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During the Mesozoic and the Cenozoic periods, Europe recorded several tectonic events that have been interpreted as far-field tectonics. This may be diffuse tectonics that occur at the onset of a major tectonic event or propagation of the deformation away from orogenic fronts. 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 “Northern Aquitaine Platform”. We combine field observation with Unmanned Aerial Vehicle (UAV) imagery and U-Pb laser dating on calcite to determine the tectonic evolution in the Vendée Coastal domain. Our results highlight the occurrence of two main tectonic events since the early Jurassic: (1) At the transition from the Late Jurassic to Early Cretaceous, WNW-ESE striking normal faults exhibit a horst and graben pattern that emplaced at the onset of the opening of the Bay of Biscay. Their association with reactivated Variscan strike-slip faults is consistent with oblique extension processes. Such a tectonic event was accompanied by fluid flow coming from depth. (2) During the Late Cretaceous, fractures, wide-open folds, veins, and joints are consistent with the N-S shortening direction that occurs during the earliest stages of the N-S Pyrenenan compression. In both cases, the early stages of the main regional deformation stages are recorded in the “Northern Aquitaine Platform” in a far-field position and before the focus of strain in rifts or orogenic belts. In the Aquitaine basin as in many other places in Europe, the tectonic study of sedimentary platforms located far from the main plate-scale deformation zone provides new constraints on the early diffuse deformation process that predate the main tectonic phases.
During the Mesozoic and the Cenozoic periods, Europe recorded several tectonic events that have been interpreted as far-field tectonics. This may be diffuse tectonics that occur at the onset of a major tectonic event or propagation of the deformation away from orogenic fronts. 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 “Northern Aquitaine Platform”. We combine field observation with Unmanned Aerial Vehicle (UAV) imagery and U-Pb laser dating on calcite to determine the tectonic evolution in the Vendée Coastal domain. Our results highlight the occurrence of two main tectonic events since the early Jurassic: (1) At the transition from the Late Jurassic to Early Cretaceous, WNW-ESE striking normal faults exhibit a horst and graben pattern that emplaced at the onset of the opening of the Bay of Biscay. Their association with reactivated Variscan strike-slip faults is consistent with oblique extension processes. Such a tectonic event was accompanied by fluid flow coming from depth. (2) During the Late Cretaceous, fractures, wide-open folds, veins, and joints are consistent with the N-S shortening direction that occurs during the earliest stages of the N-S Pyrenenan compression. In both cases, the early stages of the main regional deformation stages are recorded in the “Northern Aquitaine Platform” in a far-field position and before the focus of strain in rifts or orogenic belts. In the Aquitaine basin as in many other places in Europe, the tectonic study of sedimentary platforms located far from the main plate-scale deformation zone provides new constraints on the early diffuse deformation process that predate the main tectonic phases.
The Variscan orogen in southern Ireland and Britain is characterized by an intensely deformed, E-W−trending fold-and-thrust belt. Farther north in Ireland, the Carboniferous North Dublin Basin exhibits tight chevron folds and kinematically linked en echelon vein sets, along with bedding-parallel veins with slickenfibers. This deformation is assumed to be Variscan in age, despite lying 150 km north of the supposed Variscan “front.” The laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) U-Pb dating of these calcite veins undertaken for this study showed that relict Variscan U-Pb ages are very poorly preserved. Instead, late Eocene ages were obtained from many calcite veins, including fold hinge breccias and bedding-parallel slickenfiber veins associated with N-S shortening (flexural slip). Also, U-Pb ages from one bedding-parallel vein showed protracted flexural slip over ∼5 m.y. during late Eocene times. Detection of the growth domains within this vein was facilitated by the imaging approach to LA-ICP-MS U-Pb dating adopted in this study, which can identify age-homogeneous domains by integrating spatial U-Pb isotopic information with maps of petrogenetically diagnostic major and trace elements. The late Eocene fold reactivation phase was hitherto undetected on the Irish mainland, but regional Cenozoic N-directed shortening has been documented in Mesozoic−Cenozoic sequences of the southern Irish Sea, Celtic Sea, southern England, and the Paris Basin. We attribute late Eocene fold reactivation to far-field, N-directed shortening associated with the Alpine/Pyrenean orogenies. It is likely that many Variscan or Caledonian folds, particularly in southern onshore Ireland, were reactivated during Eocene−Oligocene shortening, which has not been recognized to date because of the lack of post-Variscan markers (e.g., dikes, Mesozoic−Cenozoic cover sequences).
The “Calcaires du Barrois” Formation is a succession of dominantly micritic limestone of Kimmeridgian to Tithonian age, outcropping in the eastern part of the Paris Basin. This is an active karstic aquifer of main interest for the Andra (French National Agency for Radioactive Waste Management) who study the feasibility of a deep geological repository of radioactive waste in an Underground Research Laboratory (URL) located approximately 450m below the surface. Surface installations of the CIGEO (Industrial Centre for Geological Disposal) project are planned to be located in the upstream recharge zone of the aquifer. It is of primary interest to characterise the “Calcaires du Barrois” Formation to provide guidelines for the planning and the sizing of these facilities, with the objective of minimising the impact on the aquifer system. An integrated study was designed for this purpose linking petrography (thin section, and SEM, Scanning Electron Microscope), C & O stable isotope geochemistry, XRD (X-Ray Diffraction), petrophysics and geomechanics, and based on the analysis of three key cored wells penetrating the formation at different relative depths. The “Calcaires du Barrois” underwent several stages of diagenesis that defined the current properties. Unconformities associated with the Jurassic-Cretaceous transition led to prolonged early subaerial exposures during which freshwater flowed efficiently through the upper half of the formation. Through mineralogical stabilisation, among other processes, microporosity was preserved in micrites in this interval consisting of clean limestone with thin marl layers. The lower half of the formation, more argillaceous, was not or only slightly affected by this early meteoric diagenesis and recrystallization and cementation of micrites occurred during burial diagenesis, involving chemical compaction. Later, during the return to the surface associated to the Cenozoic orogens, another phase of meteoric diagenesis affected the uppermost few metres below the outcropping portions of the formation, but without modifying significantly the previously acquired petrophysical properties. Consequently, an intra-formational boundary was progressively developed at around 75m (from the top reference). This boundary separates (1) a lower half of the “Calcaires du Barrois” with dense and tight micrites, showing high Young's Modulus values, and a moderate intensity of fractures, from (2) a upper half with microporous micrites showing low Young's Modulus values, and almost devoid of fractures. A transitional zone of about 30m-thick, with intermediate properties, sitting above this boundary and below the only thin metre-scale macroporous grainstone level of the formation, accommodated most of the deformation linked to the Cenozoic west-European orogens and is intensively fractured. The current hydrogeological model considers a purely sedimentological boundary to delimit two sub-aquifers within the “Calcaires du Barrois” Formation, but will have to be reappraised since it is here demonstrated that the real boundary is located significantly higher in the formation and is inherited from a multi-stage diagenetic history. These findings will complement and influence planning for engineering of the CIGEO project.
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