The link between electrical conductivity anomalies and rheological boundaries

Abstract

Interpreting magnetotelluric (MT) models requires solid modelling of the data as well as good knowledge from other geophysical data and geological constraint in the particular tectonic setting of the survey area. MT measurements, relating the natural variations of electric and magnetic field to obtain the electrical resistivity distribution of the crust and mantle appear to show that enhanced electrical conductivity zones are more abundant at certain depths. Models show that frequently enhanced conductivity zones are topping out in the upper crust at depths of about 10-15 km. These features are discrete and extend usually over a few km to tens of km laterally, and can be found across the Delamerian Orogeny, in zones of high heat flow east of the Northern Flinders Ranges and also in the central Eyre Peninsula. We interpret this to be related to recent findings on dynamic interactions between brittle and ductile layers leading to mid- to upper crustal detachment faults. A second zone of higher conductivity occasionally appears in the lower crust, as imaged east of the Flinders Ranges at depths of around 25-35 km. Geodynamic modelling indicates that permeable porosity is created through viscous grain boundary sliding, creep cavitation and precipitation. Irrespective of current of fossil fluid flow, these ductile shear zones and fluid pathways have a likely electromagnetic response to surface MT measurements. Thirdly, at 80 km depth mantle conductors appear in stable Archean and Proterozoic terranes around the world, such as in the Slave Craton, Kaapvaal Craton and the Gawler Craton. In summary, information from geodynamic modelling helps to understand the processes in the earth in regards to fluid movement and potential mapping of heat flow and corresponding shift in depth of brittle-ductile boundaries.