Tectonic implications of Paleomagnetic data from the Northland ophiolite, New Zealand

Abstract

Paleomagnetic sampling of 12 sites from two separate massifs of the Northland ophiolite, New Zealand, yields consistent results. Two characteristic NRM directions are recognised: one of normal polarity associated with weakly magnetised rocks (particularly intrusive dolerites), and another of reversed polarity associated with strongly magnetised rocks (particularly basaltic pillow lavas). Thermal and AF demagnetisation shows that the normal directions are due to a secondary viscous component which reflects the present-day field direction, whilst the reversed directions are stable (typically to 500°C) reflecting either the primary TRM or a hard secondary component. The declination and inclination of this stable magnetisation differ substantially from that of the present-day field implying significant tectonic movement or deformation of the ophiolite since formation. Anomalous declinations and inclinations are interpreted here in terms of rotations about vertical axes and, tentatively, latitudinal translation since no widespread significant tilting has been recognised within the massifs. Declinations are consistent with clockwise rotations of 50° and 120° for the Cape Reinga and Mangakahia massifs, respectively, probably associated with obduction at a convergent margin with a right-lateral slip component, situated to the northeast of Northland. The anomalously low inclination vectors (30–40°) are consistent with an origin at lower than present latitudes which, if true, implies long-lived N-S translation of oceanic crust in the southwest Pacific region between Cretaceous and Oligocene times.