DEFORMATION PARTITIONING AMONG STRUCTURAL BLOCKS IN THE PRECAMBRIAN BASEMENT OF THE NORTHERN BLUE RIDGE

A ductile shear zone within the Catoctin Formation north of the Carbaugh-Marsh Creek (CMC) fault is defined by lineated, mylonitic rhyolite. Foliation is defined by alternating bands of light and dark matrix, composed of fine grained quartz and feldspar, laminae of magnetite and locally developed laminae of muscovite. Smeared grains of magnetite and hematite define lineation. Ductile shear is heterogeneously developed within a belt approximately two km wide, distinguished by mylonite that anastomoses around less deformed pods of rhyolite. The heterogeneity of the shear zones can be recognized on the map, outcrop and thin section scale. Within zones of high strain, quartz and feldspar phenocrysts commonly record asymmetric grain tail complexes indicative of simple shear. Curved quartz fibers are locally developed in strain shadows adjacent to magnetite grains. In less deformed rocks spherulites and patches of optically continuous quartz that enclose feldspar microlites (micropoikilitc patches) may be preserved. Compositional bands are commonly convoluted and define disharmonic folds interpreted as primary volcanic structures. Locally, rocks record foliation defined by flattened micropoikilitic patches, and/or devitrified shards or pumice fragments. Adjacent to the CMC fault, where the strike of mylonitic foliation bends westward mylonitic rocks are strongly jointed. In this area the CMC fault strikes westerly across the structural grain of the Blue Ridge and is interpreted as a right-lateral strike-slip fault. The timing of movement on the CMC fault is not well established. The fault offsets quartzite ridges, which define the western limb of the Blue Ridge. This indicates that the CMC fault was active after formation of the anticlinorium. The CMC fault may have formed early in the formation of Blue Ridge with the result that separate structural domains north and south of the fault deformed independently. The absence mylonitic rocks in exposures adjacent to the CMC on the south supports previous interpretations that the CMC fault separates structural domains. In general the rocks south of the CMC fault record less ductile deformation, less foliation and less recrystallization of both groundmass and quartz and feldspar phenocrysts.