Story: Geology – overview
Page 5 – Disturbances on the edge of Gondwana
The sea floor is constantly on the move. About 250 million years ago it began to shunt offshore sediments towards the coast of Gondwana. As the sea floor was pushed beneath the edge of the supercontinent, the piles of sediment on top were broken into sections and scraped off. They piled up as stacks of steeply dipping, overlapping slabs. Slices of ocean floor were also caught up in the collision. Eventually the rocks were thrust above the surface to form a mountainous new area of land on the edge of Gondwana.
In Southland and Nelson there is a belt of rocks that affects compasses measuring earth’s magnetic field. The magnetic disturbance, known as the Stoke’s Magnetic Anomaly, is caused by a band of rocks rich in iron and magnesium, called an ophiolite belt. They are sections of old sea floor that have become sandwiched in with the volcanic and sedimentary Western Arc and Murihiku rocks. Even where these rocks are not visible at the surface, magnetic disturbances indicate where they are present deeper down.
The Haast schists
From about 200 million to 160 million years ago, as the Western Arc, Murihiku and Torlesse sediments moved into the coastal collision zone, the rocks at the base of the sediment piles were undergoing a transformation. Pressure and heat were breaking down the sediment, and it was recrystallising to form glittering new rocks containing minerals such as mica and garnet. These metamorphosed rocks are known as the Haast schists. The mineral crystals within the different types of schist are clues to the temperatures and pressures where they were formed. To change Torlesse greywacke into the most highly metamorphosed type of Haast schist, temperatures had to reach over 300°C and the rocks had to be buried to a depth of more than 10 kilometres.
Today, Haast schists form the bedrock in broad areas of Otago and Southland, and are exposed in a narrow band along the western edge of the Southern Alps. Some sections of ocean floor caught in the collision were also metamorphosed, forming rocks that include pounamu (New Zealand jade or greenstone).
Late in the cycle of mountain building, about 105 million years ago (in the Cretaceous period), some rock within the crust became hot enough to melt completely. The molten material moved upwards, solidifying to form masses of granite now found in areas such as Abel Tasman National Park.
Cretaceous granites often cannot be distinguished from older granitic rocks by their appearance in outcrop. Laboratory radiometric dating of minerals is needed to determine the age of an igneous rock.