The rotational stability of a convecting Earth: The Earth's figure and TPW over the last 100 Myr

Palaeomagnetic records spanning the last 100 Myr indicate that the reorientation of the Earth's rotation axis relative to the surface geography (or true polar wander, TPW) has been confined to a range less than 6° from its present location. This limited TPW is unexpected given that a canonical theory for the rotational stability of the Earth generally predicts that mantle convection should drive larger displacements of the pole. We argue, following earlier work, that the muted TPW is a consequence of the stable, excess flattening of the Earth's figure driven by plate subduction and deep mantle superplumes rising beneath Africa and the Pacific. In particular, we show that the TPW record is consistent with convection-induced perturbations to the Earth's inertia tensor of order 20 per cent or less of the excess flattening over the last 100 Myr; this upper bound will be higher if the Earth's lithosphere retains any significant elastic strength over such long timescales. This inferred stability of the Earth's figure has important implications for our understanding of deep mantle structure and the long-term, global-scale evolution of the Earth.