DUCTILE AND BRITTLE STRUCTURES IN THE CHIMNEY ROCK, NC, AREA AND IMPLICATIONS FOR SLOPE PROCESSES

Analysis of ductile and brittle structures is critical for understanding deformation history as well as slope failure processes. Ongoing work seeks to improve understanding of bedrock structures in the Chimney Rock, NC, area, where >1000m of topographic relief and steep slopes typical of the Blue Ridge Escarpment form major landforms like the NW-oriented Rumbling Bald ridge (RB) and NW-oriented Hickory Nut Gorge (HNG), and the risk of debris flow and rock slope failures is considered high.

Consistent amphibolite facies deformation fabric (gently SE-dipping foliation, NE-SW lineation, shear sense top to the SW) indicate the bedrock here is part of a km-scale, gently dipping, amphibolite facies, shear zone, within which primary lithological domains have been transposed by high shear strain. Granodioritic Henderson Gneiss (HG) is dominated by well-foliated medium-grained biotite ± hornblende gneiss and porphyroclastic mylonitic (“augen”) gneiss. To the NE, structurally above is moderately-foliated medium-grained felsic gneiss with rare HG lenses; to the SW, strongly-foliated, fine-grained mafic amphibolite with muscovite schist lenses.

Steeply dipping (>75°) planar joint sets dominated by NE and NW strikes occur in all lithologies and structural depths. Slickenline fiber terminations, Riedel secondary fracture sets, and step-over fracture geometries in rare fault zones indicate minor NNW-SSE to NE-SW extension in both the eastern HNG and RB. Joint or shear structure orientations or intensity do not correlate with topographic highs or lows.

Discontinuities vary with lithotextural unit and hold implications for slope sedimentation and failure processes. Amphibolite gneiss, muscovite schist, and well-foliated HG, exhibit steep joint and foliation parallel fractures (FPF) with cm-m spacing. Shallowly-dipping exfoliation shapes felsic gneiss outcrops. In contrast, gently curved, steep, 1-10m scale surfaces characterize many HG slopes. These truncate against FPF, joint fractures, or “inward” against other curved surfaces. Curved surfaces are subparallel to joints in the same outcrop areas, and are interpreted as fracture propagation in the current stress environment. We speculate that this process generates boulder-sized sediment of HG in colluvial hollows and block talus fields.