EXCITATION OF TRUE POLAR WANDER BY SUBDUCTION

TRUE polar wander-the global motion of the mantle relative to the Earth's rotation axis-is known from the analysis of palaeomagnetic data, hotspot tracks and plate motions to have occurred at velocities of up to 0.5-degrees Myr-1 since the Late Cretaceous period1-4. We address here the longstanding question of how fast episodes of true polar wander (TPW) can be excited5, by analysing the impact of the distribution and activity of subduction zones on polar motion. Using nonlinear Liouville equations, which allow us to treat large excursions of the polar axis, we show that unrealistically fast TPW is excited by subduction episodes unless the lower mantle has a viscosity at least 10 times that of the upper mantle. This need for a viscosity increase with depth in the mantle reinforces the conclusions of previous studies on post-glacial rebound and geoid anomalies, theoretical creep laws and some preliminary results on TPW induced by density anomalies embedded in the mantle6-10. The lower viscosity in the upper mantle means that upper-mantle density anomalies are most effective in exciting TPW. Changes in the pattern of subduction through time may be responsible for both episodes of fast TPW and times of quiescence in polar motion.