Geology of New Zealand - Basement Rocks

Basement Rocks

The oldest rocks in New Zealand date from mid Cambrian times (510 Ma), although there are older rocks in the Auckland Islands. The lower(basement) rocks are divided into the “Western Province”, consisting mainly of granite and gneiss, and an “Eastern Province”, consisting mainly of greywacke and schist. The provinces are further divided into terranes – large slices of crust with different geological histories that have been brought together by tectonic activity (subduction and strike-slip faulting) to form New Zealand.

The plutonic basement rocks are subdivided into the Hohonu, Karamea, Median and Paparoa batholiths, that solidified from magma deep in the crust. These rocks form the foundations beneath offshore Taranaki, and the West Coast, Buller, Northwest Nelson, Fiordland and Stewart Island. Most New Zealand granites were formed in Devonian-Carboniferous time (380-335 Ma) and Jurassic-Cretaceous time (155-100 Ma). The rocks can be matched to similar rocks in Eastern Australia.

The sedimentary basement rocks are subdivided into the Western Province Buller and Takaka terranes, and the Eastern Province Brook Street, Murihiku, Maitai, Caples, Torlesse Composite (Rakaia and Pahau) and Waipapa Composite (Morrinsville and Hunua) terranes. The Western Province sedimentary rocks form the foundations beneath the Northwestern portion of the South Island, to the West of the Alpine Fault, and Fiordland. The Eastern Province sedimentary rocks form the foundation for the rest of New Zealand. They are mostly composed of greywacke together with argillite.

The Alpine Fault that corresponds to the line of the Southern Alps has separated the basement rocks that used to be adjacent by about 460 km. For example, Nelson and Southland used to be adjacent, as were Marlborough and Otago.

In general, the sedimentary basement terranes become younger from West to East across the country, as the newer terranes were scraped off the subducting paleo-Pacific Plate, and accreted to the boundary of Gondwana over hundreds of millions of years.

The Buller and Takaka terranes of the Western Province formed in Cambrian-Devonian time (510-400 Ma) and contain New Zealand’s oldest greywacke.

The Median Batholith represents a long-lived batholith dividing the Western and Eastern Provinces, and before the separation of Gondwana, stretched from Queensland, through what is now New Zealand, into West Antarctica. It marks the site of a former subduction zone on the edge of Gondwana.

The Central Arc Terranes (Brook Street, Murihiku and Maitai Terranes) are of Carboniferous-Jurassic age (310-165 Ma), and are believed to be related to ancient volcanic arcs. They contain sandstone and mudstone derived from volcanic rock, and some limestone. The Brook Street terrane contains Glossopteris fossils, linking New Zealand to Gondwana. The Maitai terrane includes the Dun Mountain Ophiolite Belt, which contains dark serpentinised ultramafic rocks. These rocks form a line (split by the Alpine Fault) throughout the length of the country that can be detected by the magnetic anomaly it produces. Ophiolites are slices of oceanic crust and magma that have been detached from a subducting plate, and obducted onto the continental crust above the subduction zone.

The Caples, Torlesse Composite (Rakaia and Pahau) and Waipapa Composite (Morrinsville and Hunua) terranes formed in Carboniferous-Cretaceous time (330-120 Ma). Much of the rocks were deposited as submarine fans. They have different origins, as shown by different chemical compositions and different fossils.

Many rocks in the Western Province (Paparoa, Fiordland and Stewart Island) have been metamorphosed into gneiss. Many rocks in the Eastern Province have been metamorphosed into schist, due to exposure to high pressures and temperatures. Rocks grade continuously from greywacke (e.g., in Canterbury) to high-grade schist (e.g., around the Caples-Torlesse boundary in Otago and Marlborough, and Torlesse rocks just to the East of the Alpine Fault).

Refer to the GNS Map of New Zealand's Geological Foundations.