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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.

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