Abstract
Stable continental zones such as metropolitan France are characterized by slow deformation. As a result, fault slip rates are low and return periods long. However, these zones are not devoid of seismicity and can produce major earthquakes (e.g. Lambesc 1909 – Mw 6.0). The seismic hazard is therefore not negligible and needs to be better assessed. One way of improving estimates of long-term deformation (> 10 ka) is to measure large-scale displacements using certain morphological objects. In this study, we combine the use of 14C and cosmogenic 3He to date ancient alluvial terraces of the Escoutay River. This river has the particularity of crossing the Cevennes fault system (CFS) where the Le Teil earthquake (11.11.2019) of Mw 4.9 recently occurred on a fault considered to be inactive. This event has called into question our knowledge of regional geodynamics, the seismogenic potential of the CFS and the seismic hazard in the middle Rhône Valley where many sensitive infrastructures are located. By dating MIS 2, 3 and 4 river terraces we constrain an incision rate of [0.43 - 0.70] mm.y-1 along the Escoutay valley. This incision rate is consistent with previous studies carried out along the southern CFS on a 100 ka timescale. When compared to studies on the Ma timescale, this proposed incision rate is one order of magnitude faster. After discussing internal and external factors, we consider local tectonics as the primary driving mechanism of incision implying an acceleration in uplift rate since the Late-Pleistocene along the CFS. These preliminary results are a first milestone of a more extensive work that aims to compare across-fault incision rates along the CFS between its northeastern and southwestern extremities, near Montélimar and Montpellier, respectively.