Crustal structure of the northern Perth Basin, southwest margin of Australia: Insights from three-dimensional density models

The northern Perth Basin formed from the Palaeozoic toMesozoic within an obliquely orientedextensional rift system on the southwest continental margin of Australia. Knowledge of thebasin in onshore and inboard areas reflects better accessibility and the existence of provenhydrocarbon resources. In contrast, outboard parts of the basin have poorer data coverage andhydrocarbon potential remains to be proven. In order to better constrain sediment thickness andcrustal structure in the northern Perth Basin, particularly in offshore areas where coverage ofseismic data is less extensive, we adopted a 3-D density modelling approach whereby simplemodels were initially constructed as a means to highlight the level of agreement betweenmeasured and calculated gravity. These initial models are based only on available constraints,automated extrapolation of interpreted horizons into areas without constraints, and differentinterpretations of Moho depth. The initial models show that the processes leading to formationof sediment depocentres in the northern Perth Basin are not governed by simple local isostasy.We show instead that a model incorporating an existingMoho model for the Australian regionleads to a better fit. In this model, the Moho is deeper under the thick sediments of the onshoreDandaragan Trough. As a result, the crystalline crust between the Beagle Ridge and basinboundingDarling Fault has a relatively constant thickness that is consistent with crustal-scaletilting and normal displacement on a steeply west-dipping Darling Fault. In the outboard partsof the basin, our modelling suggests that the deep water Zeewyck Sub-basin is a deep andsteep-sided depocentre, but this area lacks constraints and uncertainty in its interpretationcannot be resolved without additional data. Despite this, the steep edges, thick sediments andthe large lateral variations in Moho depth are consistent with the geometry expected of atranstensional basin.We also present a map showing effective elastic thickness (T_e) variationsacross the northern Perth Basin. In the area of the Dandaragan Trough, T_e is moderately high(10-20 km), which is consistent with the suggestion of rigid block rotation on the crustal-scaleDarling Fault. In contrast, T_e in the Zeewyck Sub-basin is near-zero, which is consistent withthe weakening expected in an area subjected to strike-slip deformation.