Australia: The Land Where Time Began

A biography of the Australian continent 

Superplumes – Fine-Scale Ultra-Low Velocity Zone Layering and Core-Mantle Boundary and Superplumes

It has been detected by the use of a variety of seismic probes that at the core-mantle boundary (CMB) that is ultra-low velocity layering. It has been found by mapping that in a thin layer, 5-50 km thick, which commonly underlies reduced shear wave speeds in the overlying few hundred km of mantle, there are P- and S-waves velocity reductions of up to tens of percent. Properties consistent with partial melt of rock at the very base of the mantle are contained by ultra-low velocity zones (ULVZ). There is now strong evidence of a significant increase in density in the layer (~5-10 % greater than reference models), which must be included I dynamical scenarios which relate ULVZ partial melt to plume genesis in the deep mantle. When plat motions in the past are imposed as a surface boundary condition 3-D dynamical calculations that involve a uniform dense layer that is initially uniform in the lowermost few hundred km of the mantle result in thermos-chemical piles that are well-correlated geographically with seismic tomography low velocities. The lower mantle regions that have been found to be the hottest underlie edges of the dense thermos-chemical piles. In this paper the authors¹ propose a scenario where these piles correlate geographically with ultra-low velocity zones, and subsequent genesis of mantle plumes.

Garnero et al.¹ discussed ultra-low velocities at the CMB, and their possible relationship to megapiles or superplumes in the deep mantle.  The structure of the ULVZ may have effects on the global cycles of the Earth that are far-reaching (Muller, 2002; Dobson & Brodholt, 2005), though there are still important uncertainties in the seismic modelling. An example is the SPdKS and PKP modelling, the CMB interacts with waves entering and exiting the core. Therefore, in many cases there are large uncertainties regarding the exact location of anomalous structures. Garnero et al. suggest this can be improved by adding topside CMB reflection probes, such as PcP or ScP precursors (e.g., Mori & Helmberger, 1995; Rost et al., 2005), as well as devising new ULVZ probes, like PKKP^diff/_AB (Rost & Garnero, 2006). Most ULVZ modelling to date utilised 1D wave propagation tools, with the exception of a few notable studies, such as Wen & Helmberger, 1998a,b). Improvements in this area need to be included in future studies.

Garnero et al. have summarised recent ULVZ findings in relation to possible dense thermochemical piles in Dʺ, which may be related intimately to superplumes. Garnero et al. suggest the large dense piles may explain the degree 2 low shear velocity heterogeneity revealed by seismic tomography, and help with an explanation of several seismic observations, including the sharpness of the sides of these structures, and marked differences, even anticorrelation, between P and S heterogeneity – neither of which can be explained by thermal anomalies alone. It is suggested by geodynamical calculations that the hottest temperatures in the deep mantle occur beneath the corners of dense piles, which is the most likely region to develop ULVZ with origin of partial melt. The most likely regions for the formation of mantle plumes are the topographical ridges to the dense piles near these edges. A self-consistent story for ULVZ is presented by this scenario, large scale low velocities in the deepest mantle, and the genesis of plumes, there are several important uncertainties that have been noted, primarily in relation to resolution issues in the seismic imaging and geodynamic modelling.

Sources & Further reading

Edward J Garnero, Michael S. Thorne L, Allen McNamara, and Sebastian Rost in Yuen, D.A., Maruyama, S, Karato, Shun-ichiro & Windley, B., (Eds), 2007, Superplumes: Beyond Plate Tectonics, Springer.