Nature and Potential Origins of the Low-Rate Tectonics of the Hauraki Rift and North-Western New Zealand: A Geophysical Investigation

Abstract

Little is known about the nature or origin of tectonic activity throughout north-western New Zealand, beyond that it is a stable environment (velocities ~1 mm/yr relative to Australia) and home to the Hauraki Rift, N-S trending, 250+ km long and 25 km wide on-shore/off-shore continental rift 40 km east of Auckland City and oblique to the Hikurangi subduction margin. A three-year 40+ station campaign GNSS geodetic survey was done incorporating previously unused historic data to constrain the crustal motion throughout the north-western North Island and the Hauraki Rift. A screw dislocation rift model incorporating this data derived estimates for the far-field widening rate and mechanical thickness of the Hauraki Rift to be 1.15 +/- 0.3 mm/yr and 17.5 +/- 7.7 km respectively. Relative to a fixed Australian Plate reference frame, the results generally showed S-SE velocities east of the Hauraki Rift and W-SW velocities west of the rift with increasing magnitudes to the south towards the plate boundary. North of Auckland (~37°S) sites maintained consistent W-SW velocity magnitudes throughout, over 700 km from the plate boundary. This indicates that northernmost New Zealand may not be stable relative to the Australian Plate. Analysis of ground and satellite-based gravity anomalies in conjunction with the observation of geodetic velocity discrepancies were used to infer the location and motion of several faults not currently considered active, including N-S trending faults parallel to the west coast of New Zealand near Auckland. The observation of similar cGNSS geodetic velocities on both Norfolk Island and New Caledonia similar to those observed throughout Northland NZ may be evidence that the eastern portion of the Australian tectonic plate (east of Lord Howe Island) is mechanically separated. A Oligocene-Miocene tectonic model detailing the westward collision of the Loyalty-Three Kings arc and subsequent opening of the Norfolk Basin is presented. This model proposes that the relative motion between the Australian and "East Australian" sub-plate may be accommodated by a continuous ~2000 km lineament of weakened crust, low Bouguer Anomalies, and concentrated volcanism. This feature may extend from New Caledonia along the western Norfolk Ridge through the Reinga Basin and along the western coast of the North Island of New Zealand.