The Torlesse Composite Terrane is a plate tectonic

terrane In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust fragment formed on a tectonic plate (or broken off from it) and accreted or " sutured" to crust lying on another plate. The crustal block or fragment preserves its ow ...

forming part of the

South Island The South Island, also officially named , is the larger of the two major islands of New Zealand in surface area, the other being the smaller but more populous North Island. It is bordered to the north by Cook Strait, to the west by the Tasman ...

of New Zealand. It contains the Rakaia, Aspiring and Pahau Terranes and the Esk Head Belt.

Greywacke Greywacke or graywacke (German ''grauwacke'', signifying a grey, earthy rock) is a variety of sandstone generally characterized by its hardness, dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or lit ...

(or Torlesse Greywacke) is the dominant rock type of the composite terrane;

argillite :''"Argillite" may also refer to Argillite, Kentucky.'' Argillite () is a fine-grained sedimentary rock composed predominantly of indurated clay particles. Argillaceous rocks are basically lithified muds and oozes. They contain variable amounts ...

is less common and there are minor basalt occurrences. The Torlesse Composite Terrane is found east of the Alpine Fault in the

Southern Alps The Southern Alps (; officially Southern Alps / Kā Tiritiri o te Moana) is a mountain range extending along much of the length of New Zealand's South Island, reaching its greatest elevations near the range's western side. The name "Southern ...

of New Zealand. Its southern extent is a cryptic boundary with the Caples Terrane within the

Haast Schist The Haast Schist, which contains both the Alpine and Otago Schist, is a metamorphic unit in the South Island of New Zealand. It extends from Central Otago, along the eastern side of the Alpine Fault to Cook Strait. There are also isolated outcrops ...

s in Central Otago. It is named for the Torlesse Range in

Canterbury Canterbury (, ) is a cathedral city and UNESCO World Heritage Site, situated in the heart of the City of Canterbury local government district of Kent, England. It lies on the River Stour. The Archbishop of Canterbury is the primate of t ...

Description

The Rakaia Terrane rocks, of Permian to late Triassic age (300–200 Ma), occur south of

Rangiora Rangiora is the largest town and seat of the Waimakariri District, in Canterbury, New Zealand. It is north of Christchurch, and is part of the Christchurch metropolitan area. With an estimated population of Rangiora is the 30th largest urb ...

. The Pahau Terrane rocks, of

Late Jurassic The Late Jurassic is the third epoch of the Jurassic Period, and it spans the geologic time from 163.5 ± 1.0 to 145.0 ± 0.8 million years ago (Ma), which is preserved in Upper Jurassic strata.Owen 1987. In European lithostratigraphy, the name ...

Early Cretaceous The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145& ...

age (160–100 Ma), occur to the north, and are probably derived from the Rakaia Terrane. At the boundary between these two terranes is the Esk Head Belt, an wide

mélange In geology, a mélange is a large-scale breccia, a mappable body of rock characterized by a lack of continuous bedding and the inclusion of fragments of rock of all sizes, contained in a fine-grained deformed matrix. The mélange typically cons ...

of broken and deformed rocks. The Aspiring Terrane (Aspiring Lithologic Association) is officially included within the Torlesse Composite Terrane; however, it has a higher proportion of igneous rocks and a different sedimentary source. Its original relationship with the Rakaia Terrane is obscured by the Haast Schist.

Deposition

The greywacke of the Torlesse Composite Terrane was deposited on the eastern side of New Zealand from the

Late Carboniferous Late may refer to: * LATE, an acronym which could stand for: ** Limbic-predominant age-related TDP-43 encephalopathy, a proposed form of dementia ** Local-authority trading enterprise, a New Zealand business law ** Local average treatment effe ...

through to the Middle

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

. It was deposited in giant

deep sea fan Abyssal fans, also known as deep-sea fans, underwater deltas, and submarine fans, are underwater geological structures associated with large-scale sediment deposition and formed by turbidity currents. They can be thought of as an underwater ver ...

s that extended beyond the ends of ancient submarine canyons. A fan starts with a

submarine canyon A submarine canyon is a steep-sided valley cut into the seabed of the continental slope, sometimes extending well onto the continental shelf, having nearly vertical walls, and occasionally having canyon wall heights of up to 5 km, from c ...

on the

continental shelf A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea. Much of these shelves were exposed by drops in sea level during glacial periods. The shelf surrounding an island ...

. Then

turbidity current A turbidity current is most typically an underwater current of usually rapidly moving, sediment-laden water moving down a slope; although current research (2018) indicates that water-saturated sediment may be the primary actor in the process. T ...

s rush down the canyon like giant undersea avalanches. They carry all sorts of sediments from the shallower seafloor of the continental shelf. At the end of the canyon the turbidity current spreads out and creates giant fans of sediment that blanket the deep seafloor. These sediments may have derived in part from the granitic rocks of northeastern

Australia Australia, officially the Commonwealth of Australia, is a sovereign country comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands. With an area of , Australia is the largest country by ...

, as suggested by studies of the mineral grains.

Metamorphism

The Torlesse Composite Terrane has undergone

metamorphism Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of ch ...

and been transformed into Haast Schist. In the Haast Schists, the

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

s that make up greywacke became coarser grained and altered to other minerals including quartz, feldspar and biotite. Rare pods of pounamu (

jade Jade is a mineral used as jewellery or for ornaments. It is typically green, although may be yellow or white. Jade can refer to either of two different silicate minerals: nephrite (a silicate of calcium and magnesium in the amphibole group ...

) are found in the higher metamorphic grades near the Alpine Fault.

Fossil content

* Torlessia * Monotis * Hector's Ichthyosaur * Nothosaur *

Conodont Conodonts ( Greek ''kōnos'', " cone", + ''odont'', "tooth") are an extinct group of agnathan (jawless) vertebrates resembling eels, classified in the class Conodonta. For many years, they were known only from their tooth-like oral elements, whi ...

https://teara.govt.nz/en/fossils/page-2

References

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Description

Deposition

Metamorphism

Fossil content

See also

References

Further reading