DIRECTION AND RATE OF SHORTENING ACROSS THE OLYMPIC PENINSULA

A variety of observations suggest that the Cascadia forearc--consisting of the Oregon-Washington Coast Ranges--is being driven northward, parallel to the margin, due to oblique convergence. The Juan de Fuca/North America convergence direction is highly oblique beneath NW California and W Oregon, but becomes orthogonal beneath NW Washington and Vancouver Island. This observation has prompted the suggestion that margin-parallel transport is resolved by N-S shortening across the Olympic Mountains (NW Washington), with Vancouver Island serving as a buttress. A useful analogue is uplift of the Transverse Range in southern California due to "buttressing" across the restraining bending in the San Andreas fault. For the case of a subduction zone, however, the zone of buttress-related deformation would be expected to extend seaward to the trench. We present new work that provides quantitative information about active deformation along the Pacific coast of the Olympic Peninsula. The coast preserves a prominent wave-cut terrace, formed during the Sangamon sea level high stand at 122 ka. Broad E-W trending folds in this terrace have been cited as evidence for margin-parallel shortening. Our measurements indicate that the unconformity has shortened by 80 m over a N-S base length of 80.8 km. From this, we conclude that the N-S shortening across the Olympics uplift (N-S dimension ~100 km) accounts for <1 km/my of the N transport of the Oregon-Washington Coast Range. Furthermore, we show, based on the pattern of uplift of coastal and fluvial terraces across the Olympics, that active uplift is being driven almost entirely by NE-SW shortening, parallel to the modern convergence direction. Late Cenozoic structures in the Olympics Mountains also indicate NE-SW shortening. These conclusions are consistent with our published interpretation that the Olympics uplift is presently at steady state, defined by a balance between the accretionary flux being driven into the range by plate convergence and the erosional flux coming from the uplifted topography. We maintain that variations in rates of uplift and margin-perpendicular shortening along the Cascadia margin are related to along-strike variations in accretionary and erosional fluxes.