Paleomagnetism and geology of Eocene volcanic rocks of southwest Washington, implications for mechanisms of tectonic rotation

Journal of Geophysical Research
By:  and 



Paleomagnetic and geologic investigations in Eocene volcanic rocks of the southwest Washington Coast Range demonstrate a close relationship between tectonic rotations and the local structural geology. The allochthonous middle Eocene submarine basalt basement of the Crescent Formation consists of several fault‐bounded structural domains up to 30 km across that are characterized by different amounts of clockwise rotation (20° to perhaps as much as 65°) when compared to the Eocene reference pole for North America. Structural analysis shows the differential rotations postdate middle Eocene folding of the Crescent Formation against the continental margin and predate the unconformably overlying upper Eocene Goble Volcanics, which are rotated about 23° and do not show the same domains of rotation as the underlying Crescent Formation. Post‐Goble rotations may be accommodated by a fault pattern very similar to that expected for areas caught in a simple dextral shear couple along transcurrent faults. Major north‐northwest trending faults with several kilometers of dextral displacement form the boundaries of cross‐faulted shear domains in which the clockwise rotation of elongate crustal slices is accommodated by west‐northwest trending sinistral R′ Riedel shears. Thirty‐five to 100% of the observed post‐late Eocene rotations could have occurred by this shear rotation mechanism. Other paleomagnetic study areas in the Coast Range and western Cascades have a similar fault geometry and may also have undergone significant shear rotations. Long‐term northward oblique subduction of the Farallon plate beneath the Coast Range throughout most of the Tertiary could have been the driving force for the shear rotations and could explain the rapid eastward decrease in rotation away from the continental margin. Shear rotations could eliminate many of the structural and stratigraphic difficulties associated with models involving rotation of large, rigid plates.

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Publication type Article
Publication Subtype Journal Article
Title Paleomagnetism and geology of Eocene volcanic rocks of southwest Washington, implications for mechanisms of tectonic rotation
Series title Journal of Geophysical Research
DOI 10.1029/JB090iB02p01925
Volume 90
Issue B2
Year Published 1985
Language English
Publisher AGU
Contributing office(s) Geology, Minerals, Energy, and Geophysics Science Center
Description 23 p.
First page 1925
Last page 1947
Country United States
State Washington
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