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

Orientations of fault-bend folds are determined by the geometry of fault ramps and are independent of the regional translation direction; the axes of fault-bend folds are generally parallel to strike of the frontal ramp. Folding is complicated, however, where the translation direction is at an oblique angle to a lateral ramp. As a thrust sheet is translated over the intersection of an oblique lateral ramp and a frontal ramp, second-order compressional or extensional structures are superposed on the fault-bend folds. The second-order stress is oriented perpendicular to strike of the oblique lateral ramp, and thus is oblique to the translation direction. Drape of hanging-wall beds over a footwall lateral ramp imposes plunge on fault-bend folds. Where a footwall lateral ramp is oblique to translation direction and second-order stress is compressional, second-order folds are superposed on the first-order fault-bend folds.

In the Appalachian thrust belt in Alabama, thrust sheets of Paleozoic strata strike northeast and are imbricated northwestward. Four transverse zones (cross-strike alignments of lateral ramps) 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 trend of the leading edge of the thrust sheet (the Pell City fault) curves abruptly from approximately N40ºE to N25ºW. In the hanging wall adjacent to the north-northwesterly trending segment of the Pell City fault, the dominant structures are upright isoclinal folds that trend approximately N25ºW, oblique to the regional translation direction. The north-northwesterly striking segment of the Pell City fault corresponds to an oblique lateral ramp in the footwall. The steep north-northwesterly trending folds in the hanging wall indicate compression perpendicular to the oblique footwall lateral ramp. The magnitude of lateral shortening, as determined from folded bed length, is approximately 50%. Lateral compression and isoclinal folding of the Pell City hanging wall are independent of regional translation direction and of the geometry of the northeast-striking frontal ramp.