Paleomagnetism and ^{40}Ar/39Ar Dating of Folded Sills in the Lombard Thrust Sheet, South Central Montana: Implications for the Timing of Fold and Thrust Deformation and Vertical Axis Rotations Along the Southern Margin of the Helena Salient

We report new paleomagnetic and ^{40}Ar/³⁹Ar data from sills exposed in folds of the Lombard Thrust Sheet along the southern margin of the Helena Salient in Southwest Montana. The folds in the Doherty Mountain fold complex represent the southern extension of the Devils Fence anticline structural culmination along the Southwest Montana Transverse Zone east of Whitehall, Montana. Numerous intermediate composition sills probably temporally related to the Late Cretaceous Boulder Batholith are well exposed in complexly deformed Cambrian through Mississippian strata in the hanging wall of the Jefferson Canyon--Cave fault system. We report well-defined paleomagnetic results from 14 sites in sills exposed in four different folds. After correction for fold plunge and then bedding tilt, all four folds yield positive fold tests at 100% unfolding indicating that their remanence was acquired prior to folding. We suggest that these magnetizations are primary thermoremanent magnetizations acquired during sill emplacement prior to fold and thrust deformation. ^{40}Ar/³⁹Ar dates on biotites from two sills in two different folds yield identical plateau dates of 77.26 +/- 0.36 Ma and 77.00 +/- 0.31 Ma (2σ ), consistent with a Late Cretaceous age for deformation. Although the group mean directions from sills in each of the folds probably represent spot readings of the geomagnetic field, their structurally corrected grand-mean direction (Dec. = 344°, Inc. = 65°, k = 82, α₉₅ = 10°) is statistically indistinguishable from the expected Late Cretaceous cratonic reference direction from the Adel Mountains Volcanics. Consequently, this result suggests that hangingwall strata in the Lombard Thrust Sheet of the Helena Salient in this vicinity have probably not experienced significant vertical axis rotation (R = -8.5° \pm$ 20.5°) due to thrust movement along the Southwest Montana Transverse Zone.