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Quaternary upper plate deformation in coastal Oregon

¹ Department of Geology, Humboldt State University, Arcata, California 95521
² 74 Russell Avenue, Watertown, Massachusetts 02172
³ Department of Soil Science, University of Wisconsin, Madison, Wisconsin 53706 Clifton
⁴ Science Department, Lane Community College, Eugene, Oregon 97403-0640

The leading edge of the North American plate along the Cascadia subduction zone is deforming and rotating clockwise as a consequence of both underthrusting of the Gorda- Juan de Fuca plate and collision of the North American plate with the Pacific plate. The details of late Quaternary (125 ka) upper-plate deformation resulting from these plate interactions are largely obscured because the most recent deformation overprints earlier deformation in the Tertiary rocks of the Coast Range. However, by mapping uplifted wave-cut platforms formed during times of high sea level in the past 500 000 yr, we identify faults and folds active in the late Quaternary in central coastal Oregon. Through a long-coast correlation of these platforms using elevation and soil development, we infer that several major faults have vertically offset platforms at rates as high as 0.6 m/k.y. for the past 125 k.y. We regionally extend our analysis by incorporating all known faults and folds in southern and central coastal Oregon that deform wave-cut platforms. Most platforms along the southern and central Oregon coast have been uplifted at rates of 0.1–0.3 m/k.y. since the late Pleistocene; however, platform uplift rates approach 1 m/k.y. in the vicinity of faults. The trend and distribution of these upper-plate coastal faults are consistent with their interpreted role as left-lateral, strike-slip, block-bounding structures accommodating clockwise rotation. We speculate that these upper-plate faults have a component of dip slip because of their association, in many instances, with localized uplift. If these faults bound rotating blocks, the dip-slip component of displacement may be either contractional or extensional, depending on the orientation of the fault relative to the north-south trend of the plate margin.

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