FROM COLLISION TO EXTENSION: THE TIMING OF EXHUMATION, UPLIFT AND RIFTING IN VIRGINIA’S EASTERN PIEDMONT

The structural architecture of the central Appalachian Piedmont developed by collisional tectonics, crustal thickening and terrane accretion during the late Paleozoic, but the region was then exhumed and uplifted by continental extension and rift basin formation in the early Mesozoic. We report new thermochronology data (U-Pb zircon, rutile, apatite; (U-Th)/ He zircon; Ar-Ar hornblende, muscovite, K-feldspar) from the enigmatic Goochland Terrane, the Hylas Zone, and the Taylorsville Mesozoic basin in the Eastern Piedmont of Virginia. The Goochland Terrane consists of disparate Mesoproterozoic and early Paleozoic lithologic elements that record different thermal histories until they were juxtaposed during the Alleghanian (~300 Ma). The Goochland Terrane cooled through 350 to 400˚ C between 230 and 270 Ma, and rapid exhumation of the Taylorsville Mesozoic basin’s footwall commenced prior to 225 Ma. Zircon (U-Th)/He ages across the Goochland Terrane are variable and range from 175 to 225 Ma with a younging trend towards the Mesozoic basin. Pseudotachylite veins are exposed in the damage zone of the Fork Church normal fault, the bounding structure at the western edge Taylorsville basin (half graben). Micro-coring of pseudotachylite veins, followed by step-heating analysis yielded Ar isotopic ages of 210 to 218 Ma which are interpreted as the crystallization age K-feldspar, glass and minor biotite from the pseudotachylite melt and consequently the time of seismic activity on this basin-bounding fault. Footwall rocks, including both the basin sediments and the Piedmont basement, were reburied well into the Jurassic. Thermochronological data from the Virginia’s eastern Piedmont is consistent with rapid exhumation at the onset of crustal extension in the Mesozoic rather than orogenic collapse during the Permian.