CRUSTAL STRUCTURE, IGNEOUS INTRUSIONS, AND FAULTS IN THE SOUTH GEORGIA RIFT BASIN AND VICINITY, SOUTH CAROLINA – CONSTRAINTS FROM POTENTIAL-FIELD, SEISMIC, AND WELL DATA

Potential-field data, with its excellent spatial coverage of the southeastern North American continental margin, used in combination with seismic imaging, can help to illuminate basement and upper- to mid-crustal structures. These rocks record the large-scale geologic processes responsible for the evolution of continental lithosphere spanning a Wilson cycle, including structures associated with the Alleghanian orogeny and features attributed to Mesozoic continental rifting. Maps and two-dimensional cross-section models, constrained by deep-well and seismic data, were developed for the basement beneath the South Carolina coastal plain by forward and inverse modeling of the aeromagnetic and gravity fields.

A major crustal boundary southeast of the Dunbarton Basin and Barnwell Mafics coincides with a distinct lateral discontinuity in crustal velocity down to a depth of 8 km observed in seismic refraction data. In the topography the boundary underlies the Orangeburg Escarpment. We interpret this as a major northeast-trending basin-bounding fault for the South Georgia Rift separating Triassic sediments to the southeast from a structural high composed of metamorphic rocks and mafic igneous intrusions to the northwest. With a prominent gravity and magnetic signature, the Barnwell Mafics form a laterally extensive sheet with 2 conduits down to a depth of 6 km. The Santee mafics are interpreted to be within a rift basin, but their interpretation as Paleozoic gabbros cannot be ruled out. The Clubhouse Crossroads mafics are modeled as laterally extensive thin sills or flows fed by a relatively shallow conduit. Based on the character of its gravity anomaly, the Springfield Granite represents one of the largest buried granites on the eastern margin of North America, with an average thickness of 3 to 4 km and conduits to a depth of 7 km. We have also interpreted the coastal gravity low near Myrtle Beach as a granite buried beneath coastal plain sediments. This gravity low has previously been interpreted as a sub-basin of the South Georgia Rift, but forward modeling suggests it is more likely a buried Paleozoic Granite outside of the basin boundary. Their relatively high measured heat flow as well as their large volumes makes the granite plutons attractive low-temperature geothermal energy targets.