The Sea Floor Around New Zealand
The bathymetry of the sea floor around New Zealand provides examples of most of the main types of feature. The coast is surrounded by the Shelf, which varies in width from one area to another. The narrowest portions of the Shelf are found off the east coast between Kaikoura and Cape Kidnappers where the width varies from less than 1 mile up to about 15 miles, and off Fiordland where the variation is from 1–4 miles. Around other parts of the coast, the Shelf is much more extensive, being generally 10–40 miles wide, while in western Cook Strait and south of Stewart Island the width increases to over 100 miles.
The gradient of the Slope varies a great deal between different areas, there being a broad correlation between the steepness of the Slope and the narrowness of the Shelf. In the Fiordland sector, the Slope is very steep, with a maximum gradient of about 1 in 4; the Slope is also relatively steep off the east coast between Kaikoura and East Cape, although in this sector there occur a number of submarine ridges and basins which locally reverse the general gradient. Apart from these two areas, most of the Slope is relatively gentle, but locally there occur steeper zones, for instance, the upper part of the Slope east of Otago Peninsula.
Several submarine canyons cut the Slope between Banks Peninsula and East Cape. The largest of these are the Pegasus Canyon (north of Banks Peninsula), the Cook Strait Canyon, and the Madden Canyon off Porangahau. The Cook Strait Canyon has an unusual shape as compared with most submarine canyons in other parts of the world, with its numerous branches and other irregularities, while the Madden Canyon with its great headward expansion is also exceptional. A group of smaller submarine canyons occurs east of Otago Peninsula.
Between Kaikoura and East Cape, the Slope flattens out rather abruptly at 1,500–2,000 fathoms into the Hikurangi Trench, a feature of low relief which is replaced by the much deeper Kermadec Trench to the north-east. In the sector between East Cape and North Cape, the Slope descends slowly and somewhat irregularly into the South Fiji Basin and the Havre Trough, while it flattens out at intermediate depths opposite the prominent Kermadec Ridge and the less conspicuous Colville Ridge. West of Fiordland, the Slope flattens out rapidly at about 2,000 fathoms into the wide and flat Tasman Basin. North-west of the North Island, and south-east of the South Island, the Slope fades out at shallow or intermediate depths and passes into two large continental borderlands. That to the north-west comprises the Lord Howe Rise, New Caledonia Basin, and Norfolk Ridge, while that to the south-east comprises the Chatham Rise, Bounty Trough, and Campbell Plateau. All of the continental borderland features are of major dimensions, and the majority cover an area equal to or greater than the North or South Islands. Along their outer margins they grade downwards into the adjoining ocean basins.