The Depth of Magnetisation of Australia, and its Uncertainty, from a Hierarchical Inversion

The depth of magnetisation is the depth at which the crust or upper mantle ceases being magnetic. The loss of magnetisation is predominantly due to temperature effects, reflecting the Curie point of the constituent minerals of the region. Methods exploiting variations in the magnetic field are most often used to map variations in this depth, but the resulting uncertainty is not well characterised. In this study, we used aeromagnetic data, analysed at various scales, in combination with Monte Carlo techniques to evaluate both the maximum depth of magnetisation and its uncertainty for the Australian continent. The magnetic data have been represented with an areal hierarchy to produce stabilisation of the long wavelength anomalies that are crucial to magnetic techniques for defining the maximum depth of magnetisation. The magnetic field data are modelled using a one-dimensional fractal model to define the depth of magnetisation. For each level in the hierarchy, a contracting grid search is employed to produce an ensemble of possible solutions, from which robust estimates of the depth of magnetisation, and its uncertainty, can be produced across the continent. Variations in the depth of magnetisation for Australia are related to differences in mineralogy and thermal regimes across differing provinces of Australia, and may also be used to further our knowledge of crustal and mantle geothermal gradients, which relate to the distribution of heat producing elements.