Analyse comparée de bassins sédimentaires transtensionnels : le bassin de Santa Rosalia (Basse Californie du Sud, Mexique) et de Mendibelza (Pyrénées, France)

Cet article présente un analyse comparée de bassins sédimentaires liés a l’ouverture en cours du Golfe de Californie, avec le bassin d’ouverture ancien, jurassico-crétacés de Mendibelza dans les Pyrénées occidentales par l’ouverture du Golfe de Gascogne. Ce travail est une contribution à la compréhension de la sédimentation et des contrôles de bassins transtensionnels situés sur des marges divergentes obliques actuelles, dans le but d’utiliser ces modèles et de les comparer, pour mieux comprendre et interpréter les bassins anciens créés dans des contextes géodynamiques analogues, mais qui sont tectonisés. Le fil directeur est l’analyse comparée de ces bassins : - dans leur évolution tectonique, leur modélisation géométrique et les contraintes auxquelles ils ont été soumis, - dans l’enregistrement sédimentaire (séquences) des épisodes d’ouverture, des forçages tectoniques, climatiques et sédimentaires qui ont accompagné la naissance, l’évolution et la tectonisation de ces bassins (Mendibelza). Ainsi, le bassin de Santa Rosalia a enregistré les étapes d’évolution parallèles à la formation du rift oblique du Golfe de Californie depuis le Miocène supérieur suivant 3 séquences : T1 (Miocène supérieur-Pliocène inférieur) représente la phase syn-rift constituée de faciès de fan-delta; T2 (Pliocène inférieur à moyen) représente la transition du rifting-drifting, marquée par une rupture brusque de la sédimentation et constitué de faciès de fan-delta passant à plate forme; T3 (Pliocène supérieur-Pléistocène) est un cortège de haut niveau marin, marquant une deuxième pulsation de la subsidence du bassin de Santa Rosalia pendant l’ouverture généralisée du Golfe de Californie. La formation de Mendibelza est constituée de 3 séquences : - la séquence inférieure K1 de l’Albien inférieur, formée de faciès de front-delta et pro-delta déposés pendant la phase Syn-rift. K2, Albien inférieur-Albien moyen, regroupe des faciès de pente et de fan-delta proximal. K3, Albien supérieur, formée de faciès de chenaux distributaires en tresses répond à la phase d’ouverture généralisée rapide du Golfe de Gascogne. L’analyse tectonique comparée révèle des analogies géodynamiques de ces bassins en limites de plaques divergente obliques, dextre pour le bassin de Santa Rosalia et sénestre pour Mendibelza. La géométrie de ces bassins transtensionnels est celle de demi-grabens obliques aux failles principales du stade Rift, subissant un basculement en échelon. La sédimentation des deux bassins analysés est caractérisée par des faciès de fandelta, qui ont enregistré une partie de l’évolution du Golfe de Gascogne (formation de Mendibelza) et du Golfe de Californie (Bassin de Santa Rosalia) pendant la phase Synrift, puis Syn-rift tardif et transition Rift à Drift.Se presenta un análisis comparativo de la cuenca transtensional de Santa Rosalía, asociada a la apertura del Golfo de California Mioceno-Reciente por un lado, y de la cuenca antigua, Jurásico-Cretácico de Mendibelza, formada en los Pirineos occidentales por la apertura del Golfo de Vizcaya. Este estudio es una contribución a la comprensión de la sedimentación y los controles de las cuencas transtensionales localizadas sobre márgenes divergentes oblicuos actuales no deformados, con el objeto de utilizar esos modelos y compararlos para interpretar mejor y comprender el funcionamiento de cuencas antiguas creadas en contextos geodinámicos similares, pero tectonizados. Los sedimentos de la cuenca de Santa Rosalía registraron las diferentes etapas de su evolución paralela a la formación del rift oblicuo del Golfo de California desde el Mioceno superior. Aplicando las nociones de la estratigrafía secuencial, encontramos 3 secuencias: la secuencia T1 (Mioceno superior-Plioceno inferior) que representa la fase syn-rift constituida de facies de abanico deltaico; la secuencia T2 (Plioceno inferior a medio) que representa la transición rifting-drifting, marcada por una ruptura brusca en la sedimentación y constituida por facies de abanico deltaico a plataforma; la secuencia T3 (Plioceno superior-Pleistoceno) interpretada como un cortejo de nivel alto del mar, que marca un segundo pulso en la subsidencia de la cuenca de Santa Rosalía y la apertura generalizada del Golfo de California. En el análisis de Mendibelza, reconstruimos la zona de sedimentación de la formación de Mendibelza, que está constituida por 3 secuencias: La secuencia inferior K1 (Albiense inferior) constituida de facies de frente deltaico a prodelta, depositadas durante la fase syn-rift; la secuencia K2 (Albiense medio), que en su conjunto reagrupa facies de pendiente y de abanico deltaico proximal; la secuencia K3 (Albiense superior) que es la respuesta a la fase de apertura generalizada rápida del Golfo de Vizcaya. El análisis comparativo muestra analogías en función de su cuadro geodinámico asociado a un limite de placas divergente oblicuo dextral para la cuenca de Santa Rosalía y de divergente oblicua sinestral para Mendibelza. La geometría de las cuencas transtensionales estudiadas está constituida por semigraben oblicuos, siguiendo un basculamiento en echelon. La sedimentación de las dos cuencas está constituida de facies de abanico deltaico, que registraron parte de la evolución del Golfo de Vizcaya (formación de Mendibelza) y del Golfo de California (Cuenca de Santa Rosalía) durante la fase syn-rift, y syn-rift tardío (transición rifting-drifting)

