DEFORMATION CONDITIONS, KINEMATICS, AND CHARACTER OF THE FRIES AND GOSSAN-LEAD FAULTS, NORTH CAROLINA: IMPLICATIONS FOR THE ALLEGHANIAN OROGENY IN THE SOUTHERN APPALACHIANS

The Fries and Gossan-Lead faults are northeast striking ductile shear zones, characterized by pervasive mylonitic fabrics, located in the Blue Ridge province of Virginia and North Carolina. Both faults are considered to be Alleghanian-aged thrust faults active at greenschist-facies conditions. An alternative hypothesis is that the Gossan-Lead fault is related to the Burnsville fault, an Acadian-aged dextral transpressional fault active at amphibolite-facies conditions. This hypothesis is based on the shared structural positions of the Gossan-Lead and Burnsville faults, within the Fries thrust sheet.

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.