Early rotation and late folding in the Pennsylvania salient (U.S. Appalachians): Evidence from calcite-twinning analysis of Paleozoic carbonates

Abstract

Calcite-twinning analysis of Paleozoic limestones from 42 sites reveals that the change in regional strike along the frontal edge of the Pennsylvania salient is accompanied by an equal-magnitude rotation of paleostress directions of up to 60 degrees. The rotations, recorded at 22 reliable sites, show no discernible difference between sites with rocks of Cambrian-Ordovician and Silurian-Devonian age. Evidence for similarly fanned orientations is not present in foreland sites. Scatter in the data is attributed to grain-scale rotations and compaction overprinting, as demonstrated in prior studies, and it was reduced by data-cleaning methods as well as by the use of contouring and data-averaging methods. Comparisons of paleostress directions within the belt reveal only minor rotations in the southwest region of the salient, and the bulk of rotation is accommodated by the northern limb. We hypothesize that these rotations resulted from convergence in the thrust wedge against a northerly bounding, rigid basement block around Pennsylvanian times. This created a structural anisotropy in the evolving belt that guided the post-rotational formation of folds in Early Permian times and produced the current pattern of the salient. This model of decoupled thrusting/rotation and folding explains a variety of previously conflicting observations in the area while offering a kinematically consistent scenario that may also apply to other curved orogenic belts.