Seismic record of tectonic evolution and backarc rifting in the southern Ryukyu island arc system

Abstract

The southern Ryukyu island arc system is located at a convergent plate margin where the Philippine Sea Plate is subducting under the Eurasia Plate. We have conducted multi-channel seismic reflection surveys to study tectonic evolution and backarc rifting of the southern Ryukyu island arc system using the R/V Tansei-maru of the Ocean Research Institute, University of Tokyo, in June 1993 and 1994. We describe systematically a complete cross-section from the East China Sea continental shelf to the Ryukyu trench from the viewpoint of seismic stratigraphy. Seven major seismic units and three stages in the tectonic evolution of the system are identified: (1) during stage 1 from Late Miocene to earliest Pleistocene, pre-rift deposits of the Shimajiri Group accumulated over a wide region from the East China Sea continental shelf to the forearc region; (2) stage 2 is defined by a series of tectonic processes involving crustal doming, erosion, subsidence, and sedimentation, in association with initial rifting of the southern Okinawa Trough during most of Early Pleistocene time; and (3) the backarc rifting is still in progress and syn-rift sedimentation has been under way since the Late Pleistocene (stage 3). A new significant observation lies in the fact that the Pliocene Shimajiri Group is not distributed in most of the present-day southern Okinawa Trough. This implies that the backarc rifting of the southern Okinawa Trough was probably initiated after the deposition of the Shimajiri Group, that is, during the Early Pleistocene. Crustal-scale simple shear allowing an asymmetrical half-graben structure, rather than pure shear, governed the initial rifting of the southern Okinawa Trough. The possibility, however, cannot be excluded that pure shear associated with symmetrical rifting may be predominant in the present-day rifting of the southern Okinawa Trough as a result of northwestward migration of the rifting axis. The average extension rate of the southern Okinawa Trough is estimated to be approximately 1–2 cm/year on the basis of fault geometry observed on the acoustic basement.