Abstract

The Blue Mountains region in north-central Oregon underwent three main periods of compressional deformation during the late Cenozoic. Regional structural patterns are interpreted to have been formed by interaction between the northwestward-moving lithosphere of the northern Basin and Range and a relatively stable microplate beneath the Blue Mountains. The type of deformation and attendant volcanism changed from one period to the next, without time breaks. The age and duration of these periods have been determined from eight new K-Ar dates and published K-Ar dates. The first main period lasted from 36 to about 17 m.y. ago, the second from 17 to 10 m.y. ago, and the third from 10 to about 6 m.y. ago. The beginning of each of the three periods coincided with major changes in plate interactions in the western Pacific. A relationship between the volcano-tectonic events in north-central Oregon and plate interactions in the Pacific is proposed.

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