PRELIMINARY COMPILATION OF THE NEW LITHOTECTONIC MAP OF THE APPALACHIAN OROGEN

Compilation of a new lithotectonic map of the Appalachian Orogen is underway; ultimately the map will be digitized and produced in both hard copy and compact disc formats. We will display a 1:1,000,000 scale copy of the compilation to date. First-order division of the orogen reflects its pre-Middle Ordovician tripartite nature, including i) native rocks of the Laurentian realm, ii) accreted, oceanic-affiliated elements of the Axial realm, and iii) exotic, peri-Gondwanan, elements of the Eastern realm. Comparitively, the lithotectonic affinity of Silurian and younger rocks is at a rudimentary stage of understanding. Compilation to date has focused on accreted units of the Axial and Eastern realms. There is a significant change in the nature of both realms at approximately the latitude of New York. The southern Axial realm embraces i) a westerly belt, dominated by pre-Late Ordovician metaclastic rocks, likely representing an accretionary complex and ii) an easterly belt that appears to preserve the remnants of an Early Ordovician magmatic arc; both subdivisions may be of peri-Gondwanan origin. The northern Axial realm contains two divisions that represent distinct arc systems, including a western, peri-Laurentian tract and an eastern, peri-Gondwanan tract, separated by the tectonic Red Indian Line. Along the entire orogen, the tectonic contact between the Axial and Laurentian realms likely originated as a series of NW-directed thrust faults. The Eastern realm is a collection of dominantly Neoproterozoic volcanic arc terranes that are superficially similar. However, differences in depositional history, faunas, magmatic history, and paleomagnetic data all indicate that the northern portion of the Eastern realm is distinct from the southern segment. To the north, the contact between the Eastern and Axial realms ranges from depositional, with Axial realm rocks overlying Eastern realm rocks, to tectonic, along faults with protracted movement histories; in the southern Appalachians it is a late Paleozoic ductile thrust system termed the central Piedmont shear zone. As compilation progresses, our divisions of the orogen as well as their distribution have required second- and third-order modifications; in this vein, the ‘in progress’ map will be on display for a ‘gauntlet’ style of peer review.