Proceedings of the 12th International INQUA meeting on paleoseismology, active tectonic and archaeoseismology

278 PATA Days 2024 Fig. 3 : (a) Stereographic framework paleostresses on a fault plane in the “Grand Pré” area. (b) Calcite steps observed on a fault plane, here with left-lateral strike-slip kinematics. As suggested by the potentially active faultmap of France (Jomard et al. 2017), the MVF signature in morphology splits as it reaches the Jura mountain range, with one section crossing the area of interest around Villards d’Héria, suggesting that the distribution of the deformation is controlled by surface lithology. Where "hard" rocks (limestones) are close to the surface, the subparallel segments are numerous (Jura domain, NW part of the fault). Conversely, fault segmentation is limited, and the geometry appears rather continuous where “soft” rocks (molasse) are dominant (Savoy area, southeastern part of the fault). A splay fault structure can thus be observed in our study area, which is located at the northwestern end of theMVF (Fig. 1a). Numerous fault segments can be observed in the morphology there (Fig. 1). Field investigations confirmed numerous NW-SE fault planes with left-lateral, strike-slip kinematics, evidenced by calcite steps (Fig. 3b). When associated with their conjugate faults (right lateral, NE-SW), the paleostresses accommodation dated by these faults can be well constrained (Fig. 3a). (Fig. 3a). The paleoseismological trench has been excavated in an area about 2 km south of Lac d'Antre. This area, called "Grand Pré", has the same characteristics as the Lac d'Antre area: it is an anticlinal offset by a fault segment, with a cumulative displacement of ~500m in sinistral kinematics, which led to the trapping of a river and the creation of a lake, now dried up. We therefore found lake sediments that seem to show continuity disorders with two supposed fault traces, which correspond with what we observed before the trench, on the ERT profiles made. However, the current trench does not allow us to clearly show an offset at the fault zone. For this reason, we plan to trench again, this time down to bedrock. A set of sediment cores was already retrieved from the lake in June 2023, and their analysis show laminated sequences with seismic features. We observed several centimetres vertical offset on laminae, but also areas of boudinage, a phenomenon made possible by stretching of the level concerned, the probable consequence of horizontal ground movements. The chronological model of the cores has not yet been finalized, but our initial estimates put the age of these tectonic markers at around the Antic period.