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doi:10.14379/iodp.pr.351.2015

Preliminary scientific assessment

Expedition 351 was conceptually straightforward, targeting a single site (U1438) in the ASB, west of the KPR, a remnant arc of the intraoceanic IBM arc. Drilling penetrated a thick sediment section overlying igneous oceanic crust of normal thickness; nevertheless, the water depth (4700 m), sediment thickness (1461 m), and consequent depth to basement were technically challenging. In fact, the fourth longest drill string ever deployed by the JOIDES Resolution in the history of ODP/Integrated Ocean Drilling Program/IODP was deployed during Expedition 351. Despite these challenges, Expedition 351 was remarkably successful. We recovered the entire sedimentary section of the basin and cored 150 m of subjacent igneous oceanic basement.

Expedition 351 has given us an increased understanding of subduction initiation and subsequent arc maturation that has led to critically important new insights into these topics. All of the primary science objectives were achieved, as well as the majority of the secondary objectives. Below, we assess these objectives and highlight major results.

Primary objectives

1. Determine the nature of the original crust and mantle that existed in the region prior to the beginning of subduction in the middle Eocene

An essential boundary condition for understanding the evolution of island arcs is to know the composition, structure, and age of the crust and mantle that existed before subduction began. In the northern IBM case, fortuitously, the preexisting oceanic crust exists under 1–1.5 km of sediment in the ASB adjacent to the KPR remnant arc and perhaps also crops out on the lower fore-arc slope of the Bonin Trench (DeBari et al., 1999; Ishizuka et al., 2011a), making possible access to samples of the pre-arc igneous oceanic basement upon which the arc was constructed.

IODP Site U1438 successfully reached the sediment/basement contact at 1461 mbsf and recovered igneous basement of the ASB. The igneous basement comprises a sequence of basaltic lava flows. Although the basaltic lavas have variable degrees of alteration, they are sufficiently fresh for a variety of petrological, geochemical, and geochronological investigations to determine successfully and comprehensively the nature of the oceanic crust and the critical characteristics of the mantle source(s) from which the basalts were derived.

Decoding the nature of the magma source in the upper mantle that existed immediately before IBM arc initiation is a key to understanding the cause of the initiation of subduction zones and intraoceanic arc formation. Absolute trace element abundances together with ratios of these and various isotopic indicators of the basement rocks will be utilized to quantify models of interaction between ascending IBM arc magmas and the basement.

We have found the basement lava flows are high-MgO (mostly >8 wt%), low-TiO₂ (0.6–1.1 wt%), low-Zr (mostly <50 ppm), high-Sc (mostly >40 ppm) and high-Cr (up to ~400 ppm) Cr spinel + olivine + plagioclase + clinopyroxene + microphyric tholeiitic basalts (Fig. F14). These basalts are compositionally very similar to the ~49 Ma basalts recovered during ODP Leg 195, Site 1201D, in the WPB (Savov et al., 2006) and the ~52 Ma FAB recovered from the present-day Izu-Bonin Trench wall (Reagan et al., 2010; Ishizuka et al., 2011a). Compared with global mid-ocean-ridge basalt compositions (Jenner and O’Neill, 2012), Hole U1438E basalts have high MgO/FeO, with markedly low TiO₂, low Zr, and high Sc abundances; they are relatively primitive melts and likely derived from upper mantle sources that are more strongly depleted in terms of magmaphile trace elements than those typically tapped beneath mid-ocean ridges.

2. Identify and model the process of subduction initiation and initial arc crust formation

Understanding the response of the overriding plate during the initial stages in formation of the new subduction zone is essential for testing first-order competing proposals for subduction initiation. The sediment section overlying the igneous basement of the ASB was expected to preserve evidence for processes associated with subduction initiation. Different responses of the overriding plate during the initial stages of subduction initiation were predicted to result from either forced convergence (uplift) or spontaneous nucleation of the subduction zone.

The character of the earliest juvenile magmatic outputs of the nascent IBM arc recovered from the sedimentary sequence at Site U1438 is of first-order importance for (1) constraining the composition of the mantle wedge at this stage of arc development, (2) determining the nature of subducted inputs, and (3) comparing with the magmatic sequences recovered so far by drilling and dredging in the current fore arc (e.g., Reagan et al., 2010) to be sought during Expedition 352. We have an opportunity to explore possible across-strike variations in the nature of juvenile arc magmatic outputs and to establish whether the sequence recognized for the fore arc from earliest MORB-like FAB through boninite to the temporally more enduring and apparently stable output of island arc tholeiitic types also existed at the western margin of the earliest IBM arc.

