Tectonic evolution of the European Variscan belt constrained by palaeomagnetic, structural and anisotropy of magnetic susceptibility data from the Northern Vosges magmatic arc (eastern France)
Geochronology, structural and anisotropy of magnetic susceptibility data from the northern Vosges batholith, which belongs to the Northern Vosges–Mid-German Crystalline Rise arc, show contrasting emplacement modes of southern granodiorites and northern granites. The ENE–WSW-trending fabrics of granodiorites (346–334 Ma) are parallel to the metamorphic cleavage affecting the host rocks developed during regional compression. The NNW–SSE-trending fabrics of younger granitoids (c. 330 Ma) reveal an extensional emplacement mode, associated with a normal shear zone separating the two magmatic suites. Palaeomagnetism shows that the switch from a compressive to an extensional regime coincides with a regional counterclockwise rotation. The 330–325 Ma extension is further supported by palaeomagnetic and seismic data indicating southeastward tilt of the whole batholith. Finally the system is rotated clockwise without any structural overprint. Data from the Early Carboniferous northern Vosges magmatic arc and remote Bohemian Massif suggest that this evolution is valid for the whole eastern branch of the European Variscan belt, for which the following model is proposed: (1) Late Devonian–Early Carboniferous east–west shortening of the Variscides above the Rhenohercynian subduction zone; (2) axial NNW–SSE shortening of the assembled Variscan orogen associated with (3) ‘internal’ sinistral rotation of inherited Rhenohercynian transform faults and shortening of intervening blocks; (4) east–west extension and dextral ‘external’ rotation of blocks between dextrally reactivated transforms.