The main Variscan deformation event in the Pyrenees: new data from the structural study of the Bielsa granite

A structural and magnetic fabric study of the Bielsa granite (Axial Zone of the Pyrenees) provides new data indicating that this pluton was emplaced during the main Variscan event recognized in the Pyrenees. We argue that the later post-Triassic deformation was localized along mylonitic bands reactivating earlier Variscan shear bands. The Bielsa granite intrudes Cambro–Ordovician metasediments at its northern border, and the pluton is unconformably overlain by sediments Triassic in age to the south and east. The main structures in the igneous body are magmatic foliations and lineations defined by feldspar phenocrysts and biotite and dextral shear bands with dominant WNW–ESE trends. The WNW–ESE striking magmatic foliations have variable dips, and the magmatic lineation a subhorizontal WNW–ESE trend. The zonation of low-field magnetic susceptibility, coarsely related to mineral content, indicates a layer arrangement of rock-types, with more basic compositions at the northern border. The magnetic ellipsoids are prolate along a WNW–ESE central band, and oblate at the pluton borders, where flattening is more important. All these data allow us to interpret the Bielsa intrusion as being caused by coeval NNE shortening and dextral shear. The presence of shear bands indicates that strain localization under dextral transpression continued during late-Variscan times.