Sheeted porphyry dike swarms are concentrated along and parallel to the margins of the Republic and Keller grabens in northeast Washington. They were emplaced during graben subsidence and, in part, during mylonitization of the Kettle and Okanogan domes (core complexes) above the transition into the brittle deformation zone during Eocene regional extension. The swarms consist of older high-K biotite rhyolite and younger high-K hornblende dacite and andesite dikes. The biotite rhyoGte is correlative with the basal O'Brien Creek Formation now exposed in the grabens but has no documented plutonic equivalent in the adjacent Colville batholith. The hornblende dacite and andesite dikes (Scatter Creek Formation) are divided into (1) a low-Zr, low-K2O type that fed the lower part of the Sanpoil Volcanics and (2) a high-Zr, high-K2O type that fed the upper part of the Sanpoil Volcanics, respectively. The Scatter Creek Formation dikes are the hypabyssal equivalents of both the Devils Elbow intrusive suite and the earlier phases of the Herron Creek intrusive suite in the Colville batholith. The occurrence of the dike swarms both within and adjacent to grabens filled with comagmatic volcanic rocks demonstrates that the graben fill was deposited in situ during regional extension and is not an allochthonous fault block transported to its present location during regional compression.