STRONG DOLOMITE BEDS CONTROL THE LOCATION OF, AND DISPLACEMENT ALONG, FORELAND THRUST FAULTS OF THE SOUTHERN APPALACHIANS

The stratigraphic positions of foreland thrust faults in the southern Appalachians are commonly associated with rare dolomite beds near the base of the Cambrian Rome Formation. For example, hanging wall strata immediately above two exposures of the Copper Creek fault (displacement ~15 km) contain a thick (~3-8 m) dolomite bed while the footwall is Ordovician shale or limestone. The Town Knobs fault (displacement ~275 m), a splay in a duplex associated with the Saltville thrust, also carries Rome dolomite in the hanging wall. Footwall strata are dolomites in the Cambrian Conasauga Group. Dolomites are extensively fractured and microfractured and the density of fault parallel veins increases toward narrow (~2-20 cm) shear zones.

Along two exposures of the Copper Creek fault, fault-related shearing localized within calcite veins via a sequence from plasticity-induced fracturing that generated an ultrafine grain size followed by diffusion creep of the ultrafine calcite. Shale wall rocks lack evidence of penetrative deformation or displacement along mesoscale fractures, suggesting that deformation was primarily accommodated within the highly deformed calcite veins and not surrounding strata. Along the Town Knobs fault, which places dolomite on dolomite, deformation is localized within dolomite and potassium feldspar veins. Grain size reduction in the dolomite veins took place by fracturing and dynamic recrystallization, while feldspar deformed by fracturing and alteration to sericite.

The locations of the Copper Creek and Town Knobs thrust faults are correlated with fracturing of the dolomite at the base of the Rome. Fractures allowed migration of fluids and precipitation of veins. The Copper Creek fault, with limestone and carbonaceous shale in the footwall, contains calcite veins. In contrast, the Town Knobs fault, with dolomite in the hanging wall and footwall, has veins of dolomite and feldspar. Calcite veins, deforming by diffusion creep of ultrafine-grained calcite, allowed large displacements. Along the Town Knobs fault, deformation of dolomite and feldspar veins allowed less displacement per fault, but displacement is distributed over many faults of the duplex structure. Thus, while dolomite controls the location of the faults, displacement along the faults is controlled by the rheology of the veins.