Expedition 358 summary

International Ocean Discovery Program (IODP) Expedition 358 was carried out from October 2018 through March 2019 on the D/V Chikyu in an attempt to reach a plate boundary fault zone at seismogenic depths for the first time in scientific ocean drilling. The goal was to extend Hole C0002P from ~2900 to ~5200 meters below seafloor (mbsf ) and cross the seismically interpreted main décollement fault zone with logging while drilling, downhole stress measurements, cuttings sampling, mud gas sampling, and partial coring by drilling a sidetrack to create a new hole (C0002Q). Although drilling reached 3262.5 mbsf, the deepest to date in all of scientific ocean drilling, the effort to drill to and sample the target—the megathrust fault zone—was not successful. Operational challenges in establishing sidetrack holes and advancing them at reasonable rates of penetration limited the new cased hole interval to less than 60 m total at a depth shallower than the previously established casing depth of 2922 mbsf. Combined, the cuttings, logs, and ~60 cm of recovered core from sidetrack Holes C0002Q–C0002T revealed hemipelagic sediments and fine silty turbidites consistent in lithology and physical properties with those recovered in the same depth interval at the same site during Integrated Ocean Drilling Program Expedition 348. Cuttings revealed evidence of only weakly deformed rock, with relatively common calcite veins but few other structural indicators. Because no downhole leak-off tests were made and very little borehole imaging was performed, no further insight into the tectonic context was acquired. After riser drilling at Site C0002 was terminated, drilling at alternate contingency Sites C0024 and C0025 was carried out. Site C0024 targeted the frontal thrust region to sample and log hanging wall rocks and the shallow portion of the décollement zone, and Site C0025 accessed sediments in the Kumano fore-arc basin. At Site C0024, a dedicated logging hole was drilled and a very complete suite of logs were acquired from 0 to 869 mbsf. Preliminary interpretation of log response and images suggests the frontal thrust zone was encountered from about 813 mbsf to the base of the hole, with a zone of notably low resistivity and steep bedding from 850 mbsf to the bottom of the hole. Core samples revealed lithologic units interpreted to be hemipelagic and turbiditic basin fill, trench fill, and Shikoku Basin sediments and encountered deformation potentially associated with a back thrust imaged in seismic reflection data. However, coring had to be terminated at about 620 mbsf, well short of the frontal thrust zone. Site C0025 recovered fore-arc basin sediments underlain by those interpreted to have been deposited in a trench-slope basin setting; no clear transition into older, inner accretionary wedge material was identified during the preliminary analysis. Coring from 400 to 571 mbsf yielded datable material and possible evidence for diapiric intrusion of sediments. H. Tobin et al. Expedition 358 summary Introduction and background The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) program is a coordinated, multiexpedition drilling project designed to investigate fault mechanics and seismogenesis along the Nankai subduction megathrust (Figure F1) through direct sampling, in situ measurements, and long-term monitoring in conjunction with allied laboratory and numerical modeling studies (Tobin and Kinoshita, 2006; Tobin et al., 2015a, 2019). The fundamental scientific objectives of the NanTroSEIZE project include characterizing the nature of fault slip and strain accumulation, fault and wall rock composition, fault architecture, and state variables throughout the active plate boundary system. Eleven previous expeditions have established a transect of sites from the incoming (subducting) plate through fault zone and accretionary wedge characterization and into the central fore-arc basin (Figure F2). For reviews of many of the principal results from these sites, see Underwood and Moore (2012), Ujiie and Kimura (2014), and Tobin et al. (2015b, 2019). Site C0002 is located in the Kumano fore-arc basin above the seismogenic, partially locked portion of the plate boundary thrust system (Figures F1, F2, F3). The Kumano Basin sedimentary sequence and uppermost part of the accretionary prism were drilled, logged, and sampled during Integrated Ocean Drilling Program Expeditions 314 (logging while drilling [LWD] to 1401.5 meters below seafloor [mbsf ]), 315 (coring to 1057 mbsf), 338 (LWD to 2005 mbsf and coring to 1120 mbsf ), and 348 (LWD to 3058.5 mbsf, with limited coring from 2163 to 2218.5 mbsf ) (Expedition 314 Scientists, 2009; Expedition 315 Scientists, 2009; Strasser et al., 2014; Tobin et al., 2015b). The principal goal of International Ocean Discovery Program (IODP) Expedition 358 was to reach and sample the reflector identified as the likely plate boundary fault at Site C0002 by extending the riser borehole (Hole C0002P) established during previous Integrated Ocean Drilling Program NanTroSEIZE expeditions (Figure F3). This drilling target is also in close proximity to the location of very low frequency (VLF) earthquakes and the first tectonic tremor recorded in any accretionary prism setting (summarized in Obara and Kato, 2016), all of which suggest that fault processes related to the updip limit of megathrust seismogenic mechanics are active here. Recent results from the NanTroSEIZE long-term borehole monitoring systems and Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) seafloor observations further suggest that the plate boundary at this location is partially locked and partially releasing plate convergence strain in these slow slip events accompanied by low-frequency tectonic tremor and VLF earthquakes (Araki et al., 2017). A detailed drilling plan was developed by the Center for Deep Earth Exploration after careful analyses of the Expedition 338 and Figure F1. NanTroSEIZE transect map. Transect Lines a and b refer to composite seismic depth section of Figure F2A and Line ODKM-B of Figure F2B, respectively. Solid dots = Expedition 358 sites, open dots = previous NanTroSEIZE sites. 135°30'E 136°00' 136°30' 137°00' 137°30' 138°00' 32°30' 33°00' 33°30' 34°00' 34°30' N 0 50 km 130° 140° 150° 30° 40° 50° ~4.1–6.5 cm/y C0002 C0025