GEOLOGICAL RE-EVALUATION OF THE SOUTHERNMOST BREVARD ZONE, ALABAMA: IMPLICATIONS FOR APPALACHIAN EVOLUTION

The origin of the Brevard zone remains a classic problem in Appalachian geology. This major, polyphase, <6 km wide, ductile shear zone separates the Laurentian eastern Blue Ridge (EBR) from the suspect Inner Piedmont (IP) terrane from Virginia southward to Tallassee, Alabama, where it is buried beneath the Gulf Coastal Plain. Generally, rocks and structures defining the Brevard zone are intensely overprinted by D2, Alleghanian, right-slip brittle-plastic shears that have obliterated tectonostratigraphic relations and the earlier D1 movement history. South of Jacksons Gap, Alabama, however, retrogressive D2 shears splay from the Brevard zone lithologies (i.e., Jacksons Gap Group = JGG) such that their depositional and pre-D2 tectonometamorphic relations are preserved. South of Jacksons Gap a major, D1, right-slip contractional duplex, the Lake Martin duplex, comprises stacked, ~0.5-1 km thick repeating panels of metamorphosed quartzite, conglomerate, chert, carbonate-bearing pelites, and meta-plutonic/-volcanic rocks. The JGG siliciclastics are lithologically similar to rocks of the Talladega slate belt (TSB), the southernmost extension of the Laurentian western Blue Ridge. This is compatible with 14 SHRIMP U-Pb analyses on detrital zircons from a JGG quartzite (944, 990, 1005, 1084, 1104, 1203, 1208, 1347, and 1544 Ma) suggesting a Grenville (1.0-1.2 Ga) and mid-continent granite/rhyolite province (1.3 to 1.5 Ga) provenance; whole-rock Nd isotopic analysis of an interbedded metapelite has a depleted mantle model age of 1.51 Ga, confirming a component of at least this age is present in the source. Most plutonic rocks in the duplex are tectonic slivers from the EBR and IP. The JGG does, however, contain intermediate metavolcanics and greenstones, another lithologic similarity with the TSB; ongoing isotopic work is aimed at dating these bodies. Igneous geochemical studies further explore their tectonostratigraphic affinity and implications for tectonic development of the southernmost U.S.