PHYLLONITES OF THE BREVARD ZONE, SOUTHERN APPALACHIANS, ALABAMA

The Brevard Zone is one of the longest (~600 km) and widest (~5 km) shear zones in the southern Appalachian Mountains between the Inner Piedmont and Blue Ridge provinces (Roper and Justus, 1973) with a protracted history of metamorphism and slip across multiple orogenies (Hatcher, 2001). In this study, new detailed mapping and petrographic analysis were completed in the Ponders quadrangle near Dadeville, AL, some of the southernmost exposures of the Brevard Zone, to better understand the structural and metamorphic history of this zone. Poles to foliations fall along a NW-SE girdle defining a cylindrical best fit with a hinge line of 17à031. Kinematic indicators range from reverse with the top to the southwest or northeast, right-lateral strike-slip, to normal with top to the southeast. Textures observed in thin section indicate a large proportion of sericite, muscovite, biotite, chlorite, and, in some samples, graphite, as well as quartz and garnet. The majority of the phyllosilicates are fine grained and define the foliation. Biotite forms sigma-clasts with quartz tails. Quartz forms polygonal crystals that do not have a shape or crystallographic preferred orientation determined with a wave plate. Garnet is typically relict and altered. Rocks with phyllosilicates and stronger phases like quartz have been suggested to deform by frictional-viscous flow characterized by frictional sliding along interconnected phyllosilicate grain boundaries as well as fluid-assisted diffusive mass transfer of soluble phases (i.e., quartz; Handy, 1990). Relict garnet and rotated porphyroblasts of biotite indicate that these rocks were sheared under amphibolite grade conditions, likely during the Acadian orogeny, which recorded peak metamorphism (Goldberg and Steltenpohl, 1990). Amphibolite grade fabrics were exhumed to shallower depths, sheared, and folded. Frictional-viscous flow became the dominate deformation mechanism focusing slip along phyllosilicates during strike-slip motion becoming a zone of low effective shear strength. Fluid likely played a significant role in this deformation as shown by mass transfer of quartz to pressure shadows and the formation of quartz veins. This later shearing was likely during the Alleghenian orogeny, and defined a reactivated zone of focused, preferential strain.