Geology of the Jacksons Gap, Alabama, 7.5-Minute Quadrangle: Implications for the Evolution of the Brevard Fault Zone

Abstract

The Brevard fault zone is a major Appalachian structure that marks the boundary between the eastern Blue Ridge (EBR) and the Dadeville Complex (DC) of the Inner Piedmont (IP) terrane in Alabama. Despite over a century of geologic scrutiny, it remains an enigmatic structure with more than forty interpretations having been suggested. The southernmost exposures of the zone in Alabama contain several poorly understood features that are unique to the zone of deformation, requiring detailed 1:24,000-scale geologic mapping to clarify their significance. In the study area, the Brevard zone is lithologically defined by the Jacksons Gap Group (JGG), a distinctive group with uncertain affinities. Apparent repetition of strata within portions of the JGG implies contractional duplexing, folding, primary sedimentary repetitions, or a combination of these. The Brevard zone is folded by the late-stage Tallassee synform, providing an unusual structural setting to investigate its geometry. The JGG is fault bounded, the Abanda fault separating it from the underlying eastern Blue Ridge, and the Katy Creek fault marking the boundary with the overlying the DC. Tabular, curvilinear cataclastic zones splay out from the Abanda fault to merge westward with the Alexander City fault zone (ACFZ), which occurs in locally intense distributed ductile shears. The Abanda and ACFZ are kinematically similar, with ductile oblique, predominantly dextral, movement. The Katy Creek fault is cryptic where studied, with no recognized post-metamorphic (retrogressive) fabric disruption, implying a pre- or syn-metamorphic origin. 40Ar/39Ar isotopic analyses on muscovite from phyllonites of the Abanda fault and ACFZ are interpreted to constrain the timing of their movement histories to a mean age of ~317.5 Ma. Two-dimensional graphical methods and a simple cut-and-staple-paper model are described to help students visualize and understand complex three-dimensional fault-rock structures/fabrics such as those of the Brevard zone. Lower hemisphere, equal-area, stereographic projections are used to resolve problems recognized while mapping oblique-slip shears, and the methods explained have a general applicability for characterizing movement in ductile shear zones containing S-C composite planar fabrics. The technique allows for the solving of movement direction as dip-slip (reverse/normal), strike-slip (dextral/sinistral), or oblique-slip motion based on plots of the S and C relative to the center of the stereonet. The method is transportable for field use since it only requires one to understand and measure the outcrop relationship(s) of penetrative mylonitic fabric(s) in the rock, and then to plot and analyze the measurements on a stereogram. Digital models and a simple paper physical model provide tools to help advanced undergraduate and graduate students to better understand and grasp S-C fabrics and their applications for solving movement sense along natural fault/shear zones.