The kinematics of active deformation in the North Island, New Zealand, determined from geological strain rates

Kinematic modelling utilising the method of Haines & Holt extended to the case of cubic Bessel interpolation on curvilinear grids, allows analysis of present-day horizontal motions occurring in the Hikurangi margin, North Island, New Zealand. The velocity field solutions are derived from first order geological data; that is, rates and orientation of extension in the Taupo Volcanic Zone and rates and orientation of motion on the North Island Dextral Fault Belt, against the background of the pattern of uplift and subsidence in the margin. A basic (preferred) velocity field is presented, with four other solutions with different input data, to explain what controls features in the main solution. Every one of the five solutions is the best fitting solution for the input data in each case. All velocity fields are shown relative to the western boundary of the model, which is considered fixed, as part of the assumed non-deforming Australian plate. The velocity field in the main solution includes a strong clockwise rotation of the Hikurangi margin east of the Taupo Volcanic Zone in the north. Farther south, shear across the North Island Dextral Fault Belt facilitates the southwestward motion of the eastern part of the margin. An important boundary condition for the deformation in the North Island appears to be the higher rate of dextral shear in the Marlborough region, which accommodates the relative motion of the Australian and Pacific plates immediately south of the Hikurangi margin. The extension in the Taupo Volcanic Zone, with rates onshore of 5-10 mm/yr north of where the Taupo Volcanic Zone terminates in the centre of the North Island, and the strike-slip component of shear on the North Island Dextral Fault Belt, of c. 20 mm/yr in the south and <5 mm/yr in the north, account for most of the margin-parallel plate motion in the Hikurangi margin. No other major geological strains are required to be occurring in the North Island out of compatibility with these strains.