Relations between deformation of crystalline basement and sedimentary cover at the basement/cover transition zone of the Appalachian Blue Ridge Province

Precambrian crystalline basement of the Appalachian Blue Ridge deforms inhomogeneously by developing relatively narrow ductile deformation zones (DDZs). The Paleozoic sedimentary cover develops open to tight folds and penetrative fabrics. A transition between these two styles occurs at the base of the sedimentary cover in the Early Cambrian Chilhowee quartzites of the central Appalachians and in the Late Proterozoic arkosic sandstones of the southern Appalachians. On a mesoscopic scale, the transition zone sediments show tight to isoclinal folds with highly deformed overturned limbs analogous to mesoscopic (1 cm to 10 m wide) DDZs in crystalline basement. Deformation zones in the basement cut across the basement/cover contact and feed into the overturned limbs of tight folds. On a microscopic scale, both arkoses and granitic basement rocks show thin (5 mm) DDZs characterized by grain-size reduction and alteration of feldspars to quartz and mica. The actual style and symmetry of deformation varies with metamorphic grade, proximity to major thrust faults, and amount of tectonic shortening. In the Grandfather Mountain area of the southern Blue Ridge Province, sets of low-dipping DDZs close to major thrust faults approximate a simple shear deformation field. In the central Appalachians of northern Virginia, similar simple shear deformation features are observed close to major thrust faults, but sets of DDZs define a flattening plane perpendicular to tectonic transport direction higher up within the thrust sheets.