COSMOGENIC 26AL-10BE BURIAL DATING OF LITTLE TENNESSEE RIVER TERRACE DEPOSITS: AN APPLICATION TO THE EAST TENNESSEE SEISMIC ZONE

Seismicity from the East Tennessee seismic zone (ETSZ) trends NE-SW through Vonore, Tennessee, where the greatest concentration of earthquakes occurs. Vonore is located ~48 km SW of Knoxville, TN. Recent paleoseismic research has been focused here along the Tellico Reservoir on the Little Tennessee River. Mapping of Little Tennessee River terraces prior to the impoundment of the Tennessee Valley Authority Tellico Reservoir identified as many as nine terraces above the modern floodplain (T₀), with a nearby terrace at lake level identified as T₇. Above the mapped T₇, at least two Little Tennessee River terraces (T₈,T₉) are exposed at higher elevations.

These higher terraces were sampled for cosmogenic 26Al-10Be burial dating to determine their ages and if faults previously identified (or unknown others) in this area have displaced them. Samples were taken from eight locations on the north and south sides of the reservoir near previously identified faults. Three additional samples were taken from a Tennessee River terrace exposure at Rocky Hill (in Knoxville), Tennessee. Quartz pebble (1-10 cm) were collected from these high river terraces and processed at the cosmogenic nuclide sample preparation lab at the University of Tennessee base on standard Al-26 and Be-10 sample preparation procedures. Al-26 and Be-10 targets were measured using Accelerator Mass Spectrometer (AMS) at PRIME Lab, Purdue University.

Preliminary calculations of these samples suggest that the Vonore terrace deposits are much older than those exposed at Rocky Hill. The burial ages of the terraces in Vonore are on the order of ~0.8-1.5 Ma, while the Rocky Hill deposits have a much younger age of ~300 ka.

Previous work constrained the ages of younger French Broad River terrace deposits on Douglas Lake (E of Knoxville) based on optically stimulated luminescence (OSL) and revealed even younger ages. The series of terraces in that area is identified as T₀-T₄, with T₄ being the oldest and highest terrace dated. The ages of T₃ and T₄ terraces are as young as ~73 ka, while T₄ yielded an OSL age of ~203 ka.

Comparison of the terrace age data from the Tennessee, Little Tennessee, and French Broad River system suggest a decrease in age of the oldest terraces upstream.