Australia and North America are the only two continent-wide regions, currently, with the complete wavelength spectrum of magnetic anomalies that can address the much-debated issue of the existence and understanding the magnetic state of the upper mantle.
The depth to the base of magnetic sources (DBMS) has been extensively investigated in Australia in previous studies, yielding differing DBMS estimates across the continent. In this contribution, the DBMS in Australia is re-assed from aeromagnetic data using the defractal method, which simultaneously estimates the DBMS and the fractal parameter of the magnetic field. The effect of window size is tested on DBMS estimates for the defractal method and conclude that a window size 10 times the expected DBMS is required for the defractal method. It is also demonstrate the dependence of derived spectral slopes (and thereby depths) on the size of window by comparing spectral slopes obtained from window sizes up to 1000 km. Present analysis yields DBMS estimates between 25 and 60 km over Australia and are complexly related to temperature structure, magnetic mineralogy and petrology and geological history. Deeper DBMS estimates (∼55–60 km) are obtained for the Yilgarn Craton (West Australian Craton) and the Gawler Craton (South Australian Craton).
These estimates are significantly deeper than the Moho, suggesting the uppermost mantle is ferromagnetic for these cold and old cratonic regions. Shallower DBMSs of 25–35 km are obtained in the Pilbara Craton, the eastern and southern part of the Delamerian Orogen, the northwestern part of the Canning Basin, Officer, Cooper and Georgina Basins and the Tasmanides. The areas of shallower DBMS are positively correlated with high heat flow in the regions. On the other hand, regions of large concentrations of radiogenic elements associated with the major metallogenic deposits have high heat flow arising from the radiogenic heat, but deeper DBMSs, for example the eastern margin of the Gawler Craton covered by Neoproterozoic Adelaide Rift Complex, the McArthur Basin, the Pink Creek Inlier.
Maps of the depth to the base of magnetization (DBMS) values estimated by (a) Chopping & Kennett (2015) and (b) Li et al. (2017). As discussed in the text, there are significant differences in the DBMS derived between the two studies. The black dashed lines are used to divide the region into four parts (west, south, central, north Australia), and the red dashed line is the Tasman line delimiting eastern Australia.
Simplified geological map of Australia (Stewart et al. 2013) superimposed with the estimated DBMS presented by coloured circles at the location described in Fig. 4. The larger circles correspond to the centre of windows of size of 1000 km × 1000 km used for calculations in Yilgarn and Gawler Cratons.
Raj Kumar , A.R. Bansal , Peter G. Betts and D. Ravat : Re-assessment of the depth to the base of magnetic sources (DBMS) in Australia from aeromagnetic data using the defractal method