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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.

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