Crustal imaging of a mobile belt using magnetotellurics: An example of the Fowler Domain in South Australia

A long-period magnetotelluric data set was obtained in 2005 along a two-dimensional profile across the western part of the late Archaean-early Proterozoic Gawler Craton, South Australia. The study is aimed at delineating the electrical conductivity structure of the crust and upper mantle underneath an east-west trending profile extending from the Gawler Range Volcanics in the east, crossing the Nuyts Domain and the highly prospective Meso-Proterozoic Fowler Domain, and terminating in the Eucla Basin to the west. The resistivity model shows a very electrically resistive crust and upper mantle underneath the Nuyts and Fowler Domain, possibly representing the cratonic root of the Gawler Craton extending to depths of similar to 160 km. The resistive cratonic root is closer to the surface underneath the Fowler Domain compared to the Nuyts Domain which supports findings from outcrops of metasediments of higher metamorphic grade in the Fowler Domain. A subvertical conductor marks the western terminus of the Fowler Domain and is imaged to upper mantle depths. On the eastern side of the Fowler Domain, another subvertical conductor extends to similar depths. These features spatially coincide with the Tallacootra and Coorabie shear zone at the surface, respectively, and their higher conductivity is likely due to a reduction in grain size of olivine associated with an increase in influence of grain boundary diffusion and thus enhanced conductivity. A comparison of the results with other surveys across mobile belts worldwide shows a more resistive response of the interpreted mobile belt of the Fowler Domain raising questions as to the nature of the domain.