Geometry and tectonic history of the Northeastern Cévennes Fault System (Southeast Basin, France) : new insights from deep seismic reflection profiles

Abstract

Following the Mw4.9 Le Teil surface rupture earthquake that occurred on the north-eastern Cévennes fault system (NCFS) in France, several investigations were carried out to understand the origin of the earthquake rupture. A few studies performed local modeling of the NCFS structures in three dimensions integrating the rheology of the sedimentary layers within the hypocenter zone. However, the geometry of the NCFS at the scale of the Southeast French Basin is poorly constrained and it remains difficult to locate its trace beneath the Quaternary sediments of the Rhône river valley. To address this issue, Électricité de France (EDF) carried out a deep reflection seismic survey along the NCFS. This new set of seismic profiles was interpreted using a geological data base including surface data, well data, and previous seismic data that were reprocessed. The resulting 3D structural model allows us to reconstruct a polyphase geological history during the past 320 Ma, which we divide into three major tectonic phases. We show that all structures in the basin in the study area were initiated as normal faults during the Lower Jurassic and the Lower Cretaceous. During the Upper Cretaceous, these structures were reactivated, acting as a major transfer fault zone during the Pyrenean shortening phase, then as normal faults during the Oligocene extension. The morphology and faults at the top-Carboniferous basin initiated during the Lower Jurassic strongly shaped the final structure of the NCFS during the subsequent tectonic phases. Our new results allow updating the historical geology of the Vivaro-Cévenol region and our knowledge about the structures that have affected the Southeast Basin since the Mesozoic. In the context of the Le Teil earthquake, our new structural model provides important constraints for continuing paleoseismological works that will better assess the seismic hazard in this region.

Keywords

• le teil earthquake
• cévennes fault system
• seismic profiles
• structural model
• southeast basin
• rhône river valley