New Palaeomagnetic Constraints on Tectonic Rotations in the Marlborough Fault Zone, New Zealand

New Zealand lies on the plate boundary between the obliquely converging Australian and Pacific Plates. Off the east coast of the North Island, this motion is accommodated by subduction of the Pacific Plate along the Hikurangi Margin. This system terminates northeast of the South Island, where the motion is transferred into the transpressive Marlborough Fault Zone (MFZ). Finite offset on the faults in this system is substantially short of that on the Alpine Fault to the south. It has long been recognised that rotation about vertical axes has played an important role in the accommodation of relative plate motion. Previous palaeomagnetic studies have concentrated mainly on coastal exposures, and large clockwise rotations have been found. Two kinematic models of rotation have been proposed, though spatial coverage of the data is insufficient to discriminate between them. This study has determined declination anomalies from six new sites in Marlborough and North Canterbury, yielding improved constraints on the extent and magnitude of rotation. The central MFZ has undergone moderate rotation of 40-90°. This finding is inconsistent with both kinematic models of rotation. A zero rotation result to the south west of the MFZ provides improved limits on the extent of the rotated zone, which extends beyond the major faults of the MFZ. This study demonstrates that the pattern of rotation in the MFZ is complex, and a new kinematic model is required to fit the improved data set.