Magnetic fabric evolution in ductile shear zones: examples in metagranites of the Aar Massif (Swiss Central Alps)

The Aar Massif forms part of the polycyclic basement of the External Crystalline Massifs in central Switzerland. Strong heterogeneous Alpine deformation produced a network of broad, anastomosing shear zones, with deformation strongly localized in mylonitic domains. This study investigates the combined effects of high-strain deformation and synkinematic metamorphism on magnetic fabric evolution in Tertiary shear zones of the Aar granite and Grimsel granodiorite. In transects across several mesoscale shear zones with large strain gradients, magnetic fabric orientations are in excellent agreement with principal strain orientations determined from outcrop fabrics and strain markers. However, the magnitude and shape of the magnetic anisotropy do not change systematically with increasing finite strain, likely as a result of recrystallization and metamorphism. The overall pattern of steeply dipping fabrics is consistent with the main shortening stage of regional Alpine kinematics, while some mylonite structures reflect a local component of dextral shearing.