Accelerating Seafloor Spreading and a Crustal Production Slowdown During the Cenozoic

The motions of Earth's tectonic plates represent the surface manifestation of convection in the Earth's mantle, and, over time scales of 50-100 Myr, govern a variety of geological processes including sea level change, the carbon cycle, and changing seawater chemistry. Although the basic history of plate motions has been deduced for the Cenozoic and some of the Mesozoic, controversy has recently emerged regarding changes in global rates of ridge spreading and lithospheric production during this this time period. While global studies of ridge spreading or subduction rates suggest that lithospheric production rates at ridges have slowed during the Cenozoic, the approximately triangular distribution of present-day seafloor area per unit age suggests nearly constant rates of crustal production (e.g., Rowley, 2002). We examined spatial gradients of present-day seafloor ages and inferred ages for the subducted Farallon plate to construct a history of spreading rates in each major ocean basin since ~140 Ma. Our analysis reveals dramatic events during the Cenozoic. For example, an early Cenozoic period of slow spreading in the South Atlantic is coincident with a period of accelerated spreading in the western Indian Ocean, which suggests that plate motions must be correlated via a coupling to mantle flow. Globally, seafloor spreading rates increased by ~30% during the Cenozoic due to a nearly two-fold increase in plate speeds in the Pacific basin. At the same time, subduction of the Farallon-Pacific ridge system led to a ~20% decrease in ridge length. Taken together, we observe a ~25% slowdown in the global rate of crustal production since ~30 Ma. This rapid decrease during the Cenozoic is consistent with observations of increasing Mg/Ca ratio, decreasing sea level, and decreasing atmospheric carbon during the Cenozoic. This Cenozoic crustal production slowdown defies models of steady-state seafloor formation, and demonstrates the highly time-dependent and evolving nature of plate tectonics on Earth.