Abstract

The Snake River Plain (SRP) developed over the last 16 Ma as a bimodal volcanic province in response to the southwest movement of the North American plate over a fixed melting anomaly. Volcanism along the SRP is dominated by eruptions of explosive high-silica rhyolites and represents some of the largest eruptions known. Basaltic eruptions represent the final stages of volcanism, forming a thin cap above voluminous rhyolitic deposits. Volcanism progressed, generally from west to east, along the plain episodically in successive volcanic fields comprised of nested caldera complexes with major caldera-forming eruptions within a particular field separated by ca. 0.5–1 Ma, similar to, and in continuation with, the present-day Yellowstone Plateau volcanic field. Passage of the North American plate over the melting anomaly at a particular point in time and space was accompanied by uplift, regional tectonism, massive explosive eruptions, and caldera subsidence, and followed by basaltic volcanism and general subsidence.

The Heise volcanic field in the eastern SRP, Idaho, represents an adjacent and slightly older field immediately to the southwest of the Yellowstone Plateau volcanic field. Five large-volume (>0.5 km3) rhyolitic ignimbrites constitute a time-stratigraphic framework of late Miocene to early Pliocene volcanism for the study region. Field relations and high-precision 40Ar/39Ar age determinations establish that four of these regional ignimbrites were erupted from the Heise volcanic field and form the framework of the Heise Group. These are the Blacktail Creek Tuff (6.62 ± 0.03 Ma), Walcott Tuff (6.27 ± 0.04 Ma), Conant Creek Tuff (5.51 ± 0.13 Ma), and Kilgore Tuff (4.45 ± 0.05 Ma; all errors reported at ± 2σ). The fifth widespread ignimbrite in the region is the Arbon Valley Tuff Member of the Starlight Formation (10.21 ± 0.03 Ma), which erupted from a caldera source outside of the Heise volcanic field. These results establish the Conant Creek Tuff as a distinct and widespread ignimbrite in the Heise volcanic field, eliminating former confusion resulting from previous discordant K/Ar and fission-track dates.

New 40Ar/39Ar determinations, when combined with geochemical, lithologic, geophysical, and field data, define the volcanic and tectonic history of the Heise volcanic field and surrounding areas. Volcanic units erupted from the Heise volcanic field also provide temporal control for tectonic events associated with late Cenozoic extension in the Snake Range and with uplift of the Teton Range, Wyoming. In the Snake Range, movement of large (≥0.10 km3) slide blocks of Mississippian limestone exposed 50 km to the east of the Heise field occurred between 6.3 and 5.5 Ma and may have been catastrophically triggered by the caldera eruption of the 5.51 ± 0.13-Ma Conant Creek Tuff. This slide block movement of ∼300 vertical meters indicates that the Snake Range had significant relief by at least 5.5 Ma. In Jackson Hole, the distribution of outflow facies of the 4.45 ± 0.05-Ma Kilgore Tuff related to eruption from the Kilgore caldera in the Heise volcanic field on the eastern SRP indicates that the northern Teton Range was not a significant topographic feature at this time.

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