Variation of crustal mass over the Australian region

Updated: 2018-04-20

Seismic refraction surveys in Australia and nearby marine areas provide data on crustal velocities and layer thicknesses. The averages of these at various places have been used to estimate a density-depth structure. By applying corrections for average elevation and free-air gravity, a crustal mass deficiency (CMD) is calculated for each site; this characterizes a standard crustal column with zero elevation and zero free-air anomaly, which is considered to be in isostatic equilibrium over a mantle of uniform density 3.32 t/m^3. The CMD ranges from about 13 kt/m^2 in the south-west of the continent, to about 21 in parts of Queensland; marine values range from 15 to 17 kt/m^2. The variations imply that isostasy on a broad scale is not complete at the base of the crust. As the gravity field indicates that departures from isostasy are comparatively small, compensation must occur partly in the mantle.