HISTORY OF GLOBAL MEAN SPREADING RATE: 83 MA TO PRESENT

An up-to-date global rotation model including uncertainties is used for all major plates together with end points of major isochrons, integrated with estimates of age uncertainties at specific chron boundaries, to compute ridge length, area production, and length weighted mean global half spreading rate as a function of time since 83.0 Ma. Seafloor production from the observable ocean basins, excluding back-arc basins, is described. No attempt is made to extend the analysis to entirely subducted oceanic plates. Since the average spreading rate is area production per unit time divided by ridge length for that interval these data allow a straightforward analysis of the variation of global rate of seafloor spreading as a function of time. The analysis reveals that the directly reconstructable global mean rate of spreading has not varied by more than ±18 % since 83 Ma. The global, interval-length-weighted half spreading rate has averaged about 28.2 ± 2.5 mm/a since 83 Ma. The maximum global mean half-rate of about 33 mm/a occurred during the interval from 33.1 Ma to about 25.8 Ma, progressively decreasing to the Present. The middle (47.9 Ma) to late Eocene (40.2 Ma), and Present (since 3.2 Ma) are minima in the global mean rate history, with rates about 12% and 13%, respectively, slower than the 83 My global mean. Preserved seafloor provide no evidence for a simple secular trend in rates since the Late Cretaceous, nor is there preserved evidence of >20% or more variation in rate through this interval. These data are compatible with (1) deceleration of global plate motions since ~25 Ma recorded by variations in the vigor of mantle convection (Forte et al. 2009; GRL v. 36, p. L23301), and (2) global sea level variation over this interval in which there has been little variation in the maximum height of long-term sea level since the late Eocene, and probably less than ~100m decrease since 83 Ma, based on global paleogeographic reconstructions (Rowley, 2013, J. Geology).