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sedimentary thicknesses in the Sofu Trough reveal different rifting histories for the
eastern and western parts of the basin. The western part has thicker sediments and
structures indicating that rifting is older there than in the eastern part. Evidence for
recent rifting is also seen on both sides of the volcanic front. The rift just behind the
volcanic front may correspond to the activity of the Sumisu rift in the northern
Izu-Ogasawara arc, and the area in front of the volcanic front is the most recent rift.
We suggest that the crust is so thin beneath the central Izu-Ogasawara arc because
it has been repeatedly rifted. The crustal volume of the central arc segment is larger
than that of the northern segment, which has much thicker crust beneath the volcanic
front. This means that the basic scenario of crustal growth is similar for the Izu-
Ogasawara arc, but that extension histories vary significantly along strike.
Acknowledgement
We greatly thank R. J. Stern and M. Ishikawa for providing us with helpful
comments. Their comments significantly improved this paper. We also thank Norio Shimomura
for his support in the data analysis and Go Fujie for seismic tools that he developed. We also
appreciate support during data acquisition by the captains and crews of R/V “Kairei” and R/V
“Kaiyo”, and marine technicians of Nippon Marine Enterprises. This study was funded by the
Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and
Technology, and partially supported by Grant-in-Aid for Creative Scientific Research (19GS0211)
and Grant-in-Aid for Scientific Research (B) (20340122).
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