The Hikurangi Trench is a broad, asymmetric deep-sea trough, running along the base of the Continental Slope in the sector between Kaikoura Peninsula and East Cape. The axis of the Trench is slightly sinuous, but the general trend is NE-SW, more or less parallel to the coast. The distance of the axis from the coast increases from south to north, being about 20 miles opposite Kaikoura Peninsula, 45 miles opposite Cape Palliser, and 100 miles opposite East Cape. The depth of the axis increases from 1,000 fathoms in the south to over 2,000 fathoms in the north.
The north-western wall of the Trench is formed by the Continental Slope, which reaches the floor of the Trench at depths ranging from 1,000 fathoms in the extreme south to 1,800 fathoms in the north. The gradient levels out rather abruptly at the margin of the Trench floor: the latter is virtually flat over a wide area, although the centre is very slightly depressed with respect to the margins. The width of the floor increases northwards, reaching a maximum of about 100 miles opposite East Cape.
The south-eastern wall of the Trench, except in the south near the Chatham Rise, is poorly defined and has very low relief, with the region to the east of the Trench (part of the South-west Pacific Basin) lying only some 250 fathoms higher than the Trench itself. In contrast, the Continental Slope on the other side of the Trench has a total relief of 900–1, 700 fathoms.
As the Trench is followed southwards, it swings to the west and abuts against the Chatham Rise: in this area the southern boundary is formed by the northern slope of the Rise, which has a total relief of about 1,300 fathoms.
At its northern end, the Hikurangi Trench is replaced by the much deeper Kermadec Trench, which is offset 50 miles to the west. These Trenches are two of the major geological structures in the New Zealand region. The flat floor of the Hikurangi Trench is the result of the infilling with sediment derived from the New Zealand land mass.
by Henry Moir Pantin, B.A.(CANTAB.), PH.D.(CANTAB.), New Zealand Oceanographic Institute, Department of Scientific and Industrial Research, Wellington.