EXHUMATION AND KINEMATICS ALONG THE LEWIS AND CLARK STRIKE-SLIP SYSTEM: ANACONDA EXTENSIONAL TERRAIN, WEST-CENTRAL MONTANA

The Anaconda Extensional Terrain (AET) of west-central Montana records large-scale extension along the Lewis and Clark transform system. The AET is outlined by a >100 km long, 100-300 m thick, east-dipping mylonite zone. Throughout the complex, amphibolite-facies rocks, derived largely from Middle Proterozoic Belt sediments, were exhumed along this top-to-the-east fault system. Based on geometric and kinematic criteria, the fault system is divided into a northeast and southwest segment. In the northeast, footwall rocks are characterized by 5-8 km wide belt of highly attenuated metapelites, calc-silicates, and intrusive rocks lying subparallel to the mylonite. Metapelites show kyanite pseudomorphs that are consistently overprinted by sillimanite and low pressure andalusite-bearing assemblages. Mylonitic mineral lineations and penetrative lineations within this belt are consistent with the general strike of the Lewis and Clark line at ~104-108°. Approximately 20 km south of Anaconda, Montana the N-S striking trace of the mylonite turns abruptly west. Coincident with this change in strike, mineral lineations change to an orientation of ~125-130°. This southwest region is further distinguished by a much broader belt of high-grade footwall rocks. Kyanite-bearing metapelites, not overprinted by high-temperature and/or lower pressure are also more prevalent in footwall rocks. In our interpretation, the broader belt of exposed footwall rocks in combination with higher pressure assemblages, indicate that the southwest region shows greater extensional strain than exposures to the northeast. This complex geometric and kinematic setting might have developed as the Lewis and Clark continental transform propagated farther east toward the Beartooth and Absaroka ranges.