The Osbourn Trough, is a -long extinct

mid-ocean ridge A mid-ocean ridge (MOR) is a undersea mountain range, seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading ...

, that may have stopped spreading as recently as 79 million years ago. It is a west-to-east oriented

sea floor The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as seabeds. The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...

feature, located to the east of the present Tonga-Kermadec Ridge where the present Pacific Plate is under going

subduction Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second p ...

under a micro-plate of the

Australian Plate The Australian plate is or was a major tectonic plate in the eastern and, largely, southern hemispheres. Originally a part of the ancient continent of Gondwana, Australia remained connected to India and Antarctica until approximately when Indi ...

. The Osbourn Trough is key to understanding the postulated breakup mechanism of the historic massive

Ontong Java Ontong Java Atoll or Luangiua (formerly Lord Howe Atoll, not to be confused with Lord Howe Island) is an atoll in the Solomon Islands, and one of the largest atolls on earth. It is inhabited by a Polynesian community of about people, who spe ...

-Manihiki-Hikurangi

large igneous province A large igneous province (LIP) is an extremely large accumulation of igneous rocks, including intrusive ( sills, dikes) and extrusive (lava flows, tephra deposits), arising when magma travels through the crust towards the surface. The format ...

(LIP), as it has been shown to be the spreading centre that lead to the separation of the

Manihiki Plateau The Manihiki Plateau is an oceanic plateau in the south-west Pacific Ocean. The Manihiki Plateau was formed by volcanic activity 126 to 116 million years ago during the mid-Cretaceous period at a triple junction plate boundary called the Tongar ...

to its north and the

Hikurangi Plateau The Hikurangi Plateau is an oceanic plateau in the South Pacific Ocean east of the North Island of New Zealand. It is part of a large igneous province (LIP) together with Manihiki and Ontong Java, now located and north of Hikurangi respect ...

to its south close to

New Zealand New Zealand () is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and List of islands of New Zealand, over 600 smaller islands. It is the List of isla ...

Geology

Its basement is known to be oceanic basaltic crust with compositional affinity with a Pacific

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

( MORB ) mantle source. There is only relatively thin sediment overlay to this, often less than thick. The ocean depth ranges between to over the trough which has been characterised as having at least three segments, each of which has an axial valley up to wide bounded by ridges up to high. Its south western aspect intersects and helps separate geographically the very deep

Tonga Trench The Tonga Trench is an oceanic trench located in the southwestern Pacific Ocean. It is the deepest trench in the Southern hemisphere and the second deepest on Earth after the Mariana Trench. The fastest plate-tectonic velocity on Earth is occurri ...

to the north from the

Kermadec Trench The Kermadec Trench is a linear ocean trench in the south Pacific Ocean. It stretches about from the Louisville Seamount Chain in the north (26°S) to the Hikurangi Plateau in the south (37°S), north-east of New Zealand's North Island. Togethe ...

which is also where presently the

Louisville Ridge The Louisville Ridge, often now referred to as the Louisville Seamount Chain, is an underwater chain of over 70 seamounts located in the Southwest portion of the Pacific Ocean. One of the longest seamount chains on Earth, it stretches some Vander ...

at the Osbourn Seamount is subducting under the Kermadec Plate. It is located at about near 26°S about midway between the two large oceanic plateau. The Manihiki Plateau is now to the north and to its south is the Hikurangi Plateau, away, near

. To the east the trough finishes at the Wishbone–East Manihiki scarp. The Osbourn Trough was active during the period of the Cretaceous Normal Superchron ( C34 normal magnetic polarity period, Chron 34n ) which lasted for almost 40 million years. This means that the trough has uniform magnetic polarity unlike most spreading centres which have evidence of

geomagnetic reversal A geomagnetic reversal is a change in the Earth's Dipole magnet, dipole magnetic field such that the positions of magnetic north and magnetic south are interchanged (not to be confused with North Pole, geographic north and South Pole, geograp ...

in parallel to the axis of the spreading ridge. Further the Osbourn Trough is a more prominent feature in

gravity anomaly The gravity anomaly at a location on the Earth's surface is the difference between the observed value of gravity and the value predicted by a theoretical model. If the Earth were an ideal oblate spheroid of uniform density, then the gravity meas ...

studies than in bathymetric data. This uniformity over such a large area of ocean floor lead to its first description as a possible spreading center in 1997 and means spreading stopped by end of the Chron 34n. The

Glomar Challenger The ''Glomar Challenger'' was a deep-sea research and scientific drilling vessel designed for oceanography and marine geology studies. It was used in the Deep Sea Drilling Project for obtaining sediment cores from the ocean floor. The drills ...

had surveyed part of the area by 1987 but the global magnetism data was not available to allow an understanding that a large undersea feature existed.

Tectonics

The precise timing of the Osborne Trough formation has been a matter of some debate as it is important to dating geological events in the South Pacific region, in particular subduction cessation. The first rifting must have occurred prior to 115 million years ago. The spreading took place over about 21 million years and resulted in between to of plate convergence along the Hikurangi Plate and the

Gondwana Gondwana ( ; ) was a large landmass, sometimes referred to as a supercontinent. The remnants of Gondwana make up around two-thirds of today's continental area, including South America, Africa, Antarctica, Australia (continent), Australia, Zea ...

margin to its south. In the current best fit Pacific Plate reference frame tectonics model the Osborne Trough is modelled as a spreading centre between the Manihiki Plate and the Hikurangi Plate which later when spreading ceased became fixed components of today's Pacific Plate. Spreading between the Manihiki and Hikurangi Plateau ceased when Hikurangi LIP collided with the

Chatham Rise The Chatham Rise is an area of ocean floor to the east of New Zealand, forming part of the Zealandia continent. It stretches for some from near the South Island in the west, to the Chatham Islands in the east. It is New Zealand's most productiv ...

east of New Zealand and this event, which must have been no more distant in time than 101 million years ago, has been dated at 86 million years ago, but is now with Pacific Plate reference frame modelling believed to be up to 7 million years more recent.

Geology

Tectonics

See also

External links

References

Sources