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Eastern schist basement rocks

Eastern Schist Basement Map

The deep burial and tectonic forces that formed hard greywacke rocks from sediments were applied to greater extremes with the basement rocks of Otago, Marlborough and the western side of the Southern Alps. This resulted in the development of schist and gneiss metamorphic rocks where the original minerals in the sediment have been forced to change (metamorphose) into denser new minerals under the pressure and much higher temperatures.

Platy minerals such as the micas biotite and muscovite have grown in response to the pressure forming foliated (flaky) schist layering. The distinctively flat-lying foliation through much of central Otago is the result of aligned minerals and mineral layers.

Minerals in the highest metamorphic grade rocks such as garnet and oligoclase formed more than 15 km below surface and temperatures in excess of 400°C. Through tectonic forces and erosion of the overlying rocks these deeply buried schist rocks were uplifted close to the present day surface around 100 million years ago in Otago and Marlborough, and more recently in Westland. This uplift signalled the end of the ancient plate boundary through New Zealand.

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