The Pioneer Mountains of Idaho contain a detachment fault that separates a lower-plate core of Precambrian and Ordovician metasedimentary rocks and Cretaceous and Eocene plutonic bodies from a surrounding upper plate of Paleozoic sedimentary and Tertiary sedimentary and volcanic units. The detachment system developed during Tertiary extension, and it overprinted Paleozoic and Mesozoic east-directed compressional features. Beneath the detachment, both brittle and ductile (mylonitic) deformation can be observed. Trends of stretching lineations in the mylonite and striations along the detachment surface both cluster around N65°W. Composite planar fabrics (S- and C-surfaces) in the mylonite and development of the mylonite zone on the northwest side of the core both suggest a top-to-the-west sense of shear. Both brittle and ductile deformation affect the Eocene pluton of the core; therefore, the deformation must be Eocene or younger. Displacement is estimated at about 17 km. The extension direction in the Pioneer complex correlates with that of other core complexes in the region north and south of the Snake River Plain; this suggests that the direction of extension in core complexes was controlled by an inherited structural framework which was crustal in scale.