FRACTURE ORIENTATIONS AND CAUSES IN CONTRASTING LITHOLOGIES IN THE CENTRAL PIEDMONT OF NORTH CAROLINA, USA

In deeply weathered crystalline rocks of the Southern Appalachian Piedmont, linear topographic features are generally assumed to represent trends in lithostratigraphic variations to erosional resistance or to fracture-controlled erosion. We have compiled over 600 outcrop scale fracture orientations within an area of about 3,600 hectares in central North Carolina to show that, while general statistics do support the assumption of regional fracture dominance on local landscape evolution, there is considerable domainal variation in planar fracture expression controlled by deformation, inherited structures, rheology, and differential weathering. The rocks are Neoproterozoic to Cambrian formations in the volcanic arc-related Albemarle Group that were metamorphosed in greenschist facies during the Late Ordovician to Early Silurian Cherokee orogeny. For this report, we divide bedrock into fine-grained argillites and resistant, ridge-capping volcanogenic felsic and mafic rocks with minor pyroclastic components. All rocks have cleavage that is axial planar to regional folds. While slaty cleavage and later fractures in the argillites tend to be systematic, in felsic metavolcanic rocks cleavage is anastomosing, there are multiple weakly systematic planar fractures, abundant exfoliation surfaces, and curvi-planar discontinuities related to cracking and spallation. Analysis of the integrated fracture sets results in a mean attitude of 147°,87° (RHR) but there is considerable dispersion in trend. Part of the spread in orientation is attributed to mixing between joints and exfoliation guided by cleavage, which trends northeast and is orthogonal to true joint trends. Additionally, there are secondary true joints co-linear with bedding or cleavage trends and some outcrops contain conjugate joint families. In a few very fine-grained rhyolite outcrops we have observed multiple, co-axial surfaces that intersect to form long polygonal prisms. This geometry is inconsistent with joint set morphologies outside of these outcrops and is suggestive of columnar joints. Some felsic igneous bodies in this area are interpreted as lava domes and these unique fractures may help distinguish igneous facies in the volcanic arc. Collectively, fracture analysis in this relatively small area shows that regional fracture patterns inferred from large landscape features do not necessarily map down into identical patterns within subdomains where other processes of cracking must be considered.