A SYNTHESIS OF GEOLOGICAL RESEARCH FROM THE TALLADEGA BELT AND EASTERN BLUE RIDGE OF ALABAMA AND GEORGIA (USA): A TRIBUTE TO THE CAREER OF JAMES F. TULL

For more than 5 decades, much of the geologic research from the southernmost Appalachian western Blue Ridge-equivalent Talladega slate belt (TSB) and Ashland-Wedowee-Emuckfaw belt (eastern Blue Ridge, EBR) has been conducted by Florida State University professor James (Jim) Tull and his students-collaborators. Building on a geologic framework established in the late 1800s to mid-1900s, prior to widespread acceptance of tectonic theory, Jim collaborated with a myriad of colleagues to establish a tectonic framework for these and other lithotectonic belts in Alabama and Georgia. Conducting field work in a region covered in thick saprolite, abundant vegetation and limited roads, basic geologic maps, cross-sections, and structural data were ultimately coupled with modern geochronology, geochemistry, and geophysical techniques to produce a coherent geologic model that guides our current understanding of this segment of the orogen. That research ultimately changed one of the prevailing models for the tectonic evolution of the region – one in which the metamorphic-structural evolution of the TSB-EBR was largely a byproduct of the Taconic orogeny. The work conducted by Jim, his students and collaborators has increasingly revealed fundamental differences between the orogenic character of the northern-central and southernmost Appalachians during the early Paleozoic. Unlike the traditional Taconic collisional orogenic model developed in New England, rocks of the TSB-EBR were largely unaffected by the A-type subduction processes that obducted peri-Laurentian and exotic arcs onto the Laurentian margin during the Ordovician. Instead, the early Paleozoic southernmost Appalachians record an accretionary orogen, with rocks of an extensive, now dismembered Ordovician-Silurian suprasubduction system developing above subducting Iapetus lithosphere. In the TSB-EBR, rocks of the Laurentian shelf and an amalgamated outboard back-arc basin (Wedowee-Emuckfaw-Dahlonega basin) remained largely intact until the latest Devonian-Carboniferous, well after Taconic orogenesis had ended. Ultimately, this work indicates that a significant transform boundary must have separated A-type subduction systems of the northern-central Appalachians and B-type subduction systems of the southernmost Appalachians.