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Master of Science (MS)
Housen, Bernard Arthur
Suczek, Christopher A., 1942-2014
Schermer, Elizabeth, 1959-
The Helena salient is an arcuate curve in the southwest Montana fold and thrust belt, characterized by thin-skinned folding and thrusting. Ages from volcanic sills imply that deformation in the region began 77 million years ago during the Late Cretaceous (Harlan et al., 2008). This study investigates the nature of curvature associated with this salient using paleomagnetic techniques. Carbonate rocks of the Mississippian Madison Group were sampled from 24 sites across three folds: the Devil's Fence anticline, the Three Forks anticline and the Turner anticline (near Townsend, MT). Results from 16 sites have well defined, but very weak, magnetizations. At 100% untilting, two components of magnetization are revealed: a Mississippian primary magnetization (M-group) from at least two sites, and a Late Cretaceous chemical remanent magnetization (CRM) for 13 sites (K-group). Fold tests for the K-group indicate that each fold acquired a magnetization at 90-100% untilting. A mean direction for the K-group, in geographic coordinates and D = 35°, I = 72.9°, α95 = 8.8° is obtained for Devil's Fence anticline, D = 37.7°, I = 70.1°, α95 = 23.9° for Three forks anticline and D = 224.6°, I = 69.1°, α95 = 29.1° for Turner anticline. Using a direction (D = 335.8, I = 70.1 and ΔDx = 6.3°;) for the locality calculated from a Late Cretaceous North American (NA) reference pole (McFadden and McElhinny, 1995), the K-group indicates large (~60° CW) vertical axis rotations from Devil's Fence and Three Forks anticlines, as well as 111° counter-clockwise rotation from Turner anticline since the Late Cretaceous. Magnetic hysteresis data fall on the superparamagnetic + pseudo-single domain mixing lines, consistent with rock magnetic data from other studies sampling remagnetized carbonate units (e.g. Suk et al., 1993; Xu et al., 1998). When K-group directions are un-rotated on an equal-area plot to a Late Cretaceous NA reference pole, the M-group restores to a direction similar to a direction expected from a Mississippian NA reference pole (D = 310, I = 8.2 and ΔDx = 4.7°) and indicates a clockwise rotation of the pre-deformational sedimentary basin of 22 ± 18° to 59 ± 14°. Rotations reveal rigid block behavior of a clockwise rotating Elkhorn plate, while thrust sheets along the northern margin of the Helena salient experienced buttressing against the foreland margin resulting in counter-clockwise rotation.
Western Washington University
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Baugh, Benjamin F. (Benjamin Franklin), "Paleomagnetism and rock magnetism of remagnetized carbonate rocks from the Helena salient, Southwest Montana" (2010). WWU Graduate School Collection. 86.