Expedition 350 summary

Abstract

International Ocean Discovery Program (IODP) Hole U1436A (proposed Site IBM-4GT) lies in the western part of the Izu fore-arc basin, ~60 km east of the arc-front volcano Aogashima, ~170 km west of the axis of the Izu-Bonin Trench, and 1.5 km west of Ocean Drilling Program (ODP) Site 792, at 1776 meters below sea level (mbsl). It was drilled as a 150 m deep geotechnical test hole for potential future deep drilling (5500 meters below seafloor [mbsf ]) at proposed Site IBM-4 using the D/V Chikyu. Core from Site U1436 yielded a rich record of Late Pleistocene explosive volcanism, including a distinctive black glassy mafic ash layer that may record a large-volume subaqueous eruption on the Izu arc front. Because of the importance of this discovery, Site U1436 was drilled in three additional holes (U1436B, U1436C, and U1436D), as part of a contingency operation, in an attempt to get better recovery on the black glassy mafic ash layer and its enclosing sediments and to better constrain its thickness.IODP Site U1437 is located in the Izu rear arc, ~330 km west of the axis of the Izu-Bonin Trench and ~90 km west of the arc-front volcanoes Myojinsho and Myojin Knoll, at 2117 mbsl. The primary scientific objective for Site U1437 was to characterize "the missing half of the subduction factory" because numerous ODP/Integrated Ocean Drilling Program sites had been drilled in the arc-front to fore-arc region (i.e., ODP Site 782A Leg 126), but this was the first site to be drilled in the rear-arc region of the Izu arc. A complete view of the arc system is needed to understand the formation of oceanic arc crust and its evolution into continental crust. Site U1437 on the rear arc had excellent core recovery in Holes U1437B and U1437D, and we succeeded in hanging the longest casing ever in the history of R/V JOIDES Resolution scientific drilling (1085.6 m) in Hole U1437E and cored to 1806.5 mbsf.The stratigraphy at Site U1437 was divided into seven lithostratigraphic units (I-VII) that were distinguished from each other based on the proportions and characteristics of tuffaceous mud/mudstone and interbedded tuff, lapilli-tuff, and tuff-breccia. The section is much more mud rich than expected, with ~60% tuffaceous mud for the section as a whole (89% in the uppermost 433 m) and high sedimentation rates of 100-260 m/My for the upper 1320 m (Units I-V). The proportion (40%) and grain size of volcaniclastics are much smaller than expected for an intra-arc basin, composed half of ash/tuff and half of lapilli-tuff of fine grain size (clasts <3 cm). These volcaniclastics were deposited by suspension settling through water and from density currents, in relatively distal settings. Volcanic blocks are only sparsely scattered through the lowermost 25% of the section (Units VI and VII, 1320-1806.5 mbsf ), which includes hyaloclastite, in situ quench-fragmented blocks, and a rhyolite peperite intrusion (i.e., proximal deposits). The transition from unconsolidated to lithified rocks occurred progressively; however, sediments were consider...

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

“…On the basis of paleomagnetic results (Tamura et al 2015a), this normal fault is inferred to have caused a loss of section between the two holes ( Figure 9). Magnetostratigraphy could then be followed down as far as the top of Chron C4An (8.771 Ma) at 1302 mbsf (Unit V, Figure 9).…”

“…This site was chosen to provide a temporal record of rear-arc magma compositions, ideally from Paleogene to Neogene time, allowing comparison with the previously drilled fore-arc magmatic record and determination of across-arc geochemical variations throughout the history of the arc system. In addition to drilling in the rear arc, Expedition 350 also drilled a 150 m deep geotechnical hole in the fore-arc (Site U1436, Figure 1) for potential deep drilling; this site was chosen partly on the basis of results gotten from ODP Site 792, which is only 1.5 to the east of site U1436 (Tamura et al, 2015a). ODP Site 792 was drilled to 886 meters below the sea floor (mbsf,) and the stratigraphy of its upper 150 m (Taylor et al, 1990) is very similar to that of Site U1436.…”

Expedition 350 methods

“…Installation of the casing in Hole U1437E, which represents a record length for the JOIDES Resolution (1085.6 m), is described in Operations in the Site U1437 chapter (Tamura et al, 2015).…”

Data report: Pleistocene planktonic foraminiferal oxygen and carbon stable isotope records and their use to improve the age model of Hole U1436C cores recovered east of the Aogashima Volcano

“…B. Sedimentary description (shown wrapped in A). Orange = all ash beds but the 10 thinnest, green = layers with coring disturbances (see the Expedition 350 summary chapter [Tamura et al, 2015b]). and F4 for an additional visual reference.…”

“…Site U1436 lies 60 km east of Aogashima, an arc-front mafic volcano that forms a small and inhabited island. Figure was slightly modified from the Expedition 350 summary chapter (Tamura et al, 2015b). Figure F2.…”