OROCLINAL BENDING IN THE PENNSYLVANIA SALIENT: EVIDENCE FOR EARLY ROTATION FROM CALCITE TWINNING ANALYSIS

Calcite twinning analysis of Paleozoic limestones from 42 sites reveals that the change in regional strike along the frontal edge of the Pennsylvania Salient of the Appalachian Orogen is accompanied by an equal magnitude of rotation in paleostress directions of about 60 degrees, with no discernable difference between sites in Cambro-Ordovician and Siluro-Devonian rocks. Scatter in the data is attributed to grain-scale rotations and minor compaction effects. Data analysis reveals a layer-parallel north-northwesterly oriented stress field as recorded by twins in calcite. Comparison of paleostress directions within the rotated belt show minimal rotations in the southwest region of the salient, whereas the bulk of rotation is accommodated by the central and especially northern segments of the curved belt. We propose a model in which these pre-folding rotations, as well as those documented by previous paleomagnetic data, result mostly from impingement of the Paleozoic sheet on a northerly-bounding rigid cratonic block. This created a structural anisotropy that guided the subsequent formation of folds into today's configuration of the salient. We find no evidence for an earlier proposed detachment horizon between Cambro-Ordovician and Siluro-Devonian rocks. The post-rotational formation of folds is responsible for both the lack of tangential extension in the region and for the divergent evolution of kinematic directions observed by previous workers. We also note that comparison with similarly curved belts elsewhere (e.g., Wyoming, Spain) suggest that few, if any, oroclines are primary features.