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Basement constitutes rocks which belong to a previous orogenic cycle which have been reactivated and incorporated into a younger cycle. Basement massifs may be classified according to their relative position in an orogen as external or internal massifs. They may also be categorized according to their role in deformation, as thrust-related, fold-related and composite massifs. All Appalachian external massifs were transported following removal from the overridden edge of the ancient North American continental margin. Most of the internal massifs are also probably transported, but several (Pine Mountain and Sauratown Mountains) may exist as windows exposing parauthochthonous basement beneath the main thrust sheet. The latter reside immediately west of the low (west) to high (east) gravity gradient which probably outlines the old edge of Grenvillian crust. Reactivated crustal material generated during early Paleozoic orogeny plays the same mechanical role in reactivation as basement from the previous Grenville cycle. Basement (Grenville) massifs are distributed throughout the western Blue Ridge from Georgia to Maryland. Additionally, internal massifs are also present (Pine Mountain belt, Tallulah Falls and Toxaway domes, Sauratown Mountains anticlinorium, State Farm Gneiss dome, Baltimore Gneiss domes, and Mine Ridge anticlinorium). Basement internal massifs probably served to localize thrusts by causing them to detach and ramp over and around the massifs. Their antiformal shape may in part be as much related to thrust mechanics as to folding.

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