A new look at old debates about the Corbières (NE-Pyrenees) geology: salt tectonics and gravity gliding

In the Corbières area, a large-scale nappe has been identified at the beginning of the 20th century: the “Nappe des Corbières Orientales” (NCO) resting over a thick Triassic sole. This geological object is located at the NE of the Pyrenees, close to the Gulf of Lions. At this place, the chain changes in orientation from E-W to NE-SW and presents in detail, a great complexity. The existence of the nappe itself has never been contested. However, due to its overall complexity, several controversies exist regarding the style and chronology of deformation of its substratum in the so-called the “Pinède de Durban” in particular. We show that the new concepts of salt tectonics can clarify these old debates. Indeed, the rise of the Triassic salt during Mesozoic rifting episodes results in the development of characteristic sedimentary sequences (halokinetic sequences) on top of salt walls. It is along one of these, coinciding with the prolongation of the Cévenole Fault System, that the NCO has been individualized. During its Cenozoic emplacement, a gravity-gliding component, explaining the importance of the observed translation, could result from an uplift preceding the rifting at the origin of the Gulf of Lions

Le relief des Atlas Marocains : contribution des processus asthénosphériques et du raccourcissement crustal, aspects chronologiques.

This thesis focuses on the study of vertical movements and the characterization of the main processes controlling them in an intraplate mountain belt. The mechanisms at the origin of the relief evolution are numerous and still poorly known. The High Atlas belt of Morocco is situated at more than 600 km of the Africa-Europe plate boundary, and supports the second highest peak of Africa (Jebel Toubkal, 4165 m). The lack of a significant crustal root under the belt, a consequence of limited shortening (~20 km), implies that another process takes place to explain such elevations. Lithospheric cross-sections allow us to show that a lithospheric thinning explain 1000 m of topography in the Anti-Atlas, the Central High Atlas, the Middle Atlas. Some of the foreland basins are also affected, like the Souss, Ouarzazate and Missour basins. The thinned area is an elongated stripe running North East / South West and crossing over the main structural domains of Morocco and probably the Africa-Europe plate boundary.The second mechanism explaining the High Atlas topography is the crustal shortening. We present a structural analysis on the basis of field data in the Marrakech High Atlas. The constrated mechanical stratigraphy is described, and numerous potential décollement levels are identified. They are mostly developed in the Visean, Cambrian, Triassic and Senonian units. The complex heritage of this area, situated between the Triassico-Liassic Atlantic and Tethyan rifts, controls their activation on the belt borders. They trigger the formation of various structures, such as tectonic wedges, rabbit ears folds, imbricate fans... A basal rigid group is distinguished, including the Precambrian rocks and locally the Paleozoic and Triasic series. A complete cross section of the belt is presented. The chronology of the deformation and the age of the lithospheric thinning are finally discussed. We propose a scenario of the foreland evolution in the light of the available geological data combined with new results of fission track thermochronology. A first shortening phase occurred during Upper Eocene to Oligocene. The lithosphere is thinned between the Lower and the Middle Miocene, and a main erosionnal event affects the belt and its foreland. Finally, the crustal shortening starts again during Plio-Quaternary.L'objectif de cette thèse est l'étude des mouvements verticaux et la caractérisation des processus à l'origine de ces mouvements au sein d'une chaîne de montagnes intracontinentale. Les mécanismes contrôlant l'évolution de la topographie sont nombreux et encore mal connus. La chaîne du Haut Atlas marocain est située à plus de 600 km de la limite de plaque Afrique-Europe et supporte pourtant le deuxième sommet d'Afrique (Jbel Toubkal, 4165 m). L'absence de racine crustale développée sous la chaîne, conséquence d'un taux de raccourcissement assez faible (~20 km), implique donc l'existence d'un autre processus permettant de maintenir une telle topographie. La réalisation de profils géophysiques nous permet de montrer qu'un amincissement lithosphérique provoque un soulèvement d'environ 1000 m dans l'Anti-Atlas, le Haut Atlas Central, et le Moyen Atlas. Certains bassins d'avant-pays sont aussi affectés, comme les bassins du Souss, de Ouarzazate ou de Missour. La zone amincie est une bande d'orientation Nord-Est / Sud-Ouest recoupant les principaux domaines structuraux marocains et probablement lalimite de plaque Afrique-Europe.Le raccourcissement crustal est le deuxième mécanisme à l'origine du relief de cette chaîne. Une analyse structurale sur la base des données de terrain dans le Haut Atlas de Marrakech nous a permis de montrer l'existence d'une stratigraphie mécanique contrastée. Plusieurs niveaux de décollements potentiels y sont identifiés en particulier dans le Viséen, le Cambrien, le Trias et le Sénonien. L'héritage complexe de cette région, située à la limite entre les rifts Triasico-Liasiques Atlantique et Téthysien, contrôle leur activation sur les bordures de la chaîne. Celle-ci entraîne la formation de structures variées : zones triangulaires, plis secondaires (« rabbit ears »), imbrications. Un groupe basal rigide est distingué. Il inclut le Précambrien, et localement le Paléozoique et le Trias. Une coupe complète de lachaîne est présentée. La chronologie de la déformation et l'âge de l'amincissement de la lithosphère sous la chaîne sont finalement discutés. Une analyse détaillée de la géologie de l'avant-pays sud du Haut Atlas de Marrakech est combinée aux résultats obtenus par comptage de traces de fissions pour proposer un scénario d'évolution. Une première phase de raccourcissement est identifiée à l'Eocène Supérieur – Oligocène. Au Miocène Inférieur – Moyen, la lithosphère est amincie et une phase de dénudation affecte l'ensemble de la chaîne et son avant-pays. Enfin, une dernière phase de raccourcissement a lieu au Plio-Quaternaire

