Abstract

The Madison Mylonite Zone is a major shear zone of probable middle Proterozoic age discontinuously exposed in the Archean basement of southwestern Montana. This northeast-trending, northwest-dipping zone of reverse faulting coincides with a profound topographic and geologic lineament extending from the Red Rock Lakes to Livingston.

The only full section through the Madison Mylonite Zone is in the southern Madison Range where the 3-km-thick shear zone is centered in a 10-km-thick aureole affected by retrograde metamorphism to epidote-amphibolite facies. The shear zone contains partially recrystallized ribbon quartz, plagioclase clasts, shredded micas, garnet, chloritoid, and blue-green hornblende. Garnet-biotite geothermometry yields temperatures of 500+50°C (+10°). Reverse faulting in the shear zone is indicated by sub-horizontal fold axes, down-dip mineral aggregate lineations, and large drag folds. The high temperature of the event combined with the lack of tectonic breccias or disrupted stratigraphy suggests entirely ductile deformation.

In the northwestern side of the Beartooth Mountains, a continuation of the Madison Mylonite Zone is offset by the Mill Creek-Stillwater fault zone. Normal faults utilizing the foliation of the shear zone have resulted in the burial of the northwestern side of the shear zone. Homogeneous muscovite-plagioclase-quartz mylonite and fine-grained amphibolite grade into porphyroclastic tonalitic gneiss at the southeastern contact of the shear zone.

The Madison Mylonite Zone parallels the Mullen Creek-Nash Fork Shear Zone of Wyoming and trends toward the boundary between the Churchill and Superior Provinces. However, it is not a major suture because Archean metasediments can be correlated across the shear zone. It may parallel a Hudsonian suture in the subsurface northwest of the exposures of Archean basement in Montana.