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International Ocean Discovery Program (IODP) Expedition 399 collected new cores from the Atlantis Massif (30°N; Mid-Atlantic Ridge), an oceanic core complex that hosts the Lost City hydrothermal field (LCHF). Studies of the Atlantis Massif and the LCHF have transformed our understanding of tectonic, magmatic, hydrothermal, and microbial processes at slow-spreading ridges. The Atlantis Massif was the site of four previous expeditions (Integrated Ocean Drilling Program Expeditions 304, 305, and 340T and IODP Expedition 357) and numerous dredging and submersible expeditions. The deepest IODP hole in young (<2 My) oceanic lithosphere, Hole U1309D, was drilled ~5 km north of the LCHF and reached 1415 meters below seafloor (mbsf) through a series of primitive gabbroic rocks. A series of 17 shallow (<16.4 mbsf) holes were also drilled at 9 sites across the south wall of the massif during Expedition 357, recovering heterogeneous rock types including hydrothermally altered peridotites, gabbroic, and basaltic rocks. The hydrologic regime differs between the two locations, with a low-permeability conductive regime in Hole U1309D and a high- (and possibly deep-reaching) permeability regime along the southern wall. Expedition 399 targeted Hole U1309D and the southern wall area to collect new data on ancient processes during deformation and alteration of detachment fault rocks. The recovered rocks and fluids are providing new insights into past and ongoing water-rock interactions, processes of mantle partial melting and gabbro emplacement, deformation over a range of temperatures, abiotic organic synthesis reactions, and the extent and diversity of life in the subseafloor in an actively serpentinizing system. We sampled fluids and measured temperature in Hole U1309D before deepening it to 1498 mbsf. The thermal structure was very similar to that measured during Expedition 340T, and lithologies were comparable to those found previously in Hole U1309D. A significant zone of cataclasis and alteration was found at 1451–1474 mbsf. A new Hole U1601C (proposed Site AMDH-02A) was drilled on the southern ridge close to Expedition 357 Hole M0069A, where both deformed and undeformed serpentinites had previously been recovered. Rapid drilling rates achieved a total depth of 1267.8 mbsf through predominantly ultramafic (68%) and gabbroic (32%) rocks, far surpassing the previous drilling record in a peridotite-dominated system of 201 m. Recovery was excellent overall (71%) but particularly high in peridotite-dominated sections where recovery regularly exceeded 90%. The recovery of sizable sections of largely intact material will provide robust constraints on the architecture and composition of the oceanic mantle lithosphere. The deepest portions of the newly drilled borehole may be beyond the known limits of life, providing the means to assess the role of biological activity across the transition from a biotic to an abiotic regime. Borehole fluids from both holes were collected using both the Kuster Flow-Through Sampler and the new Multi-Temperature Fluid Sampler. Wireline logging in Hole U1601C provided information on downhole density and resistivity, imaged structural features, and documented fracture orientations. A reentry system was installed in Hole U1601C, and both it and Hole U1309D were left open for future deep drilling, fluid sampling, and potential borehole observatories. 

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. 

This chapter documents the methods used for shipboard measurements and analyses during International Ocean Discovery Program (IODP) Expedition 358. We conducted riser drilling from 2887.3 to 3262.5 meters below seafloor (mbsf) at Site C0002 (see Table T1 in the Expedition 358 summary chapter [Tobin et al., 2020a]) as a continuation of riser drilling in Hole C0002F begun during Integrated Ocean Drilling Program Expedition 326 (Expedition 326 Scientists, 2011) and deepened during Integrated Ocean Drilling Program Expeditions 338 and 348 (Strasser et al., 2014b; Tobin et al., 2015b). Please note that the top of Hole C0002Q begins from the top of the window cut into the Hole C0002P casing. Previous Integrated Ocean Drilling Program work at Site C0002 included logging and coring during Integrated Ocean Drilling Program Expeditions 314 (logging while drilling [LWD]), 315 (riserless coring), 332 (LWD and long-term monitoring observatory installation), 338 (riser drilling and riserless coring), and 348 (riser drilling) (Expedition 314 Scientists, 2009; Expedition 315 Scientists, 2009b; Expedition 332 Scientists, 2011; Strasser et al., 2014b; Tobin et al., 2015b). Riserless contingency drilling was also conducted at Site C0024 (LWD and coring) near the deformation front of the Nankai accretionary prism off the Kii Peninsula and at Site C0025 (coring only) in the Kumano fore-arc basin. Riser operations began with connection of the riser to the Hole C0002F wellhead, sidetrack drilling out the cement shoes from 2798 to 2966 mbsf to establish a new hole, and then running a cement bond log to check the integrity of the Hole C0002P casing-formation bonding. A new sidetrack was established parallel to previous Hole C0002P drilling and designated as Hole C0002Q to distinguish it from the overlapping interval in Hole C0002P. Several new kick offs were established (Holes C0002R–C0002T) in attempts to overcome problems drilling to the target depth and then, in the end, to collect core samples. During riser operations, we collected drilling mud, mud gas, cuttings, downhole logs, core samples, and drilling parameters (including mud flow rate, weight on bit [WOB], torque on bit, and downhole pressure, among others). Gas from drilling mud was analyzed in near–real time in a special mud-gas monitoring laboratory (MGML) and was sampled for further postcruise research. Continuous LWD data were transmitted on board and displayed in real time for QC and for initial assessment of borehole environment and formation properties. Recorded-mode LWD data provided higher spatial sampling of downhole parameters and conditions. Cuttings were sampled for standard shipboard analyses and shore-based research. Small-diameter rotary core barrel (SD-RCB; 8½ inch) coring in Hole C0002T provided only minimal core. Riserless coring at Sites C0024 and C0025 with a 10⅝ inch rotary core barrel (RCB) and hydraulic piston coring system (HPCS)/extended punch coring system (EPCS)/extended shoe coring system (ESCS) bottom-hole assembly (BHA) provided most of the core used for standard shipboard and shore-based research. 

During Expedition 357, cores were recovered from three sites in the central area of Atlantis Massif: Sites M0069, M0072, and M0076 (Figure F1; Table T1). Newly acquired multibeam data, combined with preexisting data sets, were evaluated prior to each site to guide the drill teams with regard to anticipated seabed conditions and slope.