BREAK-BACK KINEMATIC DEFORMATION PLAN, BESSEMER TRANSVERSE ZONE, ALLEGHANIAN THRUST BELT IN ALABAMA

Geologic map patterns of thrust faults in the Bessemer transverse zone (BTZ) in the Alleghanian thrust belt in Alabama provide evidence for sequence of thrusting. In a break-forward deformation plan, frontal ramps break successively from the internides towards the foreland. In a break-back deformation plan, frontal ramps break successively from the foreland toward the internides. Large-scale thrust sheets in the BTZ, from internides to the foreland, are the Talladega, Yellowleaf, Dry Creek, Helena, Jones Valley, Opossum Valley, and Blue Creek faults. Relations of the faults to footwall structure indicate the sequence of thrusting.

The Talladega fault truncates folds in the underlying Yellowleaf thrust sheet, indicating a break-back sequence in which folds in the Yellowleaf hanging wall pre-dated Talladega thrusting. Co-axial folding of the Talladega thrust sheet and footwall folds in the Yellowleaf thrust sheet indicates late slip on the Yellowleaf fault after emplacement of the Talladega sheet.

The Yellowleaf and Dry Creek faults truncate folds in the underlying Helena thrust sheet.

The Cahaba synclinorium in the Jones Valley thrust sheet ends northeastward up-plunge in a northwest-striking lateral ramp, which is truncated by the Helena fault northeast of the BTZ. Truncation of the lateral ramp in the Helena footwall indicates that the Helena thrust sheet was emplaced after the formation of the Jones Valley lateral ramp. In the BTZ, kinematic relationships between the Jones Valley and Helena thrust sheets are not clearly defined; however, the break-back sequence indicated by truncation of the lateral ramp in the Helena footwall defines a regional kinematic succession.

The Jones Valley fault truncates folds and a lateral ramp in the Opossum Valley thrust sheet and two transverse faults in the Blue Creek thrust sheet.

Map relationships generally indicate a break-back succession of thrusting in the BTZ. The Opossum Valley and Blue Creek faults are the oldest in the BTZ; the Jones Valley, Helena, Yellowleaf, Dry Creek, and Talladega faults are successively younger. Some non-systematic variations are indicated, as by the folding of the Talladega thrust sheet by late-stage slip on the Yellowleaf fault.