Numerical Simulation of True Polar Wander during Supercontinent Assembly

The supercontinent cycle is often accompanied by True Polar Wander (TPW) events (Evans, 2003) — reorientation of the silicate Earth relative to its spin axis in response to internal mass redistribution. During TPW events, the maximum inertia axis (I_max) aligns with the spin axis to conserve the angular momentum (Gold, 1955). While an assembled supercontinent typically reside near the equator once it has developed its own degree-2 mantle structure driven by a circum-supercontinent subduction girdle with two antipodal superplumes (Li et al., 2023), this configuration is not always instantaneous with the assembly of a supercontinent. Supercontinent is in fact believed by some to assembly over a degree-1 mantle structure: a cold downwelling beneath the supercontinent and a hemispheric superplume on the opposite hemisphere (Zhong et al., 2007; Zhong and Liu, 2016). The resulting TPW behavior during such processes remains poorly constrained. Here we report a novel computational framework that couples 3D spherical mantle convection (CitcomS) with Earth’s rotational dynamics to simulate TPW driven by both convective mass anomalies and rotational bulge readjustment. We particularly examined the effect of varying upper/lower mantle viscosity ratios (η_um/η_lm).

Our results reveal a critical dependence of TPW behavior on viscosity stratification. For high η_um/η_lm (1:30), supercontinents assemble near the pole over a degree-1 mantle structure. Subsequent formation of a subduction girdle triggers TPW, transporting the supercontinent to the equator. In contrast, low η_um/η_lm (1:100) with a mean lower-mantle viscosity of 3×10²² Pa·s promotes equatorial assembly. Here, girdle development induces TPW that transports the supercontinent toward the pole, where it stabilizes for a considerable period. However, reducing lower-mantle viscosity destabilizes this polar position, causing rapid return to the equator. These dynamics arise because viscosity stratification determines the structure of the geoid kernel, which governs the geoid’s response to mass anomalies and thereby modulates TPW pathways. Our models demonstrate that before a stable degree-2 structure (e.g., modern LLSVPs) is developed, TPW can drive complex supercontinent trajectories—including equator-to-pole-to-equator round-trip migrations. Future work integrating plate reconstruction with viscosity constraints will refine predictions for specific supercontinents.

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