Electrical properties of schist and mylonite from the South Island, New Zealand: Exploring the source of the Southern Alps Anomalous Conductor

Abstract

The Southern Alps Electrical Conductor (SAC), identified from magnetotelluric surveys of the South Island Geophysical Transect (SIGHT) in the South Island, New Zealand, has high electrical conductivity relative to surrounding lithology. This phenomenon is spatially coincident with Alpine Fault-associated shear zones and a region of anomalously reduced seismic velocity, also observed by SIGHT. The source of these geophysical anomalies is unknown.

To understand the electrical properties of the Southern Alps Conductor, this study measured the electrical properties of hand samples from outcrops located above the Southern Alps Conductor, and of the same protolith. Particular consideration was given to the more recently exhumed mylonites which have been transported up the Alpine Fault ramp from the conductive region at depth. To accomplish this, complex resistivity of samples under confining pressure was measured up to 200 MPa and over a range of salinities, from 0.01 M to 0.4 M KCl brine. Laboratory measurements were converted to and analysed in terms of complex conductivity and compared to the expected conductivity of the rock, given the associated porosity, formation factor and cementation factor. Modelled conductivity values were extrapolated to 9 km depth, with a geothermal gradient, salinity profile and overburden pressure profile from the Alpine Fault, to generate values comparable to those derived by SIGHT magnetotelluric (MT) models. This allowed laboratory analysis of hand samples to be applied in predicting electrical properties in and around the SAC. Measured conductivity values at 200 MPa effective confining pressure ranged from 4.76x10 -4 to 8.26x10-6 S/m. Experiment-based modelled conductivity values, adjusted for in situ conditions, ranged from 1.08x10-4 and 1.04x10-2 S/m between modelled depths of 0 and 9.4 km.

The aim of this study, in addition to characterizing the effect of increasing deformation grade on electric properties of rocks approaching the Alpine Fault from across the South Island, was to further constrain the source of the SAC.

It was found that porosity and conductivity of schists decreased towards the Alpine Fault and conductivity of mylonites increased towards the Alpine Fault. There was no correlation found in mylonites between porosity and distance to the Alpine Fault. Experiment-based modelled conductivity values of mylonites exhibited a critical threshold, with a maximum of 1.33x10-2 S/m at a depth of 3 km, but continued to decrease with additional effective confining pressure (peff) as pore volume diminished. The SIGHT MT model (Jiracek, 2007) exhibited a conductivity maximum of 1 to 0.1 S/m between 10 and 20 km depth (Ingham, 1997; Wannamaker, 2002; Jiracek, 2007). Due to the non-porosity based Alpine Fault-proximity based mylonite conductivity and the discrepancy between the MT model critical threshold and experiment based model critical threshold, I conclude that a conductivity parameter, such as surface conductance, is missing from my evaluations.