CONTRASTING STRUCTURAL HISTORIES OF THE SALMON RIVER BELT AND THE WALLOWA TERRANE: IMPLICATIONS FOR TERRANE ACCRETION IN WEST-CENTRAL IDAHO

The Salmon River belt (SRB) is composed of metamorphic tectonites with poorly dated Mesozoic protoliths lying between the volcanic terranes of the Blue Mountains province in NE Oregon and the North American continent in western Idaho. In the west, the SRB overlies modestly deformed volcanic rocks of the Wallowa terrane (WT) on a shallow east-dipping fault; to the east, the SRB is separated from continental rocks by the steeply-dipping western Idaho shear zone (WISZ). The SRB is divided into western and eastern assemblages defined by greenschist- and amphibolite-facies tectonites, respectively. The SRB is deformed in three and locally four generations of structures. First-generation structures are synmetamorphic, west-vergent tight to isoclinal folds with a penetrative axial-planar cleavage and down-dip stretching lineation. Second- and third-phase structures are post-metamorphic upright to overturned close to open folds with N-S and E-W trending axial-planar cleavages, respectively. The fourth-generation structures occur along eastern margin where the SRB is overprinted by fabrics of the WISZ. Low-angle faults truncate metamorphic fabric within the SRB and are deformed by second- and third-generation folds. The age of synmetamorphic structures within the SRB is constrained by penetratively deformed Norian carbonates and a post-kinematic tonalite pluton (40Ar/39Ar = 145 Ma) and along the eastern margin of the belt by the steeply east-dipping fabric of the mid-Cretaceous WISZ. Synmetamorphic structures in the SRB formed prior to Late Jurassic amalgamation of the WT and North American fringing-arc complex as part of a regional belt of intra-arc metamorphic tectonites. Implications are that (1) SRB rocks are not part of the WT, and (2) early deformation in the SRB (absent in the WT) is unrelated to terrane accretion in west-central Idaho. Pre-Cretaceous docking of the WT is represented by post-metamorphic structures in the SRB. The WISZ formed after assembly of exotic and North American volcanic arc terranes and records intraplate deformation associated with collapse of the fringing-arc and back-arc basin system.