FASTER SEAFLOOR SPREADING AND LITHOSPHERE PRODUCTION DURING THE MID-CENOZOIC

Concurrent changes in seawater chemistry, sea level, and climate since the mid-Cretaceous are thought to result from an ongoing decrease in the global rate of lithosphere production at ridges during this time. The present-day area distribution of seafloor ages, however, is most easily explained if lithosphere production rates were nearly constant during the past 180 Ma (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, revealing dramatic events during the Cenozoic. Globally, seafloor spreading rates increased by ~20% during the early Cenozoic due to an increase in plate speeds in the Pacific basin. Since then, subduction of the fast-spreading Pacific-Farallon ridge system has led to a 12% decrease in average global spreading rate and at least an 18% decrease in the total rate of lithosphere production by the most conservative estimates. These rapid changes during the Cenozoic defy models of steady- state seafloor formation, and demonstrate the highly time- dependent and evolving nature of plate tectonics on Earth.