Dynamics and stress field of the Eurasian plate: A combined lithosphere-mantle approach

This thesis presents a new combined lithosphere-mantle modeling approach to the dynamics of individual tectonic plates. This approach incorporates tractions from convective mantle flow modeling into a detailed analysis of the forces acting on a tectonic plate. Mechanical equilibrium of the plate is used as a constraint, requiring internal consistency between the model representation of both lithosphere dynamics and mantle flow. As such, it provides the opportunity to constrain remaining uncertainties, in particular concerning the nature and extent of coupling between the lithosphere and the underlying mantle. In this thesis, I focus on the Eurasian plate and find that it is approximately equally influenced by the actively convecting mantle and by resistance to plate motion. In chapter 2, lithospheric forces and their uncertainties are evaluated and the constraint of torque balance is used to estimate the net contribution of mantle tractions to the dynamics of the Eurasian plate. I conclude that the approximation of uniform shear (anti)-parallel to absolute plate motion does not give a good average of lithosphere-mantle interaction under Eurasia. Incorporation of active mantle flow in a direction other than absolute plate motion is thus indispensable for mechanical equilibrium of the Eurasian plate. In chapter 3, the analysis of Eurasian dynamics is extended by evaluating mantle tractions from convective mantle flow models. I find that only mantle flow models based on shear-wave tomography can successfully be combined with models of lithospheric body- and edge forces to mechanically balance Eurasia. Successful mantle flow models generate tractions on the base of the lithosphere that are approximately equally governed by active mantle flow and resistance to plate motion. The forces driving Eurasia are identified by comparing the net contribution of the various model forces with the direction of absolute plate motion. Forces arising from collision of Africa, Arabia and India are found to substantially deviate the direction of motion generated by the gravitational forces. A comparison of the contribution of the different forces to the dynamics of Eurasia shows that edge forces are dominant and generate a torque that is comparable to the combined contribution of mantle tractions and lithospheric body forces. Finally, in chapter 4 builds upon the results of the previous chapters by evaluating force models that mechanically balance the Eurasian plate based on their ability to reproduce observed stress directions. I find that the stress field is predominantly sensitive to the magnitude of the collisional forces on the plate's southern boundary. Stress observations require collision forces on the India-Eurasia plate boundary of about 10 TN/m.