Tracking repeated subduction of the Hikurangi Plateau beneath New Zealand

The Hikurangi Plateau has been subducted beneath New Zealand twice — firstly at ca. 100 Ma during N–S convergence with Gondwana, and currently during E–W convergence between the Pacific and Australian plates. Here we use a nationwide 3-D seismic velocity model derived from seismic tomography studies, and seismicity relocated with this model, to determine the location of the subducted western and southern edges of the plateau. In the Hikurangi subduction zone in the North Island and northern South Island we identify the western edge in the 37–140 km depth range, based on the distribution of seismic velocities in the top of the subducted Pacific plate, changes in the thickness of the dipping seismic zone, and steepening of the subducted plate downdip of the buoyant plateau. In the central South Island, where the Hikurangi Plateau is subducting for a second time, the plateau dips less steeply. Its southern edge is well-defined by the collision of the delaminated lower crust of the plateau with the northern Fiordland subduction zone, where it bends the younger subducted Australian plate to vertical. The Hikurangi Plateau was originally rifted from the Ontong Java Plateau, and the subducted portions of the Hikurangi Plateau identified here fill a gap in reconstructions of this large igneous province.