EXPLORING THE POTENTIAL TECTONIC SIGNIFICANCE OF EXTENSIONAL MOVEMENT DOCUMENTED ALONG THE BREVARD ZONE IN ALABAMA

The Brevard fault zone (BFZ) is a classical problem in Appalachian tectonic evolution that remains controversial. It extends more than 600km from the Gulf Coastal Plain onlap in Alabama to the North Carolina-Virginia border and separates the Inner Piedmont from the Blue Ridge. The BFZ in Alabama is unique in that it is marked by a distinct package of relatively low-grade miogeoclinal metasedimentary rocks that are bound below and above by faults, the Abanda fault and the Katy Creek fault, respectively. The Abanda fault was considered a NW-directed thrust but mapping and structural analysis of retrograde mylonites and phyllonites documents well-developed S -C fabrics with oblique dextral-and-normal sense of movement, which explains its emplacement of lower-on-higher grade rocks. Timing of movement along the Abanda is not tightly constrained but post-dated 315 Ma based on 40Ar/39Ar analysis of metamorphic-fabric forming muscovites. Mapping also documents two sets of subvertical brittle normal faults. Faults of the NW-SE–trending set have <2 m displacement and cut the oblique dextral-normal phyllonites of the Abanda fault. Faults of the WSW-ENE set rarely cut Abanda phyllonites and are similar to Mesozoic(?) faults known to cut the Alexander City and Towaliga mylonite zones. The geometry, kinematics, rheologies, and age range (Pennsylvanian to Jurassic) of the Abanda are similar to those of the Goodwater-Enitachopco fault. We explore whether this dextral-and-normal movement occurred as a result of Triassic-Jurassic rifting of Pangaea, Permian-Triassic Alleghanian orogenic collapse, Late Carboniferous-Early Permian extension that was passively transported to their present location, or some combination of these mechanisms.