Deformation and magnetic fabrics in isoclinal folds of the Variscan Pyrenees

Magnetic fabrics are used as strain markers since they reflect the orientation-distribution of grains in a rock. In this work we analyzed, from the point of view of anisotropy of magnetic susceptibility (at low-temperature and room temperature), Devonian (29 sites) and Ordovician-Silurian (3 sites) sedimentary rocks in the Pyrenean Axial Zone, totalling 611 samples. The main target is to determine the strain in relation to the Variscan (polyphased) and Alpine orogenies in limestones and shales. The magnetic information also derives from thermomagnetic curves, acquisition of the isothermal remanent magnetization (IRM) and coercivity of the remanence analyses, together with hysteresis loops and stepwise thermal demagnetization of induced IRM in 3 axes techniques. The analyses of magnetic properties were complemented with optical microscopy observations and calcimetries. In two of the sample areas (Sen and Llisat valleys), the minimum axes of the magnetic susceptibility ellipsoid (k_min axes) are mostly perpendicular to bedding whereas in the other two (Zinqueta valley and near Posets peak area), a more developed tectonic fabric is found: the k_min axes are not completely perpendicular to bedding, and the magnetic susceptibility ellipsoid relates to the foliation plane. Different minerals carry a concordant magnetic fabric with pyrrhotite, magnetite (ferromagnetic s.l.) and phyllosilicates (paramagnetic), and little influence of calcite grains (diamagnetic). The magnetic fabric is interpreted to form at the early stages of the Variscan deformation, prior to the dextral transpression synchronous with granite emplacement that characterizes the late stages of the Variscan Orogeny. However, contact metamorphism and associated fluid circulation can enhance or obliterate the primary magnetic fabric.