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An early to mid-Mesozoic record of sedimentation, magmatism, and metamorphism is well developed in the Blue Mountains Province of northeast Oregon. Detailed studies both north and south of the Blue Mountains Province (e.g., terranes of the Inter montane belt, Klamath Mountains, and western Sierra Nevada) have documented a complex Middle to Late Jurassic orogenic evolution. However, the timing of magmatic, metamorphic, and deformational events in the Blue Mountains, and the significance of these events in relationship to other terranes in the western North American Cordillera remain poorly understood. In this study, we investigate the structural, magmatic, and meta morphic histories of brittle to semibrittle deformation zones that indicate widespread Late Jurassic orogenesis in the Blue Mountains Province. Folding and faulting associated with contractional deformation are primarily localized along terrane boundaries (e.g., Baker-Wallowa and Baker--Izee--Olds Ferry boundaries) and within the composite Baker oceanic mélange terrane (e.g., Bourne-Greenhorn subterrane boundary). These brittle to semibrittle deformation zones are broadly characterized by the development of E-W--oriented slaty to spaced cleavage in fine-grained metasedimentary rocks of the Baker terrane (e.g., Elkhorn Ridge Argillite), approximately N-S--bivergent folding, and N- and S-dipping reverse and thrust faulting on opposite flanks of the Baker terrane. Similarly oriented contractional features are also present in late Middle Triassic to early Late Jurassic (i.e., Oxfordian Stage, ca. 159 Ma) sedimentary rocks of the John Day and Huntington areas of northeast Oregon. Radiometric age constraints from youngest detrital zircons in deformed sedimentary rocks and crystallization ages of postkinematic plutons, which intrude the deformation zones, limit deformation to between ca. 159 and ca. 154 Ma. We suggest that the widespread, approximately N-S--directed contractional features in the Blue Mountains Province record a short-lived, intense early Late Jurassic deformational event and preserve an example of upper-crustal strain localization associated with terminal arc-arc collision between the Olds Ferry and Wallowa island-arc terranes. The age interval of deformation in the Blue Mountains Province is younger than Middle Jurassic deformation in the Canadian Cordillera and Klamath Mountains (Siskiyou orogeny) and predates classic Nevadan orogenesis. [ABSTRACT FROM AUTHOR]

The Baker terrane, exposed in the Blue Mountains province of northeastern Oregon, is a long-lived, ancient (late Paleozoic--early Mesozoic) accretionary complex with an associated forearc. This composite terrane lies between the partially coeval Wallowa and Olds Ferry island-arc terranes. The northern margin of the Baker terrane is a broad zone (>25 km wide) of fault-bounded, imbricated slabs and slices of metaigneous and meta-sedimentary rocks faulted into chert-argillite mélange of the Elkhorn Ridge Argillite. Metaplutonic rocks within tectonic units in this zone crystallized between 231 and 226 Ma and have low initial 87Sr/86Sr ratios (0.7033-0.7034) and positive initial εNd values (+7.7 to +8.5). In contrast, siliceous argillites from the chert-argillite mélange have initial 87Sr/86Sr values ranging from 0.7073 to 0.7094 and initial εNd values between -4.7 and -7.8. We interpret this broad, imbricate fault zone as a fundamental tectonic boundary that separates the distal, Wallowa island-arc terrane from the Baker accretionary-complex terrane. We propose that this terrane boundary is an example of a broad zone of imbrication made up of slabs and slices of arc crust tectonically mixed within an accretionary complex, providing an on-land, ancient analog to the actualistic arc-arc collisional zone developed along the margins of the Molucca Sea of the central equatorial Indo-Pacific region. [ABSTRACT FROM AUTHOR]