GEOLOGIC MAP OF THE BREVARD ZONE FROM CENTRAL GEORGIA TO EASTERN ALABAMA

Detailed geologic mapping at 1:24,000-scale from Roanoke, Alabama, to north of Atlanta, Georgia (~128 km), detailed surface mapping along four tunnel alignments in the metro-Atlanta area, and subsurface mapping of nearly 6 km of a 5.5-meter diameter tunnel which trends along strike in the Brevard Zone have provided new insights into the nature and significance of this regional geologic feature. Exposures within this, and other tunnels excavated within the Brevard, show extreme lithologic heterogeneity and structural complexity that have not been previously documented within this zone in metro-Atlanta. Although there are many definitions and interpretations to the meaning of the Brevard Zone, we define the Brevard in western Georgia as a relatively narrow zone of doubly plunging, northwest verging, overturned folds that have been overprinted by a pervasive shear fabric. Locally, the Brevard contains characteristic phyllonite and various types of mylonites and cataclastic rocks; however, these rocks are not always present.

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.