Magnetotelluric soundings across the South Island of New Zealand: Electrical structure associated with the orogen of the Southern Alps

Magnetotelluric soundings have been made at 31 locations on two transects across the central part of the South Island of New Zealand. The calculated impedance tensor at each site has been decomposed, using an assumed tectonic strike of N35 degrees E, to yield 2-D regional apparent resistivity and phase responses. These have been inverted to produce 2-D models of electrical conductivity structure associated with the Southern Alps orogen. The resulting structure is consistent with proposed models for the orogen and shows a zone of enhanced conductivity at a depth of only 10 km beneath the Southern Alps. This correlates with suggestions of enhanced temperatures at shallow depths and can be interpreted as being due to the presence of 'wet' greenschist facies rocks upthrust from lower crustal depth by the collision of the Pacific and Australian plates. Reduced resistivity also occurs at 25 km depth beneath the Canterbury Plains and can be tentatively associated with the existence of a lower crustal detachment surface. The derived resistivity structure close to the Alpine Fault is highly complex and may be indicative of either 3-D induction in igneous intrusions, or anisotropic structure associated with the fault zone.