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The Wallowa terrane in the Blue Mountains of northeastern Oregon consists of a Permian, and possibly older, volcanoplutonic island-arc complex built on oceanic lithosphere that is also basement to a Middle to Late Triassic arc complex. The two arc complexes, separated commonly by Mesozoic thrust faults, supplied detritus to a Late Triassic–Early Jurassic marine basin. The adjacent Baker terrane is an intervening mélange containing tectonic slices of the Wallowa and the Olds Ferry terranes. Near the suture with the Olds Ferry composite terrane, the Baker terrane is blueschist bearing. Previous work indicates that three deformational episodes involving contraction affected all terranes of the Blue Mountains from Late Triassic through Late Jurassic. However, this study also documents static hydrothermal metamorphism, Mstatic, that is earlier than Late Triassic D1 deformation. In the Wallowa terrane, D1 deformation is recognized locally in the form of transpressional mylonitic shearing but has the same timing as transtensional arc volcanism and basinal development. In the adjacent Baker terrane, D1 deformation formed S1 penetrative foliation in blueschists and greenschists during subduction, probably prior to juxtaposition with the Olds Ferry terrane. Middle to early Late Jurassic folding, D2, and formation of penetrative axial planar foliation (S2) involved a separate contractional event that affected mainly the Wallowa Triassic and Jurassic arc and basin. The D3 structures refold D2 structures in the Wallowa terrane and truncate them during underthrusting of Late Triassic to Early Jurassic arc and sedimentary rocks beneath the Permian arc. The D3 thrusting may reflect southward transport of the Peninsular or Wrangellian superterrane outboard of the active Mesozoic arc and apron.

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