Magnetic susceptibility of rocks in the Nutbush Creek ductile shear zone, North Carolina

Abstract

Applicability of anisotropy of magnetic susceptibility (AMS) as a strain indicator is investigated for the Falls lineated gneiss, a strongly lineated, weakly foliated tectonite lying within the Nutbush Creek fault zone (NCFZ). The NCFZ is a major Alleghanian ductile shear zone within the southern Appalachian Piedmont and is characterized along most of its length by a pronounced, linear trend on aeromagnetic maps. Sampling was conducted along a traverse near Raleigh N.C., that extended through the lineated gneiss, across the eastern boundary of the NCFZ and into adjacent, supposedly unsheared Raleigh-belt gneiss. Petrofabric element orientations are similar within the Falls lineated gneiss and the Raleigh-belt gneiss but foliation is better developed than lineation in the Raleigh-belt gneiss. Mineral lineation parallels the strike of the NCFZ. Results of the AMS analysis are remarkably consistent along the entire traverse. Mean susceptibilty ellipsoids are uniformly prolate and display little variation in spatial orientation. In both the Falls lineated gneiss and the Raleigh-belt gneiss, mean maximum susceptibility axes are oriented along the trend of mineral lineation, and mean minimum susceptibility axes coincide with poles to foliation. Magnetic fabric in both lithologies is dominated by the alignment of magnetite grains in the direction of silicate-mineral lineation; magnetic lineation is better developed than magnetic foliation even in Raleigh-belt rocks. Similarity of the magnetic fabrics in the lineated gneiss and the Raleigh-belt gneiss suggests that both fabrics were produced by the same deformational process. The possibility that the Raleigh-belt rocks are actually part of the NCFZ is suggested by alignment of petrofabric elements along the strike of the fault zone. The presently mapped eastern boundary of the NCFZ which was defined on the basis of lithologie and mesoscopic rock fabric differences, may separate two distinct units within the fault zone. AMS results for Raleigh-belt rocks located north of the study area suggest that deformation associated with the fault zone may extend several kilometers east of its presently mapped boundary at certain locations. AMS results provide insight into the size of the strain field associated with the NCFZ but cannot be used to estimate either the strain history or the magnitude of strain experienced by rocks within the fault zone.