THE RELATIONSHIP BETWEEN THE TALLASSEE SYNFORM AND THE BREVARD ZONE AND ITS IMPLICATIONS FOR THE TECTONIC EVOLUTION OF THE SOUTHERNMOST APPALACHIANS

The Tallassee synform (TS) is a major structure that resides in a unique orogenic position within the southernmost exposures of the Appalachian orogen. It is a shallow NE-plunging, late-to-post-metamorphic synform trending parallel to the structural grain of units of the Alabama and Georgia Piedmont. Laurentian margin units of the eastern Blue Ridge (EBR) form its west-limb and Laurentian Grenville basement and attached Neoproterozoic-to-lower Paleozoic cover rocks of the Pine Mountain window (PMW) form its eastern limb. The core of the TS preserves a recently discovered Taconic (i.e., Ordovician) arc terrane (i.e., Dadeville Complex - Inner Piedmont - IP) bound by Alleghanian (late Carboniferous) faults. To the west, the Abanda shear zone is an east-dipping oblique-dextral/reverse fault juxtaposing relatively low-grade (upper greenschist- to lower amphibolite-facies) metasiliciclastics of uncertain tectonic affinity (i.e., Jacksons Gap Group) upon the higher-grade (middle amphibolite-facies) metasiliciclastic and metaplutonic rocks of the EBR (i.e., Emuckfaw Group). Locally, the Abanda shear zone is overprinted by brittle normal faults. To the east, the Towaliga fault is a west-dipping oblique-dextral/normal fault that has down-dropped EBR correlatives (i.e., Opelika Complex) upon the PMW. The footwall-hinge zone to the synform is covered by East Gulf Coastal Plain deposits, but detailed 1:24,000-scale geologic mapping in this area and along both flanks of the structure document that the TS is an open fold plunging 12^o and trending N40^oE. Structural analyses and cross-sections based on down-plunge projections, assuming cylindrical folding, place the keel of the DC at relatively shallow crustal levels in Alabama and the apparent convergence of DC units depicted on geologic maps of Georgia imply the arc complex resides in a doubly-plunging synform. Combined with the fact that the DC lies in the immediate hanging-wall to the Brevard zone, these relationships make it notably reminiscent of the Alto allochthon directly along strike in northeastern Georgia. In our presentation we further explore the similarities and differences between the DC and the Alto allochthon and speculate on their tectonic implications.