Heterogeneity and Flow in the Deep Earth
Over the past half century, study of deep regions in the Earth has revealed them to be complex and dynamic. Much of our knowledge comes from seismological data, and ultimately these observations need to be linked to results from geodynamics and mineral physics in order to make inferences about compositional heterogeneities and flow.
One example of a strongly-heterogeneous region is the lower thermal boundary layer of the mantle, i.e. the layer of several hundred kilometers thickness above the core-mantle boundary, commonly referred to as D’’. This region appears to be characterized by two large provinces of distinctive slow shear velocities, 4000-5000 km across, one beneath the Pacific and one beneath Africa. Surrounding these regions, seismic velocities are faster, and often interpreted as corresponding to a graveyard of slabs. A recently discovered phase transition from perovskite to postperovskite may also occur in this depth range and has been associated with an intermittently observed seismic discontinuity at the top of D’’ This study adds to the seismological evidence for complexities both in isotropic seismic velocities as well as in anisotropic velocities and how those can be linked to flow of material.
We map a distinctive small "pile" of slow shear velocity beneath Russia through direct waveform evidence. This "pile" is less than 1000 km across, and thus much smaller than the Pacific and African provinces. Its height is several hundreds of kilometers and its velocity reduction suggests it is composed of the same material as the large provinces of slow shear velocity.
Beneath Hawaii, at the northern edge of the Pacific province, we find an extended thin zone of ultra-low velocities. This is the first time the three-dimensional extent of such a zone is constrained with some accuracy. The constraints on its morphology come from the presence of strong postcursors to shear waves diffracted along the core-mantle boundary, delayed by 30 to 50 seconds with respect to the main diffracted arrival. The zone is almost 1000 km across and roughly 25 km high. Its shear velocity is reduced by ~20% with respect to the global average, which is possible through strong iron enrichment or the presence of partial melt. Given its location, one can speculate that this zone may represent an anchor to a whole-mantle plume reaching the surface of the Earth beneath the Hawaiian hotspot and may also represent the source of geochemical anomalies in Hawaiian basalts.
Around the southern margin of the African low shear velocity province, we map strong variations in anisotropic velocity structure. These could be interpreted as evidence for strong flow outside the region of slow velocity, and insignificant flow within. To understand observations of seismic anisotropy in terms of flow in a more general context, we adopt a multi-disciplinary approach. We forward model anisotropic texture development through geodynamical models, reflecting mineral-physical constraints on the deformational behavior and elastic constants of possible compositions. In both two and three-dimensional models, we find that perovskite and post-perovskite result in distinctive anisotropic patterns. Deformation on different crystallographic lattice planes within post-perovskite also results in different signatures. In general, the presence of post-perovskite appears to best explain seismic observations of anisotropy in the deep mantle.
One other region of strong heterogeneous and anisotropic velocity variations is the inner core. In the last chapter, we model thermal histories to assess the possibility of episodes of convection over the course of the inner core growth. Our numerical model predicts strains due to convective flow that could cause texturing and seismic anisotropy. An early termination of this convection can explain the stronger anisotropy seen in the innermost inner core (the most central 500 km).