THE PINE MOUNTAIN WINDOW, GEORGIA-ALABAMA: PRODUCT OF BUTTRESSING BY THE OUACHITA OROGEN

The Pine Mountain window (PMW) is located in the southernmost exposed Appalachians on the Alabama promontory. It exposes Laurentian Grenvillian basement and a kyanite-sillimanite-grade platform cover sequence. The window is surrounded on three sides by Inner Piedmont (IP) rocks. This complex window is framed by faults formed at three different crustal levels: the E end by the sillimanite-grade Box Ankle thrust (NW vergence), the NW flank by the garnet-grade dextral Towaliga fault, and the SE flank by the chlorite-grade dextral Dean Creek fault. Additional faults, like the dextral Bartletts Ferry and Goat Rock faults (garnet grade), splay from the framing faults and cut the interior of the window farther W in Alabama. All are Alleghanian structures. The PMW plunges NE beneath the Acadian(?) central Piedmont suture that flanks the window farther E and S. Reprocessing of the COCORP seismic reflection Line GA-15 reveals a gently SE-dipping reflector package at 3-5 s (twt, 10-15 km) that passes beneath the exposed window from the IP. Above the basal reflectors is a thick package of gently SE-dipping reflectors immediately beneath the PMW suggesting that the broad arch of the window is produced by a large duplex of platform rocks. A large synformal reflector package immediately N of the window may represent another duplex beneath the IP. The Towaliga fault is not imaged in the data because of its steep NW dip. The rocks inside the window appear to be a horse derived from the middle Paleozoic continental margin to the SE, so the entire PMW may be a product of stacking of duplex imbricates beneath a horse of continental basement and cover. The entire structure may have been nucleated here by buttressing by the Ouachita orogenic high farther west preventing the Blue Ridge-Piedmont megathrust sheet from moving efficiently onto the Laurentian platform as segments of the sheet farther NE that have much greater displacement.