CONSTRAINTS ON THE AGE AND KINEMATICS OF THE BREVARD FAULT ZONE: PLUTONIC ELEMENTS OF THE JACKSONS GAP GROUP, ALABAMA (USA)

The southern Appalachian (USA) Jacksons Gap Group of Alabama (AL) lies between the Abanda fault and the structurally higher Katy Creek fault on the northwest and southeast flanks of the Brevard fault zone (BFZ), respectively. Separating a paired Ordovician-Silurian backarc terrane (eastern Blue Ridge: Ashland-Wedowee-Emuckfaw belt) and arc terrane (Inner Piedmont: Dadeville complex), the age and tectonic affinity of the Jacksons Gap Group in the BFZ is largely unconstrained. Rocks described as part of the Jacksons Gap Group in AL include mica schists with variable garnet and graphite compositions, quartzite, metaconglomerate, orthogneiss, and uncommon marble and amphibolite. Fieldwork in Lake Martin, AL, recently revealed a coarse-grained, quartzofeldspathic orthogneiss intruding quartzites of the Jacksons Gap Group along the northwestern flank and structural base of the BFZ. This thin (<1 meter) intrusive body resembles lithologies of the batholith-scale Kowaliga Gneiss in the adjacent eastern Blue Ridge (EBR). Rocks of the Kowaliga Gneiss range from 430 Ma to 452 Ma, but have never been identified outside of the EBR (i.e. in the BFZ Jacksons Gap Group). The stratified host rock of the Kowaliga pluton in the EBR (i.e. Wedowee and Emuckfaw Groups) include mixed sedimentary-volcanic sequences that formed in an Ordovician-Silurian back-arc basin, were subsequently intruded by silicic plutons (i.e. Zana Granite and Kowaliga Gneiss), and then metamorphosed to middle-upper amphibolite facies during the Upper Mississippian (ca. 330 Ma). If the “Kowaliga-like” intrusive body in the Jacksons Gap Group is similar in age, chemistry, and modal mineralogy to the adjacent Kowaliga Gneiss, it would limit the age of the Jackson Gap Group to upper Ordovican or Silurian, and constrains displacement on the BFZ. A population of >50 prismatic zircon crystals have been extracted from the “Kowaliga-like” sample and await radiometric dating. In addition to isotopic age analysis, petrography and major element geochemistry will be used to compare and contrast the well-studied Kowaliga gneiss with the “Kowaliga-like” intrusive body identified in the Jacksons Gap Group.