STRUCTURAL RESPONSES OF THE PELL CITY THRUST SHEET TO AN OBLIQUE LATERAL RAMP IN THE FOOTWALL, APPALACHIAN THRUST BELT, ALABAMA

In the Appalachian thrust belt in Alabama, thrust sheets of Paleozoic strata strike northeast and are imbricated northwestward. Four transverse zones cross the northeasterly strike of the Appalachian thrust belt in Alabama. The large-scale Pell City thrust sheet ends southwestward at a lateral ramp within the Harpersville transverse zone, and the leading edge of the thrust sheet (the Pell City fault) curves abruptly from about N40ºE to N15ºW. In the hanging wall adjacent to the northwest-trending segment of the Pell City fault, pervasive upright isoclinal folds trend about N20ºW, oblique to the regional translation direction. The train of folds is limited to the southwestern part of the Pell City thrust sheet; farther northeast, the regional northeast strike prevails. The northwest-striking segment of the Pell City fault conforms to an oblique lateral ramp in the footwall. The isoclinal folds in the hanging wall indicate compression perpendicular to the oblique footwall lateral ramp, and lateral shortening of folded bed length ranges from ~25-40% to a maximum of ~55%. The lateral compression and isoclinal folding are independent of both regional translation and the geometry of the northeast-striking frontal ramp. At the Harpersville transverse zone, the Pell City fault cuts up section southwestward in the hanging wall, indicating a hanging-wall lateral ramp emplaced over the footwall lateral ramp.

Northeast of the Harpersville transverse zone, the northeast-striking Jacksonville fault is the trailing cutoff of the Pell City fault. The Jacksonville fault cuts up section and ends southwestward via a displacement gradient. At the southwest end of the fault, the Jacksonville thrust sheet merges with the trailing part of the Pell City thrust sheet. The northwest-trending isoclinal folds at the Harpersville transverse zone extend across the southwest part of the composite Pell City-Jacksonville thrust sheet. The southwest end of the Jacksonville fault corresponds to the northeast limit of the isoclinal fold set, as well as the northeast base of the hanging-wall lateral ramp in the Pell City thrust sheet. The displacement gradient and southwest end of the Jacksonville fault may be kinematically and mechanically constrained by the hanging-wall and footwall lateral ramps and associated folds in the Pell City thrust sheet.