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A minimum rate of underthrusting of the Juan de Fuca plate beneath North America for the late Pleistocene has been calculated to be approximately 0.7 ± 0.3 cm/yr. This estimate was determined in each of 18 CSP crossings of the continental slope off the coast of Washington by dividing the amount of shortening within the westernmost anticlinal ridge by an approximation of the time elapsed since the beginning of the ridge deformation. The latter estimate was obtained by tracing the late Pleistocene discontinuity as described by Leg 18 of the Deep Sea Drilling Project from site 174 on Astoria fan into the deformed Cascadia basin turbidites of the continental slope. Recent plate-tectonics theories predicting northwesterly movement of the Pacific plate parallel with the Sa Andreas fault at 6 cm/yr, and the spreading of the Juan de Fuca plate away from the Pacific plate at 6 cm/yr, require an underthrusting rate of 2 cm/yr measured perpendicular to the trend of the Washington continental margin. If the age of the late Pleistocene discontinuity is assumed to be a half-million years, the deformation
of the marginal ridge can account for nearly 40 percent of the theoretical crustal shortening between the Juan de Fuca and North American plates. Seismic profiles also indicate that accretion of Cascadia basin turbidites along the continental margin occurred during the Pliocene-Pleistocene and has continued to the present.
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