Title: The Role of Tectonic Inheritance
Description: The Southeastern US is an ideal location to explore the interactions between mountain-building, rifting, magmatism and intraplate deformation. It experienced the formation of the Southern Appalachians over multiple episodes of orogenesis, continental rifting that formed the South Georgia Rift Basin, and widespread magmatism associated with the Central Atlantic Magmatic Province (CAMP). CAMP was followed by the breakup of Pangea, but the suture between Laurentia and Gondwana from the Appalachian orogeny is preserved in the crust of the Southeastern US. Intraplate seismicity indicates ongoing deformation in the Southeastern US today, but the mechanisms controlling this seismicity are poorly understood. This thesis uses seismic constraints to examine the tectonic history of the Southeastern United States (US). We use new wide-angle refraction seismic data to model crustal and upper mantle velocities in order to investigate the structures formed by mountain-building, rifting, and magmatism.

Broadband seismic data are then used to detect and characterize earthquakes in the central Georgia-South Carolina region. Wide-angle seismic data were collected on three profiles crossing major geological features in Georgia to investigate the tectonic history of the Southeastern United States as a part of the SUwanee Suture and GA Rift basin experiment (SUGAR) project. We model VP and VS of the crust and upper mantle on SUGAR Line 2, which extends from the Inner Piedmont to the Georgia coast. We identify a north to south decrease in upper crustal VP/VS at the Higgins-Zietz magnetic boundary, which other recent studies have identified as the location of the suture between Laurentia and Gondwana. This boundary also lies near the northwestern edge of the South Georgia Rift Basin, the southeastern termination of the low velocity zone interpreted as the Appalachian detachment, and localized crustal thinning. Together, these results provide new evidence in support of the Alleghanian suture being located at the Higgins-Zietz magnetic boundary, and suggest that this orogenic boundary influenced the location of subsequent rifting.

We compare the VP structures of two SUGAR wide-angle transects that cross western and eastern segments of the South Georgia Rift, respectively. Elevated (>7.0 km/s) lower crustal velocities are observed where the rift basin sedimentary fill is thickest and the crust is most thinned. The quantities of mafic magmatic intrusions are consistent with decompression melting at modestly elevated mantle potential temperatures, such as those estimated for CAMP intrusions. These results suggest that, in contrast with the widespread CAMP-aged magmatism at the Earth’s surface, lower crustal magmatic intrusions in the Southeastern US are limited and localized in areas that experienced extension. These new constraints on the velocity structure and tectonic history of the Southeastern United States are then applied to understand earthquakes in the region today. Using broadband seismic data, we find that earthquakes southeast of the Eastern Tennessee Seismic Zone are concentrated within the Carolina Terrane, a particularly heterogeneous accreted terrane of the Southern Appalachians.

Within this terrane, seismicity concentrates near rivers and reservoirs, including a sequence of earthquakes in 2013 associated with an increase in water levels at Thurmond Lake on the Georgia-South Carolina border. Focal mechanisms suggest that the earthquakes are occurring on structures that are oblique to the trend of the Appalachians that are more optimally oriented in the modern stress regime.