STRUCTURAL CONTEXT OF MESOZOIC DIKES IN THE EASTERN BLUE RIDGE OF NORTH CAROLINA

Lower Wilson Creek in the eastern Blue Ridge of North Carolina is incised into Late Proterozoic Brown Mountain Granite. The granite is in a southwestern reach of the Grandfather Mountain window, an Alleghanian structure that exposes Grenville rocks, Late Proterozoic Crossnore Complex rocks, and Lower Paleozoic sedimentary rocks. Brown Mountain Granite is variably an L, S, or L/S pre-Alleghanian tectonite with primary schistosity trends of 005°-090° and southeast dips 50°-70° caused by northwest-trending folds. Phyllonite zones and quartz veins post-date schistosity. A series of intensely jointed, nearly vertical silicified breccia veins trending ~030° are parallel to the shear zones but contain phyllonite and granite clasts. The area is notable because it is one of the few places where Mesozoic diabase dikes are exposed in the Blue Ridge. These dikes are at the northwest termination of the "Pageland" dike that is over 300 km long. Dikes 0.5-2.0 m thick trend about 350°, about parallel to one of four joint sets. Though near the orientation of the stream, the dike swarm does not appear to have influenced channel alignment. Several fracture zones defined by dense jointing nucleated on east-west long joints. The fracture zones include joint arrays, shear joints, micro-faults and quartz vein networks. An earlier set of quartz veins are overprinted by these features. In addition, Brown Mountain granite in the fracture zones is pink, possibly from localized hydrothermal alteration.Similar pinking altered granite in contact zones with diabase dikes, suggesting that at least the main period of discrete fracture zones may have been concomittant with dike emplacement. The linear, cross-orogen orientation of Wilson Creek is not explained by a simple relation to existing structures, although at least one joint set is azimuthally coincident. Hydrothermal alteration of granite may be a key to establishing relative age relationships among the brittle structures.