Ridge migration and asymmetric sea-floor spreading

We propose that asymmetric sea-floor spreading occurs as a consequence of the relative motion between ridges and slow-moving mantle material below. A fluid mechanical model of asymmetric spreading predicts that the trailing flank of a ridge migrating with respect to the mantle spreads fastest. Observed asymmetries are compared to those predicted by ridge migration velocities. Although the magnitude of the asymmetry appears to depend as much on local effects as on the migration of the ridge, the direction of asymmetry agrees with our prediction in most locations. In contrast, models in which the ridge attempts to remain above a source fixed in the mantle predict the opposite direction of asymmetry. Other models, which attribute asymmetric spreading to asymmetric cooling, require large deviations from the standard depth-age relationship, while our model does not.