Limited true polar wander as evidence that Earth's nonhydrostatic shape is persistently triaxial

Abstract

Earth's spin axis follows the maximum moment of inertia axis of mantle convection, with some delay due to adjustment of the rotational bulge. Here we compute this axis for geodynamic models based on subduction history, assuming constant slab sinking speed, with another contribution due to thermochemical piles. For a wide range of parameters, a large shift of ≈90° is predicted around 80–90 Ma. It can be largely attributed to a change in circum‐Pacific subduction from predominantly in the North and South toward East and West. Actual amounts of true polar wander are much smaller, pointing toward additional inertia tensor contributions, possibly due to slabs in the lowermost mantle below both polar regions. These slabs would have been subducted before ≈150 Ma, when plate motions in the Panthalassa basin are largely unknown. Matching predicted and observed true polar wander can serve at constraining such plate motions.

What drives 20th century polar motion?

“…Pa s. These are in agreement with the average profiles of many GIA models [10,15,16], including those that account for rotational constraints [12], and with a number of mantle convection models, some of which are considered later in this study [17]. Our estimates are also consistent with viscosities inferred, for example, from Satellite Laser Ranging measurements of low-degree gravity field [18], mineral physics [19], and sinking rate of subducted lithosphere [20].…”

“…This "True Polar Wander" (TPW) can be modeled geodynamically, because the spin axis follows the maximum inertia axis, which can be determined from the degree-2 geoid. Although advanced geodynamical models now incorporate the delayed viscous adjustment of the rotational bulge [37,17], we mostly disregard this effect here. This is because the adopted viscosities and the inferred rates of TPW are such that the spin axis should follow very closely the maximum inertia axis associated with mantle convection [38], as confirmed by the one case considered in our calculations where the effect of rotational bulge adjustment is investigated.…”

“…This is because the adopted viscosities and the inferred rates of TPW are such that the spin axis should follow very closely the maximum inertia axis associated with mantle convection [38], as confirmed by the one case considered in our calculations where the effect of rotational bulge adjustment is investigated. The evolving density anomalies in geodynamical models, which give rise to geoid changes, can be inferred from global subduction histories and the sinking rate of slabs [39,17], or from density and flow models [40,38] based on seismic tomographic observations [41].…”

“…Here, we assemble "present" rates of TPW -termed mantle convection driven SPM -computed by contemporary geodynamical models without an a priori goal of reconciling the residual SPM (Supplementary Methods Section 3). We retrieve 94 modeled rates of TPW, each for the last 2 and 10 million years, from a suite of slab subduction driven forward models that were based on plate motions and aimed at reconstructing 3-D seismic structure and the geoid [17]. These are computed assuming a realistic viscosity structure [19], which is compatible with -though not precisely the same as -the inferred viscosity profile in our GIA Bayesian inversions [13].…”

“…Firstly (columns 6 and 7 in Table S6), the degree-2 geoid coefficients are adjusted such that they exactly match the present-day values, and hence the rotation pole is exactly matched. Secondly (columns 8 and 9 in Table S6), like for most cases in the forward approach [17], geoid coefficients are not modified, but the TPW curve is shifted such that it matches the north pole known at the present-day. Both cases tend to give similar azimuth, but the computed amount of motion is often quite a bit larger in the latter case.…”