Modélisation géométrique 3D des granites stéphaniens du massif du Pelvoux (Alpes, France).

International audienceLa modélisation 3D de la géométrie des granites Stéphaniens du Massif du Pelvoux a permis de mettre en évidence le contexte cisaillant associé à leur mise en place. Dans les Massifs Cristallins Externes Français, ces cisaillements se répartissent selon deux directions, N50 et N135, respectivement dextre et sénestre. Ce système décrochant Carbonifère s'intègre dans un contexte d'extension N-S connu dans l'ensemble de la chaîne Varisque

Comment on “Zemmouri earthquake rupture zone (Mw 6.8, Algeria): Aftershocks sequence relocation and 3D velocity model” by A. Ayadi et al.

International audienceAlthough often difficult to characterize, the relationship between a seismic rupture, its aftershock sequence, and cumulative subsurface or surface faulting or folding is an important challenge to modern seismology and seismotectonics. Among other benefits, it helps document fault length, slip, and magnitude relationships, reconstruct the evolution of the rupture process through historical and prehistorical times and identify the complexity of the deformation in its path toward the surface. This approach is a prerequisite to any seismic hazard assessment but is particularly difficult for faults whose surface trace projects offshore. A specific effort to identify and quantify the source parameters of large earthquakes in coastal areas is therefore needed, not only in subduction zones but also in areas of slow rate and/or diffuse deformation

Early, far-field and diffuse tectonics records in the North Aquitaine Basin (France)

In Western Europe, the deformations related to the opening of the Bay of Biscay and the formation of the Pyrenean belt are well described in the southern part of the Aquitaine Basin, but little is known about the impact of these geodynamic events towards the Northern Aquitaine Platform. In this paper, we combine field observation with Unmanned Aerial Vehicle (UAV) imagery and calcite U-Pb geochronology to determine precisely the tectonic evolution in the Vendée Coastal domain. We evidence two main tectonic events: (1) At the transition between the Late Jurassic to Early Cretaceous, WNW-ESE striking normal faults formed horsts and grabens at the onset of the opening of the Bay of Biscay. The reactivation of Variscan faults during this tectonic event is consistent with oblique extension. This event triggered ascending fluid flows that mix with Basin fluids responsible for barite-pyrite-quartz mineralizations near the unconformity. (2) During the Late Cretaceous, fractures, wide-open folds, veins, and joints are consistent with the N-S shortening direction during the earliest stages of the Pyrenenan compression. In both cases, the Northern Aquitaine Platform records the early stages of the main regional tectonic events in a far-field position. In the northern Aquitaine Basin, as in many other places in Europe, the tectonic study of sedimentary platforms located far from the plate boundaries provides new constraints on the early diffuse deformation process that predate the main tectonic phases

Évolution tectono-métamorphique et modélisation 3D le long du projet ferroviaire Lyon-Turin (Maurienne, Alpes)

