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

Abstract

Magnetotelluric models spanning the Pacific–Australian Plate boundary in New Zealand’s South Island indicate a localized zone of low electrical resistivity that is spatially coincident with theductile mid-crustal part of the Alpine Fault Zone (AFZ). We explored the source of this anomaly bymeasuring the electrical properties of samples collected from surface outcrops approaching the AFZthat have accommodated a gradient of systematic strain and deformation conditions. We investigatedthe effects of tectonite fabric, fluid saturated pore/fracture networks and surface conductivity on the bulk electrical response and the anisotropy of resistivity measured under increasing confining pressures up to 200 MPa. We find that porosity and resistivity increase while porosity and the change in anisotropy of resistivity with confining pressure (δ (ρk/ρ⊥)/δ (peff)) decreases approaching theAFZ, indicating the electrical response is controlled by pore fluid conductivity and modified during progressive metamorphism. Conversely, Alpine mylonites exhibit relatively low resistivities at low porosities, and lower δ (ρk/ρ⊥)/δ (peff) than the schists. These findings indicate a transition 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.