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Tectonic and geochronological implications of variably timed magnetizations carried by authigenic greigite in marine sediments from New Zealand

¹ Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, UK

Detailed scanning electron microscope observations, coupled with elemental microanalysis, confirm the presence of a chemical remanent magnetization carried by authigenic greigite (Fe₃S₄) in uplifted Neogene marine sediments from the Hikurangi Margin of New Zealand. Normal polarity samples from the studied section have declinations that are deflected 60° clockwise of reversed polarity samples, indicating the presence of two distinct magnetizations separated by several million years of tectonic rotation about a vertical axis. However, although multiple generations of iron sulfide growth are observed petrographically, we see no clear differences in the relative timing of greigite formation between samples carrying these two magnetizations. Not only can the diagenetic growth of greigite in fine-grained marine sediments occur long after deposition, obscuring tectonic and magnetostratigraphic information, but such remagnetizations are also difficult to distinguish from a more primary signal in the absence of constraints from field tests. Our observations emphasize that considerable care is necessary when interpreting paleomagnetic data from greigite-bearing sediments.

Key Words: greigite • remagnetization • New Zealand • tectonics • magnetostratigraphy