Le projet ferroviaire Lyon-Turin offre l opportunité de disposer de nombreuses données du sous-sol dans le secteur de Modane-Aussois (Briançonnais, France) qui, combinées aux structures géologiques de surface permettent de progresser dans la connaissance des processus de subduction continentale, d exhumation et de collision dans les Alpes. Plusieurs étapes dans l exhumation sont mises en évidence : - Un empilement des unités de couverture à la faveur de chevauchements vers le NW (D1-D2) permet l exhumation entre 37 et 35 Ma depuis des conditions de HP BT (1.02 GPa - 350C) jusqu à 0.75 GPa 380C. - Des zones de cisaillement normale à vergence est (D3) datée entre 35 et 31 Ma sont activent entre 0.75 GPa 380C et 0.4 GPa et 300C. Au cours de cette phase un réchauffement partiel (0.57 GPa, 430C) est enregistré. - Deux phases de déformations cassantes successives terminent cette histoire géologique. La première est caractérisée par une direction d extension N-S avec une direction de raccourcissement E-W à verticale qui a eu lieu entre 31 et 20 Ma. La seconde est marquée par une direction d extension E-W et une direction de raccourcissement N-S à verticale qui débute vers 5 Ma et qui est toujours active. Par ailleurs le développement de méthodes de traitement de données de reconnaissance pour la descenderie et les forages ont permis de mieux comprendre la formation et la géométrie finie des principaux contacts tectoniques. La modélisation 3D (modeleur géologique du BRGM) a été utilisée tout au long de cette thèse afin de vérifier la cohérence géométrique des données, dans un secteur où la géométrie est complexe et les données de reconnaissance abondante.The Lyon Turin railway project gives the opportunity to combine underground data with surface geological data in order to progress on the knowledge of continental subduction and exhumation processes in the Alps. Different exhumation steps have been identified: - A cover nappe stack resulting of thrusting toward the NW (D1-D2), it allows exhumation of HP-LT rocks (from 1.02 GPa - 350C to 0.75 GPa 380C) between 37 and 35 Ma. - East verging normal shear zones (D3) occurred between 35 and 31 Ma allowing the exhumation until 0.4 GPa et 300C. During this phase, a temperature increase is recorded at 0.57 GPa, and 430C.- Two successive brittle deformation phases ended the geological story. The first one occurs between 31 and 21 Ma, and is characterized by N-S extension and E-W to vertical shortening directions. This eformation phase is associated with the whole tilting of the Modane-Aussois area. The second one starts at 5 Ma and is still active today. It is characterized by E-W direction of extension and N-S to vertical direction of shortening. New methods of treatments of bore hole and tunnel dataset lead to better understand the formation and the present day geometry of the mains tectonic contacts. These include on one hand a GIS and VRML numerical treatment of the tunnel forcut pictures and on the other hand the attribution of tectonic contact to D1-D2 or D3 tectonic phases by using the chemical compositions of the phengites. 3D odelling (using 3D geomodeler) has been used during each step of this work in orde to validate the geometrical compatibility between geological scenario and geological data both on the surface and in depthLYON1-BU.Sciences (692662101) / SudocSudocFranceF

Évolution tectono-métamorphique et modélisation 3D le long du projet ferroviaire Lyon-Turin (Maurienne, Alpes)

Le projet ferroviaire Lyon-Turin offre l opportunité de disposer de nombreuses données du sous-sol dans le secteur de Modane-Aussois (Briançonnais, France) qui, combinées aux structures géologiques de surface permettent de progresser dans la connaissance des processus de subduction continentale, d exhumation et de collision dans les Alpes. Plusieurs étapes dans l exhumation sont mises en évidence : - Un empilement des unités de couverture à la faveur de chevauchements vers le NW (D1-D2) permet l exhumation entre 37 et 35 Ma depuis des conditions de HP BT (1.02 GPa - 350C) jusqu à 0.75 GPa 380C. - Des zones de cisaillement normale à vergence est (D3) datée entre 35 et 31 Ma sont activent entre 0.75 GPa 380C et 0.4 GPa et 300C. Au cours de cette phase un réchauffement partiel (0.57 GPa, 430C) est enregistré. - Deux phases de déformations cassantes successives terminent cette histoire géologique. La première est caractérisée par une direction d extension N-S avec une direction de raccourcissement E-W à verticale qui a eu lieu entre 31 et 20 Ma. La seconde est marquée par une direction d extension E-W et une direction de raccourcissement N-S à verticale qui débute vers 5 Ma et qui est toujours active. Par ailleurs le développement de méthodes de traitement de données de reconnaissance pour la descenderie et les forages ont permis de mieux comprendre la formation et la géométrie finie des principaux contacts tectoniques. La modélisation 3D (modeleur géologique du BRGM) a été utilisée tout au long de cette thèse afin de vérifier la cohérence géométrique des données, dans un secteur où la géométrie est complexe et les données de reconnaissance abondante.The Lyon Turin railway project gives the opportunity to combine underground data with surface geological data in order to progress on the knowledge of continental subduction and exhumation processes in the Alps. Different exhumation steps have been identified: - A cover nappe stack resulting of thrusting toward the NW (D1-D2), it allows exhumation of HP-LT rocks (from 1.02 GPa - 350C to 0.75 GPa 380C) between 37 and 35 Ma. - East verging normal shear zones (D3) occurred between 35 and 31 Ma allowing the exhumation until 0.4 GPa et 300C. During this phase, a temperature increase is recorded at 0.57 GPa, and 430C.- Two successive brittle deformation phases ended the geological story. The first one occurs between 31 and 21 Ma, and is characterized by N-S extension and E-W to vertical shortening directions. This eformation phase is associated with the whole tilting of the Modane-Aussois area. The second one starts at 5 Ma and is still active today. It is characterized by E-W direction of extension and N-S to vertical direction of shortening. New methods of treatments of bore hole and tunnel dataset lead to better understand the formation and the present day geometry of the mains tectonic contacts. These include on one hand a GIS and VRML numerical treatment of the tunnel forcut pictures and on the other hand the attribution of tectonic contact to D1-D2 or D3 tectonic phases by using the chemical compositions of the phengites. 3D odelling (using 3D geomodeler) has been used during each step of this work in orde to validate the geometrical compatibility between geological scenario and geological data both on the surface and in depthLYON1-BU.Sciences (692662101) / SudocSudocFranceF

