A two-dimensional interpretation of the geomagnetic coast effect of southeast Australia, observed on land and seafloor

Kellett, R.L., Lilley, F.E.M. and White, A., 1991. A two-dimensional interpretation of the geomagnetic coast effect of southeast Australia, observed on land and seafloor. Tectonophysics, 192: 367-382. Data from detailed investigations of the geomagnetic coast effect across the southeast Australian continental margin are inte~ret~ in terms of a tw~im~sional numerical model. There is both land and seafloor control on this modeI, and both E-p01 and B-p01 mode responses are incorporated. The good fit to the data of a model comprising ocean water and marine sediments is improved when the sub-ocean electrical conductivity profile is allowed to differ from the continental conductivity profile. A final model is determined by an inversion procedure based on systematic search. The main characteristic of this model is that an increase in conductivity occurs at a shallower depth beneath the ocean than beneath the land (some 100 km beneath ocean, 200 km beneath land). The southeast continental margin of Australia is considered to have formed by passive rifting, at the time of the opening of the Tasman Sea. A depth of 100 km for the base of the oceanic lithosphere corresponds well to the age of rifting some 80 Ma ago. The contrast with the continental profile suggests an electrical asthenosphere relatively deeper beneath southeast Australia. Near many coastlines, a strong correlation occurs between the vertical and horizontal components of the fluctuating magnetic field. First studied by Parkinson (1959), the phenomenon is commonly known as the geomagnetic coast effect. It is the subject of many papers, for example Everett and Hyndman (1967), Parkinson and Jones (1979), White and Polatajko (1978), and Neumann and Hermance (1985). Initially observations were restricted to those made on land, but subsequently observations on the ocean side of coastlines have become possible, with the development of instruments for measuring seafloor magnetic and electric fields (Filloux, 1988). r Department de Genie Mineral, Ecole polytechnique, Montreal, Que., H3C 3A7, Canada. 0040-1951/91/$03.50