IS PLATE TECTONICS SPEEDING UP?

There is increasing evidence for a speed-up of plate tectonics with time as the mantle cools. Consistent with increasing plate speed in the last 2 Gyr are 1) a decrease in supercontinent duration with time as estimated by three different methods, 2) a decrease in plate thickness and a corresponding increase in surface heat flux with time, and 3) possibly an increase with time in the number of multiple subduction zones in closing ocean basins. Also during the last 2 Gyr, increases are observed in angular plate speeds as estimated from paleomagnetic data and in the frequency of collisional orogens. A secular increase in plate speed should lead to increased subduction rates, which could increase the growth rate of the LLSVPs in the deep mantle; this could account for a decrease in the time between LIP activity and the initiation of ocean-basin openings in the last 2 Gyr. Because the 90-myr mantle cycle as reflected in LIP and zircon age distributions does not correlate with the supercontinent cycle, there must be a decoupling between the mantle processes controlling this mantle cycle (thermochemical destabilization in the deep mantle) and those controlling the supercontinent cycle. The large pulses of LIP and granite (zircon) production at 1900, 1100, 600 and possibly 100 Ma that approximately coincide with the onsets of supercontinent assembly may result from an increasing frequency of slab avalanche events in the upper mantle resulting from breakup of earlier supercontinents (or supercratons).