THE BLUE RIDGE-PIEDMONT BOUNDARY IN VIRGINIA: A TRANSPRESSIONAL FRONTIER

In the southern Appalachians, the Blue Ridge-Piedmont boundary has long been a source of spirited debate as to its kinematic history and tectonic significance. In Virginia, the boundary between the Blue Ridge-Piedmont geologic provinces forms both a distinct and nebulous NE-SW trending fault zone that can be traced for >300 km from the North Carolina boundary to the Culpeper Mesozoic basin in north-central Virginia. From south to north, the fault zone is variously known as the Bowens Creek, Buckmarlson, and Mountain Run zones. These zones separate medium- to low-grained Neoproterozoic cover rocks with a Laurentian affinity from various western Piedmont terranes including the Smith River Allochthon, Hardware, and Potomac terranes. Although the character of this fault zone changes along strike, it delineates a major change in the deformation plan of the orogen. Blue Ridge rocks are dominated by NW-directed contractional structures that facilitated material transport towards the Appalachian foreland, whereas rocks at, and to the east, of this boundary record dextral transpression with both NW-directed shortening and significant strike-parallel elongation and material flow.

In central Virginia, the recently named Buckmarlson zone forms a 3- to 5-km-wide transition zone between the eastern Blue Ridge and western Piedmont. The Buckmarlson transition zone is a NE-SW–trending belt with steeply dipping fabrics that record a different kinematic history than rocks to the northwest and southeast. Rocks in the Buckmarlson transition zone experienced a complex general shear-dominated deformation, and are cut by later discrete low-angle normal faults. Late normal faulting may explain the juxtaposition of low-grade western Piedmont rocks with older (400 to 420 Ma) muscovite cooling ages on higher grade Blue Ridge rocks with younger (345 to 330 Ma) muscovite cooling ages.