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 where Archean and Paleoproterozoic rocks record convergence associated with the 1.78-1.72 Ga Big Sky Orogeny. Studies in several parts of the range show that exposed ca. 1.7 Ga crustal levels were as deep as 40 km at the NW end and potentially as shallow as 10 km at the SE end across a map distance of 40 km. In documenting the nature of the internal structure of this section, we report on field mapping, kinematics, petrology, and monazite geochronology for one of several km-scale ductile shear zones in the region. The Hell Roaring Creek shear zone is a 2-3 km wide, NE-SW striking and steeply dipping amphibolite-facies structure. Stretching lineations within the shear zone appear to have a bimodal orientation distribution, with nearly down-dip orientations in most lithologies but shallowly plunging orientations in the most quartz-rich lithologies. Dextral shear sense indicators are consistently observed on subhorizontal surfaces. Observations within and adjacent to the shear zone suggest that the steep fabric elements are at least partly inherited from an earlier episode of SE-vergent thrusting and were enhanced during subsequent shear zone deformation. We interpret the shallow lineation to have developed entirely during the younger deformation. Thus, the shear zone appears to exhibit a degree of strain partitioning due to rheological contrast among lithologies and heterogeneity from inherited structures. Monazite from within the shear zone is dominated by ca. 1.74 Ga U-Th-total Pb dates, whereas data outside the shear zone support a ca. 2.55 Ga age for the earlier episode of shearing. At least one other NE-striking km-scale dextral mylonite zone has been identified in the range, but no data on its age is yet available other than that it deforms 2.8 Ga protoliths. Dextral shearing along present-day NE-striking structures late during the Big Sky orogen may reflect evolving convergence directions of accreting terranes from the west and/or a farther afield expression of late-stage convergence between the Wyoming and Superior cratons across the southern Trans-Hudson Orogen. The latter is supported by the kinematic and temporal compatibility between the Montana shear zones and similar conjugately oriented structures in the Black Hills of South Dakota.