Tillamook Volcanic Series: Further evidence for tectonic rotation of the Oregon Coast Range

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Abstract
Paleomagnetic directions from the Eocene Tillamook Volcanic Series of the Oregon Coast Range point 46° clockwise from the expected Eocene field direction. Potassium argon dating of six dikes and flows from this formation yields a mean age of 44.3 ± 0.6 m.y. These results establish that the Oregon coastal block of Simpson and Cox (1977) extends north to the Oregon‐Washington border and that this block has rotated clockwise 46° ± 13° during the past 44 m.y. The block has undergone no detectable north‐south translation. A two phase model is developed to explain the tectonic history and anomalous magnetic field directions of the Pacific northwest region. Phase I of the rotation, active between 50 and 42 m.y. B.P., results from fragmentation of the Farallon plate with rotation of a fragment during its accretion to North America. Phase II of the rotation, active between 20 m.y. b.p. and the present, occurs in association with extension in the Basin and Range province.