PSEUDOTACHYLITE IN THE FORK CHURCH FAULT ZONE, EASTERN PIEDMONT, VIRGINIA: A RECORD OF MESOZOIC SEISMIC ACTIVITY

The Fork Church fault (FCF) is a major normal fault in the eastern Piedmont of Virginia that separates Proterozoic to Paleozoic metamorphic rocks of the Goochland Terrane from the Mesozoic Taylorsville basin, a large half-graben complex with strata that dip 10° to 30° NW towards the FCF. The FCF bounds the western edge of the exposed part of the Taylorsville basin. To the west of the fault is the Hylas zone, a mylonite shear zone that records dextral transpression during the Alleghanian orogeny. The FCF formed during the Late Triassic when rifting of Pangaea began.

Pseudotachylite occurs as 1-15 cm wide injection and fault veins in the footwall of the FCF, but not in the hanging wall. They are located within 50 m of the contact with the Taylorsville basin, surrounded by foliated and fractured mylonitic quartzofeldspathic gneiss and hornblende-bearing gneiss. Pseudotachylite veins are typically northeast striking with a variable dips. Microstructures within the pseudotachylite include abundant spherulites and microlites, flow structures and cataclasite along the margins of the pseudotachylite. Xenocrysts of plagioclase, quartz and cataclasite range in size from 0.01 to 3mm with inclusions of pseudotachylite and commonly form deeply embayed grains. These observations are consistent with frictional melt origin likely generated by large magnitude slip events. Pseudotachylite veins are cut by mineralized fractures composed of quartz, calcite, and zeolites. Dating the pseudotachylite will determine the age of past seismic activity in the FCF. Quantitative analysis of mineral and matrix chemistry of the pseudotachylite aims to determine the temperature and pressure conditions associated with fault slip.