In order to investigate the detailed seismic structure of the crust and the uppermost mantle at the off-Hawaii site, the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and the University of Hawai`i conducted a 2-D marine seismic survey (an active-source refraction and reflection survey) in the central Pacific Ocean north of the Hawaiian Islands from August to November 2017 (Fig. 3). Multi-channel seismic (MCS) reflection data were acquired with a 444-channel, 6000 m long streamer cable with a 12.5 m interval between hydrophones, towed at a depth of 12 m. For the acquisition of the wide-angle seismic data, five ocean bottom seismographs (OBSs) were deployed by the R/V Marcus G. Langseth and recovered by the R/V Kilo Moana. Tuned airgun arrays (volume 7800 cubic inches) were fired by the R/V Kairei at intervals of 50 m along the EW, NS2, NS5 lines and at intervals of 50 and 200 m along the NS1 line. The total length of survey lines is 1150 km.

The preliminary results of MCS profiles and P-wave velocity structure using OBSs were reported by Ohira et al. (2018). Their results show typical oceanic crustal structure of oceanic crust from the Hawaiian Arch to the ocean basin. The P-wave velocity structure in the uppermost mantle shows strong azimuthal anisotropy. To image the detailed reflection structure, we applied the prestack migration technique using the initial velocity of the P-wave velocity structure by Ohira et al. (2018) for MCS data. The reflections from the Moho are characterized by images of a sharp, flat, continuous, and large amplitude (Fig. 4). The clear refraction phase from the boundary with an apparent velocity of more than 8 km s-1 is also observed in record sections of the OBSs (Ohira et al., 2018). Moho reflections from about every 2 s in two-way travel time below the basement are locally strong and continuous as expected for “normal” oceanic plate, whereas weak, diffuse, or no Moho reflections were observed in other parts of the seismic profile. The appearance of spatially variable Moho reflections is hereafter called Moho diversity. In order to recognize the spatial crustal characteristics for the drilling proposal, we focus on the 3-D image around the OBSs. Although the OBSs were sparsely arranged, many reflectors from the upper crust to the upper mantle are identified around the crossing point of survey lines (Figs. 5 and 6). It is important to obtain high-resolution velocity information in order to evaluate the Moho reflection for the drilling proposal.

Although seismic observations provide information of the architecture of the in situ oceanic crust, direct geological information of deeper portions of tectonically undisturbed normal oceanic crust can only be obtained by deep-ocean drilling. A long-standing question is the nature of the seismic Layer 2–3 transition. Only Hole 504B penetrated through the Layer 2–3 transition within the sheeted dike complex, which appears to be controlled by alteration mineralogy or a change in porosity (Detrick et al., 1994; Alt et al., 1996). To test whether this Layer 2–3 transition is typical and true for crust spread at faster rates, Hole 1256D was aimed to drill into the 15 Ma crust, created by spreading at 22 cm yr-1. Although the hole ultimately reached the gabbro below the sheeted dikes, seismic data suggest the Layer 2–3 transition has not been reached yet (Teagle et al., 2006). Besides these two, only limited numbers of holes have been drilled more than a few hundred meters into the basement of normal oceanic crust (Fig. 1; Michibayashi et al., 2019).

Oceanic drillings deeper than 50 m into mid-oceanic basement were skewed to young (<15 Ma) or slow-spread crust (Fig. 1), with a wide gap of crust age between 20 and 110 Ma, including the world average age of 62.5 Ma and spreading rate of 7.6 cm yr-1 (Fig. 1). For example, deep basement drillings (Holes 504B and 1256D) were conducted in young crust (< 15 Ma). Among the three candidate sites of M2M, only the off-Hawaii site can provide information about oceanic crust within the gap regarding age and spreading rate. Deep drilling at the off-Hawaii site, penetrating through the upper crust and into the gabbros, will enhance our understanding of the nature of the Layer 2–3 transition and the magmatic accretion and hydrothermal cooling at depth.

The North Arch Volcanic Field covers ∼24000 km2 of ocean floor (Fig. 3). The volume and age of North Arch volcanism is poorly constrained, but they are estimated to be 103 km3 and 0.5–1.5 Ma (Clague et al., 2002). The calculated eruption rates are ∼1 km3 kyr-1 corresponding with ∼1 %–2 % of the Kilauea eruption rate during peak times of eruption. The geochemistry of the lavas is highly alkaline basalts (Dixon and Clague, 2001). Due to the timing of eruptions, eruption rate, and chemistry, the North Arch volcanism is compared with the rejuvenated stage volcanic activity in the axial Hawaiian volcanic chain, which occurs after the shield until the post-shield stage (Garcia et al., 2010). Source mantle chemistry of the North Arch basalts is thought to connect them to the Hawaiian plume (Dixon and Clague, 2001; Garcia et al., 2010; Kimura et al., 2006). Estimated equilibrium melting pressure of the North Arch magmas corresponds with the lithosphere–asthenosphere boundary at 70–80 km (Li et al., 2004). Although the original seismic structure of the uppermost mantle is unaffected by the North Arch volcanism, crustal P-wave velocity (Vp) beneath the North Arch, which is slower than for the undeformed Pacific plate by 0.2–0.3 km s-1, is ascribed to the presence of open cracks in the upper crust by extensional stress on the flexured North Arch lithosphere (Ohira et al., 2018).

