KINEMATICS OF THE EVERONA FAULT, CENTRAL VIRGINIA

The Cenozoic Everona fault is within the Paleozoic to Mesozoic ductile Mountain Run fault zone in the western Piedmont of Virginia. Louis Pavlides and other USGS geologists discovered the Everona fault in 1983 near the town of Everona in Orange County, Virginia. The Everona fault is one fault in a relatively narrow, brittle fault zone that appears to be a successor structure on the larger, older, and originally deeper Mountain Run fault zone. The Everona fault is a reverse fault dipping about 20° NW that truncates foliation and layering in saprolite of metasedimentary rocks in the footwall of the Mountain Run fault zone. The upper part of the fault continues through a sedimentary cover of a basal quartz pebble conglomerate and overlying red clay with a net dip slip of about 2 meters. Many sub-parallel reverse and thrust faults dip 20°-30° NW and strike about 025°. Thin (a few centimeters) clay gouge with dip-parallel slickenlines mark most faults in the surficial deposits. Saprolite is mainly a phyllitic or phyllonitic part of the True Blue Formation that is everywhere very deformed and poorly exposed in the Mountain Run fault zone. The primary tectonite foliation in this shear zone might be thought to guide subsequent fault slip, especially at very shallow crustal levels, but this does not seem to be the case everywhere. The mylonite foliation near Everona is moderately to steeply dipping but often at a large angle to reverse faults in the Everona suite. A less pervasive secondary cleavage, which appears throughout the Mountain Run fault zone, is parallel to most reverse faults. Either the discrete cleavage is considerably weaker than the primary foliation or had a more favorable orientation for slip during formation of the Everona fault. The Everona fault actually places sub-unconformity saprolite over the basal gravel cover layer and updip of that brings the gravel over massive red clay. This and other faults root into the underlying saprolite. To this extent, the Everona faults involve “basement” and are not confined to the overlying surficial deposits. The footwall of the main fault is deformed into a syncline. The overall geometry of the Everona fault and affiliated structures is that of a fault propagation fold at very shallow crustal levels, and is sufficient for a tectonic origin for the structure.