Preserved history of global mean spreading rate: 83 Ma to present

Using an up-to-date global plate rotation model, applied to the endpoints of preserved major spreading ridge isochrons, we have calculated the explicitly reconstructable length-weighted mean global half-spreading rate (HSR), ridge length and area production as a function of time since the end of the Cretaceous Normal Superchron at 83.0 Ma. Our calculations integrate uncertainties in rotation parameters and chron boundary ages with the partial sampling uncertainties arising from progressive subduction of older oceanic lithosphere and its preserved spreading record. This record of directly reconstructable oceanic ridge production provides a well-constrained baseline that can be compared to reconstructions that include the largely unconstrained extrapolated histories of entirely subducted oceanic plates. The directly reconstructable global mean HSR has not varied by more than +/- 15 per cent about an average rate of 28.4 +/- 4.6 mm a(-1) since 83 Ma. No long-term secular trend is evident: a maximum global mean half-rate of 32 +/- 6 mm a(-1) occurred from 33.1 Ma to about 25.8 Ma, with minima of 26 +/- 5 mm a(-1) between about 56 and 40.2 Ma, and 24 +/- 1 mm a(-1) since 3.2 Ma. Only this most recent interval has a rate that differs significantly (at +/- 2 sigma) from the long-term mean. The global, reconstructable ridge length at 56 Ma decreases by less than 15 per cent relative to the modern ridge system; by 83 Ma it has decreased by 38 per cent. These relatively high preserved ridge fractions mean that the estimated uncertainty due to partial sampling stays roughly equivalent to the estimated rotation model uncertainties, allowing long-term spreading rate variations of >20 per cent since the Late Cretaceous to be ruled out. In contrast, prior to 83 Ma too little oceanic lithosphere is preserved to reliably reconstruct global spreading rates.