2018 Volume 124 Issue 10 Pages 805-817
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 25 years, in and around the collision area. The climax of tectonic rotation of both sortheast and southwest Japan during the back-arc opening of the Japan Sea has previously been estimated at ~15 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 17 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 17 and 15 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 17 and 15 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.
