The “central melange” terrane of eastern Oregon, consisting mainly of an argillite-chert sequence and ophiolitic rocks, and the Seven Devils volcanic-arc terrane of northeastern Oregon and western Idaho probably formed in a volcanic island-arc and back-arc basin environment. New 40Ar-39Ar ages indicate that the ophiolitic rocks formed during the Permian and Late Triassic and that metamorphism synchronous with pervasive orogenic deformation occurred at the end of Triassic time. Accretion of the terrane to the North American continent in Late Jurassic time was accompanied by a second major orogenic event.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page of <sup>40</sup>Ar-<sup>39</sup>Ar ages of some pre-Tertiary plutonic and metamorphic rocks of eastern Oregon and their geologic relationships
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.