REACTIVATION AND SUPERPOSITION OF CENOZOIC BRITTLE DEFORMATION IN OROGEN-PARALLEL SHEAR ZONES IN THE SOUTHERN APPALACHIAN MOUNTAINS: THE EVERONA FAULT AND MOUNTAIN RUN FAULT ZONE IN VIRGINIA, U.S.A

The Everona fault is a Neogene or Quaternary tectonic structure that is superimposed on an older late Paleozoic orogen-parallel shear zone, the Mountain Run fault zone, in the Blue Ridge foothills of central Virginia. The Mountain Run fault zone is comparable in orogenic position, kinematics, and age with the Brevard fault zone and together comprise a transprovince shear zone over 1,000 m long in the Southern Appalachians. The Everona fault is not historically seismic (>M1.5), but the fault is ~40 km from the epicenter of the 5.8 magnitude 23 August 2011 Mineral, Virginia, earthquake.The Everona fault is recognized as multiple displacements of an unconformity between weathered mylonites in the older Mountain Run fault zone and an overlying colluvial/fluvial sequence of pebble gravel and massive, ferruginous loamy sand forming higher terrace deposits along the Mountain Run channel. This unconformity is faulted and warped into an antiform above several reverse faults with reverse dip-slip movement up-to-the-southeast. Faults in the Everona set trend parallel to the strike of the Mountain Run fault zone, but at the mesoscopic scale the faults cut across the strong transposition foliation in the phyllonites at a moderate angle. It does not appear that the strong mylonite foliation acted as to guide reverse faulting. Rather, a set of secondary axial planar cleavage planes in the mylonites may have acted as guides during faulting. The system of reverse, near-surface faults has nearly pure dip-slip motion and approximately horizontal northwest-southeast trending P-axes. Throws on these faults are a couple of meters. The coincidence of faulted sedimentary deposits with a ridge- and stream terrace–forming soil derived from these deposits suggests that the deformation is geologically young. Potentially correlative dated stream terraces elsewhere in the western Piedmont of Virginia further suggest that motion on the Everona fault could be younger than Pliocene.