GEOMETRY, KINEMATIC EVOLUTION, AND TIMING OF CRYSTALLINE THRUST SHEETS AND ASSOCIATED STRUCTURES ALONG THE WEST FLANK OF THE SOUTHERN APPALACHIAN OROGEN IN ALABAMA AND GEORGIA, USA

Work on thin-skinned thrust faulting has typically focused on foreland thrust belts, where fault mechanics are strongly influenced by anisotropies associated with primary layering in stratified rocks. Unlike foreland thrusts, hinterland thrust faults commonly cut up and down section in previously deformed crystalline rocks and are more likely to behave as mechanically isotropic bodies due to polydeformation that spatially limits the mechanical influence of primary layering. Crystalline thrust sheets in the southern Appalachians of Alabama and Georgia incorporate at least ten regional, cross-cutting contractional deformation phases as a result of Neoacadian-Alleghanian orogenesis. Examination of a 300 km-long segment of the northwestern flank of the orogen indicates most of these far-traveled crystalline allochthons were emplaced onto Laurentian platform strata during out-of-sequence faulting that propagated through previously metamorphosed and complexly deformed rocks. The earliest record of thrust faulting, represented by the Devonian/Mississippian Hillabee thrust, emplaced a volcanic back-arc terrane onto Laurentia’s outermost shelf prior to greenschist facies metamorphism of the tectonically-assembled Talladega belt. Like the younger foreland thrusts, this fault’s trajectory was significantly influenced by primary layering in both hanging and footwall. The Devonian/Mississippian deformational wave associated with the Hillabee thrust may have penetrated as far as the northwest edge of the deformed foreland. In contrast, younger thrusts of the crystalline hinterland post-date the main phase of thrust faulting and associated deformation in the adjacent and cratonward foreland wedge. Each of these faults telescope metamorphic isograds, have low regional dips, and follow trajectories that cut regional stratigraphy and metamorphic fabrics along and across strike. These younger faults bound thin-skinned, crystalline thrust sheets of deformed metamorphic rocks of the outer Laurentian platform and dismembered segments of a Laurentia-fringing Ordovician-Silurian back-arc basin, each of which behaved as internally strong, mechanically homogeneous and brittle slabs of intact cover rocks.