FLAT TO STEEP TRANSITION IN SUBDUCTION STYLE

We use a thermal history of the mantle to calculate the evolution of o ceanic crustal thickness over Earth history and use residual depth ano malies from the present-day Pacific to find the crustal thickness rang e (9.2-11.6 km) where the subduction style changes from mainly flat to mainly steep. We find that steep subduction was well developed by 2.5 Ga, which coincides with a major change in sedimentary rare-earth ele ment patterns. Over 50% of all oceanic crust subducted steeply by 2.0- 1.6 Ga, the same interval over which the average thickness of continen tal plates declined rapidly. Because producing thick plates requires m any episodes of flat subduction, our model can explain why there are n o known thick plates < 1.6 Ga.