Can Moroccan Atlas lithospheric thinning and volcanism be induced by Edge-Driven Convection?

International audienceThe Moroccan lithosphere is characterized by an anomalously thinned area, located beneath the Atlas domains, which forms a singular narrow NE-SW directed strip overlain by Cenozoic alkaline volcanism. The origin of this thinning and volcanism is still a matter of debate. The proposed models invoke processes either related to the Mediterranean slab or mantle plumes. Herein, we propose an alternative Edge-Driven Convection (EDC) model involving small-scale convection at the boundary between the West-African craton and the Atlas lithosphere. Our comparison of the Atlas lithosphere velocity and volcanism episodes during the last 80 Ma points out that volcanism occurs when plate moves at velocities c.<1 cm a)1, a velocity sufficiently low to trigger EDC. This is the first process that could explain the c. 20 Ma volcanism shutdown separating the two volcanic episodes of the Atlas. In addition, it may successfully account for the lithosphere thinning location and geometry and volcanism geochemistry

Le relief des Atlas marocains (contribution des processus asthénosphériques et du raccourcissement crustal, aspects chronologiques)

Le but de cette thèse est l étude des mouvements verticaux et la caractérisation des processus à l origine de ces mouvements au sein d une chaîne de montagnes intracontinentale. Le Haut Atlas au Maroc culmine à plus de 4000 m à 500 km au Sud de la limite de plaque. Nous montrons que deux processus sont à l origine d un tel relief. Un amincissement de la lithosphère explique 1000 m de la topographie du Haut Atlas, de l Anti-Atlas et du Moyen Atlas. Sa géométrie est allongée NE-SW. Le raccourcissement crustal est le deuxième mécanisme à l origine de ces reliefs. Il s exprime sous la forme de structures variées en raison d une stratigraphie mécanique contrastée, contrôlée par l héritage structural complexe de cette région. Une approche thermochronologique combinée aux données géologiques m a permis d établir une chronologie des mouvements verticaux liés à deux phases de raccourcissement (Eocène Sup. et Plio-Quaternaire) et à l amincissement lithosphérique (Miocène Inférieur à Moyen).We focus on vertical movements and processes at their origin in an intracontinental mountain belt. The High Atlas belt, situated at more than 500 km from the plate boundary, culminates at more than 4000 m. Two mechanisms explain this relief. A lithospheric thinning explains 1000 m of the High Atlas, Anti Atlas and Middle Atlas belts relief. The thinned area is elongated in a NE/SW direction. Crustal shortening is the second process at the origin of the relief of the belts. In the High Atlas, a wide variety of structures can be observed. I show that the complex structural inheritance led to the development of multiple décollement levels. A thermochronological approach, combined with the geological data, allows me to propose the chronology of the vertical movements in the belts and their foreland. Two tectonic phases took place during Upper Eocene and Plio-Quaternary, and the uplift related to the thinning of the lithosphere occurred during Lower to Middle Miocene.CERGY PONTOISE-BU Les Cerclades (951272104) / SudocCERGY PONTOISE-BU Neuville (951272102) / SudocSudocFranceF

