Abstract

Paleomagnetic and 40Ar/39Ar data from intermediate composition sills exposed along the southern margin of the Helena salient in southwestern Montana help refine the age of Late Cretaceous contraction in this part of the North American Cordillera. The structures in the Doherty Mountain fold complex deform Cambrian through Mississippian strata in the hanging wall of the Jefferson Canyon–Lombard fault system. Sills exposed within these strata, which are complexly deformed into northeast- to northwest-plunging anticlines and a northwest-plunging syncline give well-defined remanent magnetizations and, after correction for fold plunge followed by bedding tilt, paleomagnetic data from 20 of 21 sites from four folds yield positive fold tests at 100 percent unfolding. The magnetizations from these sites are interpreted to be primary remanent magnetizations acquired during sill emplacement prior to fold-and-thrust deformation, although pre-folding remagnetization due to widespread latest Cretaceous magmatism in the area cannot be ruled out. One site yields a magnetization that appears to postdate folding; this site has either been remagnetized or is in a distinct intrusive phase emplaced subsequent to folding. Biotite mineral separates from sills from two folds yield identical 40Ar/39Ar plateau dates of 77 Ma. Although the group mean directions from sills in each of the folds may represent incomplete samplings of the geomagnetic field and the number of independent readings is low, their grand-mean direction (declination [Dec.] = 347°, inclination [Inc.] = 66°, k = 71.9, α95 = 9°, N = 5 independent observations based on results from 21 sites) is similar to the expected Late Cretaceous reference direction for the study area, based on results from the essentially coeval Adel Mountain Volcanics. Therefore, these data suggest that deformation began after 77 Ma (Campanian-Maestrichtian) and that upper plate strata in the Lombard thrust sheet of the Helena salient in this area have not experienced significant local vertical-axis rotation due to thrust movement and impingement with foreland structures along the Southwest Montana Transverse Zone. The absence of appreciable vertical-axis rotations is consistent with data obtained from other paleomagnetic studies in the region.

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