Paleomagnetic field directions from the basalt of Pack Sack Lookout are compared to those from the Pomona Member of the Saddle Mountains Basalt of the Columbia River Basalt Group. The Pomona crops out over a wide region on the Columbia Plateau east of the Cascade Range, and the basalt of Pack Sack Lookout crops out well to the west of the Cascades about 30 to 60 km east of the Washington coast. Our paleomagnetic results support existing geologic and geochemical evidence that both these units are part of a single 12‐m.y.‐old flow that erupted in western Idaho and flowed to the Washington coast. The magnetic data further indicate that southwest Washington has undergone clockwise rotation with respect to the Columbia Plateau and stable North America. The data suggest that both a large‐scale regional rotation of ∼16° and locally complex small‐scale rotations exist, the two being present to different degrees in the eastern and western regions of southwest Washington. The Cascade Range appears to coincide with the tectonic boundary, separating rotated and unrotated regions of Washington state. Rotation of southwest Washington appears to have been associated with the rotation of large pieces of western Oregon and southern Washington, possibly as a result of either a ridge‐push force from the Basin and Range province or shear along the Pacific‐North America plate boundary.

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.

Paleomagnetic results from three plutons of the central Sierra Nevada limit the motion of the Sierra Nevada relative to the North American craton to 5° ± 8° of poleward motion and 7° ± 11° of clockwise rotation since about 90 Ma. The indicated rotation and translation are not significant at the 95% confidence level. A new tectonic model is presented that is consistent with all available paleomagnetic data from the Sierra Nevada, Great Valley, Klamaths, Oregon Coast Range, Cascades, and western Nevada. The paleogeographic reconstructions of this model associate the displacement of major crustal blocks with Basin and Range extension starting at about 30 Ma. This model accounts for about 15° of clockwise rotation of the Klamaths and southern Cascades, about 2° of clockwise rotation of the Sierra Nevada and Great Valley, and 26° of clockwise rotation of the Oregon Coast Range since 30 Ma. The model is consistent with 40° of clockwise rotation of the Oregon Coast Range before 30 Ma, as proposed by Magill and Cox (1980). The amount of Basin and Range extension in this model at latitude 41°N is 225 km directed E‐W and at latitude 35°N is 245 km directed WSW‐ENE.

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