IODP Expedition 338: NanTroSEIZE Stage 3: NanTroSEIZE plate boundary deep riser 2

Abstract

Abstract. The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) is designed to investigate fault mechanics and seismogenesis along a subduction megathrust, with objectives that include characterizing fault slip, strain accumulation, fault and wall rock composition, fault architecture, and state variables throughout an active plate boundary system. Integrated Ocean Drilling Program (IODP) Expedition 338 was planned to extend and case riser Hole C0002F from 856 to 3600 meters below the seafloor (m b.s.f.). Riser operations extended the hole to 2005.5 m b.s.f., collecting logging-while-drilling (LWD) and measurement-while-drilling, mud gas, and cuttings data. Results reveal two lithologic units within the inner wedge of the accretionary prism that are separated by a prominent fault zone at ∼ 1640 m b.s.f. Due to damage to the riser during unfavorable winds and strong currents, riser operations were suspended, and Hole C0002F left for re-entry during future riser drilling operations.Contingency riserless operations included coring at the forearc basin site (C0002) and at two slope basin sites (C0021 and C0022), and LWD at one input site (C0012) and at three slope basin sites (C0018, C0021 and C0022). Cores and logs from these sites comprehensively characterize the alteration stage of the oceanic basement input to the subduction zone, the early stage of Kumano Basin evolution, gas hydrates in the forearc basin, and recent activity of the shallow megasplay fault zone system and associated submarine landslides.

Hydrocarbon seepage and its sources at mud volcanoes of the Kumano forearc basin, Nankai Trough subduction zone

“…The region is characterized by destructive and tsunamigenic earthquakes and nonvolcanic tremors [Ito and Obara, 2006;Obana and Kodaira, 2009;Shelly et al, 2006]. Although several sites at the Nankai Trough subduction zone have already been explored in the frame of IODP drilling campaigns [see Henry et al, 2012;Kinoshita et al, 2009;Kopf et al, 2011;Moore et al, 2013;Saffer et al, 2010;Saito, 2009], causes for the MV activity in the Kumano forearc basin and the fluid sources have been poorly constraint so far. In the present study, we analyzed hydratebound gases and dissolved methane in shallow deposits of 12 MVs in order to assign fluid formation depths and processes, and potential source rocks, respectively.…”

In situ stress magnitude and rock strength in the Nankai accretionary complex: a novel approach using paired constraints from downhole data in two wells

“…Scientific drilling in the Nankai Trough has included Ocean Drilling Program (ODP) Legs 131, 190, and 196 (Hill et al 1993;Moore et al 2001;Mikada et al 2002) offshore of Shikoku Island, and several IODP Expeditions offshore the Kii Peninsula as part of the NanTroSEIZE experiment 319,322,326,(332)(333)338, and 348) Tobin et al 2009Tobin et al , 2015Saffer et al 2010;Saito et al 2010;Kopf et al 2011;Henry et al 2012;Moore et al 2013). These efforts have provided important information about the plate boundary and outer forearc, including its overall structure, fault zone properties, and hydrogeology.…”

“…1). The boreholes penetrated ~918 m of the Kumano forearc basin fill, composed of Pliocene-to-Pleistocene mudstones and turbidites, and into the uppermost several hundred meters of the underlying accretionary wedge, composed of Miocene silty claystones with lesser amounts of sandstone and sandy siltstone (Ashi et al 2008Moore et al 2013). Holes C0002F and C0002A provide data with overlapping coverage from 872 to 1401 meters below the seafloor (mbsf ).…”

“…We focus on IODP Site C0002, located approximately 30-km landward of the trench along the NanTroSEIZE drilling transect, above the inferred seismogenic portion of the plate boundary ( Fig. 1) (Tobin et al 2015;Moore et al 2013). Operations at Site C0002 included the drilling of multiple holes (Fig.…”

Accumulation of an earthquake-induced extremely turbid layer in a terminal basin of the Nankai accretionary prism

“…This slope basin is a terminal basin that is completely enclosed by topographic highs (Figure 2). IODP Expeditions 333 and 338, as part of the NanTroSLIDE (Nankai Trough Submarine Landslide History) project for investigating landslide processes in an accretionary prism margin Moore et al 2014), drilled two sites 20 km upslope from the terminal basin (sites C0018 and C0021 in Figure 1). However, the targets were ancient mass transport deposits more than 100 m beneath the seafloor.…”