The missing half of the subduction factory: shipboard results from the Izu rear arc, IODP Expedition 350

Abstract

IODP Expedition 350 was the first to be drilled in the rear part of the Izu-Bonin, although several sites had been drilled in the arc axis to fore-arc region; the scientific objective was to understand the evolution of the Izu rear arc, by drilling a deepwater volcaniclastic section with a long temporal record (Site U1437). The Izu rear arc is dominated by a series of basaltic to dacitic seamount chains up to ~100 km long roughly perpendicular to the arc front. Dredge samples from these are geochemically distinct from arc front rocks, and drilling was undertaken to understand this arc asymmetry. Site U1437 lies in a ~20 km-wide basin between two rear arc seamount chains, ~90 km west of the arc front, and was drilled to 1804 mbsf with excellent recovery. We expected to drill a volcaniclastic apron, but the section is much more mud-rich than expected (~60%), and the remaining fraction of the section is much finer-grained than predicted from its position within the Izu arc, composed half of ashes/tuffs, and half of lapilli tuffs of fine grain size (clasts <3 cm). Volcanic blocks (>6.4 cm) are only sparsely scattered through the lowermost 25% of the section, and only one igneous unit was encountered, a rhyolite peperite intrusion at ~1390 mbsf. The lowest biostratigaphic datum is at 867 mbsf (~6.5 Ma), the lowest paleomagnetic datum is at ~1300 mbsf (~9 Ma), and the rhyolite peperite at ~1390 mbsf has yielded a U-Pb zircon concordia intercept age of 13.6 +1.6/-1.7 Ma. Both arc front and rear arc sources contributed to the fine-grained (distal) tephras of the upper 1320m, but the coarse-grained (proximal) volcaniclastics in the lowest 25% of the section are geochemically similar to the arc front, suggesting arc asymmetry is not recorded in rocks older than ~13 Ma.

Depositional setting, provenance, and tectonic-volcanic setting of Eocene–Recent deep-sea sediments of the oceanic Izu–Bonin forearc, northwest Pacific (IODP Expedition 352)

“…Our results complement and extend previous exploration, especially dredging and submersible studies of the forearc region (Reagan et al 2010(Reagan et al , 2013 and recent results from the Izu-Bonin arc (Arculus et al 2015c) and backarc (Busby et al 2017;Tamura et al 2015).…”

“…Amami Sankaku Basin; Arculus et al 2015c). Volcanism became active along rear-arc seamount chains and isolated seamounts from c. 17 to 3 Ma (Ishizuka et al 1998(Ishizuka et al , 2003Tamura et al 2014Tamura et al , 2015Busby et al 2017). During this time, the IBM arc moved northwards to its current position.…”

Postmagmatic Tectonic Evolution of the Outer Izu‐Bonin Forearc Revealed by Sediment Basin Structure and Vein Microstructure Analysis: Implications for a 15 Ma Hiatus Between Pacific Plate Subduction Initiation and Forearc Extension

“…In 2014, the International Ocean Discovery Program (IODP) conducted three closely related drilling expeditions using the R/V JOIDES Resolution to explore the Izu-Bonin-Mariana (IBM) arc system (Figure 1), and the magmatic processes related to subduction initiation. Expedition 350 (Sites U1436 and U1437) was the first expedition designed to ascertain "the missing half" of the subduction factory in the IBM rear arc (Busby et al, 2017;Tamura et al, 2015). Expedition 351 (Site U1438) drilled west of the Kyushu-Palau remnant arc ridge with focus on IBM arc origins (Arculus, Ishizuka, Bogus, Gurnis, et al, 2015).…”

Progressive and Punctuated Magnetic Mineral Diagenesis: The Rock Magnetic Record of Multiple Fluid Inputs and Progressive Pyritization in a Volcano‐Bounded Basin, IODP Site U1437, Izu Rear Arc

“…2.1. U1437 Setting, Stratigraphy, Age, and Geothermal Gradient Site U1437 (Figure 1) was drilled during IODP Expedition 350 in a volcano-sedimentary basin within the Izu-Bonin rear arc, about 300 km south of Japan (Busby et al, 2017;Tamura et al, 2015). The water depth at the site is 2,116 m. Coring extended to 439.1 mbsf in Hole U1437B and from 427.2 mbsf to 1,104.6 mbsf in Hole U1437D.…”