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The Western Cascade Series (WCS) is a 3.5-km-thick, crudely homoclinal (east dipping) calcalkaline volcanic sequence of mid-Oligocene to early Miocene age that crops out near the southern tip of the Cascade Range in northern California. The mean direction of remanent magnetization in the WCS is D, 4.9°; I, 57.6° (N, 53; k, 14.4; α95 , 5.3°). When compared to a reference direction for the North American cration, the WCS direction indicates that the southern Cascade Range has rotated 14.0° +/- 9.0° since the WCS accumulated. A difference in mean direction between the lower and upper halves of the WCS suggests that much of this rotation occurred during the late Oligocene. Six other paleomagnetic studies of rock units of an age roughly comparable to the WCS also are available from western Oregon, northwestern California, and southwestern Washington. All show statistically significant clockwise rotation (inclination of all but one are concordant). Comparison of directions indicates that the Pacific Northwest did not rotate as a rigid body, but neither did it behave as a collection of small, independently rotating domains. The amount of rotation found throughout the area increases to the west or northwest, suggesting a driving force for rotation operating at the continental margin.

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Journal of Geophysical Research - Solid Earth and Planets





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Copyright 1986 by the American Geophysical Union

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