Paleomagnetism of the Late Cretaceous Ventura Member of the Midnight Peak Formation, Methow-Pasayten Belt, North-Central Washington

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Bazard, David R. (David Richard)
Western Washington University
Paleomagnetism; Midnight Peak Formation; Remagnetization; Geology; Washington (State); masters theses
thesis / dissertation description
Paleomagnetic analysis was conducted on 31 sites of the Ventura redbed Member of the Midnight Peak Formation located in the Methow-Pasayten belt, north-central Washington. Single high-stability components of magnetization, chiefly retained by high-unblocking temperature hematite exist in specimens from all but two of the sites. Directions calculated from these components were found to form circular within-site distributions and to be consistent across several meters of section. The single high-stability components, positive inclinations, and pre-folding, thus pre-Paleocene magnetizations suggest that the Ventura Member records a single Late Cretaceous dipole-field.Proportionally untilting site-mean directions did not produce a reasonable field direction nor degree and pattern of dispersion. However, 3 distinct groups form when means are corrected for 100% of observed bed tilt. The difference between two of these corrected directions and any reasonable expected Late Cretaceous field direction suggests that the magnetization of each group was acquired in a different structural position. For this reason Ventura sites were subdivided into structural domains (A, B, and C) that appear, by virtue of common directions, to have acquired their magnetizations while in different structural orientations.Positive fold test within two of these domains suggest that the remagnetization pre-dates the pre-Paleocene folding. However, since these directions are due to a remagnetization accurately determining the inclination of the magnetizing field is not possible without knowing the structural position they occupied during remagnetization. If either a Late Cretaceous North American direction or an expected direction based on shallow inclination results from the Spuzzum Pluton is assumed, then the measured directions suggest that Late Cretaceous remagnetization in the Methow terrane occurred across a broad north-northwest trending, east-northeast dipping structure.Alternatively, if the pre-folding bed orientation of the structurally simplest domain (Domain C) is assumed to be horizontal during remagnetization, then a pole calculated from this domain indicates that the Methow terrane acquired a Late Cretaceous remagnetization about 1800 km south of its present position relative to North America. This amount of northward transport is similar to that suggested for the Spuzzum and Axlegold intrusions but the clockwise rotation (36o) indicated by this pole is about half that suggested for the intrusions.Both interpretations imply that domain B passes the fold test yet does not produce a direction upon unfolding that is representative of the magnetizing field. Analysis of the relationship between remagnetization and deformation in this domain demonstrates that directions can pass a fold test yet yield an erroneous paleopole.