FOLD-RELATED STRUCTURES: IMPLICATIONS FOR VARIABILITY IN GROUNDWATER STORAGE AND RECHARGE IN THE BLUE RIDGE, WESTERN NORTH CAROLINA

Groundwater resource availability is difficult to assess in fractured crystalline bedrock. Km-scale structures should be an indicator of the variation in fracture network orientation and intensity. Case studies of these relationships therefore improve the ability to predict heterogeneity in permeability and groundwater storage.

The ~6 km²Panthertown valley is part of the Appalachian Blue Ridge thrust complex in SW North Carolina. Neoproterozoic and Ordovician bedrock is comprised of granodioritic and micaceous gneiss. Penetrative foliation defined by aligned lithologic contacts and grain shapes was deformed by late Paleozoic contraction into a km-scale, gently NE-plunging, steeply SE-inclined, antiform outlined by a 10 m-scale micaceous layer. The fold core is granodioritic gneiss, in which both the deepest weathering and highest relief occur.

New field work results suggest the valley’s geomorphology is the result of fold-related structures within the granodioritic gneiss, which also controlled exfoliation development during exhumation. The valley’s axis is parallel to, but just NW of, the mapped fold axis. Axial planar foliation occurs in schistose outcrops at the fold closure, but foliation does not occur along the fold axis in the granodiorite. Mineral abundances, grain size, and foliation intensity elsewhere in the granodiorite gneiss core are consistent, suggesting contraction during folding removed the pre-existing foliation. Between the walls and valley axis, NW-SE trending secondary drainages create km-long lineaments perpendicular to the fold axis. Steep, NE-SW-facing and NW-SE-facing, exfoliation slopes underlain by granodiorite form both the main and secondary valley walls. The secondary drainages are consistent with a cross-fold joint set with ~0.5 km-scale spacing. The deepest weathering developed along the fold axis where cross-joints occur and pre-existing gneissic foliation has been removed. If the relative resistance to weathering corresponds to fracture intensity and therefore permeability, this case study suggests that fracture affects on recharge and flow rates can vary greatly at the <0.5 km scales, but that this heterogeneity can be modeled using constraints from km-scale geologic mapping.