The structure of New Zealand from seismic evidence

Abstract

The New Zealand region is characterised by a crust 20–25 kms in thickness, and a sub-crust extending to at least 370 kms. These are separated by a transition layer extending from the base of the crust to a depth of about 100 kms. The crust is divided into blocks by steeply dipping faults. Some blocks are seismically active and others stable. The region is bounded by the Pacific Basin to the east, and the Tasman Basin to the west. Both of these areas have oceanic crusts about 5 kms thick. The sub-crust is traversed by a major wedge-shaped structure within which the deep focus seismicity is confined. This has been named the “Sub-Crustal Rift”, and its activity is apparently an extension of that associated with the Kermadec Trench. It follows a roughly south-west course from the Bay of Plenty to Farewell Spit, when it turns abruptly south-south-east, and finishes close to the Alpine Fault. Faults with both north-east and north-west trends are at present active. The most important of the north-west faults appears to run through Cook Strait. It is considered that the main crustal faults could have developed as the result of successive positioning above the Sub-Crustal Rift. Stable blocks have not been fractured in this way. There is some evidence for a difference in crustal structure on opposite sides of the Rift. The western portion is about 5 kms thicker, and contains a layer in which the P-velocities exceed 7 km/sec. No velocities above 6 1/2 km/sec have been observed in the eastern part. There is no evidence of arcuate structure. All the crustal and sub-crustal features appear to follow linear trends.