Origin of Regular Networks of Joints: Experimental Constraints, Theoretical Background, and Numerical Modeling

International audienceExperimental data show that inelastic straining occurs even at very low pressure before and during brittle fracturing. This process is therefore investigated within the framework of elastoplasticity using 2‐D, three‐layer finite difference modeling. The constitutive model includes both tensile and shear failure mechanisms coupled at the level of the strain softening law. The modeling results show that sets of parallel joints initiate as pure dilation bands, the narrow σ3‐normal bands of localized dilatant damage (inelastic deformation). The band thickness, length, and the initial strain softening degree within it are proportional to the ductility of the material, which increases with the effective stress level (σ1) or pressure. The strength reduction within the bands is accelerated at a certain stage, and the strength locally reaches zero resulting in fracture initiation. The initial fracture then propagates in Mode I following the propagating band. The fracture (joint) appears thus as a band of damaged material with the increased porosity, which is maximum along the axial zone of the band where the material is completely broken. The damage is due to both tensile and shear mechanisms. The role of shear failure increases with the ductility (pressure) increase, which also leads to the band thickness increase. These processes can result in small (band thickness)‐scale oblique shear fractures within the band, causing the increase in the roughness of fracture walls organized in plumose patterns typical of both natural and experimentally generated joints

Surface rupture and shallow fault reactivation during the 2019 Mw 4.9 Le Teil earthquake, France

International audienceThe Rhône River Valley in France, a densely populated area with many industrial facilities including several nuclear power plants, was shaken on November 11th 2019, by the Mw 4.9 Le Teil earthquake. Here, we report field, seismological and interferometric syntheticaperture radar observations indicating that the earthquake occurred at a very shallow focal depth on a southeast-dipping reverse-fault. We show evidence of surface rupture and up to 15 cm uplift of the hanging wall along a northeast-southwest trending discontinuity with a length of about 5 km. Together, these lines of evidence suggest that the Oligocene La Rouvière fault was reactivated. Based on the absence of geomorphic evidence of cumulative compressional deformation along the fault, we suggest that it had not ruptured for several thousand or even tens of thousands of years. Our observations raise the question of whether displacement from surface rupture represents a hazard in regions with strong tectonic inheritance and very low strain rates

Where are the limits of Mesozoic intracontinental sedimentary basins of southern France?

International audienceReconstitution of the geometry of sedimentary basins is fundamental to understand the nature of present sedimentary rocks and the economic potential in hydrocarbon and mineral resources. Present-day topography of southern France shows elevations growing between the Meso-Cenozoic Aquitaine Basin to the South-East Basin across the Variscan domain and the Jurassic Causses small basin with maximum relief in the Cévennes area. Present-day elevation offset is of approximately 1000 m. This geometry questions the paleogeography and dynamics of these various domains and the relative elevation of the Variscan domain during the subsidence of adjacent Aquitaine and South-East Basins. In this study, we investigate the geological history of the Variscan basement high and the Causses small basin using paleotemperatures deduced from organic matter analysis, low temperature thermochronology and regional geological constraints.Lower Jurassic (upper Pliensbachian and Toarcian) marls sampled across the area from the Aquitaine Basin to the South-East Basin have similar depositional environments containing mainly type III oganic matter, and close Tmax values ranging between 430 and 440 ± 2 °C. These data show that the entire south Massif Central has undergone a similar burial history. considering that these values are explained by burial. Low temperature thermochronology data have been acquired on basement rocks outcropping on the borders of sedimentary basins (Rouergue, Cévennes and Margeride). Fission-track ages are ranging between 74 ± 5 and 187 ± 6 Ma and track lengths between 11.5 ± 0.3 and 13.6 ± 0.1 μm; apatite (U–Th)/He corrected ages are ranging between 65 ± 5 and 184 ± 15 Ma. Data inversion with the software QTQt indicates a cooling episode starting at the end of Early Cretaceous or beginning of Late Cretaceous from maximum temperature of 100 ± 10 °C in the Rouergue and Cévennes area and from 80 ± 10 °C in most of the Margeride area.Thermal indicators are compatible with the erosion of a Middle/Upper Jurassic and Cretaceous sedimentary cover of 1400 ± 400 m assuming a thermal paleogradient of 35 °C/km. The preserved sedimentary cover attests of a Middle and Upper Jurassic – Lower Cretaceous sedimentary cover of 1000–2000m in the Aquitaine and South-East basins. This erosion phase occurred during mid-Cretaceous and is associated to a major geodynamical event characterized by large amplitude (from the Aquitaine Basin to Durancian doming in the South-East Basin) and by kilometric offset. We interpret also these data to show that marine connections have existed between the Aquitaine Basin and the South-East Basin during the Jurassic and likely Early Cretaceous. The present-day morphology of the area has then been acquired after Cretaceous times and may result from the Pyrenean orogenic event during Eocene times