Abstract

The Mesozoic boundary separating accreted oceanic rocks and continental North America in west-central Idaho forms a prominent right-angle bend, the origin and kinematic significance of which have had a great influence on models for the evolution of the North American Cordilleran orogen. Based on structural and geochronologic relations, we conclude that the bend in the steep crustal boundary is formed by the intersection of two shear zones with different ages. Structural fabrics of the north-striking western Idaho shear zone are overprinted by penetrative structures of the west-northwest–striking Orofino shear zone. U-Pb sensitive high-resolution ion microprobe ages on metaplutonic rocks and late synkinematic leucocratic dikes constrain the age of deformation in the Orofino shear zone to ca. 90–70 Ma, distinctly younger than ca. 120–90 Ma magmatism and deformation within the western Idaho shear zone. The Orofino shear zone is interpreted as a southwest-vergent contractional relay structure that linked basement-cored foreland thrusts of the Middle and Southern Rocky Mountains with the deep crustal duplex of the Shuswap complex in the Canadian Rockies. Truncation ca. 90 Ma terminated Late Cretaceous margin-parallel translation of terranes along the western Idaho shear zone and required displacement to step outboard to poorly understood structures to the west.

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