PRELIMINARY STRUCTURAL ANALYSIS OF A HIGH STRAIN ZONE IN THE EASTERN PIEDMONT, GEORGIA

Previous work demonstrates that the tectonic units of the eastern Piedmont are key to understanding the final stages in the development of the southern Appalachians. In Georgia, the province is well studied adjacent to the Pine Mountain window and near the border with South Carolina. However, existing geologic maps show a need for more detailed investigations in the central part of the state. We present preliminary field and microstructural analysis of highly strained rocks from an exposure located in Oconee National Forest, ~40 km northwest of Milledgeville, GA. Previous mapping places this location in the Charlotte terrane, a peri-Gondwanan infrastructural terrane. The outcrop is ~35 meters (structural thickness) of pervasively deformed amphibolite and feldspathic gneiss. The main fabric in the outcrop is a vertical to steeply southeast-dipping penetrative foliation with an average strike of 057^⁰. The foliation planes contain weakly developed, sub-horizontal mineral lineations. The foliation is defined by discontinuous, quartz-rich laminae in the amphibolite and thin biotite domains in the gneiss. Interlayering of the amphibolite and feldspathic gneiss is subparallel to the foliation. Contacts between the two phases are sharp and cuspate, and in most instances, the cusps reflect boudinage of the amphibolite intervals. Boudinaged layers contain a nonsystematic mix of symmetrical and asymmetrical boudins. Asymmetrical boudins record primarily dextral shear sense, consistent with the stretch direction implied by the mineral lineations. The outcrop also contains symmetrical intrafolial folds and asymmetrical drag folds, the latter of which also record dextral shear sense. Biotite, white mica, and chlorite porphyroblasts are aligned to the foliation, and the same phases occur in strain shadow assemblages, indicating these phases grew synkinematically with respect to the main foliation. Quartz in the gneiss exhibits grain boundary migration recrystallization microstructure. Collectively, the mineral assemblages and microstructures imply that the main fabric developed under greenschist conditions. Shear sense, as recorded by shear bands and tails on feldspar porphyroclasts is ambiguous. Both dextral and sinistral shear-senses are evident in all thin sections.