STRAIN DISTRIBUTION IN THE GOLD HILL SHEAR ZONE, CENTRAL SOUTH CAROLINA

The Gold Hill shear zone (GHSZ) near the town of Carlisle in the central South Carolina Piedmont is up to 10 km. wide, strikes N60E, and lies between Carolina terrane biotite gneiss and amphibolite to the southeast, and metaplutonic and greenschist facies metavolcanic rocks to the northwest. Foliations within the zone have high angle dips; mineral lineations plunge 10 to 15 to the northeast. The GHSZ is truncated on the southwest by an east-to-southeast dipping thrust, to the west of which lies the Inner Piedmont terrane. Ductile strain is inhomogeneously distributed in the GHSZ, with approximately 100 meter thick high-strain bands of ultramylonite (grain size .004 to .01 mm) alternating with wide less-deformed bands of lineated granite gneiss, metadiorite, amphibolite, and biotite gneiss. In the lineated gneiss, quartz ribbon axis ratios are 1:3, but in mylonites the most stretched ribbons have ratios of 1:27. Temperatures at the time of deformation have been estimated using textures and mineralogy. In the lineated gneiss and protomylonites, evidence of early higher temperature (450-500°C) strain includes minor strain-twinning and undulose extinction in plagioclase. Quartz grains in lenses and ribbons are medium-sized equant grains with curved or straight boundaries which meet at equilibrium triple points. Later strain at lower temperatures (350° C) resulted in fractured plagioclase, and finer grained quartz with undulose sweeping or patchy extinction and sutured grain boundaries. The last deformation at about 250-300° C resulted in fractured quartz with undulose extinction. Biotite was stable during the higher temperature strain, supporting a 400-500° C temperature range. Paleozoic cooling of this area has been determined by ⁴⁰Ar/³⁹Ar dating of hornblende at about 300 Ma (500° C blocking temperature) and biotite at about 275 Ma, (300° C blocking temperature). Thus, although the early history of higher temperature strain in the GHSZ is not constrained, the later right-slip deformation at lower temperatures below 500° C appears to have been early Permian.