First Paleoseismological Trench in Northwestern France: A Multidisciplinary Study along the South Armorican Shear Zone.

The South Armorican Southern Shear Zone (SASSZ), located in the northwestern France within the Armorican Massif, represents a major structural feature inherited from the Variscan orogeny. Although this region is now far from active plate boundaries and characterized by very low strain rates (i.e. 10^-9 yr^-1), it’s characterized by a moderate and diffuse seismicity associated with a few large events (up to M~5), suggesting possible fault reactivations.

This study integrates high-resolution mapping, geophysical investigations, and paleoseismic trenching to decipher the SASSZ structure and its possible quaternary activity. Based on high-resolution DEMs (LiDAR, RGEALTI from IGN), the analysis of morphological scarps along the SASSZ shows a wide range of surface trace complexities (bends, secondary splays, step-overs, gaps) associated with initial ductile and more recent brittle deformation. The width of the deformation zone around the SASSZ can reach up to 4 km, alternating between a localized and a distributed shear zone from the Pointe du Raz to Nantes. These measurements are in agreement with slope measurements performed along the SASSZ: the wider the deformation zone (> 0.3 km), the lower the maximum and mean slopes associated with the scarps.

Three geophysical surveys were conducted at sites of interest, along the SASSZ, in order to connect observed scarps at the surface with variations in crustal physical properties. They reveal distinct resistivity contrasts consistent with surface scarp locations. At the Moulin Quilly site, two paleoseismic trenches were excavated across two sub-parallel scarps. Trench 1 across the main surface scarp is not associated with a clear lithological contrast. However, the foliated granitoids are affected by several families of fractures oriented from N50°E to N120°E. The main structure is located at the base of the scarp and is made of sub-horizontal goethite deposits filling a N120°E trending open fracture of 10 cm wide, in the same direction as the SASSZ. Trench 2 crosses a secondary scarp and is divided in to three main structural units: (1) a slightly weathered granite unit preserving subvertical foliation and affected by cryogenic processes dated between 20 and 30 ka (from Optically Stimulated Luminescence dates on sand deposits); (2) a narrow transition unit, associated with a high-strain zone showing sub-vertical fabrics filled by sands; and (3) a fine-grained, strongly altered ultramylonite unit dipping 15°–25° northeast. All units are covered by an undeformed modern soil. From the subvertical fabrics in the transition unit, oriented samples were collected for microstructural analysis. Thin sections in the altered fabric show well-oriented minerals, alteration veins, and kinematic indicators that document higher deformation and alteration processes than in the granite. Further analyses will be conducted to quantify the strain distribution, in close comparison with the dating results.

Our study highlights a brittle deformation phase of the SASSZ, either linked to a recent tectonic activity, or associated with the Mesozoic regional extension, but the latter raises questions about the preservation of surface morphology through geological times. Future dating results of goethite deposits will help clarify whether the brittle fractures and their subsequent infilling reflect quaternary activity or an older phase of deformation.