BOUNDARY CONDITIONS AND INTERNAL DEFORMATION IN THE CURVED SOUTHERN APPALACHIAN FORELAND FOLD-THRUST BELT

The basement surface below the southern Appalachian foreland fold-thrust belt (SAFT), the base of the Blue Ridge-Piedmont (BRP) megathrust sheet above, and the topographic free surface comprise the boundaries of the NW-vergent, thin-skinned, listric Alleghanian thrust wedge. Additional boundary conditions include T <200° C and P <300 MPa. SAFT thrusts, and the BRP sheet, reach maximum displacement in TN and decrease displacement and numbers SW and NE. Ten thrusts in the SAFT at 36° N latitude include three master faults, and the BRP thrust sheet indenter. The westernmost BRP thrust in NE TN–SW VA transported basement through Ordovician rocks, and in SE TN–NW GA is a bedding thrust in older Cambrian rocks, indicating the frontal BRP is thin skinned. Large frontal duplexes obliquely overridden along the leading edge of the BRP sheet are traceable for many km beneath the sheet, and domes in the sheet formed by footwall duplexing. Low-grade metamorphic rocks are in reentrants in the BRP sheet in GA and VA, but not in windows near its front in TN. This may indicate the up-dip westward limit of the BRP sheet is near its present-day trace and the sheet was never very thick in TN, whereas in GA it may have been thicker and extended some 30 km farther W. Seismic reflection data in the SAFT and BRP image the basement surface and base of the BRP sheet revealing a gently SE-dipping basement surface from VA to GA broken by normal faults. Basement normal faults are more common beneath the SAFT in western GA and AL (Thomas and Bayona), and beneath the BRP sheet in GA and the Carolinas. Small-displacement normal faults formed basins that did not impede thrust movement, but large-displacement normal faults localized facies changes, nucleated later thrusts, and influenced structural style. Plane strain occurred locally but not regionally in the SAFT, so particle trajectories curve in an orogen that traces a 70° arc along the BRP from N GA to SW VA, where SAFT thrusts have unequal displacements. Despite apparent balance of many 2–D cross sections, lack of plane strain regionally diminishes their use for quantifying particle trajectories. Coulomb behavior characterizes individual faults, but plastic rheology better addresses the particle trajectory problem. Neither address mechanics of localization of fault families, out-of-sequence thrusts, thrust spacing, and other attributes.