Transition zone and lower mantle (2)

This latest lecture summarizes seismic observations of radial anisotropy in the D" region at the global level, showing a correlation between regions where this is present and zones of above-average shear velocity (interpreted as representing the graveyard of tectonic plates), suggesting the presence of magnesium post-perovskite (pPv, the high-pressure form of perovskite), whose crystals are more anisotropic than those of perovskite. A number of recent studies also suggest the presence of azimuthal anisotropy at the boundaries between these regions and the two zones of below-average shear velocity, located at opposite ends of the globe, one under the Pacific Ocean and the other under Africa, and dubbed the LLSVP(large low shear velocity provinces). The second part of this lecture is devoted to state-of-the-art materials physics methodologies for studying the mechanical properties of deep mantle minerals. In particular, major advances are being made in numerical calculations of deformation in these minerals, enabling a more realistic deformation regime to be achieved than is possible through laboratory experimentation at lower mantle pressures and temperatures. Finally, we present a recently-introduced approach to combining three fields of research: materials physics (elastic properties, slip planes), geodynamics (calculation of flow shape and deformation amplitude) and seismology to provide new constraints on deep mantle dynamics via seismological observations at the Earth's surface.

For further information on anisotropy in the lower mantle :

Carrez, P., D. Ferre, P. Cordier, (2007). Implications for plastic flow in the deep mantle from modelling dislocations in MgSiO3minerals. Nature 446, 68-70.

Catalli, K., S. H. Shim, V. B. Prakapenka (2009). Thickness and Clapeyron slope of the post-perovskite boundary. Nature 462, 782-785.

Cottaar, S., M. Li, A. McNamara, B. Romanowicz, H.-R. Wenk, (2014). Synthetic seismic anisotropy models within a slab impinging on the core-mantle boundary. Geophys. J. Intern. 199, 164-177.

Cottaar, S., B. Romanowicz, (2013). Observations of changing anisotropy across the southern margin of the African LLSVP. Geophysical Journal International 195 (2). 1184-1195. DOI 10.1093/gji/ggt285

Ford, H. A., M. D. Long, X. He, C. Lynner (2015). Lowermost mantle flow at the eastern edge of the African Large Low Shear Velocity Province, Earth Planet. Sci. Lett. 420, 12-22.

French, S. W. and B. Romanowicz (2015) Broad plumes Rooted At The Base Of The Earth's Mantle Beneath Major Hotspots, Nature, 525, 95-99.

Girard, J., G. Amulele, R. Farla, A. Mohiuddin, S. I. Karato (2016) Shear deformation of bridgmanite and magnesiowüstite aggregates at lower mantle conditions, Science, 351, 144-148.

Goryaeva, A. M., P. Carrez and P. Cordier (2016) Low viscosity and high attenuation in _MgSiO3 post-perovskite inferred from atomic-scale calculations, Nature Sci. Rep. 6:34771.

Irifune T. and T. Tsuchiya (2015) Phase transitions and mineralogy of the lower mantle, Treatise on Geophysics, 2nd Ed, Elsevier, Volume 2, chap 2.03.

Kraych, A., P. Carrez, P. Hirel, E. Clouet, P. Cordier, (2016). On dislocation glide in MgSiO3bridgmanite at high-pressure and high-temperature, Earth and Planetary Science Letters 452, 60-68 (2016) doi: 10.1016/j.epsl.2016.07.035

Lay, T. (2015). Deep Earth Structure: Lower Mantle and D", in Treatise on Geophysics, Vol. 1, Eds B. Romanowicz and A. M. Dziewonski, 1.22, 684-723.

Lay, T., D. V. Helmberger (1983). The shear-wave velocity gradient at the base of the mantle. J. Geophys. Res. 88, 8160-8170.

Lynner, C., M. D. Long, (2014). Lowermost mantle anisotropy and deformation along the boundary of the African LLSVP. Geophysical Research Letters 41, 3438-3446, doi:10.1002/ 2014GL059875.