Late-stage dextral transpression in the Paleoproterozoic Big Sky Orogen of southwestern Montana: Implications for shear zone heterogeneity and evolution of the northern Wyoming craton

The northern Madison Range is one of several exposures in SW Montana that record tectonism associated with the 1.78-1.72 Ga Big Sky orogeny. Studies show that exposed 1.7 Ga crustal levels were as deep as 40 km at the NW end of the range and potentially as shallow as 10 km at the SE end. In documenting the internal structure of this oblique section, we report on the newly recognized Hell Roaring Creek shear zone. This amphibolite–facies structure is 2–3 km wide, NE–SW striking and steeply dipping. Stretching lineations have nearly downdip orientations in most lithologies but shallowly plunging orientations in quartz-rich units. Dextral shear sense indicators are consistently observed on subhorizontal surfaces. Observations suggest that the steep fabric elements were partly inherited from an earlier episode of SE-vergent thrusting. The shear zone is interpreted to exhibit strain partitioning during transpression, apparently due to rheological contrast among lithologies and heterogeneity from inherited structures. Geochronological data support a 1.74 Ga age for the shear zone and a 2.55 Ga age for the earlier episode of shearing. Dextral shear along present-day NE-striking structures late during the Big Sky orogeny may reflect evolving convergence directions of accreting terranes from the west.