USING DUCTILE STRAIN PATTERNS TO INFER DEVELOPMENT OF FRACTURES: AN EXAMPLE FROM THE PANTHERTOWN CONSERVATION AREA, NC

The orientation and connectivity of crystalline bedrock fractures are important for understanding groundwater flow and slope stability. Poor exposure and the range of bedrock types and km-scale structures in western North Carolina makes documenting the intensity and geometry of fracture sets challenging. In the Mississippian Blue Ridge thrust complex, fracture sets are typically the result of cooling temperatures during exhumation of the thrust sheets or are exfoliation fractures. The former are likely to be kinematically related to km-scale structures or lithological domains, and the latter to differential weathering ~150 million years later, meaning mapping for 7.5’ quadrangles can be used to predict and/or relate fracture sets.

Micaceous paragneiss and two-mica felsic orthogneiss underlie the Panthertown conservation area, WNC. These Neoproterozoic and Ordovician gneisses were deformed in the earlier Paleozoic, then incorporated into the uppermost Blue Ridge thrust complex. Published data for the region suggests that the Panthertown area experienced a cooling path during the Alleghanian from >600°C to <350°C. During the event, the area was folded into an NE-trending, open, gently plunging antiform and refolded about a subvertical axis. Prior work in the northern Panthertown area proposed that lineaments in the fold are evidence of fold-related fracturing along the fold axis and cross-joint fracturing during refolding.

This study further develops this model using additional field work, microstructural analysis, and interpretation of LiDAR imagery. Geologic and foliation intensity mapping have (1) relocated the southern part of the fold axis, which also corresponds with fold axis-parallel and cross-fracture lineaments, and (2) documented additional evidence of foliation intensity weakening along the fold axis to the south. The revised fold axis location suggests the refolding represents ~30° of limb rotation. The reduction of foliation intensity along the fold axis with respect to the limbs is consistent with flexural folding or the inner arc of the neutral fold model. Both of these predict contraction along the fold axis, meaning that the grain-scale response during cooling conditions would have been feldspar microfracturing and/or quartz pressure solution.