The pattern of deformation in the western part of the Columbia River flood basalt province contains two key components: (1) anticlinal uplifts of the Yakima Fold Belt with east-northeast to west-southwest trends, and (2) strike-slip fault zones with dominantly northwest trends. It is the abundance and regional extent of the latter that distinguish this area from other parts of the province. There are many northwest-striking, right-lateral, strike-slip faults in the interval from the Willamette Valley eastward to Umatilla (123°W to 119°W longitude). Some of these faults are only a few kilometers long, whereas others are of regional extent (>100 km). Conjugate northeast-striking, left-lateral, strike-slip faults have also been identified but are far less numerous. Local variations in the stress field within basins have produced sets of subsidiary structures by transtension and transpression. These occur where fault zones change trend with respect to the NNW-SSE–oriented maximum principal compressive stress. Strike-slip faulting was active early in the history of the Yakima Fold Belt uplifts, at least by emplacement of the Columbia River Basalt Group lavas, but after the Yakima Fold Belt uplift, spacing had already been firmly established. It is probable that many of these faults are episodically reactivated basement structures that have repeatedly undergone cycles of emergence, burial by flood basalts, and reemergence. Strike-slip deformation appears to have happened simultaneously within the Yakima Fold Belt uplifts and adjacent synclinal basins. However, the pattern and magnitude of deformation differ significantly in the basins compared to the uplifts. The Yakima Fold Belt uplifts have been segmented and shifted many kilometers by strike-slip faults, while displacements within adjacent basins are orders of magnitude less. Within Yakima Fold Belt uplifts, reversals of vergence sometimes occur wherein the frontal (forelimb) thrusts and fold asymmetry switch from one side of the uplift to the other. These changes are accommodated by cross-trending, right-lateral, strike-slip faults of regional extent. The pattern of strike-slip deformation as mapped within basins in many cases appears to be immature and lacking in interconnection. Eruptive vents in the Simcoe backarc volcanic field and Boring lavas are often aligned along strike-slip faults. Pliocene-age Simcoe lava flows have been deformed by both folding and strike-slip faulting within the Klickitat Valley basin. Pleistocene-age deposits are known to be cut by both the Luna Butte and Portland Hills faults. Strike-slip earthquake focal mechanisms have also been determined for some faults.

Deformation of the continental flood-basalt in the westernmost portion of the Columbia Plateau has resulted in regularly spaced anticlinal ridges. The periodic nature of the anticlines is characterized by dividing the Yakima fold belt into three domains on the basis of spacings and orientations: (1) the northern domain, made up of the eastern segments of Umtanum Ridge, the Saddle Mountains, and the Frenchman Hills; (2) the central domain, made up of segments of Rattlesnake Ridge, the eastern segments of Horse Heaven Hills, Yakima Ridge, the western segments of Umtanum Ridge, Cleman Mountain, Bethel Ridge, and Manastash Ridge; and (3) the southern domain, made up of Gordon Ridge, the Columbia Hills, the western segment of Horse Heaven Hills, Toppenish Ridge, and Ahtanum Ridge. The northern, central, and southern domains have mean spacings of 19.6, 11.6, and 27.6 km, respectively, with a total range of 4 to 36 km and a mean of 20.4 km ( n = 203). The basalts are modeled as a multilayer of thin linear elastic plates with frictionless contacts, resting on a mechanically weak elastic substrate of finite thickness, that has buckled at a critical wavelength of folding. Free slip between layers is assumed, based on the presence of thin sedimentary interbeds in the Grande Ronde Basalt separating groups of flows with an average thickness of roughly 280 m. Many of the observed spacings can be explained by this model, given that: (1) the ratio in Young’s modulus between the basalt and underlying sediments E/E o ⩾ 1,000, (2) the thickness of the Grande Ronde Basalt was between 1,200 and 2,300 m when the present wavelengths were established, and (3) the average thickness of a layer in the multilayer is between 200 and 400 m. The lack of well-developed anticline-syncline pairs in the shape of a sinusoid may be the result of plastic yielding in the cores of the anticlines after initial deformation of the basalts into low amplitude folds. Elastic buckling coupled with plastic yielding confined to the hinge area could account for the asymmetric fold geometry of many of the anticlines.