Differential timing of vertical-axis block rotations in the northern Ryukyu Arc: Paleomagnetic evidence from the Koshikijima Islands, Japan

Over 300 samples for paleomagnetic analysis and K–Ar dating were collected from 27 sites at NW–SE and NE–SW trending dike swarms (herein, NW dikes and NE dikes, respectively) in the Koshikijima Islands, northern Ryukyu Arc. The NW dikes are Middle Miocene in age and have directions (D = − 37.7∘, I = 51.8∘, α95 = 9.6∘, and κ = 40.8) that are deflected westward relative to the stable eastern Asian continent. Conversely, the NE dikes, of Late Miocene age, have directions (D = 16.1∘, I = 57.7∘, α95 = 7.1∘, and κ = 41.9) that show no such deflection. These differences are interpreted as indicating that the Koshikijima Islands underwent approximately 40∘ of counter-clockwise rotation during the Middle to Late Miocene. A synthesis of the paleomagnetic and structural data suggests a three-stage history of extensional deformation: (1) displacement upon normal faults (F1 faults) without vertical-axis block rotation, (2) strike-slip reactivation of F1 faults and oblique-normal displacement on NE–SW-trending faults (F2 faults) with vertical-axis block rotation, and (3) oblique-normal displacement on F2 faults without vertical-axis block rotation. Regional differences in the timing and amount of counter-clockwise vertical-axis block rotations indicate that the northern Ryukyu Arc rotated as several distinct rigid blocks.

Research Highlights
► We present new paleomagnetic data from the Koshikijima Islands, northern Ryukyu Arc.
► The study area rotated counter-clockwise about 40° during the Middle to Late Miocene.
► Timing of rotations several regions in the northern Ryukyu Arc are different.
► The northern Ryukyu Arc rotated as several distinct rigid blocks.