ANALYSIS AND COMPARISON OF MULTIPLE SYSTEMATIC PLANAR FRACTURES IN METAVOLCANIC AND METASEDIMENTARY ROCK SEQUENCES IN THE CAROLINA TERRANE IN NORTH CAROLINA

The Albemarle arc is in the Neoproterozoic to Cambrian Carolina terrane that was accreted to Laurentia in the Late Ordovician to Early Silurian Cherokee orogeny. The arc rocks deformed under greenschist facies into regional folds that trend northeast-southwest and verge southeast; some folds are periclinal. In Uwharrie National Forest, NC, resistant metavolcanic rocks in the Tillery and Cid formations cap ridges separated by lowlands underlain by argillaceous metasedimentary rocks. All rocks have cleavage that is axial planar to the regional folds. Of particular interest are multiple planar structures in felsic metavolcanic rocks, including anastomosing cleavage, multiple systematic planar fractures or joints, exfoliation fractures, and curvi-planar discontinuities related to cracking and spallation. Integrated vector analysis of a few hundred joints results in a mean attitude of (strike, dip) 330°,88° (RHR). That trend is consistent with long, fracture-controlled topographic lineaments in the Piedmont. A second, sparse lineament trend of about 290° is weakly represented in the joint data, too. Fracture trends in a subset of felsic metavolcanic rocks, however, are dispersed and even somewhat anti-clustered (or rotated) when compared to the integrated data. Cleavage is accentuated as weathering advances but in fresher exposures may resemble close joints. Nonetheless,the interference of regional cleavage attitudes is minimal in the fracture datasets. Tectonic joints in metavolcanic rocks contain multiple sub-maxima, the strongest one close to but not coincident with the integrated joints maximum. In a limited number of metavolcanic outcrops we have observed multiple, co-linear and closely-spaced surfaces that intersect to form long polygonal prisms. This geometry is inconsistent with joint set morphologies outside of the metavolcanic rocks and is suggestive of strained columnar joints. Some bodies in this area have been interpreted as rhyolite domes, which commonly exhibit shallow intrusion features like columnar joints. Lithologic fracture trend differences in the study area are the result of pre-regional jointing discontinuities, potential stress variations across rheological boundaries, and surficial processes.