Abstract The Yakima fold belt is a series of asymmetric, east-west anticlines separated by much broader synclines in the thick and otherwise horizontal sequence of Columbia River basalt flows. The folds have grown progressively over the last 17 Ma in response to regional north-south compression and east-west extension. This stress and strain pattern is reflected on a regional scale by the highly oriented north-northwest-south-southeast basalt feeder dykes, by the WNW- (right-lateral) and NNE- (left-lateral) trending conjugate fault system, and by the presence of a similar strain field measurable in the basalt flows today. Peripheral to the Yakima fold belt, pre-basalt structures of different orientations project under the basalt pile. Some of these structures have continued to develop during and after basalt extrusion and have influenced structures in the basalt flows which are, therefore, ‘forced’.

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.