A NEWLY RECOGNIZED THRUST FAULT AND THE BYLLESBY-FALLS FAULT ZONE, IRON MOUNTAINS, VIRGINIA: INSIGHTS INTO STRUCTURAL PROGRESSION IN THE BLUE RIDGE-VALLEY AND RIDGE TRANSITION

Structures within province transition zones accommodate tectonic changes and variation in an orogenic system and influences of inherited continental margin and terrane shapes. Analysis of structural and kinematic relationship in the Blue Ridge-Valley and Ridge transition is important to understanding accommodation of tectonic changes during development of the southern Appalachian Mountains. The Iron Mountains, southwest Virginia, expose structures developed in the Blue Ridge-Valley and Ridge transition. New mapping indicates a previously unidentified thrust fault, termed the Dry Run Gap Fault. The Dry Run Gap Fault is exposed as several klippe of older Unicoi Formation sandstone structurally overlying younger, sub-vertical to overturned beds of Hampton Formation shale. Presence of this fault resolves less-than favorable structural geometry interpretation in previous mapping as an upright anticline (e.g. Stose and Stose, 1957). Exposures of the Dry Run Gap Fault klippe are adjacent to the steeply-dipping Byllesby-Falls Fault Zone immediately to the south. Truncation by the Byllesby-Falls Fault Zone suggests that the Dry Run Gap Fault may have been active initially, then offset by reverse and dextral components of displacement in the Byllesby-Falls Fault Zone. Identification of the Dry Run Gap Fault and kinematic changes in the Byllesby-Falls Fault Zone indicate accommodation of changes during the Alleghanian Orogeny in the Blue Ridge-Valley and Ridge transition.