A zone of gently southeast-dipping low-angle Tertiary faults is exposed along the eastern margin of the Pintlar and Flint Creek ranges, located 100 km east of the Bitterroot metamorphic complex. The sinuous fault zone can be traced for 100 km from the southeastern margin of the Pintlars in the Big Hole Valley northeastward, passing around the west side of the town of Anaconda, thence northward along the east side of the Flint Creek Range. Faults in the zone variously place Mesoproterozoic through Cretaceous sedimentary rocks on younger Tertiary granitic rocks or on older sedimentary rocks. Lower plate rocks are lineated and mylonitic at the main fault and, below the mylonite, are cut by mylonitic, veinlike microfaults. The upper plate consists of an imbricate stack of younger on older sedimentary rocks that are mylonitic at the main, lowermost fault but, at greater distances above the mylonite, are characteristically strongly brecciated or broken. Mesoscopic kinematic indicators in the lineated mylonite indicate tectonic transport easterly. Sedimentary breccia derived solely from upper plate rocks and interlayered with generally poorly sorted fluvial lenses of sand and gavel was deposited locally on top of hanging wall rocks in low-lying areas between fault blocks and breccia zones. Mylonitic granitic rocks in the lower plate have not been dated; however, dates of plutonic rocks in the Pintlar Range that appear to grade into mylonitic rocks at the fault zone are middle Eocene in age (50.6 Ma) whereas the Mt. Powell pluton in the Flint Creek Range that intrudes mylonite at the fault is Paleocene (63-62 Ma). The fault zone is interpreted as a detachment fault that bounds a metamorphic core complex, here termed the Anaconda core complex, similar in age and character to the Bitterroot fault that bounds the Bitterroot core complex and associated mylonite along the Idaho-Montana state line to the west.