PALEOZOIC TERRANES AND ACCRETIONARY PROCESSES IN THE SOUTHERN AND CENTRAL APPALACHIANS: TRIBUTE TO DOUG RANKIN—MY TEACHER, PROFESSIONAL COLLEAGUE, AND FRIEND

The 1980 Nature paper by Coney, Jones, and Monger made the tectonostratigraphic terranes concept better known. Hank Williams and I shortly afterwards published two papers applying the concept to the Appalachians, and these were soon followed by numerous papers, including several by Doug Rankin, applying the concept to different parts of the chain. Existence and nature of specific terranes, and recognition of new ones was debated until modern (1990s) geochronology, particularly ion microprobe, produced large numbers of detrital zircon ages on metasedimentary rocks, providing more definitive data for terrane provenance, and the basis for recognition of several new ones (e.g., Cat Square). Today, we recognize three kinds of terranes in the southern and central Appalachians (SCA): (1) Laurentia-sourced suspect terranes formed in Iapetus offshore from Neoproterozoic to Ordovician Laurentia (now “acquitted”); (2) Ordovician to Devonian terranes and components that became parts of older terranes (allochthonous blocks and obducted thrust sheets; obducted Cat Square terrane, containing both Laurentian and Avalonian zircons, probably formed in a remnant ocean); and (3) Individual and composite Peri-Gondwanan (Panafrican) terranes accreted during the mid-Paleozoic. Terranes were accreted during two diachronous orogenies: Ordovician–Early Silurian Taconian, involving W– and E–dipping subduction; and Late Devonian–early Mississippian Acadian–Neo–Acadian, caused by N-to-S zippered obduction and accretion of Carolina (–Avalon) superterrane to already assembled Laurentian terranes, producing wholesale melting of the Inner Piedmont, and eastern Blue Ridge, with possible formation of a SW-directed orogenic channel. The late Pennsylvanian–Early Permian Alleghanian orogeny completed the Appalachian Wilson cycle with diachronous N-to-S zippered collision of Gondwana with all previously assembled terranes, and propagating the Blue Ridge–Piedmont megathrust westward through SCA cold crust.