Magnetotelluric investigations of the Tamar lineament

Magnetotelluric experiments were carried out in the Tamar lineament area of northern and central Tasmania to study subsurface electrical structure. These experiments have defined a high conductivity anomaly in the upper-middle crustal depth range and confirmed the presence of the anomaly suggested by previous authors (Buyung, 1980; Sayers, 1984; Hermanto, 1985; Parkinson, 1988). Other geophysical techniques such as de-resistivity, SIROTEM and two-dimensional gravity and magnetic modelling were also used. Results from these studies and other geophysical studies such as the seismic studies of Richardson (1985) and Vitesnik (1984) make a significant contribution to a better understanding of the shallow structure, nature, tectonic history and, most importantly, the setting of the conductivity anomaly. The measured apparent resistivities are interpreted in terms of inhomogeneous electrical structure using one- and two-dimensional modelling indicating that the top of the high conductivity anomaly is located about 500 to 700 metres from the surface. This is about the level of the basal Permian cover unconformity. The resisitivity of the anomaly ranges from 1 to 8 Ohm.m with a thickness of 13.3 to 16.3 kilometres. The shape and position of the anomaly which tapers with depth is consistent with the results suggested by the current density and two-dimensional gravity and magnetic modelling. The gravity and magnetic data indicate large fault or thrust bounded blocks of Lower Palaeozoic and Upper Precambrian rocks underlie the Permian cover. The gravity and magnetic models also show that the more magnetic Cambrian volcanic sequences, may account for the upper part (at least 5 kilometres) of the anomaly. The deeper part of the conductive structure underlies the more magnetic part of the Cambrian volcanics. This suggests that the formation of the volcanic piles may be associated with high conductivities deeper in the crust. It seems likely that the boundaries of the conductive block are marked by tectonic slices of ultramafics. The conductive anomaly does not extend west beyond the Tiers Fault which marks a fundamental structural limit affecting rocks from Cambrian to Tertiary in age. This zone is marked by a strong gravity gradient which extends SSE from Devonport to Sorell across central Tasmania. No such feature occurs along the line of the Tamar River or the Tamar Lineament of previous authors. The most likely cause of the high conductivity anomaly is a combination of the presence of high conducting fluids and graphite in pores, cracks, and or fractured rocks which provide a continuous conducting path. Geological and geophysical evidence showing a considerable tectonic histo:ry in this region support abnormal porosity and extensive fracturing of the deep crust providing an environment for deep fluid circulation and graphite precipitation. Black shales and sheared carbonates are to be expected in the Precambrian rocks implied to be present. The velocity-resistivity relation suggests that a resistivity as low as 2 Ohm.m may be obtained from the Tasmanian crust where the velocity is about 5.8 to 6.0 km s-1. This is comparable with the resistivity associated with the high conductivity anomaly.