QUATERNARY UPPER PLATE DEFORMATION IN COASTAL OREGON

The leading edge of the North American plate along the Cascadia subduc tion zone is deforming and rotating clockwise as a consequence of both underthrusting of the Gorda-Juan de Fuca plate and collision of the N orth American plate with the Pacific plate, The details of late Quater nary (less than or equal to 125 ka) upper-plate deformation resulting from these plate interactions are largely obscured because the most re cent deformation overprints earlier deformation in the Tertiary rocks of the Coast Range, However, by mapping uplifted wave-cut platforms fo rmed during times of high sea level in the east approximate to 500 000 yi; we identify faults and folds active in the late Quaternary in cen tral coastal Oregon. Through along-coast correlation of these platform s using elevation and soil development, we infer that several major fa ults have vertically offset platforms at rates as high as 0.6 m/k.y. f or the past 125 Ey. We regionally extend our analysis by incorporating all known faults and folds in southern and central coastal Oregon tha t deform wave-cut platforms. Most platforms along the southern and cen tral 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-p late coastal faults are consistent with their interpreted role as left -lateral, strike-slip, block-bounding structures accommodating clockwi se rotation. We speculate that these upper-plate faults have a compone nt of dip slip because of their association, in many instances, with l ocalized uplift, If these faults bound rotating blocks, the dip-slip c omponent of displacement may be either contractional or extensional, d epending on the orientation of the fault relative to the north-south t rend of the plate margin.