EVOLUTION OF THE BREVARD FAULT ZONE AND GEOLOGIC MAP OF THE COLLETSVILLE, NC 7.5-MINUTE QUADRANGLE

The Brevard Fault Zone (BFZ) is a terrane boundary between Laurentian margin and accreted rocks in the Grandfather Mountain Window, NC that has been reactivated multiple times throughout Appalachian tectonic history. We present a geologic map of the Colletsville, NC 7.5-minute quadrangle. The BFZ, a major NE/SW striking fault zone in the southern Appalachians, approximately bisects the quadrangle. The NW two-thirds of the mapping area are Grenville-aged and Precambrian rift-related Laurentian margin rocks exposed in the Grandfather Mountain Window and the SE one-third are peri-Laurentian/peri-Gondwanan inner Piedmont rocks. The last map of the study area was produced as part of the Lenoir, NC 15-minute quadrangle by Brant and Reed, 1970.

Using traditional and digital geologic mapping techniques, Cs-magnetometer surveys, magnetic susceptibility measurements, electron backscatter diffraction (EBSD) analysis, and whole rock major and trace element geochemistry to produce a geologic map, cross-section, and model, we provide evidence that the Brevard Fault Zone is a terrane bounding feature within the Grandfather Mountain Window and has experienced multiple episodes of reactivation. Approximately 40 km of transects of local magnetic surveys from NW to SE across the fault zone were conducted to map magnetic anomalies and create a model of the subsurface. The anomalies correlate to distinct lithologic units and were also used to help identify contacts. EBSD was used to examine differences in deformation regimes on either side of the BFZ and the results suggest that higher temperature (500-600° C) prism <a> quartz slip, and potentially prism <c> slip (650° C), was achieved within inner Piedmont rocks SE of the BFZ. In contrast, lower temperature (~ 350° C) basal <a> quartz slip was found in Brevard fault zone rocks. Finally, Laurentian margin rocks NW of the fault zone have complex EBSD pole figures suggesting both low and high-temperature deformation episodes (350° C / 600° C).