COMBINING GEOLOGICAL MAPPING AND LIDAR DATA TO ANALYZE THE CONTRIBUTION OF "LOCAL" VERSUS "REGIONAL" BEDROCK FRACTURE SETS IN WESTERN NORTH CAROLINA

In western NC, poor exposure and the range of lithologies and km-scale structures makes documenting the intensity, geometry, and lateral extent of bedrock fracture sets challenging. Here we report progress on a pilot study combining public domain LiDAR data (NC, 2005) with targeted 1:12,000 scale geological mapping in the Panthertown valley area of SW NC to characterize bedrock fracture sets at the scales needed for land use planning.

Mapping in areas of best exposure (~6 km²) indicates Panthertown valley follows the axis of a NE-trending, upright, km-scale open antiform of felsic gneiss overlain by micaceous gneiss, gently refolded about a subvertical axis. Bedrock along the fold core is incised by two sets of topographic lineaments, one following the arcuate fold axis and others in a NW-trending cross-joint geometry. A kinematic model for fracture formation during folding predicts that steeply dipping, NE and NW trending, joint fracture sets are responsible for the valley lineament sets.

Bedrock exposure is very limited on the fold limbs and along the valley axis. A LiDAR ground model (~83 km²) was used to look for lineaments indicating predicted, and possible superimposed regional, fracture sets. Despite a 1 pt/2 m² spatial resolution, fracture trace lineaments are easily identifiable and penetrative. Most lineaments have azimuths predicted by the kinematic model. To assess contribution from superimposed regional fracture sets, adjacent LiDAR tiles were added (~664 km²) and resampled to artificially lower resolution. Lineaments visible at the lowest resolution (analogous to topographic map resolution) are considered 1^st order. Those only visible at the original LiDAR resolution are considered 3^rd order; those at intermediate resolution, 2^nd order. Two 3^rd order sets are statistically parallel to the 1^st order sets and can be considered part of "regional" lineament sets. Traces not parallel to 1^st order sets are focused in the Panthertown fold area.

Geological mapping is often limited by exposure but permits development of a kinematic model relating fracture sets in 3D. LiDAR is limited by largely 2D fracture information, but permits mapping of fracture traces regardless of bedrock exposure. Therefore, their combination is a powerful way to characterize bedrock fractures at scales needed for land use planning.