Seismic refraction studies off Oregon and northern California

Abstract

Seismic refraction profiles across Juan de Fuca ridge, west of the coast of Oregon, show that the ridge is the surface expression of an upraised crust and mantle; the crustal velocity is slightly higher than normal, and the mantle velocity is low. Near the axis of the ridge the mantle velocity is below normal, and the mantle may be as shallow as 7 km below sea level. The topographic asymmetry of the ridge is caused by burial of the east flank of an essentially symmetrical ridge by the sediments of the Cascadia basin, trapped behind the ridge. The base of the sedimentary layer and the base of the oceanic crust both reach depths almost as deep as those normal for the ocean basins under the Tufts abyssal plain to the west and at the foot of the continental slope to the east. In the Gorda basin, just east of Gorda ridge, new measurements agree with older measurements by R. W. Raitt, which show very shallow depth to mantle (depths between 6 and 8 km below sea level) and low mantle velocity, both here and on the west side of Gorda ridge. Juan de Fuca ridge and Gorda ridge thus appear to have the same structure as the East Pacific rise. Profiles on the continental shelf off Washington, Oregon, and northern California show that there is a thick sedimentary section beneath the shelf. Rocks with the velocity of the oceanic crust were found at a depth of 10 km beneath the shelf off Oregon and northern California; mantle depth was determined as 17 km west of the Oregon-Washington border, where the water is 0.12 km deep, and 14 km west of central Oregon at the outer edge of the shelf, where the water is 0.7 km deep.