PRELIMINARY ANALYSIS OF ANISOTROPY OF MAGNETIC SUSCEPTIBILITY IN THE BREVARD SHEAR ZONE, NORTH CAROLINA

The Brevard Shear Zone, a major tectonic feature in the Appalachian Mountains, spans a distance of over 600 km and forms the boundary between the southern Appalachian Blue Ridge and Piedmont provinces. Ductile deformation in and around the Brevard Zone is recorded by changes in the shape and orientation of the initial rock fabric. This study uses anisotropy of magnetic susceptibility (AMS) to assess deformation history including finite strain in the Brevard Zone and its surrounding area. Due to the high degree of mylonitization and the lack of conventional strain markers in the Brevard zone, AMS is potentially a valuable method for determining finite strain. A previously documented decrease in grain size of the Henderson Gneiss towards the Brevard Zone suggests a smooth progression of mylonitization. This increase in mylonitization represents an increase in strain, which theoretically should correlate with AMS. Oriented hand samples of the Henderson Gneiss were collected at 42 sites along a transect across the Brevard Zone in Rosman, North Carolina. Anisotropy and bulk susceptibility measurements were made with a KLY-3 Kappabridge. Preliminary results show that the orientation of macro- and microscopic foliation and lineation corresponds to that of the AMS ellipsoid suggesting a qualitative correlation between finite strain and AMS fabric. Mean bulk susceptibility for the Henderson Gneiss ranges from 5.9 x 10^-5 to 4.87 x 10^-3 S.I., and increases with distance away from the shear zone. Mean k values, which range from 0.2 to 0.9, do not show a clear relationship with distance. Percent anisotropy varies in a broad range (from 5 to 85%) and shows a gradual increase with distance from the shear zone. Unlike the observed grain size reduction gradient towards the shear zone, neither mean bulk susceptibility, percent anisotropy, nor k show a smooth variation as the shear zone is approached suggesting that either strain does not occur in a progressive gradient in the vicinity of the Brevard Zone, or that the magnetic fabric of the rock does not correlate directly with finite strain.