MODELING OF THE WESTWARD DRIFT OF THE LITHOSPHERE

Global geodynamics is the result of the long-lasting action of large-scale stress sources with different origins. Plate boundary forces and mantle circulation are usually considered to be dominant in driving plate tectonics. However, these contributions are not sufficient to explain absolute plate dynamics. Plates follow a westward mainstream and the lithosphere with long-term angular velocity of about (0.1-0.3)°/Myr relative to the underlying mantle in the deep hotspot reference frame, which may increase up to 1°/Myr if the relative motion of hotspots is considered, e.g., according to the shallow hotspot reference frame model (Cuffaro and Doglioni, 2007). Plate motion is just one, even though likely the most evident global scale asymmetry in geodynamics. Together with other geophysical and geological observations, it suggests an exogenous influence on plate tectonics. After the Seventies, the interest in understanding a role of tidal forces in global geodynamics declined when it was noticed that the torque provided by tides is several orders of magnitudes weaker than the value required to directly speed up the plates. However, the interaction between the stratified mantle convection, the ultra-low viscosity within the Low Velocity Zone (LVZ) and the solid Earth tides could actually explain why plate tectonics appears to be westerly polarized. We realized a mathematical model supporting the observed westward drift of the lithosphere providing both analytical results and computational simulations. We demonstrate that the tidal drag of both Sun and Moon allows to explain why absolute plate motions, regardless of the reference frame, show a significant westward component while the variability of angular velocity among plates is controlled by the basal viscosity at the interface between the lithosphere and the mantle in the presence of a basal effective shear viscosity in the order of magnitude of 10¹⁶ Pa s within the LVZ atop the asthenosphere. With lower viscosity values, faster velocity would be possible, allowing better compatible with regional geodynamic observations. Reference: Cuffaro, M., & Doglioni, C. (2007). Global kinematics in deep versus shallow hotspot reference frames. In: Foulger, G.R., and Jurdy, D.M., eds., Plates, plumes, and planetary processes, Geol. Soc. Am. Spec. Pap., 430, 359–374, doi: 10.1130/2007.2430(18).