STRUCTURAL RE-EVALUATION OF THE SOUTHERNMOST EXPOSURES OF THE BREVARD ZONE, ALABAMA, AND ITS IMPLICATIONS FOR APPALACHIAN EVOLUTION

Although recognized in the early 1900’s, the origin of the Brevard zone remains a classic problem in Appalachian geology. This major, polyphase, up to 6 km wide, ductile shear zone, which separates the eastern Blue Ridge from the Inner Piedmont terrane, is exposed from near the Virginia/North Carolina border southward to Tallassee, Alabama, where it is buried beneath Mesozoic and younger sedimentary rocks of the Gulf Coastal Plain. As many as 23 different interpretations, ranging from the Gondwanan-Laurentian suture to a rather simple anticline/syncline fold pair, have been suggested to explain its development. Today, about all that workers agree on is that there is an early, crystal-plastic, shearing history that predated latest Carboniferous (Alleghanian), right-slip overprinting that imparted its remarkably straight, ~N55^oE trend. Our ongoing mapping in the southernmost exposures of the Brevard zone, where at Jacksons Gap it takes a sharp due-South bend, suggests several unique occurrences with implications for interpretations of its tectonic development. First, “late” Brevard zone, right-slip, brittle-plastic shears do not bend due-Southward at Jacksons Gap but, rather, continue their straight trend along S55^oW to merge with the Alexander City fault zone. What is shown on earlier maps as bending due-South is an earlier-formed, deeper-crustal-level, plastic shear zone that we interpret to be the “early” Brevard zone, which here has escaped effects of the late-stage overprint. Second, in contrast to previously interpreted geophysical data, lithologies within the “early” Brevard zone (i.e., Jacksons Gap Group) are not truncated directly south beneath the Coastal Plain onlap by the Towaliga fault separating allochthonous terranes from Laurentian Grenville basement of the Pine Mountain window. Rather, these lithologies, as well as footwall eastern Blue Ridge units appear continuous around the hinge of the NE-plunging Tallassee synform. Details of our findings are presented and we explore their implications for Brevard zone development and required reinterpretation of terranes and their contact relations.