Evolution and kinematics of an ancient décollement zone, mélange in the Shimanto accretionary complex of Okinawa Island, Ryukyu Arc

Detailed structural analysis of the mélange in the Shimanto accretionary complex of Okinawa Island in the Ryukyu Arc elucidates the spatial distribution and temporal progression of décollement-related deformation. Early deformation took place in the footwall of the décollement and is characterized by subhorizontal layer-parallel extension and subsequent heterogeneous shear of partially lithified sediments, resulting in alternation of asymmetric (e.g. S–C fabrics) and layer-parallel extensional (boudined layers) fabrics. Mud intrusions suggest that high fluid pressure was locally generated in the footwall of the décollement. Late deformation is marked by the partitioning of deformation in the mélange: the lower structural level of the mélange was pervasively sheared along microfaults whereas the upper structural level was flattened with development of planar pressure solution cleavage. Because shear is concentrated in the lower structural level, late deformation is considered to have occurred in the hanging wall of the décollement. Kinematic indicators suggest that shear directions during décollement-related deformation reflect the relative plate motion during early Tertiary. After the mélange was incorporated into the accretionary prism, crenulation cleavage and cylindrical upright folds were formed under subhorizontal shortening.