KINEMATICS OF FAULT-RELATED FOLDING IN A DUPLEX, LOST RIVER RANGE, IDAHO, USA

The Doublespring duplex, located in the Lost River Range of Idaho, is a Sevier age fault-related fold complex in massive limestones of the U pper Mississippian Scott Peak Formation. Folds within the duplex close ly resemble fault-bend fold geometries, with open interlimb angles and low-angle bed cut-offs. Narrow, widely spaced, bedding-parallel shear zones with well-developed pressure solution cleavage alternate with m assive, relatively undeformed layers on fold limbs. Shear zones are de veloped only on the limbs of anticlines, and have similar but unique m orphologies in each of three different folds, Incremental strain histo ries reconstructed from antitaxial fibrous overgrowths and veins withi n the shear zones constrain the kinematics of folding. Shear zones exp erienced distributed bedding-parallel simple shear (flexural flow) tow ards pins near axial surfaces, while adjacent massive layers experienc ed rotation through an externally fixed extension direction. The absen ce of footwall synclines and morphological differences in shear zones from adjacent folds suggest that faulting preceded folding. Kinematic histories of folds that have experienced different translational histo ries are identical, and are not compatible with strain histories predi cted from previous kinematic models of fault-bend folding. Shear zone development and fiber growth is instead interpreted to have occurred d uring low amplitude fixed-hinge buckling in response to initial resist ance to translation of the thrust sheet. Fault-bend folding with mobil e axial surfaces occurred with translation of the thrust sheets once t he initial resistance to translation was overcome and resulted in no p enetrative strain.