True Dipole Wander Driven by Mantle Convection in the Phanerozoic

Paleomagnetic directions are often interpreted by assuming that the geomagnetic field approximates a geocentric axial dipole. The axial dipole assumption is vital for reconstructing continent positions and for inferring true polar wander, rotations of the lithosphere with respect to Earth's spin axis. Here we use numerical dynamos driven by heterogeneous heat flux at the core-mantle boundary to demonstrate that mantle convection breaks the axial symmetry of the geomagnetic dipole, producing true dipole wander, rotations of the dipole axis. First, we conduct dynamo simulations using single harmonic heat flux heterogeneities at the core-mantle boundary to show that the amplitude of the nonaxisymmetric heat flux controls the dipole tilt angle. Second, we use numerical dynamo models with heat flux patterns derived from mantle global circulation models to demonstrate that nonaxisymmetric heat flux heterogeneities in the Phanerozoic likely produced detectable rotations of the dipole axis. Our calculations yield dipole rotation by more than 15 degrees about a near-equatorial axis between 200 Ma and 100 Ma, in response to changes in lower mantle convective structures, which transition from a highly nonaxisymmetric state too a nearly axial state. This dipole rotation is consistent with net rotations of the continents relative to the geomagnetic pole inferred from paleomagnetic directions during this time period, and suggests that some events, now classified as true polar wander, may instead result from rotations of the geomagnetic dipole.