Abstract

Postrift faulting in the Atlantic Coastal Plain of the eastern United States is a focus of several recent studies, particularly because of seismic activity associated with areas such as Charleston, South Carolina (earthquake of 1886). Understanding the ancient behavior of fault systems can contribute to increased awareness of earthquake potential where traditional mechanisms of earthquake activity are poorly understood. In the coastal plain of eastern North Carolina, the Graingers wrench zone, a more recently active northeast- southwest–trending fault system, overprints a Paleocene east-west fault trend and preserves evidence of episodic activity during Mesozoic and Cenozoic time. The western border of the Graingers basin is coincident with a northeast-southwest–oriented feature delineated on aeromagnetic and gravity maps. This northeast-southwest–trending western border fault records movement that initially occurred during the Paleozoic, was reactivated during the Triassic–Jurassic and the Cretaceous (Cenomanian and Maastrichtian), and has been active into the Holocene. Paleocene deformation records a shift in motion from the northeast-southwest trend to the east-west. This shift in direction controls the present-day distribution of early to late Paleocene strata of the Beaufort Group. Stratigraphic relationships between the Paleocene and underlying Cretaceous units suggest that the fault zone propagated to the southwest during the late Danian–Selandian and into the Selandian–Thanetian. Post-Paleocene activity of the northeast-southwest–trending faults exhibited a down-to-the-east displacement pattern and appears to have a minor component of strike slip, which may offset the older east-west–trending structures. Preservation of topographic features, such as a fault-line scarp with as much as 13 m of relief, triangular facets, and extensive ravinement normal to the scarp orientation, suggest Holocene movement on the faults. Historical records indicate seismicity in the area during the latter part of the nineteenth century.

The combined northeast-southwest and east-west fabric has geometry similar to those of fault zones in Georgia and seismically active areas of South Carolina. The faults that affect the area were probably formed by interaction of the Neuse hinge and the Roanoke Island–Goldsboro fault, which border the Graingers basin to the south and north, respectively. Differential movement on these larger structures may be the result of compression caused by changes in direction of motion of the North American plate.

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