THE APPALACHIAN INNER PIEDMONT: AN EXHUMED OROGENIC CHANNEL

The Appalachian Inner Piedmont (IP) extends from North Carolina to Alabama. It has been known for decades to have attributes that set it apart from other Appalachian components. Its gentle dip, high metamorphic grade, and contrasting structural style with adjacent terranes have led several to hypothesize that it is exotic to Laurentia. It contains W- and SW-directed thrust sheets and a mineral stretching lineation, sheath folds, and other indicators that define a master crustal flow pattern throughout the belt. Meso- and microfabrics suggest that S foliations were transposed into C-foliations, particularly in the western IP. New field and geochronologic data both confirm that the IP is not exotic, and it contains both a Laurentian component (eastern Tugaloo terrane) and an internal terrane (Cat Square) that contains Laurentian and Gondwanan detrital zircons, suggesting these metasedimentary rocks originally accumulated in a remnant ocean that closed during the mid-Paleozoic. The complex but consistently asymmetric, NW- to W- to SW-directed flow pattern throughout the IP probably reflects confinement beneath an ~15 km-thick overburden produced during subduction of the Cat Square remnant ocean crust, sediments, and some of the Laurentian components farther W, beneath the approaching Carolina superterrane. Oblique NE-to-SW transpressive subduction to 15 km depth may have initiated partial melting creating partially viscous material and escape in an orogenic channel out of the collision zone. The IP would have become detached from other terranes to the W along the mid-Paleozoic Brevard fault zone as the collision zone tightened and the IP mass flowed southwestward in the orogenic channel. In 3–D the orogenic channel would have had a flattened ellipsoid shape with the X axis being the horizontal NE-SW IP axis, the Y axis would have been the across-strike horizontal axis, and the Z axis would have been vertical. If the IP is an exhumed orogenic channel, the curved IP flow paths may provide an opportunity to better understand middle to lower crustal deformation and mid-crustal flow in modern orogens, like the Himalayas, where channel flow is thought to occur.