Gabriele Casale

Timing and deformation conditions of the Tallulah Falls dome, NE Georgia: Implications for the Alleghanian orogeny

Structural domes composed of crystalline rocks metamorphosed to midcrustal conditions are present in every major orogen and record deformation of the deep interior of mountain ranges. The Tallulah Falls dome in northeast Georgia is a shear zone−bounded body of metasedimentary rocks that is defined by a foliation pattern that crosscuts the regional fabric of the Eastern Blue Ridge crystalline nappe in the southern Appalachian Mountains. Previously, the Tallulah Falls dome was interpreted as the expression of duplexed underlying slices of Laurentian margin sedimentary rocks imbricated during Alleghanian emplacement of the allochthonous Blue Ridge thrust sheet under greenschist-facies conditions. We provide new observations of quartz and feldspar recrystallization fabrics and new 40 Ar/ 39 Ar dates from muscovite that complement existing thermochronometric ages to determine the timing and conditions of doming within the Tallulah Falls dome. Our microstructural observations are consistent with deformation mechanisms active dominantly at amphibolite-facies conditions, with some samples recording temperatures in excess of 650 °C. Thermochronometric ages define a period of relatively rapid cooling from ∼550 °C to 340 °C within and around the Tallulah Falls dome between 321 and 317 Ma, with differential cooling across the northwestern dome-bounding shear zone lasting until at least 313 Ma. Our observations reveal a period of early Alleghanian cooling from amphibolite- to greenschist-facies conditions associated with displacement along a shear zone bounding the Tallulah Falls dome. Across the Blue Ridge, the Alleghanian orogeny has long been characterized only by thrusting at greenschist-facies conditions. Similarly, dome formation has been interpreted as the result of duplexing within the thin-skinned Blue Ridge crystalline thrust sheet as it was emplaced upon the Laurentian margin. Both of these interpretations are inconsistent with the new data presented here, which suggest more complex Alleghanian deformation over a broader range in temperatures, and perhaps a diverse suite of structural styles. Our results are best explained by doming of the footwall of a normal-displacement shear zone at amphibolite-facies conditions during the earliest Alleghanian period.