Electrical properties and anisotropy of schists and fault rocks from New Zealand’s Southern Alps under confining pressure

Magnetotelluric inversions spanning the Pacific-Australian Plate boundary in New Zealand’s South Island indicate there is a localized zone of low electrical resistivity that is spatially co-incident with the mid-crustal part of the Alpine Fault Zone (AFZ), that currently accommodates shear strain by temperature-sensitive creep. We explored the source of this anomaly by measuring the electrical properties of samples collected from surface outcrops approaching the AFZ that have accommodated a gradient of systematic strain and deformation conditions. We investigated the effects of tectonite fabric, fluid saturated pore/fracture networks and grain surface conduction on the bulk electrical response and the anisotropy of resistivity of these samples measured under increasing confining pressures up to 200 MPa. We find that for fault rock protoliths, Haast and Alpine Schist, resistivity and change in anisotropy of resistivity with confining pressure (δ(ρ_‖/ρ_⊥)/ δ(p_eff)) increases while porosity decreases approaching the AFZ. This indicates the electrical response is controlled by pore-fluid conductivity and modified during progressive metamorphism. AFZ mylonites exhibit low electrical resistivities at low porosities, and lower δ(ρ_‖/ρ_⊥)/ δ(p_eff) than the schists. These reflect changes in both the porosity distribution and electrical charge transport processes in rocks that have experienced progressive grain size reduction and mixing of phases during development of mylonitic fabrics due to creep shear strain within the AFZ.