Structural characteristics of the northern Okinawa Trough and adjacent areas from regional seismic reflection data: Geologic and tectonic implications

Abstract

Analysis of regional multi-channel seismic data from the northern Okinawa Trough and adjacent shelf provides some important constraints on the structural development of the area. The sedimentary strata in the northern East China Sea shelf basin, separated from the Okinawa Trough by the Taiwan–Sinzi belt, are affected by the Miocene compressional tectonism and truncated by the Late Miocene unconformity. In contrast, those in the western margin of the northern Okinawa Trough are cut by numerous normal faults and the Late Miocene horizon forms a conformable surface. This suggests that the Taiwan–Sinzi belt acted as a buttress for the northern Okinawa Trough against the compressional tectonism. Our data also reveal the Ho Basin in the western margin of the northern Okinawa Trough, previously known only from proprietary industry data. The Longwan Ridge, lying between the Ho Basin and the northern Okinawa Trough, may be the youngest of the relict arcs or buried ridges in the area. The abrupt along-strike change in the fault polarity near the northern margin of the depocenter of the Ho Basin suggests a NW-trending left-lateral transfer zone or fault. The western boundary of the northern Okinawa Trough is characterized by the gently deepening seafloor, whereas that of the southern Okinawa Trough is marked by steep border faults, suggesting more rapid subsidence in the south. This, together with the well-developed symmetric axial faults in the south and the greater width and the lack of well-developed axial zone in the north, may suggest focused rifting/extension in the southern Okinawa Trough and diffuse rifting/extension in the northern Okinawa Trough. We postulate that the diffuse rifting/extension in the northern Okinawa Trough is due to the tectonic perturbation, caused by the convergence or subduction of the topographically high and buoyant Amami Plateau at the northern Ryukyu arc.