Rapport annuel 2022 de la FRABilan de sortie de crise Covid-19 : l’Agence des droits fondamentaux entre vigilance et indigence

Au sortir de la période de pandémie de Covid-19, l’Agence européenne des droits fondamentaux met un terme à la tolérance à certaines violations jusqu’alors justifiée par la crise sanitaire. Elle souligne avec fermeté les défaillances des États membres en matière de respect des droits fondamentaux. Toutefois, la vigilance réaffirmée de l’Agence est à relativiser, en ce que cette dernière demeure dépourvue de moyens coercitifs

Stratigraphy, facies and geodynamic settings of Jurassic formations in the Bükk Mountains, North Hungary: its relations with the other areas of the Neotethyan realm.

Jurassic mélange complexes related to the subduction of the Neotethys Ocean occur in the Bükk Mountains, North Hungary. This paper characterizes the sedimentary sequence of basin and slope facies that occur in the southwestern part of the Bükk Mountains, placing special emphasis on the redeposited sedimentary rocks (olistostromes, olistoliths: Mónosbél Group) in order to obtain information on the provenance of the clasts, and the mode and time of their redeposition. The series of formations studied shows a general coarsening-upwards trend. Based on radiolarians and foraminifera, the Mónosbél Group formed in Early to Late Bathonian time. The lower part of the complex is typified by a predominance of pelagic carbonates, shale and radiolarite with andesitic volcaniclastic intercalations. The higher part of the succession is characterized by polymictic olistostromes. Large olistoliths that are predominantly blocks of Bathonian shallow marine limestone (Bükkzsérc Limestone) appear in the upper part of the sequence. Based on the biostratigraphic and sedimentological data, results of analyses of the redeposited clasts and taking into consideration the concepts of the development of the western Neotethys domain, the evolutionary stages of the sedimentary basins were defined. The onset of the compressional stage led to initiation of nappe stacking that led to the formation of polymict olistostromes and then to the redeposition of large blocks derived from out-of-sequence nappes of the former platform foreland

Pro- vs. retro-foreland basins

Alpine-type mountain belts formed by continental collision are characterised by a strong cross-sectional asymmetry driven by the dominant underthrusting of one plate beneath the other. Such mountain belts are £anked on either side by two peripheral foreland basins, one over the underthrust plate and one over the over-riding plate; these have been termed pro- and retro-foreland basins, respectively. Numerical modelling that incorporates suitable tectonic boundary conditions, and models orogenesis from growth to a steady-state form (i.e. where accretionary in£ux equals erosional out£ux), predicts contrasting basin development to these two end-member basin types. Pro-foreland basins are characterised by: (1)Accelerating tectonic subsidence driven primarily by the translation of the basin ¢ll towards the mountain belt at the convergence rate. (2) Stratigraphic onlap onto the cratonic margin at a rate at least equal to the plate convergence rate. (3) A basin in¢ll that records the most recent development of the mountain belt with a preserved interval determined by the width of the basin divided by the convergence rate. In contrast, retro-foreland basins are relatively stable, are not translated into the mountain belt once steady-state is achieved, and are consequently characterised by: (1) A constant tectonic subsidence rate during growth of the thrustwedge, with zero tectonic subsidence during the steady-state phase (i.e. ongoing accretion-erosion, but constant load). (2)Relatively little stratigraphic onlap driven only by the growth of the retro-wedge. (3)Abasin ¢ll that records the entire growth phase of the mountain belt, but only a condensed representation of steady-state conditions. Examples of pro-foreland basins include the Appalachian foredeep, the west Taiwan foreland basin, theNorthAlpine ForelandBasin and the EbroBasin (southern Pyrenees). Examples of retro-foreland basins include the SouthWestlandBasin (SouthernAlps,NewZealand), theAquitaine Basin (northern Pyrenees), and the Po Basin (southern European Alps).We discuss how this new insight into the variability of collisional foreland basins can be used to better interpret mountain belt evolution and the hydrocarbon potential of these basins types

