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Quaternary Research Center, University of Washington, Seattle, WA 98195, USA
Department of Geology, Idaho State University, Pocatello, ID 83209, USA thacglen{at}isu.edu
Quaternary tectonism on the central Olympic coast is dominated by the Kalaloch syncline, a broad (>32 km wavelength), low-amplitude (c. 45 m) fold deforming the Pleistocene glacial-nonglacial stratigraphic sequence. The stratigraphy includes two sequences of glacial and nonglacial sediments, one overlying and one underlying a prominent wave-cut surface. Stratigraphic and chronological data indicate that the buried wave-cut surface formed during isotope stage 7 or 5 sea-level highstands, probably during substage 5e or 5c. The elevation pattern of the wave-cut surface expresses the syncline most clearly. The outcrop patterns of overlying strata also reflect the syncline, as does outcrop-scale deformation in the underlying strata. Geological uplift rates for the wave-cut surface on the syncline’s north limb contrast in magnitude with, but are similar in pattern to, geodetically measured 1931–1974 uplift rates. Geological uplift rates range from –0.03 to 0.7 mm/year. The difference at each location between geological and geodetic uplift rates ranges from 0.4 to 1.0 mm/year, and is inferred to reflect interseismic strain accumulation. For each assumed age for the wave-cut surface, the inferred interseismic strain accumulation is nearly constant across the syncline’s north limb. Thus, the syncline accounts for most of the spatial variability in the 1931–1974 uplift rates. Comparison of the geological uplift rates with alternative representations of 1931–1974 uplift does not result in the same patterns, but does reveal a significant component of interseismic strain accumulation.
