Isostatic response of the Australian lithosphere:
Estimation of effective elastic thickness and anisotropy
using multitaper spectral analysis
Department of Earth, Atmospheric and Planetary
J. Geoph. Res.105 (B8), 19163-19184, 2000
Massachusetts Institute of Technology (MIT)
Cambridge MA 0213 USA
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.
- Figure 01
The necessity of averaging in the coherence calculation.
- Figure 02
One-dimensional Slepian tapers for spectral analysis.
- Figure 03
Two-dimensional Slepian tapers for spectral analysis: spatial domain.
- Figure 04
Two-dimensional Slepian tapers for spectral analysis: spectral domain.
- Figure 05
Synthetic coherence data set: 1-D isotropic case.
- Figure 06
Synthetic coherence data set: 2-D (an)isotropic case.
- Figure 07
Tectonic map of Australia
- Figure 08
Australian gravity and topography data set.
- Figure 09
Coherence calculation for three domains in Australia; one and two dimensions.
- Figure 10
Comparison of free-air anomaly with gravity from uncompensated topography.
- Figure 11
Influence of wavenumber resolution on Central Australia coherence.
- Figure 12
Error in Te determination for the three domains in Australia.
- Figure 13
Coherence calculation for three domains in Australia. Comparison of periodogram and multitaper methods.
Last modified: Wed Apr 12 23:06:25 EDT 2023