THE STRUCTURE AND STRATIGRAPHY OF THE MULBERRY ROCK GNEISS STRUCTURAL RECESS OF NORTHWESTERN GEORGIA

The Allatoona fault, a terrane boundary formed very late in the kinematic sequence, is the southeastern boundary of the southern western Blue Ridge and northern Talladega belt (TB), maintains a relatively straight trace for >110 km, and remains undeflected past the Cartersville recess. However, on the Yorkville, GA 7.5' quadrangle, a large, convex south structural recess occurs, where the Allatoona fault takes a sharp, north-south trending jog, indenting rocks of the Talladega Group(?)/ Great Smokey Group(?) (TG-GSG; This stratigraphic relationship remains unresolved.) ~10 km into a bimodal metavolcanic complex (Pumpkinvine Creek formation), which is overlain in probable fault contact by metaplutonic and metapelitic sequences intruded by bodies of felsic gneiss. Detailed mapping suggests that a structural window (eyelid window) exists, where rocks of the eastern Blue Ridge and the terrane bounding Allatoona fault are tightly folded into the structural recess. Large culminations of plutonic rock, termed Mulberry Rock Gneiss (MRG), occur within this recess. Previous work suggested that these yet undated rocks are Grenvillian basement overlain nonconformably by the TG-GSG, making this the only known exposure of basement rocks in the TB. The MRG is a medium grained, two-mica orthogneiss unlike the megacrystic Corbin metagranite of well documented Grenville age, which cores the large basement culmination east and northeast of Cartersville, GA. Lithologically the MRG is similar to Paleozoic granitic gneisses of the adjacent eastern Blue Ridge. The surrounding cover sequence consists of sericite phyllite, which is quite different than the coarse metaclastic sequence nonconformably above the Corbin basement rocks only 30 km to the northeast in the western Blue Ridge. Tectono-stratigraphic models of the MRG structural recess being tested include: 1) The MRG is Grenville basement lithologically dissimilar to other basement culminations to the northeast, surrounded by cover rocks of the TG-GSG, and 2) the MRG represents a Paleozoic intrusion within the eastern Blue Ridge terrane and implies a previously unrecognized tectonic break to the northwest between the MRG and its cover sequence.