A composite section of eight Grande Ronde Basalt flows delineates the margin of the Columbia River Basalt Group on this portion of the eastern flank of the Cascade Range. The Grande Ronde Basalt flows belong to the following units (in descending stratigraphic order): Sentinel Bluffs Member (Basalt of Museum 2 and Museum 1; Basalt of Stember Creek; and upper and lower flows of the Basalt of McCoy Canyon), Ortley member (informal), Grouse Creek member (informal “Meeks Table” flow), and Wapshilla Ridge Member. All these Grande Ronde Basalt flows display similar intraflow structures (cooling joint patterns) and lithology, but they are separable by chemical compositions (i.e., TiO 2 , MgO, P 2 O 5 , Cr, Ba, and Zr). Individual Grande Ronde Basalt flows can range in thickness from 8 to 180 m, with the maximum total thickness of the Grande Ronde Basalt section being 555 m. As the Grande Ronde Basalt flows advanced into the map area, they covered plains, filled stream-cut valleys and canyons up to 160 m deep, and surrounded extinct volcanoes 750 m tall. In post–Grande Ronde Basalt time, the Grande Ronde Basalt flows were deformed into a series of ENE-striking anticlines, synclines, and associated faults that define this portion of the Yakima Fold Belt. During this same time, transpressional deformation activity increased folding and thrust faulting in the Cle Elum–Wallula Lineament, a structural segment of the Olympic-Wallowa Lineament. In addition, series of NNW-striking, dextral strike-slip and normal faults were developed with displacements up to 4.5 km on the strike-slip faults and 1 km on the normal faults. The N-striking Goat Creek and NW-striking Indian Flat and Devils Slide faults merge with the White River fault to the west. These faults, along with the E-NE–striking Bethel Ridge anticline and NNW-striking Cleman Mountain anticline, form the major structures in this area.

Recent mapping of pre-basalt rocks along the northwestern Columbia River basalt margin and well logs from Shell Oil Company gas wells provide new information about the rocks and structure underlying the Yakima fold belt. Pre-basalt rocks along the margin range in age from Jurassic to lower Miocene, with early to middle Tertiary arkosic and volcaniclastic strata concentrated in three fault-bounded basins. With one exception, pre-basalt rocks cut by the Shell Oil Company wells (Yakima Minerals, Bissa, and Saddle Mountains) can be correlated with rocks found in the basins along the margin. These rocks extend under the Columbia River Basalt Group almost to the center of the Columbia Basin. Two major features, the Leavenworth–Hog Ranch cross-structure and the White River–Naches River fault zone, affect the distribution of sedimentary rock types. Based on well and geophysical data, the Columbia River Basalt Group thins across the Hog Ranch–Naneum Ridge structure, suggesting that this feature was active during Miocene time. The northwestern Columbia River basalt margin is the focus of major structural elements that converge on the Yakima fold belt, including the Olympic-Wallowa lineament (OWL), the Cle Elum–Wallula lineament (CLEW), the Hog Ranch–Naneum Ridge cross-structure, the Chiwaukum graben, and the White River–Naches River fault zone. In the area of CLEW, splays of the Straight Creek fault turn southeast and pass under the Columbia River Basalt Group, aligning with folds of the Yakima fold belt. Elsewhere along the margin, there is little expression of sub-basalt structure in the overlying Columbia River basalt. The Columbia River Basalt Group, at the margin, exhibits an absence of faulting and displays only broad, gentle folds. Closely spaced, tight folds and associated faults in the interior of the Yakima fold belt either die out before reaching the margin or become broad, gentle flexures.

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.