Terrane boundaries in the southeastern United States: Insights from seismic anisotropy

Over the course of a full Wilson cycle, the southeastern United States experienced several orogenic episodes that resulted in terrane accretion. Subsequent impact with Gondwana translated the uppermost 5 - 10 km of crust up to several hundred kilometers to the west. Due to this displacement, it has been difficult to positively identify the locations of crustal terrane boundaries at depth. While some terrane boundaries have been inferred from magnetic anomalies (e.g. the Brunswick Magnetic Anomaly/Suwannee Suture), others, like the Carolinia terrane boundary, are clearly expressed at the surface but their locations at depth remain debated. We examine regional patterns of seismic anisotropy as recorded by SKS and SKKS splitting and compare these to magnetic anomalies and Bouguer gravity anomalies. Data for this study come from regional permanent stations and four temporary broadband seismic deployments: the Appalachian Seismic Transect (AST), the Test Experiment for Eastern North America (TEENA), the South Carolina Earth Physics Project (SCEPP), and the Florida to Edmonton Array (FLED). We find broad general patterns of seismic anisotropy that have been noted before: strong anisotropy aligned parallel to North American plate motion to the north and west, and largely null splitting measurements south and east of the Appalachians. However, we find notable exceptions within these patterns that closely align themselves with magnetic anomalies, some of which have been interpreted as terrane boundaries. We suggest that some of the anisotropic signature observed may be due to frozen lithospheric fabric formed along transpressive terrane boundaries. In particular, we argue that the Carolinia terrane boundary at depth is not far removed from its surface expression despite the displacement of the upper crust during the Alleghanian. Future research using data from the SESAME deployment in Georgia and from the EarthScope Transportable Array will allow for a closer evaluation of the correlation between terrane boundaries and seismic anisotropy in the southeastern United States.