Structural analysis of the original Everona fault excavation and Cenozoic deformation in the Mountain Run fault zone, central Virginia
Published:January 01, 2015
Andy R. Bobyarchick, 2015. "Structural analysis of the original Everona fault excavation and Cenozoic deformation in the Mountain Run fault zone, central Virginia", The 2011 Mineral, Virginia, Earthquake, and Its Significance for Seismic Hazards in Eastern North America, J. Wright Horton, Jr., Martin C. Chapman, Russell A. Green
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The Everona fault is a Cenozoic, possibly Pliocene, tectonic structure that is superimposed on an older orogen-parallel shear zone called the Mountain Run fault zone in the Blue Ridge foothills of central Virginia (USA). The Everona fault was originally defined from an excavation in Orange County, Virginia, that uncovered an unconformity between highly weathered mylonitic rocks in the older Mountain Run fault zone, and an overlying colluvial and fluvial sequence of pebble gravel and massive, ferruginous loamy sand reminiscent of higher terrace deposits in the greater Rapidan River basin. This unconformity is faulted and the channel morphology of the sedimentary deposits is warped into an antiform above several reverse faults. Some of these faults we examined that define the Everona fault have undergone reverse dip-slip movement with up-to-the-southeast displacement, based on striae on polished red and gray clay-rich fault gouge.
Although the Everona fault is not known to be historically seismic (>M1.5), and is outside the normal extent of the Central Virginia seismic zone, the excavation site is only 44 km from the epicenter of the 23 August 2011 Mineral, Virginia, earthquake. From structural analysis of exposures at the original Everona excavation, it has been determined that there is a system of related reverse motion, near-surface tectonic faults with nearly pure dip-slip motion and nearly horizontal northwest-southeast trending P (pressure) axes. Observed throws on these faults approximate a couple of meters. The coincidence of faulted sedimentary deposits with a ridge- and stream terrace–forming soil (the Hiwassee series) 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.