Aging of lithosphere is the cause of the diversity of the Moho in the Pacific plate (Ohira et al., 2018). The Moho reflection is weak in magma-starved segments and becomes obscured by deep crustal intrusions and by hydration and deformation of the lithosphere. Correlation of magma geochemistry, alteration, stress conditions, and seismic structures investigated by drilling enables us to understand the relationship between the Moho diversity and the aging of oceanic lithosphere, as well as by comparing these results with data for younger crust at Sites 504 and 1256 and older crust at Site 801.

P-wave velocity in the uppermost mantle obtained from the forward analyses in the seismic lines of Fig. 3 is 8.50–8.65 and 7.9–8.0 km s-1 across and along the paleo-ridge direction, respectively (Fig. 9 of Ohira et al., 2018), which is common for the lithospheric upper mantle produced at fast-spreading ridges. This suggests that the uppermost mantle preserves its original structure formed at the ridge. The seismic cross section shows a reduction of crustal P-wave velocity from the un-deformed Pacific plate of ∼7.0 km s-1 to the deformed North Arch with a Vp  of ∼6.7 km s-1 (Fig. 9 of Ohira et al., 2018). However, shallow faults found in the Layer 1 sediment do not differ much between the two areas. Further, the intra-crustal and Moho reflectors are considerably heterogeneous over these areas, and differences between the two areas are obscure (Ohira et al., 2018).

The North Arch magmatism also potentially modified the original architecture of the Pacific plate. The temperature of an 80 Ma oceanic crust is low (100–200 ∘C; McKenzie et al., 2005). Therefore, the magma chambers that developed in such cold oceanic crust are unlikely to be long-lived tholeiitic shallow systems like Kilauea (Poland et al., 2014), but rather, they are likely deep-seated chambers near the lithosphere–asthenosphere boundary, as estimated by the post-shield alkali basalts (Kimura et al., 2006). Basalt feeder dikes are likely present within the Layer 3 gabbro. Such alkali basalt dikes appear to provide little thermal and chemical overprints of the surrounding crust. All those assumptions can be tested by drilling because of the contrasting compositions of the ∼80 Myr old mid-ocean ridge-type Pacific plate basalts and the ∼0.5–1.5 Myr old alkaline North Arch basalts.

The relatively old oceanic plate results in temperatures of < 150 ∘C even at the Moho depths, which is close to the upper temperature limit of life. Thus, the potential drilling sites are intriguing targets for studying a deep subseafloor biosphere in aged, hydrothermally overprinted igneous rocks of the lower crust.

In contrast to sedimentary rocks, crystalline igneous rocks are generally low in organic carbon and porosity, thus providing extremely low nutrient and energy supplies. However, microorganisms are possibly transported and migrate to fractures and interconnected pores. Compared to sedimentary rocks, the habitability of igneous rocks and their altered equivalents in the lower oceanic crust remain poorly understood and limited to slow-spreading ridges (Mason et al., 2010). Exploration of the deep subseafloor biosphere from the lower crust of intermediate-to-fast-spreading oceanic plates will provide a unique opportunity to investigate the diversity and survival strategies of microbial ecosystems in rocks that make up the majority of the Pacific seafloor.

No primitive basalt was found from the North Arch area, suggesting either primitive basalts are undiscovered beneath sedimentary cover or efficiently differentiated (Dixon and Clague, 2001; Garcia et al., 2010). The presence of cracks in the upper crust on the flexured North Arch lithosphere reduces the bulk density of the upper crust, leading to the presence of apparent LNB in the mid-crust, where magma is trapped, fractionates crystals, and interacts with the wall rocks. Fundamental issues are if there is any primitive basalt hidden beneath differentiated lavas on the surface, how any mid-crustal magma chambers formed associated with the flexure, and to what extent and how the lithosphere is geochemically altered and structurally disturbed by the North Arch volcanism. Assimilation of Layer 2–3 (basalt–gabbro) characteristics by the North Arch basalts can also be explored by geochemical signatures, such as lower O, Sr, and Pb isotopes. Those can also be tested by drilling to recover more primitive North Arch basalt samples.

The enormous size (up to 8.5 km of relief and 80 000 km3) and rapid growth (∼1 to 1.5 Myr) of Hawaiian volcanoes causes them to become gravitationally unstable and collapse (Moore et al., 1989). These collapses have generated some of the largest landslides on Earth, and they have undoubtedly produced colossal tsunami (e.g., Satake et al., 2002). Dozens of giant landslides, some with deposits extending more than 200 km from their source and with volumes >1000 km3, have been recognized along the Hawaiian Ridge (Moore et al., 1994). Considering only the deposits exposed on the ocean floor, the Hawaiian Ridge has major landslides every 32 km along its length. This suggests that a major landslide has occurred about every 350 kyr (Moore et al., 1994). However, drilling at ODP Site 1223 revealed that Ko`olau Volcano, a moderately sized Hawaiian volcano, produced at least four major and three other slides during a period of ∼0.7 Myr, and many more potential landslide deposits appear to be buried at depth (Garcia et al., 2006). Thus, existing data argue that large landslides are a common occurrence (once in ∼100 kyr) and are an important geologic hazard that requires additional investigation to assess their impact on the circum-Pacific regions. Drilling at the proposed site north of the main Hawaiian Islands will allow the frequency, size, and possible failure mechanics of such landslides to be better understood.