Northland, geology - Alexandre SCHOHN

The Northland (in māori: Te Tai-tokerau or Te Hiku-o-te-Ika, “the tail of the fish”) is the most northerly of the regions of New Zealand. Located north of Auckland, taking Route 1, it is a region that brings a lot of surprises if you stay there a little.

Geology setting of Northland

Allochtones and obduction

Obduction is a peculiar phenomenon in geology that could be defined as the inverse of a subduction … Well, it must not be as clear as that. For allochtones is an element from another place / territory, this word is the opposite of the native (“allo” meaning alien and “ochthones” meaning earth).

The Northland is composed of oceanic sediments from the Cretaceous (145-65 million years), Eocene (56-23 million years) and Oligocene (23-6 million years). At that time, New Zealand was below the level of the oceans. Indeed, there exists under the ocean a continent called Zealandia, larger than India or Greenland. New Zealand represents only a small part of this continent.

Continent Zealandia at the bottom of the ocean. In red, oragne and green is New-Zéalande

23 million years ago, Northland was in the seabed. At that time, the Pacific plate collided with the Australian Indian plate. This resulted in an obduction of the Pacific plate on the Australo-Indian plate.

Obduction is the inverse of the subduction. Either instead of a plate passing under another plate of the density differences. The two plates have the same density which causes a collision and a superposition of one plate on the other with friction.

Obduction between Pacifique plate (at right) and Australo-Indienne plate (at left)

Allochtones

The overlapping plate will bring a discontinuity between the ages of the rocks. Indeed, a plate is composed of several layers of rocks called strata, which range from the youngest at the surface to the oldest in-depth. When a plate will overlap one second, a contact will be made between the oldest stratum with the younger stratum of the overlapping plate. The rocks of this overlapping plate are called allochthones.

Discontinuity of sediment ages. Oligocene (the youngest) is found under the Cretaceous and Eocene which forms a discontinuity.

The subduction

This obduction takes place at the bottom of the ocean, New Zealand is a set of small islands 23 million years ago. Following the obduction, the Pacific plate will subdue under the Austrlo-Indian plate. We then have an obduction and then a subduction. This will lead to the beginning of a volcanic activity directly following at 20-15 million years. The erupted magma will be positioned on the allochthones layer. This will lead to the emergence of the North Island of New Zealand!