Abstract

Near the South Carolina and Georgia border, the southeastern parts of the Charlotte belt and Carolina slate belt near the Coastal Plain overlap are coincident with the northwestern edge of an isolated negative Bouguer gravity anomaly. This anomaly is interpreted as evidence for a detached fragment of continental crust. The pattern of the magnetic and gravity anomaly contours for the northwestern edge of this fragment is similar to the pattern of magnetic anomaly contours and the Piedmont gravity gradient in central Georgia. This geometric similarity suggests that the fragment may have been separated from a larger continental block by a rift zone nearly 160 km wide. The Carolina slate belt in Georgia and South Carolina may delineate the axis of this continental rift or rift system and may represent remnants of rift-derived volcanic and sedimentary rocks. Gravity profiles across the edges of the hypothesized rift indicate the existence of anomalous high-density rocks in the upper 5 km of the crust in the proposed rift and low-density rocks at the base of the adjacent continental crust. Seismic data in Georgia indicate a typical rift-zone velocity structure that consists of a high-velocity (6.3 km/s) discontinuous surface layer 0.0 to 5.0 km thick over a low-velocity (6.0 km/s) crustal layer which extends to depths of at least 30 km. Magnetic lineations provide evidence for the faults and/or depositional layering of the rift and outline the structures and margins of the rift. The rift developed from late Precambrian through Cambrian time (650 to 520 m.y. B.P.), as determined from radiometric dating of Carolina slate belt rocks. In North Carolina the rift was reactivated during the Triassic, but in Georgia and South Carolina, Triassic extension was restricted to the southeastern edges of the detached crustal fragment.

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