Nevin M. Fenneman first described the Blue Mountains Province in a 1931 book, “Physiography of the Western United States.” The Blue Mountains Province, in general, is included within a polygon defined by boundary lines that extend from Oroville, Idaho on the northeast to Huntington, Oregon on the south, from there west to Mitchell, Oregon, then northeast to Dayton, Washington, and east again to Oroville. The Blue Mountains Province, as described, includes the Seven Devils, Wallowa, Elkhorn, Greenhorn and Blue mountain ranges and Canyon Mountain near the town of John Day, Oregon, and Hells Canyon.
The Blue Mountains island arc, in contrast to the Blue Mountains Province, comprises most of the older Permian to Late Jurassic rocks within the Blue Mountains Province. The island arc formed in the ancestral Pacific Ocean as a consequence of two or more oceanic tectonic plates colliding and building the island arc above one or more complex subduction zones. The island arc was accreted to the older North American continent in the Early and Middle Cretaceous.
The Blue Mountains island arc is divided into four major tectonic terranes (Wallowa, Baker, Olds Ferry, and Izee), which are generally in fault contact with one another and represent different parts of one or more volcanic arcs. The Wallowa terrane is defined as the magmatic, or volcanic, axis of an intra-oceanic island arc, like the main volcanic axes of today’s Aleutian and Tonga island arcs, and includes most of the pre-Cenozoic rocks in Hells Canyon and the Wallowa Mountains. The Baker terrane is the fore-arc region (seaward side of the magmatic axis) and is made up of rocks not only deposited on the ancient shelves and slopes of the arc, but also incorporated within the subduction zone of the arc, particularly within an accretionary prism.
Hells Canyon lies within the eastern part of the Blue Mountains Province where the Snake River has cut through older rocks of Paleozoic and Mesozoic ages and younger Miocene rocks of the Columbia River Basalt. Late Cenozoic sediments, including landslide debris, alluvial fan deposits, and Bonneville Flood deposits, cover small parts of the canyon’s floor and lower slopes.
The Olds Ferry terrane may have been a more distant part of the Blue Mountains island arc, but new evidence suggests that the terrane is part of a separate arc which was attached to North America during the Triassic and Jurassic and subsequently added tectonically to the terranes of the intra-oceanic arc. More work is being done on the relationship between the two volcanic arcs.
The Izee terrane is mainly an overlap assemblage of sediments that was shuffled during the island arc’s later movements and during the final accretion to ancient North America. The small Grindstone terrane on the west is included here as part of the Baker terrane.