Contemporary horizontal velocity and strain rate fields of the Pacific-Australian plate boundary zone through New Zealand

We have inverted velocity solutions from nine geodetic networks distributed across New Zealand to derive present-day continuous horizontal velocity an d strain rate fields at the Earth's surface throughout the country. The nin e networks contain a total of 362 Global Positioning System (GPS) stations that have been observed at least twice and at least a year apart between 19 91 and 1998. The model velocity field is expanded as bicubic spline interpo lation functions defined within a curvilinear grid that covers the country and extends into the assumed rigid Australian and Pacific plates to west an d east. The inversion jointly minimizes the magnitudes of fitted strain rat es and the misfit to the observed velocity data. The spline technique allow s high spatial resolution of strain rate variations, especially in regions with spatially dense GPS data. Expansion of the model velocity field on the surface of a sphere allows arbitrarily large areas to be studied. Previous ly known aspects of New Zealand plate boundary deformation are highlighted with improved resolution, including back-are extension in the Taupo Volcani c Zone, rotation of the Hikurangi forearc away from this zone in the North Island, and a band of high shear strain rate under the Southern Alps to the southeast of the Alpine fault. Our results reveal several new features, in cluding a region of enhanced shear straining apparently associated with str ike-slip faults in the southern North Island and a band of contractional st raining subparallel and well east of the Alpine fault that is similar to fe atures found in numerical and sandbox models of continental collision.