RELATIONSHIP BETWEEN THE ORIENTATION OF THE NASHVILLE RIFT IN CENTRAL TENNESSEE AND THE ORIENTATION OF MACROSCALE FOLDS IN POST-RIFT PALEOZOIC STRATA: EVIDENCE FOR RIFT FAULT REACTIVATION

The Nashville Rift is a ~375 km long inferred Precambrian or Cambrian basement structure thought by some to be time-correlative with the East Continent Rift Basin and the Mesoproterozoic Midcontinent Rift. The Nashville Rift runs from northwestern Alabama through the Nashville Dome in central Tennessee to south-central Kentucky. The extent and overall north-northeast orientation of the rift are based on a published density model derived from gravity and seismic wave velocity. In and near the Nashville Dome, macroscale folds deform post-rift Paleozoic strata, and some folds likely formed during reactivation of rift faults. The Stones River syncline is the longest (~25 km) macroscale fold, and this syncline is associated with the largest amount of structural relief (~40 m on the middle Ordovician Lebanon-Ridley contact). This fold trends ~356^o, paralleling the trend of the rift in the immediate vicinity of the fold. The Marshall Knobs syncline is the second longest (~16.3 km) macroscale fold, and this syncline is associated with the second largest amount of structural relief (~35 m). This fold trends ~283^o, forming a right angle with the overall trend of the rift. The Marshall Knobs syncline likely formed during reactivation of what was originally a transverse fault separating two rift segments. However, shorter folds and minor faults of the Harpeth River fault zone trend ~327-358^o, generally forming oblique angles with the overall trend of the rift. Also, the ~6.7 km long Freeman Hollow Road fault and minor faults of the Northern Highland Rim fault zone strike ~240-265^o, forming oblique angles with the overall trend of the rift. In summary, the longest folds are sub-parallel or perpendicular to the trend of the rift, and many shorter folds and faults are oblique to the trend of the rift. These findings support the inference that some macroscale folds formed during reactivation of rift faults.