The northern boundary is the Cook fracture zone and the southern is the Regina ridge projecting from Northland Peninsula, New Zealand.
[3] Seamounts along the line of the Nepean saddle and Bates plateau approach within about 1 km (0.62 mi) of the seas surface.
[b] The basin is a Cretaceous to Miocene structure at the western edge of the continental crust of the submerged continent of Zealandia.
[13] Definite oceanic crust samples have been obtained from much of the basins sea floor as have some continental type rocks.
[14] The Nepean saddle, that separates the two main basins, the Cook fracture zone to the north, and the Three Kings Ridge, and the adjacent Cagou Trough to the east are not magnetically quiet, and this relates to magmatic intrusions and seamount formation.
[19] The Cagou Trough as a major north–south oriented feature at the eastern side of the basin has a normal fault visible on seismic reflection on its western flank.
[21] Some seamounts in the area of the basin have atypical Oligocene age magma geochemistry, so while being subduction-related, are possibly caused by reactivation and roll-back of a slab that had been already subducted and partially dehydrated in the Cretaceous.
[19] Other seamounts to the west of the basin on the western side of the Norfolk Ridge have ocean island basalt characteristics that mean they are not subduction related.
[23] Pacific basin subducting slab dipped southwest beneath the eastern margin of Gondwana in the period from 260 to 110 million years ago.
[26] By the Late Cretaceous (83.5 million years ago) the eastern opening of the Tasman Sea from the Australian plate started.
So at either end of the extension Norfolk Basin geological processes were dominated by convergence and obduction since the Eocene.