ALTERNATING DUCTILE-BRITTLE DEFORMATION IN ALLEGHANIAN SHEAR ZONES, NORTHWESTERN NORTH CAROLINA AND EASTERN TENNESSEE

Deformation in Alleghanian shear zones in northwestern North Carolina and northeastern Tennessee is generally characterized by ductile deformation, locally overprinted by cataclastic features. Past workers have recognized the cataclastic overprint primarily in the westernmost shear zones (i.e., toward the foreland) and have suggested this as a manifestation of the faults bringing mylonitic rocks to shallower crustal levels and into a more brittle deformation regime. Our more recent work suggests a more complicated deformation history for these rocks. Cataclastic deformation occurs at least locally in all of the shear zones between the Grandfather Mountain and Mountain City windows. Additionally, rocks from several locations show evidence of a sequence of ductile-brittle-ductile-brittle deformation. Outcrop-scale features include veinlets of ultracataclasite (or pseudotachylite?) that were subsequently transposed into a mylonitic fabric. This mylonitic fabric shows later brittle fracturing. Thin sections of mylonitic granitic rocks show ribbons of dynamically recrystallized quartz that have been broken and displaced by cataclastic processes. The broken clasts of quartz ribbons show evidence of a subsequent rotation in a ductile matrix that is further overprinted by fracturing. We suggest that the multiple episodes of contrasting deformation styles occurred at equivalent structural positions, but resulted from variations in other conditions such as strain rates, fluid pressures, and/or strain hardening.