Large Low Shear Velocity Provinces
Large Low Shear Velocity Provinces (LLSVPs ) are characteristic structures of the lowermost mantle (the region right above the outer core of the Earth). These provinces are characterized by slow shear wave velocities and were discovered by seismic tomography of the 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 1000 km vertically from the core-mantle boundary. These zones represent around 3% of the volume of the Earth. LLSVPs are also called superplumes, thermo-chemical piles or hidden reservoirs. Many of these names, however, are more interpretive of their geodynamical or geochemical effects, while many questions remain about their nature.
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. The LLSVPs lie around the equator, but mostly on the southern hemisphere. Global tomography models inherently result in smooth features; local waveform modeling of body waves, however, has shown that the LLSVPs have sharp boundaries. The sharpness of the boundaries makes it difficult to explain the features by temperature alone; the LLSVPs need to be compositionally distinct to explain the velocity jump. Ultra Low Velocity Zones (ULVZ) at smaller scales have been discovered mainly at the edges of these LLSVPs.
One hypothesis for the origin of the LLSVPs is the accumulation of subducted oceanic slabs. However, it is difficult to stabilize the subducted oceanic material on the core-mantle boundary, without it heating up and upwelling. It is also difficult to explain their slow velocities in this manner.
Another hypothesis is that the regions are the remnants of a basal magma ocean. In this scenario, when the early earth solidifies, it excludes silica and iron in the lowermost mantle. Eventually this material solidifies too, but is too dense to mix in with the rest of the material, and is swept up into piles. Additionally, it might leave patches of partially molten, iron-rich material, explaining the observation of ULVZs.
Geodynamic mantle convection models have included compositional distinctive material. The material tends to get swept up in ridges or piles. When including realistic past plate motions into the modeling, the material gets swept up in locations that are remarkably similar to the present day location of the LLSVPs. These types of models, as well as the observation that the degree two structure of the LLSVPs is orthogonal to the path of true polar wander, suggest these mantle structures have been stable over large amounts of time.
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