Evidence of Westerly Polarized Plate Tectonics

Slab-pull is considered the force with the greatest impact on plate motions, but also ridge-push, trench suction and physico-chemical heterogeneities are thought to play an important role in driving lithospheric movements. However, several arguments suggest that these mechanisms are insufficient to explain global-scale tectonics. Moreover, there are tectonic lines of evidence that plates follow a mainstream and the lithosphere has a 0.2-1.2°/Myr westerly drift relative to the asthenospheric mantle. Tidal forces are usually considered too weak to directly speed up plates. However, the interaction between the stratified mantle convection, the ultra-low viscosity within the Low Velocity Zone (LVZ) atop the asthenosphere and solid Earth tides could explain the asymmetry of plate boundaries and fuel plate tectonics. Tidal drag allows to explain why absolute plate motions, regardless of the reference frame, show a significant west-ward drift of the lithosphere with respect to the underlying mantle and it also provides a reason for large evidence of polarization, which otherwise should be attributed to chance. Variable angular velocities among plates can be controlled by the viscosity gradients within the LVZ. GNSS measurements are now accurate and long-lasting enough that both secular plate motions and periodic tidal displacements can be detected, although with still large uncertainties, allowing more accurate analysis of the contribution of the horizontal component of the body tide in shifting the lithosphere. We review the data and show that the speed of lithospheric plates shows a correlation with tidal harmonics (Zaccagnino et al., 2020). Seismic activity also correlates with tides. The low-frequency body tides rather show horizontal residuals equal to the relative motion among plates, suggesting a possible astronomical tuning on plate dynamics. The high-frequency body tides are mostly buffered by the high viscosity of the lithosphere and the underlying mantle, whereas low-frequency harmonics (e.g, 18.6 years period tide) are compatible with the relaxation time of the LVZ and can allow the westerly drag of the lithosphere over the asthenospheric mantle.

Bibliography

Zaccagnino, D., Vespe, F., & Doglioni, C. (2020). Tidal modulation of plate motions. Earth-science reviews, 205, 103179.