Assessing post-pliocene deformation in a context of slow tectonic deformation: insights from paleoseismology, remote sensing and shallow geophysics in Provence, France

Abstract

The Provence region, located in the southeast of France, has experienced a few destructive earthquakes during the last centuries, such as the 1909 Lambesc earthquake or the 1509 and 1708 Manosque earthquakes. However, faults in the area experience slow slip rates (< 0.1 mm/yr) that preclude quantification of deformation using geodetic measurements. Active faults in Provence have long recurrence intervals, and the region experiences erosion under a Mediterranean climate where surface markers of such deformation are rapidly erased. As a consequence, several faults in the region may go unnoticed despite having the potential to generate earthquakes. This work focuses on the Vinon-sur-Verdon area in Provence, where a relatively narrow fault structure, the Maragrate fault, brings into contact Miocene clays and Pliocene conglomerates. Because of its proximity to a major nuclear research facility, this fault poses a potential earthquake hazard that justifies studies aimed at gaining insight into its mechanics and Quaternary seismic activity. In this context, a multidisciplinary approach has been implemented. A LiDAR campaign was undertaken to generate a 30-cm high-resolution Digital Elevation Model that depicts in detail the current fault morphology. In parallel, an electrical resistivity campaign was conducted and three trenches were opened. These investigations revealed a fault contact expressed by a fault gouge zone surmounted and sealed by numerous recent Quaternary periglacial and high-energy channel units. Radiocarbon and optically stimulated luminescence dating allowed the chronological reconstruction of sediment deposition and erosion cycles and of the most recent deformation event, older than 20 ka. These investigations did not allow us to demonstrate with certainty that these deformations are of a co-seismic nature. This work highlights the difficulty of identifying faults and studying the seismic hazard in intra-continental contexts.