| Norfolk Ridge | |
|---|---|
| Stratigraphic range:85–2 Ma[1] | |
The Norfolk Ridge in relation to other Pacific Ocean seafloor features. TheZealandia continental margin is shown in black. | |
| Lithology | |
| Primary | Mesozoic basement |
| Other | Limestone,mudstone withandesite,trachyandesite,shoshonite lavas |
| Location | |
| Coordinates | 27°00′S167°30′E / 27°S 167.5°E /-27; 167.5[2] |
| Region | South Pacific |
| Extent | 1,500 km (930 mi) |
| Type section | |
| Named for | Norfolk Island |
| Year defined | 2023 |
| Region | South Pacific Ocean |
TheNorfolk Ridge (Norfolk Island Ridge, Norfolk Rise, South New Caledonia Ridge[2]) is a longsubmarine ridge running betweenNew Caledonia andNew Zealand, about 1,300 km (810 mi) off the east-coast ofAustralia.[3]
It is part of a complex region of ridges between the oceanic crust of thePacific plate and the continental crust of Australia. Little has been known about the Norfolk Ridge; however, it generally lies about 1,000–1,200 m (3,300–3,900 ft) belowsea level and consists ofLate Cretaceouscontinental crust.[4] It is part ofZealandia, acontinent that was submerged 60-85 million years ago.[5][6][7]
The Norfolk Ridge is about 1,500 km (930 mi) long and is up to 100 km (62 mi) wide.[4] It is between theNew Caledonia Trough andNorfolk Basin which has central and southern parts. The Norfolk Ridge emerges above the sea surface atGrande Terre ofNew Caledonia,Norfolk Island, and part of the northernNorth Island ofNew Zealand.[8] At about 31°S proceeding towards New Zealand, the ridge widens and splits into the Reinga Ridge and West Norfolk Ridge, which are separated by the Reinga Basin.[9]
The geology has only been well characterised near Grande Terre, Norfolk Island and near Northland but there is now sufficient evidence from oceanographic studies to revise the former view that the tectonics were just those of the two ends of the ridge. The Norfolk Ridge formed as adepocenter along the eastern margin of Zealandia prior to theEocene. This formation is suggested to beMesozoic to earlyCenozoic. The ridge itself came into being from then to theOligocene as part of a spontaneous mode of subduction initiation associated with theKermadec–Tonga subduction zone. This was previously not understood from the geological studies near Grande Terre and Northland which fitted best an induced subduction initiation model.[5][10]
The Norfolk Ridge basement is 20–25 km (12–16 mi) thick with continental crust characteristics.[4] It can be seen as a continuation of the mesozoic basement of Grande Terre (Tremena and Central Chainterranes) through to the Waipapa and Murihiku terranes of New Zealand. There is no evidence of theallochthons that are found in New Caledonia and Northland, except for one limitednappe area on the ridge's eastern flank.[11] The limited rocks dredged include undatedandesitic totrachyandesitic lava, a 26.3 ± 0.1 Mashoshonite, 23 – 16 Ma or probablePliocenelimestone,Cretaceous blackshales, lateEocenemudstone and middle to late Eocene sedimentarybreccias. There are at least seven seamounts located on the western flank of the ridge with three samples from their flanks dated between 33 and 21.5 Ma.[4][a] Quaternary coral reefs and intraplate basalts that are much younger at 2–3 Ma are found on Norfolk Island.[4]
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