Pleistocene tectonic accretion of the continental slope off Washington

Interpretation of reflection profiles across the Washington continental margin suggests deformation of Cascadia basin strata against the continental slope. Individual reflecting horizons can be traced across the slope-basin boundary. The sense of offset along faults on the continental slope is predominantly, but not entirely, west side up. Two faults of small displacement are seen to be west-dipping reverse faults. Magnetic anomalies on the Juan de Fuca plate can be traced 40–100 km eastward under the slope, and structural interpretation combined with calculated rates of subduction suggests that approximately 50 km of the outer continental slope may have been formed in Pleistocene time. Rocks of Pleistocene age dredge from a ridge exposing acoustic “basement” on the slope, plus the results of deep-sea drilling off northern Oregon, are consistent with this interpretation. The question of whether or not subduction is occurring at present is unresolved because significant strain has not affected the upper 200 m of section in the Cascadia basin. However, deformation of the outer part of the slope has been episodic and may reflect episodic yield, deposition rate, subduction rate, or some combination of these factors.