High Resolution Aeromagnetics Clarifies Structuring in the Vlaming Sub-Basin, Western Australia

Woodside has conducted two high resolution aeromagnetic surveys in the northern and southern extremities of the Vlaming Sub-basin. The northern survey also covered a portion of the Edwards Island Block and the southern extremity of the Abrolhos Sub-basin. The acquisition of the data highlighted the problem and characteristics of ocean swell generated magnetic noise, which had amplitudes similar to the structurally related signal.

The aeromagnetic data defined the broad basin architecture. Image processing helped to enhance many of the weaker magnetic features which correlated with intra-sedimentary structures observed on the seismic data. The aeromagnetic data showed that the structuring and the structural orientations are slightly different between the two survey areas. The northern area had a wider envelope of fault azimuths with trends of NW to NNE with the NW to NNW trends dominant. The southern area had a narrower envelope of fault azimuths with trends predominantly NNW to NNE.

In the northern area NW lineaments, parallel with the oceanic transforms, offset other lineations in a right-lateral direction. These are interpreted to be right lateral strike-slip zones related to oblique faulting during breakup. Similar NNW strike-slip features are observed in the southern survey data set. East-west lineaments are interpreted as antithetic strike-slip faults. These are believe to be related to basement block rotations which caused localised areas of compression, observed on seismic data.

The aeromagnetic and seismic data show the Vlaming Sub-basin underwent right lateral oblique faulting during the breakup between Australia and India. Changes in fault drag on the Darling Fault, possibly related to variations in basement between the two survey areas, are proposed as the reason for the differences in fault azimuths.