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

Extent: 18p.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 ∼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.Stephan Thiel and Graham Heinso

Electrical conductors in Archean mantle - Result of plume interaction?

From long-period magnetotelluric data across the Gawler Craton, we obtain a three-dimensional resistivity image of the lithosphere and provide constraints on tectonothermal events dating back to the Proterozoic. Contrary to common observations of Archean cratons displaying high electrical resistivity in the mantle lithosphere, the magnetotelluric data show low resistivity of around 10Ω m at 80km depth underneath the 1595Ma Gawler Range Volcanics, a silicic large igneous province. The resistivity distribution appears to be a signature of plume-modified orogenesis with low-degree partial melting at the base of the lithosphere in a back-arc setting. The enhanced conductivity is explained through higher hydrogen and iron content in the crystal lattice and also along the grain boundaries of the mantle constituting minerals. Older, arc-related magmatism to the southwest across 1620-1610Ma St. Peter Suite does not display enhanced conductivity and suggests a depleted mantle lithosphere. The data show that Yellowstone-type mantle plume analogues are preserved through time and still display an elevated electrical signature in the lithosphere today. Key Points 3D resistivity model of the South Australian Gawler Craton is produced Mantle conductor at 80 km depth in tectonically quiet Archean craton Conductor is result of Proterozoic plume interaction and led to IOCG deposits ©2013. American Geophysical Union. All Rights Reserved.Stephan Thiel and Graham Heinso