Creation and evolution of the Columbia River Basalt Province occurred in late Cenozoic time, during an 11-m.y. interval in which most of the volcanism was centered in the east and southeast part of the province. Tectonism contemporaneous with volcanism resulted in: (1) progressive rise of the eastern margin of the Columbia Plateau and creation of a westerly dipping regional paleoslope; (2) development of uplifted plateau surfaces (Nez Perce plateau and Joseph plains) isolated from further (younger) volcanism; (3) initiation of the Lewiston, Troy, and Stites structural basins; and (4) initiation of the Blue Mountains uplift of southeast Washington and northeast Oregon. Although deformation was continuous throughout the eruptive episode, it is most evident during its later, waning stages. Deformation continued after cessation of volcanism about 6.0 m.y. B.P. A combination of epeirogeny and a stress regime, having a horizontal NNW-SSE axis of maximum compression and a horizontal WSW-ENE axis of minimum compression (tension), imposed on basalt overlying an older structural grain can account for most structural elements in the southeast part of the province.

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