AN INTRA-ARC DEFORMATION HISTORY OF THE NORTHERN RYUKYU ARC (JAPAN) DURING BACKARC EXTENSION

We present a deformation history of the northern Ryukyu Arc, Japan on the basis of fault systems and paleomagnetic directions of Cenozoic strata in this area. Cenozoic tectonics of the eastern margin of the Asia is characterized by backarc extension [e.g., Ren et al., 2002]. Many geological and geophysical dataset of backarc region revealed the tectonic history and kinematics of the backarc basins [e.g., Otofuji et al., 1985]. On the other hand, the deformations of intra-arc region during backarc extension are not enough understood. In the Ryukyu Arc, sedimentary rocks since Paleogene are largely distributed and suffered from deformations in multi-stages. These geological structures also likely to be correspond to extensional deformation in the backarc region. Several geological data of the northern Ryukyu Arc show that deformation style in this area changed accompanied with progress of crustal extension. Many fault systems could be observed in the Cenozoic strata of this area. Characteristics of the fault zone and crosscut relationships of them show that there are several systems with different activity ages. In addition, these fault systems involve many minor faults of which displacements can be confirmed at one outcrop. Paleostress analysis using an inversion technique to fault-slip data indicate that at least two normal stresses affected faulting in this area after the Paleogene sedimentation. Some minor faults have different directed slickensides on their fault planes. These suggest reactivation of these minor faults. Moreover, comparison of ages between the paleomagnetic directions and the geologic structures suggest that the vertical-axis block rotation was caused by the reactivation of faults. Thus, we divided the extensional deformation process of the northern Ryukyu Arc since Paleogene into three major stages; (1) extensional deformation with tilting of strata, (2) extensional deformation with vertical-axis block rotation, and (3) extensional deformation with no significant block rotation. These temporal changes of deformation style in this area would correspond to extensional tectonics in the backarc region. In addition, geologic structures and paleomagnetic directions of some other arc region with backarc extension show that the deformation process similar to that of the northern Ryukyu Arc may have been took place in there.