Kinematic Vorticity of Brevard Fault Zone Mylonite, South Carolina

The Brevard fault zone (BFZ) extends ~750 km from Virginia to Alabama, separating the Blue Ridge and Inner Piedmont (IP). The BFZ originated at high temperature during the Neoacadian orogeny (407-350 Ma) as a broad dextral shear zone in the IP when Carolina superterrane A-subducted Tugaloo and other more westerly terranes. Greenschist facies retrograde metamorphism and fluid-flux occurred during ductile dextral reactivation of a 1-3 km-wide zone along the BFZ ~280 Ma (Alleghanian). Brittle dip-slip reactivation occurred later, producing the out-of-sequence Rosman fault along the NW boundary of the BFZ, during final collision with Gondwana.

Fifteen oriented samples of Henderson Gneiss mylonite (HGM) were collected from the best mapped and best exposed segment of the fault, in NW South Carolina (Whetstone and Tamassee quadrangles), and were sectioned parallel and normal to mineral lineation. Retrograde HGM ranges from coarse porphyroclastic mylonite to ultramylonite, and contains rigid feldspar prophyroclasts in a matrix of annealed quartz and micas. The retrograde BFZ here consists of NE-striking narrow belts of phyllonite, impure marble, orthogneiss mylonite, and other fault rocks: mostly strongly NE-oriented C-L tectonites. Mineral lineation is subparallel to the dominant C foliation, plunging gently NE and SW. Asymmetric feldspar and muscovite porphyroclasts in HGM and S-C fabrics in all units record dextral, SW-directed motion. Both dextral and sinistral mica fish were observed in one thin section, suggesting a pure shear component.

Kinematic analysis includes vorticity analysis using the rigid grain net method and measuring crystallographic fabrics. Preliminary vorticity estimates range from 0.47 to 0.83: from pure to simple shear. Quartz c-axis plots indicate plane strain, possibly the result of annealing. These imply that local transpressional deformation may be related to zippered collision of Gondwana and Laurentia. Local transtensional fabrics may result from outcrop- to map-scale heterogeneity. The dominant 3D strain mechanism is probably triclinic.