LATE PERMIAN 40AR/39AR AGE OF BRITTLE-DUCTILE DEFORMATION WITHIN THE JONESBORO FAULT ZONE ADJACENT TO THE MESOZOIC DEEP RIVER BASIN, NORTH CAROLINA

In the eastern Piedmont of North Carolina, brittle-ductile mylonitic deformation and cataclasis commonly overprint Paleozoic transpressional structures. Locally, this style of late-stage deformation has a clear spatial and geometric relationship to extension and normal faulting associated with development of early Mesozoic rift basins. In order to evaluate the absolute age for such brittle-ductile deformation and its bearing on rift basin development, we collected samples from the Mesozoic Jonesboro fault zone exposed in the Holly Springs quarry, approximately 200 meters southeast of the contact with Late Triassic clastic rocks. The samples are characterized by a prominent stretch and slickenline lineation; these are associated with asymmetric fabrics that indicate normal displacement to the northwest. Dynamic recrystallization of quartz, recrystallization and kink-bands in muscovite, and brittle deformation of feldspars collectively suggest that normal faulting occurred at sub-greenschist to greenschist facies conditions.

The presence of mineral assemblages containing varying proportions of relict porphyroclasts and recrystallized grains commonly hampers bulk-sample geochronologic studies of mylonites, with the result that ages intermediate between the timing of deformation and the protolith age are obtained. Using modern laser ⁴⁰Ar/³⁹Ar techniques for the study of single crystals, we find that muscovite from the Holly Springs quarry yields a range of Permian ages that can be interpreted clearly in the context of sample petrography. Laser spot-fusion analyses along the edges of deformed porphyroclasts and in small, recrystallized grains yield an age of 255 ± 2 Ma (n=14; 2-sigma intralaboratory precision). We interpret this Late Permian age to represent the timing of low-temperature deformation in the fault zone. Spot-fusion analyses in the cores of larger porphyroclasts yields ages of ca. 280 Ma, a result that provides a minimum age estimate for muscovite in the mylonite protolith. Single-crystal analytical techniques as used in this study offer the ability to resolve differing generations of mineral development, and precisely date regional structures that played an important role in the transition from Paleozoic to Mesozoic tectonic events.