ABSTRACT The southern Appalachian orogen is a Paleozoic accretionary-collisional orogen that formed as the result of three Paleozoic orogenies, Taconic, Acadian and Neoacadian, and Alleghanian orogenies. The Blue Ridge–Piedmont megathrust sheet exposes various crystalline terranes of the Blue Ridge and Inner Piedmont that record the different effects of these orogenies. The western Blue Ridge is the Neoproterozoic to Ordovician Laurentian margin. Constructed on Mesoproterozoic basement, 1.2–1.0 Ga, the western Blue Ridge transitions from two rifting events at ca. 750 Ma and ca. 565 Ma to an Early Cambrian passive margin and then carbonate bank. The Hayesville fault marks the Taconic suture and separates the western Blue Ridge from distal peri-Laurentian terranes of the central and eastern Blue Ridge, which are the Cartoogechaye, Cowrock, Dahlonega gold belt, and Tugaloo terranes. The central and eastern Blue Ridge terranes are dominantly clastic in composition, intruded by Ordovician to Mississippian granitoids, and contain ultramafic and mafic rocks, suggesting deposition on oceanic crust. These terranes accreted to the western Blue Ridge during the Taconic orogeny at 462–448 Ma, resulting in metamorphism dated with SHRIMP (sensitive high-resolution ion microprobe) U-Pb ages of metamorphic zircon. The Inner Piedmont, which is separated from the Blue Ridge by the Brevard fault zone, experienced upper amphibolite, sillimanite I and higher-grade ­metamorphism during the Acadian and Neoacadian orogenies, 395–345 Ma. These events also affected the eastern Blue Ridge, and parts of the western Blue Ridge. The Acadian and Neoacadian orogeny is the result of the oblique collision and accretion of the peri-Gondwanan Carolina superterrane overriding the Inner Piedmont. During this collision, the Inner Piedmont was a forced mid-crustal orogenic channel that flowed NW-, W-, and SW-directed from underneath the Carolina superterrane. The Alleghanian orogeny thrust these terranes northwestward as part of the Blue Ridge–Piedmont megathrust sheet during the collision of Gondwana (Africa) and the formation of Pangea.

The Inner Piedmont is a large, composite, sillimanite-grade terrane that extends from near the Virginia–North Carolina border to central Alabama and consists of the eastern Tugaloo and Cat Square terranes. It is bound to the west by the Brevard fault zone and to the east by the central Piedmont suture. It is the core of the Neoacadian (360–350 Ma) orogen in the southern Appalachians and records Late Devonian–Mississippian closure and high-grade metamorphism (sillimanite I and II) of Siluro-Devonian sediments deposited in the remnant Rheic ocean basin. The Cat Square terrane is bounded by the younger-over-older Brindle Creek fault to the west and the central Piedmont suture to the east. It consists of a unique sequence of Siluro-Devonian metapsammite and pelitic schist that was intruded by Devonian anatectic granitoids (Toluca Granite, ∼378 Ma, and Walker Top Granite, ∼366 or ∼407 Ma). Rare mafic and ultramafic rocks occur in the eastern Cat Square terrane. Minimum sediment thickness is estimated at 4 km (13,000 ft). Detrital zircons indicate that Cat Square terrane rocks have a maximum age of ∼430 Ma, with both Laurentian (2.8, 1.8, 1.4, 1.1 Ga) and peri-Gondwanan (600, 500 Ma) affinities. Deposition on oceanic crust explains the existence of several mafic and ultramafic bodies and the absence of continental basement in the Cat Square terrane. The Cat Square terrane petrotectonic assemblage represents a Siluro-Devonian remnant ocean basin between Laurentia and the approaching Carolina superterrane. Metapsammite and pelitic schist may represent turbidites shed from approaching tectonic highlands on both flanks of the closing ocean. Palinspastic restoration of the Inner Piedmont constrains the location of the Cat Square basin to the Pennsylvania embayment, and links the mid-Devonian to Mississippian deformation in the Neoacadian core to the SW-migrating pulses of the diachronous Acadian-Neoacadian clastic wedge. Location and SW migration of the clastic wedge in concert with structural patterns in the Inner Piedmont support a transpressive NW-directed collision of the Carolina superterrane with the New York promontory and zippering the basin shut from NE to SW.