Relations fluides-fracturation en contexte d’extension: exemple des minéralisations à Baryte du Bassin Aquitain au Jurassique supérieur

International audienceEn bordure des bassins sédimentaires, les premiers dépôts forment souvent des gisements d’importance économique (e.g. Pb-Zn pour les gisements de type Mississippi). Le mécanisme à l’origine de ces gisements implique des fluides expulsés depuis le centre du bassin et circulant à la base du bassin, en dessous d’un niveau imperméable.Des gisements de ce type, à barytine et fluorine, sont connus au niveau de la bordure nord du Bassin aquitain (France). Les minéralisations se sont formées au Jurassique supérieur dans un contexte d’extension N-S marqué par le début de l’individualisation des sous bassins de Parentis et de Charentes. Les niveaux minéralisés et imperméables sont datés du Jurassique inférieur. Les études géochimiques ont mis en évidence le mélange de trois fluides à l’origine de ces minéralisations : i) un fluide d’origine diagénétique issu du centre du bassin, ii) un fluide plus chaud provenant du socle hercynien et iii) un fluide possédant la signature géochimique de l’eau de mer.Dans le secteur de Jard-sur-Mer, ces minéralisations ont aussi été reconnues dans les niveaux du jurassique Moyen suggérant que les fluides minéralisateurs ont traversés la couche imperméable. Afin de mieux contraindre les relations entre les fluides et les structures tectoniques, nous avons réalisé une étude structurale du socle et de la couverture sédimentaire.Les résultats mettent en évidence un découplage mécanique entre socle et couverture sédimentaire. Dans cette dernière, l’orientation des veines et des failles traduit une phase d’extension N-S contemporaine des circulations de fluides. Du haut vers le bas de la série sédimentaire, le pendage des failles normales diminue jusqu’à devenir listrique au niveau de la discordance. Dans le socle, ce sont des failles normales, héritées de la fin de l’orogénèse hercynienne, qui drainent les fluides. Dans le niveau minéralisé, un réseau de veines à la fois verticales et horizontales évolue localement en brèches hydrauliques. L’ensemble est compatible avec la mise en surpression des fluides dans le niveau minéralisé et le découplage mécanique du socle et de la couverture sédimentaire.Nous proposons que l’origine de ce découplage soit liée à la tectonique distensive du Jurassique supérieur : L’accumulation de sédiments au centre du bassin permet la mise en mouvement des fluides d’origine diagénétique et la fracturation du socle permet également la mobilisation de fluides profonds le long de structures héritées

Initial geometry of western Himalaya and ultrahigh-pressure metamorphic evolution

