DEFORMATION CONDITIONS, KINEMATICS, AND CHARACTER OF THE FRIES AND GOSSAN-LEAD FAULTS, NORTH CAROLINA: IMPLICATIONS FOR THE ALLEGHANIAN OROGENY IN THE SOUTHERN APPALACHIANS
Mapping along the Fries fault in northwestern North Carolina reveals a region characterized by high and low strain zones within a broader zone of distributed strain that is tens of kilometers wide. Consequently, it is not a discrete fault zone, but a region of distributed strain containing zones of highly-strained mylonites surrounding lenses of less-deformed rocks. Kinematic indicators identified in the field and in thin section record top-to-the-NW motion, consistent with thrusting. In contrast, the Gossan-Lead fault is a discrete shear zone separating Grenville-aged basement rocks from rocks of the Ashe Metamorphic Suite. Moreover, shear sense indicators along the Gossan-Lead fault are complex; mineral stretching lineations, mylonitic fabrics, porphyroblasts, and porphyroclasts record fault parallel, perpendicular, and oblique motion.
Microstructural observation of quartz and feldspar provide evidence for deformation conditions ranging from greenschist- to lowermost amphibolite-facies. Quartz displays dominantly bulging recrystallization and subgrain rotation recrystallization, with rare examples of high-temperature grain boundary migration, and feldspar is characterized by brittle fracture and some incipient bulging recrystallization.
Our mapping and microstructural analysis along the Fries and Gossan-Lead faults provide three important interpretations. First, both shear zones experienced deformation at conditions exceeding greenschist-facies, higher than expected for Alleghanian orogenesis. Second, the Fries fault is characterized by a wide zone of distributed strain, rather than a discrete shear zone. Third, the Gossan-Lead fault records pre-Alleghanian motion.