STRUCTURAL/ PETROGRAPHIC ANALYSES PAIRED WITH TARGETED 40AR/39AR ISOTOPIC DATING AIMED AT CONSTRAINING THE TIMING OF MOVEMENT ALONG THE BREVARD AND ALEXANDER CITY FAULT ZONES, ALABAMA

We report field/structural and petrographic analyses, paired with ongoing ⁴⁰Ar/³⁹Ar investigations on rocks from the southernmost exposures of two major SE-dipping Appalachian fault zones, the Brevard and Alexander City fault zones (BZ and ACFZ, respectively). The BZ marks the boundary between the eastern Blue Ridge (EBR) and Inner Piedmont (IP) terranes, whereas the ACFZ is an internal EBR fault. In Alabama, metasiliciclastic lithologies of the Jacksons Gap Group (JGG), defining the Brevard zone, are sandwiched between two faults, the Abanda (to the NW, and tectonostratigraphically below) and Katy Creek (SE, above) faults. Rocks of the EBR, the JGG, and the IP are polydeformed and generally metamorphosed to middle-amphibolite-facies conditions. ⁴⁰Ar/³⁹Ar mineral cooling dates on metamorphic-fabric-forming muscovite range in the Late Carboniferous (~320-295 ma), respectively, without any recognizable age-distribution patterns within or between the three terranes. Peak metamorphic assemblages have been retrograded to upper-greenschist or lower-amphibolite facies along mostly right-slip mylonites of the BZ, Abanda fault, ACFZ, and in locally intense ductile shears distributed between the BZ and ACFZ. The Katy Creek fault is a cryptic fault where studied, with no recognized post-metamorphic (retrogressive) fabric disruption but with map-scale truncation of hanging wall units, implying a pre- or synmetamorphic origin. Likewise, numerous ductile shears cutting plutonic igneous bodies (i.e., Late Ordovician Kowaliga and Zana granitic gneisses) have mylonitic fabrics formed under temperature conditions only slightly below the metamorphic peak. Timing of movement along any of the faults or shear zones is not well constrained but is being investigated through careful petrographic characterization of shear fabrics and associated mineral phases and dating them using single-crystal fusion 40Ar/39Ar isotopic methods at the Auburn Noble Gas Isotopic Mass Analysis Laboratory (ANIMAL).