Looking for Quaternary Fault Activity in the Armorican Massif (R8 FACT region): Preliminary Results of a Geophysical Analysis Along the Southern Armorican Shear Zone.

The recent surface ruptures observed after the Mw4.9 Le Teil earthquake in 2019 has rekindled the need to better characterize active faults in France. The major lesson from this earthquake is that the activity of most quaternary faults is not well expressed at the surface due to the combination of surface processes (erosion, anthropization, etc) and very low strain rates. Thus, it is necessary to use a paleo-seismological approach to uncover the signs of quaternary deformations below the ground surface. The "FACT" (Failles ACTives) initiative, part of the "Action Transverse Sismicité" of the French seismological and geodetical network (Résif), is a national group of French scientists working on these research topics. In this study, we focus in the northwestern part of France, which is cut by a set of major faults and shear zones, mostly inherited from Protero-Paleozoic orogens (Cadomian, Variscan), and extensional (Meso-Cenozoic) and compressional (Cenozoic) tectonic phases. Despite the apparent low strain rates, the region has experienced significant earthquakes in historical times, and continues to record a fairly high activity of moderate size instrumental earthquakes (M~3-4). Among the numerous faults in the region, the Southern Armorican Shear Zone (SASZ) experienced these past tectonic phases and still has a well-expressed morphology. We use high resolution topographic data to study multiple fault scarps and their relation with both river beds and recent alluvial deposits to understand if there has been any quaternary deformation. We will present sites of interest for potential paleoseismological investigations. We will show preliminary results of a geophysical analysis performed at one of these sites where a scarp of the SASZ cross-cuts a small quaternary alluvial fan. The across-strike geo-electric profiles indicate a large variation of apparent resistivity in good agreement with the surface scarp, which would highlight the transition between granitoids and quaternary sediments indicating possible recent movement on this structure.