Stratigraphic and seismic data indicate that basement faults occur beneath the Appalachian foreland fold and thrust belt in Alabama. Some of the high-relief basement faults control the locations of thrust-related

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structures in cover rocks, and in part may be a cause of polyphase, breakback thrust development.

Basement faults are of three types: (1) normal faults of Precambrian and Cambrian age that are related to Precambrian continental rifting, (2) normal faults that were reactivated or initiated during Late Mississippian, and (3) Alleghanian thrust faults.

A basement fault system that moved during Cambrian and Mississippian-Pennsylvanian defines the northwest border of a basement low referred to as the Birmingham trough. The trough is a narrow, northeast-trending structure as much as 135 mi (215 km) long, up to 25 mi (40 km) wide, and has more than 7,000 ft (2,100 m) of displacement along the faulted northwest border. The southeast edge of the trough is bounded by Alleghanian basement thrust faults that exhibit as much as 2 mi (3 km) of shortening. The southwest-plunging trough loses relief to the southwest beneath the Gulf Coastal Plain onlap. On the northeast in northwestern Georgia, the trough is bordered by shallow basement.

The basement normal faults exerted significant control on the configuration of Alleghanian structures as well as on sedimentation during the Paleozoic. Basement faults control the magnitude of duplexes, thrust ramps, and associated ramp anticlines to varying degrees, depending upon local basement structural relief. Break-back thrusts in the Alabama Appalachians may be a result of buttressing of large-scale thrust sheets against the high-relief basement fault system at the northwest border of the Birmingham trough.

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