Thomas (1977) presents a powerful paradigm for the interpretation of long-lived accretionary orogens. Curvature of Appalachian F&T belts and thicknesses of stratigraphic packages are concluded to result from the original rift-transform geometry of the Iapetan margin (e.g., Rankin, 1976). Stratigraphic packages are recognized 1) as belonging to rift-drift phases of opening Iapetus or 2) as clastic wedges recording accretion of exotic terranes or displaced ribbon continents. Yet another type of sedimentary package has been discerned within the App's, and recognition of these basins has been critical to understanding accretion and later terrane dispersal along the length of the orogen. Successor basins were deposited across Laurentian and exotic basements, accepting detritus from both terranes (e.g., Williams, 1978, his 17c). These basins must post-date accretion, and a variety of lines of evidence suggest they record extension and rifting along a suture. Particularly notable are Wenlock-Ludlow basins now recognized from NFLD to the Carolinas and GA (e.g., Tremblay & Pinet, 2005, Dennis, 2006, Rankin et al, Wintsch et al, 2007). These basins record the rifting of the Ganderia- and Carolinia-Laurentian sutures. Along the length of the orogen magmatism coeval with basin deposition records two stages of this rifting:1) crustal anatexis, followed by 2) mantle-derived melts.

Is middle Paleozoic successor basin geometry inherited from the Iapetan Laurentian margin? The Gaspé-Central Maine-Connecticut Valley basins appear to have formed south of the St Lawrence Promontory; restored late Silurian position of the Cat Square basin appears to be south of the New York Promontory. At least two Grenvillian (?) basement terranes are recognized in the central and southern Piedmont: the Goochland (NC-VA) terrane and the Brandywine (PA-DE-MD) massifs. Elements of the Peach Bottom structure and (exotic?) portions of the southeastern PA Piedmont and adjacent states may be part of a similar Ordovician suture, rifted and overlain by a Late Silurian successor basin complex; age constraints in these rocks are generally lacking but dated Sil rift-related rocks are present (Smith et al, 2004). Restoration of these internal basement terranes and their cover sequences to their Ordo-Sil locations will improve our understanding of the Wenlock-Ludlow event.