Abstract

Structural analysis of two areas in central Idaho indicates the presence of large, steep-plunging sigmoidal folds. These structures bend schistosity and early fold axes, and one of the structures is cut by Cretaceous quartz monzonite of the Idaho batholith. A large, synkinematic orthogneiss body which intrudes the core of one of these folds has a weak schistosity parallel to the axial surface of the fold. The sigmoidal folds may be from latest Precambrian to early Cretaceous in age.

These large sigmoidal folds are similar in style, attitude, and direction of tectonic transport to the much larger Cordilleran Orocline proposed by Carey (1958). The presence of the folds in central Idaho supports Carey's concept of translational rotation of the northwestern United States in response to deep crustal movements.

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