THE ALLEGHANIAN APPALACHIAN FORELAND-FOLD THRUST BELT

The Alleghanian (AG) foreland fold-thrust belt (FFTB) of the southern and central Appalachians (AL to NY), and the Blue Ridge-Piedmont (BRP) megathrust sheet indentor that pushed foreland deformation in front of it, formed during the latter stages of Early Permian collision of Gondwana with Laurentia. Salients and recesses also occur here, which likely inherited the shape of the Neoproterozoic rifted margin, with greatest shortening (>50%) in the TN (greatest number of thrusts) and PA (rootless fold dominated) salients. This is one of the world’s classic FFTBs, consisting of foreland-vergent thrusts and folds that comprise a foreshortened, eastward-thickening wedge that formed above a master Lower Cambrian décollement, which rests on undeformed basement. Classic fault-bend and fault-propagation folds occur here, but the dominant fold mechanism is thrust-related buckling with flexural slip. Rootless folds occur at present erosion level above an intermediate décollement atop a regional duplex from northern VA to NY, where layer-parallel shortening above the intermediate décollement occurs in the Allegheny Plateau. Thrust faults propagated parallel to bedding in the master décollement until a mechanical discontinuity forced them to ramp at a higher angle across strong, mostly carbonate units to a higher weak unit. Oldest thrusts formed closest to the hinterland in a transition zone between the hinterland, where thrusts involve basement, and the foreland. Successively younger thrusts formed as faults locked, causing renewed propagation along the basal décollement; the process was repeated until energy from the indentor was expended, forming the last rootless folds and thrusts in the AG clastic wedge in the Cumberland–Allegheny Plateau. Where large numbers of thrusts occur, one to three dominant thrusts will have large displacements (10s of km). Shortening recorded by FFTB folds and thrusts should be less than or equal to the displacement on the BRP indentor that drove foreland deformation.