PATTERNS OF VOLCANIC AND TECTONIC EVENTS IN THE WAKE OF THE YELLOWSTONE HOT SPOT

The Snake River Plain-Yellowstone Plateau (SRP-YP) and volcanic extension to the southwest 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-YP 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. Most major caldera-forming eruptions within a particular field are separated by about 0.2 to 1 Ma, similar to the present-day Yellowstone Plateau volcanic field. Volcanic fields may be separated in time by as much as 2 Ma and in space by 100-150 km from center to center. Passage of the North American plate over the melting anomaly at a particular point in time and space resulted in uplift, regional tectonism, massive explosive eruptions and caldera subsidence, followed by basaltic volcanism and subsidence.

The Heise volcanic field in the eastern SRP, Idaho immediately to the southwest of the Yellowstone Plateau volcanic field, provides a definitive example of SRP-YP volcanic field cycle. Five large-volume rhyolitic ignimbrites constitute a time-stratigraphic framework of late Miocene to early Pliocene volcanism in the eastern SRP. Field relations and high-precision 40Ar/39Ar age determinations establish that four of these regional ignimbrites were erupted from the Heise volcanic field forming the framework of the Heise Group; the source of the fifth unit is outside the Heise field. 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 ± 2 sigma). Facies and magnetic fabric data indicate that these units erupted from separate calderas, each with a set of discrete vents. Many of the volcanic fields along the SRP evolved in a similar fashion following this model.