Lithosphere-mantle coupling and the dynamics of the Eurasian Plate

Abstract

Mechanical equilibrium of tectonic plates implies that lithospheric edge and body forces are balanced by forces arising from interaction with the underlying mantle. We use this quantitative physical relation to integrate existing modelling approaches of lithosphere dynamics and mantle flow into a new combined approach applied to the Eurasian Plate. By combining a thorough analysis of lithospheric forces with the requirement of torque balance we constrain the orientation of the torque on Eurasia arising from mantle tractions. We use this constraint to evaluate convective mantle flow models driven by tomographic or subduction history model anomalies and observed plate motion. Mantle forcing is considered through both shear at the bottom of the plate and induced dynamic topography. We find that instantaneous semi-analytic flow models without lateral viscosity variations generate tractions that meet the constraint from Eurasian lithosphere dynamics, but only for specific ranges of mantle flow parameters. Of the explored set of mantle anomaly models, only mantle flow models based on S-wave tomography anomalies can balance Eurasia for realistic viscosity profiles and velocity–density scaling. Choices in mantle density forcing and viscosity are crucial in that they govern the relative magnitude of tractions due to convective mantle flow (‘active tractions’) and resistive tractions due to plate motion (‘passive tractions’). We find mechanical balance is only achieved for similar torque magnitudes of active and passive shear. The two shear contributions do however in no case balance each other and a considerable, dominant, net torque from edge forces is required to balance total mantle tractions and lithospheric body forces (LBFs). Our analysis provides a range of mechanically consistent total force sets acting on the Eurasian Plate. Using this result we find that mantle buoyancy forces and LBFs acting on Eurasia itself are important driving forces but do not drive Eurasia in the observed direction. Continental collision at Eurasia’s southern boundary significantly deviates Eurasia northwards. Our combined torque balance approach, in which mantle tractions from convective mantle flow modelling are combined with explicitly applied edge forces, thus emphasizes the role of plate interactions to the dynamics of tectonic plates