TEMPORAL CONSTRAINTS ON DEFORMATION IN THE SOUTHERN APPALACHIAN FOLD-THRUST BELT, USA: INSIGHTS FROM RADIOMETRIC DATING OF FAULT ROCKS AND PALEOMAGNETISM

The age of deformation in the Southern Appalachian fold-thrust belt is difficult to constrain, primarily due to a lack of syntectonic sediments. To better resolve the deformation history of the Southern Appalachians, brittle fault rock samples from thrust exposures in Tennessee and Virginia were analyzed via the well established Illite Age Analysis (IAA) technique, which provides a means to directly date illite rich fault rocks by incorporating illite polytypism and radiometric dating. The extrapolated age of the neomineralized component of illite is generally interpreted to represent the timing of the most recent activity on the fault. Ages were obtained for gouges from the Copper Creek (279.5 ± 11.3 Ma), the St. Clair (276.6 ± 6.8 Ma), Knoxville (276.3 ± 26.9 Ma) and the Great Smoky (277.1 ± 34.0 Ma) thrusts, and for a cataclasite from the Saltville (354.2 ± 9.9 Ma) thrust. The date from the Saltville cannot characterize the timing of fault activity, since the thrust cuts younger Mississippian-aged rocks along strike. Instead, the older age likely represents the age of illitization of the wall rock, as the analyzed sample was a highly polished cataclasite derived primarily from the Devonian Chattanooga Shale in the footwall. However, the four gouge samples have ages that are Early Permian, consistent with the timing of Alleghanian deformation. These results have implications for the mechanics of the Alleghanian orogenic wedge, as the entire foreland thrust sequence was internally deforming around 275-280 Ma. Addtionally, paleomagnetic data from Chickamauga Group carbonates exhibit a syntilting remagnetization in the thrust belt, which, when compared to the Apparent Polar Wander Path (APWP) for Laurentia, suggest a Pennsylvanian age for both the remagnetization and folding. Consequently, the Early Permian illite ages derived from the fault gouges likely represent the final pulse of deformation. Moreover, the thrust ages along with the age of remagnetization reveal a long lived tectonic history in the Southern Appalachian fold-thrust belt.