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Effective Elastic Thicknesses of the Lithosphere and Mechanisms of Isostatic Compensation in Australia

The isostatic compensation of the Australian lithosphere is examined using the coherence of the two-dimensional Fourier transforms of Bouguer gravity and topography. The isostatic model assumes regional compensation of an elastic plate that undergoes flexure in response to surface and subsurface loading. A least squares inversion of the observed coherence for continental Australia yields an effective elastic thickness of 130 km (corresponding flexural rigidity of 2.0×1025 N m). The model indicates that loads with wavelengths of greater than 1500 km are locally compensated, loads with wavelengths in the range 600–1500 km are partially supported by the strength of the lithosphere, and loads with wavelengths of less than 600 km are almost completely supported by lithospheric strength. Regions within the continent with different ages and tectonic histories exhibit effective elastic thicknesses that increase with the time since lithospheric stabilization. Precambrian shields in western, northern, and southern Australia are well fit by an elastic plate with a thickness of the order of 100 km. The Phanerozoic Interior Lowlands have elastic thicknesses that range from 30 to 80 km, and the Eastern Highlands have thicknesses of 15–35 km. The theoretical model of predicted coherence, which assumes statistically independent surface and subsurface loads, does not provide a good fit to the coherence spectrum for the intracratonic basins of central Australia. The disagreement of the model and observations is most simply explained by statistically correlated surface and subsurface loads. Calculations of load distributions show that most of the elevation of the Western Shield can be represented as a locally compensated plateau with wavelengths greater than 1000 km; smaller-amplitude, shorter-wavelength relief appears almost entirely as surface loading. Approximately half of the topographic amplitude of the Southeastern Highlands is locally compensated, and both surface and subsurface loading contribute significantly to the regionally compensated topography. The low rigidity and subsurface loading in southeastern Australia are consistent with crustal underplating, igneous intrusion, and thermal perturbation of the upper mantle as mechanisms for the uplift and compensation of the highlands.