ABSTRACT This guide explores relationships among macroscale folds, mesoscale structures, the Nashville dome, and an inferred Precambrian or Cambrian rift in the basement beneath the dome. The Nashville dome, central Tennessee, is an ~12,000 km 2 north-northeast–trending, elliptical cratonic uplift. A published crustal density model shows that a previously undescribed Precambrian or Cambrian rift, herein named the Nashville rift, probably runs from northwestern Alabama through the Nashville dome to southern Kentucky. Within the Nashville dome, macroscale folds and mesoscale structures of the Stones River and Harpeth River fault zones have been interpreted previously as the surface manifestation of subsurface normal faults. This road guide describes two previously undescribed inferred subsurface fault zones: the Marshall Knobs fault zone and the Northern Highland Rim fault zone. The Marshall Knobs fault zone, which is ~16.3 km long, is associated with ~35 m of structural relief, trends east-southeast, is down on the north side, and is inside the geophysically defined rift. The Northern Highland Rim fault zone consists of east-northeast–­striking minor normal and reverse faults and a minor strike-slip fault exposed above the western margin of the geophysically defined rift. The authors hypothesize that the Northern Highland Rim fault zone may be the surface manifestation of the subsurface continuation of a macroscale fault previously mapped at the surface 25 km to the southwest. All of the inferred faults fit into a tectonic model in which they originally formed within a rift and later reactivated, accommodating extension of the uppermost crust during uplift of the Nashville dome.

Abstract The authors use mesoscale structures and existing 1:24,000 scale geologic maps to infer the locations of four macroscale NNW-striking blind normal faults on the northwest flank of the Nashville dome ~30 km south of downtown Nashville. The Harpeth River fault zone has an across-strike width of ~6 km, and, from west to east, includes the Peytonsville, Arno, McClory Creek, and McDaniel fault zones. All of the fault zones are east-side-down except for the west-side-down Peytonsville fault zone. Mesoscale structures are exposed within each fault zone and are observed at three stops along Tennessee State Route (S.R.)-840 and at an additional stop 1.8 km south of the highway. These structures include minor normal faults (maximum dip separation 3.8 m), non-vertical joints, and mesoscale folds. No faults are depicted on existing geologic maps of the zone, but these maps reveal macroscale folding of the contact between the Ordovician Carters Formation and the overlying Hermitage Formation. The authors use the orientation and amplitude of these folds to constrain the orientation and length of the inferred blind fault zones and the amount of structural relief across the zones. The longest fault zones are the Arno (13.2 km long) and McDaniel (11.6 km) fault zones, and the amount of structural relief across these zones peaks at 27 m and 24 m, respectively. The authors also use existing geologic maps to hypothesize that a second east-side-down blind normal fault zone (Stones River fault zone) is located ~27 km northeast of the Harpeth River fault zone. The authors interpret non-vertical joints at one stop as fault-related, and they interpret joints at a second stop as related to a hanging wall syncline. Both of these stops are within 4 km of S.R.-840.