MICROSTRUCTURAL AND KINEMATIC INVESTIGATIONS OF THE BREVARD FAULT ZONE NEAR ROSMAN, NC: IMPLICATIONS FOR MATERIAL MOVEMENT OBLIQUE TO OROGENIC STRIKE

The mid-Paleozoic multiply reactivated Brevard fault zone is approximately 15-20 km wide, and in the Carolinas it separates Tugaloo terrane rocks of the eastern Blue Ridge from those of the western Inner Piedmont. This boundary, however, also represents a profound break in structural style. In the Inner Piedmont, to east of the BFZ, rocks display a gentle dip of the dominant foliation (S2), meso- and map-scale sheath folds are common, and a curved map-scale lineation pattern is observed. Based on this evidence, recent studies have proposed that the western IP represents a curved crustal channel that resulted from tectonically forced, SW directed strike-parallel “escape” flow of partially migmatitic melt-weakened crust during Late Devonian to early Mississippian accretion of the Carolina superterrane.

In the vicinity of Rosman, NC, outstanding exposures of the Brevard fault zone afford the opportunity to collect samples for kinematic (vorticity and strain symmetry) analysis in a number of along and across strike transects. Preliminary vorticity estimates (Wk = 0.65 to 0.80) from sections cut perpendicular to foliation and parallel to the lineation (dominantly oriented SW-NE) indicate that a pure shear component was significant (40 to 55% pure shear), and higher vorticity estimates (ex. Wk = 0.80) occur only in samples collected within ~100 m of the fault surface. These preliminary estimates may suggest that if “escape” flow did occur, it was driven by transpressional tectonism, and material movement either toward the surface or to the SW was not driven by Himalayan-type channel flow. Instead, it is likely the result of complex horizontal wedge extrusion, with simple shear focused on the margins of the wedge (i.e., Brevard fault zone and Central Piedmont suture) and pure shear dominant within the wedge itself. Lastly, since there is little or no obvious lithologic and/or rheologic change across the Brevard fault zone, it is unclear how or why this boundary surface created a buttressing effect to drive material flow to the southwest.