Upper mantle velocities on the Northern Cocos Plate

Refraction lines on the Northern Cocos Plate between the Orozco and Clipperton Fracture Zones have been used to determine upper mantle velocities over the plate. The velocities range from 7.50 to 8.43 km/sec. Azimuthal variations are found near the rise crest with low velocities parallel to the rise crest. The low velocities increase with age, lessening the observed azimuthal dependence away from the rise crest. A low-velocity zone is found in the mantle and may extend over a considerable portion of the plate near the rise crest. A 7.1-km/sec basal crustal layer is also observed and makes up a substantial portion of the crustal thickness.