The electrical properties of the Woodroffe Thrust: a resistive shear zone
thesisposted on 2022-03-29, 00:17 authored by Tasman Gillfeather-Clark
Shear zones are a geological feature of interest due to their unique electrochemical and structural properties. However, consensus on the causation of their conductivity anomalies at large is not unified. There are many factors to consider when understanding the electrical features of a given shear zone. Composition, including minerals and grain boundary films, as well as particle size and the presence of fluids are some key factors. Generally, shear zones are conductive in geophysical exploration and the above factors are sometimes used to explain conductivity features associated with certain shear zones. However, the cause of this conductivity is often poorly constrained. My work uses rock samples and geophysical data gathered at a surface expression of the Woodroffe Thrust, NT, Australia. Using micro X-ray fluorescence, I evaluate compositional (mineral) and structural properties (grainsize) of my survey area. The conductivities of samples from the thrust are examined in lab conditions using electrochemical impedance spectroscopy. I then compare these conductivity values with the field geophysical data (time domain electromagnetics and magnetotellurics). By constraining the data in this way, I can consider in detail the causation and expression of the electrical response. My conductivity results highlight a disparity between my field and lab results of over seven orders of magnitude whilst also hinting at an anisotropy of conductivity mechanisms. These results contribute to a larger narrative of conductivity, with lab/petrophysical analysis on one side and field/geophysical results on the other. My work highlights that there is no property of a shear zone that makes it inherently conductive and that understanding the electrochemical response of a shear zone requires detailed analysis.