摘要：Tectonic and volcanic structures of the northern Ryukyu arc are investigated on the basis of multichannel seismic
(MCS) refection data. The study area forms an active volcanic front in parallel to the non-volcanic island chain in the
eastern margin of the Eurasian plate and has been undergoing regional extension on its back-arc side. We carried out
a MCS refection experiment along two across-arc lines, and one of the profles was laid out across the Tokara Channel,
a linear bathymetric depression which demarcates the northern and central Ryukyu arcs. The refection image reveals
that beneath this topographic valley there exists a~3-km-deep sedimentary basin atop the arc crust, suggesting that
the arc segment boundary was formed by rapid and focused subsidence of the arc crust driven by the arc-parallel
extension. Around the volcanic front, magmatic conduits represented by tubular transparent bodies in the refection
images are well developed within the shallow sediments and some of them are accompanied by small fragments
of dipping seismic refectors indicating intruded sills at their bottoms. The spatial distribution of the conduits may
suggest that the arc volcanism has multiple active outlets on the seafoor which bifurcate at crustal depths and/or
that the location of the volcanic front has been migrating trenchward over time. Further distant from the volcanic
front toward the back-arc (>30 km away), these volcanic features vanish, and alternatively wide rift basins become
predominant where rapid transitions from normal-fault-dominant regions to strike-slip-fault-dominant regions occur.
This spatial variation in faulting patterns indicates complex stress regimes associated with arc/back-arc rifting in the
northern Okinawa Trough.
关键词：Continental rift; Volcanic arc; Back-arc basin; Seismic reflection; Magma intrusion; Fault development
