SOME THEORETICAL PREDICTIONS ON THE RELATIONSHIPS AMONG SPREADING RATE, MANTLE TEMPERATURE, AND CRUSTAL THICKNESS

A series of numerical experiments on mantle flow and melting predict a positive relation between mantle temperature and crustal thickness. T he models also demonstrate that crust formed at slow spreading rates i s more sensitive to variations in mantle temperature than crust formed at fast rates so that the range of calculated thicknesses is much gre ater for crust formed at slower rates. An instantaneous mantle tempera ture increase results in a transient pulse of melt production that is also more pronounced at slower spreading rates. The predicted behavior is caused by the interplay between mantle flow driven by plate separa tion and that driven by thermal, compositional, and melt-related buoya ncy. A temperature increase results in a decrease in mantle viscosity and an increase in the depth at which melting begins. A lower viscosit y leads to stronger buoyancy-driven flow that carries more mantle to s hallow depths below the ridge. Thermal buoyancy effects, which may res ult in cooling and mixing of depleted and undepleted material under th e ridge, appear to be of greater importance at slower spreading rates. The steady state results are broadly consistent with global compilati ons of oceanic crustal thickness that show larger variations in crusta l thickness at slower spreading rates than at faster rates. Thicknesse s estimated from seismic refraction data from crust formed within a si ngle segment of the Mid-Atlantic Ridge but at different spreading rate s (1.0 to 1.9 cm/yr) are consistent with (but do not prove) the model results. The transient pulse of melt production associated with a rapi d increase in mantle temperature might occur when a ridge becomes prox imal to a hot spot.