AbstractsEarth & Environmental Science

Geomagnetic variations were measured at 35 temporary stations in the north, north-east and centre of Tasmania. The surveys were carried out from early in 1983 to the middle of 1984. The surveys were designed to map the southern extension of the anomaly discovered by Buyung (1980) and revealed two types of anomalies, i.e. inland and coast effect. The inland anomaly is characterized by oppositely directed in-phase induction arrows at close locations for periods from 4 to 20 minutes, indicating the presence of a concentration of current in a highly conductive zone. This anomaly is coincident with the Tamar Fracture Zone. Meanwhile, the gradual change in the direction of induction arrows with increasing period from various directions to almost south-east illustrates the significant effect of the coast. This is probably due either to current induced in the conducting ocean water or to high conductivity contrasts between the resistive block in the eastern part of Tasmania and the conducting ocean floor. The finite difference method of Jones and Pascoe (1971) and Pascoe and Jones (1972) was used to calculate the induction response of twodimensional models and to interpret the observations. Modelling reveals that a highly conductive zone, responsible for the inland anomaly, is located at a depth, of about 2 km from the surface. This zone, which is bounded by resistive blocks to the east and west, has a resistivity of 0.5.SL m and thickness of 2 to 3 km. This model agrees with magneto-telluric observations of previous workers. The most likely cause of this highly conductive zone is a large quantity of saline water or conducting liquid in porous or cracked rocks. Analogue modelling of the coast effect around Tasmania by Dosso et al. (in press) has been used in an attempt to isolate the effect of inland structures. This was only partly successful, probably because the analogue models assume that the island is a perfect insulator.