Daiichi-Kashima Seamount is a guyot in the Pacific Ocean off Japan. It is about high and reaches a depth of . 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 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 east of Cape Inubō and Chōshi off the eastern coast of Honshu, Japan. Other seamounts in the area are Katori Seamount northeast of Daiichi-Kashima and Daini-Kashima Seamount east of Katori Seamount and the Kashima Fracture Zone ends southeast of the seamount.
Local
Daiichi-Kashima is a high and wide guyot and rises to a depth of . On the eastern part of the volcano lies an at least thick platform of clay and reef limestone with traces of past barrier reefs at its margins. The summit platform of Daiichi-Kashima covers an area of .
It is cut by several normal faults that run approximately parallel to the trench and have an offset of about in the central sector of the volcano; the carbonate platform is also offset in such a manner by a normal fault represented by a scarp into a lower western and a higher eastern part. This fault, which appears to be split in two or three subsidiary faults separated by grabens, extends past the Daiichi-Kashima seamount and covers a length of ; evidently Daiichi-Kashima has been split in half by the fault, which is much younger than the ocean floor and moved at a rate of but does not appear to be presently active in light of the sediment cover on the scarp. Aside from a normal motion, the western half of the seamount has also been moved away from the eastern half and is tilted west.
The seamount appears to be part of a seamount chain called Joban Seamount Chain or Kashima-Ryofu No.1 that formed during the Cretaceous in the Equatorial Pacific and about 30° south of their present-day position. Based on isotope ratios it was once inferred that Daiichi-Kashima consists of two separate volcanoes but a later theory indicates that these are two separate stages of the same volcano.
Relation to the Japan Trench
Daiichi-Kashima lies south of the Japan Trench on a seafloor of Valanginian age, very close to the trench. The Pacific Plate is subducting beneath Japan at a rate of and close to the Daiichi-Kashima Seamount lies the Boso Triple Junction between the Japan Trench, the Sagami Trench and the Izu–Bonin Trench. The subduction process may cause the downgoing oceanic plate to buckle and form normal faults that run parallel to the trench.
Since about 100,000 years, the western half of Daiichi-Kashima is being subducted in the Japan Trench and about one third to one quarter of the seamount has been subducted already. Part of the landward margin of the trench close to Daiichi-Kashima is uplifted, perhaps as a consequence of the subduction of the seamount, and there is periodic earthquake activity in front of Daiichi-Kashima seamount with magnitude 7 earthquakes about every 20 years. 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. The other seamounts in the area will likely be subducted after Daiichi-Kashima has been.
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. Phenocrysts identified in sampled rocks include aegirine–augite, alkali feldspar, amphibole, chromium spinel, clinopyroxene, magnetite, olivine and plagioclase.
Dredging has found limestones on Daiichi-Kashima which have been subdivided into an upper and a lower formation. Especially on its western part ferromanganese crusts and phosphorites have been encountered as well. Other rocks are rudistid-coral floatstones, oolithic grainstones and peloidal wackestones containing algal pisolites and other algal remnants. Other fossils include bivalves, corals, echinoids, foraminifers and stromatoporoidea. The rudist Praecaprotina kashimae is named after the seamount.