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doi:10.2204/iodp.proc.333.101.2012

Sites C0011 and C0012

Scientific objectives

As mentioned previously, the upper stratigraphic intervals of the Shikoku Basin facies were not adequately sampled during Expedition 322. Therefore, one of the priorities for Expedition 333 was to fill in the coring gaps and expand the age-depth models into the Pliocene and Quaternary. The shallow section is also important for comprehensive profiles of physical properties and organic and interstitial water geochemistry. Thermal structure, including the effects of fluid circulation in the basement, is another of the critical input variables to document because of its influence on sediment diagenesis and fluid chemistry (Spinelli and Underwood, 2005; Saffer and McKiernan, 2009; Spinelli and Wang, 2008). The age of subducting lithosphere within the Kumano transect area is ~20 Ma (Okino et al., 1994), as verified by coring at Site C0012 (Underwood et al., 2010). The Kashinosaki Knoll lies west of the Zenisu Ridge intraoceanic thrust, which brings backarc basin crust to crop out at the seafloor further to the east (Lallemant et al., 1989; Henry et al., 1997). However, the respective contributions of active compressive tectonics and seamount volcanism to the Kashinosaki Knoll morphology are unclear, and the exact timing of volcanic activity responsible for the birth of the Kashinosaki Knoll (Ike et al., 2008a) still needs to be established by radiometric dating of the basalt. Dense near-surface heat flow measurements around the Kashinosaki Knoll indicate significantly higher value than the theoretical value estimated from the age of the Shikoku Basin (Kinoshita et al., 2008). Deeper thermal structure is required to document the entire heat flow pattern around the sites with high-quality borehole temperature measurement.

As subduction carries Shikoku Basin strata toward and beneath the accretionary prism, we expect fluids and physical properties to change downsection and downdip in response to hydration reactions (e.g., volcanic glass to zeolite + smectite), dehydration reactions (e.g., opal-to-quartz and smectite-to-illite), and crystalline cement precipitation (carbonates, zeolites, and silica). Documenting such changes is an essential ingredient of the NanTroSEIZE science plan. Sharp diagenetic fronts (especially opal-to-quartz) have been linked to anomalous offsets in profiles of porosity, P-wave velocity, and other geotechnical properties (Spinelli et al., 2007). LWD data from Site C0011 show offsets at ~250 m LWD depth below seafloor (LSF) that may be caused by this type of reaction (Underwood et al., 2010). Alteration of dispersed volcanic glass is also potentially important during diagenesis but, as yet, this component of the sediment budget is poorly understood (Scudder et al., 2009). Similarly, hydrous authigenic phases in the basalt (e.g., saponite from ridge-flank hydrothermal alteration) are susceptible to diagenetic reactions at higher temperatures. Updip migration of fluids (including hydrocarbons) toward the Shikoku Basin from landward zones of deeper seated dehydration reactions is a distinct possibility (Saffer et al., 2008), and this idea can be tested through a comprehensive program of geochemical analyses. Interpretation of the geochemistry, however, requires constraints on the in situ temperature.

Characterization of basement composition and structure is a high priority for NanTroSEIZE. Permeability and fluid flow within oceanic basalt are affected by many variables (Fisher, 1998). A long-term goal is to monitor and sample fluids in the basement using subseafloor observatories, but design of those experiments hinges on coring and logging results. As a prelude, we planned to concentrate first on documenting the basement’s structural architecture, hydrologic properties, and early alteration products. Products of early alteration within the uppermost basalt (e.g., saponite and calcite) change the rock’s bulk chemistry and physical properties (porosity and permeability). The extent of this alteration is important for constraining the volatile content of subducting crust. In addition, coring at least 100 m into basement and wireline logging during a future expedition will capture heterogeneities in fracture patterns and porosity that might be involved in delamination of the basalt downdip in the seismogenic zone.

The specific set of questions addressed by additional drilling at input sites are

• Is fluid circulation in basement and permeable sedimentary layers influencing heat flow and diagenesis at Sites C0011 and C0012?

• How does contrasting interstitial fluid chemistry at Sites C0011 and C0012 relate with in situ diagenesis and fluid flow?

• Can a change of physical properties between 200 and 250 mbsf at Site C0011 be related to lithologic variation or silica diagenesis? Does the same transition occur at Site C0012?

• Was magmatic activity heterogeneous in composition and age on the backarc basin basement high (Kashinosaki Knoll)?

• Is alteration of the upper oceanic basement heterogeneous and how does such alteration influence geochemical and fluid budgets?

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