Abstract

Near Wolf Creek, Montana, the fold and thrust belt intersects the central Montana trough at the northern edge of the Helena salient. Remanent magnetizations from 18 of 31 paleomagnetic sites in the Late Cretaceous Two Medicine Formation (∼79 Ma) document counterclockwise rotations of thrust-bound rocks along the margin of the salient. Results from 12 reliable sites in the upper of two thrust sheets gives a mean direction of Dec = 328.4°, Inc = 67.9°, α95 = 5.63°. Results from 6 reliable sites in the underlying thrust sheet yield Dec = 321.9°, Inc = 69.3°, α95 = 11.87°. The difference in these directions is not significant at the 95% confidence level. The thrust sheets show no relative rotation. Combining all 18 sites gives an average direction of Dec = 326.3°, Inc = 68.4°, α95= 4.94°. This average direction is 25.2° ± 14.0° counterclockwise from a Late Cretaceous reference direction (Dec = 351.5°, Inc = 69.3°) established from the adjacent Adel Mountain volcanic rocks (sim;76 Ma). Comparison to an older (∼88 Ma) pole established from the Niobrara Formation indicates 11.7° ± 11.4° of counterclockwise rotation. The paleomagnetic results, with other geologic evidence, show that both thrust sheets interacted with an oblique, lateral foreland ramp.

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