Resistivity structure of the central Taupo Volcanic Zone, New Zealand

Abstract

The Taupo Fault Belt (TFB) is a 15-km-wide zone of active normal faulting, which is located in the central portion of the Taupo Volcanic Zone (TVZ), New Zealand, and is aligned sub-parallel to the strike of this zone. A large-scale multiple-source bipole–dipole resistivity survey has shown that the TFB is characterised by high resistivity (ca. 250 Ωm) to depths of ca. 7 km, in contrast to the adjacent regions in the east and west of the TVZ where resistivities are much lower (<50 Ωm). The eastern edge of the TFB is marked by the Paeroa fault. East of the fault, the Taupo–Reporoa depression is filled with volcaniclastic rocks that have low resistivities believed to be caused by the presence of clay alteration. At the Paeroa fault, the conductive volcaniclastics are downthrown to the west by about 1 km but they can be traced no more than 2.5 km west into the TFB. To the west of the TFB, a low-resistivity layer about 500 m deep has been identified as a sequence of older (>1 Ma) ignimbrites that have undergone slow diagenetic alteration that has produced clay minerals. The volcaniclastics in the TFB have high resistivities which suggests that they have not undergone significant alteration and are thus younger than the volcaniclastics in the regions to both east and west. These high resistivities are also consistent with the absence of surface geothermal activity and low heat flow. The TFB also exhibits high seismicity and active faulting in contrast with its surrounding. It was not possible to determine the depth to basement rocks in the TFB because of the lack of resistivity contrast between the basement and the overlying volcanics, but gravity data indicate that the young volcanics are no more than 3 km thick.