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45893-AC8
Curvature of Orogenic Fronts: A Study of the US Appalachians

Ben van der Pluijm, University of Michigan

To test the origin of curvature in the Tennessee salient of the southern Appalachian fold-thrust belt, we conducted an integrated paleomagnetic and calcite twinning study of limestones in the region, as well as paleomagnetic sampling of redbeds.  Samples from thirty-two sites in the Middle to Upper Ordovician Chickamauga Groups limestones and twenty sites from the Middle Cambrian Rome Formation redbeds were analyzed using paleomagnetic methods to constrain the relative age of magnetization as well as the nature of curvature in the Tennessee salient.   Results from three sites of the Silurian Red Mountain Formation were added to an existing dataset in order to determine whether the southern limb had rotated.  After thermal demagnetization, all three sample suites display a down and southeasterly direction, albeit carried by different magnetic minerals.  The syn-tilting direction of the Chickamauga limestones lies on the Late Carboniferous segment of the North American apparent polar wander path (APWP), indicating that deformation was about half completed by the Pennsylvanian.  The Rome and Red Mountain Formations were also remagnetized during the Late Carboniferous.  Both the Chickamauga limestones and Rome redbeds fail to show a correlation between strike and declination along the salient, suggesting either that the salient was a primary, non-rotational feature or that secondary curvature occurred prior to remagnetization. Moreover, remagnetized directions from the Red Mountain sites show no statistical difference between the southern limb of the salient and the more northeasterly trending portion of the fold-thrust belt in Alabama.  All of the studied units in the Tennessee salient are remagnetized and show no evidence for rotation.  This implies that remagnetization was widespread in the southern Appalachians and that any potential orogenic rotation must have occurred prior to the Late Carboniferous.

In order to determine whether early orogenic rotation occurred, we sampled coarse grained limestones for calcite twinning analysis, both from within the thrust belt and in the minimally deformed foreland.  Layer-parallel paleostress orientations at 30 sites within the thrust belt reveal a radial pattern that is systematic along the orogenic front.  However, the degree of fanning (~80°) exceeds the belt’s curvature (<50°), indicating that passive rotation of originally parallel paleostress directions is unlikely.  In addition, results from 27 foreland sites display a similarly fanned pattern of paleostress directions, showing that a radial stress regime was imparted on rocks of this region.  Fanning of paleostress directions preserved in foreland limestones exceeds the degree of curvature of the Tennessee Salient, which, together with the results from the paleomagnetic analysis, 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 and that early orogenic rotation did not occur at anytime during the development of the Tennessee salient. 

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