The “central melange” terrane of eastern Oregon, consisting mainly of an argillite-chert sequence and ophiolitic rocks, and the Seven Devils volcanic-arc terrane of northeastern Oregon and western Idaho probably formed in a volcanic island-arc and back-arc basin environment. New 40Ar-39Ar ages indicate that the ophiolitic rocks formed during the Permian and Late Triassic and that metamorphism synchronous with pervasive orogenic deformation occurred at the end of Triassic time. Accretion of the terrane to the North American continent in Late Jurassic time was accompanied by a second major orogenic event.

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