Abstract
Tectonic and volcanic structures of the northern Ryukyu arc are investigated on the basis of multichannel seismic (MCS) reflection 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 reflection experiment along two across-arc lines, and one of the profiles was laid out across the Tokara Channel, a linear bathymetric depression which demarcates the northern and central Ryukyu arcs. The reflection 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 reflection images are well developed within the shallow sediments and some of them are accompanied by small fragments of dipping seismic reflectors indicating intruded sills at their bottoms. The spatial distribution of the conduits may suggest that the arc volcanism has multiple active outlets on the seafloor 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.
Highlights
The northwestern Pacific region is bounded by several island arcs and provides an appropriate field for studying evolution of continental lithospheres associated with oceanic plate subduction, arc volcanism and back-arc spreading/rifting
Since we find a variety of structural features reflecting complex tectonic and magmatic events, we divide the study area into three domains and describe them separately namely the Tokara Channel (“Tokara Channel” and “Arc segmentation” sections, and Fig. 4), the volcanic front (“Volcanic front” and “Active magmatic intrusions around the volcanic front” sections, and Figs. 5 and 6) and the back-arc basins (“Back-arc basins” and “Fault development and rifting style in the northern Okinawa Trough” sections, and Figs. 7 and 8)
Tokara Channel One of the most important findings from our reflection data is the deep sedimentary basin beneath the Tokara Channel (Fig. 4): The acoustic basement marked by highamplitude reflections forms a ~ 30-km-wide bowl-shaped basin with the maximum depth of ~ 2 s two-way travel time (~ 3 km deep below the seafloor) relative to the adjacent arc segment
Summary
The northwestern Pacific region is bounded by several island arcs and provides an appropriate field for studying evolution of continental lithospheres associated with oceanic plate subduction, arc volcanism and back-arc spreading/rifting. Bathymetric data show that these tectonic boundaries form wide acrossarc valleys which are roughly 1 km deeper than the adjacent arc segments (Fig. 1) They are thought to be a key to characterize the architecture and the subsidence history of the overall Ryukyu arc, structure and tectonic controls on the arc deformation there remains unclarified yet. Another subject of debate is that active volcanoes are unevenly distributed along this arc, and the highest density occurs in the northern part: The northern Ryukyu volcanic arc, including the Kikai caldera (which has caused one of the deadliest volcanic disasters in Japanese history) and the Suwanose Island (which caused multiple eruptions in 2015), is one of the most active volcanic areas in Japan (Machida and Arai 1978; Yokose et al 2010) (Fig. 1b). A full understanding of these magmatic systems requires large-scale close-in observations of subsurface structures
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