KINEMATICS OF DECOLLEMENT FOLDING IN THE LOST RIVER RANGE, IDAHO

Decollement folding of massive (> 2 m thick) carbonate beds of the Sco tt Peak Formation, within the Willow Creek anticline, northern Lost Ri ver Range, Idaho was associated with the development of thin (<0.25 m) , widely-spaced deformation zones. Incremental strain histories determ ined from fibrous overgrowths in strain fringes from the deformation z ones provide a basis for kinematic analysis of the anticline. Plane st rain is indicated by coaxial up-dip extension on cleavage (XY) planes and a lack of extension in the Y direction. In XZ sections, coaxial in cremental strain histories characterize fold hinges and document pinne d hinges during folding. The sense of rotation on opposing limbs indic ates that the fiber curvature records primarily the spin of layers thr ough fixed, moderately steep (66 degrees+/-12 degrees(1 sigma)), diver ging extension directions. The magnitude of the external rotation for each sample is typically less than the bedding dip, suggesting that th e external rotation also includes a component of internal rotation rel ated to layer-parallel shear toward the upper flat panel or the hinge pin line. The magnitude of this internal rotation varies within and be tween layers. (C) 1997 Elsevier Science Ltd.