ARE MAJOR TOPOGRAPHIC FEATURES IN THE CHIMNEY ROCK, NC, AREA STRUCTURALLY CONTROLLED?

W- to NW-oriented, steep-sided, linear valleys and catchments within the Blue Ridge Escarpment of western NC are generally interpreted to be structurally-controlled topographic features in a NW-retreating escarpment. This study is part of ongoing work using geologic mapping and airborne LiDAR digital elevation maps to test this idea in the Chimney Rock, NC, area, where the Hickory Nut Gorge (HNG), Rumbling Bald (RB) Ridge, and the Shumont Mountain (SM) catchment, are examples of such features.

Bedrock in the study area is penetratively deformed with gently SE-dipping foliation, NE-SW-oriented stretching lineation, and shear sense indicating top to the SW. The Henderson Gneiss, most commonly well-foliated and porphyroclastic, is the major bedrock unit. In the NE of the field area, structurally above is medium-grained, weakly- to moderately-foliated felsic gneiss interlayered with well-foliated fine-grained mafic amphibolite gneiss, schist, and rarely Henderson Gneiss. Along strike to the SW in the field area, interlayered schist and amphibolite gneiss are structurally above Henderson Gneiss. The most common brittle deformation structures are steeply-dipping (>70°) joint fractures with cm- to m-scale trace lengths and spacing. Most strike NW and NE; minor sets strike W and N. Shear structures are minor and uncommon with rare, isolated minor faults indicate approximately SE-NW extension. Results so far find no correlation between lithology, ductile fabric, or the type or intensity of brittle deformation structures, and the location of topographic highs and lows in the field area.

However, the orientation of these major topographic features does seem to be influenced by bedrock structures favorably oriented for NW escarpment retreat. Steep, NW-oriented, 1- to 10-m scale, convex fractures and lenticular fracture sets are observed in some slopes of Henderson Gneiss. They truncate vertically against foliation-parallel fractures and laterally against N-NE joints. These curved fractures are interpreted to record “stress release” propagation of NW joints. The 10-m-scale, steep, gently curved, slopes of Henderson Gneiss along HNG, RB ridge, and forming narrow NW-oriented slot valleys within the E-facing side of SM ridge, may result from slope failure of these curved fracture zones.