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New 40 Ar-39 Ar geochronology, bulk rock geochemical data, and physical characteristics for representative stratigraphic sections of rhyolite ignimbrites and lavas from the west-central Snake River Plain (SRP) are combined to develop a coherent stratigraphic framework for Miocene silicic magmatism in this part of the Yellowstone 'hotspot track'. The magmatic record differs from that in areas to the west and east with regard to its unusually large extrusive volume, broad lateral scale, and extended duration. We infer that the magmatic systems developed in response to largescale and repeated injections of basaltic magma into the crust, resulting in significant reconstitution of large volumes of the crust, wide distribution of crustal melt zones, and complex feeder systems for individual eruptive events. Some eruptive episodes or 'events' appear to be contemporaneous with major normal faulting, and perhaps catastrophic crustal foundering, that may have triggered concurrent evacuations of separate silicic magma reservoirs. This behavior and cumulative time-composition relations are difficult to relate to simple caldera-style single-source feeder systems and imply complex temporal-spatial development of the silicic magma systems. Inferred volumes and timing of mafic magma inputs, as the driving energy source, require a significant component of lithospheric extension on NNWtrending Basin and Range style faults (i.e., roughly parallel to the SW-NE orientation of the eastern SRP). This is needed to accommodate basaltic inputs at crustal levels, and is likely to play a role in generation of those magmas. Anomalously high magma production in the SRP compared to that in adjacent areas (e.g., northern Basin and Range Province) may require additional sub-lithospheric processes.

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 fi xed 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 fi nal 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 fi elds comprised of nested caldera complexes with major caldera-forming eruptions within a particular fi eld separated by ca. 0.5-1 Ma, similar to, and in continuation with, the present-day Yellowstone Plateau volcanic fi eld. 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 fi eld in the eastern SRP, Idaho, represents an adjacent and slightly older fi eld immediately to the southwest of the Yellowstone Plateau volcanic fi eld. Five large-volume (>0.5 km 3) rhyolitic ignimbrites constitute a time-stratigraphic framework of late Miocene to early Pliocene volcanism for the study region. Field relations and high-precision 40 Ar/ 39 Ar age determinations establish that four of these regional ignimbrites were erupted from the Heise volcanic fi eld 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 fi fth 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 fi eld. These results establish the Conant Creek Tuff as a distinct and widespread ignimbrite in the Heise volcanic fi eld, eliminating former confusion resulting from previous discordant K/Ar and fi ssion-track dates. New 40 Ar/ 39 Ar determinations, when combined with geochemical, lithologic, geophysical, and fi eld data, defi ne the volcanic and tectonic history of the Heise volcanic fi eld and surrounding areas. Volcanic units erupted from the Heise volcanic fi eld 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 km 3) slide blocks of Mississippian limestone exposed 50 km to the east of the Heise fi eld 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 signifi cant relief by at least 5.5 Ma. In Jackson Hole, the distribution of outfl ow facies of the 4.45 ± 0.05-Ma Kilgore Tuff rel...

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