GEOLOGIC MAP OF THE BARTLETTS FERRY FAULT ZONE, SOUTHWEST QUARTER OF BARTLETTS FERRY DAM, 7.5 MINUTE QUADRANGLE, GA-AL

The Bartletts Ferry Fault Zone (BFFZ) has been interpreted to be the major south-bounding fault of the Pine Mountain Window (cored by Grenville basement rock), and possibly a part of the master decollement of the original thrust fault equivalent to that of the Towaliga Fault Zone on the northern side of the Pine Mountain Window. The BFFZ is dextrally sheared with a strike-slip component and is signified by a major mylonite zone approximately 2.8 km (1.75 mi) wide. The mylonite zone is characterized by proto-mylonite, mylonite, ultramylonite, blastomylonite, and phyllonites. Shear fabrics within the mylonite zone have an average strike of N55°E, dipping to the southeast around 47°; however, these are locally folded.

To the north of the zone of most intense mylonitization is a megacrystic, granitic, massive, biotite gneiss unit that shows slight to moderate shearing. Shearing of the feldspar megacrysts forms discrete augens that locally develop sigmoidal tails showing shear sense. The shear foliation in the magacrystic-biotite gneiss is undulatory, but has an average strike of N52°E; however, dip varies as a function to proximity to the BFFZ. Near the BFFZ, dips are high-angle (~80°) to the northwest; away from the BFFZ, dips are moderately inclined (45°) to the northwest.

South of the zone of mylonitinization is a hornblende gneiss/amphibolite that exhibits polyphase deformation as indicated by pervasive interference folds. Weak development of plagioclase porphyroclasts suggests minor shearing. Due to the complex deformation of this unit, foliation is variable, with an average strike of N53°E and dips to the southeast near 50°.

Kinematic analysis of shear sense indicators and geochemical analyses of each unit are currently underway to further characterize the stress regime and to help determine the protoliths of the different mylonites within the BFFZ.