TECTONOMAGMATIC ARCHITECTURE OF THE SEVIER OROGENIC WEDGE OF WESTERN MONTANA

From ~79 to 75 Ma the Sevier orogenic wedge in western Montana developed large basement culminations along the tapered edge of the Middle Proterozoic Belt Basin. At ~75 Ma basement culminations were replaced by silicic plutons, including the large volume Boulder batholith, that invaded the fold and thrust belt at all crustal levels.

The Eocene Anaconda detachment, which translated the Boulder batholith tens of kilometers eastward, exposes a footwall showing high-temperature deformation, metamorphism, crustal anatexis, and plutonism in the middle crust of the Montana hinterland. Early burial beneath thick thrust sheets is indicated by high-pressure assemblages in kyanite-bearing metapelitic rocks. These fabrics are overprinted by a polyphase extensional history that post-dates hinterland thickening. A NE trending structural dome composed of amphibolite-facies calc-silicate and quartzite gneiss. Encircling the dome is the 1-2 kilometer thick Lake of the Isle shear zone (LISZ). Strain is heterogeneously distributed within the shear zone, yet overall there is extreme tectonic attenuation of Middle Proterozoic and Cambrian stratigraphy. Shear zone lineations plunge gently in conjunction with younger over older stratigraphic relationships. Within the LISZ, early kyanite-bearing assemblages are overprinted by high-temperature, lower pressure sillimanite and andalusite assemblages. The age of the LISZ is constrained by the Late Cretaceous Storm Lake pluton.

High-temperature, Cretaceous footwall structures are cut by the Anaconda detachment, which places unmetamorphosed sedimentary rocks against the high-grade gneisses. This moderately dipping greenschist mylonite zone accommodated top-to-the-ESE (~106°) sense of shear, exhumed mid-crustal rocks, and translated the Boulder batholith tens of kilometers eastward. New data constrain movement within the Anaconda mylonite at ~53-47 Ma by ages of Tertiary plutons.

We interpret the tectonic history as follows: 1) early tectonic wedge growth through basement culminations 2) construction of a “magmatic culmination” in conjunction attenuation and lateral mid-crustal flow and continued contraction along the thrust front 3) Eocene detachment-style extension possibly related to changes in plate configurations at the continental margin.