Crust and Upper Mantle Inhomogeneities Beneath Western North Island, New Zealand: Evidence from Seismological and Electromagnetic Data.
Three geophysical techniques have been used to investigate the location and the nature of a large-scale change in crust and uppermost mantle properties below the western North Island of New Zealand. Receiver function analysis reveals a step like change in crustal thickness from ~ 25 km below the northwestern North Island to ≥ 32 km in the southwestern North Island. P-wave attenuation is elevated north of this change in crustal thickness (1000/Qp ≈ 1.9 for α = 0) and is compatible with a wet mantle at near solidus temperatures (T ≈0.97 melting temperature). Attenuation decreases by at least a factor of 2 for the southwestern North Island to values closer to those expected for normal continental lithosphere (1000/Qp ≤ 1 for α = 0). A region of extremely high attenuation (1000/Qp ≈ 5 for α = 0) is observed below the Central Volcanic Region. This value of attenuation is compatible with a wet mantle at temperatures just above melting (T ≈ 1.02 melting temperature). Finally 2D modelling of magnetotelluric data reveals a region of low electrical resistivity (100 Ωm) in the mantle below the region of thinned crust. Like the P-wave attenuation, this region of low resistivity can be explained by a water-saturated mantle at near solidus temperatures (T=0.88-0.97 melting temperature). The changes in crustal thickness, attenuation and electrical resistivity are all coincident with the southern limit of volcanism (~ 39.3°S) at a boundary that runs approximately east-west, perpendicular to the present plate boundary. The only surface expressions of this boundary are the termination of volcanism and the dome-like uplift of the North Island, which has previously been explained by the presence of a buoyant low-density mantle beneath the northwestern North Island. Elevated temperatures and water content inferred from this study are in agreement with this explanation. The sudden transition displayed in all three data sets, but particularly the crustal thickness step seen in the receiver function, calls for a special explanation. Thermal processes are too diffuse to explain the step and instead a mechanical process is called for. One possibility is that the step was created by convective removal of thickened lithosphere.