Seismic reflection, magnetic, and gravity data from the Blue Ridge and Inner Piedmont of Georgia, North Carolina, and South Carolina indicate that southern Appalachian basement contains two sets of steeply dipping normal faults that bound rotated and tilted blocks. The predominant faults strike about N35°E, subparallel to the axis of the orogen, whereas the oblique set strikes about N10°W. Vertical offsets of as much as 1.3 km across individual N10°W-striking faults have been observed on seismic profiles. Regionally, the pattern is probably similar to normal faults in Precambrian rocks exposed in the Adiron-dacks or along other rifted margins. The initial age of faulting was Late Proterozoic, 570 m.y. or older, inferred from comparison of seismic reflectors in the eastern Blue Ridge and Inner Piedmont with stratigraphy along the Great Smoky thrust. Basement beneath the western portion of the Inner Piedmont in South Carolina and northeastern Georgia has been stable at least since the Middle Triassic and did not reactivate during the Mesozoic breakup of Pangea,based on the absence of crosscutting relationships between basement faults and the Brevard zone reflection.
Basement structure related to rifting may partially explain the focal mechanisms of regional earthquakes. The oblique fault set is consistent with well-constrained north-south-striking, subvertical nodal planes obtained for many basement earthquakes in the Valley and Ridge of eastern Tennessee. Valley and Ridge earthquakes in southwestern Virginia are consistent with reactivation of the faults that strike parallel to the orogen. We suggest that the basement structures survived Phanerozoic orogeny because Alleghanian continental collision was interpreted in the southern Appalachians, so that convergence between Laurentia and Gondwana stopped about the time their respective lithospheres came into contact.