Australian Continent – the Lithosphere-Asthenosphere Transition and Radial Anisotropy Beneath the Continent

Australia: The Land Where Time Began

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.

Sources & Further reading