The electrical resistivity of the Australian lower crust

Abstract

The resistivity structure of the crust is broadly expected to be homogeneous, with highly resistive lower crustal rock overlain by more conductive rock in the upper crust. However, observed data shows that although the upper crust is typically resistive, the lower crust can be much more conductive. The presence of such high electrical conductivity in the lower crust is remarkable and suggests a substantial highly connected material, melt or fluid. Has the low resistivity structure been present since inception, or is it the result of a later overprinting event. The secondary objective to establish how such a low resistivity region can be preserved over such an extended time scale. Data has been collated from magnetotelluric (MT), and geomagnetic depth sounding (GDS) surveys collected over the last thirty years. Three different methods have been used to model thousands of data points. A thin-sheet inversion of thousands of GDS data has been used to place constraints on the regional scale electrical conductance. Inversions of the MT data in both 2D and 3D have provided more detailed models of how the Moho is connected to the upper crust. Strong correlations were observed between major tectonic domains (such as the Gawler Craton) and regions of high resistivity within the crust. The 2D profiles show broad regions of low resistivity at the boundary between the upper and lower crust (10–15 km depth), with low resistivity zones extending for tens of km. Above the boundary, the low resistivity regions transform in to narrow pathways penetrating through the resistive upper crust and the areas with the lowest resistivity were found to have a strong correlation with known major mineral provinces. This leads to the suggestion that crustal low resistivity anomalies are likely a product of fluxes of fluid and possibly melt from the upper mantle and lower crust.