Electromagnetic induction in the Australian crust: results from the Australia-wide array of geomagnetic stations

Abstract

An array of 57 three-component magnetometers has been deployed over the Australian continent to record geomagnetic field fluctuations at one-minute intervals from November 1989 until June 1990. The inductive response of the continent to external field variations at various frequencies is dominated by a prominent coast effect around the western, southern and eastern coasts. Albany displays a particularly large induction effect. The east and west coast display very similar coast effects that are detectable up to 500 km inland. The northern margin of the continent displays a smaller, more complicated inductive response with contributions from crustal conductivity anomalies in the Canning Basin and the Gulf of Carpentaria, and, possibly, from offshore shallow-water sediments. Contour mapping of the A and B transfer function coefficients reveals evidence for an conductive zone surrounding the North-Central cratonic area. The zone sweeps through the main sedimentary basins that separate the Australian cratonic blocks and includes several of the conductors mapped previously. Electric currents induced in the sedimentary basins appear to be locally concentrated by shear zones and basement highs. An earlier suggestion for an Australia-wide north-south conductor appears to be supported by the data and there is evidence for its continuation into the Gulf of Carpentaria. Our array results suggest a possible east-west continuation of a conductive zone through the Officer Basin and into the Canning Basin. The presently available data are consistent with the existance of a large U-shaped lntercratonic Conductive Zone that curves around the central part of the country. Thin sheet modelling by Corkery and Lilley indicates that the required conductivity contrasts are geologically realistic.