Late Cretaceous palaeomagnetic results from Southwest Japan: New insights for early Cenozoic clockwise rotation

Palaeomagnetic samples in the Upper Cretaceous Arima Group have been collected from two areas in the middle of Southwest Japan. Tuffaceous sediments were sampled at eleven sites in the Arima area and welded tuffs were sampled at four sites in the Ikuno area. Thermal demagnetization isolated characteristic remanent magnetization components with unblocking temperatures of 500-580°C from all the sites. After tilt correction, the characteristic directions provide Late Cretaceous mean directions for the Arima area (D = 46.6°, I = 53.7°, α₉₅ = 3.7°) and the Ikuno area (D = 78.4°, I =44.7°, α₉₅ = 9.5°). I interpret these data to show that the palaeomagnetic pole from the Arima area has been rotated counter-clockwise relative to those from the central part of Southwest Japan, while the pole from the Ikuno area is in good agreement with palaeomagnetic poles from the central part of Southwest Japan. The pole for the Ikuno area together with previously reported ones give a new Late Cretaceous pole representative of the central part of Southwest Japan. This pole is rotated 64.6° ± 15.5° clockwise with respect to the coeval one for the North China Block. In my model, the tectonic history of Southwest Japan includes Early Tertiary clockwise rotation of about 23° and Middle Miocene clockwise rotation of about 42°, resulting in a total rotation of Southwest Japan by 65°. The Early Tertiary clockwise rotation occurred in an area incorporating the Korean Peninsula and Southwest Japan, and the Middle Miocene clockwise rotation was associated with the opening of the Japan Sea.