Snvelocities and the composition of the lithosphere in the regionalized Atlantic

Sn is a short-period shear wave that propagates in the lithosphere. For long paths, the wave may travel well below the M discontinuity as it seeks a least-time path. It cannot penetrate the partially molten asthenosphere. More than 130 Sn phases from earthquakes in the North Atlantic, recorded by stations on islands and surrounding coasts, have been analyzed by regression to obtain regionalized velocities corresponding to different age zones of the Atlantic lithosphere. The highest velocity, 4.71 ± 0.01 km/sec, was found for sea floor older than 50 m.y. The average velocity for younger sea floor, age 0 to 50 m.y., was 4.58 ± 0.02 km/sec. For continental paths, Sn velocity was 4.61 ± 0.02 km/sec. These velocities are interpreted in terms of a lithosphere evolving into a colder, denser mineral assemblage as part of the spreading process. Rocks with elastic velocity higher than that of peridotite must be present in the suboceanic lithosphere to account for the high value of 4.71 ± 0.01 km/sec found in this study.