GROWTH AND COLLAPSE OF THE SEVIER OROGENIC WEDGE: NEW EVIDENCE FROM THE CORDILLERA OF WESTERN MONTANA

The Sevier orogenic wedge in western Montana grew through a unique spatial and temporal overlap between arc magmatism and regional crustal shortening. In the Late Cretaceous numerous silicic plutons, including the large volume Boulder batholith, invaded the fold and thrust belt at all crustal levels. As a result, a 50-60 km thick hinterland culmination was formed by ~70 Ma.

Footwall sections of the Anaconda metamorphic core complex expose mid-crustal levels of the Sevier hinterland with a complex record of high-temperature deformation, metamorphism, and crustal anatexis. Crustal thickening is indicated by kyanite-bearing metapelitic rocks overlain by the Georgetown thrust sheet, which is folded and further disrupted by numerous low-angle detachment faults.

Footwall rocks also record a polyphase extensional history that post-dates crustal thickening. Beneath the Georgetown thrust, lies 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). This structure shows kilometers of stratigraphic offset and tectonic attenuation of 1-4 km of stratigraphic section. Shear zone lineations plunge gently in conjunction with younger over older stratigraphic relationships. Within the LISZ, kyanite-bearing assemblages are overprinted by high-temperature, lower pressure sillimanite and andalusite assemblages. The LISZ is constrained as Late Cretaceous by the synkinematic Storm Lake pluton.

The central gneiss dome and LISZ are truncated 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) Late-K construction of a magmatically and tectonically thickened orogenic wedge 2) Late-K extensional collapse of the hinterland contemporaneous with foreland thrusting 3) Eocene detachment-style extension possibly related to changes in plate configurations at the continental margin.