INVESTIGATIONS OF THE BREVARD ZONE, A COMPILATION OF 1:24,000 GEOLOGIC MAPS TRACING THE FEATURE THROUGH ALABAMA

The Brevard fault zone is a major Appalachian structure that marks the boundary between the eastern Blue Ridge (EBR) and Inner Piedmont (IP) terranes. Despite over 100 years of geologic scrutiny it remains an enigmatic structure with more than 30 interpretations having been suggested. The southernmost exposures of the zone in Alabama contain several poorly understood problems and/or features that are unique to the zone, requiring detailed 1:24,000 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 of relatively low grade and low strain metasiliciclastics and metapelites, which have uncertain affinities in comparison to known terranes in this region. Sporadic granitoids near the base of the JGG makes differentiating its boundary with the EBR difficult. Apparent repetition of strata within some portions of the JGG implies contractional duplexing, folding, or primary sedimentary repetitions. 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, with the Abanda fault separating it from the underlying EBR and the Katy Creek fault, marking the boundary with the overlying IP. We report kinematic and rheological information for both of these faults. Tabular, curvilinear cataclastic zones of questionable origin splay out from the Abanda fault to merge westward with the Alexander City fault zone (ACFZ). The Abanda and ACFZ are kinematically similar in that they are polydeformed, ductile oblique fault/shear zones with predominantly dextral movement, but the ACFZ differs in that it occurs in locally intense distributed ductile shears. The Katy Creek fault is cryptic where studied, with no recognized post-metamorphic (retrogressive) fabric disruption, implying a pre- or synmetamorphic origin. Dates of the fault movement along the Abanda and ACFZ are not well constrained and are currently being investigated. The aim of this study is to define and reevaluate the criteria used to define this large-scale, complex structure.