Magnetic fabric of Late Jurassic arc plutons and kinematics of terrane accretion in the Blue Mountains, northeastern Oregon

The Blue Mountains Province of northeastern Oregon, western Idaho, and southeastern Washington (USA) consists of the amalgamated Wallowa distal island arc, Baker mélange-bearing accretionary wedge–forearc, and Olds Ferry fringing island arc terranes. Anisotropy of magnetic susceptibility (AMS) and particularly the orientation of magnetic lineations in Middle to Late Jurassic diorite–tonalite–granodiorite plutons intruding the Baker terrane indicate a change in tectonic regime from flattening with principal stretching at a high angle to the local orientation of the terrane boundaries at around 160 Ma to constriction with boundary-subparallel subhorizontal stretching at ca. 146 Ma. The former magnetic fabric is compatible with magnetic lineations recording strain related to top-to-the-southwest back-thrusting onto the Olds Ferry arc. In contrast, the ~ 146 Ma boundary-parallel stretching may record strain partitioning whereby a part of the Baker terrane was extruded laterally parallel to terrane boundaries after the terrane collision. After restoration for the clockwise post-mid-Cretaceous terrane rotation, the ~ 160 Ma restored principal stretching directions are compatible with the overall left-oblique subduction and convergence of the Wallowa distal and Olds Ferry fringing arc terranes. Magnetic fabric data from the plutons thus support the interpretation that the early displacement of the Blue Mountains Superterrane, and perhaps also its possible correlative Intermontane Superterrane of the Canadian Cordillera, was southward along the truncated North American continental margin during the late Middle Jurassic.

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► AMS in Baker terrane records thrusting at 160 Ma and lateral transport at 146 Ma.
► AMS is compatible with sinistral terrane convergence.
► Mid-Jurassic terrane displacement was southward along North American craton.