Kanto Syntaxis: when did it begin to grow?

Abstract

The Kanto Syntaxis in central Japan is a cuspate, convex-northward deformation structure of zonal geologic belts and belt-bounding faults, and its formation can likely be attributed to the Neogene collision of the Honshu and Izu-Ogasawara (Bonin)-Mariana (IBM) arcs. However, it remains unclear as to how and when the deformation occurred. To determine the age of onset of syntaxis formation, this review summarizes recent geological and paleomagnetic studies, published during the past years, in and around the collision area. The climax of tectonic rotation of both sortheast and southwest Ja-pan during the back-arc opening of the Japan Sea has previously been estimated at ~ Ma, with this age potentially representing the onset of syntaxis formation. However, sedimentary records in the earliest accreted volcanic edifice indicate that arc-arc collision began at Ma or shortly later. This edifice, named the Kushigatayama (or Koma) block, originally formed on the IBM arc and is now present in the Izu collision zone. Paleomagnetic analyses, which included an orocline test that utilized early Miocene data from both limbs of the Kanto Syntaxis, suggest that it began to grow between and Ma. This is consistent with the age estimate of the onset of arc-arc collision inferred from the geology of the collision zone. Thus, it is concluded that the arc-arc collision, which began between and Ma, caused the coeval onset of the syntaxis formation. Hypotheses relating to the development of the syntaxis are introduced, and these need to be tested in future studies.