vallées tunnel ordoviciennes sont localisées en Afrique du Nord et au Moyen-Orient. Les sédiments glaciaires ordoviciens représentent un important objectif pour l exploration des hydrocarbures et les vallées tunnel, qui constituent des hétérogénéités lithologiques dans l enregistrement sédimentaire, sont d excellentes roches-réservoirs. En Europe, les anciennes vallées tunnel quaternaires constituent des aquifères dont la mise en évidence et la préservation constituent un véritable enjeu. Un intérêt majeur réside donc dans une meilleure connaissance de l architecture du remplissage de ces vallées, mais également une compréhension des paramètres qui contrôlent leur distribution et des mécanismes qui participent à leur formation sous les calottes de glace. Le travail de recherche présenté dans ce mémoire est principalement basé sur une étude de terrain, dont les résultats recouvrent trois grands axes : (1) l environnement sous-glaciaire, (2) la dynamique de remplissage des vallées tunnel et (3) les facteurs contrôlant leur distribution.The icehouse period during the lower Palaeozoic led to the development of large continental ice-sheet over the Gondwana. This ice-sheet fluctuated in size and repeatedly recovered the north-Gondwana platform. The resulting glacial record includes major erosional surfaces of regional extent, with subglacial landforms and morphologies, as well as glacial valleys and specific glaciogenic sedimentary record. Among these valleys, tunnel valleys refer to Quaternary analogues, associated with the development of ice-sheets over Europe and North-America. Tunnel valley defines elongated, linear to slightly sinuous depressions, measuring few kilometers in width and several kilometres in length. They start and terminate abruptly, are generally a few hundred of meters deep, and display frequent overdeepening along the floor. Ordovician tunnel valleys are described from North Africa to the Middle East. Ordovician glacial deposits are considered as a major target for hydrocarbon exploration in these regions, with tunnel valleys forming lithological heterogeneities with excellent reservoir properties. In Europe, Quaternary tunnel valleys are targeted for groundwater resources as they form reliable aquifers. A major interest therefore exists for these valleys, as well as a need for better understanding of the nature and the stratigraphy of the infill, their origin and the parameters controlling their distribution. The recherché project presented in this thesis is based on an extensive fieldwork that focused on three main issues, regarding tunnel valleys: (1) the subglacial environment, (2) the sedimentary processes and depositional environments associated with their infill and (3) the parameters controlling their distributionDIJON-BU Doc.électronique (212319901) / SudocSudocFranceF

Continental weathering as a driver of Late Cretaceous cooling : new insights from clay mineralogy of Campanian sediments from the southern Tethyan margin to the Boreal realm

21 pagesInternational audienceNew clay mineralogical analyses have been performed on Campanian sediments from the Tethyan and Boreal realms along a palaeolatitudinal transect from 45° to 20°N (Danish Basin, North Sea, Paris Basin, Mons Basin, Aquitaine Basin, Umbria-Marche Basin and Tunisian Atlas). Significant terrigenous inputs are evidenced by increasing proportions of detrital clay minerals such as illite, kaolinite and chlorite at various levels in the mid- to upper Campanian, while smectitic minerals predominate and represented the background of the Late Cretaceous clay sedimentation. Our new results highlight a distinct latitudinal distribution of clay minerals, with the occurrence of kaolinite in southern sections and an almost total absence of this mineral in northern areas. This latitudinal trend points to an at least partial climatic control on clay mineral sedimentation, with a humid zone developed between 20° and 35°N. The association and co-evolution of illite, chlorite and kaolinite in most sections suggest a reworking of these minerals from basement rocks weathered by hydrolysis, which we link to the formation of relief around the Tethys due to compression associated with incipient Tethyan closure. Diachronism in the occurrence of detrital minerals between sections, with detrital input starting earlier during the Santonian in the south than in the north, highlights the northward progression of the deformation related to the anticlockwise rotation of Africa. Increasing continental weathering and erosion, evidenced by our clay mineralogical data through the Campanian, may have resulted in enhanced CO2 consumption by silicate weathering, thereby contributing to Late Cretaceous climatic cooling

Marine Incursion: The Freshwater Herring of Lake Tanganyika Are the Product of a Marine Invasion into West Africa

The spectacular marine-like diversity of the endemic fauna of Lake Tanganyika, the oldest of the African Great Lakes, led early researchers to suggest that the lake must have once been connected to the ocean. Recent geophysical reconstructions clearly indicate that Lake Tanganyika formed by rifting in the African subcontinent and was never directly linked to the sea. Although the Lake has a high proportion of specialized endemics, the absence of close relatives outside Tanganyika has complicated phylogeographic reconstructions of the timing of lake colonization and intralacustrine diversification. The freshwater herring of Lake Tanganyika are members of a large group of pellonuline herring found in western and southern Africa, offering one of the best opportunities to trace the evolutionary history of members of Tanganyika's biota. Molecular phylogenetic reconstructions indicate that herring colonized West Africa 25–50MYA, at the end of a major marine incursion in the region. Pellonuline herring subsequently experienced an evolutionary radiation in West Africa, spreading across the continent and reaching East Africa's Lake Tanganyika during its early formation. While Lake Tanganyika has never been directly connected with the sea, the endemic freshwater herring of the lake are the descendents of an ancient marine incursion, a scenario which may also explain the origin of other Tanganyikan endemics

