Neogene paleomagnetism and geodynamics of the Hikurangi margin, East Coast, New Zealand

Abstract

Vertical-axis rotations are an important component of Neogene deformation in the New Zealand plate boundary region, and potentially offer fundamental insights into the rheology of continental crust. Extensive paleomagnetic sampling along the Hikurangi margin, on the East Coast of the North Island, has provided new insights into the patterns, rates and timings of tectonic rotation, and also an improved understanding of the magnetic signature of New Zealand Cenozoic mudstones. Rigorous field tests reveal numerous late remagnetizations, which haveoften formed several million years after deposition and can be irregularly distributed within an outcrop. Scanning electron microscopy and rock magnetic analyses indicate that the remanence carrier is predominantly the ferrimagnetic iron sulphide, greigite, which is present as a mixed population of single domain and superparamagnetic grains that are characteristic of arrested authigenic growth. Strong viscous overprints are the result of later, usually recent, oxidation of these sulphides. The recognition of late-forming magnetizations leads to a completely new view of the Neogene tectonic evolution of the Hikurangi margin, with no tectonic rotations being evident prior to 8–10 Ma; coherent rotation of most of the Hikurangi margin since that time refutes the existence of the independently rotating ‘domains’ that were inferred from earlier paleomagnetic data. This pattern is more consistent with the short-term velocity field, and allows all Neogene rotation to be more simply explained as a large-scale response to realignment of the subducting Pacific plate. Tectonic rotations have been accommodated by a variety of structures since 10 Ma; in the Late Miocene and Pliocene, rates of tectonic rotation were 3–4 times faster than presently observed and possibly involved a much larger region, before initiation of the North Island Dextral Fault Belt and the Taupo Volcanic Zone at 1-2 Ma instigated the current tectonic regime. Collision of the Hikurangi Plateau in the Late Miocene is interpreted to have caused both the initiation of tectonic rotation, and the widespread remagnetization of sediments, making it a key event in the Neogene evolution of the plate boundary region.

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