Fault architecture and growth in clay-limestone alternations: insights from field observations in the SE Basin, France

Document available in extended abstract form only. The Callovo-Oxfordian (COX) clayey formation is currently studied by Andra in 'Meuse/Haute- Marne' (MHM), eastern Paris basin (France), for hosting a disposal of high level and intermediate, long-lived radioactive waste. As an independent organisation performing safety reviews for the Nuclear Safety Authority, IRSN conducts studies in support of the review of this disposal project. This nearly 130 m-thick clayey formation is surrounded by two 250 m-thick limestone formations. In such limestone/clay alternations, tectonic fracturing is often observed within limestones and propagates in some cases to clay layers. Such a propagation through the COX within or close to the disposal area could diminish its containment ability by creating preferential pathways of radioactive solute towards limestones. Nevertheless, minor to moderate fracturing is difficult to investigate in hectometre scale multilayer systems such as COX: seismic reflexion surveys only provide data on major faults, drilling data are too localised and clays have a 'bad-land' aspect at surface. The aim of this study is to provide a model of fracturing across clay-limestone alternations so as to strengthen the assessment of their possible development. We thus investigated fracturing within decametre-sized clay-limestone alternations, located in the South-Eastern Basin (France), to determine the evolution of fault architecture during its growth. After analysis of the possible scale effects using data from other analogous fields, an application to the COX in MHM is presented. We studied minor normal faults that reflect various stages of development, from simple fault planes restricted to limestones to complex fault zones propagated across several clay-limestone layers. The analysis of the fault characteristics, the construction of displacement profiles and the results obtained using numerical models enlighten fault growth processes, i.e. nucleation, restriction and propagation. We showed that studied faults nucleated in the limestone layers. After their nucleation, the faults propagated within a homogeneous medium with a constant displacement gradient until their vertical propagation is stopped by a 'restrictor'. The evidenced restrictors correspond to limestone-clay interfaces or to sub-horizontal faults in clays. During the slip accumulation along the fault, the displacement gradients stay constant and small in the fault centre, and gradually increase at near-tip up to a threshold, leading to the fault propagation across the restrictor. Fault restriction may be related to the contrasts of stiffness and strength between the clay and limestone layers. Displacement gradients along the faults are specific of each lithology, with larger values in clays (0.06 to 0.2) than in limestone layers (≤0.08), which indicate that clays discourage the fault propagation. The vertical propagation of faults appears to be continuous, i.e. it does not result from the development of pre-existing fractures in limestones. The fault segments exhibit refractions as a function of lithology: dips are less important in clays (40-55 deg.) than in limestones (62-85 deg.). During the fault growth, its architecture evolution depends on the layer thickness and the existence of restrictors. The transposition of these metre-sized observations to hectometre-thick layers such as in MHM raises the problem of possible scale effects. Other studies were carried out by IRSN on layers showing larger thicknesses. In the Maltese Islands, the 70 m thick Blue Clays outcropping along the seashore between two 10's m thick limestone formations of Oligo-Miocene age was affected by slight extensional tectonics. The minor normal faults present refractions, and appear to spread following a continuous model. In addition, the Blue Clays/underlying limestone interface appears to play the role of restrictor. In several sites, the faults propagated in Blue Clays with constant displacement gradients close to 0.1, while gradients in limestones appear to be almost null. Fracturing was also examined in a tunnel and drifts located in IRSN's Tournemire experimental station, crossing a 200 m-thick Toarcian clayey formation, located between two hectometre-thick limestone formations. In the Toarcian clays, joints are absent and minor normal faults have less important size and thickness as well as slightly lower dips (40-50 deg.) than observed on the plateau in the overlying limestones (> 60 deg. dip), likely confirming fault refraction. The main normal fault zone crossing the Tournemire Station shows a slightly weaker dip in clays than in limestones and the vertical throw appears dampen with an estimated gradient of 0.11. These results, associated with the bibliography that shows that the majority of the characteristics of faults described above occur at all scales, suggest that the scale effect is not prohibitive for the transposition of our results to the MHM multilayer system. We investigated the propagation depth in the COX of a hypothetic normal fault showing a throw of 10 m in one surrounding limestone formation; such a fault would be at the detection limit of classical 2D seismic-reflexion surveys. We postulate that the MHM system only consists of three homogeneous layers and that the layer interfaces are sharp. We also solely focus on a hypothetical non-reactivated normal fault. Finally, even if it is not fully verified, we assume that all the faults initiated in one of the surrounding limestones rather than in the COX. Taking into account the rock physical properties and the layer thicknesses, we estimated possible displacement gradients in limestones and in the COX, as well as a threshold gradient for propagation across the limestone/COX interfaces. We then deduced at which distance from one limestone/clay interface the fault needs to initiate in limestones to propagate 30 m or 60 m in the COX, or to crosscut the whole COX