Differential rotation of the eastern part of southwest Japan inferred from paleomagnetism of Cretaceous and Neogene rocks

Abstract

Paleomagnetic directions have been determined on Late Cretaceous and Miocene rocks in the Nohi area, in the eastern part of southwest Japan, in an effort to clarify within‐block deformation of the rotated part of southwest Japan. Twenty‐two sites in the area are found to have a reliable primary magnetic component through progressive demagnetization tests using thermal and alternating field methods. Tiltcorrected paleomagnetic data in the Nohi area are divided into three stages based on geologic age: Late Cretaceous, early Miocene (23–15 Ma), and middle Miocene (12–10 Ma). The formation‐mean direction of 10 sites obtained from Late Cretaceous Nohi rhyolite is D=17.0°, I=49.8°, and α95=6.1°. Neogene rock units underlain by Nohi rhyolite show two distinct formation‐mean directions: D=13.0°, I=52.7°, and α95=7.2° for eight sites of early Miocene, and D=−2.5°, I=55.6°, and α95=6.6° for four sites of middle Miocene. Differential rotation is proposed between the eastern part of southwest Japan including the Nohi area and the central part of southwest Japan, which is known to have rotated clockwise through about 50° around 15 Ma. The spatial distribution of contemporaneous paleomagnetic data in southwest Japan suggests that two differentially rotated parts are connected in the western neighborhood of the Nohi area. It is concluded that the Nohi area had rapidly rotated counterclockwise through 48° relative to the central part of southwest Japan between 15 and 12 Ma. Together with previous paleomagnetic data from early Miocene rocks in the Kanto area, which is situated to the east of the Nohi area, the present results suggest that differential rotation of the eastern part of southwest Japan is attributed to the formation of northward cuspate shapes of pre‐Neogene terranes which constitute the framework of the Japan arc. One probable explanation of the formation of cusps is a crustal indentation caused by the collision of landmasses on the Philippine Sea plate against the Japan arc in the middle Miocene.