TERRANES AND TERRANE ACCRETION IN THE SOUTHERN APPALACHIANS: AN EVOLVED WORKING HYPOTHESIS

Tectonostratigraphic terranes are defined based on their stratigraphic (including volcanic) assemblages, basement, metamorphic, and plutonic histories. Appalachian terranes have also been defined on this basis, but many of the small terranes defined in the orogen are partially based on incomplete or incorrectly interpreted data. The southern Appalachians contain two major exposed terranes, the suspect Piedmont and exotic Carolina terranes. The Piedmont terrane, now acquitted, contains Grenville detrital zircons (from numerous samples) and can be divided into the central Blue Ridge (BR) Cartoogechaye terrane and the eastern BR-Inner Piedmont (IP)-Smith River allochthon Tugaloo terrane. The Suwannee terrane is another exotic terrane beneath parts of Florida, Georgia, and Alabama containing African basement and fossiliferous Paleozoic cover. Terranes were accreted during the three major Appalachian orogenies. The Allatoona-Hayesville-Gossan Lead fault is a Taconian terrane boundary that produced obduction of ophiolites (?), arc volcanics, and eclogite onto Laurentia and the Ordovician eastern BR plutons. Although this boundary is traceable to Newfoundland, it may represent closing of a forearc basin next to Laurentia and not Iapetus at 460 Ma. The central Piedmont suture is probably the Iapetus suture, if evidence from the northern Appalachians and British Caledonides is any indication. At the time the small Ordovician forearc basin next to Laurentia closed, a west-dipping subduction zone may have formed at the present site of the CPS producing the 460-420 Ma central-western IP plutons. West-dipping subduction and collision of the Carolina terrane with Laurentia closed Iapetus here about 360 Ma and produced the Devonian eastern BR plutons; Chattahoochee, primordial Brevard, and IP faults; and Tugaloo terrane-IP regional metamorphism. Difficulties exist, however, in developing IP structures with a W-dipping subduction zone. The Alleghanian collision involved zippered N-S closing of the Rheic ocean, and first produced dextral faults (Brevard, CPS, Modoc-Nutbush Creek) and later the thrust array that dominates southern Appalachians.