Magnetotelluric soundings across the Southern Alps orogen, South island of New Zealand: Data presentation and preliminary interpretation

As part of a major geophysical study of the orogen of the Southern Alps, magnetotelluric soundings have been made on two transects across the South Island of New Zealand. Initial results are presented and indicate the existence of significant electrical conductivity variations across the island. In particular, one-dimensional inversion of invariant responses from the central part of the southern transect, in the Southern Alps, shows the existence of a region of enhanced conductivity at a depth of around 10 km. This depth correlates well with that to the top of the aseismic lower crust and can be related to enhanced crustal temperatures and the dehydration of greenschist-zeolite facies rocks upthrust from depth. The absence of a similar conductive structure beneath the equivalent area of the northern transect suggests lateral variation in structure along the Southern Alps. One-dimensional inversions of short period responses from sites on the Canterbury Plains and north-west of the Alpine Fault show good correlation with depths to basement derived from other geophysical and geological data. Close to the Alpine Fault itself data show strong distortions. These may result from induction in more complex three-dimensional structures, the deviation of telluric currents by electrically resistive intrusives or possibly from electrical anisotropy associated with the Alpine Fault shear zone.