MIOCENE TO PLEISTOCENE VOLCANISM AND RIFTING, WESTERN SNAKE RIVER PLAIN, SOUTHWESTERN IDAHO

The western Snake River Plain (SRP) is a 160-by-60-km complex graben that stretches NW from the middle of the main SRP trend. While the main SRP follows the Yellowstone hotspot track, the western SRP is a rift that was widened parallel to the hotspot path. Displaced structural markers on its margins suggest crustal extension accounts for nearly half of the rift's width. Between 11.6 and about 10.0 Ma ago, when the western SRP started to form, rhyolite lava flows were erupted. The western SRP rift merges into the Bruneau-Jarbidge eruptive center (BJEC) on the main SRP trend. The BJEC is a 55-by-95-km structural basin which coalesced as successive calderas formed during the eruption of high-temperature rhyolitic ignimbrites, also about 11.6 to 10.0 Ma ago. More than 30 rhyolite flows occur along the western SRP margins. Many of the rhyolite vents show spatter accumulations, and phreatomagmatic deposits accompany some.

Two basaltic eruptive episodes accompanied the western SRP rifting, producing more than 130 vents. The first episode was from about 9 to 7 Ma ago, and the second was from about 2.2 to 0.1 Ma ago. Much of the 9-7 Ma basalt erupted subaqueously, whereas most of the 2.2-0.1 Ma basalt erupted phreatomagmatically, or more passively in groundwater-poor areas. Most of the basalt is olivine tholeiite or ferrobasalt. Toward the end of the younger basalt episode, however, the basalt became increasingly alkalic. Much of the ferobasalt probably was melted from differentiated mafic plutons that had been emplaced earlier beneath the western SRP, whereas the late alkalic basalt may have risen from deeper in the Earth.