International audienceAn 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 Intermontane 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 metamorphic 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 melange 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 orogenesi

Sedimentary and volcanic rocks in the Blue Mountains province (BMP) of northeastern Oregon preserve a well studied record of Triassic-Jurassic magmatism, basin evolution, and terrane accretion. Terranes of the BMP represent two magmatic arcs (Wallowa and Olds Ferry terranes), an intervening oceanic subduction and accretionary complex (Baker terrane), and a complex thick succession of sedimentary rocks commonly known as the Izee terrane. We divide volcanic and sedimentary rocks into two regionally correlative, unconformity-bounded megasequences: (1) MS-1, Late Triassic to Early Jurassic deposits that change up section from (1a) older volcanic and volcaniclastic deposits of the Wallowa and Olds Ferry arcs to (1b) marine turbidites, shale, and argillite with chert-clast conglomerate and olistostromes derived from the emergent Baker terrane; and (2) MS-2, Early to Late Jurassic marine deposits that overlap older rocks and structures and record ϳ20 to 40 m.y. of deep crustal subsidence in a large marine basin. Many of the known stratigraphic relationships in the Blue Mountains cannot be explained using the existing model for a Middle Triassic to Late Jurassic west-facing, non-collisional volcanic arc and forearc basin. We propose a new tectonic model for the BMP based on prior studies and comparison to modern analogues, which includes: (1) Middle Triassic magmatism in the Wallowa and Olds Ferry arcs during subduction and progressive closure of an ocean basin; (2) Late Triassic collision between facing accretionary wedges of the Wallowa and Olds Ferry arcs, and growth of marine basins on both sides of the emergent Baker terrane thrust belt; (3) Early to Middle Jurassic terrane-continent collision which resulted in closure of a wide back-arc basin, crustal thickening and loading in Nevada, and growth of a large marine collisional basin in the BMP; and (4) Late Jurassic thrusting, regional shortening, and final accretion of the basin and underlying terranes to western North America. This analysis suggests that collisional tectonics may have played a significant role in plate interactions that drove Triassic-Jurassic crustal thickening, mountain building, and basin development in the western North American Cordillera. introduction 1175 collisional tectonics in the Blue Mountains province, northeastern Oregon

This study assesses early Mesozoic provenance linkages and paleogeographic-tectonic models for the western United States based on new petrographic and detrital zircon data from Triassic andJurassic sandstones of the "Izee" and Olds Ferry terranes of the Blue Mountains Province, northeastern Oregon. Triassic sediments were likely derived from the Baker terrane offshore accretionary subduction complex and are dominated by Late Archean (ca. 2.7-2.5 Ga), Late Paleo proterozoic (ca. 2.2-1.6 Ga), and Paleozoic (ca. 380-255 Ma) detrital zircon grains. These detrital ages suggest that portions of the Baker terrane have a genetic affinity with other Cordilleran accretionary subduction complexes of the western United States, including those in the Northern Sierra and Eastern Klamath terranes. The abundance of Precambrian grains in detritus derived from an offshore complex highlights the importance of sediment reworking. Jurassic sediments are dominated by Mesozoic detrital ages (ca. 230-160 Ma), contain significant amounts of Paleozoic (ca. 290, 380-350, 480-415 Ma), Neoproterozoic (ca. 675-575 Ma), and Mesoprotero zoic grains (ca. 1.4-1.0 Ga), and have lesser quantities of Late Paleoproterozoic grains (ca. 2.1-1.7 Ga). Detrital zircon ages in Jurassic sediments closely resemble well-documented age distributions in transcontinental sands of Ouachita-Appalachian provenance that were transported across the southwestern United States and modified by input from cratonal, miogeoclinal, and Cordilleran-arc sources during Triassic and Jurassic time. Jurassic sediments likely were derived from the Cordilleran arc and an orogenic highland in Nevada that yielded recycled sand from uplifted Triassic backarc basin deposits. Our data suggest that numerous Jurassic Cordilleran basins formed close to the Cordilleran margin and support a model for moderate post-Jurassic translation (~400 km) of the Blue Mountains Province.

Tectonic models for the Mesozoic evolution of accreted island arcs and subduction complexes in the Blue Mountain Province (BMP) of eastern Oregon and western Idaho vary widely. The BMP is situated between coeval accreted terranes of southern British Columbia and the western U.S., and is critical for interpreting Mesozoic paleogeography of the U.S. Cordillera. In this study, we interpret the Triassic-Jurassic evolution of the BMP using geochemistry as a tool to evaluate the provenance of mudrocks in fine-grained turbidites. Temporal and spatial variations in mudrock geochemistry indicate that during Late Triassic to Early Jurassic time the Wallowa terrane was an intra-oceanic island arc and the Olds Ferry terrane was a pericratonic (continental-fringing) island arc. Subsequently, a regionally extensive Middle to Late Jurassic marine basin received input from continental sources and sediment may also have been derived from coeval terranes to the south. Comparison of provenance data from the BMP with similar data from accreted terranes to the north and south suggests that Triassic-Jurassic basins of the western U.S. share a long-term provenance history within a complex system of continent-fringing and intra-oceanic island arcs. In contrast, it appears that the Quesnel terrane of southern British Columbia had a distinct provenance with a more direct link to Laurentia during Triassic time. Early to Middle Jurassic sedimentary rocks from southern British Columbia and the western U.S. show a high degree of geochemical provenance similarity. This may indicate reorganization of formerly discrete tectonic regions into a nascent, integrated western Laurentian margin prior to complete development of a Cretaceous Andean-type margin. The results of this study demonstrate the utility of mudrock geochemistry in tectonic analyses and show that major-and trace-element geochemistry of mudrocks is useful for developing a comprehensive understanding of regional tectonic evolution.

Nd data from the Blue Mountains province, eastern Oregon and western Idaho, clarify terrane correlations and regional evolution of the western Laurentian plate margin during Mesozoic time. We report an Early Jurassic age for a red tuff unit at Pittsburg Landing, Idaho, which is 25 m.yr. older than previous Middle Jurassic estimates. In the Coon Hollow Formation at Pittsburg Landing and at the type location on the Snake River, chemical abrasion thermal ionization mass spectrometry U-Pb zircon ages on interbedded tuff and detrital zircon U-Pb maximum depositional ages indicate that deposition spanned ca. 160-150 Ma, entirely during Late Jurassic time. Detrital zircon U-Pb ages represent local Wallowa arc basement and regional magmatic sources spanning ca. 290-140 Ma. Mudrock Nd isotope compositions of the Coon Hollow Formation record an increase in juvenile magmatism consistent with regional Late Jurassic trends in western North American magmatic systems. These data show that the Coon Hollow Formation is not part of a Middle Jurassic overlap assemblage, as has been historically interpreted. Instead, we propose that the Coon Hollow Formation is part of a belt of suprasubduction-zone extensional back-arc basins that formed in latest Jurassic time due to a well-documented period of trench retreat in the western United States. Our new data require that the underlying Wallowa terrane was accreted to and received detritus from western North America by ca. 160 Ma (early Late Jurassic). This minimum estimate for the age of terrane accretion in western Idaho and eastern Oregon is substantially earlier than previous estimates (∼135-118 Ma). In the Blue Mountains region, westward expansion of Laurentia was accomplished by accretion of arc terranes to the North American craton prior to Late Jurassic time.