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Australia: The Land Where Time Began |
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Australian Continent – the Lithosphere-Asthenosphere Transition
and Radial Anisotropy Beneath the Continent
Yoshizawa & Kennett examined the nature of the lithosphere-asthenosphere
transition (LAT) and its relation to radial anisotropy by using a new
3-D S wave speed model for the Australian region that was derived from
multi-mode surface waves. The estimated depth of the LAT in eastern
Australia during the Phanerozoic ties in well with receiver functions.
The lithosphere during the
Archaean and
Proterozoic in central and western Australia, however, the LAT
derived from the surface wave model is in general much deeper than the
discontinuities that were recognised from receiver functions, and also
shows a smooth transition. In the LAT and the underlying asthenosphere
as well as in the upper lithosphere there is significant radial
anisotropy (SH>SV). The effects of present shear flow in the mantle
beneath the continent are reflected in the strong anisotropy in the
asthenosphere. Yoshizawa & Kennett suggest the lateral variation of
lithospheric anisotropy is well correlated with the suture zones between
cratonic blocks which represent frozen anisotropy that is associated
with the ancient tectonics of Australia.
A key to an understanding of the interactions between continents and
plate tectonics is provided by the nature of the transition from the
lithosphere to the asthenosphere (LAT). The LAT represents a mechanical
or thermal boundary layer rather than a simple interface, as the LAT is
fundamentally associated with changing rheology and not with elastic
properties, which causes it to be difficult to detect with reflection or
transmission of seismic waves. A change in wave speeds is often the
result of physical properties that are responsible for a change in
rheological properties, and this allows the unravelling of this elusive
boundary layer by the combination of a variety of seismological
observations.
Conclusions
I global studies (Gung et al.,
2003; Debayle et al., 2005),
it has long been recognised that there are very fast shear wave speeds
in central and western Australia, and beneath the continent, anomalous
anisotropy. There are, however, clear indications of substructure and
distinctiveness between cratonic blocks (Kennett et
al., 2013), when analysed at
a regional scale by portable instrument deployments across the
continent. In this study Yoshizawa & Kennett were able to link the
broader scale features across the continent in terms of the nature of
the LAT and the variations of radial anisotropy.
In western parts of Australia the ancient cratons from the Archaean show
a gradual transition from lithosphere to asthenosphere with modest
anisotropy, and a thinner transition with a smaller velocity drop is
indicated by the surrounding Proterozoic belts.
In the cratons of Western Australia the radial anisotropy is somewhat
weaker than most other areas of the lithosphere of the Australian
continent. Similar results of weaker radial anisotropy in western parts
of Australia are shown by the radial anisotropy model of Fichtner et
al. (2010); though in the
west the horizontal resolution is weak as a result of a limited number
of paths used in the full-waveform modelling.
There is a dramatic change to the east. There is thinning and deepening
of the LAT which is accompanied by strong radial asymmetry (SH>SV),
particularly above 90 km in the upper lithosphere and the LAT and the
underlying asthenosphere. In the suture zones between the cratons and
their Proterozoic borders these features are most strongly developed.
This region displays the high shear wave speeds in the lithosphere that
are expected for cratonic regions, though it has repeatedly been
deformed (Cawood & Korsch, 2008), the most recent episode of which
occurred about 400 Ma in the
Alice Springs
Orogeny. It has been estimated (Kennett & Iaffaldano, 2013) that at
least 300 Myr would be required for full re-establishment of the
lithosphere in Central Australia following the orogeny, which would
allow for the distinctive local properties. In the upper lithosphere the
strong radial anisotropy correlates well with the geographical extent of
the extrusion of the lower crust that occurred during the Alice Springs
Orogeny, as has been proposed (Klootwijk, 2013).
It is suggested by a number of lines of evidence that there may be a
change in the character of lithospheric heterogeneity at
mid-lithospheric discontinuity (MLD) depths (Kennett, 1987; Thybo, 2006;
Kennett & Furumura, 2008). The apparent strength of radial anisotropy
would be enhanced by the presence of fine-scale heterogeneity (Fichtner
et al., 2013), and rapid
variation of lithospheric anisotropy can result from changes in the
style of heterogeneity with depth. It is suggested based on recent work
(Selway et al., 2015) that
vertical change in the character of radial anisotropy can produce a
negative peak in the S receiver function. Yoshizawa & Kennett say this
is consistent with their model, where the MLD depth from receiver
functions (RF) coincides well with the depth at which rapid change in
radial anisotropy occurs.
Across Australia one of the most striking features is the anisotropy
that is anomalously strong immediately beneath the estimate of the
deeper bound of the LAT, with the anisotropy peak strength coinciding
with the depth of the deeper bound in central Australia. This peak
anisotropy arises mostly primarily from an SV wave speed that is rather
slow, which strongly influences the estimated depth of the LAT from the
velocity gradient, while the fast SH wave speeds extend even below this
transition.
Yoshizawa & Kennett suggest it appears such strong anisotropy in the
asthenosphere is absent beneath the cratons of Archaean age in western
Australia. The absolute shear wave speed in central Australia is faster
than the average even below the LAT, which suggests a lithosphere root
that is mechanically more resistive and diffusive in central Australia,
which may be a cause of the anisotropy that is anomalously strong,
associated with basal drag or shear flow in the asthenosphere.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |