INFERRED PALEOZOIC REACTIVATION OF PRECAMBRIAN BASEMENT FAULTS WITHIN A CRATONIC FLEXURAL BULGE, NASHVILLE DOME, CENTRAL TENNESSEE

The investigators examined a largely aseismic area on the Nashville dome where no macroscale faults appear on published geologic and geophysical maps. To search for folds related to inactive subsurface faults, they scanned and georeferenced 7.5 min. quadrangle maps, digitized contacts between Ordovician formations, and used the National Elevation Dataset (NED) to assign elevations to numerous points on these contacts. Elevation variations revealed six synclines interpreted by the investigators as having formed through the movement of subsurface normal faults. Four of these inferred faults are within the Harpeth River fault zone approx. 30 km south of downtown Nashville. They strike 331-358º, are 1.0-13.2 km in length, and are responsible for maximum structural relief of 8-27 m. The Stones River fault is a fifth inferred fault approx. 35 km southeast of Nashville, striking 356º and having a length of 25.0 km and associated maximum structural relief of perhaps 40 m. With one exception within the Harpeth River fault zone, all of these faults are east-side-down. The sixth inferred fault is the northeast-side-down Marshall Knobs fault which is approx. 55 km southeast of Nashville and has a strike of 283º, a length of 16.3 km, and associated maximum structural relief of 35 m. As would be expected for faults, displacement (d) increases with length (L): d=10^-2.5L with r²=0.86. (This relationship assumes that structural relief was caused by dip-slip on faults dipping 60º.) Field observations are also consistent with the fault interpretation because three fault-parallel joints dipping 64-76º have been found at one location along the Marshall Knobs fault, and minor normal faults have been found along all of the others.

Limited published drill hole data is consistent with basement faulting along the Harpeth River fault zone and Stones River fault, and a published interpretation of aeromagnetic data indicates greater depth to magnetic basement within the Harpeth River fault zone. The deviation of the strike of the faults from the 050º trend of the Nashville dome is also consistent with reactivation of basement structures. Consequently, the investigators think these faults are Precambrian basement structures which reactivated when the lithosphere flexed during Paleozoic orogenic loading of the Laurentian margin.