BASEMENT CONTROL ON THE DEVELOPMENT OF EXTENSIONAL SYSTEMS IN AUSTRALIA'S TIMOR SEA: AN EXAMPLE OF HYBRID HARD LINKED/SOFT LINKED FAULTING?

Abstract

A series of rift models has been developed for basin systems in Australia's Timor Sea, via the interpretation of newly acquired deep crustal seismic and high resolution aeromagnetic data. These models, which incorporate observations on rift architecture, fault geometries, fault orientation, basement grain and composition, extensional transport direction and reactivation history, have then been iteratively tested by sophisticated analogue modelling experiments. This work has led to the development of a hybrid hard linked/soft linked (basement-involved/basement-detached) fault model. In this model, basement grain is the principal control on the rift architecture that develops, with pre-existing fracture systems acting to establish discrete offsets (hard linkages) between adjacent extensional faults. It is these basement features that produce the recti-linear features which are so common in aeromagnetic data around the Australian margin. With progressively greater extension, the basement-involved, hard linked system exerts no through-going (transfer fault-type) influence over the faulting within the overlying syn-rift phase, with the linkages between the syn-rift faults being 'soft' (via relay ramps, etc). However, as the hard links do act to relay the extensional faults or to flip their polarity (thereby typically producing cross-basinal highs), hard links strongly segment the extensional system into compartments of similar extensional style, and do control the relative positions of source rocks, fluid migration pathways and reservoirs within the rift. During basin reactivation (particularly inversion), the location and geometry of the underpinning, hard linked basement features closely control the locations of the traps that develop in the syn-and post-rift section, and the late-stage fluid flow history. When combined with aeromagnetic data, which define the location of the under-pinning, recti-linear, hard linked basement features, and some regional seismic data, these observations provide a first-pass predictive tool for determining where source depocentres, reservoirs and major structures are likely to be developed in a frontier basin, or where more subtle structural and/or stratigraphic traps might be found in a mature province.