De la collision continentale à l'extension tardi-orogénique, 100 millions d'années d'histoire varisque dans le Massif central français‎. Une étude chronologique par la méthode 40Ar-39Ar

Abstract : The 40Ar-39Ar technique is applied to a chronological study of the successive tectono-metamorphic episodes which took place in the French Massif Central from the beginning of the continental collision (390 Ma ago) until the end of the late-orogenic extension (290 Ma ago). Several regions are investigated in the eastern part of this Variscan massif, along a north-south profile, parallel to the general polarity of the belt. This "chronological profile" first transects high-grade metamorphic nappes from the internal parts of the belt (Monts du Lyonnais, Marvejols, Rouergue), then less metamorphic nappes from the external zones (Albigeois, Montagne Noire). (i) Radiometric data obtained along this profile argue the north-south migration of the major thrust deformation during the intra-continental development of the French Massif Central. Thus, ages vary between 360 Ma in the inner-most high-grade units of the northern part and 320 Ma in the most external epizonal fold belt of the southern part. (ii) The east-west diachronism of the collisional stage is emphasized in the French Massif Central. Indeed, the emplacement of the high-grade nappes took place at least 360-340 Ma ago in the eastern part of the massif and 390-370 Ma ago in the western part. This diachronism is also a characteristic feature of the southern branch of the European Variscan Belt. An interpretative scheme is tentatively proposed in order to explain the different contrasts (lithological, metamorphic, chronological) noticed from east to west of the orogen. (iii) Between 320 and 290 Ma, the late evolution of the belt is characterized by extensional tectonics (Mont Pilat, Montagne Noire). This process is compared with the granulitic metamorphism of the lower crust which occurred at the same period. It is suggested that both the extensional tectonics observed in the upper part of the crust and the granulitization of the lower crust reflect a unique mechanism of late-orogenic extension, favouring the thinning of the Variscan continental crust, previously thickened by the collision.
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Sylvie Costa. De la collision continentale à l'extension tardi-orogénique, 100 millions d'années d'histoire varisque dans le Massif central français‎. Une étude chronologique par la méthode 40Ar-39Ar. Sciences de la Terre. Université Montpellier 2 Sciences et Techniques du Languedoc, 1990. Français. ⟨tel-01986574v2⟩

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