Lower crust and upper mantle electrical anisotropy in southeastern Australia

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.