QUATERNARY FAULTING IN BLOUNT COUNTY TENNESSEE AND ITS IMPLICATIONS FOR ACTIVE TECTONISM IN THE EASTERN TENNESSEE SEISMIC ZONE

The East Tennessee Seismic Zone (ETSZ) in the southern Appalachians is the second most active east of the Rockies. The trend of seismicity approximates the regional NE-SW strike of Paleozoic bedrock and is confined to mostly the Valley and Ridge, bounded to the east by the Blue Ridge and the Cumberland Plateau to the west. Surficial deposits in this area include: Quaternary fluvial strath or cut and fill terraces, colluvium, and residuum.

Recent fieldwork within the ETSZ documents a N60°E zone of Quaternary faulting containing several NE-SW striking thrust, normal, and strike-slip faults in exposures of Quaternary alluvium as well as numerous NE-SW trending Quaternary cobble-filled fractures in Paleozoic shale and shale saprolite along the Little Tennessee River, French Broad River, and now the Little River. Available OSL dates delimit faulting to <25 ka. A newly recognized fault (N70°E, 40°SE) exposed along the Little River in Blount County, TN reveals two episodes of thrust displacement of Quaternary alluvium with 1 m of cumulative dip slip and 80 cm of structural relief. Each event produced a scarp and colluvial wedge. The event horizon of the first paleo-earthquake shows no sign of weathering or soil formation, so the two events may have been relatively close in time. Based on dip slip, magnitudes of these events were >M6. OSL ages for alluvium bracketing these two paleo-earthquakes are forthcoming.

Some ETSZ Quaternary faults have been found in Paleozoic shales with approximately the same bedding attitude as that of the Quaternary faults (N45-075°E, dip SE), suggesting there may be local co-seismic slip on bedding during seismogenic faulting at depth. The modern N70°E S_Hmax may exploit favorably oriented, pre-existing planes of weakness in near-surface Paleozoic shales during rupture on a N60°E fault at depth. Thus, Paleozoic structural trends and lithologic strength may place controls on the orientation and formation of active surface-rupturing faults.