Presentation on theme: "Aotearoa The birth of a country. Gondwana This reconstruction of the Gondwana supercontinent about 200 million years ago shows the position of New Zealand."— Presentation transcript:
Aotearoa The birth of a country
Gondwana This reconstruction of the Gondwana supercontinent about 200 million years ago shows the position of New Zealand adjacent to Australia and Antarctica.
Where did it start? Greywacke forming at the edge of Gondwana This diagram is a reconstruction of how Torlesse greywacke and associated sediments formed at the edge of the Gondwana supercontinent. Rivers carried sediment from land out to sea on the continental shelf, where it was then transported down channels as turbidity currents onto huge submarine fans on the deep ocean floor. Source: Glen Coates, The rise and fall of the Southern Alps. Christchurch: Canterbury University Press, 2002
Humble beginnings… Over 200 million years, tens of thousands of metres of these sediments built up off the edge of Gondwana. They were eventually buried, deformed and hardened to become the rocks known as the Torlesse greywackes. Today, Torlesse rocks make up more than half of the New Zealand landmass. They cover a vast area, extending from Otago to East Cape, and below the ocean across to the Chatham and Auckland Islands.
New Zealand region 150 million years ago The rocks that underlie much of New Zealand were formed beneath the oceans between 300 and 150 million years ago. Sediments eroded from the Gondwana supercontinent were carried by rivers to the sea and deposited offshore, while volcanic activity built up oceanic islands. By 150 m.y.a., the rocks that had built up offshore had been plastered onto the edge of Gondwana and uplifted, forming a mountainous region.
New Zealand adrift About 85 million years ago a spreading ridge formed in the future Tasman Sea, causing a continental fragment, including the future New Zealand, to break away from Gondwana. This reconstruction shows that at about 70 million years ago the Tasman Sea has opened up, and the continental fragment (Zealandia) is quite separate from Gondwana.
New Zealand region 10 million years ago This is a reconstruction of the New Zealand region about 10 million years ago. The green area represents land. The major tectonic features we recognise today had formed, including the Alpine Fault in the South Island and the subduction zone, where the ocean floor is descending beneath the North Island.
Sunken Zealandia splits up About 25 million years ago, a shift in plate movements began to wrench apart the largely submerged New Zealand continent, Zealandia. In the north, sections of ocean floor of the Pacific Plate began to sink beneath continental rocks of the Australian Plate. Within the continent, pressure caused major cracks to develop. These cracks would eventually join to become New Zealand’s great Alpine Fault, splitting the continental mass in two. New Zealand now lay across two separate plates. These plates began to rotate. A sideswiping collision began, with the plates sliding past and running into each other. New land began to rise above the sea along the plate margins as colliding sections began to crumple. Volcanic activity and uplift increased, and substantial mountain building began about 5 million years ago.
Underwater basins and uplift While many areas were being uplifted, parts of the New Zealand landmass were warped downward, creating large basin areas. As more land was pushed above the sea it began to erode and shed more sediment into the surrounding ocean. Layers of soft, grey mudstones and fine sandstones were deposited, with particularly thick accumulations along the east coast of both islands As the land rose, the surface layers of younger rocks such as limestones, sandstones and coal were fractured and folded. In the rising ranges of the Southern Alps, however, most of the younger rocks were eroded away, exposing the underlying Torlesse rocks.