HALF A CENTURY OF MODERN APPALACHIAN TECTONICS—ARE WE THERE YET?

The fossiliferous Appalachian (AP) foreland yielded a basic stratigraphic-structural framework before the 20^th century; enormous strides have been made in understanding the APs interior since 1970, when the first plate tectonics models appeared. This was the time of the beginnings of publication of modern plate tectonics-based papers on Newfoundland (NFL) geology and Williams’ formulation of the “NFL model.” More recently, detailed geologic mapping (DGM) in interior Canadian Maritimes yielded new tectonic models. Billings’ and Thompson’s students produced good geologic maps in New England before 1970, so a basic stratigraphic framework, major boundaries, distributions of plutons, and metamorphic zonation became known, and a conceptual framework evolved earlier than in other parts of the APs. The central APs benefitted from high-quality work by the Stoses, Cloos, and others before 1970, so a stratigraphic and structural framework emerged. Interior southern APs, however, remained the least known segment with huge areas known only by reconnaissance, and only a few small islands of DGM sufficient to construct a local stratigraphic framework. Conventional wisdom said the Piedmont and Blue Ridge are so poorly exposed it was not worth venturing off of the roads.

Islands of DGM began to grow in all of the interior APs in the late 20^th and early 21^st centuries. Using shear-sense indicators in mapping resolved many disputes about the kinematics and motion sense of major faults. This ground truth permitted great strides to be made with TIMS, SIMS, ICP-MS geochronologic techniques from the 1990s forward, and correct ages of plutons and basement, provenance of metasedimentary rocks, and timing of metamorphism were determined. This permitted confirmation of distribution of exotic and suspect terranes that, with DGM, refined the character of major tectonic boundaries.

New tectonic problems and models will emerge when islands are enlarged and resolved with modern geochronology, structural and petrologic studies, but all advances will depend on a DGM foundation. Perhaps the largest remaining AP tectonics problem is the renewed Mesozoic- and Cenozoic uplift and erosion of the APs. This requires better geophysical resolution of mantle structure, coupled with geomorphic data, modern geochronologic, and stratigraphic data.