THE GREAT VOLCANIC OUTBURST INITIATING THE YELLOWSTONE HOTSPOT

Miocene initiation of the Yellowstone hotspot began in southeastern Oregon with an outburst of basaltic volcanism that continued, unabatedly, over six paleomagnetic intervals (R0-N2), from ~16.6 to ~15.0 Ma. This massive volcanic disturbance represents the main phase of the Columbia River Basalt (CRB) eruptions, generating over 220,000 km³ of Steens, Imnaha, Grande Ronde, and Picture Gorge basalt in ~1.5 m.y., the greatest eruptive volume of continental basalt on Earth over the past 30 million years. Current knowledge of this well-exposed succession, connected in space and time to the Yellowstone hotspot track, provides an unusual opportunity to describe the gross chemical and physical evolution of a continental plume head-plume tail pair.

The main-phase eruptions contain two age-progressive, and partly contemporaneous, chemical trends: (1) primitive tholeiitic to mildly alkalic lavas of lower and upper Steens basalt, respectively, and (2) more evolved tholeiitic lavas of Imnaha and Grande Ronde basalts. The initial eruptions of lower Steens basalt (R0) were spread over a broad region, from opposing sides of the Western Snake River Plain in Idaho to as far west as the Warner Range in northeastern California. The eruptions quickly became more localized by N0 time, as upper Steens lavas began to erupt in the vicinity of the Steens and Pueblo Mountains, and Imnaha lavas began to erupt simultaneously north of Steens Mountain. By R1 time, the upper Steens alkalic volcanism had ceased, and the main focus of tholeiitic volcanism began to shift rapidly to the north into the Chief Joseph and Monument dike swarms. This rapid migration is revealed in the progressive northward thickening and offlap of younger magnetostratigraphic units of the Grande Ronde (R1-N2) and Picture Gorge (N1-R2) basalts, respectively.

Lava flows associated with these two chemical trends are distinct in both their eruptive style and physical volcanology. The initial Steens basalt eruptions generated thin, complex flows from widespread sources, becoming more centralized with time at the Steens Mountain shield volcano. These early lavas are similar in form to those found along the remainder of the hotspot track, but they bear little resemblance to the massive, fissure-fed, and often inflated sheet flows associated with the younger CRB eruptions.