The first Shinkai dive study of the southwestern Mariana arc system
Abstract:The 3000 km long Izu-Bonin-Mariana (IBM) arc system is an outstanding example of an intraoceanic convergent plate margin. The IBM forearc is a typical nonaccretionary convergent plate margin; the inner trench slope exposes lithologies found in many ophiolites. To more clearly delineate the geology of the forearc, we have been investigating a ~500 km long region of the Mariana forearc south of ~13°N using the DSV Shinkai 6500 and deep-tow camera since 2006. Discoveries includes the presence of MORB-like basalts that formed during subduction initiation (~51 Ma) [Reagan et al., 2010, G3], a region of forearc rifting unusually close to the trench axis, the Southeast Mariana Forearc Rift [Ribeiro et al., 2013, G3], and a serpentinite-hosted ecosystem near the Challenger Deep, the Shinkai Seep Field [Ohara et al., 2012, PNAS].
However, there have been no studies on the southern Mariana area west of the Challenger Deep except one [Hawkins and Batiza, 1977, EPSL], hindering our understanding of the IBM system. To advance our biogeoscientific understanding of this region, a Shinkai 6500 diving cruise (YK14-13) was conducted in July 2014 on two major sites: the inner trench slope west of the Challenger Deep (Site A), and the southwesternmost tip of the Mariana Trough (Site B). Dives at Site A recovered very fresh mantle peridotite associated with troctolite and limestone. The limestone preserves the remnants of corals, clearly indicating that the limestone is an accreted material originating from the incoming (colliding) Caroline Ridge. The freshness of the peridotites also indicates that the collision is an ongoing event, resulting in a protruding peridotite ridge along the inner trench slope west of the Challenger Deep. Dives at Site B recovered basalt and gabbro, which is either new backarc basin crust or rifted West Mariana Ridge crust. This cruise allowed for continued sampling of the inner trench slope of the Mariana Trench, from south of Guam to the Yap Trench junction, a distance of ~800 km. The future synthesis of the cruise results will bring us the first comprehensive biogeoscientific view of the southern Mariana area.
YK14-13 science party included Y. Ohara, F. Martinez, M. Brounce, I. Pujana, T. Ishii, H. Watanabe, T. Okumura, S. Oya, T. Mizuno, U. Konno, Y. Onishi, Y. Miyajima, and S. Minamizawa.