The role of grain size and temperature on viscous anisotropy in the mantle

Over 60% of the Earth’s upper mantle is built up by olivine. A particular characteristic of olivine crystals is their anisotropic behaviour: they deform (elastically and viscously) more easily in some directions than others. If many crystals align with each other, this anisotropic behaviour can have a macroscopic effect, which means that elastic anisotropy can be detected through seismic waves and viscous anisotropy can modify tectonic processes. The alignment of olivine crystals (development of LPO – lattice preferred orientation) is thought to be a consequence of deformation on the mantle (i.e. mantle flow). Therefore, seismic anisotropy from the asthenosphere is often used to interpret mantle flow directions. On the other hand, viscous anisotropy and LPO development are highly coupled as deformation depends on the viscosity, while the viscosity depends on the LPO.

The viscosity of the mantle controls many dynamic processes, from slab subduction to dripping instabilities, and the movement of tectonic plates on the surface. Viscosity is a function of many factors, including composition, stress, fluid content, temperature, grain size, anisotropic texture, etc. Temperature and grain size evolution is particularly important when studying the evolution of anisotropic textures and viscosity. So far, laboratory experiments on olivine and numerical models have demonstrated the importance of both grain size evolution or anisotropic viscous behaviour, but the two have never been studied in together.

This MSc project would be made in close collaboration with multiple CEED team members (Earth Modelling, Dynamic Earth and Deep Earth). The project will involve running state-of-the-art numerical models and analysing their output, mainly using MATLAB, to explore a variety of deformation and temperature paths through time.

The goal will be to analyse the development of both grain size and LPO and to evaluate how viscous anisotropy evolves dynamically. A particular focus will be to predict changes in the speeds of plate motions, which would be compared back to global plate reconstruction studies.

Project organisation

  • Literature review and required course work
  • Familiarization with using MATLAB
  • Familiarization with, and further development of MATLAB code that models viscous anisotropy
  • Running a selected series of numerical simulations
  • Post-processing and visualization of the model data
  • Geodynamic interpretation of the model results in conjunction with observables from seismic tomography and previous studies
  • Preparing a manuscript about the results for publication

Recommended skills

  • Basic knowledge of computer programming in order to modify a MATLAB code, setup the model runs, and handle the resulting data sets
  • Basic knowledge of plate tectonics and mantle dynamics

Learning outcomes

  • Computer programming
  • State-of-the-art numeric geodynamic modelling
  • Post-processing and visualising large datasets
  • Quantitative and qualitative geophysical analysis
  • Broad understanding of rheology, including anisotropy and grain size dependence.

This skillset, including scientific writing and critical thinking, is highly valuable for a career in both industry and academia.

Published Sep. 16, 2020 10:25 AM - Last modified Sep. 16, 2020 10:26 AM

Scope (credits)