INTERACTION OF THE APPALACHIAN-OUACHITA OROGEN WITH THE PRE-OROGENIC IAPETAN RIFTED PLATE MARGIN OF LAURENTIA

Sinuous curves of the late Paleozoic Appalachian-Ouachita orogen reflect tectonic inheritance of the zigzag trace of the Iapetan rifted margin of Laurentia. Specifically, the composite Alabama thrust-belt recess mimics the Alabama promontory.

Unlike typical narrow, sharply bent structural recesses, components of the Alabama recess spread >400 km along the thrust belt. In northwestern Georgia, the interior structures bend abruptly from ~005 in the southern arm of the Tennessee salient to ~065 in the eastern part of a generally 040-trending sector of the Alabama recess. The contrasting orientations are absorbed toward the foreland, where nearly straight frontal ramps trend 040. Farther west, beneath post-orogenic cover of the Gulf Coastal Plain in eastern Mississippi, the 040-trending frontal Appalachian structures curve westward and truncate the 335-trending Ouachita thrust front.

The northwest-trending Alabama-Oklahoma transform fault and northeast-trending Blue Ridge rift frame the Alabama promontory of the Iapetan rifted margin of Laurentia. The Georgia transform separates an upper-plate rift on the Alabama promontory from a lower-plate rift in the Tennessee embayment. Down-to-southwest tectonic-load-driven foreland subsidence and northeastward clastic-wedge progradation in the Black Warrior basin in the ~335-striking Ouachita foreland along the Alabama-Oklahoma transform margin of Laurentia began in Late Mississippian. At approximately the same time, down-to-east load-driven subsidence and westward clastic-wedge progradation began in the 005-striking southern Appalachian foreland along the rift margin in the Tennessee embayment. The two oppositely directed clastic wedges merged onto a Mississippian carbonate shelf on the Alabama promontory. Later, in the Early Pennsylvanian, down-to-southeast subsidence and northwestward clastic-wedge progradation began across the Alabama recess in response to continent-continent collision of the African sector of Gondwana with the Alabama promontory of Laurentia at the ~065-trending Suwannee-Wiggins suture, linking the arms of two otherwise unconnected salients. The trace of the thrust belt reflects a diachronous history of collision and tectonic loading, which was forced by the trace and structure of the pre-orogenic rifted margin.