GEOPHYSICAL EVIDENCE FOR TERRANE TRANSFER DURING THE GRENVILLE OROGENY

Geophysical techniques including local earthquake tomography and potential field analysis are used to investigate a terrane boundary established during the Grenville orogeny. The earthquakes form the active eastern Tennessee seismic zone (ETSZ) and are located in Grenville basement rock. The ETSZ trends NE-SW and is about 300 km long. Unlike other intraplate seismic zones, the ETSZ is not associated with an ancient rift structure but is associated with a major basement structural feature of some kind. This is suggested by a remarkable correlation of the ETSZ with the prominent New York – Alabama (NY-AL) magnetic lineament. The NE-SW trending NY-AL magnetic lineament probably represents a major basement fault that played an important but enigmatic role in the construction of the supercontinent Rodinia during the Grenville orogeny. High-resolution velocity models determined for the ETSZ reveal the presence of a sharp velocity contrast associated with the vertical projection of the NY-AL magnetic lineament that extends to depths of at least 24 km. A band of anomalously low velocity is present to the NW of the lineament and anomalously high velocity is present to the SE. The low velocity band is interpreted as a major basement fault. The high velocity region hosts the earthquake activity and is characterized by very low magnetic anomalies and slightly elevated Bouguer gravity anomalies. Plausible rock types for the high velocity region include granodiorite and felsic granulite suggesting that earthquake activity is occurring in basement rocks that are similar to Grenville basement exposures in the Appalachians. Relocated earthquakes align in near-vertical segments trending roughly parallel to the NY-AL magnetic lineament. Earthquakes in the most seismogenic portion of the ETSZ delineate another set of near-vertical faults trending roughly E-ESE. These apparent trends and steep dips are compatible with ETSZ focal mechanism solutions. The solutions are remarkably consistent and indicate strike-slip motion along the entire length of the seismic zone. We suggest that ETSZ earthquakes represent reactivation of a particularly well-preserved, distributed zone of deformation that involved transfer of the southern Appalachian basement to Laurentia along a major strike-slip fault during the Grenville orogeny.