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The Kermadec Trench is a linear ocean trench in the south Pacific Ocean. It stretches about from the Louisville Ridge in the north (26°S) to the Hikurangi Plateau in the south (37°S), north-east of New Zealand's North Island. Together with the Tonga Trench to the north, it forms the -long, near-linear Kermadec-Tonga subduction system, which began to evolve in the Eocene when the Pacific plate started to subduct beneath the Australian plate. Convergence rates along this subduction system are among the fastest on Earth, /yr in the north and /yr in the south.
The Kermadec Trench has a southern continuation in the turbidite-filled Hikurangi Trough, but a series of seamounts on the Australian plate act as a dam and prevents this turbidity from reaching the sediment-starved Kermadec Trench. Debris from a larger subducted seamount probably dammed the trench from 2 Ma to 0.5 Ma and similar events probably redirected sediments in similar ways before that.
Two oceanic plates meet at the Kermadec Trench which is located far from any larger landmass. Because of this, the Pacific plate as well as the trench itself is only covered by of sediments. The trench is almost perfectly straight and its simple geometry is the result of the uniformity of the subducting sea-floor. This sea-floor formed at the extinct Osbourn Trough, located just north of the Louisville Ridge. Abyssal hills on the subducting sea-floor are oriented perpendicular to the old spreading centre and the sea-floor is 72–80 Ma near the Louisville seamounts at the northern end and more than 100 Ma near Hikurangi Plateau at the southern end. There are no seamounts on the sea-floor near the Kermadec Trench except one sitting on the trench slope at which has been dated to 54.8±1.9 Ma.
The Hikurangi Plateau formed part of the Ontong-Java-Manihiki Plateau-Hikurangi large igneous province (LIP) during the Ontong Java Event 120 . The Manihiki Plateau is currently subducting under the southern part of the Kermadec Arc but most of it has already been subducted. The LIP-arc collision occurred north of its present location, but oblique plate convergence has migrated the subducted plateau southward.
The second-deepest fish, the Hadal zone snailfish Notoliparis kermadecensis, is endemic to the trench and occupies a very limited depth range, .
A species of pearlfish, Echiodon, has been caught in the Kermadec Trench at a depth of . All other known pearlfishes live in the range and the presence of E. neotes at this depth remains unexplained.
In December 2022, a research team from the Chinese Academy of Sciences and National Institute of Water and Atmospheric Research reached the bottom of the Scholl Deep nearly 10 km below the surface.
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