WHITHER THE ALLEGHANIAN SUTURE?

Recognition of subsurface Paleozoic strata in the southeastern U.S., exotic to North America, led Wilson to the then-striking conclusion that opening of the Atlantic Ocean basin left former parts of Africa stranded in the western Atlantic. Ever since, considered effort has been spent on identifying the boundary that formed between the former Laurentia and Gondwana continents in this region during the Alleghanian orogeny. Historically, no less than six major boundaries have been proposed, only three of which potentially separate Laurentia and/or accreted peri-Gondwanan terranes from Gondwana, i.e. the Alleghanian suture. These boundaries include the Brevard zone, the Central Piedmont shear zone, the Eastern Piedmont fault system, the Carolina-Mississippi fault, and the Suwannee suture zone, identified as either a zone of moderately-dipping crustal-scale reflectivity or a sub-horizontal boundary defined by crustal velocity gradients. Recent work demonstrates that crustal-scale reflectivity previously attributed to the inferred Alleghanian Suwannee suture zone is Neoproterozoic, and renamed the Brunswick suture zone. The Brevard zone, Central Piedmont shear zone, and Eastern Piedmont fault zone all record major (primarily thrust, dextral slip) Alleghanian deformation, but (1) are within and separating terranes exotic to both Laurentia and Gondwana, and (2) are, along with the Brunswick suture zone, cut by the Carolina-Mississippi fault. The latter appears to juxtapose crust of Gondwanan affinity against crust of Laurentian or accreted terrane affinity across a relatively narrow transcurrent fault zone. However, recent identification of a Neoproterozoic arc system (Osceola arc in the subsurface of Florida, Georgia and Alabama) which may be correlative with a Carolinian arc (Albemarle) of similar age, composition, and tectonic origin raises the question whether there can be an Alleghanian suture in the southeastern U.S., despite widespread deformation of this age.