Refinement of Miocene sea level and monsoon events from the sedimentary archive of the Maldives (Indian Ocean)

International Ocean Discovery Program (IODP) Expedition 359 cored sediments from eight borehole locations in the carbonate platform of the Maldives in the Indian Ocean. The expedition set out to unravel the timing of Neogene climate changes, in particular the evolution of the South Asian monsoon and fluctuations of the sea level. The timing of these changes are assessed by dating resultant sedimentary alterations that mark stratigraphic turning points in the Neogene Maldives platform system. The first four turning points during the early and middle Miocene are related to sea-level changes. These are reliably recorded in the stratigraphy of the carbonate sequences in which sequence boundaries provide the ages of the sea-level lowstand. Phases of aggradational platform growth give precise age brackets of long-term sea-level high stands during the early Miocene and the early to middle Miocene Climate Optimum that is dated here between 17 to 15.1 Ma. The subsequent middle Miocene cooling coincident with the eastern Antarctic ice sheet expansion resulted in a long-term lowering of sea level that is reflected by a progradational platform growth. The change in platform architecture from aggradation to progradation marks this turning point at 15.1 Ma. An abrupt change in sedimentation pattern is recognized across the entire archipelago at a sequence boundary dated as 12.9–13 Ma. At this turning point, the platform sedimentation switched to a current-controlled mode when the monsoon-wind-driven circulation started in the Indian Ocean. The similar age of the onset of drift deposition from monsoon-wind-driven circulation across the entire archipelago indicates an abrupt onset of monsoon winds in the Indian Ocean. Ten unconformities dissect the drift sequences, attesting changes in current strength or direction that are likely caused by the combined product of changes in the monsoon-wind intensity and sea level fluctuations in the last 13 Ma. A major shift in the drift packages is dated with 3.8 Ma that coincides with the end of stepwise platform drowning and a reduction of the oxygen minimum zone in the Inner Sea. The strata of the Maldives platform provides a detailed record of the extrinsic controlling factors on carbonate platform growth through time. This potential of carbonate platforms for dating the Neogene climate and current changes has been exploited in other platforms drilled by the Ocean Drilling Program. For example, Great Bahama Bank, the Queensland Plateau, and the platforms on the Marion Plateau show similar histories with sediment architectures driven by sea level in their early history (early to middle Miocene) replaced by current-driven drowning or partial drowning during their later history (Late Miocene). In all three platform systems, the influence of currents on sedimentations is reported between 11 and 13 Ma

Environnements et paléoenvironnements carbonatés marins

L’interaction des enveloppes externes de la Terre (atmosphère, océans, continents) et de la vie déterminent des espaces limités appelés “environnements”. Les limites d’un environnement sont physico-chimiques et biologiques. Ces caractères varient au cours de la journée, de la nuit, de l’année, au rythme des révolutions de la Terre, de la Lune, au rythme des climats, etc. Ces caractères ont également varié tout au long de l’histoire de la Terre, qui est née il y a 4,6 milliards d’années. La plupart des environnements anciens ou “paléoenvironnements” qui se sont formés au cours de cette longue histoire ont disparu. Il en reste des forêts “primitives”, des “fossiles vivants” et des héritages. Au moyen des carbonates intertropicaux, qui sont un enregistreur privilégié des variations physico-chimiques et biologiques de la Terre, nous donnons quelques exemples d’environnements et de paléoenvironnements.Environments are the result of the interaction between the external Earth envelopes such as the atmosphere, oceans, continents, and life. The limits and characteristics of environments are physico-chemical and biological. These characteristics are subject to changes during day and night, seasons, climates, etc. and they are mainly related to the Earth, Moon and Sun revolutions. They are also related to the Earth history, which began 4,6 billion years ago. Most of the ancient environments created all along the long history of the Earth, have now disappeared. However, there remain ancient or so-called "primitive" forests, "living fossils", and inheritages. By means of intertropical carbonates, which are a selected marker of the physico-chemical and biological Earth variations, we will give some examples of environments and paleoenvironments