GPR Imaging of Fault Related Folds in a Gold-Bearing Metasedimentary Sequence, Carolina Terrane, Southern Appalachian Mountains

Abstract:

The Carolina terrane comprises Ediacaran to earliest Paleozoic mixed magmatic and sedimentary assemblages in the central and eastern Piedmont of the Southern Appalachian Mountains. The terrane was primarily deformed during the Late Ordovician Cherokee orogeny, that reached greenschist facies metamorphism. The Albemarle arc, a younger component of the Carolina terrane, contains volcanogenic metasedimentary rocks with intercalated mainly rhyolitic volcanic rocks. Regional inclined to overturned folds with axial planar cleavage verge southeast. At mesoscopic scales (exposures of a few square meters), folds sympathetic with regional folds are attenuated or truncated by ductile shear zones or contractional faults. Shear and fault zones are most abundant near highly silicified strataform zones in metagraywacke of the Tillery Formation; these zones are also auriferous. GPR profiles were collected across strike of two silicified, gold-bearing zones and enclosing metagraywacke to characterize the scale and extent of folding in the vicinity of ore horizons. Several GSSI SIR-3000 / 100 MHz monostatic GPR profiles were collected in profiles up to 260 meters long. In pre-migration lines processed for time zero and background removal, several clusters of shallow, rolling sigmoidal reflectors appeared separated by sets of parallel, northwest-dipping reflective discontinuities. These features are inferred to be reverse faults carrying contractional folds. After migration with an average velocity of 0.105 m/ns, vertical heights of the inferred folds became attenuated but not removed, and contractional fault reflections remained prominent. After migration, a highly convex-up cluster of reflections initially assumed to be a fold culmination resolved to an elliptical patch of high amplitudes. The patch is likely an undisclosed shaft or covered trench left by earlier gold prospecting. In this survey, useful detail appeared to a depth of 7.5 meters, and only a few gently inclined, laterally extensive reflectors are present inside the noise blanket. Also, convex-up planar reflectors greater than about 8 meters deep and tens of meters long suggest fold culminations much longer in wavelength than the shallow imaged folds. For these metasedimentary rocks GPR provides a valuable tool for imaging sub-surface structure.