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Australia: The Land Where Time Began |
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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 PKKPdiff/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. 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.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |