MULTIPLE BRITTLE TECTONIC AND WEATHERING PROCESSES IN METARHYOLITE

The Neoproterozoic peri-Gondwana Albemarle arc in central North Carolina contains greenschist facies argillitic sedimentary rocks intercalated with bimodal volcanic units. Resistant metarhyolite and metabasalt units underlie topographic highlands and the fine-grained sedimentary sequence crops out in relative lowlands. In addition to an axial planar cleavage related to northeast-trending, southeast-verging folds there are two systematic joint sets in all rock units. Geomorphic expression of these planar structures consequently includes long northeast-trending ridges and cross-structure linear valleys maintained by joints. Fin-like exposures of rhyolite and basalt cap several ridges and colluvial blankets cover steep slopes on the flanks of these ridges, particularly those held up by rhyolite. While fins trend parallel to cleavage strike and some fractures correlate with tectonic joints, many planar to curvi-planar fractures are complex features resulting from the interactions of mechanical weathering and pre-cracking foliation anisotropy. The lateral surfaces of these fins do not parallel cleavage dip angles or directions; those surfaces appear to form in response to exfoliation by mode I cracks normal to evolving exposure faces by a progression of concave-outward divots that create sheets of flakes in the colluvial cover. While fin formation is initiated by cleavage trends, near-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.