The recovery of an extensive sediment sequence of early Eocene age in Unit IV at Site U1438, coeval with the putative initiation of the IBM arc at ~52 Ma (Fig. F13) will allow comprehensive analysis of the provenance, geochemical and petrological characteristics, and style of earliest arc magmatic activity. The geochemical and petrological equivalence of the lithostratigraphic Unit 1 lava flows with the IBM FAB has critical implications regarding the style of magmatism accompanying arc inception and the lateral (across-strike) extent of this type of igneous activity. The apparent absence of boninites at Site U1438, however, may indicate petrologic provinciality was established within the first few million years of the IBM arc’s existence.

3. Determine the compositional evolution during the Paleogene of the IBM arc

The complete tephra record of fore-arc/arc/back-arc volcanism subsequent to arc initiation in the middle Eocene to the isolation of the KPR as a remnant arc accompanying inception of the Shikoku back-arc basin at ~25 Ma (and possibly sporadically thereafter) was obtained at Site U1438. Comprehensive analytical data exist for Neogene ash and pyroclastic materials recovered from the IBM fore arc (e.g., Bryant et al., 1999; Straub, 2003; Straub et al., 2004) and indicate remarkable stability (a function of subducted slab inputs, mantle wedge replenishment, and overriding plate inputs) of the northern part of this system, but the Paleogene record is sparse. In combination with the known Eocene and younger lava/plutonic products (Haraguchi et al., 2003; Ishizuka et al., 2011b), the ash and pyroclast record during this interval will allow us to determine the output variation through time transect across-arc of the northern IBM arc compared to Pacific plate inputs.

The voluminous volcaniclastic materials recovered from the Eocene through Oligocene sequence of Units II, III, and IV, in sediments varying from silt through sand to gravel, contain sufficiently fresh glass (at least in the shallower section) and igneous minerals to allow comprehensive postexpedition petrological and geochemical studies to satisfy this third primary objective. Noteworthy features of the mineral assemblage are the ubiquitous occurrence of amphibole. This phase is rare in the tephra recovered from previously drilled sites in the IBM fore arc and may indicate significant across-strike compositional variability of magma composition was established early in the Paleogene.

4. Establish geophysical properties of the ASB

The basement of the IBM arc comprises sedimentary and underlying igneous rock types accessible for direct geophysical measurements through study of recovered cores and downhole logging at Site U1438. With heat flow measurements at Site U1438, we have been able to establish the thermal age of the lithosphere (Fig. F15). The geothermal gradient is 77.6°C/km, and the heat flux is 73.7 mW/m², implying a thermal age of 40–60 Ma (Sclater et al., 1980). This result will become even more important once the geochronological age of lithostratigraphic Unit 1 is established. The thermal age is important to ascertain the initial mechanical properties at the time of subduction initiation and to better inform estimates of initial water depth and subsequent bathymetric evolution. The thermal age is young compared to previous suggestions and implies a near zero age and extremely low plate strength during the initiation of subduction. Existing plate tectonic models position the Pacific plate (with an age of ~50 Ma at that time) on the other side of the nascent KPR during subduction initiation. With a thermal age of ~50 Ma today for the ASB, a substantial age difference would have existed across the new subduction boundary.

A preliminary estimate of the sediment load–corrected depth of the basement at Site U1438 gives a depth of ~5527 mbsf; with an expected depth of 5000 mbsf for a 50 Ma age plate (Sclater et al., 1980), a residual depth of ~550 m is calculated. This is consistent with other estimates for the PSP, which have excess depths of 400–600 m (Flament et al., 2013). These depths are consistent with predictions of dynamic topography from mantle flow that have such depths for the PSP. The excess depth for the region provides an explanation for the lack of calcareous fossils through much of the core and why the region was below the CCD. If the basement has an age of about 50 Ma, then we would have expected intervals of time from about 40–45 Ma when our sediment column would have been above the CCD (Palike et al., 2012). Dynamic subsidence since the time of subduction initiation (Gurnis, 1992) would provide a natural explanation for the sub-CCD depths.

Basement logging was not achieved at Site U1438, but 1200 m of logs were obtained, recovering a maximum age equivalent to ~40 Ma. The best coverage is from 100 to 700 mbsf (~35 Ma). The logs (gamma radiation, velocity, density, porosity, resistivity, and magnetic susceptibility) were correlated with lithostratigraphic units. The FMS images display bedding and other features that may help characterize the large-scale tectonic development of the ASB. Additionally, orientation of these structures might help characterize the source of the mass wasting deposits described in some of the cores. Sonic data (vertical seismic images and P-wave velocity) and density data from logs and cores give a time-depth relationship for the site and thus provide characterization of seismic boundaries. We will thus be able to make accurate tie-points between core/log data and seismic data, which will help to understand the general nature of the ASB in particular and the velocity structure of the IBM system more generally.

