HR: 0830h
AN: T31E-0879    [PDF]
TI: Preliminary structural analysis of the newly discovered Anaconda metamorphic core complex, western Montana, USA
AU: * Lageson, D R
EM: lageson@montana.edu
AF: Montana State University, Department of Earth Sciences, Bozeman, MT 59717 United States
AU: O'Neill, J M
EM: jmoneill@usgs.gov
AF: U.S. Geological Survey, P.O. Box 25046, MS 964 Federal Center, Denver, CO 80225 United States
AB: A sinuous zone of gently, generally east-dipping, low-angle Tertiary normal faults is exposed for 100 km along the eastern margins of the Anaconda and Flint Creek Ranges in southwest Montana. Faults in the zone variously place Mesoproterozoic through Cretaceous sedimentary rocks on younger Tertiary granitic rocks, or on sedimentary strata older than the overlying detached units. Lower plate rocks are lineated and mylonitic at the main fault and, below the penetratively deformed mylonite, are cut by mylonitic, vein-like micro faults. The upper plate consists of an extensional imbricate array of younger-on-older sedimentary rocks that are locally mylonitic at the main, lowermost detachment fault, but are characteristically strongly brecciated or broken. Kinematic indicators in the lineated mylonite indicate tectonic transport to the east-southeast. Syntectonic sedimentary breccia and coarse conglomerate derived solely from upper plate rocks were deposited locally on top of hanging wall rocks in low-lying areas between fault blocks and breccia zones. Muscovite occurs locally as mica fish in mylonitic quartzites at or near the main detachment. 40Ar/39Ar age spectrum obtained from muscovite in one mylonitic quartzite yielded an age of 47.2, plus/minus 0.14 Ma, interpreted to be the age of mylonitization. The fault zone is interpreted as a detachment fault that bounds a newly discovered Cordilleran metamorphic core complex, herein termed the Anaconda metamorphic core complex, similar in age and character to the Bitterroot mylonite that bounds the Bitterroot metamorphic core complex along the Idaho-Montana state line 100 km to the west.
DE: 8015 Local crustal structure
DE: 8109 Continental tectonics--extensional (0905)
DE: 9350 North America
DE: 9604 Cenozoic
SC: Tectonophysics [T]
MN: 2003 Fall Meeting