Abstract
We present a new model of 3-D variations of shear wave speed in the Australian upper mantle, obtained from the
dispersion of fundamental and higher-mode surface waves. We used nearly 1600 Rayleigh wave data from the portable
arrays of the SKIPPY project and from permanent stations (from AGSO, IRIS and GEOSCOPE). AGSO
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data have not been used
before and provide better data coverage of the Archean cratons in western Australia. Compared to previous studies we
improved the vertical parameterization, the weighting scheme that accounts for variations in data quality and reduced the
influence of epicenter mislocation on velocity structure. The dense sampling by seismic waves provides for unprecedented
resolution of continental structure, but the wave speed beneath westernmost Australia is not well constrained. Global
compilations of geological and seismological data (using regionalizations based on tectonic behavior or crustal age) suggest
a correlation between crustal age and the thickness and composition of the continental lithosphere. However, the age and the
tectonic history of crustal elements vary on wavelengths much smaller than have been resolved with global seismological
studies. Using our regional upper mantle model we investigate how the seismic signature of tectonic units changes with
increasing depth. At large wavelengths, and to a depth of about 200 km, the inferred velocity anomalies corroborate the
global pattern and display a progression of wave speed with crustal age: slow wave propagation prevails beneath the
Paleozoic fold belts in eastern Australia and wave speeds increase westward across the Proterozoic and reach a maximum in
the Archean cratons. The high wave speeds associated with Precambrian shields extend beyond the Tasman Line, which
marks the eastern limit of Proterozoic outcrop. This suggests that parts of the Paleozoic fold belts are underlain by
Proterozoic lithosphere. We also infer that the North Australia craton extends off-shore into Papua New Guinea and beneath
the Indian Ocean. For depths in excess of 200 km a regionalization with smaller units reveals that some tectonic subregions
of Proterozoic age are marked by pronounced velocity highs to depths exceeding 300 km, but others do not and,
surprisingly, the Archean units do not seem to be marked by such a thick high wave speed structure either. The Precambrian
cratons that lack a thick high wave speed ‘‘keel’’ are located near passive margins, suggesting that convective processes
associated with continental break-up may have destroyed a once present tectosphere. Our study suggests that deep
lithospheric structure varies as much within domains of similar crustal age as between units of different ages, which hampers
attempts to find a unifying relationship between seismic signature and lithospheric age.
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