A ductile shear zone defined by lineated, mylonitic, volcanic rocks, principally rhyolitic quartz-feldspar porphyry, crops out in the Precambrian basement of the northern Blue Ridge anticlinorium in south central PA. These mylonitic rocks occur north of the Carbaugh-Marsh Creek (CMC) fault. In this area the CMC fault trends east-west across the structural grain of the Blue Ridge and is interpreted as a right-lateral strike-slip fault. North of the CMC fault, mylonitic foliation strikes northeast and dips moderately southeast. Adjacent to the CMC fault, the strike is more eastward and the mylonitic rocks are cut by numerous joint sets that are prominent close to the fault. Mylonitic volcanic rocks are not known south of the CMC fault. The absence of mylonitic rocks may indicate offset during right-lateral movement along CMC fault compatible with the more eastward strike caused by drag along the CMC fault. Displacements along Triassic normal faults may also obscure the mylonite zone south of CMC fault. An alternative interpretation is that the ductile fault bends into a transverse tear at an oblique thrust ramp that has been reactivated during later right-lateral movement on the CMC fault under brittle deformation conditions. A regionally extensive horizon of mylonitic beds, the Keedysville mylonite, has previously been recognized along the west flank of the Blue Ridge at the base of the Cambrian carbonate section in the northern Blue Ridge (Campbell and Anderson, 1996). The Keedysville mylonite is interpreted to be a fundamental detachment surface in the central Appalachians that is folded and cut by younger faults. The sheared volcanic rocks that crop out north of the CMC fault may be a detachment stratigraphically lower than the Keedysville within the Cambrian carbonates. If these zones of ductile deformation are correlative, then they may represent the footwall cutoff of a thrust ramp along which basement rocks were carried across the platform margin onto the Keedysville flat.