A North-Pacific slab-flux pulse drove the ~50 Ma TPW reversal

True polar wander (TPW) records displacements of Earth’s rotation axis induced by mantle convective redistribution of internal mass anomalies. A TPW reversal near ~50 Ma inferred from paleomagnetic data remains debated, particularly its cause and its robustness across reference frames. We present 70-million-year, tomography-assimilative mantle-convection reconstructions that evolve present-day seismic structure backwards in time, with an energy-consistent flow formulation, yielding time-dependent density, inertia tensor, and TPW. Three independent diagnostics converge on a single, time-localized driver: (i) maps of the long-wavelength geoid-rate (∂N/∂t) show a focused Aleutian–Kamchatka lobe at 50 Ma; (ii) off-diagonal inertia-tensor time derivatives peak contemporaneously at this time; and (iii) cap-blanking experiments that zero anomalies within a 30–40° North-Pacific cap erase the U-turn, whereas comparable caps elsewhere do not. We interpret the causative structure as a coherent North-Pacific (“Kula–Izanagi” sensu lato) slab-flux pulse entering the lower mantle.

Predicted TPW paths quantitatively match palaeomagnetic trajectories across multiple mantle frames (reduced χ² ≈ 0.6; mean path-averaged angular misfit ≈ 1.7°) and reproduce the observed ~50 Ma U-turn bracketed by twin maxima in TPW speed. Present-day mantle-driven TPW rates of 0.2–0.4° Ma^-1 imply ~20–40% of the 20th-century geodetic rate. In head-to-head tests, slab-history reconstructions (with or without hotspot-fixed “domes”) differ markedly in azimuth and TPW-speed evolution, tend to distribute path reorientation over 60–45 Ma, and yield substantially larger misfits to the same data.

These results (i) isolate a geographically localized, time-specific mantle driver of the ~50 Ma TPW reversal, (ii) demonstrate reference-frame robustness using explicit misfit metrics, and (iii) provide a transferable workflow – geoid-rate mapping, inertia-tensor derivatives, and cap-blanking – for attributing TPW events to concrete mantle processes.