COMBINING GEOLOGICAL MAPPING AND LIDAR DATA TO CHARACTERIZE FRACTURE PATTERNS IN POORLY EXPOSED CRYSTALLINE BEDROCK IN THE SOUTHERN APPALACHIANS, WESTERN NC

In the Appalachians of western NC, poor exposure and the range of bedrock types and km-scale bedrock structures makes documenting the intensity, geometry, and lateral extent of bedrock fracture sets challenging. Public domain LiDAR data (NC, 2005) has been combined with 1:12,000 scale geological mapping in a pilot study in the Panthertown valley area of SW NC, where the terrain, bedrock exposure and lithologies, km-scale bedrock structures, and vegetative cover are typical of western NC. Mapping in the areas of best exposure (~6 km²) documents that Panthertown valley follows the axis of a NE-trending, upright, km-scale open antiform that is gently refolded about a subvertical axis. Felsic gneiss is moderately well-exposed along the core of the fold because it is incised by two sets of valley lineaments, one following the arcuate fold axis and others in a NW-trending cross-joint geometry. Structurally and topographically above, poorly exposed micaceous bedrock underlies the fold limbs but does not exhibit valley lineaments. Regional pressure-temperature history and field relationships indicate that folding and refolding occurred at temperatures cooling into the brittle-ductile transition for feldspar. The fold axis-parallel valley is consistent with steep fractures related to formation of the antiform; the axis-perpendicular lineaments are consistent with steep fractures related to refolding. However, erosion and poor exposure means these fractures are rare in outcrop.

LiDAR data have been analyzed over the mapped and adjacent areas (~83 km²). Ground models generated using FUSION software (USFS, 2009) were imported into ArcMap (ESRI, 2010) to create grayscale hillshade images. Manually-identified lineaments were analyzed for orientation, length, and spacing. Preliminary results using lineaments >1,500 ft (475.2 m) long indicate two main orientations, which are subparallel to the dominant NE trend of the axis-parallel valley lineament and the NW trend of the cross-joint valley lineaments. These lineaments are interpreted as the traces of bedrock fractures. Geological mapping in a small area can therefore provide a structural model for the geometry of, and genetic relationships between, bedrock fractures detectable in LiDAR data over a much larger area.