BASEMENT STRUCTURE ASSOCIATED WITH THE EASTERN TENNESSEE SEISMIC ZONE AND POSSIBLE EXTENSION INTO EASTERN KENTUCKY AND WESTERN VIRGINIA

A seismotectonic model for the eastern Tennessee seismic zone (ETSZ) is discussed and evidence is presented for extension of the basement structure into eastern Kentucky and western Virginia. The ETSZ is over 300 km long, extending from northeastern Tennessee into Alabama. In the seismotectonic model, the ETSZ is attributed to reactivation of a major shear zone established during the formation of the super continent Rodinia during the Grenville orogeny. The model is based upon P- and S-wave velocity models for the crust and hypocenter relocations determined using local earthquake tomography. A major feature is the presence of a narrow zone of low velocity trending NE-SW and extending to a depth of at least 24 km. This low velocity zone coincides with the vertical projection of the New York - Alabama (NY-AL) magnetic lineament and is interpreted as a major basement fault. The fault forms the NW boundary of a very seismogenic portion of the basement. Relocated hypocenters tend to align in vertical planes that trend NE-SW and ESE, suggesting reactivation of a distributed zone of deformation associated with a major strike-slip fault. The seismotectonic model is compatible with a diverse set of geophysical and geological observations as well as isotopic constraints on the growth of southeastern Laurentia during the Grenville orogeny. For example, the NY-AL magnetic lineament represents a major Grenville-age basement feature. ETSZ focal mechanisms display strike-sip motion on steeply dipping nodal planes consistent with the trends of the aligned hypocenters. Paleomagnetic polar wander curves for the Grenville orogeny indicate a 2000 km along-strike migration of the Amazon craton relative to Laurentia during the formation of Rodinia. The basement shear zone may extend north of the ETSZ. This is suggested by the focal depth and focal mechanism determined for the 2012 Perry County Kentucky earthquake as well as the close proximity of this earthquake to the NY-AL magnetic lineament. Available focal mechanisms and the distribution of earthquakes on near-vertical planes suggest that the same basement structure is also associated with the Giles County Virginia seismic zone.