Global upper mantle azimuthal anisotropy and the peculiar behavior of the Australian plate

1
Global upper mantle azimuthal anisotropy and the
peculiar behavior of the Australian plate

• Eric Debayle1, Brian Kennett 2 and Keith
  Priestley3
• 1 Ecole et Observatoire des Sciences de la
  Terre, CNRS and Université Louis Pasteur,
  Strasbourg
• 2 Research School of Earth Sciences, The
  Australian National University, Canberra,
  Australia.
• 2 Bullard Laboratories, University of Cambridge,
  Cambridge, United Kingdom.
• This work is subject to press embargo

2
Our global SV-wave heterogeneities azimuthal
anisotropy model

• 100779 Rayleigh waveforms (fundamental plus 4
  higher modes) matched between 50 s and 160 s.
• Short paths (1200-6000 km)
• improve the lateral resolution ( 800 km -1500
  km)

Number of paths
Paths length (km)
3
100 km Vs ref 4.41 km/s
200 km Vs ref 4.44 km/s
4
Depth-distribution of azimuthal anisotropy
5
Synthetic experiment
Input model
6
Azimuthal distribution of rays
Optimized Voronoi diagram (Debayle and
Sambridge, JGR 2004) each geographical point
belongs to the smallest cell for which the
azimuthal variation of Sv waves can be resolved.
7
Correlation between fast anisotropic direction
and absolute plate motion
good correlation
bad correlation
FastSVAPMcos(2f)
8
Averaged correlation between fast anisotropic
direction and absolute plate motion
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Debayke, Kennett and Prietsley (Nature, 2005)
Australia
Other continents
(VSH/VSV)2
SKS
SKS
(VSH/VSV)2
1.1
1
0
gt1
1
crust
crust
Moho
Moho
50
Frozen in anisotropy (east-west)
Frozen in anisotropy
?
Debayle and Kennett (EPSL 2000) Gaherty and
Jordan (Science 1995)
100
150
MBL
MBL
Plate-motion azimuthal anisotropy (north-south)
Weak or null azimuthal anisotropy
200
HVL
HVL
250
Depth (km)
HVL Base of the High Velocity Lid MBL
Mechanical Boundary Layer
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Conclusions

• The fast-moving Australian plate would contain
  the only continental region with a sufficiently
  large deformation at its base to be transformed
  into azimuthal anisotropy.

• For continents other than Australia, weak
  influence of basal drag on the lithosphere may
  explain why azimuthal anisotropy is observed only
  in a layer located in the uppermost 100 km of the
  mantle. This layer shows a complex organisation
  of azimuthal anisotropy suggesting a frozen in
  origin of deformation, compatible with SKS
  observations.
• NB we have performed various tests on our model
  that will be available on line on the Nature
  website in early 2005 as supplementary
  information on the paper.

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Effect of the non-inverted 4q azimuthal terms
a) C0 2q terms inversion (only 2q terms shown)
b) C0 2q 4q terms inversion (only C0 and 2q
shown)