The hikurangi margin magnetotelluric investigation: presentation, inversion and interpretation of line C-C'

Abstract

Data are presented and interpreted from a profile of fourteen MT sites across the lower North Island. As part of the 1997-2000 MT investigation of the Hikurangi Margin run by VUW in conjunction with the University of Edinburgh, measurements of the earth's magnetic and electric fields were made from Kapiti in the west to the Wairarapa coast in the east. 1D modelling of MT and dc resistivity data show that static shift has not occurred within the MT data and that correction for such is unnecessary. 1D joint inversion combines both dc resistivity and MT data producing a continuous model of structure from the surface to depths of c. 10km. 1 and 2D inversion and modelling of the MT impedances have been used to image the subsurface electrical structure of the Wairarapa Valley region. Mohr circles have indicated that the data are predominantly 2D in nature and that the orientation of the principal axis of the impedance tensors is consistent with a strike angle of N035° in agreement with the general gross tectonic strike of the North Island. Due to the structural nature of the Wairarapa Valley region only the TM orientation of MT data (perpendicular to strike) has been used for the final 2D modelling. The major resistivity structures inferred through the two dimensional profile being the Wairarapa Fault, two sedimentary basins of less resistive material separated by a resistive ridge (that can be compared to a fault and anticline feature), and the conductive Tertiary strata of the east coast ranges. These features agree with several earlier geophysical investigations of the same area. Though unable to image the subducting Pacific Plate this analysis has revealed sufficient resistivity contrast to suggest that future, more detailed MT soundings in conjunction with gravity surveying should provide a higher resolution picture of the actual geological structure of both the Wairarapa Fault and the Te Maire Ridge anticline/fault.