A magnetotelluric transect of the Wairarapa region, New Zealand

Abstract

Magnetotelluric (MT) soundings have been made at 14 sites along a traverse of the Wairarapa region at the southwest end of the active Hikurangi margin. Joint inversion of direct current resistivity soundings and the MT data indicates that the MT data are unaffected by static‐shift. Analysis of the dimensionality of the data suggests that the transverse electric (TE) mode data (derived from an electric field parallel to the strike of the margin) may be distorted by 3D effects arising from the presence of Cook Strait and the non‐continuity of structure along the margin. 2D inversion of the more robust transverse magnetic (TM) mode responses has therefore been used to derive a model of electrical resistivity structure beneath the transect. This shows the existence of two depressions containing low‐resistivity sediments separated by a more resistive region which is inferred to correlate with a spur of Mesozoic greywacke extending north from the Aorangi Mountains. The same features were previously identified from gravity data. The thickness of sediments adjacent to the Wairarapa Fault is of the order of 3–4 km, whereas that in the western arm of the Wairarapa trough is c. 2–3 km. The resistivity structure of the southwest end of the Hikurangi margin is more complex than the structure determined farther north along the margin.

Keywords:

• magnetotellurics
• Hikurangi margin
• resistivity