Late Orogenic Carboniferous Extensions in the Variscan French Massif Central Michel Faure

Late orogenic carboniferous extensions in the Variscan French Massif Central Michel Faure To cite this version: Michel Faure. Late orogenic carboniferous extensions in the Variscan French Massif Central. Tectonics, American Geophysical Union (AGU), 1995, 14 (1), pp.132-153. 10.1029/94TC02021. insu-00716156 HAL Id: insu-00716156 https://hal-insu.archives-ouvertes.fr/insu-00716156 Submitted on 10 Jul 2012 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. TECTONICS, VOL. 14, NO. 1, PAGES 132-153, FEBRUARY 1995 Late orogenic carboniferous extensions in the Variscan French Massif Central Michel Faure InstitutUniversitaire de France,D6partement des Sciences de la Terre, Universit6d'Or16ans, Orl6ans, France Abstract. The Variscan French Massif Central nappestacking and is responsiblefor partial meltingat deep experienced two successivestages of extension from Middle levels, induces gravitational instability. According to mass Carboniferous to Early Permian. In the northern Massif balancedeductions from the peripheralmolasse basins and the Central, the first stage began in the late Visean, immediately uplift rate, erosionalone appearsunable to denudethe main after nappe stacking, and is well recorded by Namurian- part of the uppercrust and to exhumethe deepestparts of the Westphalian synkinematic plutonism. The Middle orogen [Thompson and Ridley, 1987; Dewey, 1988]. Carboniferous leucogranites widespread in the NW Massif Extension removes the instability by reducing the thickness Central (Limousin and Sioule area) were emplaced within a of the crustalroot and allowing the continentalcrust to recover crustextending along a NE-SW direction.At the sametime, the a standard thickness of about 30 km. hanging wall or "Gu6ret extensionalallochton" moved toward The Variscan Belt of Western Europe is interpretedas a the SE. Several examples of the synextensionalplutonism are collisional belt between the northern Laurussia and southern also recognized in central Limousin: Saint Mathieu dome, La Gondwanacontinents [Dewey and Burke, 1973, Matte, 1986]. Porcherie, and Cornil leucogranites.These examples illustrate In the FrenchMassif Central,which belongsto Gondwana,the the relationshipbetween granite emplacementand crustal scale Variscanorogenic events range from Late Silurian(circa. 410 deformation characterized by NW-SE stretching and NE-SW Ma), which is the average age for the high-pressure shortening. In the central and southern Massif Central metamorphism,up to Late Carboniferous-EarlyPermian (ca (C6vennes,Chhtaigneraie, and Margeride areas), plutonismis 300 Ma) which is the depositiontime of the late-orogenic, dominantly granodioritic and exhibits the same structural frequently coal-bearing, intramontanemolassic sediments. features: NW-SE maximum stretchingand overturning to the Crustal thickeningin the Massif Central is well documented SE. Middle Carboniferous (Namurian-Westphalian)extension [Ledru et al., 1989 and referencestherein] by (1) southverging was parallel to the Variscan belt both in the Massif Central and deep seated metamorphic nappes, (2) high pressure southern Armorican area. This extensional regime was active metamorphism,and (3) crustalmelting and magmatism. from the late Visean in the north,while compression The width of similar tectoniczones is nearly 4 times larger dominated in the southernmostdomains (Montagne Noire and in the Massif Central than in the nearby Armorican massif. Pyren6es). The second extensional stage occurred from Late Such a sudden change, occurring within a few tens of Carboniferous to Early Permian. This event was responsible kilometers, suggestseither paleogeographicvariations that for the opening of intramontane coal basins, brittle are not recorded in the rock facies or differential modifications deformation in the upper crust, and ductile normal faulting of the initial compression-relatedstructures. The tectonic localized on the margin of cordieritegranite-migmatite domes. zonation seems to have been dilated in the Massif Central. Data from the coal basins show that the half-graben is the Indeed, extensionaltectonics due to gravity collapse of the dominant structural style, except for basins located along thickened crust has been described in many places of the submeridianal left-lateral faults which have pull-apart Massif Central, [e.g., Mattauer et al., 1988; Mdnard and geometries. Late Carboniferousextension occurred along the Molnar, 1988; Van den Driessche and Brun, 1989, 1991; NE-SW direction. The NE-SW maximum