Décollement folding of massive (> 2 m thick) carbonate beds of the Scott Peak Formation, within the Willow Creek anticline, northern Lost River Range, Idaho was associated with the development of thin (<0.25 m), widely-spaced deformation zones. Incremental strain histories determined from fibrous overgrowths in strain fringes from the deformation zones provide a basis for kinematic analysis of the anticline. Plane strain is indicated by coaxial up-dip extension on cleavage (XY) planes and a lack of extension in the Y direction. In XZ sections, coaxial incremental strain histories characterize fold hinges and document pinned hinges during folding. The sense of rotation on opposing limbs indicates that the fiber curvature records primarily the spin of layers through fixed, moderately steep (66° ± 12° (1σ)), diverging extension directions. The magnitude of the external rotation for each sample is typically less than the bedding dip, suggesting that the external rotation also includes a component of internal rotation related to layer-parallel shear toward the upper flat panel or the hinge pin line. The magnitude of this internal rotation varies within and between layers.
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