Fluid circulation at basin–basement interfaces in rift systems is primarily controlled by brittle fault networks evolution and major interfaces. In the Upper Rhine Graben (Vosges, France), fractured crystalline basement rocks host major geothermal reservoirs, although the relationship between present-day fluid circulation and inherited, relict hydrothermal pathways remains debated. This study investigates the structural and hydrothermal evolution of the Villé Basin (Vosges Massif, France), located on the western shoulder of the Upper Rhine Graben, and provides a natural analogue for fractured basement reservoirs. The Villé Basin exposes the unconformity between the granitic basement and Stephanian sedimentary cover, allowing direct observations of faulted basement–cover interfaces. An integrated approach combining structural field analysis, petro-mineralogical characterization, and geochronological dating was applied to constrain fault activity and fluid circulation episodes. Two main brittle fault systems are identified: (i) An E–W-oriented fault network is pervasive at the basin scale and connects the basement to the sedimentary cover. These normal faults with metric vertical displacements are associated with hematite mineralization, and localized late barite mineralization event, indicating several hydrothermal circulations. Rb–Sr ages on muscovite and biotite suggest fault activity during Carboniferous–Permian times associated with N020°E extensional constrains. (ii) A later N–S-oriented fault network is localized along major basin-bounding faults associated to kilometric vertical displacement related to the formation of the Rhine Graben. U–Pb dating of newly-formed anatase (TiO2) yields Jurassic-Cretaceous ages (142 ± 66 Ma) questioning the activation of brittle N-S faults, low-temperature clay assemblages (kaolinite, illite/smectite, calcite) indicate late fluid circulation during late stages of URG (Upper Rhine Graben) evolution. These results demonstrate that the structural architecture of fractured basement reservoirs is controlled by evolution of fault systems and their episodic (re-)activation. The Villé Basin highlights the importance of structural control when interpreting geochronological data and provides key insights into fluid flow processes relevant to better understand complex geothermal systems of the Upper Rhine Graben.
We report on the area of the Jausiers-Col de la Bonette road, which is the highest of Europe, based on personal observations and the literature, including the published geological maps (BRGM Infoterre application). The tectonic pile comprises, 1) the folded Barcelonnette Autochthon (Keuper evaporites to marly Upper Jurassic-Lower Cretaceous); 2) various Subbriançonnais units (Séolanes platform facies), 3) some slivers from the classical Briançonnais (e.g., Ventebrun sliver); 4) the Roche Chevalière sliver described since years as originating from the Acceglio domain of eastern Briançonnais ; 5) the unconformable Neoauochthon Priabonian-Rupelian of the Grès d’Annot Basin ; 6) various sub-units of the Parpaillon Flysch, i.e., the Cretaceous-Paleocene turbidites that form the bulk of the Embrunais-Ubaye (Emb-Ub) Nappe (Flysch and basal Subbriançonnais-Briançonnais slivers). The Chevalier sub-unit of the Parpaillon Flysch (ParC) extends on top of and below the Roche Chevalière sliver, whereas the main Parpaillon sub-units extend further ENE. The sheared, brecciated « Schistes à blocs » and « Restefond Tectonic mélange » (uncorrectly described as « Autapie nappe » in previous works) underlie the Parpaillon units when they extend on top of the Neoautochthon formations. The Roche Chevalière sliver is non-metamorphic contrary to the blueschist facies Acceglio-type units of eastern Briançonnais. Sampling of Roche Chevalière by the WNW-going Flysch prism occurred before the eastward subduction of the Briançonnais margin at c. 37 Ma. The Roche Chevalière sliver remained at low temperature close to 260 °C when it crossed the exhumed Briançonnais units (cf. Furfande klippes) ; it reached its frontal position at c. 30 Ma, when the Rupelian sediments were still rich in interstitial water. The complex setting of the area is linked to the post-Rupelian folding and faulting that affected the Flysch and underlying units.
