Post 12 m.y. rotation of southwest Washington

Journal of Geophysical Research B: Solid Earth
By: , and 



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.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Post 12 m.y. rotation of southwest Washington
Series title Journal of Geophysical Research B: Solid Earth
DOI 10.1029/JB087iB05p03761
Volume 87
Issue B5
Year Published 1982
Language English
Publisher AGU
Contributing office(s) Geology, Minerals, Energy, and Geophysics Science Center
Description 16 p.
First page 3761
Last page 3776
Country United States
State Washington
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