Geology of the Dadeville Quadrangle and the Tallassee Synform in Characterizing the Dog River Window

Abstract

Structural and lithologic observations are reported for southernmost Appalachian shear zones and Blue Ridge and Inner Piedmont metamorphic rocks exposed on the Dadeville 7.5-minute quadrangle map. Brevard fault zone lithologies, called the Jacksons Gap Group, separate the dominantly metasedimentary eastern Blue Ridge from the overlying metavolcanic-metaplutonic Dadeville Complex (Inner Piedmont). The Jacksons Gap Group is sandwiched between the syn-metamorphic Katy Creek fault, above, and the post-metamorphic Abanda fault, beneath. Deformation along the Brevard fault zone is characterized by multiple reactivations through time under varying crustal conditions. Brevard fault zone faulting initiated at near peak-Neoacadian (Late Devonian to Early Carbonifereous) metamorphic (middle-amphibolite facies) conditions yielding mylonites with oblique right-slip and thrust movement indicators. S-C mylonitic fabric along the Abanda fault records Alleghanian (Middle Carboniferous) dextral strike-slip fault reactivation under upper-greenschist to- lower-amphibolite facies conditions. Mylonites reflecting both of these crystal-plastic shearing events were overprinted by Mesozoic subvertical cataclastic faults. The new geologic map is combined with existing 1:24,000-scale geologic maps and structural analyses conducted on quadrangles framing the hinge zone of the regional northeast plunging Tallassee synform, confirming that lithologies and structures of the eastern Blue Ridge (Emuckfaw Group) on the west limb are continous with those previously assigned to the Inner Piedmont (Opelika Complex) on the east limb. Aeromagnetic and gravity geophysical signatures are consistent with the surface geology, further indicating closure of the Tallassee synform beneath a thin veneer of Mesozoic Gulf Coastal Plain sediments. U-Pb dating of granites that intruded both limbs of the synform (Kowaliga Gneiss and Farmville Metagranite) confirm similar Late Ordovician-Early Silurian crystallization ages verifying common magmatic histories. U-Pb detrital zircon analyses of distinctive orthoquartzites of the Jacksons Gap Group confirm similar source areas and tectonostratigraphic continuity with regionally distinct marker quartzites along strike in the Blue Ridge and Inner Piedmont of Georgia. Findings document a large (1,029 km2) tectonic window through the Inner Piedmont allochthon, herein called the Dog River window that exposes a vast area of previously unrecognized Laurentian Blue Ridge rocks spanning most of the Alabama Promontory.