Séminaire de Mélanie Gérault (post-doctorante au MIT, CAMBRIDGE, Etats-Unis)

Net rotation of the lithosphere in mantle convection models with self-consistent plate generation

Lateral variations in the Earth’s viscosity structure give rise to a global net rotation between the lithosphere and the mantle. Plate motion reconstructions, mantle flow computations, and inferences from seismic anisotropy all indicate some amount of net rotation based on various mantle reference frames. For the present-day, while the direction of rotation is somewhat consistent across studies, the predicted amplitudes range from ~0.1 deg/Myr to ~0.3 deg/Myr and more. Further back in time, the discrepancies are even greater. Such a lack of contraints is a major impediment to a deeper understanding of fundamental topics that rely on absolute surface kinematic data. Besides, the dynamics that govern the net rotation, its fluctuations in amplitude and direction, remain largely unidentified.
In this presentation, I will show the first time-dependent assessment of the net rotation in 3-D spherical mantle convection models with self-consistent plate generation. We run the computations for billions of years of numerical integration. The mantle convection problem is solved with the finite volume code StagYY using a visco-pseudo-plastic rheology [Tackley, 2008]. We look into how sensitive the net rotation is to heterogeneities in the upper boundary layer, such as the presence of continents of variable thickness. We also explore the links between net rotation and the initiation, development and cessation of subduction.
In all models, large fluctuations in net rotation occur over a few tenth of millions of years. The amplitudes vary from nearly zero to over 0.3 deg/Myr, with time averages toward the low end of the present-day estimates. These variations are generally associated with the initiation or cessation of subduction. The results suggest that the net rotation is closely related to the tectonic make-up of the surface, evolving with the nature of plate boundaries and the physical arrangement of the plates. A rapid increase (decrease) in net rotation is sometimes associated with the acceleration (deceleration) of one or two plates. Yet, the fastest mean surface velocity does not always correlate with the fastest lithospheric net rotation. The results highlight some of the interplays between deep mantle and surface dynamics over time.

Tackley, P. J. (2008). Modelling compressible mantle convection with large viscosity contrasts in a three-dimensional spherical shell using the yin-yang grid. Phys. Earth Planet. Inter., 171(1-4), 7-18.