DEVELOPMENT OF THE CURVED SEGMENTS OF THE US APPALACHIANS: A COMPARISON OF THE PENNSYLVANIA AND TENNESSEE SALIENTS

It has long been suggested that the sinuous trend of the Appalachian thrust belt reflects preexisting margin irregularities and associated depositional facies distributions (Thomas, 1977). However, the manner in which these curved segments formed is still a topic of debate, particularly with regard to rotations. Here we compare results from the Pennsylvania and Tennessee salients, both of which exhibit a curvature of ~60°. Primary paleomagnetic directions from redbeds in the Pennsylvania salient revealed a clockwise rotation of the northern limb of the salient relative to the southern limb. Structural data, including recent calcite twinning analysis, support this hypothesis. However, secondary magnetizations in redbeds and carbonate rocks in Pennsylvania were acquired after these rotations took place, since they do not show significant declination deviations. New paleomagnetic data from redbed units of the Tennessee salient, including the Cambrian Rome and the Silurian Red Mountain formations, were obtained. Paleomagnetic analysis of three sites in the Red Mountain Formation along the southern limb of the Tennessee salient (strike ~10°) reveals a prefolding, characteristic remanent magnetization (ChRM) that is statistically indistinguishable from Red Mountain ChRM's of previous workers from the adjacent Alabama recess (strike ~45°). Additionally, twenty sites were collected in the Rome Formation along the salient. Whereas the data show some scatter, no significant correlation of declination with regional strike is observed along the curved front of the Tennessee salient. Both the Red Mountain and Rome formations are likely secondary magnetizations of Carboniferous-Permian age. In contrast, calcite twinning analysis from limestone samples along the salient display a systematically fanned distribution of paleostress orientations, similar to the Pennsylvania salient. Thus, the paleomagnetic declination patterns in the Tennessee salient preclude late Paleozoic rotations, but permit earlier rotation as indicated by calcite twinning orientations; this is remarkably analogous to the scenario of the Pennsylvania salient.