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We present a new model of three-dimensional
variations of shear wave speed in the Australian upper
mantle, obtained from the dispersion of fundamental and
higher-mode surface waves. We used Rayleigh wave data
from the portable arrays of the SKIPPY project
and from permanent stations (from AGSO,
IRIS and GEOSCOPE), amounting to about
1600 source-receiver combinations. AGSO data
have not been used before and provide better data
coverage of the Archean cratons in western Australia.
Compared to previous studies we also improved the
vertical parameterization and the weighting scheme that
accounts for variations in data quality and we 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 owing to insufficient station
coverage. 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 detailed 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 that we associate with Precambrian shields extend
beyond the so-called 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 southwestern
Papua New Guinea and beneath the Indian Ocean. For
depths in excess of 200 km a regionalization with
smaller units reveals a more complex pattern. 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 can vary as much within domains
of similar crustal age as between units of different
ages, which hampers attempts to find a unifying
relationship between seismological units and crustal age
domains. |
