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Forest Vines to Snow Tussocks: The Story of New Zealand Plants

Crustal Movements

Crustal Movements

The modern floras are dominated by flowering plants with conifer and fern components, so we can begin our journey from the past at the time when flowering plants first began to achieve prominence — the middle of the Cretaceous period (Fig. 123) about 100 million years ago. page 241By this time Africa and India had long since separated from the great southern continent, Gondwana, but Australia, Antarctica and the New Zealand-New Caledonia crustal complex (Tasmantis) were still united (Fig. 3). The southern end of South America was probably joined with Antarctica at this stage and remained so until the Oligocene, 30 million years ago.

During the middle Cretaceous, the Australasian portion of Gondwana lay in higher southern latitudes with the northern parts of New Zealand and Australia at about 60 °S and 45 °S respectively.

In the late Cretaceous (80 million years ago) the New Zealand crustal block began to separate from Australia and Antarctica, moving like a hinge with the northern end of the Lord Howe Rise remaining adjacent to Australia. By the middle Paleocene (60 million years ago) northern New Zealand had reached 55°S and the Norfolk Ridge had separated from the Lord Howe Rise. It is possible that there were dry land connections at this time with Queensland, via the Lord Howe Rise, and New Caledonia via the Norfolk Ridge.

In the late Paleocene at about 50 million years ago Australia and Antarctica were moving apart with north Australia reaching 30°S. Northern New Zealand had attained 40°S by this time. One result of these crustal movements was the formation of the Lau and Tonga volcanic ridges near to and beyond the northern Norfolk Ridge. Fiji lay at the northern end of these ridges.

By the late Eocene at 3 8 million years ago Australia had not changed much in position. New Zealand had moved further north to approach its present northernmost position of 35°S and the Lau and Tonga ridges had moved eastwards away from the Norfolk Ridge.

The late Oligocene (29 million years ago) saw north Australia reaching 25°S and the activation of the Alpine Fault in the New Zealand region with movement southwards along it of most of the present South Island and the Campbell Plateau and Chatham Rise. By this time, most of the New Zealand-New Caledonia crustal block was submerged.

By the Late Miocene (10 million years) north Australia was at 15°S and the rearrangement of the New Zealand region along the alpine fault continued. Between this time and the present the current configuration was attained with north Australia reaching 10°S and New Guinea almost reaching the equator. In the last few million years crustal movements have raised the New Zealand and New Guinea mountains.

Figure 123 Geological time scale.

Figure 123 Geological time scale.