Geomorphic and tectonic development of Swan Valley, SE Idaho, since the eruption of the Basalt of Antelope Flat: new 40Ar/39Ar, major element, and paleomagnetic data
Swan Valley is a graben in eastern Idaho that was formed by extension along the Grand Valley and Snake River faults. It is located within the seismically inactive southeastern arm of the tectonic parabola generated from surface uplift associated with the Yellowstone Hotspot and 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 intersected the South Fork of the Snake River and generated a temporary hyaloclastite dam. Previous workers proposed that the lava dam allowed for the accumulation of water that led to the generation of paleo-Swan Lake. Although lacustrine deposits from paleo-Swan Lake have not been described or mapped, several-meters thick intercalated hyaloclastites and pillow lavas require the interaction between continued volcanism and standing water. In this work, we present new geochemical, geochronologic, and paleomagnetic data to reinterpret the geologic, geomorphic, and structural evolution of Swan Valley. Geochemical and paleomagnetic data show that the deposits mapped as the Basalt of Antelope Flat comprise at least three temporally distinct flows. We obtained a new 40Ar/39Ar age of 0.904 + 0.011 Ma (2σ) for the Basalt of Antelope Flat; this age is markedly younger and significantly more precise than the previously reported K/Ar age of 1.5 ± 0.16 Ma (2σ). Correlation of several dam facies samples across the valley enables us to use the Basalt of Antelope flat as a tiltmeter, from which we conclude that there has been <1° of horizontal rotation of the footwall along the Swan Valley segment of the Gran Valley Fault System since the eruption of the basalt. We also propose that shallow, marshy, wetland conditions existed locally to produce pillow lavas, rather than the previously hypothesized existence of Swan Lake.