MESOZOIC TO RECENT FAULTING IN THE HYLAS ZONE, EASTERN PIEDMONT, VIRGINIA: FROM PSEUDOTACHYLYTE TO SURFACE RUPTURE

The Hylas zone is a regional-scale structure in the eastern Piedmont of Virginia that separates metamorphic rocks of the Goochland Terrane to the west from the ~300 Ma Petersburg batholith and Richmond/Taylorsville Mesozoic basins to the east. The Hylas zone occurs at the eastern edge of the diffuse central Virginia seismic zone, but both the temporal history of faulting and the seismic hazard associated with this structure remains poorly understood.

Regionally, two to three fracture sets cut metamorphic and mylonitic rocks in the Hylas zone including: 1) an older ENE set of mineralized dextral shear fractures, 2) a NNE set of extension fractures and normal faults, and 3) a younger NNW to NW set of extension fractures. The NNW to NW set is sub-parallel to Jurassic diabase dikes. Both the NNE and NNW sets cut Mesozoic sedimentary rocks. Fracture density increases from W to E across the Hylas zone into the Fork Church fault, a large displacement (>1 km) normal fault at the western edge of the Taylorsville basin.

Pseudotachylyte occurs in the footwall of the Fork Church fault, typically within 50 m of the contact with Triassic rocks, and consists of 1-15 cm wide injection veins. The volume of pseudotachylyte in the Fork Church fault damage zone suggests they are associated with large magnitude slip events, likely during basin formation in the late Triassic.

Faulting in the Hylas zone continued throughout the Cenozoic. Near Doswell, Miocene and Pliocene marine strata are offset by both normal and reverse faults. Boulder beds in the Eastover and Calvert formations formed adjacent to emergent fault scarps. Surface displacement created marine embayments and controlled subsidence patterns facilitating the preservation of a world-class Miocene bone bed in the Carmel Church Quarry.