GEOLOGIC MAP OF THE BREVARD ZONE FROM CENTRAL GEORGIA TO EASTERN ALABAMA
In Alabama, the Brevard is defined as a terrane boundary between the eastern Blue Ridge and Inner Piedmont/Dadeville Complex. This interpretation is not consistent with our detailed geologic mapping, in that continuous stratigraphic units are transected by Brevard shearing. For example, the Long Island Creek Gneiss (LICG) is north of the Brevard in the metro-Atlanta area. Further west along the Brevard trend, the LICG is within the Brevard and has been intensely sheared and silicified (flinty crush rock). Further southwest, near the Alabama state line, the LICG is south of the Brevard and has a similar character as the exposures in the metro-Atlanta area.
The Brevard has been overthrust in the metro-Atlanta area by the Clairmont Formation (mélange). Rocks within the Clairmont show no evidence of Brevard shearing and are interpreted to have been emplaced after Brevard shearing culminated. Both the Clairmont and the Brevard are intruded by the Ben Hill Granite. The Ben Hill cuts across button schist, phyllonite, and mylonitic rocks of the Brevard, and itself is unsheared. Preliminary age dates of mica growth in the Brevard of Alabama by Poole (per com) suggest that the last shearing occurred ~317 Ma. This, coupled with an age of the Ben Hill (~284 Ma), constrains emplacement of the Clairmont thrust sheet.