STRAIN AND KINEMATICS INFERRED FROM MAGNETIC FABRICS: NEW INSIGHTS FROM THE BREVARD SHEAR ZONE, NORTH CAROLINA

The Brevard Shear Zone, a major tectonic line of the Appalachian Belt, separates the Blue Ridge to the NW from the Inner Piedmont, to the SE. A considerable number of structural, magnetic and tectonic studies have focussed on this > 600 km-long ductile shear zone. Most previous magnetic fabric investigations have underestimated the importance of lithological and mineralogical variations across the shear zone. These studies have generally attributed variations in the degree of magnetic anisotropy (Pj) to strain localization. We present new evidence documenting the large effect that lithological variations across the shear zone have on magnetic fabrics, even within a relatively small area near Rosman, NC. Regardless of deformation regime and tectonic history, strain in the Brevard Shear Zone is likely to be strongly dependent on rock type. In turn, the magnetic mineralogy, i.e. relative abundance of ferromagnetic and paramagnetic species, depends on the rock type. For example, in ferromagnetic rocks, the anisotropy of magnetic susceptibility (AMS) is determined primarily by the shape of large ferromagnetic grains (Mt, Po). In contrast, in paramagnetic rocks, the AMS is determined by the lattice preferred orientation (LPO) of mafic silicates such as biotite. The origin of the AMS is fundamentaly different in these two cases. Pj in a rock specimen depends on the magnetic carrier and can only be as high as the anisotropy of the most anisotropic consituting mineral. Therefore, Pj cannot be considered a proxy for strain. Instead of using Pj as a strain gage, we propose to first determine the ratio between ferromagnetic and paramagnetic minerals and then to use the angle between the mylonitic foliation and the principal axis of the AMS ellipsoid (K1), measured in the K1-K3 plane. This approach allows better quantification of strain localization within the shear zone. In addition to this new angular quantification of strain, we separate instrumentally the paramagnetic and the ferromagnetic components of the bulk AMS by using low and high field AMS measurements. In cases where the two subfabrics differ, their angular relationship can be used as a shear sense criteria, hence providing an independent kinematic indicator. This type of magnetostructural indicator might be particularly useful in fine-grained (mylonitic) shear zones.