Abstract
Following the Mw 4.9 Le Teil earthquake on November 11, 2019, which occurred along the La Rouvière fault, a segment of the North Cévennes Fault System (NCFS), significant data were collected to assess its Quaternary activity (Ritz et al., 2020; Ampuero et al., in prep.). However, the activity of other segments of the NCFS, including the Marsanne Fault, identified as potentially active by Jomard et al. (2017), remains unclear. To address this issue, we conducted paleoseismological investigations in Quaternary soils of the Rhône Valley. A deeper understanding of the overall NCFS structure is needed to better define its geometry and identify suitable trenching sites beneath Quaternary deposits. Using deep and shallow seismic profiles, we examined the tectonic history of the NCFS since the late Paleozoic, aiming to decipher its multiple deformation phases. Our findings indicate that the major faults of the NCFS are listric normal faults, which were predominantly active from the Early Cretaceous (135 Ma) to the Oligocene (30 Ma) under extensional and sinistral strike-slip regimes. These primary fault segments are rooted in a Triassic detachment level and interact with basement faults. Importantly, we demonstrate that the NCFS system was not significantly influenced by Miocene Alpine shortening. This model, applied to near-surface Plio-Quaternary sediments, enabled the identification of three trenching sites along the Marsanne Fault. Combined with other geophysical methods, we were able to better constrain the fault zone's morphology and the characteristics of the Quaternary deposits. Paleoseismological interpretation of these trenches revealed Quaternary activity on the Marsanne Fault, with the main tectonic evidence suggesting reverse faulting between 320 ka and 271 ka. Based on the surface displacement, without considering surficial deformation processes, we estimated a magnitude Mw 5.8 for this event. However, trenching in Holocene Rhône sediments showed no signs of fault reactivation during the Holocene to present. Additionally, a second surface rupture indicator was identified on another NCFS segment, marked by fissures in alluvial deposits. However, due to incomplete dating, the precise age and origin of this deformation remain uncertain. The integration of these new data updates the active fault mapping of the Cévennes, enhances our understanding of the geodynamics of the Southeast Basin, and provides insights into the surface rupture hazard in the Rhône Valley. These findings, combined with the work of N. Cathelin's thesis, will be incorporated into seismicity rate calculations, contributing to the refinement of seismic hazard assessments in the middle Rhône Valley.