Following compression, uplift and erosion of the rocks of the Rangitata Orogen (Carboniferous-Lower Cretaceous) a change to extensional tectonics occured in mid-Cretaceous times throughout the New Zealand region. A rift system developed on the West
Coast of the South Island, while a tectonically-controlled basin developed on the east coast of both islands. This early extensional phase was accompanied by volcanism ranging from rhyolitic to basaltic. The Late Cretaceous transgression and pene