Английская Википедия:Daiichi-Kashima Seamount
Шаблон:Short description Шаблон:Infobox seamount Daiichi-Kashima Seamount is a guyot in the Pacific Ocean off Japan. It is about Шаблон:Convert high and reaches a depth of Шаблон:Convert. Daiichi-Kashima formed during the Barremian as a result of volcanic activity; during the Albian reefs formed on the seamount and generated a limestone cap. The seamount later.
The seamount has been approaching the Japan Trench and a noticeable vertical offset of about Шаблон:Convert between the eastern and western halves of Daiichi-Kashima appears to be the result of normal faulting as the seamount enters the trench, with the western half dropping down; it may also reflect a past sector collapse when the volcano was still active.
Geography and geology
Regional
The Daiichi-Kashima seamount lies Шаблон:Convert east of Cape InubōШаблон:Sfn and ChōshiШаблон:Sfn off the eastern coast of Honshu, Japan.Шаблон:Sfn Other seamounts in the area are Katori Seamount northeast of Daiichi-Kashima and Daini-Kashima Seamount east of Katori SeamountШаблон:Sfn and the Kashima Fracture Zone ends southeast of the seamount.[1]
Local
Daiichi-Kashima is a Шаблон:Convert high and Шаблон:Convert wideШаблон:Sfn guyotШаблон:Sfn and rises to a depth of Шаблон:Convert.Шаблон:Sfn On the eastern part of the volcano lies an at least Шаблон:Convert thick platform of clay and reef limestoneШаблон:Sfn with traces of past barrier reefs at its margins.Шаблон:Sfn The summit platform of Daiichi-Kashima covers an area of Шаблон:Convert.Шаблон:Sfn
It is cut by several normal faults that run approximately parallel to the trench and have an offset of about Шаблон:Convert in the central sector of the volcano; the carbonate platform is also offset in such a mannerШаблон:Sfn by a normal fault represented by aШаблон:Sfn scarp into a lower western and a higher eastern part.Шаблон:Sfn This fault, which appears to be split in two or three subsidiary faults separated by grabens,Шаблон:Sfn extends past the Daiichi-Kashima seamountШаблон:Sfn and covers a length of Шаблон:Convert; evidently Daiichi-Kashima has been split in half by the fault, which is much younger than the ocean floorШаблон:Sfn and moved at a rate of Шаблон:ConvertШаблон:Sfn but does not appear to be presently active in light of the sediment cover on the scarp.Шаблон:Sfn Aside from a normal motion, the western half of the seamount has also been moved away from the eastern half and is tilted west.Шаблон:Sfn
The seamount appears to be part of a seamount chain called Joban Seamount ChainШаблон:Sfn or Kashima-Ryofu No.1 that formed during the Cretaceous in the Equatorial PacificШаблон:Sfn and about 30° south of their present-day position.Шаблон:Sfn Based on isotope ratios it was once inferred that Daiichi-Kashima consists of two separate volcanoesШаблон:SfnШаблон:Sfn but a later theory indicates that these are two separate stages of the same volcano.Шаблон:Sfn
Relation to the Japan Trench
Daiichi-Kashima lies south of the Japan Trench on a seafloor of Valanginian age,Шаблон:Sfn very close to the trench.Шаблон:Sfn The Pacific Plate is subducting beneath Japan at a rate of Шаблон:ConvertШаблон:Sfn and close to the Daiichi-Kashima Seamount lies the Boso Triple Junction between the Japan Trench, the Sagami Trench and the Izu–Bonin Trench.Шаблон:Sfn The subduction process may cause the downgoing oceanic plate to buckle and form normal faults that run parallel to the trench.Шаблон:Sfn
Since about 100,000 years, the western half of Daiichi-Kashima is being subducted in the Japan TrenchШаблон:Sfn and about one thirdШаблон:Sfn to one quarter of the seamount has been subducted already.Шаблон:Sfn Part of the landward margin of the trench close to Daiichi-Kashima is uplifted, perhaps as a consequence of the subduction of the seamount,Шаблон:Sfn and there is periodic earthquake activity in front of Daiichi-Kashima seamount with magnitude 7 earthquakes about every 20 years.[2] The seamount might also influence the segmentation of the trench and its earthquakes, considering that the rupture of the 2011 Tohoku earthquake spanned the trench length between Erimo Seamount and Daiichi-Kashima.[3] The other seamounts in the area will likely be subducted after Daiichi-Kashima has been.[4]
