STRUCTURALLY INFLUENCED WEATHERING AND CRACKING OF METAMORPHOSED VOLCANIC ROCKS IN THE CENTRAL NORTH CAROLINA CAROLINA TERRANE

The Neoproterozoic Albemarle arc in the central North Carolina Piedmont comprises distinct lithostratigraphic and volcanic units, here including the Tillery Formation (argillite and rhyolite members) and overlying Cid Formation (rhyolitic and mafic volcanic members and very thin mudstone sheets). All rocks were metamorphosed to greenschist facies and contain a variably developed axial planar cleavage. Topographic highlands are held up by erosionally resistant volcanic rocks, particularly by rhyolite. Highland ridges generally trend parallel to the northeast-southwest strike of cleavage but the ridge systems are also dissected by multiple fracture-controlled valleys and streams, the most prevalent being regionally extensive and northwest-southeast in orientation. At the outcrop scale, however, fracture and joint geometries are quite complex, especially in ridge-forming rhyolitic rocks, that have as many as six fracture orientation maxima compared to two maxima in lowland argillites. In these rhyolites, some fractures correlate with tectonic joints but many planar to curviplanar fractures are composite features resulting from the interactions of mechanical weathering and pre-cracking foliation anisotropy. Weathering of rhyolite bedrock islands produced isolated fins that are oriented parallel to cleavage strike but the lateral surfaces of these fins do not parallel cleavage dip angles or directions. Rather, those surfaces appear to form in response to exfoliation by mode I cracks normal to the evolving exposure face by successive concave-outward divots that produce flake sheets. At some later stage, the scalloped surface is also modified by other weathering processes that attack sharp flake scars to produce more characteristically rounded and mature boulders, cobbles, and finer relict particles. Evidently, exposure surface cracking processes energetically favor surface-normal expansion over cleavage acting as a stress guide for crack propagation. Progressive disintegration of rhyolite exposure massifs and gravity- or water-assisted erosion has produced extensive colluvial/alluvial blankets of rhyolite on steeper slopes.