Double seismic zone of the Nazca plate in northern Chile: High‐resolution velocity structure, petrological implications, and thermomechanical modeling

This paper presents an interdisciplinary study of the northern Chile double seismic zone. First, a high‐resolution velocity structure of the subducting Nazca plate has been obtained by the tomoDD double‐difference tomography method. The double seismic zone (DSZ) is observed between 80 and 140 km depth, and the two seismic planes is 20 km apart. Then, the chemical and petrologic characteristics of the oceanic lithosphere associated with this DSZ are deduced by using current thermal‐petrological‐seismological models and are compared to pressure‐temperature conditions provided by a numerical thermomechanical model. Our results agree with the common hypothesis that seismicity in both upper and lower planes is related to fluid releases associated with metamorphic dehydration reactions. In the seismic upper plane located within the upper crust, these reactions would affect material of basaltic (MORB) composition and document different metamorphic reactions occurring within high‐P (>2.4 GPa) and low‐T (130 km), lawsonite‐amphibole eclogite conditions. The lower plane lying in the oceanic mantle can be associated with serpentinite dehydration reactions. The Vp and Vs characteristics of the region in between both planes are consistent with a partially (∼25–30 vol % antigorite, ∼0–10% vol % brucite, and ∼4–10 vol % chlorite) hydrated harzburgitic material. Discrepancies persist that we attribute to complexities inherent to heterogeneous structural compositions. While various geophysical indicators evidence particularly cold conditions in both the descending Nazca plate and the continental fore arc, thermomechanical models indicate that both seismic planes delimit the inner slab compressional zone around the 400°C (±50°C) isotherm. Lower plane earthquakes are predicted to occur in the slab's flexural neutral plane, where fluids released from surrounding metamorphic reactions could accumulate and trigger seismicity. Fluids migrating upward from the tensile zone below could be blocked in their ascension by the compressive zone above this plane, thus producing a sheeted layer of free fluids, or a serpentinized layer. Therefore earthquakes may present either downdip compression and downdip tensile characteristics. Numerical tests indicate that the slab's thermal structure is not the only factor that controls the occurrence of inner slab compression. (1) A weak ductile subduction channel and (2) a cold mantle fore arc both favor inner slab compression by facilitating transmission of compressional stresses from the continental lithosphere into the slab. (3) Decreasing the radius of curvature of the slab broadens the depth of inner slab compression, whereas (4) decreasing upper plate convergence diminishes its intensity. All these factors indicate that if DSZs indeed contour inner slab compression, they cannot be linked only to slab unbending, but also to the transmission of high compressional stresses from the upper plate into the slab

Application des méthodes d'inversion stochastique au calcul des formules structurales des minéraux

A method for calculating the structural formulae of pyroxenes is presented, involving the use of the algorithm of Tarantola and Valette (1982a and b). A model of pyroxene, involving perfect stoichiometry and an accurate analysis is adopted but with the possibility of non-stoichiometry if this is required for a particular mineral analysis to attain charge balance. This model is used with the inverse method to estimate directly from the microprobe analysis the distribution of Fe2+, Fe3+ and Mg on the octahedral sites and the best structural formula consistent with experimental errors. The reliability of this method was established for different types of pyroxenes. The validity of the distribution of Fe2+ and Mg on the octahedral sites depends on the constraints and the a priori values. The estimation of Fe2+/Fe3+ ratio is useful when the pyroxene is non-stoichiometric. Lastly, the principles of the generalization of this approach to other minerals are discussed.Une méthode de calcul des formules structurales des minéraux basée sur l'algorithme de Tarantola et Valette (1982a et b) est appliquée aux pyroxènes. Le calcul de la formule structurale est établi sur un modèle de pyroxène parfaitement stoechiométrique d'où sont déduites les équations de contraintes et les valeurs a priori. Ces valeurs a priori sont modifiées lorsque l'analyse chimique rend compte de la non stoechiométrie du pyroxène. A l'aide de l'algorithme de Tarantola et Valette (1982 a et b), on peut alors estimer la répartition de Fe2+, Fe3+ et Mg dans les sites octaédriques à partir de l'analyse microsonde, ce qui conduit à la meilleure formule structurale possible l'intérieur de l'erreur expérimentale. Appliquée à différents types de pyroxènes, cette méthode montre d'une part que la répartition de Fe2+, Fe3+ et Mg dans les sites octaédriques dépend à la fois des équations de contraintes et des valeurs a priori, et d'autre part que l'estimation du rapport Fe2+/Fe3+ est correcte mais n'est réellement utile que dans le cas des pyroxènes non stoechiométriques. Enfin la généralisation de cette approche à d'autres minéraux est discutée.Guiraud Michel. Application des méthodes d'inversion stochastique au calcul des formules structurales des minéraux. In: Bulletin de Minéralogie, volume 109, 3, 1986. pp. 289-299

Marqueurs de tectonique synsédimentaire dans les séries syn- et post-rift du bassin intérieur du Gabon.

9 pagesInternational audienc

Marseille, cité-refuge des écrivains et des artistes

Guiraud Jean-Michel. Marseille, cité-refuge des écrivains et des artistes. In: Les Cahiers de l'Institut d'Histoire du Temps Présent, n°8, juin 1988. Politiques et pratiques culturelles dans la France de Vichy. pp. 247-261