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Datations par paléomagnétisme des paléoaltérations du Massif Central et de ses bordures : implications géodynamiques

Caroline Ricordel-Prognon 1
1 Direction des GéoRessources, Unité Géologie de l'Aménagement des Territoires
BRGM - Bureau de Recherches Géologiques et Minières (BRGM)
Abstract : Continental unconformities are essential to estimate erosion rates of crystalline basement and to model the crust dynamics that control subsidence of surrounding sedimentary basins. Dating such unconformities has always been the stumbling block to constrain geodynamical models. Paleomagnetism is a great tool to date ferrugineous paleoweathering. This method has been applied: to paleoweathering occurrences resting on the Massif central crystalline basement and reaching the Morvan crystalline basement ; and to the Cretaceous ferrugineous sandstones of the Southeast border of the Paris Basin. Relative dating of these azoic formations have been acquired by comparing the recorded paleomagnetic poles from the samples with the apparent polar wander path (APWP) of the Eurasian plate The thick red kaolinitic formations from the Massif central have classically been ascribed to the "Siderolithic" (Eocene-Oligocene). These formations are arranged in clear paleolandscape features: around basement paleoreliefs, leaned against fault scarps, infilling paleovalleys, etc. It show well preserved pedogenic features (termites burrows, illuviation and hydromorphic features, nodules...). Macro and micromorphological observations show that these formations are in situ paleosoils. Paleomagnetic datings provide ages ranging from 160 Ma (Late Jurassic) in the Naussac Basin (centre of the Massif central) to 140 Ma (Early Cretaceous) in the Lembron region (Southwest border of the Limagne graben). The Cretaceous ferrugineous deposits from the Southeast border of the Paris Basin, in the Puisaye and Sologne areas are made up of sandstones with ferrugineous cement (Early Albian Sands and Ferruginous Sandstones, La Puisaye Sands). The cement is mainly goethitic. No dating were obtained because of the instable magnetization of this mineral. A hematitic layer located at the base of the Myennes Clays gave a recent age (0-3 Ma). It could be related to the oxidization of a pyretic layer subsequent to groundwater flow during the formation of the present-day landscapes. The Massif central and the Morvan crystalline basements show albitized facies. It affects granites, gneiss, rhyolites and Permo-Carboniferous sediments. On granites, albitized facies are layered, with pinkish mottling at the base and reddish spotting to the top. These horizons are marked by albitization of primary plagioclases and chloritization of primary biotites. Secondary albite and chlorite contain secondary minute hematite inclusions, which have been dated by paleomagnetism to be Triassic in age. The albitized facies are related to the Triassic unconformity. Both datings and geographic distribution of the albitized formations argue for a superficial origin of these facies. Therefore, the post-magmatic or hydrothermal hypothesis classicaly proposed to explain their origin can be excluded. Paleomagnetic analyses allowed to date a large range of paleoweathering formations for which no objective datings was available. The space and time distribution of the formations and the related unconformities gives elements to highlight the geodynamic evolution of the Massif central. * Triassic, Late Jurassic and Tertiary unconformities are superimposed on large areas of the Massif central. It implies very little erosion of the crystalline basement since Triassic times, as shown by the widespread albitized triassic unconformity outcroping at present. * Datings points out that no Mesozoic cover was present on the basement of Massif central during development of the red kaolinitic paleosoils during the Late Jurassic/Early Cretaceous period. Consequently, the Massif central did probably not support an important (more than 500 m) sedimentary cover during the Mesozoic. In conclusion, identification and dating of such continental unconformities give evidences for long lasting continental evolution and landscape stability of the Massif central all along the Mesozoic.
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Submitted on : Monday, October 15, 2012 - 11:55:54 AM
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Caroline Ricordel-Prognon. Datations par paléomagnétisme des paléoaltérations du Massif Central et de ses bordures : implications géodynamiques. Sciences de la Terre. Ecole Nationale Supérieure des Mines de Paris, 2007. Français. ⟨tel-00741773⟩

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