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 multi

Thirty years of paleoseismic research in metropolitan France

“…Analysis of geological marker offsets on balanced cross sections [Bestani et al, 2015, Champion et al, 2000, [Chardon and Bellier, 2003] confirm that tectonic deformation is slow ( 1 mm/yr) [e.g., Siame et al, 2004, Chardon et al, 2005, Chardon and Bellier, 2003, Cushing et al, 2008. In addition, assessment of fault activity based on morphological evidences is subdued because denudation rates are comparable to or faster than fault slip rates [about 30-60 mm/ka: Godard et al, 2016, Siame et al, 2004, Thomas et al, 2020, 2018. This implies that tectonic geomorphic markers tend to be erased, hence, no single stand-alone method can be used to accurately evaluate seismic deformation.…”

“…These trenches were not conclusive given the very poor Quaternary sedimentary settings specific to this region. It should be noted that as part of the ITER preliminary program, a trench showing a fault affecting Pliocene up to early Quaternary deposits was also excavated in 1995, in the south of Vinon-sur-Verdon [see in Thomas et al, 2020].…”

“…The north Vinon thrust does not display Quaternary tectonic activity as Miocene and Pliocene units are lying unconformably on top of folded Cretaceous and Jurassic units, sealing the fault plane. Thomas et al [2020] proposed that the Maragrate Fault is a sub-vertical fault affecting the sedimentary sequences up to the Pliocene. Thomas et al [2020] proposed a depth anchoring of this fault connected to the north Vinon thrust approximately 500 meters below the surface, suggesting strain partitioning between the two faults.…”

“…Thomas et al [2020] proposed that the Maragrate Fault is a sub-vertical fault affecting the sedimentary sequences up to the Pliocene. Thomas et al [2020] proposed a depth anchoring of this fault connected to the north Vinon thrust approximately 500 meters below the surface, suggesting strain partitioning between the two faults. F3).…”

“…Even if the Maragrate contact has been interpreted as testifying for a fault, the question for the relationship between this Quaternary deformation and seismic activities has long been raised. A recent multidisciplinary study has been implemented along this fault [Thomas et al, 2020]. The first challenge in this analysis was uncovering potential deformed Quaternary surfaces and fault scarp morphologies, often found under dense vegetation cover or in urban areas.…”