A QUATERNARY RECORD OF VOLCANISM, LAKES, AND SEISMICITY IN SWAN VALLEY, IDAHO

Swan Valley is a graben in eastern Idaho that was formed by extension along the Grand Valley and Snake River faults. The area is within the southeastern arm of the tectonic parabola generated from surface uplift associated with the Yellowstone hotspot. The valley is regionally significant because it preserves a record of explosive rhyolitic volcanism sourced from the Yellowstone and Heise Volcanic Fields, as well as locally sourced basaltic lavas. The Basalt of Antelope Flat flowed into the South Fork and Pine Creek Canyons, through which the modern Snake River flows; thus, the Basalt of Antelope Flat may have temporarily dammed the Snake River, thereby forming paleo-Swan Lake. Although sedimentary lacustrine deposits of paleo-Swan Lake have not been described or mapped, several-meters thick intercalated hyaloclastites and pillow lavas indicate the interaction between continued volcanism and deep standing water that could have been paleo-Swan Lake. If so, then the lake must have eventually overtopped, incised, and eroded the volcanic dam to form the present-day Snake River Canyon. Previous whole-rock K/Ar dating of the Basalt of Antelope Flat determined an eruption age of 1.5 ± 0.8 Ma (1σ); this large uncertainty neither precisely constrains the age of the lake, nor permits an accurate incision rate calculation of the Snake River Canyon. Here, we determine a new ⁴⁰Ar/³⁹Ar eruption age for the Basalt of Antelope Flat via incremental heating of basaltic groundmass to better estimate the age of paleo-Swan Lake. We supplement this work with paleomagnetic data to precisely correlate our field locality with other dam facies outcrops within the valley, which enables us to estimate the regional attitude of the unit. By comparing the dip of the younger dam facies with older mapped units in Swan Valley, we provide first-order estimates on the Quaternary slip and tilting rates within the graben associated with the Grand Valley Fault.