The crust-mantle boundary is defined seismologically by the Mohorovicic discontinuity (or Mono), where the velocity of seismic waves increases from typical crustal values to typical mantle values. The depth of the Moho beneath Australia has been mapped using all available seismic data in order to study the crustal thickness patterns and their relationship to tectonic regions. There are significant variations in crustal thickness in Australia. Generally the crust in Archaean regions of Western Australia is relatively thin, with large velocity contrasts at the transition from the crust to the mantle. Parts of Tasmania and the New England Fold Belt also have relatively thin crust. The crust is significantly thicker in the Proterozoic north and central Australia and in Phanerozoic southeastern Australia. In these areas there is a very broad transition from crustal to mantle velocities. Other regions of Australia are generally intermediate in character. The crustal thickness ranges between 24 and 56 km; the average crustal thickness in Australia is about 38 km. Previous studies have regionalised the Australian continent into seven ‘mega-elements’ on the basis of geological and geophysical character and tectonic age. However, the pattern of crustal thickness does not match the mega-elements with the possible exception of the North East Australia mega-element. Differences in crustal thickness that may exist between the mega-elements generally do not occur where the mega-element boundaries are mapped but may be offset by a considerable distance. A major dislocation of the Moho in central Australia is possibly an example of this.

There is now a comprehensive coverage of magnetic and gravity potential-field datasets for the Australian region. Onshore government regional airborne magnetic surveys and ground gravity surveys commenced about fifty years ago, and these datasets now cover the whole continent. Marine magnetic and gravity data have been acquired over the same period, and recently these data have been combined with the onshore data to provide single coherent compilations for much of the Australian region. Digital processing, enhancement and display of these datasets provide geoscientists with an unparalleled opportunity to unravel the tectonic framework of the Australian Plate, particularly in areas obscured by recent cover material. The datasets are being used extensively in the minerals, petroleum, coal, groundwater, environmental and educational industries. This paper discusses several enhancements of the national magnetic and gravity datasets in comparison to the geophysical interpretation published as the Australian Crustal Elements map. A detailed example, the Tasman Line in the Broken Hill region, shows how these datasets can be used to interpret geology under cover.

Granites and their associated comagmatic felsic volcanic rocks occur in most Proterozoic provinces of Australia. Using multi-element, primordial-mantle-normalised abundance diagrams and various petrological characteristics, Australian Proterozoic granites can be subdivided into five groups: (i) I-type, Sr-depleted, Y-undepleted, restite-dominated, (ii) I-type, Sr-depleted, Y-undepleted, fractionated, low in incompatible elements, (iii) I-type Sr-depleted, Y-undepleted, enriched in incompatible elements (anorogenic granites), (iv) I-type, Sr-undepleted, Y-depleted, (v) S-type, Sr-depleted, Y-undepleted. The four Sr-depleted groups dominate, and group (iv) is of very limited extent. A comparison of these Proterozoic granites with Australian and Papua New Guinean granites of other time periods shows that these characteristic Sr-depleted Y-undepleted patterns are also dominant in early Palaeozoic granites. They are significantly different from those of granites in modern island arcs associated with subduction, and with most granites from Archaean terranes, where the multi-element diagrams are dominated by Sr-undepleted, Y-depleted patterns. The Sr-depleted, Y-undepleted patterns are thought to indicate source regions that contained plagioclase but not garnet, whilst the Sr-undepleted, Y-depleted patterns are taken to correspond with the presence of garnet, but not plagioclase, in the source rocks. The Sr-depleted, Y-undepleted patterns also only occur in regions where the lower crustal structure is dominated by an underplated mafic layer with a P-wave velocity of 7.2 – 7.4 km/s. In contrast, in regions where the granites are dominated by Sr-undepleted, Y-depleted patterns, such as in the Archaean and in Cainozoic island arcs, this intermediate velocity layer is not present, and the crust-mantle boundary is very sharp. Two other distinctive compositional changes have been noted among the I-type granites of different age. Firstly, Na is highest in Archaean and Cainozoic granites, and lowest in early Proterozoic granites; Palaeozoic and Mesozoic granites have intermediate values. Secondly, late Archaean and Proterozoic granites are the most enriched in K, Th and U, while the Cainozoic and early Archaean tonalites are the most depleted; Palaeozoic and Mesozoic granites again contain intermediate amounts of those elements.