The response of gravity to topography provides
important insights regarding the degree and mechanisms
of isostatic compensation. The azimuthally isotropic
coherence function between the Bouguer gravity anomaly
and topography evolves from high to low for increasing
wave number, a diagnostic that can be predicted for a
variety of lithospheric loading models and used in
inversions for flexural rigidity thereof. In this study
we investigate the isostatic response of continental
Australia. We consider the effects of directionally
anisotropic plate strength on the coherence. The
anisotropic coherence function is calculated for three
regions of Australia that have distinctive geological
and geophysical properties. The coherence estimation is
performed by the Thomson multiple-Slepian-taper spectral
analysis method extended to two-dimensional fields. Our
analysis reveals the existence of flexural anisotropy in
central Australia, indicative of a weaker N-S direction
of lower Te. This observation is consistent with the
suggestion that the parallel faults in that area act to
make the lithosphere weaker in the direction
perpendicular to them. It can also be related to the
N-S direction of maximum stress and possibly the
presence of E-W running zones weakened due to
differential sediment burial rates. We also demonstrate
that the multitaper method has distinct advantages for
computing the isotropic coherence function. The ability
to make many independent estimates of the isostatic
response that are minimally affected by spectral
leakage, particularly at low wavenumbers, results in a
more robust estimate of coherence. Our analysis
elucidates the reasons for discrepancies in previous
estimates of effective elastic thickness, Te, of the
Australian lithosphere. In isotropic inversions for Te,
we obtain values that are as much as a factor of two
less than those obtained in standard inversions using
Bouguer gravity and topography, but greater than those
obtained by inversions that utilize free-air rather than
Bouguer gravity and ignore the presence of subsurface
loads. However, owing to the low spectral power of the
Australian topography, the uncertainty on any estimate
of Te is substantial.
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