Contrasting structural histories of the Salmon River belt and Wallowa terrane: Implications for terrane accretion in northeastern Oregon and west-central Idaho
The Salmon River belt is composed of metamorphic tectonites with poorly dated Mesozoic and Paleozoic protoliths that lie between volcanic arc assemblages of northeastern Oregon and the western margin of the North American continent in west-central Idaho. To the west, the Salmon River belt overlies late Paleozoic and Mesozoic volcanic arc rocks of the Wallowa terrane on a shallowly east-dipping fault; to the east the belt is separated from North American rocks by the western Idaho shear zone. The Salmon River belt is divided into western and eastern assemblages composed of greenschist- and amphibolit-facies metamorphic tectonites, respectively, and is deformed in three and locally four generations of penetrative structures. First-generation structures are synmetamorphic, west-vergent, tight to isoclinal folds with a penetrative axial-planar cleavage and downdip stretching lineation. Second- and third-phase structures are postmetamorphic and consist of open to close, upright to slightly overturned folds with north-south and east-west trending penetrative axial-planar cleavages, respectively. Low-angle faults truncate metamorphic structures within the Salmon River belt and juxtapose rocks with different metamorphic grades. The faults formed together with second-generation structures and are deformed by third-generation folds. The age of metamorphic and postmetamorphic structures is constrained by deformed carbonate rocks of Late Triassic age and clastic rocks of late Early Jurassic age and by the ca. 145 Ma emplacement of an undeformed tonalitic pluton. Along the eastern margin of the Salmon River belt, structures are overprinted and transposed by a younger, steeply east-dipping mylonitic shear zone, the mid-Cretaceous western Idaho shear zone. Synmetamorphic structures within the Salmon River belt formed before amalgamation of the Wallowa terrane and the North American fringing-arc complex and developed as part of a regional belt of intra-arc metamorphic tectonites. The pr-Cretaceous docking of the Wallowa terrane is recorded within the Salmon River belt by development of postmetamorphic structures and predates formation of the western Idaho shear zone. The western Idaho shear zone formed after the amalgamation of the arc terranes and records the intraplate deformation associated with the collapse of the fringing-arc and back-arc-basin system.