Distribution and geochronology of Oregon Plateau (U.S.A.) flood basalt volcanism: The Steens Basalt revisited

The timing and petrogenesis of mid-Miocene flood basalt volcanism in the northwest United States has been extensively addressed, yet the chemical characteristics and temporal details of the Steens Basalt, exposed on the Oregon Plateau, are poorly defined. Steens Basalt volcanism has generally been accepted to have occurred at ∼ 16.6 Ma, coeval and/or just prior to the onset of Columbia River Basalt Group volcanism to the north. New major and trace element analyses and nine 40Ar/39Ar ages ranging from 15.51 ± 0.28 to 16.58 ± 0.18 Ma were obtained on Oregon Plateau flood basalt lava flows from stratigraphic sections in close proximity to Steens Mountain. Additionally, new 40Ar/39Ar ages were obtained on the uppermost and thirty-first lava flow down from the top of the ∼ 1 km section of Steens Basalt exposed at Steens Mountain and yield eruption ages of 16.59 ± 0.10 and 16.55 ±0.10 Ma, respectively. Field relations between these basalt sections suggest that multiple eruptive centers were present in the vicinity of Steens Mountain.The chemical and chronologic data presented in this study illustrate that flood basalts with the “Steens Basalt” chemical signature erupted across the southern Oregon Plateau over a much greater timespan than what is typically quoted for the Steens Mountain type section. These data suggest that the main volume of Steens Basalt volcanism erupted over at least an ∼ 1 m.y. duration from eruptive loci in the vicinity of Steens Mountain, while likely much less voluminous volcanism and lithospheric input of mafic magma appears to have occurred for > 2 Ma across the Oregon Plateau. These new geochemical and geochronological constraints verify a common temporal link between Steens, Malheur Gorge-region, and Columbia River Basalt Group (CRBG) volcanism. This direct temporal link requires that petrogenetic and tectonic models of mid-Miocene northwestern U.S. flood basalt volcanism recognize that the northern (Columbia Plateau) and southern (Oregon Plateau) portions of this mid-Miocene basalt province were erupting simultaneously during portions of the regional event.