stretchingdirection Faure, 1989; Echtler and Malavieille, 1990; Malavieille et al., can be found in the whole Massif Central but is more developed 1990; Faure et al., 1990; Faure and Pons, 1991]. As pointed in the eastern part. The extensional direction is transverseto out by Faure and Becq-Giraudon [1993], post-Variscan the general trend of the belt, and top-to-the-NE shearing is extension in the Massif Central can be subdivided dominant. Correlations of these two extension directions with chronologically and structurally into two successivestages. neighboringVariscan massifsare discussed. The Middle Carboniferous(late Visean-Namurian-Westphalian, 330-315 Ma) extensionalperiod is characterizedin the north Introduction Massif Centralby a NW-SE maximumstretching direction and a NE-SW intermediate axis of finite strain. It is followed by a Late orogenic extension is presently recognized in most secondextensional stage in Late Carboniferous(Stephanian)- collisional belts. Crustal thickening, which is caused by Early Permian (Autunian) times characterizedby a NE-SW maximumstretching axis and a NW-SE intermediateaxis. This Copyright1995 by the AmericanGeophysical Union. paperaims to showthe spatialsuperposition and the temporal successionof the two extensionalregimes. New structuraldata Paper number 94TC02021. andprevious metamorphic, magmatic, and sedimentarydata are 0278-7407/95/94TC-02021 $10.00 synthetizedand reinterpreted in the frameof this model. 132 FAURE:VARISCAN EXTENSIONS IN THE FRENCH MASSIF CENTRAL 133 The Middle Carboniferous NW-SE extension lineations occur in all the ductile faults that bound the Gu6ret allochton. Shear criteria consistentlyindicate a top-to-the-SE In spite of lack of Namurian-Westphaliansedimentary motion for the Gu6ret allochton with respect to the basins in the Massif Central, evidence for extensional surrounding units. tectonicsof this age is found in the ductile crust by a The time of the displacementis given by the radiometricage conspicuoussynmetamorphic deformation and the shapeof of the syntectonic leucogranitic plutons: 318_+5Ma (Rb-Sr numeroussyntectonic plutons (Figure 1). In the Massif methodon whole rock) for the Saint Sylvestrepluton [Duthou Central, two groups of Carboniferous granitoids are et al., 1984] and 324_+18 Ma (U-Pb method on zircon and recognized, namely, leucogranitesand granodiorite- monazite) for the Brame pluton [Hollinger et al., 1986]. The monzograniteassociations [Didier and Lameyre, 1969]. The syntectonic character of the leucogranitesis observed in the former is more common in the northern Massif Central, field, hand specimen, and thin section by the progressive whereasthe latter is widespreadin the centraland southern evolution at the scale of each pluton, from a mylonitic fabric parts.In the following,examples from several parts of the it, the pluton cortex to a magmaticfabric (i.e., orientedtexture Massif Central are presented to show how Middle with weak or no plastic strain) in the core [Molllet and Carboniferoussyntectonic granitoids recorded the regional Bouchez, 1982; Faure and Pons, 1991]. To the west and SW, extensionaldeformation during their emplacement. the Gu6ret extensional allochton is bounded by the Namurian leucogranitesof Brame, Saint Sylvestre, and Saint Goussaud. This last pluton is asymmetricwith a subverticalnortheastern Northern Massif Central side and a gently dipping southwesternone. It has an elliptical The Gu6ret extensional allochton. Ductile outcrop shape with its long axis parallel to the right-lateral extensionis well demonstratedin the NW part of the Massif AttUnes fault and also to the regional stretchinglineation in Central, or Limousin area. Northern Limousin (Figure 2) is the host rock mica schist.The Saint Goussaudpluton intrudes occupiedby the Gu6ret massif, which consistsof pre-upper the Devonian stack of nappes. The early tectonic contact Visean cordierite-bearing granitoid, migmatite, and gneiss between the mica schist and orthogneiss,which is parallel to radiometricallydated around360 Ma by the Rb-Sr methodon the regional foliation, is refolded by SW verging low-angle whole rock [Bertbier et al., 1979]. The granitoid and axial planar microfolds.These structuresare interpretedas drag metamorphic rocks that bear a subhorizontalfoliation are folds related to the emplacementof the Saint Goussaudmassif. geometrically underlain by flat-lying mica schist and gneiss The contactis thereforea detachmentfault (Figure 3). (Figure 3). To the east, the Gu6ret massifis sharplytruncated The Brame massif

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