Scientific objectives

The primary drilling plan for Expedition 338 was to extend Hole C0002F from 860 (20 inch casing set point) to 3600 mbsf (13⅜ inch casing set point) through riser drilling. However, riser operations during Expedition 338 were suspended because of riser damage during unfavorable winds and strong current conditions. Contingency operations were then conducted for the remainder of Expedition 338.

Site C0002 objectives

At Site C0002, LWD data already exist from 0 to 1400 mbsf from Expeditions 314 and 332 (Expedition 314 Scientists, 2009a; Expedition 332 Scientists, 2011b) and core data exist for 0–204 and 475–1057 mbsf from Expedition 315 (Expedition 315 Scientists, 2009). The Kumano Basin sedimentary package composes the interval from 0 to ~940 mbsf, which is underlain by the inner accretionary wedge. The seismic reflection character of the entire zone from ~940 mbsf to the megasplay reflection at ~5000 mbsf exhibits virtually no coherent reflections that would indicate intact stratal packages, which is in contrast to the outer accretionary wedge seaward of the megasplay fault system (Fig. F2) (also see Moore et al., 2009).

The main research objectives for combined primary riser operations and contingency riserless operations at Site C0002 were to sample the upper part of the forearc basin sediment and gas hydrate zone, the basal Kumano Basin-to-accretionary prism unconformity, and the upper portion of the inner wedge with cores, drill cuttings, mud-gas sampling, and an extensive suite of LWD logs. Sampling these intervals, which are either previously unsampled or undersampled, allows the (1) determination of composition, age, stratigraphy, and internal style of deformation of the Pliocene–Recent Kumano forearc basin and underlying Miocene accretionary complex; (2) characterization of the gas hydrate zone in the forearc basin; (3) reconstruction of thermal, diagenetic, and metamorphic history and comparison with present pressure and temperature conditions; (4) determination of minimal horizontal stress within the inner wedge; (5) investigation of the mechanical state and behavior of the formation; (6) characterization of the overall structural evolution of the Nankai accretionary prism; and (7) characterization of the current state of the upper plate above seismogenic plate boundary thrust.

Site C0012 objectives

The primary objectives at IODP Site C0012 were to characterize the sedimentary section and the upper portion of the oceanic crust with a full suite of LWD logs. Site C0012 is located in the Shikoku Basin on the crest of a prominent basement high (Kashinosaki Knoll; Ike et al., 2008) on the subducting Philippine Sea plate (Figs. F1, F2). This location provides access to the uppermost igneous crust with modest penetration below the seafloor. Previously, Expedition 322 collected core samples to 576 mbsf, which included sampling sediment and basement (Expedition 322 Scientists, 2010b) and Expedition 333 sampled the sediment and igneous basement to 630.5 mbsf (Expedition 333 Scientists, 2012b). As part of contingency operations, Expedition 338 collected LWD logs to 709 mbsf.

LWD operations at Site C0012 were performed to provide key data to understand (1) how compressional velocity relates to compaction state and fluid sources, (2) how chemical profiles relate to physical and chemical properties, (3) how the structures of the sedimentary section relates to slumping, and (4) how structures in igneous basement relate to the alteration state. Ultimately, these analyses will help define the fluid and chemical budgets of subduction inputs, which are important toward the understanding of fluids in the accretionary prism and subducted materials.

Site C0018 and C0021 objectives

A slope basin seaward of the megasplay fault was drilled and sampled during Expedition 333 (IODP Hole C0018A) (Figs. F2, F4) targeting mass transport deposits (MTDs) to understand how submarine landslides relate to tectonic activity and evolution of the slope basin and shallow megasplay fault system and to evaluate deformation and transport mechanisms of MTDs (Expedition 333 Scientists, 2012c). Site C0018 is situated within a depocenter for downslope mass transport, and the sedimentary succession is dominated by stacked MTDs that are seismically imaged as acoustically transparent-to-chaotic bodies with ponded geometries (Fig. F5) (Strasser et al., 2011). Hole C0018A was drilled at a location where the MTD bodies wedge out and basal erosion is minimal. Coring to ~314.15 mbsf in Hole C0018A sampled six MTDs, which record more than 1 million years of submarine landslide history (Strasser et al., 2012).

The primary goals of Expedition 338 operations at IODP Sites C0018 and C0021 were (1) to characterize the sedimentary section and MTDs at Site C0018 with LWD logs and (2) to add LWD and coring at Site C0021, which is located ~2 km northwest of Site C0018 at a more proximal site for MTDs observed at Site C0018 (Figs. F4, F5). Logging and coring at Site C0021 were designed to provide data for correlation to Site C0018. Together, the sites provide constraints on the lateral variability of MTDs within the basin. This variability relates to the nature, provenance, and kinematics of the landslides. Logging at Sites C0018 and C0021 enables us to correlate and integrate these data with core and seismic data, and hence, to understand the comprehensive nature of MTDs and their bearing on sliding dynamics and tsunamigenic potential.

Site C0022 objectives

LWD and coring were carried out at IODP Site C0022, located in the slope basin between previously drilled IODP Sites C0004 and C0008 (Figs. F1, F4) (Expedition 314 Scientists, 2009b; Expedition 316 Scientists, 2009a, 2009b) to target the uppermost 400 mbsf near the projected fault tip of the megasplay fault. The seismic reflection data had previously identified this region as the tip of the megasplay fault that emplaced the block drilled at Site C0004 over slope basin strata (Fig. F6) (Moore et al., 2009). This megasplay fault is thought to coincide with the rupture area of the 1944 Tonankai earthquake, and its slip was likely responsible for the associated devastating tsunami (Park et al., 2002; Moore et al., 2007).

Reconstruction of splay fault activity through time, however, indicates that the surface layers younger than ~1.24 Ma are not displaced by the megasplay fault (Fig. F6) (Strasser et al., 2009), which implies that this fault has been inactive recently. Alternatively, further work by Kimura et al. (2011) defined the lateral extent and characteristics of this fault zone and predicted that its tip should extend into the slope basin strata. According to the authors, ongoing splay fault activity may not only be inferred by stratal ages and architecture across the fault itself, but also by broader and distributed deformation, apparent in (1) deformation of the lower slope basin, (2) erosion and redeposition of slope sediment by successive oversteepening and mass transport, and (3) accumulation of MTDs overlying the fault. Thus, Site C0022 was cored and logged to test these ideas.