ELECTROMAGNETIC INDUCTION STUDIES IN THE EYRE PENINSULA, SOUTH AUSTRALIA

Magnetic held fluctuations have been recorded by an array of portable three-component magnetometers at 60 sites across the Eyre Peninsula in South Australia between December 1993 and March 1995. An additional 5 4 magnetometer data records, collected prior to 1989 and described by Milligan (1989) and Milligan, White & Chamalaun (1989), were included in the analysis. major conductive feature in the crust, first noted by White & Milligan (1984) as the Eyre Peninsula Anomaly (EPA), is re-ex amined to assess its continuity to the north of the original arrays an d to investigate its relationship with major tectonic features. Magnet ic-field time-series were converted to induction arrows in the frequen cy domain. These Induction arrows were initially inverted using the mi nimum-structure 2-D Occam approach to estimate the electrical conducta nce of the crust. Following this, thin-sheet forward modelling was use d to examine the relationship between the conductance and the dominant tectonic features. The principal results of the modelling are that a narrow conductive feature extends inland from the coast about 160 km b efore terminating, and the conductance is in the range 3000 to 10 000 S, which decreases inland. A strong correlation exists between the ele ctrical conductance of the Eyre Peninsula and Bouguer gravity anomalie s, and in particular the EPA is coincident with a significant Bouguer gravity gradient. There is also good agreement between the locations o f the foci of earthquakes of magnitude greater than 4.0 and the EPA. W e believe that the anomaly is associated with a geological fracture in the Precambrian upper crust as a result of crustal extension prior to the rifting of Australia from Antarctica in the Jurassic (160 Ma).