There has been a long-standing debate concerning the initial geographic orientation and location of the IBM system during subduction initiation (argued to be ~90°) and its subsequent postulated 90° clockwise rotation to a present-day north–south strike during 50 My (Hall et al., 1995; Yamazaki et al., 2010, and references therein). The initial orientation of the IBM system and any subsequent rotations are important with respect to establishing the boundary conditions for subduction initiation and later plate interactions. Archive-half core remanent inclination data produced during this expedition are insufficient to allow accurate determination of changes in paleolatitude of the PSP (as suggested by previous workers; e.g., Yamazaki et al., 2010), as isolation of characteristic remanences in many intervals requires demagnetization to higher levels than can be achieved using shipboard systems. However, analyses of additional discrete sample demagnetization postexpedition will allow a robust determination of plate latitudinal motion. Discrete samples obtained from APC cores will provide additional constraints on plate rotation, as these cores were oriented using the FlexIT tool. Finally, postcruise integration of remanence data and analysis of wireline FMS logs may potentially allow magnetic declinations to be recovered in some deeper intervals, allowing plate rotation to be documented beyond 25 Ma.

Secondary objectives

1. Recover sedimentary records of paleoceanographic conditions from Pliocene–Pleistocene to early Tertiary and possibly Late Cretaceous in the eastern Tethys–western Pacific

The paleographic position of the ASB adjacent to Cretaceous-age arcs and back-arc basins had the potential to provide an excellent opportunity to recover records of paleoceanographic conditions in the far eastern Tethys–western Pacific in the Late Cretaceous (and possibly older) through the Tertiary. Among the opportunities were the transection of the Cretaceous/Tertiary boundary, the organic-rich shales of ocean anoxic events, the Paleocene/Eocene Thermal Maximum, and Eocene hyperthermals. However, the oldest sediments recovered at Site U1438, immediately overlying the igneous basement, are ~50–53 Ma. This result has critical significance in terms of achieving the primary objectives of Expedition 351 (see above), but means that none of the objectives related to the recovery of Cretaceous or older Paleogene sedimentary rocks could be realized. Despite not recovering sedimentary records of climate intervals/events, the quality of the cores obtained indicates an excellent Neogene paleoceanographic sedimentary record. In particular, the Oligocene–Miocene transition and sediments deposited during the mid-Miocene climatic optimum were recovered. The Eocene/Oligocene boundary and a high-resolution record of Pliocene–Pleistocene environmental changes will also provide exciting opportunities for paleoclimate studies.

2. Recover sedimentary records of onset and persistence of the East Asian Monsoon and other climate-modulated land-sea correlations

Integrated Ocean Drilling Program Expedition 346 conducted two latitudinal transects to test hypotheses concerning tectonic linkages with the onset of the Asian Monsoon. Site U1438 is located southeast of these sites and may be complementary to them. Although our site is distal from the point of Yangtze River discharge, evidence of fluvial input is possible. More likely, eolian inputs to the ASB, such as loess from East Asia, dominate the terrestrial signal. High-quality cores recovered from Site U1438 span the Miocene to recent, include a large clay fraction, and should provide a complete record of East Asian monsoon onset and intensity changes over a long timescale. This will provide important information regarding timing and geographic distribution of terrestrial material in the western Pacific marine record.

3. Recover an ash record of the evolution of the Ryukyu-Kyushu arc

Subduction of the PSP, on which the ASB now is situated, takes place in part along the Ryukyu-Kyushu arc to the northwest of Site U1438. We anticipated an ash record from this arc to be preserved within the sediments. Some particularly significant explosive events from the Ryukyu-Kyushu arc produced extensive ash deposits that are used as critical stratigraphic marker horizons in the Japanese islands. In addition to this feature, the temporal evolution of the Ryukyu-Kyushu arc located on the east Asian continental margin will provide important complementary evidence of arc evolution with the IBM system.

Numerous layers of volcanic ash were recovered in the recent to Miocene sediments of the uppermost Unit I at Site U1438, comprising fresh glass shards and igneous minerals; layers range in thickness from 1 to 13 cm and in grain size up to lapilli. The pyroclastic components are dominantly colorless vitric pumice and shards with traces of brown vitric fragments. Phenocrysts and isolated volcanogenic crystals include pyroxene, opaque minerals, plagioclase, minor biotite, and quartz. Some ash fractions are notably enriched in lithic fragments and crystals. Comparison with glass and mineral compositions with published data for the Ryukyu-Kyushu, Honshu, and Neogene IBM arcs will permit identification of the sources of volcanic ash at Site U1438. An opportunity exists with good age constraints to provide new physical volcanogenic data for modeling volumes, explosivity, and geochemical properties of the explosive output of the respective arcs surrounding the ASB.

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