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

Magnetotelluric soundings have been made a t 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 a n assumed tectonic strike of N35"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 a n d shows a zone of enhanced conductivity at a depth of only 10 k m beneath the Southern Alps. This correlates with suggestions of enhanced temperatures a t shallow depths and can be interpreted as being due t o 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 k m depth beneath the Canterbury Plains and can be tentatively associated with the existence of a lower crustal detachment surface. The derived resistivity structure close t o the Alpine Fault is highly complex a n d may be indicative of either 3-D induction in igneous intrusions, or anisotropic structure associated with the fault zone.