THE CAROLINA-MISSISSIPPI FAULT: LATE-ALLEGHANIAN TRANSCURRENT FAULT?

The Carolina-Mississippi fault (CMF) was first identified by Higgins and Zietz (1983) on the basis of a prominent change in the signature of aeromagnetic data along an ~1,500 km east-northeast–west-southwest trending boundary from offshore North Carolina to eastern Mississippi. These authors noted that the CMF truncates magnetic anomalies both north and south of the boundary, including the prominent Brunswick magnetic anomaly through southern Georgia and Alabama, and inferred that the CMF represents the Alleghanian suture between Laurentia and Gondwana. More recently, results from the SESAME broadband seismic experiment (Hopper et al 2017) have been interpreted to represent a regionally extensive subhorizontal mid-crustal boundary within the southeastern U.S. defined by a major seismic velocity gradient. These authors interpreted this feature, which projects to the CMF in the shallow crust, to be the Alleghanian suture, with significant low-angle thrusting of Gondwana above Laurentia. While these two features could be related, there are several reasons why they are probably not. The CMF appears to sharply truncate all Appalachian provinces (Valley & Ridge, Blue Ridge, Piedmont) and structures (Brevard Zone, Central Piedmont fault system, Eastern Piedmont fault system) at an oblique angle along a fairly linear path. The sharp linear truncation of these Appalachian features and the apparent sense of dextral shear suggests the CMF is a strike-slip fault which was active in the latest stages of Alleghanian deformation. In contrast, the subhorizontal boundary of Hopper et al with significant structural relief along strike (1) would be an unlikely surface on which to accommodate significant strike slip, and (2) would not be consistent with the linear shallow-crustal trace exhibited by the CMF. An interpretation consistent with these observations would be that the subhorizontal boundary of Hopper et al may be related to the Alleghanian suture, but like the rest of the structure in the Appalachians, is cut by the younger CMF.