Effect of lower mantle viscosity on the time-dependence of plate velocities in three-dimensional mantle convection models

Earth's evolution has featured stages of relatively steady plate motion interrupted by brief periods of rapid change in plate direction. Previous studies have shown that vigorously convecting internally heated systems featuring dynamically determined plate- like surface motion can also be characterized by predominantly steady periods punctuated by comparatively short reorganization events. Here, we investigate time- dependence in two mantle convection models featuring significantly contrasting viscosity profiles and assess the influence of lower mantle viscosity on plate velocity time- dependence. We model a system featuring nine finite thickness viscous plates at the top of a 6 x 6 x 1 Cartesian geometry solution domain. We find that plate reorganization events involving between 1 and 3 plates occur with geologically relevant frequency in a calculation featuring a factor of 36 increase in lower mantle viscosity relative to the upper mantle. Specifying a mantle viscosity at a depth of 2000 km that is 3000 times greater than the upper mantle viscosity suppresses this time-dependence.