Long Period Magnetotelluric Measurements in Southern South Island, New Zealand: The Resistivity Structure of Eastern Province Terranes

As part of a project to assess the risk posed by geomagnetically induced currents to the New Zealand electrical transmission network, long period magnetotelluric (MT) measurements have been made at 62 sites in southern South Island of New Zealand, a region where there was previously a complete absence of MT data. Analysis of the data using phase tensors and dimensionality indicators show that the data are largely 3-dimensional in character, but show distinct features which can be related to the known tectonic and geological structure. A 3-dimensional inversion of the data, using 2 independent codes, shows low resistivity to extend from the Moonlight Tectonic Zone, marking an spreading center which was active from 45 to 25 Myr, across the sandstones/siltstones of the adjacent Murihiku terrane. High resistivity is observed on the much older Fiordland Median Batholith and can also be traced along the length of the Dun Mountain Ophiolite Belt, a region of obducted oceanic crust which forms part of the Stokes Magnetic Anomaly. Variations in resistivity with depth agree well with features of the gravity field across the area as well as with seismic estimates of crustal thickness Resistivity contrasts across faults also mirror changes in seismic attenuation.