Electrical structure of the crust in Southeast Tasmania

In the south-east of Tasmania, Jurassic dolerite forms a partial cover over much of the area. The sedimentary and volcanic rocks in this region are mainly Permian or younger. The topography is largely dominated by the local structure of the dolerite. Utilizing the magnetotelluric and magnetovariational methods, an investigation was made of the electrical structure along two cross-sections to lower crust / upper mantle depths. In conjunction with this study the potential field and transient electromagnetic methods were used. Two-dimensional gravity and magnetic modellings delineated the geometry and possible structural origin of several rock sequences associated with a basin structure. The use of the transient electromagnetic method placed constraints on the thicknesses and resistivities of the surface layer. As part of the magnetotelluric analysis package, a new rotation angle and dimensionality calculation method is introduced and tested with different geometrical structures. The results when compared to other conventional and the Mohr circle methods show this new method works well and is simpler and faster. Two types of anomalies, inland and coastal effect, are revealed from magnetovariational observation. Correction for the ocean were applied to induction vectors at periods of 10 and 60 minutes. The inland anomaly is characterized by corrected in-phase induction vectors at stations to the east of the Huon River, pointing north-west. Those to the west of the river point in a north-east direction, indicating the presence of a gradient anomaly zone which lies along the Huon River. Meanwhile the gradual swing in direction from southeast for observed vector to east and almost perpendicular to the coast-line for corrected vector at the eastern-most station of crosssection II illustrates the remaining effect of the coast. This can be explained by high conductivity contrasts between the resistive block in the eastern part of Tasmania and the conducting ocean floor. The results from the one and two-dimensional modelling of magnetotelluric, gravity and magnetic data indicate that the base of the Permo-Triassic cover with its stockwork of massive dolerite intrusions, is probably never less than 500 metres below the surface at the northern cross-section and dips south reaching a depth of about 800 metres on the southern cross-section. The Ordovician limestone, which may be a possible source of hydrocarbon deposits, has a bulk apparent resistivity value of 40 Ohm-m inferred from transient electromagnetic modelling. This rock has a thickness of about 300 metres and its distribution is restricted to the western part of the study area which is consistent with the results of the two-dimensional gravity and magnetic modelling. The dipping discontinuity needed in the models to match the magnetotelluric data, results in a trough-like structure with depth from the surface to the bottom of about 6 kilometres. This structure is reflected by a broad and large gravity anomaly together with slightly negative magnetic anomaly and is believed to be associated with a trough of Cambrian volcanics. This trough has a northwest - southeast direction and becomes wider and has more conductive flanks to the south. The magnetotelluric results also indicate the presence of a low resistivity layer at middle-lower crustal depths. There is broad but not exact correlation with the position of the Cambrian trough above. In combination with other geophysical evidence gained from the magnetovariational method, i.e. the short corrected induction vectors at all stations at periods of 60 minutes, the layer is inferred to exist beneath the entire study area. The likely cause of the low resistivity associated with this layer is believed to be the presence of free carbon along grain boundaries or fractured rocks which provide a continuous conducting path. Another possible cause is the presence of anomalously high temperatures in the deeper crust expected from previous geothermal measurements.