The evolution of the upper mantle beneath mid-ocean ridges
Abstract
Several recent measurements of Rayleigh wave dispersion in the Pacific and Atlantic oceans have demonstrated conclusively that phase and group velocities in the period range 20-200 seconds increase systematically with increasing age of the sea floor. The increase in velocity can be explained by the growth and cooling of the oceanic lithosphere as it moves away from the mid-ocean ridges. If the base of the lithosphere is defined as the top of the low-velocity zone, the average lithospheric thickness can be no more than 25 km for a region from the ridge axis to an isochron of 5 million years. Within 10 m.y. after formation, the lithosphere reaches a thickness of about 60 km. As the mantle continues to cool, the thickness gradually increases, reaching a maximum of about 90 km in old ocean basins. In addition to the slow growth after 10 m.y., there is a gradual increase in shear velocity within the lithosphere from about 4.4 km s -1 at 10 m.y. to 4.6 to 4.7 km s -1 beneath old sea floor. The increase has also been found in regional studies of S n velocity in the Atlantic. Beneath the lithosphere extending to a depth of about 175 km, there is a zone of very low shear velocities apparently associated with partial melting. There is a significant increase in the shear velocity within this low-velocity zone with increasing age. A narrow zone of low Q and increased partial melting has been outlined directly beneath the ridge axis by observations of lateral variation in attenuation of longperiod S-waves and Rayleigh waves. This zone is roughly coincident with the elevated region of low compressional and shear velocities under sea floor which is less than 10 m.y. old. The seismic data are consistent with a model of the oceanic lithosphere as a thermal boundary layer growing at the expense of a laterally homogeneous asthenosphere. Substantial partial melting occurs at shallow depths beneath the ridge axis where the asthenospheric mantle material rises near the surface. The boundary-layer model is consistent with the change in elevation of the sea floor with age and with recently revised estimates of oceanic heat flow.
- Publication:
-
Tectonophysics
- Pub Date:
- March 1977
- DOI:
- 10.1016/0040-1951(77)90202-5
- Bibcode:
- 1977Tectp..38...89F