Three-dimensional kinematic modelling of the magnetic field of the southern Joseph Bonaparte Gulf

Kinematic modelling differs from traditional modelling techniques in that model geometries are obtained by subjecting an initial horizontal stratigraphy to specified structural deformations rather than being input as a pre-specified shape. The magnetic field over the southern Joseph Bonaparte Gulf of northwestern Australia is ideally suited to a kinematic modelling approach because the magnetic effect (anomalies) of the area is almost entirely due to an extensive sheet of Proterozoic Carson Volcanics, which apparently originally covered a large proportion of the area. That unit has subsequently been fractured and warped during Palaeozoic rifting. A three-dimensional kinematic model has been produced for the present geometry of the Carson Volcanics such that the computed magnetic field for the model closely duplicates the observed magnetic field. The modelling largely confirmed earlier interpretations of the data based on image analysis and modelling of profile data. Refinements indicated by the three-dimensional modelling were a magnetic feeder pipe to explain a large localised magnetic high and a salt accumulation with a diamagnetic susceptibility giving a negative magnetic susceptibility contrast relative to adjacent sedimentary rocks to explain a magnetic low previously interpreted as being due to a localised absence of the magnetic sheet. The interpretation of the salt body is consistent with drill and seismic data, thus demonstrating the effectiveness of kinematic modelling of magnetic fields.