KINEMATIC VORTICITY ANALYSIS ACROSS THE TALLULAH FALLS DOME, NE GEORGIA, USA

The Tallulah Falls dome (TFD) is a crystalline dome located in northeastern Georgia, USA that was formed during the early Alleghenian orogeny between 330-308 Ma under amphibolite facies conditions. Previous authors have either suggested that the dome was formed by pure shear dominated underplating or by simple shear dominated unroofing. To distinguish between these two end-member hypotheses we conducted a kinematic vorticity analysis on quartz fabrics. Internal deformation within quartz grains causes dispersion of the crystallographic axes that can be used to fit a vorticity axis, and the bulk vorticity axis orientation at the specimen scale can be estimated using grain populations. We interpret the bulk kinematic vorticity from the pole figure where a lineation parallel vorticity axes indicates pure shear and a lineation perpendicular vorticity axes indicates simple shear.

We used electron backscatter diffraction (EBSD) to determine quartz c-axis orientations. Using MTEX in MATLAB we used the previously published Crystallographic Vorticity Axis (CVA) analysis code to quantify dispersion and determine bulk vorticity. We conducted CVA analysis on 27 samples that were collected across the TFD dome and cut parallel to lineation and perpendicular to foliation. Of the twenty-seven samples, seven exhibited strong pure shear, thirteen had obvious simple shear patterns, and the remaining seven were in between the two endmembers. Samples displaying pure shear are located along both the eastern and western dome boundaries and outside of the dome, whereas samples with evidence for simple shear are ubiquitous across the dome, particularly along the eastern dome boundary.

Our CVA analysis indicates that the Tallulah Falls dome is dominated by simple shear with local pure shear along the western edge of the dome. These results are more consistent with a simple shear dominated dome formation mechanism.