RELATIONSHIPS BETWEEN FRACTURE PATTERNS AND KM-SCALE STRUCTURES IN LOW-POROSITY ROCK AS INFERRED FROM GEOMORPHOLOGY: IMPLICATIONS FOR FLUID/GAS STORAGE AND PERMEABILITY

Fracture networks help determine fluid/gas storage and permeability, especially in rock types with typically low porosity. Where km-scale structures occur, fracture networks vary in orientation and intensity likely causing storage and permeability to also vary. Case studies of these relationships improve the ability to predict the nature of storage and flow networks, which is relevant to many questions in the geosciences.

The ~6 km² Panthertown study area is in the Appalachian Blue Ridge thrust complex, western North Carolina, USA. Neoproterozoic and Ordovician bedrock is comprised of granodioritic gneiss overlain by micaceous gneiss. Lithologic contacts parallel foliation, which together were deformed by late Paleozoic contraction into a km-scale, gently NE-plunging, steeply SE-inclined, antiform outlined by a 10 m-scale micaceous layer and cored by granodioritic gneiss. Axial planar foliation occurs in schistose outcrops at the fold closure, but disappears in the granodiorite to the SW. Elsewhere, mineral abundances, grain size, and foliation intensity in the granodiorite gneiss are consistent, suggesting contraction during folding removed the pre-existing foliation. The deepest weathering and highest relief occurs in the granodiorite core. The valley is an elongate depression whose axis is parallel to, but just NW of, the mapped fold axis. Steep exfoliation slopes forming the valley walls are also underlain by granodiorite, at varying distances from the contact with the micaceous rock unit. Between the walls and valley axis, NW-SE trending secondary drainages dissect exfoliated granodiorite creating km-long lineaments perpendicular to the fold axis, consistent with a cross-fold joint set with ~0.5 km-scale spacing.

Relative resistance to weathering indicates significant storage/permeability variability primarily related to fold-related fractures. The deepest weathering developed along the fold axis where cross-joints occur and gneissic foliation has been removed. The next deepest weathering occurs where jointing is most intense and gneissic foliation is preserved. Cross-jointing intensity decreases ~1 km away from the fold axis, where the valley walls begin. These fractures likely facilitated weathering and erosion of the granodiorite material, generating the high valley relief.