HOME

TheInfoList



OR:

The d'Entrecasteaux () Ridge (DER) is a double oceanic ridge in the south-west Pacific Ocean, north of
New Caledonia New Caledonia ( ; ) is a group of islands in the southwest Pacific Ocean, southwest of Vanuatu and east of Australia. Located from Metropolitan France, it forms a Overseas France#Sui generis collectivity, ''sui generis'' collectivity of t ...
and west of Vanuatu Islands. It forms the northern extension of the New Caledonia– Loyalty Islands arc, and is now actively subducting in the Vanuatu subduction zone under the Vanuatu/New Hebrides arc. The subduction of the DER is responsible for the anomalous morphology of the central part of New Hebrides arc whose movement more closely matches the north-east direction of the subducting Australian Plate (the rest of the New Hebrides arc rotate west in front of the southward expanding North Fiji Basin). The name honours French naval officer
Antoine Bruni d'Entrecasteaux Antoine Raymond Joseph de Bruni, chevalier d'Entrecasteaux (; 8 November 1737 – 21 July 1793) was a French Navy officer, explorer and colonial administrator who served as the Governor of Isle de France (Mauritius), governor of Isle de Fran ...
, explorer of the south-west Pacific in the late 18th century.


Geological setting

The DER extends north from the New Caledonia ridge to the New Hebrides/Vanuatu Trench and thus separates the North Loyalty Basin ( ) to the south from the West Santos Basin ( bsl) to the north. The DER has a western and an eastern part with distinct morphologies: the arcuate western part is an elongated
graben In geology, a graben () is a depression (geology), depressed block of the Crust (geology), crust of a planet or moon, bordered by parallel normal faults. Etymology ''Graben'' is a loan word from German language, German, meaning 'ditch' or 't ...
flanked by two near-parallel horsts; the eastern part widens to the east where it has been described to split into a southern and northern ridge. These are separated by the Central d'Entrecasteaux Basin This has a smooth surface covered by sediments, which is deepening eastward to west of Santo Island. The southern ridge is composed of 40 to 36 -old extinct
stratovolcano A stratovolcano, also known as a composite volcano, is a typically conical volcano built up by many alternating layers (strata) of hardened lava and tephra. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile with ...
es. Some have called it therefore the South d'Entrecasteaux Chain. It was considered to be a prolongation of the Loyalty Ridge by many, but this is questioned by reason of a gap between the westernmost South d'Entrecasteaux Chain seamount and the northernmost seamount in the Loyalty Ridge and the divergence in magnetic anomaly azimuths. It is a
seamount A seamount is a large submarine landform that rises from the ocean floor without reaching the water surface (sea level), and thus is not an island, islet, or cliff-rock. Seamounts are typically formed from extinct volcanoes that rise abruptly a ...
chain on-top of a broad east–west-trending ridge. The seamounts, including the Sabine Bank which reaches bsl, are the volcanic remnants of an old island arc. A -deep scarp separates the southern ridge from the North Loyalty Basin to the south. The Bougainville Guyot, south of Santos, forms the eastern continuation of the seamount chain and is a Middle
Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...
andesitic volcano, aged 40 ± 2 Ma, covered by
Oligocene The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...
to Miocene lagoonal limestone. The northern ridge, also known as the North d'Entrecasteaux Ridge, is a more continuous ridge, mostly more similar in composition to the Mariana fore-arc, forms part of an abandoned, north-facing
oceanic trench Oceanic trenches are prominent, long, narrow topography, topographic depression (geology), depressions of the seabed, ocean floor. They are typically wide and below the level of the surrounding oceanic floor, but can be thousands of kilometers ...
which has been subducting under the New Hebrides
island arc Island arcs are long archipelago, chains of active volcanoes with intense earthquake, seismic activity found along convergent boundary, convergent plate tectonics, tectonic plate boundaries. Most island arcs originate on oceanic crust and have re ...
during the last 2–3 Ma. The northern DER reaches bsl. Its eastern part has had its basalts reinterpreted by modern compositional reanalysis as
Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...
basalts that are primitive arc
tholeiite The tholeiitic magma series () is one of two main magma series in subalkaline igneous rocks, the other being the Calc-alkaline magma series, calc-alkaline series. A magma series is a chemically distinct range of magma compositions that describes ...
s between 21 and 38  Ma in age. Accordingly the eastern part of the DER is an abandoned plate boundary which was part of a south-dipping subduction zone before the Miocene. A single dredged sample closest to the New Caledonia Nappe, as yet obtained, where the western part starts a curve that transitions to the eastern part, is of E-MORB back-arc basin-type composition and has an age of 49 ± 10 Ma. This western part of the DER, has an average depth of , and is bounded to the north by a scarp. The western platform has experienced an Eocene compressional phase followed by Middle Miocene to
Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS), as well as the current and most recent of the twelve periods of the ...
uplift and tilt, and indeed the next dredged sample to the north east has a poorly constrained age that is greater than 21 Ma and of primitive arc
tholeiite The tholeiitic magma series () is one of two main magma series in subalkaline igneous rocks, the other being the Calc-alkaline magma series, calc-alkaline series. A magma series is a chemically distinct range of magma compositions that describes ...
composition consistent with subduction related formation.


References


Notes


Sources

* * * * * * {{Coord, 16, S, 165, E, display=title Underwater ridges of the Pacific Ocean