RHEOLOGICAL AND TIMING CONTRASTS OF MAJOR FAULTS FRAMING THE NORTHEASTERN END OF THE PINE MOUNTAIN WINDOW, CENTRAL GEORGIA

The Pine Mountain window (PMW) is the southernmost basement massif in the Appalachian orogen that exposes Grenville orthogneiss and a Neoproterozoic-Paleozoic(?) cover sequence. It is bounded by several Alleghanian(?) faults of different rheologies and therefore different ages. At its northeastern end, the PMW is separated from Inner Piedmont (Cat Square terrane) rocks to the east by the Box Ankle fault, to the northwest by the Towaliga fault, and to the southeast by the Dean Creek fault. New detailed geologic mapping in this area has provided fresh insights into field relations between the Towaliga and Box Ankle faults, and the southwestern end of the Cat Square terrane. The Box Ankle fault is an ~2 km thick, gently southeast and northwest dipping, northwest vergent, thrust and terrane boundary that separates footwall PMW rocks from hanging wall Cat Square terrane rocks. Mylonite within the fault contains an upper amphibolite facies mineral assemblage, a well-developed S-C fabric, rotated feldspar porphyroclasts with abundant myrmekite rims and recrystallized tails. It is truncated to the northwest by the younger Towaliga fault, which is a dextral strike-slip fault that is characterized by garnet-grade mylonite with rounded feldspar porphyroclasts that deformed brittlely in a very fine-grained biotite-rich matrix. Feldspar porphyroclasts in this mylonite exhibit quartz-mica pressure shadows with no feldspar tails; however, several σ- and δ-type porphyroclasts, asymmetric folds of quartz ribbons, and S-C fabrics provide shear sense. The Towaliga fault was also reactivated brittlely during the Late Triassic-Early Jurassic by sinistral motion producing numerous rhomb-shaped step-overs filled with multiply broken hydrothermal quartz; in areas between the rhombs a brittle fabric has overprinted the mylonite, creating a cataclasite. Small-scale brittle faults (1-2 cm displacement) also exist in the Box Ankle mylonite, and can possibly be attributed to this same event. Several late brittle faults with 1-2 km of displacement have been traced into the PMW as well. The PMW is thus a complex window that was emplaced during a long history of terrane accretion and fault reactivation.