Age of the Fontainebleau sandstones: a tectonic point of view

The age of the cementation of the Fontainebleau sandstones, located in the upper part of the Rupelian Fontainebleau Sand Formation and largely outcropping in the south of the center of the Paris Basin, remains a matter of debate: did the silicification occurred at early times during Miocene, following sedimentation, or did it occurred during Quaternary cold climate episodes? In this work, we determined an orthogonal fracture network (main directions N115° ±5° and N025° ±5°) over an area of ∼6000 km2. The fractures are oblique to the adjacent valley orientation and to the quarry working face orientation, discarding a gravitational origin. This tectonic fracturing is superimposed on regional scale antiforms and synforms that may be at least partly controlled by inherited basement faults reactivation during Alpine episodes. The whole Fontainebleau Sand Formation seems to be folded, including the Fontainebleau sandstones. We establish a relative chronology of the various phenomena and propose that silicification at the origin of the Fontainebleau quartzite occurred during early or middle Miocene. Alpine stresses then induced Fontainebleau sand and quartzite folding and fracturing during late Miocene and Pliocene. Finally, the fracture network facilitated fluid circulations and secondary carbonate sandstones or quartzite precipitation probably during Quaternary cold climate episodes

A new look at old debates about the Corbières (NE-Pyrenees) geology: salt tectonics and gravity gliding

In the Corbières area, a large-scale nappe has been identified at the beginning of the 20th century: the “Nappe des Corbières Orientales” (NCO) resting over a thick Triassic sole. This geological object is located at the NE of the Pyrenees, close to the Gulf of Lions. At this place, the chain changes in orientation from E-W to NE-SW and presents in detail, a great complexity. The existence of the nappe itself has never been contested. However, due to its overall complexity, several controversies exist regarding the style and chronology of deformation of its substratum in the so-called the “Pinède de Durban” in particular. We show that the new concepts of salt tectonics can clarify these old debates. Indeed, the rise of the Triassic salt during Mesozoic rifting episodes results in the development of characteristic sedimentary sequences (halokinetic sequences) on top of salt walls. It is along one of these, coinciding with the prolongation of the Cévenole Fault System, that the NCO has been individualized. During its Cenozoic emplacement, a gravity-gliding component, explaining the importance of the observed translation, could result from an uplift preceding the rifting at the origin of the Gulf of Lions

Apatite (U-Th-Sm)/He date dispersion: First insights from machine learning algorithms

International audienceNumerous parameters impact apatite (U-Th-Sm)/He (AHe) thermochronological dates, such as radiation damage, chemical content, crystal size and geometry, and their knowledge is essential for better geological interpretations. The present study investigates a new method based on advanced data mining techniques, to unravel the parameters that could play a role in He retention and thus on AHe date. The purpose is to decipher which factors influence the AHe date dispersion, and to exclude the impact of other parameters on helium retention. As an example, we use a dataset previously collected on apatite from basements rocks, sampled in French Brittany, where all samples underwent the same thermal history, and for which were reported a set of physical and chemical parameters. The dataset includes dimension and geometry, He, U, Th, Sm and major and trace element content for ∼35 crystals. The algorithm ranks the parameters according to their influence on helium retention, using predictive trees, which are commonly used in computing sciences. After looking at 100 simultaneous predictions, we compared the predicted and measured He content for each analyzed apatite crystal. For this particular case, the predictions confirmed the prominent role of the parent nuclides in the He production, as AHe dates can be predicted accurately with these parameters (especially U and Th). Additionally, the predictions without knowledge of the apatite chemical composition and dimension provided better results than using all available parameters (median error of 14% instead of 18%). Therefore, for this specific study, the apatite chemistry and crystal dimensions do not influence significantly He retention nor AHe date dispersion. Nevertheless, detailed inspection of analysis results suggests which parameters have the most discrimination ability, which in this study include crystal length, height, and Mn content. The latter may reveal an eventual influence on alpha damage annealing kinetics. Finally, this approach shows that some grains could never achieve good predictions, indicating that for these crystals the input parameters are not enough to predict the He content. We propose that such crystals are statistically different from the remaining dataset, and this suggests that machine learning has a strong potential to correct errors, or to detect anomalies