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A marine deep seismic sounding survey over Winona Basin Thorleifson, Allan James


During the summer of 1975 a deep seismic sounding survey was carried out over Winona Basin, a deep water sedimentary basin located off the northern end of Vancouver Island. Three reversed refraction profiles were shot, one parallel and two perpendicular to the axis of the basin, with penetration from the ocean bottom to the upper mantle. Several sub-critical reflection profiles were also shot in an attempt to delineate the sedimentary structure of the basin. The two sub-critical reflection profiles shot over the central part of the basin were analyzed using the T²-X² method. The data sets gave layer velocities and thicknesses for 2 km of sediments for one of the profiles and .6 km for the other although petroleum industry data indicate that neither profile penetrated to the volcanic basement. The remaining reflection profiles were shot on the sides of the basin. On the western flank of Paul Revere Ridge, approximately 1 km of sediments with velocity in the range 2.5 to 3.5 km/s overlies volcanic basement. Over the continental slope on the east the seismic energy is strongly scattered below an upper 0.7 km of sediments. Refraction profile 75-1,1R, along the axis of the basin, was analyzed in a previous study using synthetic seismograms. However, the severe lateral inhomogeneities across the basin necessitated the use of ray tracing for the cross basin refraction profiles, 75-2,2R and 75-3,35. The final models are non-unique but they satisfy the seismic data very well and are consistent with profile 75-1,1R, gravity data and current views on plate tectonics. They show deep crustal layers dipping from both sides of the basin towards the center. Evidence for subduction as well as lateral motion between the Explorer and American plates has led to the conclusion that oblique subduction is occurring at Winona Basin.

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