Abstract

Paleomagnetic directions from 34 widely distributed sampling sites in presumed Oligocene volcanic rocks from the Cascade Mountains of southern Washington are well grouped with a mean and confidence interval of declination = 27°, inclination = 64°, and α95 = 4.5°. When compared to an expected direction, computed from the Oligocene reference pole for stable North America, a clockwise rotation of 33.5° ± 14° has apparently occurred. This result is not significantly different from several other studies from neighboring parts of the Cascades and Coast Range of southern Washington and suggests that the two ranges have rotated as a single unit during post-Eocene time. Comparison with the results from rocks of similar age in Oregon suggests that at least two crustal blocks are involved in Pacific Northwest tectonics. Inclinations in most Coast Range and Cascade Paleogene rocks indicate that these provinces have been translated slightly northward, as well as rotated.

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