L'etude geologique detaillee, basee sur une cartographie precise, des Caledonides de Laponie occidentale a permis la mise en evidence de huit d'unites tectoniques allochtones qui n'avaient pas ete reconnues anterieurement. Une carte geologique au 1/100 000e accompagnee de nombreuses coupes detaillees et d'un schema structural au 1/250 000e illustrent l'organisation de ce bâti orogenique. L'etude presente une reconstitution tectono-stratigraphique basee sur des analyses petrographiques, geochimiques et geochronologiques. Il est ainsi confirme l'existence de nappes allochtones de materiel precambrien (lames cratoniques) separant les Externides des Internides a materiel paleozoique. L'âge de mise en place majeure des nappes se situe au cours de la phase scandinave principale, au Silurien superieur. Les migmatites des Internides sont originales (phase potassique representee par la muscovite et non par du feldspath), le facteur dominant de cette migmatisation est une fusion anatectique limitee (systeme quartz-plagioclase sodique-muscovite-vapeur associe a liquide-silicate d'alumine). L'etude des mylonites montre que la deformation intime des tissus cristallophylliens est precoce et independante de la mise en place des nappes mais qu'elle se produit au cours d'un meme continuum de deformation. Par ailleurs, l'intensite du taux de deformation est independante de la distance aux plans de chevauchement. La generalisation de cette etude regionale a pu etre effectuee par la realisation de coupes systematiques au travers de l'orogene, sur pres de 1 000 km. Il est ainsi presente une synthese originale des Caledonides scandinaves arctiques qui s'appuie sur un schema structural au 1/ 1 000 000e ou l'importance des lames cratoniques est soulignee. Cette ceinture arctique resulte, comme les domaines meridionaux, d'une evolution polycyclique reprenant du materiel precambrien deja structure. Sa place dans le domaine caledono-appalachien est discutee, sur les bases d'une reconstruction paleo geodynamique. On insiste enfin, a partir d'exemples concrets, sur la necessite d'etablir la geometrie des corps geologiques et la chronologie des evenements avant d'en proposer des modeles d'evolution.
Summary The Plateau d’Aigurande represents the north-westernmost part of the French Massif Central. It is overlain by the sediments of the Paris Basin to the north and bounded by the La Marche shear zone to the south. Detailed mapping, mainly from the Creuse Valley region, indicates thrust and nappe tectonics. Beginning at the base of the sequence one can recognize: (1) the Fougères unit (schists of low to medium grade) intruded by syntectonic leucogranites; (2) the Eguzon unit (medium- to high-grade schists which are partly diaphtoretic in the lower part of the unit); (3) the Gargilesse unit (high-grade schists); and (4) the Migmatitic unit. The metasediments of the Fougères, Eguzon and Gargilesse units were originally coarse grained clastics. They are interbanded with magmatic units (orthogneisses and amphibolites). The migmatites are derived from greywacke-type rocks intruded by magmatic rocks (orthogneisses and amphibolites). Following a high-pressure (Silurian?) tectonometamorphic event, the piling up of tectonostratigraphic units occurred during two periods of deformation. The main one, probably of late Caledonian (Acadian?) age, was synchronous with or slightly after the climax of metamorphism. The second, of Westphalian age, was accompanied by the emplacement of leucogranitic magma and by retrograde metamorphism. The shear-sense appears to have been from SW to NE during the Acadian phase and from NW to SE during the Westphalian. Structural and lithological studies suggest that the three lowermost units may have been derived from the same palaeogeographic domain while the migmatites have a distinctly different history.
Abstract The Variscan continental collision has led to the development of large strike-slip shear zones in western Europe. Our study focuses on the regional deformation and shear zone patterns in the Massif Armoricain and the French Massif Central. The synthesis of granite emplacement ages associated to granite deformation fields, allow us to propose a geodynamic model for the tectonic evolution of this part of the Variscan belt between 370 Ma – 320 Ma (Late Devonian – Namurian). After the first steps of the continental subduction-collision, leading to high temperature and anatexis associated with N-S shortening at 380-370 Ma (Frasnian to Famennian), the southern part of the Massif Armoricain and western part of French Massif Central underwent large dextral shearing along N100-N130 trending shear zones up to early Visean time. These large-scale displacements progressively decreased at around 350-340 Ma, during the first emplacements of biotite bearing granites (Moulins-les Aubiers-Gourgé massif and Guéret massif intrusions). During middle Visean times, the shortening axis direction rotated towards a NNE-SSW direction implying changes in the regional deformation field. The occurrence of N070-N100 sinistral and N110-N130 dextral conjugate shear zones within leucogranites are related to that time. Finally, new N150-N160 dextral shear zones appeared in middle to late Visean times: as for examples, the Parthenay and the Pradines shear zones in the SE Massif Armoricain and the Millevaches massif, respectively. These shear zones were conjugated to the sinistral N020 Sillon Houiller in the French Massif Central. They reflect large scale brittle continental indentation in the French Variscan belt during the middle to late Visean.
Other| January 01, 1979 La tectonique tangentielle dans le Troms oriental (Caledonides de Laponie); ses manifestations mega-, meso- et microscopiques J. M. Quenardel; J. M. Quenardel Author Search for other works by this author on: GSW Google Scholar A. M. Boullier A. M. Boullier Author Search for other works by this author on: GSW Google Scholar Bulletin de la Société Géologique de France (1979) S7-XXI (4): 457–466. https://doi.org/10.2113/gssgfbull.S7-XXI.4.457 Article history first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation J. M. Quenardel, A. M. Boullier; La tectonique tangentielle dans le Troms oriental (Caledonides de Laponie); ses manifestations mega-, meso- et microscopiques. Bulletin de la Société Géologique de France 1979;; S7-XXI (4): 457–466. doi: https://doi.org/10.2113/gssgfbull.S7-XXI.4.457 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin de la Société Géologique de France Search Advanced Search This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Other| January 01, 1978 Successions orogeniques en Fenno-Scandie septentrionale J. M. Quenardel; J. M. Quenardel Author Search for other works by this author on: GSW Google Scholar A. Ploquin; A. Ploquin Author Search for other works by this author on: GSW Google Scholar P. Dangla P. Dangla Author Search for other works by this author on: GSW Google Scholar Bulletin de la Société Géologique de France (1978) S7-XX (5): 715–719. https://doi.org/10.2113/gssgfbull.S7-XX.5.715 Article history first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation J. M. Quenardel, A. Ploquin, P. Dangla; Successions orogeniques en Fenno-Scandie septentrionale. Bulletin de la Société Géologique de France 1978;; S7-XX (5): 715–719. doi: https://doi.org/10.2113/gssgfbull.S7-XX.5.715 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietyBulletin de la Société Géologique de France Search Advanced Search This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not currently have access to this article.
