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During Leg 4 of Project EEZ-SCAN, long-range side-scan sonographs and seismic-reflection profiles were collected off Oregon and Washington, from the edge of the continental shelf to the boundary of the United States Exclusive Economic Zone (375 km from shore). The survey was extended seaward where necessary to include the Juan de Fuca Ridge. The project utilized the British GLORIA side-scan sonar system. The records were slant-range corrected and anamorphosed, and mosaics were constructed at a scale of 1:375,000.
The sonographs display precise geometry of the major geomorphic features of the area: accretionary ridges, submarine canyons, and fan valleys on the continental slope; deep-sea fans and channels in Cascadia basin; and elongate volcanic ridges making up Gorda and Juan de Fuca Ridges. Canyons with gullied walls deeply incise the upper continental slope off Washington. On the lower slope, the regime apparently changes from one of downcutting to one of overbank deposition. Cascadia basin and Cascadia Channel record intricate and complex drainage histories. The channel is not evident as a major feature on Nitinat Fan but becomes more prominent to the south, especially where it crosses Blanco Fracture Zone and enters Tufts Abyssal Plain.
Recent tectonic deformation of oceanic crust in the vicinity of Gorda Ridge is evident in the sonographs. For example, long, linear volcanic ridges flanking the spreading center are distorted and rotated westward at the north end where the Gorda Ridge meets the Blanco Fracture Zone.
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