THE SEISMO-TECTONICS OF NORTH CAROLINA: WHAT WE KNOW FROM EXISTING BROADBAND SEISMIC DATA

Over the course of a full Wilson cycle, the southeastern United States experienced several orogenic episodes that resulted in terrane accretion, followed by the impact with Gondwana that translated the uppermost 5 – 10 km of crust up to several hundred kilometers inland. This translation had made investigations of lower crustal and mantle lithospheric structures across the southeastern United States particularly challenging. The state of North Carolina from west to east ranges from Laurentian basement rocks across the Tugaloo Terrane, Cat Square terrane, and Central Piedmont Suture into Carolinia which likely extends to the coast (at least at the surface). Subsequent Triassic rifting produced the Deep River and Dan River Basins, both of which have recently been considered for potential natural gas exploration. In total, North Carolina provides a remarkable record of a full Wilson’s cycle worth of tectonic activity, including terrane accretions, a continent-continent collision, and subsequent opening of the Atlantic ocean. Despite it’s advantage as a natural laboratory, North Carolina has historically been poorly instrumented with broadband seismic equipment. In 2009 – 2010, we deployed the Appalachian Seismic Transect across the Blue Ridge Escarpment, crossing from Laurentian basement, across the Tugaloo Terrane into the Cat Square Terrane. Our receiver function analyses of these data indicate that the crustal structure in the Blue Ridge is far more complex than previously suspected, and may involve extensive tectonic wedging. A broader analysis of shear wave splitting across the southeast, including limited data from two permanent stations in North Carolina (one of which is no longer operational) may indicate the location of the Central Piedmont Suture at depth in North Carolina, though more data are needed. More recently, we have deployed 12 broadband seismometers in and surrounding the Deep River Basin near Sanford, NC. Preliminary results suggest that this area may be more seismically active than previously expected. The arrival of EarthScope Transporatable Array data and possible future active source research from GeoPRISMS will help answer some of the many questions raised by these small preliminary investigations.