Influences of plate-boundary forces on the regional intraplate stress field of continental Australia

Abstract

The Indo-Australian Plate is unique compared to other plates in that the maximum horizontal stress (aHmax) orientation is not uniform and does not parallel the direction of absolute plate velocity. Consequently, the Australian continent provides an ideal setting in which to study the interaction between tectonic forces and the intraplate stress field. Finite-element modelling of the intraplate stress field of the Indo-Australian Plate using a new 'basis-set' approach enables the evaluation of a very large number (several million) of boundary and potential energy force combinations acting on the plate. Constraint for the modelling is provided by an 'observed' regional stress field based on observations in 12 stress provinces, which is greatly enhanced from the observed data used in previous modelling efforts. The results of the present study indicate that modelling of the Australian intraplate stress field is inherently non-unique in that a large number of different boundary-force combinations can produce similar predicted stress fields. Nevertheless a number of fundamental conclusions may be drawn about the tectonic forces acting along the principal plate-boundary segments: (i) the Himalayan and New Guinean boundaries exert a compresslonal force on the Indo- Australian Plate producing a stress focusing normal to the boundaries and rotating between them; (ii) fitting the stress field in the Bowen Basin requires compressional boundary forces along the Solomon and New Hebrides subduction zones directed towards the interior of the Indo-Australian Plate; (iii) east-west compression in eastern Australia requires only a small compressional force along the Tonga-Kermadec subduction zone; and (iv) fitting the stress field in southeastern Australia (Otway Basin and Gippsland Basin stress provinces) requires compressional forces along the New Zealand and south of New Zealand boundary segments. The orientation of the modelled stress field over most of Australia is robust with northeastern and southern Australia the most sensitive to variations in the plate-boundary force combination. Furthermore, the modelling suggests that large sections of eastern Australia exhibit a relatively isotroplc stress field compared to the rest of Australia.