Foreland Signature of Indenter Tectonics: Results from the Tennessee Salient of the Appalachians

Most fold-thrust belts worldwide are curved, but the mechanisms for the development of arcuate belts remain ambiguous. In the Appalachians, most studies have focused on curvature of the Pennsylvania Salient rather than the similarly curved Tennessee Salient. We have completed an integrated paleomagnetism and calcite twinning study of Mississippian and Ordovician limestones, both within the thrust belt and in the minimally deformed foreland, to elucidate the origin of curvature in this part of the Appalachians.

Layer-parallel paleostress orientations at 27 sites within the thrust belt reveal a radial pattern that is systematic along the orogenic front. However, the degree of fanning (~85°) exceeds the belt's curvature (<60°), indicating that passive rotation of originally parallel paleostress directions is unlikely. In addition, results from 13 foreland sites display a similarly fanned pattern of paleostress directions, showing that a radial stress regime was imparted on rocks of this region. Differential stress values are comparable to previous results, with decreasing σ_d values from >100MPa in the thrust belt to ~35MPa at 40km from the orogenic front, regardless of orientation.

Fanning of paleostress directions is preserved in foreland limestones and exceeds the degree of curvature of the Tennessee Salient, which together is best explained by indenter tectonics. Moreover, the predicted indenter geometry from stress directions matches the shape of the Blue Ridge to the east. We conclude, therefore, that radial paleostress directions were imparted in response to the advancing Blue Ridge, with differential thrust displacement occurring instead of secondary rotation during shortening. This produced the present-day geometry of the Tennessee Salient, and also explains the increase in displacement and number of major thrusts near the indenter's apex, as others have noted. Paleomagnetic results also support a primary origin for curvature in the Tennessee Salient, which contrasts with secondary rotation in the Pennsylvania Salient.