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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 asso ciated 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 metasedimentary 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 87 Sr/ 86 Sr ratios (0.7033-0.7034) and positive initial ε Nd values (+7.7 to +8.5). In contrast, siliceous argillites from the chert-argillite mélange have initial 87 Sr/ 86 Sr values ranging from 0.7073 to 0.7094 and initial ε Nd 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-Pacifi c region.

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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

[1] The Brazilian Lithosphere Seismic Project (BLSP, a joint project by University of São Paulo and Carnegie Institution, 1992-1999) operated more than 20 temporary broadband stations in the southeastern Brazilian shield. The area, a transect $1000 km long and 300 km wide, covers different geological provinces: the Precambrian São Francisco craton, the adjacent Brasiliano (700 -500 Ma) fold belts, and the Paraná basin of Paleozoic origin. Crustal thicknesses were estimated for 23 sites using receiver functions. For each station, receiver functions were stacked for different sets of earthquakes according to azimuth and distance. The P-to-S Moho converted phase was clearly identified at most sites. Crustal thicknesses were estimated using an average crustal P wave velocity of 6.5 km/s. Poisson's ratio of 0.23 (Vp/Vs = 1.70) was used for the São Francisco craton and adjacent fold belt (based on travel times from small, local earthquakes) and 0.25 was used for the Paraná basin and coastal belt. Crustal thicknesses ranged from 35 -47 km. Although there is a clear inverse correlation between topography and Bouguer gravity anomalies in the study area, Moho depths show the opposite pattern from that expected: areas of low topography and less negative Bouguer anomalies, such as the Paraná basin, have thicker crust (40 -47 km) compared with the high elevation areas of the craton and fold belt (37 -43 km). Two hypothesis are proposed to explain the data: (1) A lower density, by 30 -40 kg/m 3 , in the lithospheric mantle under the Archean block of the São Francisco craton relative to the Proterozoic lithosphere is responsible for maintaining the high elevations in the plateau area. Relatively low density and high P wave velocity are compatible with a depleted (low FeO) composition for the Archean lithosphere. (2) Alternatively, if the density contrasts between Archean and Proterozoic lithospheres are smaller than the values above, then the crust beneath the Paraná basin must be more dense than that of the craton. Higher crustal density and high Poisson's ratio would be consistent with magmatic underplating in the lower crust beneath the Paraná basin, as inferred from other studies.