GEOLOGIC MAPPING TRANSECT ALONG THE ROANOKE RIVER IN NORTHEASTERN NORTH CAROLINA: INSIGHTS INTO THE ROANOKE RAPIDS TERRANE

New bedrock mapping, funded by USGS FEDMAP, in the eastern Piedmont of North Carolina brings insight to the structure and composition of the Roanoke Rapids terrane (RRt). Intermediate to felsic metaplutonic, mafic to felsic metavolcanic, and metavolcaniclastic rocks are exposed in a ~10 km traverse of the Roanoke River below the Roanoke Rapids Dam. A swarm of andesitic “greenstone” dikes and/or sills intrudes all units except the westernmost granite.

A greenschist-facies (chlorite zone) metamorphic overprint is pervasive, but strain is heterogeneously distributed and more intense to the west in the upstream section, where foliated metavolcanic and metavolcaniclastic rocks are folded and faulted against metagranitoids. Outcrop-scale folds plunge steeply, have subvertical axial surfaces striking N30-60W, and commonly show dextral asymmetry. An axial planar crenulation cleavage (Sc) is locally well developed and shows a sinistral sense of shear. Mylonitic zones, parallel to Sc, are up to several meters thick. Brittle faults, many reactivating ductile structures, are marked by breccia, fault gouge, and networks of calcite, quartz, and chlorite veins.

Downstream, >2km from the dam, high strain structures are largely absent. Rocks exposed here, from west to east, are metaconglomerate, metabasalt flow(s), metamudstone, and granodiorite, all crosscut by andesite intrusions. Folds are open and symmetric with subvertical, N10W-striking axial planes and moderately NNW-plunging axes. An axial planar cleavage is moderately well developed, but primary structures such as vesicular lenses and overturned cross-bedding are well preserved.

Andesite intrusions are massive to well foliated. Larger bodies are zoned with fine grained margins and coarser grained cores. Some are crosscut by later dikes. A general increase from west to east in the size of intrusions and in the size and abundance of lath-like plagioclase phenocrysts (up to 1 cm long) is interpreted to correlate with primary structural depth. Trace element compositions indicate an arc affinity, consistent with new geochemical results for volcanic rocks across the RRt.

Available geochronology constrains magmatism and sedimentation to the Neoproterozoic. The timing of deformation is unknown, but a weak foliation is present in dikes that crosscut mylonite.