Environmental Engineering Reference
In-Depth Information
Secondly, this surface-to-CMB pattern is recognized from both a moving hot-
spot frame (black squares in
Figure 3.2a
), and a TPW-corrected palaeomagnetic
frame (white squares in
Figure 3.2a
) before 125 Ma that uses Africa as a reference
plate that has remained quasi-stationary with respect to longitude since Pangea
formation.
Thirdly, using the surface-to-CMB correlation for the past 300 Myr we have
also relocated one additional LIP
-
the Emeishan CLIP in South China
-
above
the western margin of the Jason LLSVP.
Fourthly, similar conclusions have previously been reached about the majority
of these LIPs (see, e.g., Torsvik
et al
.,
2006
;
2010
; Burke
et al
.,
2008
; and Lekic
et al
.,
2012
). What is new about this study is that new TPW results and a new
global moving hotspot model have been applied together to yield a consistent and
comparable set of results for the locations of the 30 LIPs.
Fifthly, the availability of a new set of restored locations for LIP eruption sites
at the surface has enabled us to drop perpendiculars to the CMB from all those
30 eruption sites to
find which, if any, of eight different seismological maps
of shear-wave velocity variation distribution on the CMB performed best as
indicators of the locations of PGZs on the CMB.
Figure 3.2
shows that all eight
maps gave closely comparable results in both the
'
mantle and TPW-corrected
frame
. The SMEAN map performed differently
because of different mapping criteria rather than from inadequacy in displaying
shear-wave velocity distribution. In summary, all the seismologically prepared
and various shear-wave velocity maps of the CMB performed comparably well
in displaying the loci of PGZs on the CMB.
Sixthly, the Earth presently contains two anti-podal, equatorial LLSVPs in
the deep mantle, i.e. the Earth is of degree-2 mode in that respect. However,
that mass distribution has been suggested to date from a fairly recent time in the
past (since about 200 Ma) and to have been preceded by degree-1 mode with
only a single equatorial-centred mass. Because our newly controlled 30 LIP
dataset straddles in age the time during which a transition from degree 1 to
degree 2 is suggested to have occurred (e.g. Zhong
et al
.,
2007
; Zhang
et al
.,
2010
) we took the opportunity, by separately treating the LIP and kimberlite
populations of 100 Ma to 200 Ma and 200 Ma to 300 Ma, to test whether there
was evidence of change, during the postulated degree-1-to-degree-2 transition in
mapable PGZ distribution. Our results are shown in
Figure 3.4
. No signi
'
and in the
'
palaeomagnetic frame
'
cant
differences in PGZ distribution between or within the two intervals are discern-
ible. We conclude that there has not been a change in deep mantle mass
distribution from degree 1 to degree 2 during the Pangea assembly and dispersal
and that the deep mantle mass distribution has always been of degree 2 since
the Late Palaeozoic.
