KINEMATIC HISTORY OF BRITTLE DEFORMATION & PSEUDOTACHYLYTE IN THE HYLAS FAULT ZONE, EASTERN PIEDMONT, VIRGINIA

The Hylas fault zone is a Paleozoic transpressional zone in the eastern Piedmont of Virginia that separates the Goochland Terrane to the west from the Petersburg granite and Richmond/Taylorsville Mesozoic basins to the east. The Hylas fault zone occurs within the central Virginia seismic zone. Although ductile deformation in the Hylas fault zone is well studied, the timing and kinematics of later brittle deformation in the region are poorly understood.

Three regionally extensive fracture sets cut rocks in the Hylas fault zone and Petersburg granite. These include: an older ENE set of mineralized dextral shear fractures, a NNE set of extension fractures and normal faults, and a younger NNW set of extension fractures and normal faults. The NNW set is sub-parallel to Jurassic age diabase dikes. Both the NNE and NNW sets cut Mesozoic sedimentary rocks. The kinematics of the older ENE set is consistent with E/W shortening during the Alleghanian. The NNE set formed during early Mesozoic rifting, with σ₃ oriented WNW/ESE; and the NNW set is concurrent with a Jurassic shift of σ₃ to the WSW/ENE.

The shallowly dipping foliation geometry in the Hylas fault zone is inconsistent with a steeply dipping transpressional high-strain zone. Retro-deformation of the ductile Hylas fault zone rocks indicates that the present orientation of foliation is the result of tilting in the hanging wall of listric normal faults.

Pseudotachylyte occurs in the Hylas fault zone and consists of 1-15 cm wide injection veins. Microstructures in the pseudotachylyte include abundant microlites and spherulites, clasts of cataclasite, xenocrysts of plagioclase and quartz and later veins filled with quartz and calcite. Investigations into the mineralogy, chemistry, and age of the pseudotachylyte are ongoing.