The Appalachian Inner Piedmont: an exhumed strike-parallel, tectonically forced orogenic channel

Abstract

Abstract The Appalachian Inner Piedmont (IP) extends along orogenic strike some 700 km from North Carolina to Alabama. Its physical attributes contrast with those of other Appalachian tectonic elements: gentle dip of dominant foliation; imbricate stack of fold nappes; dominant sillimanite-grade metamorphism and near ubiquitous migmatization; heterogeneous, non-plane deformation; and earlier S-foliations transposed to C-foliations southeast of the mid-Palaeozoic Brevard fault zone forming a 10–20 km wide amphibolite-facies shear zone along the western flank of the IP. The IP contains west- and SW-directed thrust sheets and mineral stretching lineation, sheath folds on all scales, and other indicators that define a curved crustal flow pattern throughout the belt. Field and modern geochronologic data confirm that the IP is not exotic. It contains a Laurentian component (eastern Tugaloo terrane) and an internal terrane (Cat Square) that contains both Laurentian and Gondwanan detrital zircons, separated by the Brindle Creek fault. Cat Square terrane rocks likely accumulated in a Devonian remnant ocean that closed beginning c. 400 Ma. The complex but consistently asymmetric, NW- to west- to SW-directed flow pattern throughout the IP reflects confinement beneath a > 15 km thick overburden produced during subduction of Cat Square and Laurentian components beneath the approaching Carolina superterrane along the Central Piedmont suture. Oblique NE-to-SW transpressive subduction to > 15 km depth initiated partial melting, forcing escape from the collision zone in an along-strike orogenic channel. The IP detached from rocks to the west of the mid-Palaeozoic Brevard fault zone as the collision zone tightened and the IP mass flowed c. 200 km southwestward in the channel. The top of the channel is preserved at the NE end of the IP, and the base (Brevard fault zone) is preserved to the west and SW. As an exhumed orogenic channel, the curved IP flow paths may provide insight for middle to lower crustal deformation and flow in modern orogens.