IODP Expedition 322 Drills Two Sites to Document Inputs to The Nankai Trough Subduction Zone

Abstract

The primary goals during Expedition 322 of the Integrated Ocean Drilling Program were to sample and log the incoming sedimentary strata and uppermost igneous basement of the Shikoku Basin, seaward of the Nankai Trough (southwestern Japan). Characterization of these subduction inputs is one piece of the overall science plan for the Nankai Trough Seismogenic Zone Experiment. Before we can assess how various material properties evolve down the dip of the plate interface, and potentially change the fault's behavior from stable sliding to seismogenic slip, we must determine the initial pre-subduction conditions. Two sites were drilled seaward of the trench to demonstrate how facies character and sedimentation rates responded to bathymetric architecture. Site C0011 is located on the northwest flank of a prominent basement high (Kashinosaki Knoll), and Site C0012 is located near the crest of the seamount. Even though significant gaps remain in the coring record, and attempts to recover wireline logs at Site C0012 failed, correlations can be made between stratigraphic units at the two sites. Sedimentation rates slowed down throughout the condensed section above the basement high, but the seafloor relief was never high enough during the basin's evolution to prevent the accumulation of sandy turbidites near the crest of the seamount. We discovered a new stratigraphic unit, the middle Shikoku Basin facies, which is typified by late Miocene volcaniclastic turbidites. The sediment-basalt contact was recovered intact at Site C0012, giving a minimum basement age of 18.9 Ma. Samples of interstitial water show a familiar freshening trend with depth at Site C0011, but chlorinity values at Site C0012 increase above the values for seawater toward the basement contact. The geochemical trends at Site C0012 are probably a response to hydration reactions in the volcaniclastic sediment and diffusional Science Reports

Evolution of (Bio‐)Geochemical Processes and Diagenetic Alteration of Sediments Along the Tectonic Migration of Ocean Floor in the Shikoku Basin off Japan

“…Chloride concentrations at Site C0012 significantly increase above a typical seawater value of 558 mM below 300 mbsf until the sediment‐basement interface. Thus, the observed increase in sulfate concentrations with depth at Site C0012 is likely not related to a relict sulfate pool but attributed to a diffusional sulfate supply from the basement aquifer (Torres et al., 2015; Underwood et al., 2010). This comparison demonstrates that the sedimentary packages in the Nankai‐Shikoku‐subduction system are exposed to different geochemical controls even on comparably small‐scale spatial distances.…”

Paleothermal structure of the Nankai inner accretionary wedge estimated from vitrinite reflectance of cuttings

“…The resistivity images obtained during logging-while-drilling suggest that the dip angles of Holes C0002N and C0002P are 60-908 [Tobin et al, 2015;Boston et al, 2016], while sediments, before evolving into the inner accretionary wedge, possess subhorizontal bedding [Kinoshita et al, 2009]. The site C0002 sediments below 1700 mbsf (approximately 11-6 Ma) have porosities of approximately 0.20-0.35 [Tobin et al, 2015], while sediments of the same age at Sites C0011 and C0012 (off-Kumano input sites) have porosities of approximately 0.35-0.50 Underwood et al, 2010]. Assuming that the formations were heated when they were subhorizontal, and then subsequently inclined to approximately 608, with a porosity reduction from 0.35-0.50 to 0.20-0.35 ( Figure 6; cases in between the largest porosity reduction of 0.50-0.20, or no porosity reduction of 0.35-0.35), the paleogeothermal gradients at 1590.5-2405.5 and 2605.5-3045.5 mbsf are estimated to be 60-1118C/km (mean: 868C/km) and 91-1518C/km (mean: 1218C/km), respectively.…”

Data report: permeability, compressibility, and microstructure of resedimented mudstone from IODP Expedition 322, Site C0011

“…Integrated Ocean Drilling Program (IODP) Expedition 322 is part of the Nankai Trough Seismogenic Zone Experiment (NanTro-SEIZE) project and aimed at understanding the initial presubduction conditions because the downdip evolution of those initial properties is what ultimately changes slip behavior along the plate interface from aseismic to seismic (Vrolijk, 1990;Hyndman et al, 1997;Moore and Saffer, 2001;Tobin and Kinoshita, 2006). Expedition 322 sampled and logged the incoming sedimentary strata and uppermost igneous basement of the Shikoku Basin, seaward of the Nankai Trough (southwestern Japan) prior to their arrival and burial at the Nankai subduction front (Saito et al, 2009;Underwood et al, 2010). Two sites were drilled: IODP Site C0011 on the northwest flank of the bathymetric high called Kashinosaki Knoll and IODP Site C0012 near the crest of the seamount (Fig.…”