Browsing by Author "Malcles, O"

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    Burial ages, ghost-rocks and karst network structure. Insights from the Vis canyon (Southern France)
    (Groupe français de géomorphologie, 2020-10-21) Malcles, O; Vernant, P; Chéry, J; Ritz, JF; Cazes, G; Fink, D
    New burial ages by measuring 26 Al / 10 Be isotopic ratios in three cavities near the Vis canyon (south of France, south-eastern limit of Grands-Causses) challenge the classic karstogenesis model. used to explain the structuring of networks. Indeed, the subhorizontal levels of underground galleries are generally used as markers of the position of the regional base level ( ie , river). If the relationship between burial age and height relative to the regional base level expected using the classical model per descensumfor the Scorpions caves (1.2 ± 0.4 Ma, +105 m relative to the base level -rnb-) and Bergougnous (1.2 ± 0.3 Ma, +117 m rnb) is in agreement with the quantification of the average regional incision rate (83 ± 35 m / Ma), it does not explain the age of 0.94 ± 0.07 Ma obtained for the infills of the horizontal levels of the Rocas located much higher in altitude (+317 m rnb). The first two cavities open directly into the gorges, while the third opens onto the plateau less than 4 km from the gorges. To our knowledge, this is the first time that quantitative data support the proposition that the model of regional karstogenesis controlled by the position of the base level is not universal. These dates make it possible to suggest that ghosting plays a primordial role in controlling the location of the sub-horizontal levels. This process internal to the karst relegates the role of the position of the regional base level to that of a simple external triggering of the hollowing out of the alteration products generated by the phantomization. © French group of geomorphology
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    Challenging intraplate orogens: from geomorphology to lithospheric dynamic. The French Massif Central case study
    (Copernicus GmbH, 2020-05-04) Malcles, O; Vernant, P; Ritz, JF; Fink, D; Cazes, G; Fujioka, T; Braucher, R; Camps, P
    In the 60’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. 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 examples. Traditionally, such regions are interpreted as old structures that are slowly eroded, interpretations that are most of the time weakly constrained. 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. Using the Massif Central (France) as study area, we show that this region is currently deforming. 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.
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    Determining the Plio-Quaternary uplift of the southern French Massif Central; a new insight for intraplate orogen dynamics
    (European Geosciences Union, 2020-02-26) Malcles, O; Vernant, P; Chéry, J; Camps, P; Cazes, G; Ritz, JF; Fink, D
    The evolution of intraplate orogens is still poorly understood. Yet, it is of major importance for understanding the Earth and plate dynamics, as well as the link between surface and deep geodynamic processes. The French Massif Central is an intraplate orogen with a mean elevation of 1000  m , with the highest peak elevations ranging from 1500 to 1885  m. However, active deformation of the region is still debated due to scarce evidence either from geomorphological or geodetic and seismologic data. We focus our study on the southern part of the Massif Central, known as the Cévennes and Grands Causses, which is a key area to study the relationship between the recent geological deformation and landscape evolution. This can be done through the study of numerous karst systems with trapped sediments combined with the analysis of a high-resolution digital elevation model (DEM). Using the ability of karst to durably record morphological evolution, we first quantify the incision rates. We then investigate tilting of geomorphological benchmarks by means of a high-resolution DEM. We finally use the newly quantified incision rates to constrain numerical models and compare the results with the geomorphometric study. We show that absolute burial age ( 10 Be ∕ 26 Al on quartz cobbles) and the paleomagnetic analysis of karstic clay deposits for multiple cave system over a large elevation range correlate consistently. This correlation indicates a regional incision rate of 83+ 17 / - 5 m Ma −1 during the last ca. 4  Myr (Pliocene – Quaternary). Moreover, we point out through the analysis of 55 morphological benchmarks that the studied region has undergone a regional southward tilting. This tilting is expected as being due to a differential vertical motion between the northern and southern part of the studied area. Numerical models show that erosion-induced isostatic rebound can explain up to two-thirds of the regional uplift deduced from the geochronological results and are consistent with the southward tilting derived from morphological analysis. We presume that the remaining unexplained uplift is related to dynamic topography or thermal isostasy due to the Massif Central Pliocene – Quaternary magmatism. Integrating both geochronology and morphometrical results into lithospheric-scale numerical models allows a better understanding of this intraplate – orogen evolution and dynamic. We assume that the main conclusions are true to the general case of intraplate deformation. That is to say, once the topography has been generated by a triggering process, rock uplift is then enhanced by erosion and isostatic adjustment leading to a significant accumulation of mainly vertical deformation. © Author (s) 2020
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    The French Massif-Central, example of a not so inactive intraplate region
    (Australian Nuclear Science and Technology Organisation, 2021-11-17) Malcles, O; Phillipe, V; Ritz, JF; Fink, D; Cazes, G
    Plate tectonic theory postulates that intraplate areas are geodynamically inactive regions, active geologic and seismic deformations being concentrated along more or less narrow areas: the plate boundaries. The tectonic plates are supposed rigid, allowing stress transfer from one boundary to another. Therefore, no deformation is expected within the intraplate regions. True inactivity of intraplate areas is however refuted by evidences of active deformation. Many cases of intraplate earthquakes are known as for instance the lake Muir earthquake in 2018 (Mw = 5.3), the Botswana earthquake in 2017 (Mw= 6.5) or the New-Madrid sequence in 1811-1812 (4 events with Mw > 7). Recent propositions tend to decouple plate tectonics dynamic from intraplate earthquakes (e.g. [1]). In this case, the processes generating the stresses are local (e.g. fluid migration) or transient (e.g. GIA) and therefore long-term intraplate deformations are unlikely to happen. Significant intraplate deformations are however easily recognizable at the earth surface using the topography. Indeed, an important regional relief (Mountains) is the first evidence of earth surface long-term deformation and examples can be found for almost each intraplate region, for example: the Appalachians mountains (Northern America), the Great Dividing Range (Australia) or the Guiana highlands (Southern America). The origins of these topographic features are highly debated and almost every explanation has been given: tectonic stress, past tectonic frontier with old relief, dynamic topography, etc. In many cases, the lack of absolute dating precludes the determination of the landscape evolution rates leading to inaccurate and sometimes physically unsound geomorphologic models. Using the example of the French Massif-Central, we study if the long-term surface processes (erosion and sedimentation) can be responsible for intraplate deformation. Quantification of the surface erosion and incision rates were performed using Terrestrial Cosmogenic Nuclides (TCN), mainly 10Be and 26Al. Using both mean watersheds derived denudation rates (covering the last ~ 15 ka) and long-term incision rates using endokarstic infilling (covering the last ~ 5 Ma) we show, despite local variations due to specific morphology and possible climatic variations, that the region is affected by significant erosion (s.l.) with a mean denudation rate of ~ 60 m/Ma of and an incision rate of ~ 90 m/Ma. Given the current ~ 300 m depth of the valleys, we conclude that this mountainous region is the consequence of a Plio-Quaternay uplift and therefore that intraplate area can be associated with active long-term processes leading to consequent finite deformation. First order numerical model addresses the question of driving processes and show that a combination of thermal isostasy and erosion driven isostatic adjustment can explain both long-term uplift rate and distributed volcanic activity of the area. Such relatively constant long-term uplift is expected to be at the origin of long-term stress concentrations and therefore intraplate earthquakes could be associated with low-frequency seismic cycles modulated by transient or local processes.