MICROSTRUCTURAL ANALYSIS OF THE TALLULAH FALLS DOME: DEFORMATION CONDITIONS AND KINEMATICS ASSOCIATED WITH ALLEGHANIAN DOME FORMATION

The Tallulah Falls Dome is located in northeast Georgia within the Eastern Blue Ridge thrust sheet. The Blue Ridge thrust sheet was emplaced on the Laurentian margin during the Paleozoic Alleghanian Orogeny. Deformation associated with this collisional event is generally considered to have occurred under greenschist facies conditions. Doming in the Eastern Blue Ridge, including formation of the Tallulah Falls Dome, is interpreted to be the result of Alleghanian duplexing of Valley and Ridge sediments. The goal of this research is to determine the deformation conditions as well as kinematic shear sense during dome formation.

We determined the conditions of deformation by optically analyzing quartz and feldspar deformation fabrics and through EBSD analysis of quartz c-axis orientations. Quartz has undergone high-temperature (fast) grain boundary migration as evidenced by pinned micas in quartz, coarse grain sizes with amoeboid shapes, and the formation of quartz ribbons with high aspect ratios. Chessboard extinction is present in several samples, with coarse, equant grains of quartz, which provides evidence for uppermost amphibolite to lowermost granulite facies metamorphism. Coarse grained foliation-parallel quartz ribbons with amoeboid grain boundaries are common and provide evidence for deformation at uppermost amphibolite facies conditions. EBSD analysis using quartz opening angle thermometry provides evidence for temperatures above the alpha-beta transition in quartz, consistent with our optical observations of quartz deformation fabrics. Feldspar grains display recrystallization, kinking, and occasional fractures which supports the temperature range seen in quartz deformation fabrics. We determined kinematic shear sense in mylonitic fabrics throughout the dome with analysis of S-C fabrics, mantled porphyroclasts, mica fish, and rotated garnets. The shear sense showed no consistent pattern, suggesting coaxial deformation.

Our observations suggest that deformation within the Tallulah Falls Dome occurred at amphibolite facies to lowermost granulite facies conditions. This conclusion indicates that during the Alleghanian Orogeny, internal deformation within the Eastern Blue Ridge occurred at a higher metamorphic grade then previously recognized.