Isostatic response of the Australian lithosphere: Estimation of effective elastic thickness and anisotropy using multitaper spectral analysis

Gravity and topography provide important insights regarding the degree and mechanisms of isostatic compensation. The azimuthally isotropic coherence f unction between the Bouguer gravity anomaly and topography evolves from hig h to low for increasing wavenumber, a diagnostic that can be predicted for a variety of lithospheric loading models and used in inversions for flexura l 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 cal culated for regions of Australia that have distinctive geological and geoph ysical properties. The coherence estimation is performed by the Thomson mul tiple-Slepian-taper spectral analysis method extended to two-dimensional fi elds. Our analysis reveals the existence of flexural anisotropy in central Australia, indicative of a weaker N-S direction of lower T-e. This observat ion 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 bu rial rates. We also demonstrate that the multitaper method has distinct adv antages for computing the isotropic coherence function. The ability to make many independent estimates of the isostatic response that are minimally af fected by spectral leakage results in a coherence that is more robust than with modified periodogram methods, particularly at low wavenumbers. Our ana lysis elucidates the reasons for discrepancies in previous estimates of eff ective elastic thickness T-e of the Australian lithosphere. In isotropic in versions for T-e, we obtain values that are as much as a factor of 2 less t han those obtained in standard inversions of the periodogram coherence usin g Bouguer gravity and topography but greater than those obtained by inversi ons that utilize free-air rather than Bouguer gravity and ignore the presen ce of subsurface loads. However, owing to the low spectral power of the Aus tralian topography, the uncertainty on any estimate of T-e is substantial.