Seismic constraints of the formation process on the back‐arc basin in the southeastern Japan Sea

Abstract

To clarify the formation process of the back-arc basin in the Japan Sea, which is located next to the northwestern Pacific, a seismic survey using ocean bottom seismographs and an air gun array was undertaken in areas from the northern Yamato Basin to the coast of the northeastern Japan Island Arc off Awa-shima Island. The crust beneath the northern Yamato Basin off Awa-shima Island is approximately 16 km thick. The upper and lower crusts are, respectively, about 5 km thick with a steep velocity gradient and about 10 km thick with a gentle velocity gradient. In the basin, there are very few units with P wave velocity of 5.4-6.0 km/s, corresponding to the continental upper crust. The crustal structure of the northern Yamato Basin has characteristics of thicker oceanic crust. The high-velocity lowermost crust in the northern Yamato Basin with 7.2-7.4 km/s might show melt formed by a slightly high mantle temperature during the opening of the basin. However, the crust beneath the areas from the Sado Ridge to the coast, which is approximately 25-26 km thick, is slightly thinner than that of the continental crust and island arc crust. The crustal structure beneath this area is inferred to be a rifted continental and/or a rifted island arc crust.

Petrogenesis and structure of oceanic crust in the Lau back-arc basin

“…Similar high velocity lower crusts have been reported in other back-arc regions, including the Yamato Basin in the Japan Sea (Sato et al, 2014;Hirahara et al, 2015), the Izu-Bonin-Mariana Arc (Kodaira et al, 2007;Takahashi et al, 2008Takahashi et al, , 2009, and the Aleutian arc (e.g., Shillington et al, 2004;Behn and Kelemen, 2006), as well as rifted continental margins (Korenaga et al, 2000). Sato et al (2014) propose that the anomalous back-arc crust in the Japan Sea is the result of higher mantle potential temperature (e.g., Kelemen and Holbrook, 1995;Korenaga et al, 2002).…”

“…However, our MELTS calculations suggest that even moderate amounts of water (>0.5 wt.%) in the parental magma may have an observable effect on the seismic structure. It is possible this process plays a role in other water-rich back-arcs (e.g., Havre Trough, Mariana Trough, Manus Basin), as well as contributes to the crustal structure of the arcs themselves (e.g., Aleutian arc, Izu-Bonin-Mariana arc) where deep high velocity lower crusts have been observed (e.g., Shillington et al, 2004;Behn and Kelemen, 2006;Kodaira et al, 2007;Takahashi et al, 2008Takahashi et al, , 2009.…”

Evolution of the Sea of Japan back-arc and some unsolved issues

“…The Yamato and Tsushima basins show a velocity structure that resembles that of typical ocean crust, but the crust is anomalously thick, ~15-19 km No et al, 2014a;Sato, T. et al, 2014). Outside of the areas identified as typical or anomalous ocean crust, the Japan Sea contains areas of extended continental crust, 22-26 km thick (Kurashimo et al, 1996;Nakahigashi et al, 2012).…”

“…In the Yamato and Tsushima Basins, an anomalously thick ocean crust (15-19 km) developed whose origin is not well-understood (Hirata et al, 1989;No et al, 2014a;Sato et al, 2014;Shinohara et al, 1992). Its thickness may be a primary feature or a result of later intrusive processes (Section 4.5).…”

“…Its thickness may be a primary feature or a result of later intrusive processes (Section 4.5). High P-wave velocity zones are reported from the lower crust in the Yamato Basin suggesting magmatic additions (underplating) although their distribution appears limited (Hirata et al, 1989;No et al, 2014a;Sato T. et al, 2014).…”