THE SUWANNEE SUTURE: WHAT IS IT AND WHERE IS IT?

The Alleghanian orogeny is well represented in the igneous, metamorphic, and sedimentary rock records of the southern Appalachian orogen (SAO). Nonetheless, critical aspects of Alleghanian tectonic evolution remain ambiguous. One of the most important of these is the nature and location of the Gondwana-Laurentia contact. Geochemical and paleontologic data indicate that the lower Paleozoic strata of the Suwannee basin (subsurface of northern Florida) originated in Gondwana, making the Suwannee terrane exotic to Laurentia. In addition, the lack of metamorphism and deformation in the Suwannee basin requires a major crustal discontinuity between it and southern Piedmont terranes that record intense Alleghanian deformation. A magnetic high that trends NE beneath the Coastal Plain of Florida and Georgia has been interpreted to mark this transition and, as such, also mark the location of the Alleghanian suture between Gondwana and Laurentia, i.e., the “Suwannee suture”. Recent geochronologic and geochemical data from the Pine Mountain and Uchee terranes, however, suggest that the Uchee is also exotic to Laurentia, implying that the Laurentia-Gondwana boundary lies within the Goat Rock-Bartletts Ferry fault zone, i.e., between the Uchee and Pine Mountain terranes. The buried transition between the Suwannee and Uchee terranes, then, is likely an intra-Gondwanan plate feature that was exploited as the crust of the northern Suwannee terrane over-rode all or part of the southern Piedmont during Alleghanian collision. The presence of Alleghanian-age granites both north and south of the magnetic high add credence to this model. In addition, a 300 Ma granitoid sampled by drilling south of the proposed suture (Seminole County, GA) contains Grenville-age zircons, suggesting derivation by melting of Laurentian crust at depth. Extending this interpretation to larger areas of the SAO obviates the need for the “Alleghanian granites” to be suprasubduction plutons. Instead, burial and crustal thickening associated with accretion of the Suwannee plate resulted in lower crustal melting. Delamination may or may not have contributed to the heat budget needed to melt the lower crust, however, the lack of mafic, Alleghanian magmatism suggests minimal petrologic involvement of the underlying mantle.