Numerical modelling of crustal stresses in the eastern Australian passive margin

In the Australian continent, particularly in eastern Australia, the intraplate stresses and the absolute plate velocity trajectory are not aligned. This is in contrast to some lithospheric plates (e.g. North America), for which the intraplate crustal stress appears to be related to the plate boundary driving forces. In an attempt to understand this unique stress pattern, a computer model has been constructed for part of the eastern Australian passive margin. The model adopts the regional tectonic-geometric configuration (an east-west cross-section 700 km long and 150 km deep) and the material properties for the relevant rocks as input. Different tectonic situations are simulated using variable boundary conditions. The modelling results show that gravity-induced body forces cause the materials in the various layers of the section to creep towards the continent. This process alone is sufficient to create horizontal compression of sufficient magnitude in eastern Australia to account for the observed crustal seismicity. The upper plate passive-margin geometry of eastern Australia and the involvement of an upper mantle wedge are decisive in the creation of gravitational creep movement towards the continent in the lithosphere, and any east-west remnant or plate boundary stress further enhances the horizontal compression. The gravitational creep movement also gives rise to uplift in the shallow crust. The location of the predicted uplift is consistent with geological observation. Furthermore, brittle yielding has been observed at certain locations in the modelled section, roughly confined to a 190 km wide zone across the coastline, consistent with present day seismic observations. The present day intraplate stress system in eastern Australia appears to result from a combination of that due to the upper plate passive margin geometry (resulting in roughly east-west compression or normal to the continental-oceanic crustal boundary) and that due to plate motion (resulting in approximately north-south compression). The resultant stress distribution is therefore highly variable with east-west compression more commonly dominant than north-south compression.