CONSTRAINING THE NEOGENE ROTATION HISTORY OF THE HIKURANGI MARGIN, NEW ZEALAND, USING COMBINED MAGNETIC FABRIC AND PALEOMAGNETIC DATA

Geodetic and paleomagnetic data both indicate that clockwise vertical-axis rotation of the Hikurangi Margin has been an important component of deformation across the New Zealand sector of the Pacific-Australian plate boundary during the Neogene. However, the magnitude, timing and structural accommodation of this rotation remains poorly constrained, largely due to the widespread occurrence of remagnetizations involving late diagenetic growth of greigite (Fe₃S₄) in uplifted forearc sediments. In many cases, the actual age of the characteristic remanent magnetization (ChRM) is unconstrained or only poorly constrained by field tests, which complicates interpretation of the inferred tectonic rotations. In order to robustly determine the rotation history of the Hikurangi margin, we have taken a two-pronged approach. First, the age of magnetization has been better constrained by calculating synthetic Apparent Polar Wander Paths that combine the absolute motion of the Australian plate with a range of possible local rotation models, and determining at what age interval the direction of the synthetic predicted remanence intersects with the pre-, syn- or post-tilt ChRMs for each locality. Second, the orientations of compressional tectonic fabrics developed in forearc rocks, derived from measurements of anisotropy of magnetic susceptibility (AMS), allowed recovery of the full rotation history at remagnetized localities. These independent datasets both indicate that large-scale clockwise rotation of the Hikurangi Margin did not initiate until the Late Miocene, and was initially much more rapid than the modern rate of ~3°/Myr with respect to the Australian Plate. Contemporary rotation of the eastern North Island is driven by differential coupling along the subducting Hikurangi Plateau, which is estimated to have collided with the Hikurangi Trough at ~10 Ma: our rotation history therefore allows for a consistent driving mechanism for regional rotation, with changes in rotation rate and the extent of the deforming region occurring as accommodating structures were rotated out of alignment with the regional stress field and new structures developed to accommodate them.