The Early Structural Evolution and Anisotropy of the Oceanic Upper Mantle.

摘要

The dispersion of Love and Rayleigh waves in the period range 17-167s is used to detect the change in the structure of the upper mantle as the age of the sea-floor increases away from the mid-ocean ridge. Using the single station method, the group and phase velocities of Rayleigh waves were measured for 78 paths in the east Pacific. In order to describe the observed Rayleigh wave dispersion, both a systematic increase in velocities with the age of the sea-floor and anisotropy of propagation are required. The maximum change in velocity with age is about 5 per cent, with the contrast between age zones decreasing with increasing period. The greatest change occurs in the first few million years, due to the rapid cooling and solidification of the upper part of the lithosphere. In the 0-5 My age zone, the average thickness of the lithosphere can be no greater than 30 km, including the water and crustal layers. Within 10 My after formation, the lithosphere reaches a thickness of about 60 km. As the mantle continues to cool, the shear velocity within the lithosphere increases. Within the area of this study, no change occurs in the upper mantle deeper than about 80km.