STYLE AND TIMING OF CRUSTAL ATTENUATION IN THE ANACONDA METAMORPHIC CORE COMPLEX, WESTERN MONTANA

The Anaconda metamorphic core complex (AMCC) is subdivided into three structural domains; the hanging wall, the detachment/mylonite zone, and the footwall. The hanging wall consists largely of unmetamorphosed volcanic and volcaniclastic rocks that correlate with the Lowland Creek volcanic sequence and are variably tilted within fault-bound basins. The ~ 100 km long east-dipping, top-to-the-east (102°-108°) detachment is characterized by a 300-500 m thick amphibolite to greenschist facies mylonitic shear zone overprinted by a sequence of brittle, listric-style normal faults that also dip to the east. Beneath the detachment lies a footwall consisting of intensely deformed amphibolite-grade Middle Proterozoic and Cambrian metasediments along with metaplutonic and plutonic rocks. Peak metamorphic conditions in the footwall are estimated to have been ~ 600-700°C and 5-7 kb.

Timing of extension in the main detachment zone of the AMCC is constrained by two syn-kinematic plutons. U-Pb analyses (SHRIMP II) of zircon give ages of 53.0 ± 0.6 Ma and 47.1 ± 0.4 Ma. These correlate with a previously published 40Ar/39Ar plateau age of 47.2 ± 0.14 Ma for a muscovite porphyroblast within the mylonitic shear zone. These data suggest the main detachment initiated at ~ 53 Ma and the mylonitic shear zone continued to development until at least 47 Ma.

Recent field work suggests that high-temperature mid-crustal attenuation occurred in footwall rocks prior to Eocene detachment faulting. A prominent shear zone, the Lake of the Isle shear zone (LISZ) has been mapped within the AMCC. Within the complex LISZ, rocks of the Proterozoic and Cambrian section are highly attenuated. Strain is distinctly partitioned between different metasedimentary horizons. Near Storm Lake (western AMCC), the LISZ is overprinted by the Storm Lake stock. Biotite from the Storm Lake stocks gives a 40Ar/39Ar age of 78.7 Ma. The shear zone also deforms a Late Cretaceous quartz diorite. Therefore, ductile attenuation in footwall of the AMCC likely began in Cretaceous time, preceding the development of the main detachment and mylonite by >20 million years. The timing of such extension is still under investigation.