ASSEMBLY OF FORELAND AND HINTERLAND ALLOCHTHONS IN THE SOUTHERN APPALACHIANS OF ALABAMA AND GEORGIA: IN-SEQUENCE TO OUT-OF-SEQUENCE THRUSTING AT THE MISSISSIPPIAN-PENNSYLVANIAN BOUNDARY

A comprehensive analysis of southern Appalachian structural features in Alabama and Georgia reveals an initial pulse of in-sequence (e.g., forward breaking thrusts) Mississippian deformation stretching from the hinterland (Blue Ridge) to the northwestern flank of the foreland fold-and-thrust-belt. That in-sequence deformation was followed by an extensive interval of out-of-sequence, backward breaking thrust faults that developed during the earliest Pennsylvanian through the Permian. The out-of-sequence thrust faults, which stretch from the Sequatchie anticline on the northwestern flank of the foreland fold-and-thrust-belt, southeast to the Brevard fault zone at the eastern Blue Ridge-western Inner Piedmont boundary, dominate the structural architecture of the southern Appalachian orogen in this region. The transition from in-sequence to out-of-sequence thrust faulting at the Mississippian-Pennsylvanian boundary suggests the southern Appalachian orogenic wedge achieved critical taper during the Mississippian, but transitioned to sub-critical taper by the Pennsylvanian. Importantly, the switch from in-sequence to out-of-sequence thrusting occurred during the same time frame as uplift and erosion of the Upper Mississippian-Lower Pennsylvanian Parkwood Formation, subsequent deposition of the Pottsville Formation clastic wedge, and the waning stages of peak metamorphism in the southern Appalachian Blue Ridge (330-320 Ma). The Mississippian-Pennsylvanian boundary also coincides with a significant climatic shift from monsoonal, wet-dry conditions to “ever-wet” conditions in this segment of Pangea. We present evidence for at least 20 out-of-sequence thrust faults spanning the foreland fold-and-thrust-belt and Blue Ridge of Alabama and Georgia that likely developed as a result of climate-tectonic interactions that influenced the topographic slope and critical taper of the southern Appalachian, Alleghanian orogenic wedge. Similar climate-tectonic interactions have been noted in the Eastern Alpine Molasse Basin of Germany.