The Columbia River flood basalt province covers an area greater than 210,000 km 2 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.

A review of the structures mapped across the Columbia River flood-basalt province reveals a consistent strain pattern from the beginning of the eruptions of the Columbia River Basalt Group (CRBG) until the end of the eruptions of Grande Ronde Basalt (~17.5 to 15.5 Ma). The observed strain is one of north-northwest shortening and west-southwest extension. The degree of strain is small (i.e., extension <<1 percent) and resulted in north-northwest tensional fissures (feeder dikes), approximately east-west folds associated with steep reverse faults, and northwest (right-lateral) and northeast (left-lateral) strike-slip faults. This strain pattern is present from the Brothers fault zone in central Oregon to the northern margin of the CRBG in northern Washington and varies only in its intensity, a factor that can be correlated with the nature of the underlying crust. From approximately 15.5 Ma to the present the same stress pattern continued to deform the flows of the CRBG as they formed north of the Olympic-Wallowa Lineament (OWL), which bisects the Columbia Plateau in a west-northwest direction. South of the OWL the structural pattern changed abruptly at ~15.5 Ma. North-northwest feeder dikes, east-west folds, and northwest and northeast strike-slip faults are replaced by well-developed north-northwest–trending grabens, which indicate a much greater degree of crustal extension (~ 2 0 percent) accompanied by crustal thinning. The change in strain corresponds to a change in the type of volcanicity: from the pre–15.5 Ma flood eruptions of tholeiitic basalt to post–15.5 Ma small volumes of olivine basalt and intermediate to alkalic and silicic compositions erupted locally along the graben faults. The increased crustal extension south of the OWL at ~15.5 Ma implies right-lateral strike-slip motion along that zone, and it is shown that structures previously mapped along the OWL are similar to those mapped along the Brothers fault zone to the south. In both zones the structures are consistent with their interpretation as right-lateral megashears. A tentative model is introduced in which the deformation pattern of the whole Columbia River flood-basalt province is related to oblique subduction to the west and the back-arc spreading associated with the Basin and Range crustal extension to the east.