DEXTRAL STRIKE-SLIP OVERPRINT ON THE BYLLESBY-FALLS FAULT ZONE, IRON MOUNTAINS, SOUTHWESTERN VIRGINIA

The Byllesby-Falls Fault Zone trends east-west across the Iron Mountains, southwestern Virginia, and lie at the western edge of the Blue Ridge tectonic province in the transition to the Valley and Ridge. The Byllesby and Falls faults are are in close proximity to each other, and have been previously identified as thrust faults (Stose and Stose, 1957). Outcrop at Hale Lake, Virginia—southwest corner of the Speedwell Quadrangle—exposes two faults which are part of the Byllesby-Falls Fault Zone, and show kinematic changes during the course of displacement. Riedel fracture set geometry in the fault exposures indicate a kinematic change from reverse-slip to dextral strike-slip motion. Orientations of a dominant set of P-, R-, and T-fractures within the fault walls indicate a period of dextral motion in the fault zones following reverse motion. The dextral set of Riedel fractures overprint and cross-cut older T-fractures associated with a lineated reverse fault plane. Change from reverse to dextral shear sense represents a more complex kinematic history, and indicates that the Byllesby-Falls Fault Zone accommodated a dextral motion component following convergent motion during the Alleghanian Orogeny. Presence of overprinting dextral strike-slip structures in Byllesby-Falls Fault Zone suggest that other structures in the Iron Mountains may have formed in part to such kinematic changes. Structures in the Blue Ridge-Valley and Ridge transition may have accommodated progressive kinematic changes during convergence around the Virginia Promontory.