Foreland signature of indenter tectonics: Insights from calcite twinning analysis in the Tennessee salient of the Southern Appalachians, USA

Abstract

The Tennessee salient in the Southern Appalachian fold-and-thrust belt is defined by a regional variation in strike of ∼55° from southwestern Virginia to northern Georgia. Determination of a primary or secondary origin for the arcuate nature of this part of the belt is the focus of this study. Oriented limestone samples were collected from the thrust belt and the “undeformed” foreland for calcite twinning analysis. Layer-parallel paleostress orientations from 25 sites within the thrust belt reveal a radial pattern that varies systematically along the orogenic front. However, the amount of fanning (∼75°) significantly exceeds the belt9s frontal curvature, indicating that passive rotation of originally parallel paleostress directions, as proposed elsewhere, is unlikely here. In addition, results from 27 foreland sites display a similarly fanned pattern of paleostress directions, collectively showing that a radial stress regime was imparted on rocks along this part of the Appalachian margin by the onset of thrusting. Differential stress values are comparable to previous results, with decreasing σ d values from >100 MPa within the thrust belt to ∼50 MPa at 40 km from the orogenic front, regardless of orientation. Instead of the extent of frontal curvature, fanning of paleostress directions in limestones matches the current indenter geometry of the Blue Ridge to the east. We propose, therefore, that radial paleostress directions were imparted in response to the advancing Blue Ridge thrust sheet, with differential thrust displacement creating (primary) curvature instead of secondary rotation during shortening. This resulted in the present-day geometry of the Tennessee salient, which is supported by basic geometric modeling of a sand wedge. It also explains the previously noted increase in displacement and number of major thrusts near the indenter9s apex. However, this scenario contrasts with buttress-induced, secondary curvature in the Pennsylvania salient to the north in the Appalachian chain, showing that curvature is not explained by a single mechanism in the Appalachian belt.