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Métamophisme tardi-orogénique dans le domaine externe d’une chaîne de montagnes. Nouvelles contraintes par thermométrie RSCM et datation ⁴⁰Ar/³⁹Ar dans le sud de chaîne varisque (Cévennes et Montagne Noire)

Abstract : The P and T conditions of the internal zones of an orogen are well constrained by the study of metamorphic assemblages. But it is more difficult to estimate the metamorphic conditions of its external domains and the thermo-barometric calculations are subject to a significant error. The Raman Spectrometry of Carbonaceous Matter (RSCM) has made it possible to obtain the maximum temperature recorded by rocks. In this study, this methodology was applied to the external Variscan belt the French Massif Central.The Montagne Noire represents the outermost part of the belt. A migmatitic dome superimposed on the recumbent Fold-and-Thrust belt series between at 300-325 Ma. The RSCM reveal a decreasing thermal gradient from 400 to 250 ° C from the edge of the dome to 10km to the south. This temperature gradient complies with those determined by the illite crystallinity or the altered colour of conodonts. In all cases, the thermicity linked to the nappes stacking was overprinted by the heat provided by the dome.This study shows that the Cévennes Para-autochthonous Unit recorded a stage of high T (~ 500-550 °C) and mid P (~0.5 GPa) at 320-325 Ma, with relatively uniform conditions over the whole area. In the geodynamic framework of Massif Central evolution, this stage is interpreted as corresponding to a phase of high heat flux, when the orogen experienced the transition between shortening/thickening and collapse/extension. The measured T is not the direct result of the heat transferred from the adjacent plutons; on the contrary, the Cévennes metamorphism and the slightly later-stage pluton emplacement find a common origin in the high T field of the whole orogen at that period. However, in situ dating of the biotite cores suggests an early crystallization around 330-335 Ma that might correspond to the true age of the southward thrusting.
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Submitted on : Thursday, September 30, 2021 - 5:20:11 PM
Last modification on : Wednesday, November 17, 2021 - 12:32:13 PM

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  • HAL Id : tel-03360303, version 1

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Clément Montmartin. Métamophisme tardi-orogénique dans le domaine externe d’une chaîne de montagnes. Nouvelles contraintes par thermométrie RSCM et datation ⁴⁰Ar/³⁹Ar dans le sud de chaîne varisque (Cévennes et Montagne Noire). Géomorphologie. Université d'Orléans, 2021. Français. ⟨NNT : 2021ORLE3109⟩. ⟨tel-03360303⟩

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