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Concept
Large low-shear-velocity provinces
Summary
Large low-shear-velocity provinces, LLSVPs, also called LLVPs or superplumes, are characteristic structures of parts of the lowermost mantle (the region surrounding the outer core) of Earth. These provinces are characterized by slow shear wave velocities and were discovered by seismic tomography of deep Earth. There are two main provinces: the African LLSVP and the Pacific LLSVP. Both extend laterally for thousands of kilometers and possibly up to 1,000 kilometres vertically from the core–mantle boundary. The Pacific LLSVP is across, and underlies four hotspots that suggest multiple mantle plumes underneath. These zones represent around 8% of the volume of the mantle (6% of Earth). Other names for LLSVPs include "superswells", "thermo-chemical piles", or "hidden reservoirs". Most of these names, however, are more interpretive of their proposed geodynamical or geochemical effects. For example, the name "thermo-chemical pile" interprets LLSVPs as lower-mantle piles of thermally hot and/or chemically distinct material. LLSVPs are still relatively mysterious, and many questions remain about their nature, origin, and geodynamic effects.
LLSVPs were discovered in full mantle seismic tomographic models of shear velocity as slow features in the lowermost mantle beneath Africa and the Pacific. The boundaries of these features appear fairly consistent across models when applying objective k-means clustering. The global spherical harmonic degree two structure is strong and aligns with its smallest moments of inertia along with the two LLSVPs. This means, by using shear wave velocities, the established locations of the LLSVPs are not only verified, a stable pattern for mantle convection emerges. This stable configuration is responsible for the geometry of plate motions at the surface due as well as mantle convection. Another name for the degree two structure, a roughly thick layer of the lower mantle directly above the core–mantle boundary, is the D′′ ("D double-prime" or "D prime prime").
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