Electrical resistivity of the Northern Australian lithosphere: Crustal anisotropy or mantle heterogeneity?

Interpretations of continental magnetotelluric (MT) data are increasingly invoking electrical resistivity anisotropy in the crustal and/or mantle lithosphere, or deeper in the asthenosphere to explain non-uniform responses. In this paper, we show that seven shallow marine (<60-m water depth) MT and geomagnetic depth sounding (GDS) sites over an array of length 300 km in the Gulf of Carpentaria, above the North Australian Craton, can be interpreted as either anisotropic resistivity of a factor of more than 100 in the lower crust, or alternatively by regional-scale lithosphere variations in upper mantle resistivity of a factor of 10. All of the MT data with errors of ∼2–5% can be fit with anisotropic 1D smooth inversions or an isotropic 2D smooth inversion to a root-mean square (rms) misfit of 2.5 or less. However, from geological and geophysical constraints, the model of mantle resistivity that is isotropic but heterogeneous is preferred. In this model, the significant difference in MT apparent resistivity and phase in orthogonal modes of induction over periods of 100–10 000 s with a small GDS response can be attributed to resistive lithosphere of >10 000 Ω m in the Proterozoic western end of the survey grading to ∼1000 Ω m beneath the eastern Australian Phanerozoic. Shear-wave velocity tomographic models show 10% higher velocities beneath the Proterozoic craton compared with Phanerozoic continental lithosphere, suggesting that the variations in resistivity may be due to higher temperatures beneath the eastern side and/or an increased volatile content.