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The Columbia River flood basalt province covers an area greater than 210,000 km2 in the Pacific Northwest. The province is subdivided into the Oregon Plateau and the Columbia Basin based on significant differences in the style of deformation. The Oregon Plateau contains four structural-tectonic regions: (1) the northern Basin and Range, (2) the High Lava Plains, (3) the Owyhee Plateau, and (4) the Oregon-Idaho graben. The Columbia Basin covers a broader region and consists mainly of the Yakima Fold Belt and the Palouse Slope. Volcanism began in the Oregon Plateau and quickly spread north to the Columbia Basin. In the Oregon Plateau, flood basalt eruptions were contemporaneous with rhyolitic volcanism at the western end of the Snake River Plain hotspot track and with a major period of crustal extension in northern Nevada that began at ca. 16–17 Ma. In the Columbia Basin, a new phase of rapid subsidence folding and faulting of the basalt commenced with the initiation of volcanism but declined as volcanism waned. The coeval development of broad uplifts, subsiding basins, and flood basalt volcanism in the province is consistent with geodynamic models of plume emplacement. However, more specific structures in the province can be linked to older structures in the prebasalt basement. We attribute mid-Miocene deformation and the northward migration of volcanism to a rapidly spreading plume head that reactivated these preexisting structures. Exploitation of such structures may have also played a role in the orientation of many fissure dikes, including rapid eruption of the Steens Mountain shield volcano.

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