Regardless, the Allatoona is one of the kinematically latest and longest faults in the southern Appalachian orogen. Although that sequence extends another 120 km into North Carolina, continuation of the Allatoona fault that additional distance is in debate. To the northeast, the Allatoona fault lies at the base of the Dahlonega gold belt, becoming an internal eastern Blue Ridge thrust at Dawsonville, Georgia. The Allatoona fault cuts down obliquely into the tectonostratigraphy progressively deeper both to the northeast and northwest, locally approaching underlying foreland thrust sheets, and cutting older regional structures. Structural branch points between the Allatoona and Mulberry Rock thrusts indicate that the Mulberry Rock allochthon is a large north-trending horse beneath the Allatoona fault, centered on the Mulberry Rock window, which is likely the result of oblique ramp thrusting over the massive Mulberry Rock Gneiss. Two earlier thrusts within the window (Mulberry Rock and Burnt Hickory Ridge thrusts, with a combined minimum horizontal net slip component of 27 km) are cut by the Allatoona fault, which is a ~15 m-wide high strain zone with top-to-the-northwest displacement, and a >17.2 km horizontal net slip vector. In the central segment of the fault, where it marks the western/eastern Blue Ridge domain boundary, a ~20 km-long eyelid window (Mulberry Rock window) framed by three amphibolite facies thrust sheets overlying the greenschist facies Talladega belt allochthon, allows a 3-D view into the structural architecture, kinematics, and trajectories of the regional thrusts. To the southeast, however, units within the fault hanging wall maintain a nearly constant width resulting in a significant change in the regional structural architecture of the orogen. On the northwest, the fault cuts Pennsylvanian or younger(?) regional cross antiforms that cause significant orogenic curvature of older underlying thrust sheets and is likely Permian in age. It is an out-of-sequence, craton-directed thrust fault that cuts metamorphic isograds and earlier thrusts, and it has a nearly linear trace of ≥280 km, making it one of the major thrust faults in the orogen. ![]() The Allatoona thrust fault in the southernmost hinterland of the Appalachian Blue Ridge-Piedmont megathrust sheet is among the latest structures in the kinematic sequence of events along the west flank of the orogen. The orientation of the array of Riedel shears indicates top to the northwest (thrust) displacement on the fault (blue arrows). Fault zone is ~15 m thick and is bounded on southeast by rocks of the Wedowee Group and on the northwest by rocks of the Higgins Ferry Group. “P” shears are synthetic minor faults symmetrically oriented to the R shears with respect to the main fault plane. “R’” shears are conjugate, antithetic to the “R” shears, and are oriented at a high angle to the main fault plane. The low angle “R” Riedel shears develop at a small angle (typically 10–20°) to the main fault and are synthetic to the main fault. Shear surfaces (red lines) are interpreted as brittle Riedel shears that are subsidiary to the main thrust. B) Black lines are traces of compositional layering/schistosity within individual fault slices and generally dip to the southeast. Saprolite exposure shows subsidiary brittle shears (red lines) within the fault zone between fault slices of different lithology, including graphitic schist, granitic gneiss, and mafic rocks. A) Oblique view toward the south southwest (~55° from strike of fault) of vertical road ditch exposure showing fault zone features of the Allatoona fault (Goodwater-Enitachopco fault of Tull, 1978) southeast of Millerville, AL.
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