ALLEGHANIAN-AGED HIGH-TEMPERATURE DEFORMATION CONDITIONS ASSOCIATED WITH FORMATION OF THE TALLULAH FALLS DOME, NORTHEASTERN GEORGIA

The Tallulah Falls Dome (TFD) situated in the Eastern Blue ridge of NE Georgia is an elliptical structural dome defined by a foliation that dips shallowly outward and is delineated from overlying metasedimentary rocks by a high strain ductile shear zone. Outside the TFD, the regional deformation is characterized by tight-isoclinal folding, a pattern that predates and is cut by the TFD foliation. Previous models suggest that fabrics within the Neoproterozoic-Ordovician aged rocks (quartzite/metagraywacke and muscovite schist) found in the TFD record peak metamorphism during the Neoacadian orogeny, with dome forming at lower temperatures during Alleghanian-aged duplexing.

We have determined deformation conditions across the TFD based on both microstructural analysis and crystallographic preferred orientation of quartz using electron backscatter diffraction. All analyzed samples preserve quartz grains elongated parallel to the well-developed dome-defining foliation. In general, quartz grains are coarse, commonly forming ribbons with aspect ratios of up to 5:1, and preserve amoeboid grain boundaries and pinned micas characteristic of high-temperature grain boundary migration. The majority of our samples also display well-developed chessboard extinction, a texture consistent with the alpha-beta transition in quartz and deformation temperatures exceeding 650° C at moderate pressures. Quartz crystallographic preferred orientation patterns show large opening angles consistent with deformation at uppermost amphibolite-facies to granulite-facies conditions.

Regional isoclinal folding that predates and is interrupted by the TFD foliation has a maximum age of 335 Ma, as determined by the crystallization age of the deformed Rabun granite.⁴⁰Ar/³⁹Ar data from muscovite inside the dome record ages of 312 Ma. These age constraints indicate that rocks inside the dome cooled from the conditions we observe in deformation fabrics (>650 °C) to 340 °C in approximately 20 m.y. These timing constraints support previous interpretations that doming is Alleghanian rather than Neoacadian, but suggest substantially higher temperature (upper amphibolite-granulite-facies) conditions than previously suggested during this younger orogeny.