The Cigéo project, led by the French National Radioactive Waste Management Agency (Andra), involves the future disposal of high-level and long-lived radioactive waste in a deep geological repository. The site is located between the departments of Meuse and Haute-Marne in north-eastern France. In order to better monitor local seismicity and enhance the discrimination between natural and anthropogenic seismic events in the vicinity (<40 km) of the future Cigéo site, a dedicated network of four seismic stations – known as the Réseau d'Ecoute Sismique (RES) – was installed in 2002 and has been operated since then by the French Alternative Energies and Atomic Energy Commission (CEA). Over the past 60 years, the CEA has recorded 268 natural seismic events within a 90 km radius of the Cigéo site, with the largest reaching a local magnitude (ML) of 3.9. The closest recorded earthquake occurred about 20 km from the site, with a magnitude of ML 1.7. Historical and early instrumental records indicated that seismic activity in this region was low, a finding confirmed following the implementation of the RES network which significantly improved the detection and location of small earthquakes and identification of numerous quarry explosions in this region. This enhancement has finally increased the number of seismic events processed annually with more than 1,000 quarry blasts detected annually and allowed to reduce the completeness local magnitude down to approximately ML 1.5. We highlight the improvements in terms of event location and hypocentral depth accuracy provided by the RES network by comparing the median primary and secondary azimuthal gaps and the dimensions of the error ellipses before and after its deployment. Furthermore, we relocate the entire catalogue using a 3D location algorithm along with a 3D global velocity model to obtain more reliable estimates of event locations and hypocentre depth and to better quantify the uncertainties.
Although 3D and even 4D seismic imaging have advanced significantly, some acoustic patterns in carbonate reservoirs remain poorly understood. Diagenetic effects are still largely overlooked in interpreting reflectors within these complex sedimentary systems. We present a field study based on an outcrop of a well-known carbonate formation, the “Oolithe Blanche” of the Paris Basin (Burgundy, France). This formation is currently one of the preferred targets for medium enthalpy geothermal exploitation in the Paris Basin. A quarry working face is meshed with a decimetric resolution. Each cell of the grid is characterized from a sedimentological point of view and assigned to a dominant facies. Using a portative device, the 2D section is acoustically investigated by measuring horizontal and vertical P- and S- waves velocities. P-wave velocities measurements contribute to calculate synthetic seismic using a wavelet frequency allowing to highlight reflectors and seismic facies inside the section. Comparison between the synthetic seismic profile and the facies description provides us three main results. Firstly, the study allows us to characterize the acoustic role of the cross-beds visible in two carbonate facies of the site. These structures preferentially accelerate acoustic waves, suggesting lower porosity and permeability in this facies. Secondly, the seismic reflectors align with the boundaries of rudstone facies. The initial lithological contrast is amplified by a firm ground atop the underlying structure, increasing acoustic impedance and enhancing the seismic response beyond that of a simple facies change. Finally, small-scale karst structures have been described and appear to generate a chaotic seismic facies at the outcrop scale.
The Malta Horst is a NW-SE trending, 30 km-wide structural high situated on the southern Hyblean-Malta Plateau, which is an African continental indenter in collision with Eurasia. Sediments consist of a shallow marine carbonate platform succession (Mesozoic to Oligocene) capped by Miocene pelagic carbonates and marl. Utilizing seismic profiles, well data, and outcrop observations, this study provides the first description of kilometre-scale contractional structures within the horst and analyses the reactivation of normal faults by transcurrent movement under NW compression. The tectonic evolution of this foreland region is defined by five distinct phases (A through E), alternating between extension and compression. This cyclicity reflects the interplay between the migrating Calabrian Arc and the converging African craton. Initial NE-SW trending faults (Phases A and B) developed during the Late Oligocene to Early Miocene, coinciding with platform drowning. Following Tortonian uplift (Phase C), accelerated migration of the Calabrian Arc during Phase D triggered N-S extension, establishing the NW-SE and NE-SW normal faults that bound the Malta Horst. The neotectonic regime (Phase E) marks a return to dominance of the NW-directed compression by the African craton as the Calabrian Arc migration decelerated. This regional stress field has reactivated the NW-SE marginal normal faults through strike-slip motion. The combination of transcurrent drag and regional compression has inverted Phase A NE-SW normal faults into oblique reverse faults. These thrusts sole along a weak top Eocene evaporite décollement, producing a series of folds and inverted basins within 10 km of the horst's northeast margin. Onshore, these structures are manifest as en echelon, non-cylindrical, and doubly plunging folds that define the topography of Malta's northeast coast. These shear zone structures are thin-skinned deformations in Oligo-Miocene sediments controlled by thick-skinned, W-E transcurrent movement in the crust and Mesozoic sediments. Ongoing compression suggests an increase in seismic risk proximal to the Maltese Islands, with significant implications for local geohazard frequency and magnitude.
