Lower crust and upper mantle electrical anisotropy in southeastern Australia Zara R. Dennis 1,3 Stephan Thiel 2 James P. Cull 1 1 School of Geoscience, Monash University, Vic. 3800, Australia. 2 The Centre for Tectonics, Resources and Exploration, University of Adelaide, SA 5005, Australia. 3 Corresponding author. Email: zara.dennis@monash.edu Abstract. The dominant north–south strike of the Palaeozoic outcrop of central Victoria has been well documented, but to the north, these rocks are covered by the Cainozoic sedimentary deposits of the Murray Basin. Two magnetotelluric surveys were completed to assist in extrapolation of the known structure and to identify possible new targets for mineral discovery. Supporting the results from previous seismic interpretations for the region, the 2D MT inversion models substantiate an intrazone thrust fault system of listric geometries in the Bendigo Zone connected in the mid-crust. With the zone boundary clearly defined the electrical resistivity structure is distinct between the major subdivisions, indicating a different tectonic evolution for the Bendigo and Melbourne Zones. However, the conductive overburden in the region poses complications for the generation of the 2D resistivity models. Static shifts and electrical anisotropy were identified as distortions in the dataset, with further processing needed to attain a complete picture of the underlying geology. The difficulties caused by galvanic distortion were allayed by using the phase tensor response in place of the distorted amplitude response. Phase tensor analysis of MT data has been completed subsequently, the results of which we present here, along with the original 2D inversion models, confirming that electrical anisotropy persists into the mantle. Key words: anisotropy, induction vectors, magnetotellurics, phase tensor, resistivity, strike, western Lachlan Fold Belt. Received 16 May 2011, accepted 27 July 2012, published online 31 August 2012 Introduction During April/May 2007 and August/September 2008, two parallel magnetotelluric (MT) surveys were completed in north central Victoria as part of GeoScience Victoria’s Gold Undercover Initiative. Located within the western subdivision of the Lachlan Fold Belt, the geology in the area is typically north–south trending, however much of the structure to the north-west is obscured by thick sedimentary cover of the Murray Basin (Figure 1) where little or no surface expression can be seen. The lack of outcrop has restricted the success of traditional exploration methods and, consequently, several additional geophysical surveys have been commissioned (GeoScience Victoria) to penetrate the sedimentary cover and to assist in delineating the hidden structure. The results we present here expand on earlier 2D inversions of two MT datasets (MT07 and MT08 – Dennis et al., 2011b); comprising a total of 119 soundings. Both profiles follow an east–west, cross- strike orientation, approximately in parallel with each other with an offset of ~70 km between the separate lines (see Figure 1 for regional location). Previously considered as individual transects, the results have been combined and are presented here as a single dataset; accompanied by a third shorter line, offset south-west from the major transects. The vast range of penetrable depths achievable by the MT method (from a few hundred metres below the Earth’s surface to depth infiltrating the upper mantle), lends the technique to use in areas where the deep crustal architecture may be partly or wholly obscured. With a sounding range similar to that of seismic reflection methods, MT thus provides resolution of the subsurface structure in areas which may be seismically poor or logistically difficult/expensive to interrogate. Data can be obtained with negligible environmental impact (Vozoff, 1991) while reducing the complications of depth ambiguity that can be faced by potential field techniques (Strack, 1992), such as gravity and magnetics. Both electrical anisotropy and localised heterogeneities in resistivity near the Earth’s surface however, can distort the amplitude response of the MT signal (Caldwell et al., 2004; Heise et al., 2006), commonly influencing the orthogonal modes by different amounts. Although the amplitude of the observed electric field may be distorted, however, the phase relationships between the magnetic field vectors will remain virtually unaffected if the distortion is galvanic (Caldwell et al., 2004). Phase tensor analysis of MT data therefore presents the interpreter with an alternative, more robust tool for analysis of datasets in complex, multi-dimensional regions (refer to Ingham et al., 2009 for a recent example); sensitive only to lateral resistivity variations, they allow the interpreter to identify the presence of off-line resistivity gradients or spatial differences (Heise et al., 2006). Focusing not on the intrinsic bulk resistivity of a section therefore, but on local resistivity gradients, the additional phase tensor analysis presented here will be used to expand on previous 2D inversion results and the resistivity cross- sections presented in Dennis et al. (2011b) – with particular emphasis on dimensionality in the datasets and removing the effects of galvanic distortions. A final geological interpretation is thus presented; a first for the electrical resistivity structure of north central Victoria as a whole, with further considerations for complex responses and off-line extrapolations connecting the datasets incorporated into the final conclusions. CSIRO PUBLISHING Exploration Geophysics http://dx.doi.org/10.1071/EG11022 Journal compilation Ó ASEG 2012 www.publish.csiro.au/journals/eg