Super-panaches or LLSVPs (2)

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Ultra-low velocity zones (ULVZ), their characteristics, physicochemical nature and role. Anisotropy at the base of the mantle.

We have outlined the three-dimensional structure of the Earth's mantle, as seen in tomographic S-wave models, contrasting the structure at 100 km depth, which reflects surface tectonics, with that deeper down, which first shows plunging plates in the extension of subduction zones (between 400 and 670 km depth), then a shorter-scale structure in the middle of the mantle, and finally, once again a structure dominated by long wavelengths, as we approach the CMB (around 2500-2800 km depth). There is also an increase in the amplitude of lateral variations as we approach the CMB. This remarkable structure near the CMB is organized into two quasi-equatorial and antipodal zones of shear velocities below the global average, surrounded by a ring of faster velocities. The structure is dominated by a "degree 2" component, oriented in a manner corresponding to a stable configuration of the Earth's moments of inertia. The two slow zones are known as LLSVPs(Large Low Shear Velocity Provinces), and are sometimes also nicknamed "mega-panaches". Their nature (thermal/compositional?) and their role in global dynamics are not yet well understood. There is an asymmetry in the profile of shear velocities with depth, between the LLSVPs, where the velocity gradient is very steep near the CMB, and the fast-velocity regions surrounding them, where velocity increases moderately with depth. This is a possible indication of a compositional component to these structures. Another observation that supports the notion of distinct composition is the very steep character of the edges of LLSVPs. Some authors have proposed that LLSVPs are denser than the surrounding regions, based on observations of eigenmodes, but this result remains controversial.

On the other hand, the structure at the base of the mantle is correlated at its longest wavelengths with the distribution of plunging plates in the upper mantle, the attenuation structure in the transition zone and the distribution of hot spots at the surface, indicating that there is a thermal component to the slow shear rates of the LLSVPs.

Two contrasting scenarios are proposed to explain the presence of LLSVPs: they are either "geochemical reservoirs" of primordial material, dating from the time when a magma ocean was present in a large part of the mantle, or the manifestation of compositional and thermal heterogeneity introduced by plates that have fallen to the bottom of the mantle.

We have also described the observations that led to the detection of a discontinuity around 200 km above the CMB in certain regions, which generally correspond to areas outside the LLSVPs. These are mainly observations of precursors to waves reflected on the CMB, of type S (ScS) or P (PcP), made using regional seismic networks. The discontinuity is more pronounced in S (~3% velocity jump) than in P (1% jump), and occurs over a vertical distance of less than 70 km. The jump in density is more difficult to constrain, and in general the observations of discontinuity in S and P are not correlated, which is surprising. However, the reality of this discontinuity was recently (2004) confirmed by the independent discovery of the perovskite → post-perovskite (Pv → pPv) phase transition, made simultaneously experimentally and by theoretical calculations. If the seismic discontinuity does indeed correspond to this phase transition, this will provide very precise constraints on the temperature at the base of the mantle in the near future.