New Zealand

Zealandia is a submerged continent with a vast surface area of 4.9 to 5.37 million square kilometers located in the southwest Pacific Ocean, encompassing the islands of New Zealand and New Caledonia. Approximately 94–95 percent of the continental mass lies below sea level. Recognized as the eighth continent, Zealandia was situated along the southern edge of the Gondwana supercontinent during earlier geological periods and later detached from the Australia-Antarctica margin to become an independent continent.^【1】

Although the presence of continental crust in this region has been known for many years, the term “Zealandia” was first used in 1995 by Luyendyk to collectively describe New Zealand and surrounding submarine plateaus. The Māori-Pasifika-derived name “Te Riu-a-Māui,” proposed by academic Manuka Henare, refers to the legendary explorer Māui and evokes the image of a canoe’s hull (riu) alongside its valleys and plains.^【2】

^{Bathymetric Map of Zealandia (}mapasmilhaud.com⁾

Before Zealandia’s existence was confirmed, in 1977 Nur and Ben-Avraham, followed by Kamp in 1980, proposed the hypothetical continent “Pacifica,” which was thought to have fragmented and laterally dispersed across the Pacific Ocean. Data collected during the 1980s revealed that the Pacific Ocean ridges consisted not of continental crust but of thickened oceanic crust, leading to the abandonment of the Pacifica hypothesis.^【3】

Zealandia possesses all the principal characteristics required for a landmass to be classified as a continent. These criteria include a distinct elevation above surrounding oceanic crust, a wide variety of silicic continental rocks, a thick and low-seismic-velocity crust compared to oceanic basins, and well-defined physical boundaries exceeding one million square kilometers.^【4】

The average modal elevation of the submerged portion of the continent is approximately −1100 meters. The highest peak of Zealandia is Aoraki/Mount Cook, rising 3724 meters above sea level. The continental crust ranges in thickness from 10 to 30 kilometers, typically 20–25 kilometers, but exceeds 40 kilometers beneath certain regions such as the South Island. Seismic velocity data indicate P-wave velocities generally below 7.0 km/s.^【5】

The oldest exposed portions of Zealandia’s crust date back approximately 500 million years. However, studies of lithospheric mantle xenoliths and detrital zircons have revealed materials from the Precambrian era ranging in age from 1200 to 3526 million years.

The basement rocks formed through episodic subduction and accretion processes along the Gondwana margin during the Phanerozoic. Approximately 105 million years ago, the collision of the Hikurangi oceanic plateau with the Gondwana continental margin, and its failure to subduct, terminated prolonged subduction and initiated extensional tectonics. Zealandia gradually rifted away from Gondwana between 80 and 60 million years ago and became fully separated around 55 million years ago.^【6】

In the current tectonic configuration, the continental lithosphere is bounded by the Pacific and Australian plates. To the north, the Hikurangi subduction zone marks the westward subduction of the Pacific plate, while in the south, the Australian plate subducts eastward beneath Fiordland to form the Puysegur subduction zone. The Alpine Fault, connecting these two subduction zones, exhibits an oblique continental-continental collision with a right-lateral strike-slip motion of 28 mm per year, resulting in the vertical uplift of the Southern Alps at a rate of 6–9 mm per year.^【7】

The surface geology of Zealandia is divided into two main structural groups: a folded crystalline basement formed between 540 and 105 million years ago, and overlying stratified units of the Zealandia Megasequence, deposited unconformably between 105 and 0 million years ago.^【8】

The crystalline basement units are divided into three broad regions from west to east:

^{Mapped Zealandia Continent (}GNS Science⁾

• Western Province Terranes: Includes the Buller Terrane, composed of Early Ordovician continental margin turbidite fans, and the Takaka Terrane, containing remnants of Early to Middle Cambrian subduction-related magmatism and sediments.
• Central Arc Terranes: Units associated with magmatic arcs and subduction processes. This includes the Brook Street Terrane, an Early Permian volcanic arc remnant; the Murihiku Terrane, with fossil-rich sedimentary sequences; and the Maitai Terrane, which contains the Dun Mountain Ophiolite Belt (DMOB). The DMOB ophiolite belt is the source of the prominent “Junction Magnetic Anomaly” (JMA), a clearly traceable magnetic anomaly across the continent.
• Eastern Accretionary Terranes: Comprises the Caples, Waipapa, and Torlesse composite terranes, formed by the accretion of oceanic sediments. These areas are partially overlain by the Haast Schist, a widespread regional metamorphic belt representing pre-Cretaceous events. Additionally, across the continent, massive plutonic bodies such as the Median Batholith, which extends for 4000 kilometers and cuts through the basement rocks, are observed.

Post-Cretaceous cover units are classified into five main supergroups:

• Momotu Supergroup (~105–80 Ma): Includes marine and terrestrial sediments and clastic sequences deposited during the rifting of Gondwana.
• Haerenga Supergroup (~80–35 Ma): A sedimentary sequence reflecting the continental drift phase of Zealandia, generally showing marine transgression.
• Waka Supergroup (~35–25 Ma): Represents the Oligocene period, when Zealandia was maximally submerged, and consists primarily of limestone-dominated sequences.
• Māui Supergroup (~25–5 Ma): Formed during the Miocene as sea levels regressed and continental landmasses began to re-emerge, comprising terrestrial and shallow marine sediments.
• Pākihi Supergroup (5–0 Ma): Consists of gravels, alluvium, and unconsolidated cover materials formed during the Pliocene and Holocene, when sea-level regression reached its peak and was influenced by tectonic uplift and glaciation. Within the Megasequence, active volcanic centers related to the subduction zone, such as the current Taupo Volcanic Zone (TVZ), are also present.

The “depth to base of magnetic sources” (DBMS) method is widely used to investigate the thermal structure of the continent. Analytical DBMS depth indicates the Curie temperature at which rocks within the crust lose their magnetism, corresponding generally to the reference value for magnetite (580°C).

Spectral mapping results over Zealandia show that magnetic base depths vary between 8.9 and 28.5 kilometers across the region. In the deeper regions beneath the South Island, this boundary reaches 28.5 kilometers, while in magnetically quieter areas it is found at depths of 26–28 kilometers. In the Taupo Volcanic Zone (TVZ), where magmatic rifting and a thin crust dominate, the thermal gradient is significantly higher, and DBMS depths shallower to between 8.9 and 12 kilometers. Discrepancies between heat flow analysis data and geothermal borehole measurements in areas dominated by plutonic rocks suggest that the titanomagnetite chemistry within plutons lowers the Curie decomposition temperature to approximately 500°C, below the assumed value.^【9】