Basement deformation: tertiary folding and fracturing of the Variscan Bielsa granite (Axial zone,central Pyrenees) |
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| Institution: | 1. Departamento de Ciencias de la Tierra, Universidad de Zaragoza, 50009 Saragossa, Spain;2. Department of Paleomagnetism, University of Leoben, Gams, Austria;1. LPG – Le Mans, UMR 6112, Université du Maine, Avenue Olivier Messiaen, 72085 Le Mans, France;2. Géosciences Rennes, UMR 6118, Université de Rennes 1, CNRS 263, avenue du Général Leclerc, 35042 Rennes cedex, France;3. Helmholtz Centre Potsdam, German Research Center for Geosciences (GFZ), Telegrafenberg, Potsdam, Germany;4. BRGM, Centre scientifique et technique, 3 avenue Claude-Guillemin, BP 360009, 45060 Orléans cedex 02, France;5. BRGM, Direction régionale Bretagne, Rennes Atalante Beaulieu, 2 rue de Jouanet, 35700 Rennes, France;1. Sorbonne Universités, UPMC Univ Paris 06, CNRS, Institut des Sciences de la Terre de Paris (iSTeP), 4 place Jussieu 75005 Paris, France;2. Univ. d’Orléans, UMR 7327, ISTO, F-45071 Orleans, France;3. CNRS/INSU, UMR 7327, ISTO, F-45071 Orleans, France;4. BRGM, ISTO, UMR 7327, F-45071 Orleans, France;5. Université de Franche Comté, UMR 6249, Laboratoire Chrono-environnement, F-25030 Besancon, France |
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| Abstract: | The Bielsa thrust sheet is a south-verging unit of the Axial zone in the central Pyrenees. The Bielsa thrust sheet consists predominantly of a Variscan granite unconformably overlain by a thin cover of Triassic and Cretaceous deposits. During the Eocene–Oligocene, Pyrenean compression, displacement of the Bielsa thrust sheet generated a large-scale south-verging monocline. Low temperature deformation of the Bielsa thrust sheet resulted in the development of: (1) E–W trending, asymmetric folds in the Triassic cover with amplitudes up to 1.5 km; these folds of the cover are related with normal and reverse faults in the granite and with rigid-body block rotations. (2) Pervasive fracturing within the Bielsa granite is also attributed to Pyrenean deformation and is consistent with a NNE to ENE shortening direction; two main, conjugate fault systems are associated with this direction of shortening, as is a subvertical strike-slip system with shallow-plunging slickenside lineations and a moderately dipping fault system with reverse movement; and (3) in addition, we recognise strike-slip and reverse shear bands, associated with sericitisation and brittle deformation of quartz and feldspar in the granite, that enclose Triassic rocks. Basement deformation within the Bielsa thrust sheet can be related to movement of faults developed to accommodate internal deformation of the hanging wall. Several models are proposed to account for this deformation during the southward displacement of the thrust. |
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