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 degrees, I=51.8 degrees, alpha(95)=9.6 degrees, and kappa=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 degrees, I=57.7 degrees, alpha(95)=7.1 degrees, and kappa=41.9) that show no such deflection. These differences are interpreted as indicating that the Koshikijima Islands underwent approximately 40 degrees 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 (F(1) faults) without vertical-axis block rotation, (2) strike-slip reactivation of F(1) faults and oblique-normal displacement on NE-SW-trending faults (F(2) faults) with vertical-axis block rotation, and (3) oblique-normal displacement on F(2) 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. (c) 2010 Elsevier B.V. All rights reserved.