Abstract
We present new analytical data from lavas and associated rocks dredged and/or drilled from the South Fiji Basin, Northland Plateau, Colville Ridge and Havre Trough. These results provide much-needed ground truth about the geology, age and tectonic evolution of the Cenozoic submarine ridges and basins between the active intraoceanic TongaKermadec arc, and rifted continental borderlands of New Zealand, the Norfolk Ridge and New Caledonia. Key results from this study include: (1) ArAr dates on Minerva Abyssal Plain oceanic crust suggest that the ages of magnetic anomalies in the South Fiji Basin have been overestimated by earlier workers; (2) subduction-related lavas are widespread across the region, are not presently organised into arc-like chains, and cluster in the age range 2218 Ma (Early Miocene); (3) the oldest subduction-related lavas occur in the western part of the region (3226 Ma: Norfolk and Three Kings Ridge); (4) shoshonites, interpreted as rifted arc lavas, were erupted in a narrow 2021 Ma interval over a wide area. Put together, these results indicate high magmatic flux and large and rapid horizontal tectonic translations and basin opening from 1823 Ma in the region immediately north of New Zealand. We explain the Miocene tectonomagmatic development of the region by a model of rapid rollback of a single, east-facing Pacific arctrench system that became established after Northland Allochthon emplacement. Critical testing of this, versus other, tectonic models must await drilling and dating of thus-far unsampled Kupe Abyssal Plain crust.