International audienceUltrahigh-pressure metamorphic rocks on both sides of the western Himalayan syntaxis show different P–T–t paths. The Kaghan unit was metamorphosed under the UHP conditions significantly later ( 46 Ma) than the Tso Morari unit ( 53 Ma), implying that the Tso Morari was subducted earlier ( 57 Ma) than the Kaghan unit ( 52 Ma). The age difference likely reflects the initial shape of Greater India, with the Kaghan unit located greater than 300 km south of the Tso Morari before the collision of two continents. We calculate the dip of the subducting plate using two independent methods. The results show gentle dipping subduction east of the western syntaxis, and steep subduction west of the syntaxis since the time of India–Eurasia collision to the present time. We propose that the steep subduction in the western part is likely related to the proto-Chaman and Karakorum faults along which the Indian plate moved northward. In the eastern part, the overlying Eurasian plate extruded to east, which allowed gentle dipping subduction of the Indian continent. Although the main period of eastward extrusion of the Eurasian continent occurred between 30 and 15 Ma, our results suggest that this was likely taking place since the early India–Asia collision. Using those geometrical constraints, a 3D image of the slab is reconstructed in the western part, showing the sharp bending of the western syntaxis along the proto-Chaman fault. This bending resulted in the warping of the slab surface to form a conical fold with a north-dipping axis located near the western syntaxis

Tectonic Inversion and Geomorphic Evolution of the Algerian Margin Since Messinian Times: Insights From New Onshore/Offshore Analog Modeling Experiments

International audienceTectonic inversion of passive margins is a common but poorly documented process preceding subduction inception. We perform here a comprehensive land-sea experimental modeling of this key process by reproducing the morphotectonic and sedimentary evolution of the central Algerian margin over the last 6 Myr. Our approach is based on scaled analog models integrating interactions between crustal shortening and surface processes, including erosion, water transport, sedimentation, gravitational instabilities, and base-level changes. A challenge was to simulate the effects of the Messinian Salinity Crisis (MSC) through a major sea-level oscillation and halite deposition. By using realistic boundary conditions, adapted analog material, and robust, first-order parameters for physiography setups, we successfully reproduce the morphotectonic domains and the time-dependent geometrical relationships between fluvio-deltaic sedimentary systems, erosional surfaces, and thrust faults as observed since Messinian times. Our results highlight (1) the key role played by the MSC sea-level oscillation on an ultra-fast building, destruction and re-sedimentation of fans and deltas from the upper slope to the abyssal plain; (2) the development of a large popup structure subparallel to the coastline, with progressive strain migration from the backthrust on land toward a frontal thrust of opposite vergence at mid-slope and the margin toe; and (3) the importance of lateral changes in initial wedge shape and strain distribution for determining the non-cylindrical geometry of the margin and progradation of piggy-back basins during tectonic inversion. Our results support that the central Algerian margin is witnessing the early building of an accretionary wedge combining thin-skinned and thick-skinned tectonic styles

Brittle tectonics and fluids overpressure during the early stage of the Bay of Biscay opening in the Jard-sur-Mer area, (northern Aquitaine Basin, France)

International audienceBa, F, Pb, Ag, Zn mineral deposits are widespread at the northern and eastern boundaries of the Aquitaine Basin. In most cases, they are hosted within high permeability carbonates that rest over the Hercynian basement and below an impermeable layer. Such a position suggests a Mississippi Valley Type (MVT) model for the formation of these deposits. This model is characterized by the lateral flow of sedimentary fluids expelled from the deeper part of the basin and mixed with other sources of water as they reach the basin boundaries. In the Jard-sur-Mer area, which sits in the north of the Basin, these deposits are also found higher in the sedimentary series suggesting that fluids have flown through the impermeable layer. Our field observations demonstrate that a brittle deformation episode, compatible with an upper-Jurassic N-S direction of extension, occurred as the mineralizing fluids were over pressured. The overpressure was the result of a large input of hydrothermal water ascending along inherited faults affecting the Hercynian basement and released at the onset of the tectonics event. When compared with the rest of the basin, these new results at the northern boundary suggests that the Aquitaine Basin recorded several stages of fluid overpressure both at the onset and during the opening of the Bay of Biscay