Present tectonics and Late Cenozoic evolution of New Zealand

Summary. The boundary between the Pacific and Indian Plates in New Zealand is a 200‐km‐wide zone of pervasive deformation manifest in seismic activity, distortion of old triangulation nets and differential vertical movements of the land. The nature of the deformation within the zone is investigated using geodetic observations and earthquake slip vectors. Two components of shear strain have been measured at 22 locations using changes in the observed angles of retriangulation nets. Earthquake slip vectors for 20 published fault‐plane solutions show the direction of slip at the time and location of the earthquake. Together these data indicate an axial belt of rapid dextral shear some 70–100 km in width crossing diagonally from the Hikurangi active margin on the north‐east to the Fiordland margin on the south‐west. Within this belt maximum shear rates reach values of 7 × 10 –7 yr −1 . The rates and the directions of the principal axis of horizontal compression are the same, within the limits of error, as those deduced for the relative movement between the Pacific and Indian Plates from the Minster et al. instantaneous pole of rotation. Two zones of almost pure compression occur to the west of the axial tectonic belt; they are here named the Nelson and Milford tectonic zones. Behind the two active margins are zones of extension named the Taupo rift in the North Island and the Waiau rift in the South Island. The present rate of compression across the boundary zone, taken up by crustal thickening, can account for the Quaternary uplift of the mountains of the tectonic belt and zones. Two types of deformation in crustal rocks are inferred: a seismic, elastic deformation resulting in fault displacements, and an aseismic, ductile flow. The relative proportions of these varies along the length of the boundary zone. In central South Island flow is dominant, fault displacements are small and seismicity low. In the south, fault displacements account for most, and flow a minor part, of the movement between the plates. Extrapolation of present‐day deformation into the past indicates that much of the observed deformation of New Zealand is of Late Cenozoic age and includes the displacement of the Alpine Fault and the bending of an original straight Mesozoic geosyncline into its present arc. Crustal thickening, however, is unlikely to absorb all the surplus material due to compression across the boundary zone in the last 10 Myr. It is postulated that material caught between the converging plates has been squeezed north‐eastward to the Hikurangi subduction zone. The Alpine, Wairau Faults and Cook Strait were originally colinear. About 15 Myr ago they formed a major fault connecting the Kermadec and Macquarie active margins and have since become disrupted and bent. Only parts of this early fault are still active, most of the movement between plates being taken up in the axial tectonic belt.