An evaluation of tectonic rotation in the Pacific Northwest

Abstract

Columbia Arc is proposed as a formal name for the arcuate portion of the Nevadan orogenic belt in the states of Oregon, Idaho, and Washington. Absence of distortion throughout a gigantic dike swarm of basalt in eastern Oregon, southeastern Washington, and western Idaho is a cardinal argument against paleomagnetic interpretations of post-Miocene tectonic rotation of the Columbia Arc. Other arguments against post-Miocene tectonic rotation are the undeformed Blue Mountain Front, the undisturbed alignment of Pliocene-Pleistocene volcanoes in the Cascade Mountains of Oregon and Washington, and the lack of disruption of a zone of Pliocene faults in western Idaho. No evidence is recognized by the writer for lateral offset along the Olympic-Wallowa lineament in northeastern Oregon. The undisturbed character of a prominent Mesozoic shear zone in northeastern Oregon and western Idaho indicates that the Olympic-Wallowa lineament near the Oregon-Idaho boundary was tectonically inactive throughout the Mesozoic era. A primary origin for the configuration of the Columbia Arc is the preferred hypothesis.