Composition
Volcanic rocks from Daiichi-Kashima include basanite, benmoreite and mugearite. There is a distinction between the eastern and western sectors of the volcano, with the western one consisting mainly of mugearite.Шаблон:Sfn Phenocrysts identified in sampled rocks include aegirine–augite, alkali feldspar, amphibole, chromium spinel, clinopyroxene, magnetite, olivine and plagioclase.Шаблон:Sfn
Dredging has found limestones on Daiichi-KashimaШаблон:Sfn which have been subdivided into an upper and a lower formation.Шаблон:Sfn Especially on its western part ferromanganese crusts and phosphorites have been encountered as well.Шаблон:Sfn Other rocks are rudistid-coral floatstones, oolithic grainstonesШаблон:Sfn and peloidal wackestones containing algal pisolitesШаблон:Sfn and other algal remnants. Other fossils include bivalves, corals, echinoids, foraminifers and stromatoporoidea.Шаблон:SfnШаблон:Sfn The rudist Praecaprotina kashimae is named after the seamount.Шаблон:Sfn
Geological history
The volcano is of Barremian age, the limestones are of Aptian to Albian age.Шаблон:Sfn Magnetic traits in the seamount suggest that it formed 140–120 million years ago close to a spreading center, which is older than the age of 100 million years inferred from fossilsШаблон:Sfn but comparable to ages inferred from radiometric dating.Шаблон:Sfn Radiometric dating has yielded ages of 120.4 ± 2.7 million years ago for the eastern and of 117.8 ± 8.4 million years ago for the western side of the seamount.Шаблон:Sfn The seamount is thus considered to be 120–100 million years old, while the underlying crust is about 20 million years older.Шаблон:Sfn At the time of its formation, Daiichi-Kashima was located between 7.6° northern and 1° southern latitude, with one proposed coordinate being Шаблон:Coord.[5]
One proposal envisages that volcanism took place in two separate stages, between which the western flank of the volcano underwent a large-scale collapse. In the first stage, basalts formed a volcanic island that eventually erupted trachytes. In a second stage, the western flank of the volcano collapsed and subsequently chemically different lavas and pyroclastics were emplaced, covering the bulk of the volcano and giving it a paired appearance similar to Reunion.Шаблон:Sfn
During the Albian erosion and subsidence levelled the volcano, forming a flat surface. A carbonate platform developed on this surface first with fringing reefs and then with barrier reefs.Шаблон:Sfn The carbonate platform continued to be active for 10 million years.Шаблон:Sfn A research group of the Tokai University after studying dredged samples proposed that the limestones west and east of the central scarp are of different ages and developed at different sea levels: The western part would be of Barremian age and the eastern one of Albian age. This would explain why they lie at distinct depths.Шаблон:Sfn
After its drowning, Daiichi-Kashima continued to subside until it arrived at the Japan TrenchШаблон:Sfn between 250,000 and 150,000 years ago.[2] The buckling of the ocean crust as it approached the trench induced faulting across Daiichi-Kashima and eventually another collapse of the western flank took place.Шаблон:Sfn
References
Sources
- Шаблон:Cite journal
- Шаблон:Cite journal
- Шаблон:Cite journal
- Шаблон:Cite conference
- Шаблон:Cite journal
- Шаблон:Cite journal
- Шаблон:Cite journal
- Шаблон:Cite journal
- Шаблон:Cite journal
- ↑ Ошибка цитирования Неверный тег
<ref>; для сносокFerrand2018не указан текст - ↑ 2,0 2,1 Ошибка цитирования Неверный тег
<ref>; для сносокKanazawa2008не указан текст - ↑ Ошибка цитирования Неверный тег
<ref>; для сносокCatherine2012не указан текст - ↑ Ошибка цитирования Неверный тег
<ref>; для сносокYamazuki1989не указан текст - ↑ Ошибка цитирования Неверный тег
<ref>; для сносокUchiyama2005не указан текст
