The Structural Evolution of an Oceanic Plate

Abstract

The dispersion of Rayleigh waves in the period range 15-170 s is used to detect the change in the structure of the lithosphere as it spreads away from the mid-ocean ridge. Using the single station method, group and phase velocities were measured for 42 paths within the Nazca plate. The focal mechanisms of the 11 source events were determined from P-wave first motion data and the azimuthal variation in Rayleigh wave amplitudes. The Nazca plate was divided into several regions whose boundaries are isochrons indicated by sea-floor spreading data. A characteristic pure-path dispersion curve was then computed for each age zone by least squares regression. A systematic variation in velocities with the age of the sea floor was observed. The greatest change in velocities for short periods (20-40 s) occurs in the first 10 My after the sea floor is generated at the ridge axis, presumably due to the rapid cooling of the upper part of the lithosphere. The phase and group velocities for periods greater than 40 s continue to increase for the next 50 My indicating a continued cooling and gradual thickening of the lithosphere. An apparent 2 per cent anisotropy in velocity was also found, with the maximum in the direction of spreading. Investigations into the source of this effect are in progress.