Northland ophiolite, New Zealand, and implications for plate- tectonic evolution of the southwest Pacific

Abstract

The Northland ophiolite and coeval ocean-floor sedimentary rock form the Northland allochthon obducted from the northeast onto New Zealand in the late Oligocene. The ophiolite was probably emplaced as a single sheet and separated into individual massifs during subsequent movement of the allochthon. Chemically, the bulk of the igneous rocks are normal mid-ocean-ridge basalts, but the ophiolite also includes a younger suite of hornblende modal within-plate alkalic rocks believed to represent seamounts. Two alternative models, one involving a subduction flip, the other involving continuous westward subduction, are proposed to account for the obduction of the upper part of the Northland ophiolite. The oceanic crust from which the ophiolite originated was formed simultaneously with Tasman Sea spreading on the western rim of a once much larger South Fiji plate assemblage. -Authors