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. The Pacific LLSVP has specific dimensions of 3000 km across and 300 m higher than the surrounding ocean-floor, and is situated over four hotspots that suggest multiple mantle plumes underneath.[1] These zones represent around 3% of the volume of the Earth. LLSVPs are also called superplumes, superwells, 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.

Cartoon of the Pacific LLSVP
Cartoon of the African LLSVP

Seismological constraints

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.[2] The global spherical harmonic degree two structure is strong.[3] 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.[4] 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.[5]

Possible origin

The current leading hypothesis for the LLSVPs is the accumulation of the lithospheric subducted oceanic slab. This correspond with the locations of known slab graveyards surrounding the Pacific LLSVP. These graveyards are thought to be the reason for the high velocity zone anomalies surrounding the Pacific LLSVP that is thought to have formed by seduction zones that were around long before for the dispersion of the supercontinent Rodinia. Aided by the phase transformation, the temperature would partially melt the slabs, to form a dense heavy melt that pools and forms the ULVZ structures at the bottom of the CMB closer to the LLSVP than the slab graveyards. The rest of the material is then upswept due to chemical buoyancy contributing to the high levels of Basalt found at the MOR. The resulting motion forms small clusters of small plumes right above the CMB that combine to form larger plumes and then contribute to superplumes. The Pacific and African LLSVP, in this scenario, are originally created by a discharge of heat from the core(4000K) to the much colder mantle(2000K), the recycled lithosphere is only fuel that helps drive the superplume convection. Since it would be difficult for the Earth's core to maintain this high heat by itself, it gives support for radiogenic elements in the core, as well as the indication that if fertile subducted lithosphere stops subducting in locations preferable for superplume consumption, it will mark the demise of that superplume.[1]

Dynamics

Geodynamic mantle convection models have included compositional distinctive material. The material tends to get swept up in ridges or piles.[5] 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.[6] 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. This geometrical relationship is also consistent with the position of the supercontinent Pangaea, and the formation of the current geoid pattern due to continental break-up from the superswell below.[3]

References

  1. ↑ 1.0 1.1 Maruyama; Santosh; Zhao (4 June 2006). "Superplume, supercontinent, and post-perovskite: Mantle dynamis and anti-plate tectonics on the Core-Mantle Boundary" (PDF). Gondwana Research 11 (1-2): 7–37. doi:10.1016/j.gr.2006.06.003. Retrieved 17 August 2006.
  2. ↑ Lekic, V., Cottaar, S., Dziewonski, A., and Romanowicz, B. (2012). "Cluster analysis of global lower mantle". EPSL.
  3. ↑ 3.0 3.1 Dziewonski, A.M., Lekic, V., Romanowicz, B. (2010). "Mantle Anchor Structure: An argument for bottom up tectonics" (PDF). EPSL.
  4. ↑ To, A., Romanowicz, B., Capdeville, Y., Takeuchi, N. (2005). "3D effects of sharp boundaries at the borders of the African and Pacific Superplumes: Observation and modeling". EPSL.
  5. ↑ 5.0 5.1 McNamara, A.M., Garnero, E.J., Rost, S. (2010). "Tracking deep mantle reservoirs with ultra-low velocity zones" (PDF). EPSL.
  6. ↑ Steinberger, B., Torsvik, T.H. (2012). "A geodynamic model of plumes from the margins of Large Low Shear Velocity Provinces" (PDF). G^3.

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