Torlesse Composite Terrane contains the Rakaia, Aspiring and Pahau Terranes and the Esk Head Belt. Greywacke is the dominant rock type of the composite terrane, argillite is less common and there are minor basalt occurrences. The Torlesse Composite Terrane is found east of the Alpine Fault in the Southern Alps of New Zealand. Its southern extent is a cryptic boundary with the Caples Terrane within the Haast Schists in Central Otago. It is named for the Torlesse Range in Canterbury. The Rakaia Terrane rocks, of Permian to late Triassic age, occur south of Rangiora.
Description
The Pahau Terrane rocks, of Late Jurassic to Early Cretaceous age, 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 of broken and deformed rocks. The Aspiring Terrane 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 through to the Middle Cretaceous. It was deposited in giant deep sea fans that extended beyond the ends of ancient submarine canyons. A fan starts with a submarine canyon on the continental shelf. Then turbidity currents rush down the canyon like giant undersea avalanches. As it does this it carries 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 were derived in part from the granitic rocks of northeastern Australia has been suggested by detailed studies of the mineral grains.
Metamorphism
The Torlesse Composite Terrane has undergone metamorphism and been transformed into Haast Schist. In the Haast Schists, the minerals that make up greywacke became coarser grained and altered to other minerals including quartz, feldspar and biotite. Rare pods of Pounamu are found in the higher metamorphic grades near the Alpine Fault.
