Encyclopedia of geology / Геологическая энциклопедия

The south-west Pacific Ocean is a region of isolated islands and submerged plateaus and ridges (Figure 1). The three main islands of New Zealand (North, South, and Stewart) make up the largest landmass group in the region. Schists, greywackes, and granitoids are exposed in islands on the Challenger Plateau and Chatham Rise and have been sampled in dredges on the Campbell Plateau and Norfolk Ridge, thus demonstrating their continental geological character. Abyssal Pacific oceanic-crustal floor typically lies at water depths of about 5000 m, and the boundary between continental crust and oceanic crust is marked by a generally pronounced slope break at about 2500 m water depth. The wider area of continental crust in the New Zealand region (Figure 1) is about one-third the area of on-land Australia and is commonly referred to as Zealandia.

On-land New Zealand contains a wide variety of Phanerozoic rocks (Figure 2), which preserve a detailed record of the Cambrian to early Early Cretaceous convergent margin of southern Gondwana, late Early Cretaceous rifting, a Late Cretaceous– Palaeogene passive margin, and the Neogene–Holocene active convergent and strike-slip margin. So much of continental Zealandia is submerged because of the widespread Cretaceous extension and rifting. It was only with the development of the Neogene–Holocene convergent plate boundary that about 10% of Zealandia emerged above sea-level. A distinction is generally drawn in New Zealand between pre-late Early Cretaceous (more than 105 Ma) ‘basement’ rocks, which are commonly metamorphosed and generally highly deformed, and ‘cover’ rocks, which are younger than 105 Ma, poorly indurated, well stratified, and less deformed. Palaeogeographical Reconstructions The shape of the continental crust of Zealandia has changed throughout the Phanerozoic. From the Cambrian to the Early Cretaceous, the New Zealand part of the Gondwanan margin grew by the magmatic and tectonic addition of batholiths and terranes. In the last 100 Ma this continental crust has been thinned, rotated, and translated in response to multiple tectonic events. Reconstructions (Figure 3) involve subtracting the 480 km Neogene dextral strike-slip movement on the Alpine Fault, the 45
Oligocene– Miocene rotation between the Pacific and Australian plates, and 4000 km of northwards drift. Small crustal blocks within 100 km of the Alpine Fault (i.e. most of on-land New Zealand) have undergone strong Cenozoic deformation. In the pre-rift (100 Ma) palaeogeography (Figure 3A) Zealandia is in a near-polar position and contiguous with Tasmania and Antarctica. By 10 Ma, some movement on the Alpine Fault had taken place and modern-day New Zealand had been isolated by seafloor spreading. <...>