BASEMENT FAULTS IN THE EAST TENNESSEE SEISMIC ZONE: OBSERVATIONS FROM THE SWAN CREEK GAS FIELD

New seismic reflection and well data from the Swan Creek gas field, East Tennessee, provide an unusual opportunity to study basement structure and its relation to the New York – Alabama magnetic lineament in the East Tennessee seismic zone (ETSZ). Seismic reflection profiles reveal a system of normal faults or a possibly monoclinal scarp having an aggregate ~450 m of vertical offset, down to the SE, in Precambrian crystalline basement NW of the field. The location and trend of the basement offset are locally coincident with the NY-AL lineament. Crosscutting relationships between the Rome-basement reflector and the Wallen Valley thrust sheet interpreted from seismic profiles show that the basement offset predates the Alleghanian orogeny. The NY-AL lineament is widely thought to be an important boundary in the ETSZ, possibly a through going strike-slip fault along one side of a block of faulted, seismogenic crust. Faults that cut nearly the entire Paleozoic section, from the Rome-basement reflector to Knox unconformity, are visible on four seismic profiles, two dip lines and two strike lines, over the NW limb of the Swan Creek anticline. Taken together, these four occurrences indicate a single, left-lateral strike-slip fault trending east-west. The two directions of basement faults observed at Swan Creek are consistent with the dominant directions (N50°E and N95°E) of seismogenic basement faults in the ETSZ recently interpreted from a statistical analysis of earthquake focal mechanisms, using the best-constrained events. Deformation of the Paleozoic strata above the EW fault is interpreted to result from persistent strike-slip motion along a deeper basement fault. The age of this fault is poorly constrained, but would be younger than the NW movement of the Wallen Valley thrust sheet during the Alleghanian orogeny. No evidence was found in the Swan Creek data for movement along the NE fault, corresponding to the NY-AL lineament, after Alleghanian time.