The role of intraplate stress in tertiary (and Mesozoic) deformation of the Australian continent and its margins: A key factor in petroleum trap formation

The horizontal stress field within plate interiors is largely the result of interactions at plate boundaries. There is considerable geological evidence and theoretical support for the hypothesis that large horizontal stresses can propagate for thousands of km into plate interiors, and that changes in plate geometry or boundary configurations therefore lead to significant variations in stress in plate interiors. Because there is a global interdependence of all plate motions, a major change in the nature of one plate boundary (e.g., the India-Asia collision) may have global implications and observable geological consequences many thousands of km from the source. There are two important consequences of large horizontal intraplate stresses and stress variations for petroleum exploration. First, flexural distortions of the plate will be localized by variations in strength and/or thickness of the plate, such as at sedimentary basin boundaries. These distortions may give rise to transgressions/regressions and unconformities that may be confused with eustatic sea-level effects. Second, an increase in stress or change in stress orientation may be relieved by reactivation of a pre-existing structure in the plate interior. Reactivation of basin-forming faults is a particularly widespread consequence of intraplate stresses. Analysis of the structural petroleum traps in Australia’s main producing basins shows that a large proportion of the traps were generated by reactivation of underlying, usually basin-forming faults. We discuss examples from the Carnarvon, Bonaparte and Gippsland Basins, and relate them to the global and regional plate tectonic history in the Tertiary and Mesozoic.