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.