ORCID Profile
0000-0002-7351-8877
Current Organisation
CNRS Délégation Languedoc-Roussillon
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Publisher: Copernicus GmbH
Date: 23-06-2023
DOI: 10.5194/EGUSPHERE-2023-765
Abstract: Abstract. We present 27 new burial ages based on 26Al / 10Be ratios of terrestrial cosmogenic radionuclides measured in clasts and sediments deep within 12 caves in the southern Massif Central, France. Our results together with previously published burial ages, verifies that cave morphogenesis has been continuously active in this region for at least the past ~6 Myrs. Combining s le burial ages with their associated cave elevation above modern stream bed gives a mean regional incision rate of 88 ± 5 m/Ma for the Grands Causses area. South of the Cevennes Fault zone bordering the Grands Causses, the incision rate is 43 ± 5 m/Ma, suggesting that this difference might be accommodated by the fault zone. Sediment burial ages from caves which are not located on river valley flanks or cliff walls are surprisingly too young relatively to the expected age calculated using this regional average river incision rate. This suggests that the classical epigenic speleogenesis model that presumes a direct correlation between cave level development and regional base level lowering does not apply for the study area. Therefore, we propose that regional speleogenesis is mainly controlled by removal of ghost-rocks by regressive erosion from river canyons to central parts of the plateaus, emptying incipient primokarst passages to create cave systems. Our results suggest a continuum process from hypogene primokarst composed of ghost-rocks filled passages to epigene karst dynamic emptying these passages and creating cave networks. We propose this is a major process in the southern Massif Central that initiates the speleogenesis and control the geometry of the networks. In this region tiered karst cannot be associated with major incising rivers but must be explained by former ghost-rocks (or hypogene) processes.
Publisher: Copernicus GmbH
Date: 23-03-2020
DOI: 10.5194/EGUSPHERE-EGU2020-9099
Abstract: & & Although more and more processes are discussed and discovered on the genesis and evolution of cave systems, the tiered karsts are often explained by a control of the base level evolution. In this classical model, the horizontal galleries are explained by a stability of the base level elevation. To the contrary, the shafts and network segments with steep slopes are related to incision periods with a base level lowering.& & & & We use Terrestrial Cosmogenic Nuclide Geochronology to estimate burial ages of alluvium trapped in several caves of the Larzac plateau in Southern France. All the s les are collected in horizontal cave levels, sometimes located between steeper segments. Some caves are opened in river gorge walls, while others are located below the Larzac plateau not farther than 5km away from the river gorges.& & & & The burial ages for the caves opening in the gorges are consistent with the incision rates given for the area and could be interpreted using the classical model. However, the cave within the plateau show a horizontal level with alluvium deposited 200m above the caves in the gorge with the same burial ages (~1 Myr). Since then, new shafts have been opened without alluvium and are hydrologically connected to the river by deeper[jfr1]& hypogenic galleries. The cave morphologies and the geochronological data suggest that the classical model fails to explain the horizontal levels in cave below the plateau. We postulate that the geometry of the caves in these limestone and dolomite plateaus are related to a previous period of ghost-rock and alteration roots formations. Without the opening of an efficient connection between this primokarst and the valley, no alluvium can flow through the cave. Therefore, we think that our burial ages constrain the emptying of the ghost-rocks leading to the genesis of the cave where water and possibly alluvium can flow through. Furthermore, these new finding explain why the horizontal levels in the caves are not clearly related to horizontal markers in the surface geomorphology and why large shafts (& m) exist in the area without evidences of long periods of base level stability followed by large drop of the regional base level.& & & & & & & & & & / & & / & & / &
Publisher: Copernicus GmbH
Date: 23-06-2023
Publisher: Copernicus GmbH
Date: 23-03-2020
DOI: 10.5194/EGUSPHERE-EGU2020-8547
Abstract: & & & span& & span& In the 60& #8217 s, the formulation of the plate tectonic theory changed our understanding of the Earth dynamics. Aiming at explaining the earth first order kinematics, this primary theory of plate tectonic assumed rigid plates, a necessity to efficiently transfer stress from one boundary to another.& & /span& & /span& & & & & & span& & span& If successful to explain, at first order, the plate-boundary evolutions, this theory fails when compared to the unpredicted but identified deformation located inside the plate-domains: the intraplate orogens. Indeed, the intraplate regions are thought to be slowly, if at all, deforming. Therefore, it is expected that intraplate regions do not show important finite deformation, that is to say, no mountains. Some intraplate regions, however, have important relief: the Snowy Mountains (Australia), the Ural Mountains (Russia) or the Massif Central (France) for ex les. Traditionally, such regions are interpreted as old structures that are slowly eroded, interpretations that are most of the time weakly constrained.& & /span& & /span& & & & & & & & & & & span& & span& Our study is aiming at providing stronger constraints and then a better understanding of such challenging area that are the intraplate orogen domains. Because direct measurements of deformations (e.g. GNSS: Global Navigation Satellite System or InSAR: Interferometric Synthetic Aperture Radar) are most of the time below the precision level, it is necessary to derive this information from the landscape evolution. To do so, terrestrial cosmogenic nuclide (TCN) technics are a key method, allowing to constraint the temporal landscape evolution. Classically, two TCN-based approaches are used to quantify the landscape evolution rate: burial ages and watershed-wide denudation rates, based on measurement in quartz sediment of 10Be and 26Al concentrations, two radioactive cosmogenic isotopes.& /span& & /span& & & & & & & & & & & span& & span& Using the Massif Central (France) as study area, we show that this region is currently deforming.& /span& & /span& & & & & & span& & span& From new geochronological constraints and a geomorphometric study, we propose that the region undergoes an active uplift encompassing the last c.a. 4 Ma. It can be explained by the combination of at least two phenomena: the first one is the uplift triggering event, that has yet to be clearly identified, and the second one: the erosional isostatic adjustment enhancing the first one and possibly continuing after the end of the first one.& & /span& & /span& & &
No related grants have been discovered for Jean-Francois Ritz.