INVESTIGATIONS OF THE BREVARD FAULT ZONE WITHIN THE GRANDFATHER MOUNTAIN WINDOW, NC USING POTENTIAL FIELD GEOPHYSICS: TERRANE SUTURE?

Potential field geophysics can be used to study and define subsurface geometry in areas where rocks are heavily weathered, poly-deformed, or remain cryptic. The Appalachian Mountains are a long-lived mountain chain with orogenic events ranging from Proterozoic to late Paleozoic. Understanding the geometry of faults and terrane boundaries can help piece together the lithotectonic history of the North America margin and provide insight into orogenic processes generally.

Located in the southern Appalachians, the Brevard zone is a series of linear NE/SW trending faults that extend from Virginia to Alabama. This project examines it in the Colletsville, NC area using geologic mapping and potential field geophysics. The fault was first activated during the Acadian orogeny as the sole of an orogenic channel and the last movement occurred during the Alleghenian orogeny with the final docking of Gondwana to form the super continent Pangea. There have been many investigations of this extensive fault zone, but questions remain regarding its geometry and nature. Many researchers do not regard the Brevard as a terrane boundary. However, in the Grandfather Mountain window of North Carolina, it bounds Proterozoic Laurentian crust and the Ordovician Tugaloo Terrane.

Both magnetic and gravity observations delineate the window and fault zone, indicating that the overriding thrust sheets that surround the window are comprised of lithologies with different density and magnetic susceptibility. Proterozoic rocks within the Grandfather Mountain window have an absolute Bouguer anomaly of -100 mgal and a local anomaly 20-30 mgal lower than rocks outside the window. Also, preliminary mapping demonstrates that the rocks have magnetic susceptibility contrasts from -0.038 to 27.50 * 10^-3SI, indicating that it is possible to model the subsurface geometry using higher resolution potential field surveys.

Finally, geologic observations south of the fault zone show mineral lineations subparallel to foliation with a plunge of ~ 9º, whereas lineations the north of the fault zone are down dip and plunge more steeply (~52º). Such a structural change is consistent with the Brevard zone being a significant material boundary within the southern Appalachian Orogen.