ALLEGHANIAN DISASSEMBLY OF THE EASTERN BLUE RIDGE ORDOVICIAN BACK-ARC BASIN: PERSPECTIVES FROM THE MULBERRY ROCK RECESS (34°N), NORTHWEST GEORGIA

The Mulberry Rock recess (MRR) in NW Georgia (34°N) is a 20 km-long N-S indentation into the E. Blue Ridge (EBR) frontal fault system, marked by the breached Mulberry Rock antiform (MRA) which forms a ½ window exposing the underlying Talladega belt (TB). An array of low-angle thrusts frame the composite window, carrying EBR units, including from top to bottom: the Middle Ordovician Wedowee/Emuckfaw/New Georgia Groups (WENG), the Middle Ordovician Pumpkinvine Creek/Canton Fms. (PCCF), underlain by two 7 km-long horses of the Early Silurian Mulberry Rock Gneiss (MRG). The thrusts at the base of these units are the Burnt Hickory Ridge fault (BHRF), Allatoona fault (AF), and Mulberry Rock fault (MRF) respectively, with the AF the youngest. Rocks of the PCCF rim the MRR and extend NE >125 km, but are not found SW of the MRR. The MRG only occurs on the E. flank of the MRR, whereas rocks of the lower WENG and structurally underlying Hillabee Greenstone (HG) (emplaced on the TB much earlier) only occur to the SW. In addition, structural and stratigraphic cutoffs along the base of the BHRF and AF trend N-S, parallel to the MRR. These relationships are interpreted to be the result of the 3 thrusts advancing over a large oblique N-S footwall ramp in the MRR’s hinterland. Constriction at the intersection of this oblique ramp and an adjacent frontal ramp produced the N-S open MRA which arched the thrusts. TB stratigraphic cutoffs in the AF footwall trend NE-SW, suggesting that final emplacement involved cutting >3 km down section through the TB in the displacement direction. Minimum horizontal net slips on both the AF and the BHRF are ~14 km. These faults merge at the NW margin of the MRR, with the AF cutting up through the PCCF, across the BHRF and into the Wedowee Gp. (WG). Farther SW the AF emplaces the WG against the TB, and the WG locally overlies the HG at Tallapoosa, resulting in a footwall geometry and lithostratigraphy similar to that of the Hollins Line thrust (HLT) to the SW. Farther SW however, the AF cuts the HLT west of the AL/GA state line in a large reentrant at Hightower and continues SW, faulting the WG against the Ashland Supergroup (hangingwall of the HLT). Retrodeformation of the regional thrust sheets brings the HG and PCCF into along-strike parallelism, suggesting that they represent segments of the same Middle Ordovician bimodal volcanic sequence.