New insights into mantle convection true polar wander and rotational bulge readjustment

The Earth's rotation axis is constantly tracking the main inertia axis of the planet that evolves due to internal and surface mass rearrangements. This motion called True Polar Wander (TPW) is due to mantle convection on the million years time scale. Most studies assumed that on this long time scale the planet readjusts without delay and that the Earth's rotation axis and the Maximum Inertia Direction of Mantle Convection (MID-MC) coincide. We herein overcome this approximation that leads to inaccurate TPW predictions and we provide a new treatment of Earth's rotation discussing both analytical and numerical solutions. We obtain an average TPW rate in the range [0.5°-1.5°]Myr-1 and a sizeable offset of several degrees between the rotation axis and the MID-MC. This is in distinct contrast with the general belief that these two axes should coincide or that the delay of the readjustment of the rotational bulge can be neglected in TPW studies. We thus clarify a fundamental issue related to mantle mass heterogeneities and to TPW dynamics.