THE NATURE OF LONG-LIVED APPALACHIAN CRUSTAL BOUNDARIES: GEOLOGIC AND GEOPHYSICAL CONSTRAINTS ON THE BREVARD ZONE IN NORTH CAROLINA

Understanding long-lived crustal boundaries is important to understanding how orogenic systems work globally. The Brevard fault, extending from Virginia to Alabama, within the Southern Appalachian orogen presents an excellent natural laboratory in which to examine these processes. In particular, we focus on the Brevard fault near the Grandfather Mountain window in North Carolina. At this location, the fault forms a boundary between 1.1-1.3 Ga Laurentian crust and the Ordovician Tugaloo terrane. However, at other locations Tugaloo terrane rocks can be found on both sides of the Brevard fault, complicating interpretations.

Along the southern edge of the Grandfather Mountain window, a new geologic map of the Collettsville Quadrangle has been produced. The Brevard fault cuts across the mapping area allowing for detailed investigation of: 1) Map and outcrop-scale structural observations, 2) Magnetically constrained and modeled cross-sections across the Brevard fault, 3) Regional-scale gravity constrained cross-sections, 4) Microstructural deformation processes using thin-sections and EBSD, and 5) Trace-element fingerprinting to better identify and group individual units and tectonic provenance.

At this location, the Brevard fault dips 50-60° to the southeast with a stack of more shallowly dipping (40°) brittle thrusts located immediately to the north. The imbricated thrust stack contains slices of the Alligator Back Fm. followed by thrust sheets of the Proterozoic Grandfather Mountain Fm., and the Laurentian Wilson Creek and Blowing Rock Gneisses. Microstructural and EBSD data indicate at least two major deformation episodes with temperatures of >550 C° (quartz prism <a> slip) and a later one at 350-400 C° (quartz basal <a> slip). However, the timing of the earlier higher temperature episode may be substantially different north (Taconic or earlier) vs. south (Acadian) of the Brevard fault. Overall, the Brevard zone is interpreted as acting as a rigid backstop in the Acadian orogen against which lateral crustal flow occurred, and as a crustal ramp in the Alleghanian orogeny, over which the Tugaloo terrane was thrust.